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NATL INST. OF STAND & TECH

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NBS TECHNICAL NOTE 1139

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

RADON TRANSPORT THROUGH AND EXHALATION

FROM BUILDING MATERIALS:

A Review and Assessment

QC

100

U5753

NO. 1139

1981

OT^^>^
NATIONAL BUREAU OF STANDARDS

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RADON TRANSPORT THROUGH AND EXHALATION

FROM BUILDING MATERIALS:

A Review and Assessment

/.u^t.-

R. Colli

Center for Radiation Research

RJ. Rubin

Center for Materials Science

LI. Knab

Center for Building Technology

J.M.R. Hutchinson

Center for Radiation Research

National Bureau of Standards

Washington, DC 20234

Prepared for the

Office of Radiation Programs

U.S. Environmental Protection Agency

Washington, DC 20460

NATIONAL BUBBAU

OF 8TANOARD8

LIBRARY

OCT 2 1981

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U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued September 1981
National Bureau of Standards Technical Note 1139

Nat. Bur. Stand. (U.S.), Tech. Note 1139, 101 pages (Sept. 1981) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1981

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $4.50

(Add 25 percent for other than U.S. mailing)
ABSTRACT

This report was prepared, at the request of the U.S. Environmental Protection Agency, for the

purpose of reviewing, assessing, and sunmarizing what is currently known about radon transport through

and exhalation from building materials. In four chapters, the report 1) considers the routes of entry

of radon into buildings, describes the basic models for radcn transport through building materials,

critically reviews the small number of existing values for the necessary transport coefficients, and

summarizes the solutions of both steady-state and time-dependent transport cases; 2) reviews and

considers how the microstructural properties and internal characteristics of building materials may

affect the transport and exhalation of radon; 3) considers the exhalation process from a more macroscopic, phenomenological viewpoint, and summarizes selected experimental data on radium concentrations

in building materials, radon flux and exhalation rates from soils and building materials, and the

effects of meteorological variables on radon exhalation; and 4) reviews and assesses various measurement

methodologies that are used for laboratory and in situ studies of radon transport and exhalation. Needs

for further research in each area are also identified.

Keywords: Buildings; concrete; diffusion; exhalation; materials; measurement; permeability; radon;

transport.

m
ACKNOWLEDGMENTS

This review and assessment was prepared under the direction of a study group consisting of the

authors as well as P. W. Brown (NBS Center for Building Technology), W. A. Cassatt and E. H. Eisenhower

(NBS Center for Radiation Research), and I. C. Sanchez and L. E. Smith (NBS Center for Materials

Science). We want to thank these members of the study group for their contributions to this report.

Special thanks are due Robert Schofield, our summer student assistant, who painstakingly and efficiently

accumulated and organized our extensive reference collection. His contributions were appreciated long

after his brief stay at NBS.

Many other persons contributed by carefully reviewing and commenting on an earlier version. Dr. R.

A. Helmuth (Chemical and Physical Research Department, Portland Cement Association) was particularly

helpful in reviewing Chapter II and in giving us advice. We also benefited from comments and suggestions by Drs. R. Bruno (U.S. Environmental Protection Agency), Niels Jonassen (Technical University of

Denmark), W. W. Nazaroff (Lawrence Berkeley Laboratory), J. R. Noyce (NBS Center for Radiation Research),

and A. G. Scott (American Atcon, Inc.); and especially from the valuable suggestions and criticisms by

Allan Tanner (U.S. Geological Survey). Many of their comments have been incorporated, but, of course,

they share no responsibility for any errors in fact or interpretation that may remain.

Lastly, we gratefully acknowledge the NBS National Measurement Laboratory Text-Editing staff and

the NBS Technical Information and Publication Division for their excellent assistance in preparing this

report.

IV
TABLE OF CONTENTS

Page

Abstract.

Acknowledgments.

Table of Contents.

Introduction.

Glossary.

I. Assessment of Transport Mechanisms 9

A. The Transport Model 11

1. Qualitative Description of Model 11

2. Quantitative Characterization of Model 12

B. Transport Coefficients for Building Materials 14

1. Diffusion Coefficient 14

2. Permeability Coefficient 15

C. Related Transport Problems 16

1. Transport Through Surface Seal ants 16

2. Identification of the Dominant Transport Mechanism 17

3. Time-Dependent Effects 18

D. Quantitative Discussion of Various Transport Cases 19

1. Case I: Exhalation from the Ground into Open Air 21

2. Case II: Exhalation from the Ground into a Closed Space 22

3. Case III: Exhalation from One Side of a Slab 23

4. Case IV: Exhalation from a Slab with an Impervious Coating Applied to One Side.. 25

5. Case V: Influence of Darcy Flow on Exhalation from a Slab 26

6. Case VI: Time Development of Steady-State Concentration Profile in a Slab 28

7. Case VII: Time Development of Steady-State Pressure Profile in a Slab 29

8. Case VIII: Exhalation from a Slab Resting on the Ground 30

E. Conclusions and Recommendations 32
Page

II. The Role of the Internal Characteristics of Building Materials 33

A. A Microstructural View of the Emanation Process Within a Material 33

B. Influence of Radium Content of Materials 35

C. Pore Geometry and Water Characteristics 36

D. A Case Study: Hardened Concrete 37

1. Characteristics of Hardened Concrete 37

2. Movement of Pore Fluids in Concrete 41

3. Effect of Concrete Properties on Radon Transport 43

E. Conclusions 44

III. A Review of Experimental Data 45

A. Radium Content of Building Materials 47

B. Radon Exhalation from Soils and Building Materials 48

C. Meteorological Factors Affecting Radon Exhalation 53

1. Moisture Content of Materials and Humidity Effects 54

2. Pressure Effects 55

3. Temperature Effects 56

4. Other Factors 58

D. Conclusions and Recommendations 58

IV. Assessment of Measurement Methodologies 61

A. Assay of Gaseous Radon Samples 62

1. Internal Gas Counting Techniques 62

2. Scintillation Counting Techniques 64

B. Laboratory Measurement of Radon Exhalation 65

C. In Situ Measurement of Radon Flux Density and Concentration in Soil Gas and Pore

Spaces 68

1. Radon Flux Density 69

2. Radon Concentration in Soil Gas and Pore Spaces 77

D. In Situ Measurement of Radon Concentration in Indoor Air 78

1. Instantaneous Methods 79

2. Time-Integral Methods 80

3. Continuous Methods 83

E. Conclusions and Recommendations 86

Cited References 89

vi
INTRODUCTION

The potential health hazards of radiation exposure to the general public due to radon in buildings

has, in the past few years, received increasing attention. This concern about indoor radon has been

documented in extenso (UN 77; HEW 79: CoM 80; RPC 80), and substantial efforts are being made to study

and understand various aspects of the problem. Although several thousand publications relating to

indoor radon exist in the open literature, the prediction of indoor radon concentrations is still in a

very rudimentary stage. The reasons for this situation arise from the complex nature of the sources and

behavior of radon including its release and subsequent movement from the earth and from other repositories of the radium parent, and the dynamics within indoor atmospheres. Another important factor lies

in the fragmentary nature of many previous investigations often conducted under uncontrolled conditions

and with measurement methodologies of undemonstrated reliability.

One of the most poorly understood and critically needed areas of research concerns the sources of

indoor radon and the relation to radon transport through and exhalation from building materials. An

understanding of the transport - exhalation mechanism is necessary in order to predict the sources of

radon and routes of entry into buildings. Essentially, what is needed is a phenomenological basis or

model that can be used (i) to characterize building materials as sources of radon, and (ii) to characterize the mechanisms of radon transport through building materials. This type of predictive capability does not presently exist for several basic reasons:

1. no description of radon transport has been verified mainly because sufficiently accurate

transport coefficients and other data (i.e., diffusion coefficients, permeability coefficients,

porosity of the building material, and radon production rate per unit pore volume) are not

available; and

2. the role of the detailed intrinsic and microstructural properties of building materials (e.g.,

porosity, pore size and distribution, water content and state) in influencing the entrainment

and transport of radon is not well understood;

* 222 Unless otherwise stated, the term "radon" will be used in this report to refer to the Rn isotope, 220 and "thoron" to Rn. It should also be mentioned that radioactivity is specified throughout this

report in units of a submultiple of the "curie" (e.g., pCi) and not in the SI unit "becquerel" (Bq).

This was done for convenience since most, if not all, of the existing radon literature uses the older,

special units.

1
3. the influences of external variables such as meteorological conditions on exhalation rates,

while qualitatively understood in most cases, are not sufficiently well quantified.

4. radon measurement methodologies, especially those used in situ ("in the field") are frequently

not of adequately proven accuracy.

In short, many of the basic processes are not well understood and existing data are not sufficient

to quantitatively predict or estimate the flux or exhalation rate of radon from building materials.

The Environmental Protection Agency requested that the National Bureau of Standards address and

investigate the above concerns. This report has been prepared, as a part of this overall effort, for

the purpose of reviewing, assessing, and summarizing what is currently known about radon transport

through and exhalation from building materials. The approach has been to evaluate the literature to the

extent possible within very severe time constraints, and to consult with experts in the field. One must

recognize that this review is not exhaustive, but, of necessity, selectively considers only those areas

that were believed to be most directly applicable to understanding radon transport and exhalation. It

must be emphasized that no NBS-based research was undertaken at this stage to verify or validate

existing data and information.

This report has been structured into four chapters corresponding to the main areas of concern.

Within each chapter, an attempt has been made to identify those areas requiring further study.

The assessment of transport mechanisms is covered in Chapter I. It considers the routes of entry

of radon into buildings, describes the basic models for the transport of radon through building materials,

critically reviews the small number of existing values for the necessary transport coefficients, and

summarizes the solutions of both the steady-state and time-dependent transport equations for various

transport cases.

Chapter II reviews and considers how the microstructural properties and internal characteristics of

building materials may affect the entrainment and transport of radon through the pore system of the

materials. These characteristics include radium distribution, internal geometry, pore size and distribution, and water content and state. Because concrete and other cementitious materials are the basic

components of most buildings, hardened concrete is used as an important and representative example to

more fully explore the role of these internal characteristics.
Chapter III reviews and summarizes data on radium concentrations in building materials, radon flux

and exhalation rates from soils and building materials, and the effects of 'meteorological variables on

radon exhalation. In effect, it focuses on the radon exhalation process from a more macroscopic,

phenomenological viewpoint, and addresses whether it is possible to predict or estimate the radon flux

or exhalation rate from building materials based on existing data.

Most of what is known about radon transport through or exhalation from building materials is based

upon observations and measurements in laboratory or in situ (field) settings. Therefore, an understanding of these previously employed measurement methodologies is necessary in order to evaluate the

applicability and validity of existing transport and exhalation data, as well as to plan future laboratory transport and exhalation experiments. In addition, it is reasonable to presume that any predictive

phenomenological model will be based in part on field-based measurements. A review and assessment of

various measurement methodologies is provided in Chapter IV.

A number of very specific and specialized terms are used throughout this report. Inasmuch as

readers may be unfamiliar with some of these terms, the following glossary is provided.

GLOSSARY

DECAY CONSTANT , X, is a proportionality constant which relates the number of radioactive events per unit

time (dn/dt) to the number of radioactive atoms present (n), i.e.,

= -X

1 /dn\

n \dt)

The halflife , which is the length of time (T) required for a given number of radioactive atoms to

decrease by half, is related to the decay constant by T = (In 2)A.

DIFFUSION (or DIFFUSIVE TRANSPORT ), which is governed by Fick's Law, refers to the movement of a gaseous

component such as radon through the air-filled pore system of a porous material in which the driving

force for the movement is a concentration gradient. For a thin slab of material separating regions in

which the concentration of radon is different, Fick's Law states that the diffusive flux density of

radon through the slab is proportional to the concentration difference and inversely proportional to the

slab thickness.

DIFFUSION COEFFICIENT . D , is the effective transport coefficient for diffusive transport, and is the

proportionality constant, in Fick's law, that relates the diffusive flux density through a porous

material to the concentration gradient.

DIFFUSION LENGTH is a characteristic length, £, associated with diffusion of a radioactive species.

EMANATION POWER , ti, for a material is equal to the quotient of the number of radon atoms liberated (n,)

from the material by the number of radon atoms formed (n ) by radioactive decay of radium in the

material in unit time (i.e., fractional escape). In effect, it is just the fraction of radon that is

"missing" from the material, and may be very dependent on the shape and state of the material. Note

that it is also just equal to the ratio of the radon radioactivity released (A,) to the radon radioactivity produced (A ) in the material under steady-state conditions.

n A^

It is historically the most unequivocal term, (see, for example, Hahn [Hah 36]), and is probably the

most intrinsically fundamental exhalation quantity for a unit element of material. Tanner [Tan 78]

notes that the Soviet literature uses the term "coefficient of emanation" , and that other equivalent

terms include "escape ratio" , "escape-to-production ratio" , and "percent emanation" .
EMANATION RATE , following the definition of emanation power, is just the time rate of change of the

emanation power . It is given by the quotient of 6n by dt where dm is the change in the emanation power

in the time interval dt. It may also be expressed in terms of radioactivity

dt dt dt

In the absence of external influences, the emanation rate (for a given quantity and type of material) is

time invariant. Since the formation of radon is relatively constant compared to the effect of external

influences on the rate of release of radon, the emanation rate may be approximated by ^ dn^/dt dA^/dt

dt - ~"n "^ ~A

Obviously, the concept of an emanation rate is a meaningless concept for an unbounded amount of material

(e.g., an open field or concrete wall).

EXHALATION refers to the liberation or release of radon from a material. The use of the term "emanation"

for this concept is deprecated.

EXHALATION RATE , R, is just the time rate of release of radon from a material, and is normally expressed

either in units of the number of radon atoms per unit time (e.g., atoms-s ) or in units of a submultiple of the curie by a suitable unit of time (e.g., pCi-s ). It is either given by dn,/dt or

dA,/dt where dn, is the number of radon atoms released from the material in time dt, and dA, is the

corresponding radon radioactivity (with decay constant X).

dn^ _ ^/dA^\ dt \ \dt /

The exhalation rate is not normalized to area, mass, radon content of the material, etc. Obviously, the

concept of exhalation rate is applicable to both bounded or unbounded quantities of a material.

FLUID FLOW (or PERMEATION ), which is governed by Darcy's law, refers to the movement of a fluid through

the pore system of a porous material in which the driving force is a pressure gradient. For a thin slab

of porous material separating regions maintained at different fluid pressures, Darcy's Law states that

the fluid flux density through the porous slab is proportional to the pressure difference and inversely

proportional to the slab thickness.

FLUX DENSITY is the net number of atoms which pass through a unit cross-sectional area per unit time.

For radioactive species, the number of atoms may be specified in terms of its radioactivity.
PERMEABILITY COEFFICIENT , k, (often termed just PERMEABILITY ) is the transport coefficient for permeability, and is the proportionality constant, in Darcy's law, that relates fluid flux through a porous

material to the pressure gradient.

POROSITY , e, is a characteristic property of a porous material, which is given by the ratio of the

volume of the pore spaces in the material to the total volume of the material.

POROUS MEDIUM is a term used to describe a heterogeneous material in which any given point either lies

in a void space or in a solid phase, and in which it is possible to move between widely separated points

in the void space without crossing a solid boundary.

RADIUM , as used in this report, refers to the Ra isotope which is the radioactive progenitor of radon

(22^Rn).

222 RADON , as used in this report, refers to the Rn isotope (unless explicitly stated otherwise); thoron

220

is used to refer to the Rn isotope.

RADON FLUX DENSITY , E, (or AREAL EXHALATION RATE ) for the surface of a material (bounded or unbounded)

is the quotient of dR by da where dR is the exhalation rate from the surface of the material with area

da.

F dR

" da

It is just the exhalation rate normalized to unit area, and is normally expressed either as the number

-2 -1

of radon atoms per umt area and time (e.g., atoms-m -s ) or in a submultiple of the curie by a -2 -1 suitable unit of time and a suitable unit of area (e.g., pCi-m -s ).

TRANSPORT is used generically to refer to any mechanism by which radon moves through or from a material.

UNDERPRESSURE (OVERPRESSURE) refers to a pressure difference measured with respect to atmospheric

pressure.

VENTILATION RATE (or AIR EXCHANGE RATE ) is a term applied to a room or a building and is the number of

times the air in a unit volume is replaced per unit time.

CHAPTER I

ASSESSMENT OF TRANSPORT MECHANISMS

The purpose of this Chapter is to assess critically the current state of knowledge of radon

transport through and exhalation from building materials.

Before proceeding, however, it may be useful to consider the role of transport of radon in or

through building materials in the broader context of its contribution to the indoor radon concentration.

The radon flux density from the indoor surfaces of a building constitutes only one of several sources of

indoor radon. For a closed space such as a room in a building, a theoretical correlation may be

established between radon concentration and radon input from various sources. The basis for this

correlation is the following balance equation for the different source contributions to the rate of

change of the indoor radon concentration [UN 80],

dC.(t)

—1^— = (ES/V) + (q/V) + (a^C;+agC;)X^ - C.(t)(X+\^) , (1.1)

where C.(t) is the radon concentration in a room at time t;

E is the radon flux density from unit surface of the room;

S is the exhaling surface area of the room;

V is the volume of the room;

q is the radon release rate from a source in the room (water, natural gas);

C' is the radon concentration in the inlet or ventilating air;

C' is the radon concentration in soil gas which might be drawn into a basement room through, for

example, a major crack or a hole in the basement envelope which serves as an entry port for

utilities;

a and a = 1 - a are the relative weights of these two external air components of the building

ventilation;

\ is the ventilation rate (air changes per unit time); and

k is the radon decay constant.

Under steady conditions in which the source terms remain constant fn balance eq. (1.1),

dC. dr = 0'

*This equation, adapted from one appearing in a draft of the revised UNSCEAR report [UN 80], has been

modified slightly to suit our present purpose. It should be emphasized that the relative weights, a

and a , which we have introduced may be a function of the ventilation rate.
and the steady- state concentration of radon is

(ES/V) + (q/V) + (a C'+a C:)\

In a typical house, \ , which lies in the range 0.1<\ <3 h , is much greater than the decay constant

of radon (\=0.01 h~ ), so

C. 2 (ES/V\^) + (q/V\^) + a^C; + a^c; . (1.3)

The draft UNSCEAR report [UN 80] lists the relative significance of radon sources in a "reference

3 2 house" which is defined as having a volume of 200 m and an inner surface area of 350 m with a ventilation rate of 0.5 h . The relative indoor radon contributions attributable to various sources are

given as follows:

building materials 80% [first term in eq. (1.3)]

outside air 10% [last two terms in eq. (1.3)]

water (toilets, showers, etc.) 5%

natural gas 4% } [contributors to second terra in eq. (1.3)]

liquified petroleum gas < 1%

It should be emphasized, however, that it is not clear what is contained in the "building materials"

source category. The report [UN 80] does not specifically address whether this is only the contribution

of radon from the building material itself, or whether it also includes transport of radon from soil gas

through the building material.

Each of the terms in eq. (1.3) which contributes to the indoor radon concentration is associated

with a form of transport. The first and last of these represent two physically different kinds of

transport through building materials. The magnitude of the contribution of the last term is completely

independent of the physical properties of the building materials, but depends instead on the dimensions

of the crack(s) and the pressure difference between inside and outside. For this reason, such a form of

transport is not considered in this report. Instead, this assessment is limited to the first transport

source where the flux density, E, of radon atoms per second per square meter exhaling from a surface

depends upon intrinsic physical properties of the material, such as effective diffusion coefficient,

permeability, porosity, and radium-226 content.

Successive sections of this chapter include:

A. a description of the qualitative and quantitative characteristics of the physical model which

forms the basis of our understanding of radon transport through building materials;

10

I
B. a critical review of previous work on the measurement of effective diffusion coefficients and

permeabilities of building materials;

C. a brief discussion of some related transport problems which have been considered, or should be

considered, in evaluating the source term, E, in eqs. (1.1) - (1.3) (e.g., transport through

multilayered materials, or the effect of time variation of external parameters such as

pressure or external radon concentration);

D. a summary of the time-dependent transport equations for radon in a porous medium, and a list

of a set of solutions of both the steady-state and time-dependent transport equations for

various transport cases; and

E. some conclusions and a recommendation that a series of measurements be made which are intended

to determine a complete set of transport coefficients and related physical properties for

several different building materials.

I. A. The Transport Model

I.A.I. Qualitative Description of Model

A building material is assumed to be a homogeneous porous medium, and transport of radon through

the material is assumed to take place through Its air-filled pore system. Transport of radon takes

place in response to two different driving forces. A radon concentration difference is the driving

force for diffusive transport through the air-filled pore system; and the associated transport coefficient is the effective diffusion coefficient of the material for radon. A second driving force is a

pressure difference. Air in the pore system moves in response to a pressure difference and if radon is

present, it moves with the air. The associated transport coefficient is the permeability coefficient of

the material for air. In addition to these transport coefficients, which are unique for each building

material, two other material constants which enter the transport equations for radon are the porosity

(the ratio of pore volume to total volume), and the volume production rate of radon which is free to

move through the pore system. It will be seen in the next section that there are very few measurements

of the transport coefficients of radon in building materials. It should be recognized, however, that

there is an extensive geological literature on radon migration in the ground which is comprehensively

reviewed by Tanner [Tan 64; Tan 78]. Many of the qualitative aspects of radon migration in the ground,

described in Tanner's reviews, are transferable to the discussion of transport of radon in building

materials.

In addition to Tanner's reviews of the transport of radon in porous media, the monographs by

Scheidegger [Sch 74], Beran [Ber 68], and Carman [Car 56], and the book edited by DeWiest [DeW 69] deal

11
with aspects of the problem from different points of view. Diffusion or mass transport by fluid flow

through porous or granular materials is influenced by pore shape, size, and volume, particle packing and

size distribution (see, for example. Chapter II which describes the pore system in concrete). A

hierarchy of parameters can be introduced to characterize a pore system [Sch 74; Ber 68; Car 56]. The

simplest of these, and the only one which we will use explicitly, is the porosity which is defined as

the ratio of the volume of the pore spaces in the material to the total volume of the material.

If the pore volume in a material such as concrete or soil is filled with a fluid such as air,

diffusive transport of radon through the pore system takes place primarily via diffusion through the

air-filled pores because the coefficient of diffusion of radon through air is at least 8 or 9 orders of

magnitude greater than the coefficient of diffusion through the crystalline or glassy materials composing

the matrix of the porous medium [Tan 64; Tan 78]. The influence of external variables, such as temperature

and pressure on the one hand and humidity on the other, on the effective diffusion coefficient of radon

in a porous material may be quite different. In the case of moderate temperature and pressure changes,

the configuration of the pore system would not be expected to change. Thus, any variation of the

effective diffusion coefficient with changes in these variables would simply reflect the weak dependence

1/2 of the diffusion coefficient of radon in air (D) on these same variables (e.g., D -^ T where T is the

absolute temperature). On the other hand, when the external humidity is increased in the case of a

material such as concrete (see Section II.D.), one would expect additional condensation of water in the

finer capillaries of the pore system. Thus, although the dependence of the diffusion coefficient of

radon in air on water content is weak, an alteration in the characteristics of the pore system in

concrete with increasing water vapor pressure may signal a strong dependence on humidity of the effective

diffusion coefficient of radon in concrete. Moisture content has a different effect which is not

related to the transport of radon in the sense in which we have been using it. Tanner [Tan 64; Tan 78]

has discussed the exhalation of radon into the pore system from trace amounts of radium in the matrix

material. Viewed from the point of view of the pore system, there is a pore-volume production rate of

radon which appears to be enhanced if the "humidity in the pores" is increased. [Tanner also mentions

observations of exhalation of radon from the ground in the extreme case of saturation by a heavy

rainfall. In such a case, there is a temporary decrease in exhalation which results from complete

filling or blocking of the pore system near the surface by water (in which radon is less mobile)].

I. A. 2. Quantitative Characterization of Model

As indicated, the proposed model for the transport of radon in or through a building material

assumes that the building material is a porous medium and that transport of radon takes place through

12
the air-filled pore system of the material. In response to a concentration difference of radon, there

is diffusive transport of the radon in a direction which tends to equalize concentration. The empirical

law which relates the diffusive current to a concentration difference is known as Pick's law. Given a

thin slab of building material of thickness AZ and given fixed concentrations C^ to the right of the

slab and C, to the left, then the steady diffusive flux density, j„, (net number of gas molecules

passing from left to right through unit area of the slab per second) is proportional to the concentration

difference AC =

^o'^l' ^'^^ inversely proportional to the thickness, AZ. The limiting form for small

thicknesses and small concentration differences is a statement of Pick's law,

(S)-

where D is the effective diffusion coefficient of the gas through the air in the pore system of the

material.

Pick's law has been verified experimentally for diffusion of gases in homogeneous media such as

gases, liquids, and crystalline or glassy solids. However, in a detailed study of diffusive transport

through a given building material, it should be established experimentally rather than be assumed that

there is an effective diffusion coefficient relating diffusive flux to concentration gradient in that

material. Thus, if both the concentration difference, AC, and the thickness AZ are increased proportionally, the diffusive flux should be unaltered (except for a possible radioactive decay correction in

the case of radon). It should be noted that if the validity of Pick's law is established for one or

more simple gases in a given building material, then there is no reason to doubt that Pick's law would

also hold for radon.

There is a similar operational procedure for establishing an empirical transport relation between

air flow through a slab of porous material and pressure difference. The empirical law which relates the

3 2 air volume current, v, (cm per second per cm ) through a porous material to the pressure difference,

AP=P»-P, , maintained across a slab of thickness AZ is known as Darcy's law,

k dP ., c\

where t] is the dynamic viscosity of the gas (air) in the pore system and k is the permeability coefficient of the porous material. Similar questions regarding the validity of Darcy's law for a building

material arise as in the case of Pick's law. However, the validity of either law should imply the

validity of the other.

13
I.B. Transport Coefficients for Building Materials

Very few measurements of the transport coefficients of radon in building materials have been made.

A critical review and evaluation of this previous work follows.

I.B.I. Diffusion Coefficient

Culot, Olson, and Schiager [COS 76; COS 73] have determined an effective diffusion coefficient, D ,

for radon in a particular concrete of unmeasured porosity, e. For a range of assumed values of the

-5 -5 porosity (0.05-0.25), their value of the diffusion coefficient ranged from 1.69 x 10 to 3.08 x 10

2 -1

cm -s

Jonassen and McLaughlin [JoM 76; JoM 78], in a more complete study of a particular concrete

containing trace amounts of radium-226, determined values for the effective diffusion coefficient,

-5 2 -1 porosity, and the production rate of radon. The values obtained for D and e were 3.07 x 10 cm -s

and 0.265, respectively. It should be noted that this determination [JoM 76; JoM 78] was an integral

part of the measurement of the exhalation rate of the concrete. In the course of carrying out the

exhalation measurements (by following the build-up of radon in a closed pressure chamber containing the

concrete, see Section IV. B), Jonassen and McLaughlin also obtained indirect evidence for the validity of

the assumed model for diffusion of radon in concrete. Namely, concrete appears to behave like an ideal

porous solid for the diffusive transport of radon. This conclusion is based on the agreement between

the observed radon concentration build-up with time and the form of the theoretically predicted build-up

curve.

In the same report, Jonassen and McLaughlin [JoM 78] carried out the difficult experiment of

measuring the steady-state concentration profile of radon across a 30-cm thick concrete wall. The

measurements were made by drilling holes to different depths in the wall to insert sampling tubes

through which air-radon samples could be collected in evacuated liter flasks. This admittedly crude

experiment, which sampled pore air from an ill -defined volume surrounding the open end of each sampling

tube, yielded a concentration profile similar in form to the theoretically predicted one (see Case III

in Section I.D.3).

More recently, Stranden and Berteig [StB 80] have determined a diffusion coefficient and porosity

for radon in concrete, brick, and LECA (a light expended clay aggregate). There are no other known

measurements of diffusion coefficients of radon in concrete or any other building material such as

phosphogypsum board.

14
I.B.2. Permeability Coefficient

Data for the permeability coefficient of air in concrete are available mainly because concrete

structures such as sewage tanks, gas purifiers and containment vessels in nuclear reactors are required

to be airtight under a specified internal pressure. For example, the steady-state flow of air through

a certain concrete which is 10-cm thick is 1.45 x 10 cm -s -cm for a pressure difference of 26 mm Hg

[Nev 71]. (The time to establish steady-state for concrete up to 23-cm thick is given as "several hours".)

Assuming that the concrete contains trace amounts of radium- 226, one can solve, in the steady- state, the

combined transport equation for diffusion of radon and transport of radon by air flow under a pressure

gradient across a 10-cm thick slab of concrete. If we assume the above stated flow velocity (i.e.,

1.45 X 10 cm 'S -cm ), an effective diffusion coefficient of D = 3.1 x 10 cm -s , and a

porosity of e = 0.1, then we can calculate the ratio of the radon fluxes from the two surfaces of the

* slab of concrete (see Case V in Section I.D.5) . This ratio is 1.16.

The only experimental investigation of the influence of a pressure change on the exhalation of

radon from concrete known to us is by McLaughlin and Jonassen [McJ 78]. They sealed 60-cm diameter

exhalation cans on a 30-cm thick concrete wall and maintained constant underpressures relative to

atmospheric pressure in the range 5-25 mm Hg during the build-up of radon in the cans. They found that

the exhalation rate at an underpressure of 25 ram Hg exceeded the zero underpressure or atmospheric

pressure exhalation rate by a factor of 6. One difficulty with the interpretation of this measurement

is that in order to prevent leaks, an epoxy resin sealant was applied to the wall surface surrounding

the 60-cm diameter exhalation can to a distance of approximately 30 cm. The steady-state concentration

of radon near the wall surface under the epoxy coating should be considerably enhanced, and the effect

of this distortion in the concentration field on the exhalation rate measurement is difficult to

estimate.

It should also be noted that for a 25 ram Hg underpressure, the observed total flow velocity into

-1 -4 3 -1 -2 the exhalation can of 2 L-h is equivalent to 2 x 10 cm -s -cm . This flow velocity [McJ 78] is

approximately 400 times greater than that for the thickness-scaled flow velocity of air through 30

-6 3 -1 -2 centimeters of the concrete mentioned at the beginning of this section (1/3 x 1.45 x 10 cm -s -cm )

[Nev 71]. This large difference in permeability may reflect either a large difference in the permeability properties of the two concretes or the fact that the flow velocity, which must be measured under

specified high-pressure difference conditions [Nev 71], does not scale linearly to the small pressure

differences of interest here, or both. Any nonlinearity for large pressure differences would, most

This particular solution of the steady-state transport equation has not appeared in the literature.

15
likely, lead to an underestimate of the flow velocity at low pressure differences. The resolution of

the nature of the above difference requires further investigation. It is shown in Section I.D.5 that

if the permeability of a concrete is as large as that indicated in the McLaughl in-Jonassen experiment

[McJ 78], then the radon transport properties of a 10-cm slab should be dominated by the effect of a

25 mm Hg pressure difference.

I.e. Related Transport Problems

I.C.I. Transport Through Surface Sealants

In many circumstances, a building material in a basement floor or wall or a room wall has a

multilayered or composite character. This composite nature may arise in the fabrication process, the

esthetic addition of paint or other surface coverings, or the application of a surface sealant as a

remedial measure to inhibit the transport of radon through concrete. There appear to be relatively few

measurements of the diffusion coefficient of radon and the solubility coefficient of radon in a surface

sealant [BCP 74; HEG 75]. The two examples where measurements have been made were for polycarbonate

film [HEG 75] and for asphalt emulsion [BCP 74]. In this connection, there is an extensive literature

devoted to the measurement of the product of the diffusion coefficient and the solubility for simple

gases in polymeric films (see, for example, the review by Hwang, Choi, and Kammermeyer [HCK 74]. This

product, which occurs naturally in the analysis of the experiments, has also been given the name

"permeability". It should not be confused with the permeability k in Darcy's law.

In addition to the foregoing measurements, qualitative searches have been made [COS 73] to select

from among those sealants which are available commercially the one(s) most effective in inhibiting the

diffusive transport of radon through a 3-inch layer of concrete.

The modeling of diffusive transport through multilayer slabs is straightforward. A different class

of more complex radon transport problems arises if flaws such as pinholes or cracks exist in one of

the layers. Then the steady-state concentration field in the vicinity of the flaw is no longer onedimensional. However, mathematical techniques analogous to those used in determining edge corrections

in capacitor configurations are available for estimating the diffusive flux through the flaw. Other

direct, brute-force numerical solution methods are also available [EdB 80].

16
I.e. 2. Identification of the Dominant Transport Mechanism

There is no clear-cut answer to the question: Is the diffusion of radon through building materials

the dominant transport mechanism contributing to the steady-state indoor concentration of radon? In

part, this is because the dominant radon sources for a given building are very situational and sitespecific depending on the radon concentration in the soil gas surrounding the building's foundation, the

integrity of the building envelope, concentration of radon in the building's services (water, natural

gas), etc. It is possible, however, to estimate the steady-state indoor radon concentration if diffusion is the only transport mechanism. This estimate requires knowing: 1) the ventilation rate of the

indoor air; 2) the source strengths of radon (i.e., the production rate in the building material and the

external concentrations of radon at the foundation wall and floors); and 3) the diffusive transport

coefficients, porosities, and thicknesses of slabs of building materials (recall eq. (1.3)).

One example of such an estimate is a report to the Atomic Energy Control Board of Canada [DSM 78].

From measurements of external concentrations of radon and radon flux density, assumed source strength of

radon in the concrete, diffusion coefficient and porosity estimates of Culot et al . [COS 76] for

concrete, estimates of ventilation rates in 20 houses at Elliot Lake, and an average indoor radon

concentration of 4 pCi-L , the two principal conclusions of the report were: first, that the dominant

source of diffusing radon had to be in the concrete itself; and second, since the measured diffusive

flux densities were insufficient to maintain the observed indoor radon concentration, that radon

diffusion from or through concrete could not be a significant source of radon at Elliot Lake. This

conclusion was based on a relatively high assumed value of the ventilation rate (1 air change per hour).

However, if the assumptions are accepted, then the useful conclusion is reached that at Elliot Lake,

radon must be entering the houses through cracks in basement walls, or holes through which utilities

enter. The major source of radon in the buildings was not concrete or other building materials, but

-rather was soil gas containing radon which entered through discontinuities in the construction, cracks

in floors and basement walls, holes through which utilities enter, drains with improper traps, etc.

This may not be a typical situation for houses, however, since Elliot Lake is in a uranium district and

may be geologically atypical.

In contradistinction, a twenty-house study in the New York-New Jersey area, performed by the

Environmental Measurements Laboratory [GeB 78], showed that the indoor radon concentration could be

accounted for reasonably well on the basis of two radon sources, the radon flux density through the

basement floor and the radon concentration in the water supply, and an inferred average ventilation

rate. Measurement data for these variables were subjected to multiple regression analyses, and the

17
results were then used to obtain calculated values of the average annual radon concentrations which were

in rough agreement with directly determined values. Although the observed mean indoor radon concentrations

correlated reasonably well with radon exhalation rates from basements, this does not necessarily imply

that there is a causal relationship, and it may only be an indication of the internal consistency in the

radon concentration and radon flux measurement data. In this work [GeB 78], exhalation rates were

measured over floor cracks in older houses as well as over intact concrete floors in newer houses.

Within any given building, wide variations in radon exhalation were observed among simultaneous measurements and between successive sets of measurements. Although the radium-226 concentration in the soil

around the houses is fairly uniform at approximately 1 pCi-g , and all building codes in the New

York-New Jersey area require a 4-inch thick concrete floor in the basements, a 10-fold range in the

average radon flux from the floors was found. This implies a correspondingly large range of radon input

rates. The causes of this variation, part but not all of which can be explained by cracks, are unknown.

It was suggested [CoM 80] that these wide variations, as well as similar 100-fold ones seen in houses

with unvented crawl spaces, indicate that the rate depends on the type of soil. It was also pointed out

that soil type is only one of many factors. Depending on meteorological conditions above the soil,

100-fold changes in a 12-hour period can be observed (see, for example. Chapter III). Soil is a major

source in many cases, but it appears that this is not necessarily universal.

Most European investigators believe that the dominant source of indoor radon is from diffusion from

building rtiaterials. In Denmark, for example, Jonassen and McLaughlin [JoM 76] concluded that radon

originated primarily from building materials since indoor radon concentrations calculated from exhalation rates of walls and floors were in reasonable agreement with direct measurements of the average

radon concentration. Significantly, no differences in the exhalation rates between floors and walls in

contact with soils and walls not facing soils were found.

More recently, Goldsmith, et al . [GrtA 80] investigated the routes of entry of radon into two

buildings in the Canonsburg (Pennsylvania) Industrial Park which were built on low-level radiumcontaminated ground. They identified and investigated a number of sources and pathways, and concluded

that approximately 40-50% of the indoor radon concentration was due to diffusion through the intact

concrete floors, and the remainder was from cracks, floor drains, and other openings in the building

envelopes.

I.e. 3. Time-Dependent Effects

Most of the investigations of the balance eq. (1.1) and the transport eq. (1.6) focus on steady-state

behavior. It is possible to analyze these equations and determine effective relaxation times to

18
establish steady-state conditions. For example, if the external radon concentration changes to a

different steady value (in response to a change in meteorological conditions, or simply as a diurnal

variation), then the relaxation time for establishing a new steady-state concentration profile in a

10-cm thick concrete slab is of the order of 3.5 days (see Case VI in Section I.D.6). In such a case,

the effect of diurnal variations in radon concentration is "averaged out" and the transport of radon

through the concrete slab will depend only on the time-averaged radon concentrations outside the slab.

In a similar analysis of the relaxation of the pressure in the pores of the concrete following a change

in the external pressure, it is shown in Case VII of Section I.D.7 that the relaxation time (for the

same 10-cm thick concrete slab) to establish steady-state conditions is of the order of 1 hour.

I.D. Quantitative Discussion of Various Transport Cases

The simplest possible geometrical configuration in which to discuss the transport of radon in a

porous medium will be considered. Namely, we consider one-dimensional transport through a slab where it

is assumed that all concentrations and physical properties are constant in directions parallel to the

surface of the slab and that all characteristic lengths in the normal direction are small compared to

the transverse dimensions. Even with the restriction to one-dimensional configurations, there is a

large number of interesting cases of steady-state transport which* have been analyzed in different

contexts and which are relevant to the discussion of radon transport in and through building materials.

Before enumerating these illustrative cases, the equation for one-dimensional transport of a radioactive

gas such as radon in a porous medium in the presence of a concentration gradient and a pressure gradient

will be presented without derivation. The equations are contained in Clements and Wilkening [CIW 74]

where some earlier references may be found. A derivation is given in Clements' thesis [Cle 74] which is

similar to the derivation of Bulashevich and Khairitdinov [Buk 59]. Other references to the older

literature may be found in Tanner [Tan 64; Tan 78]. Additional discussion and references to the

literature on the transport equation are given by Culot et al . [COS 76]. This last reference is

particularly useful in pointing to confusion in the literature regarding values quoted for the effective

diffusion coefficient D . e

The following definitions are used throughout this section, and the boundary conditions will be

stated for each case:

£ porosity, ratio of pore volume to bulk volume

2

k permeability coefficient in expression for Darcy's law, m

n dynamic viscosity of fluid in pores, kg-m -s -3

p density of fluid in pores, kg-m

2 -1

D effective diffusion coefficient in porous medium, coefficient in Pick's law, m -s

19
222

C Rn concentration in the pore system, in units of radon activity per unit volume of pore

system, pCi-m

Z distance measured in direction of transport, m

k decay constant of ^^^Rn, \=(1n 2)/T=2.1 x lO"^ s"-*" where T = ^^^Rn halflife

222 222

4> Rn production rate, number of Rn atoms produced per unit time per unit pore volume that

222 ??fi are free to migrate through the pore system (i.e., Rn produced by the effective Ra),

pCi's" -m"

S. diffusion length, or lifetime diffusion-distance, a convenient length scale characterizing

diffusion of atoms which undergo radioactive decay,

V fluid volume current density, volume of fluid in pore system flowing per unit time per unit

3 -1 -2 -1 geometric area, m -s -m = m-s ; for pressure-induced flow, according to Darcy's law ^--_ k ap naz

-1 -2 where P is the pressure, kg-m -s .

Jq diffusion current density or flux density of radon, radon activity per unit time per unit

geometric area transported by molecular diffusion through the pore system, depends on the -1 -2 concentration gradient through Pick's law, pCi-s -m ,

i = - D ^ Jd "e az

jj transport current density of radon, number of radon atoms per unit time per unit geometric

-1 -2 area transported by fluid flow, pCi-s -m ,

jj = vC

P ,p the pressure P is assumed to be the sum of a steady component, P , and a small time varying

component, p, in deriving eq. (1.8) using Darcy's law.

The time-dependent one-dimensional transport equations can be written as:

ac _ ''e a^c 1 3(vC) xp . . n c-. at

- r

^

i -az

- ^c + $ (1-6)

V = - ^ II , (Darcy's law) . (1.7)

In the case where the effect of small atmospheric pressure variations, p, are being considered, the

variation of pressure in the porous medium is governed by the equation [CIW 74; Cle 74; Fuk 55]

a^ ^er]_ ae n 8)

az2 kPo ^^

20
Most of the discussion to follow will deal with the steady-state condition, in which case eq. (1.6)

becomes

^ d^ _ 1 d(vC)

£ j-,2

' e dZ

d/ AC + (]) = (1.9)

A few remarks regarding the material parameters may be useful. For transport of radon in unsaturated (with water) materials, the fluid filling the pore system is air, so the values of p,n (the

density and viscosity) are those for air. The material constants of the building material (porous

medium) are: e, k, D , and ((>. There is no simple relation between the diffusion coefficient of radon

in air and the effective diffusion coefficient of radon through the air-filled pore system of a porous

material such as concrete. The porosity of concrete is a measure of the volume fraction of the air-pore

system. However, the average number of pores which connect opposite faces of a concrete slab and the

average length of such pores effectively reduce the diffusion coefficient of radon in air by four orders

of magnitude.

The first few cases are intended to familiarize the reader with the notion of exhalation of radon

from a (building) material, and to emphasize that the radon flux density from a surface is not an

intrinsic property of a material but can depend on the boundary condition at another surface. Until

stated otherwise in these cases, the air volume current density is assumed to be v = 0.

I.D.I. Case I: Exhalation from the Ground into Open Air

This case considers:

The concentration profile of radon in

the soil and radon flux density from

the ground into the open air assuming

that the soil contains a uniform distribution of radium-226 giving rise

to a volume production rate ^ which is

constant everywhere below the soil -air

interface, and further assuming that the

concentration of radon at the open soilair interface is maintained at C = by

natural circulation of the air.

C(o)=0 - r\

X\\\ \ ,

- U

z

I

21
The concentration profile of radon in the soil is then

C(Z) = fjl - exp(Z/£)| , Z < (1.10)

1/2 where C(Z) ranges from at Z = to ^/K deep in the soil. The characteristic length, £ = (D /eA.)

diffusion length or lifetime-diffusion distance, is a measure of the distance in which there is an

e-fold variation in radon concentration near the soil surface. The diffusion current density of radon

at the soil surface (i.e., the radon flux density in pCi-s -m ) is

i = - D ^ h ^e 8Z Z=0 = $(Dg£A)-''^ = ({ie£

I.D.2. Case II: Exhalation from the Ground into a Closed Space

(1.11)

This case considers:

the concentration profile of radon in

the soil and radon flux density from

the ground into a closed collecting

container assuming all other conditions

are identical to Case I.

h «

C(0) = Cc

=

z

r

IE

^X c(z) \XwX^

1

This case is similar to Case I except that it models the presence of a collecting container which is

used for in situ measurements of radon flux density by the accumulation method (see Section IV.C.l. ).

For these measurements, the accumulator (typically a cylindrical can closed on top and open on the

bottom) is placed in close contact with the ground. If the diameter of the can is made sufficiently

large, then the steady-state diffusion field underneath the can will be one-dimensional. In this case,'

if it is further assumed that the air in the can is well-mixed so that no concentration gradients of

radon develop, then the steady-state diffusion flux density from the soil-surface into the can will just

compensate the volume decay rate of radon in the can. The appropriate boundary condition at the

soil -air interface is

A'^ D ^ = Ah C \ (1.12)

"e 8Z Z=0

where A = cross-sectional area of the can, h = height of the can. C = concentration of radon in the can

as well as the limiting value of C(Z) at the soil-air interface. The concentration profile of radon in

the soil in this case is

22
cm = * ji-gur} Z < (1.13)

and the flux density of radon into the can (i.e., the constrained areal exhalation rate) is

j(j = e(t)£[l+e£h'^]'^ (1.14)

which is just the free areal exhalation rate in Case I [eq. (1.11)] reduced by the factor [l+££h ] .

The concentration of radon in the collecting can is C,=C(0) ^

or

C, = C(-»)

y e£+h J

\

£je+h J where the factor ££/(e£+h) is the ratio of two volumes, the volume of the pore space under the container

to a depth of one diffusion length to the sum of that volume and that of the container. In the limit in

which the height of the collecting can is small compared to the diffusion length H, the concentration

profile C(Z) in eq. (1.13) is simply

C(Z) = (t)A

and the flux density into the can, eq. (1.14), is

JD = h^. .

In this connection, two remarks may be made: 1) In the limit in which the height of the can approaches

3C

zero, the boundary condition [eq. (1.12)] becomes g^

= 0. This boundary condition is appropriate

Z=0

at a surface which is impermeable to the diffusing gas and leads to the constant concentration profile,

C(Z) = (t)A. The production rate of radon, $, is balanced by the decay rate, XC. 2) The concept of free

and constrained exhalation rates has also been used by Jonassen and McLaughlin [JoM 78] in an experimental determination of exhalation properties of building materials. Some of their modeling results are

summarized in Case III. Jonassen and McLaughlin also attribute the use of eq. (1.9) (with v=0) to

Krisiuk etal. [KTS 71].

I.D.3. Case III: Exhalation from One Side of a Slab

This case considers:

the concentration profile of radon and

radon flux density from a slab of thickness L containing a uniform concentration

of radium-226 and, in addition, the radon

concentrations above and below the slab

are maintained at C, and C , respectively .

23
This configuration has been considered in an Atomic Energy Control Board of Canada Report [DSM 78] to

determine the principal sources of indoor radon in a survey of 20 houses in Elliot Lake. Other special

cases of this configuration have been treated by Culot et a1 . [COS 76], Jonassen and McLaughlin [JoM 78]^

and Clements [Cle 74]. The concentration profile in this case is

c(z) = i , sinh(Z/£) + sinh [(L-Z)/£]

^ sinh(L/£)

. p sinh(Z/A) . p sinh[(L-Z)/£]

^I sinh(L/£) ^S sinh(L/£)

(1.15)

for < Z < L. This solution of the steady-state eq. (1.9) satisfies the boundary conditions C(0) = C

and C(L) = C, below and above the slab, respectively. The flux density of radon across the surface of

the slab at Z = L is

J = - D ^ ^D "^e aZ Z=L

= (eADg) 1/2 ± cosh(L/£)-l s p cosh(L/£)

X sinh(L/£) sinh(L/£) ''I sinh(L/£) (1.16)

If we identify with the radon production rate in a slab of concrete, C with the radon concentration

of soil gas in contact with the outside surface of the slab and C, with the inside radon concentration,

it can be seen in eq. (1.16) that these parameters enter separate, independent contributions to the flux

density of radon at the interior surface of the slab. The relative contributions to J,^ in eq. (1.16)

from "typical" values of $ and C are displayed graphically in [DSM 78] for a range of thicknesses of

concrete (5cm < L < 20cm), and a range of typical values of the effective diffusion coefficient of

concrete [COS 76] (1.7 x lO'^ cm^-s"''" < D < 3.1 x lo"^ cm^-s"''') and porosity of concrete (0.05 < e <

0.25). The diffusion length £ = 10 cm for D = 2.1 x lo"^ cm^-s"""-, \ = 2.1 x lO"^ s"-"" for radon-222,

and e = 0.1. If the thickness, L, is appreciably larger than the diffusion length, S., then the flux

density jj, in eq. (1.16) can be written approximately as

vl/2 Jn = (e^D.)

= (eXDg) 1/2

f

(l-2e-L/^) . 2C e-L/^ - C,(l ^ Ze'^^^'') v

$ + 2C e-L/^ - C, k -"s" "I The first term in eq. (1.17) is identical with the expression for the exhalation of radon from the

L » £ (1.17)

ground into the open air in Case I, eq. (1.11). The negative contribution, -Cj, in eqs. (1.16) and

(1.17) represents a back-diffusion contribution to the flux density jp. from the radon above the slab.

The contribution of the external radon concentration C to the flux density, jp, in eq. (1.17) is atten- -L/£ uated by the factor e . If the thickness, L, is appreciably less than £, then the flux density, j^,

in eq. (1.16) is approximately

jp s e(tiL/2 + Dg (Cg - Cj)/L, L « £ . (1.18)

The first term in eq. (1.18) is simply 1/2 of the volumetric rate of radon production corrected to

geometric volume since <() is defined on the basis of pore volume. (The other 1/2 is the flux density in

24
the opposite direction.) The second term in eq. (1.18) is simply the definition of the diffusive flux

density in response to the concentration difference C -C, across the slab of thickness, L.

I.D.4. Case IV: Exhalation from a Slab with an Impervious Coating Applied to One Side

This case considers:

the radon concentration profile inside,

and radon flux density from a slab containing a uniform distribution of radium226 which is coated on one side with a

material impervious to the transport of

radon. In addition, there is a concentration C^ of radon in the air in contact

with the uncoated side of the slab.

In this case, we are interested in calculating the surface flux density into a room from a layer or slab

of building material containing radium- 226 which is sealed on the external side (examples are a waterproof

coating on a concrete surface or a vapor barrier on phosphogypsum board). The concentration profile in

this case can be derived from that in Case III if the boundary concentration C in eq. (1.15) is set

equal to C,. For equal external concentrations, the concentration profile is symmetric about the

midplane of the slab at Z = L/2. Therefore, the flux density across the mid-plane is zero and the

concentration profile in the upper half of the slab (from Z = L/2 to Z = L) is the same as the profile

for a slab of thickness L/2 with an impermeable layer at Z = L/2. The flux density through the exposed

surface is, according to eq. (1.16),

Jr Ms^D)l/2/j-c\ "^^(-/i>) - 1

e"^ l\ ly sinh(L/£) (1.19)

If we recognize that L in eq. (1.19) is equal to 2L where L is the thickness of the coated slab, we

can rewrite eq. (1.19) as

J'd = (e^D^)

1/2

(! - ^i) ^-h (L^/£) (1.20)

Jonassen and McLaughlin [JoM 78] treated a configuration analogous to Case II, namely exhalation of

radon from a bottom-surface coated slab into a closed space.

25
I.D.5. Case V: Influence of Darcy Flow on Exhalation from a Slab

This case considers:

the radon concentration profile and

radon flux density from a slab containing a uniform distribution of

radium-226 and, in addition; 1) the

concentration to the left of the slab

is maintained at C and to the right at

C = 0, and 2) there is a steady forced

permeation of air through the slab by a

constant pressure difference across the

slab.

C{0)=Cs

z

-> C(Z) ^

=0

^^

Z ^ Z

Pq-AP

>s

^ rotoi

-->

C(L)=0

31

In this case, the steady-state transport equation is, from eq. (1.9),

dZ

< Z < L , (1.21)

where v > corresponds to flow from left to right and v < corresponds to the reverse. The steadystate concentration profile is

C(z) 4 |l - exp[vZ/2Dg]sinhC(L-Z)/A]+ exp[-v(L-Z)/2Dg] sinh[Z/A]

sinh[L/A] (1.22)

+ C exp[vZ/2D ] ^•'"'^tJL-ZVA]

s ^^

e sinh[L/A]

where A = 'W^\

1/2 r ,2 -il/2

+ £ 4D The length. A, is the diffusion length, l, modified

by the superimposed flow velocity, v. The total flow of radon per unit area through the surface at

Z=L is

^total

I'

Dg cosh(L/A) - exp(vL/2Dg)) D^ exp(vL/2Dg)

" ~a" sinh(L/A) (^'^s"a" sinh(L/A) (1.23)

There are two simple illustrative comparisons which can be made with the expression for current density,

^total' ^" ®*^" ^1-23) at the surface Z = L. If there is no production of radon, the ratio of j^.^-] fo""

velocity v in the positive direction to Jt^+ai when v is reversed is

26
j (V < 0) = exp[Iv|L/Dg] . d. = . (1.24a)

If j. . ,(v) is compared to the radon flux density when there is no impressed flow velocity, Jtotal^*^^' the corresponding ratio is

Jtotal^^^

= exp(vL/2D,) (f) I^JI^ . ^0. (1.24b)

Jtotal^O)

If the external concentrations of radon on both sides of the slab are maintained at zero, then the

corresponding ratios of the flux densities of radon through the surface at Z=L obtained from eq. (1.23)

are

Jtotal^^^Q)

J'total^^^O)

and

Jtotal(^)

^/cosh(L/A) - exp(|vlL/2Dg)\ j^|

A ysinh(L/A) ) * 2Dg

a\

^/cosh(L/A)

sinh(L/A)

- exp(-|v|L/2DA

) ' ^20^

^/cosh(L/A) - exp(vL/2Dg)\ ^ A I sinh(L/A) /

^ 1 / cosh(L/&) - 1 \

Z

I sinh(L/£) j

Cg=0 (1.25a)

J\o^al(0) 1 / cosh(L/&) - 1 \ • "s

, C^ = . (1.25b)

The air volume current density for a particular concrete [Nev 71] is v = 1.45 x 10 cm -s -cm when

the pressure difference is 26 mm Hg and the thickness of the concrete is 10 cm. If we further assume

that Dg = 3.1 X lO"^ cm^-s'-*- and e = 0.1, then the values of the ratios in eqs. (1.24a), (1.24b), (1.25a)

and (1.25b) are, respectively, 1.60, 1.25, 1.16 and 1.08. Because of the symmetry of the special case of

eq. (1.24a), that expression also is the ratio of the exhalations from the two surfaces (at Z=L and Z=0)

when a flow velocity v is imposed from left to right. Using the foregoing permeability data in Darcy's

law, one can obtain the relation |v|L = 5.58 x 10 |AP|where the units on each side are (cm -s ), but

AP is expressed in mm Hg. Substituting this expression for |v|L in eq. (1.24a), one obtains the alternate

form,

Jtotal^^ ^ 0)

^total

(V < 0) = ^''P (-OlSlAPl) (1.24c)

It can be seen from the values we have obtained for the ratios in eqs. (1.24b) and (1.25b) that imposing

a pressure difference of 26 mm Hg increases the exhalation of radon by 25 and 8 percent, respectively.

These increases follow by combining the permeability data of Neville [Nev 71] with the diffusion

-5 2-1

coefficient of 3.1 x 10 cm -s and porosity of 0.1. The flow-modified diffusion length, A, in the

present case where v = 1.45 x 10 cm-s is

A = .962 £

27
1/2 where the diffusion length H = (D /eX) = 12.15 cm. We noted in Section 1.B.2 that McLaughlin and

Jonassen [McJ 78] obtained a permeability (and therefore a flow velocity) approximately 400 times

greater than that quoted by Neville. The flow-modified diffusion length A is completely dominated

-4 -1

by such a large velocity. Thus for v = 5.8 x 10 cm-s , the value of A is

A = 8.80 X 10"^ H;

,-4^ and the value of the ratio J+ntal^^'® ^ "'"^ ^'^''total^^^ ^" ^^' (l-24b) is 208.2. For concrete with

such a large permeability, pressure effects clearly dominate diffusion effects.

I.D.6. Case VI: Time Development of Steady-State Concentration Profile in a Slab

This case considers:

the time development of the steady- state

radon concentration profile in Case V when

4) = and the initial concentration in

the slab is C(Z,0) = 0.

In this case, the time-dependent transport equation is

Po Po-AP

C(o,t)

Z'

^/C(Z,0)«0

=0 2 ,Z

C(L,t)«0

«L

3ZI

3C(Z.t) _ \ 3^C(Z.t) _ V 8C(Z,t) . .p,, .. at " £ 3^2 e az -^^U.^J, < Z < L (1.26)

with C(Z,0) = 0, 0(0, t) = C , and C(L,t) = 0. The solution to this initial value problem is [CaJ 59]

00 rn +^ - r »«,, / vZ\ Uinh[(L-Z)/A] In V nsin(nnZ/L) ^^^. ..-2,^2 2,. 2. „ +n / ri 97^ C(Z,t) - C^ exp

(^2D;JJ sinh(L/A) '

"[2 ^^ [A'^+n^n^/L^] ^

i

'

The first term on the right in eq. (1.27) is the steady-state concentration profile already obtained in

Case V, eq. (1.22) (with (S) - 0). If, in addition, the velocity v is set equal to zero in eq. (1.27),

then the steady-state concentration profile in Case III, eq. (1.15) is recovered (with <() = and

C, = 0). Thus the solution of eq. (1.27) also includes the special case of the time development in

Case III.

If the time, t, is sufficiently large compared to

tp = [Dg(A'V/L2)]'^ = [Dg(iv\'2 + £'2 + n^L"2)]'^

28
then the time-dependent sum In eq. (1.27) may be neglected. Thus tp is a measure of the "relaxation

time" to the steady-state concentration profile (which includes both Cases HI and V). In Case III

where v = and for D = 3.1 x lO" cm -s' , e = 0.1, and X = 2.10 x 10~ s' , the "relaxation time" is

tp = [2.10 X lO"^ + 3.06 x lO"^ l'^]"-"^, seconds

5 where L is measured in cm. Thus for L = 10 cm, t„ = 3.06 x 10 s (3.54 days). In Case V where we

used the value v = 1.45 x 10 cm -s -cm and the same values of D and e, t„ is not significantly

different from the value obtained above

tp = [2.27 x 10"^ + 3.06 X lo"* L"^]"-"^ , seconds.

It appears from these numerical estimates that diurnal variations in radon concentrations outside a

concrete slab will be averaged out because of the relatively large value of tp -v 3.5 d; and so

transport of radon through the concrete will depend only on the time-averaged (over 3.5 days) values of

radon concentrations outside the slab.

I.D.7. Case VII: Time Development of Steady-State Pressure Profile in a Slab

This case considers

the time-dependent pressure profile

inside a slab of thickness L when the

initial pressure inside the pore system

and to the left of the slab is P , and

at time t = the pressure on the right

is increased suddenly to P + AP.

Po Pq+AP

P(o,t) '^^^^^ ^y'^

=

z=

/p(Z,o)sO

=0 z >

p(L,t)«

= L

AP

^ 3ZIL

In" this case, at any time t > 0, the pressure inside the slab is P + p(Z,t). The transport equation

for the time-varying component of the pressure inside the porous slab is given by eq. (1.8)

kP 2 ap - aS

3Z2

(1.8)

an equation which is formally identical to the diffusion equation. The solution of the above initial

value problem for eq. (1.8) is [Cra 75]

p(Z.t) = ^ AP - IAP 2. (-1) - sin -^ exp \- —^ tj (1.28)

29
kP where D = —- . The first term on the right side in eq. (1.28) is the steady-state pressure profile,

a linear increase in pressure from P to (P + AP) across the slab. If the time t is large compared to

2 2 2 2

L /Dn , then every term in the time-dependent sum in eq. (1.28) may be neglected. Thus, L /Dn is a

rough measure of the "relaxation time" for approach to the steady-state following a sudden change in

pressure. Using Darcy's law to evaluate k/q from the permeability data cited in Case VI [Nev 71], one

obtains a value for the relaxation time (in seconds):

L^/Dn^ = 24 L^, seconds ,

where L is the slab thickness in cm. Thus the time required for the establishment of the steady-state

pressure distribution in the pore system is of the order of one hour for 10 cm-thick concrete. It

appears that, in contrast to the results in Case VI, the relaxation time in the present case is relatively

short. This result will be useful in solving the time-dependent transport eq. (1.26) because the

coefficient v can be assumed to adjust instantaneously to changes in pressure and maintain a steady-state

profile.

I. 0.8 Case VIII: Exhalation from a Slab Resting on the Ground

This case considers:

the radon concentration profile and

radon flux density from the top surface of a slab of thickness L resting

on the ground in the case where the soil

beneath the slab contains a uniform distribution of radium-226, but the slab does

not contain radium, and where the concentration of radon in the air above the slab

is maintained at C = 0.

This configuration can be used to estimate the reduction in the flux density of radon from the ground

which results when a concrete slab is interposed. The steady-state concentration profiles in the slab

and in the ground are governed by the equations

.,2, °3

'9 D) \C +4=0

g

z < (1.29)

30
and

S \dz2/

- XC =

s < Z < L (1.30)

The solutions of eqs. (1.29) and (1.30) must satisfy the following boundary conditions:

at Z = L : Cgd) =

at Z = : 0^(0) = Cg(0)

^^s -^ dZ 1=0 1=0

at Z = -»: C (-») = ^/\

(1.31)

(1.32)

(1.33)

(1.34)

The conditions (1.32) and (1.33) at the ground-slab interface express the requirement that the radon

concentration and flux density vary continuously there. The concentration profile in the ground is

(for Z < 0)

V" = t

exp(Z/A^)

(9©""^(y 1 +

(1.35)

and the concentration profile in the slab is

C3(Z) = f m) -sinh(Z/£^) + cosh(Z/£g)tanh(L/Ag)

-md-id g/ \ s'

The flux density of radon through the top surface of the slab is

dC

(1.36)

^D = -h W

= 6 £ S. ^ s s

Z = L

cosh fe)hft)(^)-^fe)]| (1.37)

According to Case I, the flux density of radon through the bare ground-air interface is <^^^^^, i.e.,

the coefficient on the right-hand side of eq. (1.37). Thus, the ratio of the flux density of radon

through the soil-air interface in the absence of the slab to the flux density through the top surface

of the slab is

*^^s

hfc)^©(y=-M (1.38)

If the slab thickness, L, is large compared to the slab diffusion length, S, , then the approximate

relation obtained from eq. (1.38) is *Vs. 1

1 + fe)(yl-fc) L»£. (1.39)

31
For illustrative purposes, consider the reduction of the radon flux density from soil by the

interposition of a concrete slab of thickness L = 10 cm, with a diffusion length S. = 10 cm and dif-

-5 2 -1 fusion coefficient D = 2 x 10 cm -s . From Tanner [Tan 64, see correction in Tan 78], the ratio,

2 -1

D /e , for a variety of soils lies in the range: 0.01 to 0.05 cm -s It follows that the diffusion

length, S, , in soil lies in the range: 60 to 154 cm. If we take 0.25 as a typical value for the soil

2 -1 porosity, e , then the corresponding range of values of D is 0.0025 to 0.0125 cm -s Hence, for

(.Us, ) = 1, the flux density ratio given by eq. (1.38) is in the range of 23 to 49. This indicates that

one may expect perhaps a 25- to 50-fold diminution in the radon flux density from soils by the introduction of a 10-cm concrete slab (assuming that there is no contribution to the flux density from the

concrete itself).

I. E. Conclusions and Recommendations

A basic premise of the model of the transport of radon through a building material such as concrete

or phosphogypsum board is that the building material behaves like a porous medium. Any comprehensive

study of radon transport in a building material should have two objectives: first, to verify that the

transport equations for a porous medium (i.e., Fick's law and Darcy's law) provide a correct description

of transport in the material; and if so, second, to measure the transport coefficients and material

constants for the material, i.e., measure the diffusion coefficient for radon, the permeability coefficient of air, the porosity of the material, and the production rate of radon per unit pore volume.

In view of the absence of such a comprehensive study for any building material, it is recommended

that such studies of a series of concretes and phosphogypsum boards be undertaken.

32
CHAPTER II

THE ROLE OF THE INTERNAL CHARACTERISTICS OF BUILDING MATERIALS

The transport of radon in building materials can be classified as:

(i) flow, where the fluid (liquid water, air, or water vapor) within the interstitial pore spaces

of the material entrains the radon and acts as a carrier; and

(i1) diffusion, where the radon moves relative to the fluid (liquid water, air, or water vapor) in

the pores.

Building materials may be subjected to liquid or vapor water or air pressure gradients, osmotic gradients (for example, a dissolved alkali concentration gradient), temperature gradients, and radon gas

concentration gradients. These gradients, depending on their magnitude and the corresponding building

material porosity, can result in significant flow and diffusion of radon. In addition, since radon is

relatively soluble in liquid water , mechanisms involving liquid water flow may be significant, depending on the flow rate. The internal characteristics and properties of building materials, such as the

location of the radium progenitor of radon in the material and the internal geometry of the pore system,

can significantly affect the transport of radon.

This chapter, therefore, reviews and considers: the emanation process from a microstructural

viewpoint (Section II. A.); the role of the location and distribution of radium in materials (Section

II. B.); and the effects of internal geometry (e.g., pore size, distribution, accessibility and continuity), and the content and state of internal water as they may affect the entrainment and transport of

radon in materials (Section II. C). Because concrete and other cementitious materials are the basic

components of most buildings, their characteristics with respect to radon transport are particularly

important. The characteristics of hardened concrete are therefore more fully considered and developed

as a case study (Section II. D.).

II. A. A Microstructural View of the Emanation Process Within a Material

The importance of the internal characteristics of building materials becomes more apparent when one

considers the emanation process from a microstructural viewpoint. Tanner [Tan 64; Tan 78] reviewed and

discussed the role and relative importance of internal characteristics as related to radon emanation.

In a closed two-phase air-radon-water system in equilibrium at atmospheric pressure and with the

-4 partial pressure of radon approximately 10 mm Hg, the ratio, concentration of radon dissolved in the

liquid water phase divided by concentration of radon in the vapor phase, decreases from 0.5 at °C to

approximately 0.1 at 60 °C [McK 74; lUP 79]. 33
entrainment, and transport in the ground. Much of this information is also applicable to building

materials. The following microstructural view is almost wholly adopted from the review of Tanner

[Tan 78].

The release of radon atoms into the interstitial pore spaces of a material is inherently related to 226

the recoil process following radioactive decay of radium. When radium ( Ra) atoms inside a material

222 decay by a-particle emission, radon ( Rn) is formed as recoiling atomic species. The fate of a

recoiling radon atom depends not only on the location of the original radium progenitor, but also on the

recoil path and the composition of the material in the path. In air, a recoiling radon atom has a range

of approximately 60 pm; and in water, the range is approximately 1000 times shorter. Within minerals of

common density, a recoiling radon atom has a range of 20-70 nm.

4^ 4

1 i. R y'

/

1

/

\ ^ "^ "^ ^x^^'

• wf

/

/ i

Aa

^^^^^

^^p>*'^ ^^^fcsi. L . 1 ^

'O.lftm •

Figure 2.1 Illustration of several scenarios for the release of radon

into the interstitial pore spaces of a material. Adopted from Tanner

[Tan 78]. For details, see text.

Figure 2.1, adopted from Tanner [Tan 78], illustrates several scenarios for the release of radon

into the interstitial pore spaces of a material. The figure represents two spherical grains of 2-\m

diameter which are in contact. The interstitial spaces between the grains contain liquid water (lined

area) and air (stippled area). The figure shows 5 different paths for recoiling radon atoms directed

toward the outer surface of the upper grain. Since the radium progenitor of radon atom A lies at a

depth greater than the recoil range R, radon atom A loses all of its recoil energy inside the grain and

34
remains entrapped. Radon atom B recoils out of the upper grain, but embeds into the lower grain. Radon

atoms C and D recoil from the upper grain and lose the remainder of their kinetic energy in the pore

water and air, respectively. These radon atoms are now free either to diffuse through the pores or to

be transported by pore fluid flow. Radon atom E recoils out of the upper grain, loses very little of

its recoil energy in the pore air, and buries itself into the lower grain. Radon atoms B and E on

penetrating the lower grain, damage the surface and create a "pocket" from which they may escape back

into the pores. These mechanisms are discussed in much greater detail by Tanner [Tan 78]. Those radon

atoms that terminate their recoil paths in the pores (e.g., atoms C and D) constitute the "direct-recoil

fraction" of emanating power; while those that diffuse out of a recoil pocket (e.g., atoms B and E)

compose the "indirect-recoil fraction".

Tanner has noted:

"In compacted natural materials either, 1) the grain sizes are much larger than the recoil range in

the grains, so that few recoil atoms escape from the grains in which they originate; or 2) the

pores are much smaller than the recoil range in air, so that the recoil paths do not terminate in

the pores if the pores are gas filled. The direct-recoil fraction of emanating power is consequently less than 1% in dry compacted granular materials. If the pores contain water or another

liquid, however, the range of the recoil atom is only about 0.1 pm [0.06 pm] and the probability

that it will stop in the pore is greatly increased. The presence of a liquid in the pores thus

increases the direct-recoil fraction of emanating power". [Tan 78]

Hence, the emanating power (particularly, the direct-recoil fraction) of grains within a material and

the subsequent transport of radon within the pores of that material is very dependent on the size and

structure of the material's pore system and on the composition of the fluids within those pores. These

effects will be considered more fully in Section II. C.

II. B. Influence of Radium Content of Materials

Obviously, the amount of radium in a building material is a limiting and critically important

intrinsic characteristic of the material with respect to the emanation power of grains in the material,

and subsequent radon transport through the material's pore system and exhalation of radon from the

material's surface. When considering the microstructural scale of the processes involved, however, it

should be apparent that this dependence on the radium concentration is extremely complex. Since, as

noted, the recoil range of radon in most common mineral grains is in the range 20-70 nm, the fraction of

radon free to move through the pore system of a material will certainly depend on the grain size. But

even the relationship between grain size and emanation power is not simple (see, for example. Tanner

[Tan 78]); and the effects of the distribution of radium on the surface and within the grains and the

35
distribution of grains in a material are even more confounding. Andrews and Wood [AnW 72], for example,

observed two entirely different relationships between emanation power and grain size for two kinds of

sandstones because of differing radium distributions in the sandstones.

Known results for the radium content in building materials are summarized in Section III. A. It

must be emphasized, however, that existing studies that have attempted to relate the radium content of

building materials to radon flux or exhalation rates have taken only a very macroscopic or phenomenological approach. This limitation is perhaps understandable since it would be exceedingly difficult, if

not impossible, to attempt to characterize on a microscopic basis the distribution of radium within

these materials. The composition of common building materials is highly variable both within a specimen

and between specimens, and cannot be considered to be homogeneous on the micron and submicron scale that

is of importance.

II. C. Pore Geometry and Water Characteristics

The microstructural viewpoint is similarly useful for understanding the effect of pore geometry and

the composition of pore fluids on radon transport in the pore system of a building material. The

permeability of building materials (defined as the steady-state flow of a fluid induced by a pressure

gradient) is not a simple function of its porosity (ratio of pore volume to total volume), but depends

on the size, distribution, accessibility, and continuity of the pores. Accessible pores which run

through the specimen primarily control the movement of fluids. Further, the distribution of pore sizes

is important. A building material may have a relatively high porosity but a relatively low permeability

because of the size and distribution of the pores. For example, in hardened cement gel (discussed in

Section II.D.), the gel has a porosity of about 28%, and yet is relatively impermeable because its pores

are extremely small. Conversely, a building material, such as natural rock could have a low porosity

and a relatively high permeability because some of the pores are accessible, sufficiently large, and

continuous through the rock.

If the pore size is on the order of the mean free path of the gas diffusing through the pore, the

diffusion will be reduced relative to diffusion in a larger pore [You 70]. Smaller pores tend to retain

and fill with liquid water because of the stronger capillary forces associated with smaller pores.

Further, the diffusion of gas through the liquid water in the pores is significantly less than diffusion

through the air in the pores [Tan 64].

Fluids, including liquid water, water vapor, or air, in the pores may or may not significantly

contribute to the transport of radon by flow (fluid acts as a carrier for radon). The contribution of

36
fluid flow mechanisffls to the overall radon transport will depend on the relative volume flow rate of the

fluid as compared to the transport of radon by diffusion.

As noted previously, the presence of water serves to increase the exhalation of radon from radon

producing minerals [Tan 64]. Water in the pores of building materials which contain radium enhances the

capture of recoiled radon atoms in the pores (after the atoms escape from the pore wall and enter the

pore, see fig. 2.1). When a recoiling radon atom escapes from a solid pore wall and encounters liquid

water, it will be slowed down considerably as compared to the case when no water is present. The extent

to which the radon atom is slowed down or stopped depends on the size of the pore and the amount of

water in the pore. Based on the mechanisms discussed by Tanner (fig. 2.1), the higher the fraction of

liquid water in the pores and the larger the fraction of water-filled (partially or totally filled)

pores in the material, the greater the probability of radon being entrained in the pores and free to

move.

In summary, one may conclude that pore accessibility, continuity, size distribution and water

content are major factors that affect fluid transport, and thus radon transport, through building

materials. Liquid water in the pores will significantly decrease the diffusion of radon as compared to

the diffusion of radon in air-filled pores. Pore water content and size distribution appear to be major

parameters in affecting radon entrainment in the pores. To illustrate the effects of the variables

being discussed, a building material may have a high radium content and a high porosity, but may have

low exhalation of radon due to pore size distribution (many small pores) and have low radon capture due

to a lack of water in the pores.

II. D. A Case Study: Hardened Concrete

This section, using hardened concrete as an important and representative example of a building

material, more fully reviews the role of internal characteristics in affecting radon entrainment and

transport. It includes definitions of the important properties of hardened concrete, typical values for

these properties, and a discussion of their role.

II.D.l. Characteristics of Hardened Concrete

a) Composition

Hardened concrete consists primarily of mineral rock (aggregate) and hardened cement paste which is

used to coat and bond the aggregate together. By volume, about 60 to 80% of concrete is aggregate

37
[PCA 79]. In addition to the aggregate and paste, there is considerable void space. The void spaces

(pores) contain no solid material but are filled with fluids, typically air, water vapor, liquid water

(which contains dissolved alkalies), or a combination of one or more of these.

b) Porosity of Hardened Cement Paste

The void spaces in the hardened cement paste and concrete can be classified [Woo 58] as shown in

Table 2.1. Capillaries and gel pores deserve special discussion. Capillaries are cavities which were

originally water-filled spaces in unhardened concrete. As hydration proceeds some of the water is used

to form hydrated cement gel. The remaining space not occupied by cement gel is termed the capillary

porosity. Figure 2.2 taken from Powers [Pow 62], illustrates a simplified model of the cement gel

structure and void space.

Table 2.1 Void Classification and Characteristics for Hardened Concrete [Woo 68].

Class Size Shape

Volume

Percent

Entrapped air Barely Visible to about 1/2 in (1 cm) or more

Irregular Minimum of 1^

by volume of concrete

Entrained air About 0.003 to 0.004 in (0.008 to 0.01 cm)Nearly

Spherical

Up to 5% or more

by volume of concrete

Capillaries

2 4

° From about 10 to 10 A

[VeH 69]

(1 A = lO"^ cm)

Irregular with

tendency to interconnect

Typically 15-40% or more by volume of the cement paste [Pow 78]

Gel Pores

o On the order of 18 A 28% by volume

of hydrated cement gel

The hydration and hardening of cement has been studied at length (see, for example. Powers [Pow 62]

and Williamson [Wil 73]). Hardened cement paste consists of the products of hydration (gel and calcium

hydroxide crystals), unhydrated cement, and the capillaries. Hydrated cement gel is the "solid" part of

hardened cement paste, including calcium hydroxide crystals, but excluding capillaries and unhydrated

cement. As hydration proceeds, part of the water-filled space becomes filled with solid cement gel,

constricting the capillaries. Whether complete isolation of the capillaries can occur, even with low

water/cement ratios and complete hydration, is not known. Recent evidence [VeH 69, CoV 76] suggests

that, although the capillaries become constricted, they never become isolated.

The pores of the cement gel are very small and numerous and usually contain water. The gel pores

have a characteristic porosity of about 28% by volume of the cement gel. The capillary porosity of

38
hardened cement paste (excluding gel porosity) in ordinary concrete ranges from about 15 to 40% or more

by volume of the cement paste [Pow 78]. Thus the total porosity (gel and capillary) typically ranges

from about 40 to 55% or more by volume of the cement paste.

1 %

^ '-;; %

^i'!*')

T» .'

, J^evllfcC.-' *%*i

• ,,' fe *

, '-#,'J, i*'^ j,'<>-*"'Mlk.

'. ,-(', -* -

-'

. ' '1' 'y^.-t^i-r}^<^.^ '^ *,

^ ' \ 'C^-i^^^^^.l'

~ •

'-'>^.^\,

:'Ji-'. - ->;• ..^/

\J^'' f' <hJ -''-%' «-' • . t

.

' .,

Figure 2.2. Illustration of a simplified model of paste structure

in cement gel. Masses of randomly oriented groups of black lines

represent cement gel. Spaces like those marked C represent capillary

cavities. Upper drawing represents mature paste, water/cement ratio =

0.5, capillary porosity 20%; lower drawing represents nearly mature

paste, water/cement ratio = 0.3, capillary porosity 7%. From Powers

[Pow 52].

c) Porosity of Aggregates and Concrete

The porosity of most acceptable natural aggregate lies below 10% and commonly below 3% [Woo 68], in

contrast to the 15 to 40% or more (capillary porosity) for hardened cement paste. Although the porosity

of the aggregate can be considerably less than that of the hardened cement paste, the water permeability

of the aggregate can be on the order of, or even higher than that found for mature cement paste with

water/cement ratios in the range of about 0.4 to 0.7. This is because on the average the pore size in

39
aggregates Is considerably larger (though larger pores in aggregates are usually present in much smaller

proportion) than that for hardened cement paste [Woo 68]. Thus, the effective porosity of concrete is a

weighted average of the porosity of the aggregate and hardened cement paste.

d) Measurement of Porosity

Copeland and Verbeck [CoV 76] and Verbeck and Helmuth [VeH 69] reviewed the determination of

porosity and pore-size distribution. Pore-size distributions can be calculated, provided an assumption

concerning pore shape or pore "hydraulic radius" (pore volume/pore wall area) is made. Two methods are

typically used:

i) porosimetry, in which the volume of mercury that is forced into the porous sytem is measured

as a function of the applied pressure; and

ii) capillary condensation of vapors from either an adsorption or desorption isotherm.

o Sorption results are limited to pore diameters less than about 300 A (0.03 iim) while the mercury

porosimetry method can measure pore diameters as high as 1 pm [CoV 76]. Copeland and Verbeck [CoV 76]

indicate that the significance of pore-size distribution as determined by either technique needs

additional clarification.

e) Water Retention in Capillaries

Woods [Woo 68] points out that the tenacity with which the various classes of voids retain water is

inversely proportional to their size. Water in the gel pores is so firmly fixed that it does not

evaporate under drying conditions that will remove nearly all the capillary water; yet this water, to

a limited extent, participates in the flow of water [Pow 62]. At the other extreme, are the larger

entrapped and entrained air voids which have little propensity for retaining water and thus are usually

empty. Capillaries, which represent the intermediate void pore size, have an intermediate affinity for

water. Thus, a competition exists between the various void sizes for the available water. It has been

observed that the larger pores (including entrained air voids) within a body of concrete subjected to

wetting, will remain unfilled, even in water-soaked concrete [Woo 68].

f) The State of Water in Hardened Cement Paste

Water in hardened cement paste can be arbitarily classified as "evaporable" and "non-evaporable"

(that is, water not removed under a standard drying condition, which is coranonly performed in a vacuum

over dry ice (-79 °C) [VeH 69]). The drying is intended to remove nearly all the "physically adsorbed"

40
water and not the "chemically bound" water of hydration. The "evaporable" water can be further classified as "gel" water and "capillary" water, where "capillary" water is defined as the difference

between the total evaporated water of a paste and its gel water content. Copeland and Verbeck [CoV 76]

classify the water in hardened cement paste in more detail as:

i) water chemically combined in the hydration products;

ii) hydrate water in crystallites;

iii) adsorbed water on solid surfaces; and

iv) water "in capillary pores large enough to have space outside the force fields of solid

surfaces."

They indicate that experimental differentiation among these classes of water has not been achieved

except for the case of the chemically combined water which can be approximately distinguished from the

other three classes. They also indicate, however, that some hydrate water is removed under drying and

"part of the molecular water may remain". For example, using quasi-elastic neutron scattering, Harris

et al . [HWL 74] observed that saturated cement pastes had free water fractions smaller than the values

determined by simple drying weight- loss experiments. They conclude from their data that free and

combined water are lost simultaneously during evacuation (drying).

Copeland and Verbeck [CoV 76] discuss the state of information on the density and thickness of the

multilayers of adsorbed water on the capillary walls caused by the interaction between the solid phase

and water at the interface. Exact values for the specific volume of the adsorbed water are not known,

but estimates indicate that in saturated pastes they are about the same as, or a little larger than, the

values of pure bulk liquid water. Copeland and Verbeck [CoV 76] indicate that the significance of

knowing the specific volume of the adsorbed water is that the pore volume of saturated hardened paste

can be determined with relatively little error by determining the volume of evaporable water in a unit

volume of saturated paste. There is reasonable evidence suggesting that more than one layer of adsorbed

water exists in capillary pores of concrete, provided some degree of saturation exists in the pores [Pow

62].

II. D. 2. Movement of Pore Fluids in Concrete

Concrete can be subjected to various fluid (liquid water, water vapor or air) pressure gradients.

These gradients can result in the flow and diffusion of radon in the liquid water, water vapor, or air

of the concrete.

41
a) Movement of Water

-13 -1 Although the cement gel has a porosity of about 28%, its permeability is of the order of 10 cm-s

[PCH 54]. The cement paste as a whole, including capillaries, in mature, ordinary concrete can be

considerably more permeable (several orders of magnitude or more) than the gel [PCH 54], since water can

flow through the capillaries more easily than through the extremely small gel pores. Thus the permeability

of hardened cement paste is controlled by its capillary porosity. Verbeck and Helmuth [VeH 69] present

for hardened cement paste a relationship between permeability and total porosity (gel and capillary

porosity), water viscosity, and temperature.

As discussed earlier, the permeability of the aggregates may equal or exceed that of the cement

paste, even though the porosity of the aggregate may be considerably lower than that of the paste. Thus

the permeability of concrete will be dependent on the permeability of the hardened cement paste, the

aggregate, and the aggregate-cement paste interface. If the permeability of the aggregate is lower than

that of the cement paste, the presence of aggregate (60 to 80% by volume of concrete) will reduce the

area through which flow can occur. In effect, the length of the flow path has been increased, since the

flow has to circumvent the aggregate.

The most likely mechanism to cause water migration through a concrete wall or floor slab is that

resulting from a relative humidity gradient across the thickness. The primary mechanism is believed to

be that as described by the following Building Research Advisory Board statement [NAS 58] on transfer of

moisture through concrete slabs-on-ground:

"The Committee believes that moisture is transferred through partially dry concrete in

the adsorbed or condensed state by surface diffusion and does not move as vapor through

the concrete. Also, where good practice has been followed in the design and curing of

concrete, it is believed that moisture does not move through the concrete by capillarity

as it is usually understood."

A relevant Portland Cement Association laboratory study [Bre 55], showed that measurable moisture

movement occurred through 4-inch (10.2 cm) thick concrete slabs exposed to liquid water on one side and

50% relative humidity on the other. Both sides of the slabs were maintained at 73 °F (23 °C). Volume

flow rates (volume of liquid water) measured over a wide range of water/cement ratio values were on the

_g 2 -1 -9 order of 10 cm -s -cm through the 4-Inch (10.2 cm) thickness. 1

imate steady state flow conditions attained after a year of exposure.

order of 10 cm -s -cm through the 4-Inch (10.2 cm) thickness. These values correspond to approx42
b) Movement of Air

As with the permeability of liquid water in concrete in which the pores are filled with liquid

water, the flow rate of air depends on the section thickness and the pressure gradient [Nev 71]. There

is, however, no unique relationship between the permeability for air and water for all concretes. The

steady state flow rate of air can be achieved in several hours depending on the porosity and pore size

distribution. Neville [Nev 71] lists some data on the flow rate of air for concrete with a water/cement

3 -2 -1 -1 -2 ratio of 0.62. The rate of flow values, in units of in -(Ib'in ) 'h 'ft ranged from 0.58 to 0.24

3 -1 -1 -2

[i.e, 0.015 to 0.0062 cm -(Pa) -h -m ] for the thickness ranging from 4 to 9 inches (10 to 23 cm),

respectively. Loughborough [Lou 66] reported air permeability coefficient values for concrete at

various moisture contents and having a water/cement ratio of 0.50. The values ranged from 0.04 to 0.23

3 3 expressed as the volume of air in cubic inches (1 in =16.4 cm ), at atmospheric pressure and zero "C,

2 that in 1 minute flows through a 1 square foot (0.0929 m ) area of 1 inch (2.54 cm) thick concrete, at a

pressure differential of 1 psi (6895. Pa) across the specimen. Four inch (10.2 cm) thick specimens

subjected to a 14 psi (96530. Pa) pressure difference were used. With a pressure difference of 25 mm Hg

and a concrete thickness of 4 inches (10.2 cm), the corresponding volume flow rate based on Neville's

and Loughborough's data is of the order of 10 cm -s -cm

II. D. 3. Effect of Concrete Properties on Radon Transport

a) Transport by Flow

Based on the data presented, air can act as a carrier for radon gas. The volume flow rate of air

corresponding to a 25 mm Hg air pressure difference across a 4-inch (10.2 cm) concrete thickness is of

~5 3 -1 -2 the order of 10 cm -s -cm . Liquid adsorbed water is believed to act as a carrier where there is a

relative humidity gradient across the concrete thickness. With a 4-inch (10.2 cm) concrete thickness at

73 °F (23 °C), the volume flow rate (volume of liquid water) corresponding to one face wet and the other

-8 3 -1 -2 face at 50% relative humidity, is of the order of 10 cm 'S -cm , which is several orders of magnitude smaller than the air flow rate. The relative significance of radon transport by flow resulting

from air or moisture movement would depend on the intensity and proportion of time the driving force

(air pressure or relative humidity gradient) acts across the concrete thickness. In field conditions an

air pressure difference of 25 mm Hg will not always be present. In contrast, a relative humidity

gradient can often be present much of the time. Thus, further research is needed to determine which

flow mechanism (air or moisture) would contribute more to the transport of radon by flow.

43
b) Transport by Diffusion

Considering only diffusion, the presence of liquid water in the pores significantly reduces radon

diffusion, due to the large difference between diffusion coefficients for radon in air (on the order of

10 cm -s ) and liquid water (on the order of 10 cm -s ) [Tan 64]. Thus, radon diffusion in pores

partially filled with liquid water would probably be primarily controlled by the diffusion of radon in

the air rather than the diffusion in the adsorbed water layers.

II. E. Conclusions

The effects of the internal characteristics of building materials (including radium content and

distribution, internal geometry, pore size and distribution, and water content and state) on the

entrainment and transport of radon through the pore system have been discussed. As indicated, these

effects are exceedingly complex. Radon transport by flow, resulting from the movement of air and

moisture through the pore system, is possible. The relative contribution of the air and moisture

movement to radon transport would depend on the intensity and proportion of time the driving force (air

pressure or relative humidity gradient) acts across the material thickness. The presence of liquid

water in the pores can significantly reduce the diffusion of radon, but at the same time, the presence

of liquid water in the pores can enhance the entrainment of recoiling radon atoms.

44
CHAPTER III

A REVIEW OF EXPERIMENTAL DATA

This chapter considers the radon exhalation process from a more macroscopic, phenomenological

viewpoint. In part, it focuses on the question: Is it possible to predict or estimate radon exhalation from building materials based on existing data? Three categories of data have been reviewed and

summarized:

A. radium concentrations in concrete and other building materials;

B. radon exhalation from soils, concretes and other building materials; and

C. radon exhalation as a function of meteorological parameters.

Data were selected to show typical values or ranges of values. In addition, values for radium content

and meteorological parameters are addressed in terms of their potential effect on radon flux densities

and exhalation rates. Since it is often of interest in a review such as this, the criterion used to

select the data is indicated in each case. Lastly, in Section III.D. , these data are summarized in

tabular form to highlight not only the expected range of values for radon flux densities and exhalation

rates, but also to identify some of the most critical factors.

Although this chapter is based on a survey of several hundred papers, a number of extensive review

articles have also been relied on. These reviews, such as those by UNSCEAR [UN 77; UN 80], CRCPD

[CRC 78], Harley [Har 78], Eadie [Ead 75], Wilkening, et al . [WCS 72], and Tanner [Tan 64; Tan 78],

cover virtually all of the literature relevant to this chapter. The survey made for this study and the

previous reviews generally concur that there Is very little information that is directly usable for

predicting radon flux densities or exhalation rates.

Before discussing the existing data, however, it may be useful to evaluate the magnitude of the

radon flux densities which may be considered to be a significant contributor to the indoor radon concen-

* tration. Consider a single-room structure 2.5 m in height and with a 10-cm concrete slab floor located

-2 -1 directly on a soil surface. A typical radon density flux from dry soil is 0.43 pCi*m -s (i.e., 0.75

-2 -1 atoms'cm -s ) [WCS 72]. If all of the exhaled radon from the soil enters the room without diminution

This evaluation was made under the assumption that the radon flux density from open soil provides an

upper limit on the radon from soil that can enter a building. W. W. Nazaroff (Lawrence Berkeley Laboratory, Berkeley, California) has suggested that this assumption may not be valid, and has offered an

unpublished alternative "stack effect" mechanism that leads to different conclusions. Readers may wish

to contact him directly for further details.

45
2 by the floor, then approximately 1500 pCi of radon will enter through each m per hour. If this is the

only source of indoor radon, then, with a ventilation rate of k = 0.5 h , the steady-state concentration of radon in the room, given by the first term of eq. (1.2), would be 1.2 pCi-L . Assuming an

equilibrium factor of 0.5, this radon concentration corresponds to an indoor working level of 0.006

WL . This is somewhat lower than levels currently conceived to be significant and possibly warranting

remedial action (e.g., 0.01-0.02 WL). In addition, the reduction of the radon flux density from the

soil due to an intact 10-cm concrete slab is roughly a factor of 25 to 50 (see Section I.D.8). Hence,

-2 -1

it appears that the radon flux density from soils near the worldwide average of 0.43 pCi-m -s should

not be of great concern as a source of indoor radon. This may not be applicable in buildings that have

cracks or unsealed service entrances in the foundation, or have considerably reduced ventilation rates.

Also, some "technologically-enhanced" soils and radium-bearing building materials exhibit considerably

elevated radon flux densities and may be significant sources to indoor radon concentrations. There

are, of course, other contributing sources (see introduction to Chapter I) which may exacerbate the

significance of radon exhaled from soils and building materials. Radon flux densities from soils and

-2 -1 building materials are commonly within a factor of 10 of 5 pCi-m -s which may lead to indoor levels

within a factor of 10 of 0.02 WL. This suggests that field measurements should be accurate to at least

a factor of 2. Furthermore, on this same basis, the influence of a given variable affecting the flux

density might be considered significant if a typical change in that variable results in a 10-20% change

in the flux density. In this way, even if the individual effects of the possible affecting variables

add together, the overall effect on the radon flux density could still be relatively unimportant.

Reported variations of more than 10-20% (e.g., due to changing atmospheric conditions) are common.

Descriptions of how these observations can be used to estimate or predict radon flux densities are much

less common. Generally, only a qualitative understanding of the dependence of the radon flux density

on a given variable can be inferred because in most studies other variables were not controlled.

* 218 214 214 The WL unit is defined as any combination of the short-lived radon progeny ( Po, Pb, Bi , and 214 5 Po) in one liter of air that will result in the ultimate emission of 1.3 x 10 MeV of alpha-particle

5 energy [UN 77]. The numerical value of the WL (1.3 x 10 MeV per liter) is derived from the total 222 of-particle energy ultimately emitted in the decay of the short-lived Rn progeny that are in radio222 active equilibrium with 100 pCi of Rn per liter of air. Although this numerical value is derived

-1 222 with the assumption of radioactive equilibrium (i.e., 100 pCi-L of each Rn progeny), the unit is 222 applicable for any mixture of short-lived Rn progeny including nonequilibrium situations. Hence,

with an equilibrium factor of 0.5, the working level corresponding to a radon concentration of 1 pCi-L

is 0.005 WL.

46
III. A. Radium Content of Building Materials

Values for the radium content of building materials have been reported or summarized by Hultqvist

[Hul 56], Krisiuk, et al . [KTS 71], Eadie [Ead 75], Hamilton [Ham 71], Kolb [Kol 74], CRCPD [CRC 78],

UNSCEAR [UN 77], Harley [Har 78], Fitzgerald and Sensintaffer [FiS 78], and others. Despite this

seemingly substantial list, the amount of available information is actually relatively scant. Eadie

[Ead 75], for example, in reviewing 275 references pertaining to radioactivity in building materials

states as his first "conclusion" that "the article by Hamilton [Ham 71] is the only significant report

of data on the radioactivity of specific building materials." Similarly, Harley [Har 78] states: "In

reviewing the papers available, it is apparent that relatively few present data for amounts of radioactive substances and for the radiation exposure within the building. More data of this type would be

useful " A Task Force of the CRCPD [CRC 78] noted, however, that by 1978 there had been analyses

of the radioactivity concentrations of commonly used building materials in seven countries, but not in

the United States. The most recent summaries of results for the radium content of building materials

appear in the UNSCEAR compilations [UN 77; UN 80].

Table 3.1, adapted from Harley [Har 78], summarizes the results for some select building materials.

The data were selected to better illustrate typical ranges and the distribution for a few important

materials, as well as to facilitate comparisons between different types of material (bricks, concretes,

etc.). Thus, for example, the "clay brick" in the U.K., the "red brick" in the U.S.S.R. , and the

"brick" in West Germany had radium concentrations of 1.4, 1.5, and 1.7 pCi-g , respectively; while the

"silica brick" in the U.S.S.R. and "silicate brick" in the U.K. were about a factor of 3 to 7 less (0.5

and 0.2 pCi*g , respectively). Similar comparisons can be made for other building materials such as

Table 3.1 Radium Concentrations in Selected Building Materials

-72S Reference Material Ra Concentration

(pCi-g'^)

Hamilton [Ham 71] (U.K.)

Krisiuk, et al . [KTS 71]

(U.S.S.R75~

Kolb [Kol 74]

(W. Germany)

Clay brick 1.4 Silicate brick (Gravel) 0.2 Granite 2.4 Aerated concrete 2.4

Natural gypsum 0.6 Concrete block (Fly Ash) (0.2-2)

Red brick 1.5 Silica brick 0.5 Light concrete 2.0 Granite 3.0 Sand 0.4-1 Cement 0.7

Granite 2.8 Brick 1.7 Sand, gravel 0.4 Cement 0.5

Natural Gypsum 0.5 Concrete 0.6

47
concretes which range from 0.6 to 2.4 pCi*g . As a result of these variations, typical values for the

radon flux density from these materials vary from roughly 30% to factors of 2 or more. In addition,

many components of building materials (e.g., the aggregate in concrete) are locally supplied, and, as

noted by the CRCPD Task Force, the "data on radionuclide concentrations in many construction materials

showed large variability from region to region" [CRC 78].

The radioactivity content of "phosphogypsum," a potentially important building material, has been

reviewed by Fitzgerald and Sensintaffer [FiS 78]. Massive quantities of this by-product of phosphate

processing operations are available, and pose a major disposal problem. Its use in building materials

has not been widespread in the U.S., but has been contemplated. The radium concentration of phosphogypsum, as shown in Table 3.2, is approximately an order of magnitude greater than that for naturallymined gypsum. The National Radiological Protection Board in the United Kingdom recommended that sources

of raw materials having concentrations "significantly in excess of 25 pCi-g should be avoided" [CDS 72].

The variability in radium content and radon exhalation rates 'from phosphogypsum products' shouTd'be

studied more closely since, as seen in Table 3.2, average radium concentrations are at or near this

limit.

Table 3.2

Radium Concentrations in Phosphogypsum [FiS 78]

Phosphogypsum

Source

226 Average Ra . Concentration

(pCi-g"b

Florida Morocco

Idaho Khoaribga

3325

2317

III.B. Radon Exhalation from Soils and Building Materials

Radon exhalation from soil has been studied extensively since the early part of this century; the

first value being reported in 1909. Numerous results are available, and Wilkening, et al . [WCS 72]

reviewed the data from the standpoint of obtaining a worldwide average radon flux density. Based on a

review of nearly 1000 measurements of the radon flux density from soils of various types at locations

-2 -1 across the earth, they obtained a worldwide overall average of 0.43 pCi-m -s with a range from 0.005

-? -1 to 1.4 pCi-m -s [WCS 72]. Table 3.3, abstracted from this paper, tabulates some of the results for

the radon flux density from soils in the U.S. Typical values vary by a factor of 2-3, and are not

substantially different from the worldwide average. For example, the overall mean for the sampled New

-7 -1 -2 -1 Mexico locations is 0.65 ± 0.19 pCi-m -s compared with 0.28 ± 0.09 pCi-m -s for Texas, 0.19 +

0.06 pCi-m'^-s"-'^ for Alaska and 0.30 ± 0.09 pCi •m'^-s'"'" for Hawaii.

48
Table 3.3

Radon Flux Density from Soils in the U.S.

Location^ Soil Type

Measurement

Method'^

^^hn Flux Densi (pCi-m"^ -s'^)ty

Alaska

Tanana basin (14) Tanana basin (3)

Yukon basin (1)

Podsolic

Podsolic

Podsolic

A

VP

VP

0.13 ± 0.04 0.21 ± 0.06 0.22 ± 0.07

Hawaii

Deep soils (6)

Thin soils (18) Lavas (5)

Latosolic

Mountain

Mountain

A

A

A

0.85 ± 0.23

0.05 ± 0.01

0.06 ± 0.002

Illinois

Champaign County (472)

Argonne (8)

Chernozemic

Chernozemic

F

F 1.4 0.57 ± 0.05

Massachusetts

Lincoln (10) Podsolic A 1.4

Nevada

Yucca Flats Desertic A 0.48

New Mexico

Socorro (10) Socorro (6) . Valley (130)°

Plains (4). Plains (8)° . Mountains (10)

Others (3) Socorro (3)

Desertic

Desertic

Desertic

Desertic

Desertic

Mountain

Desertic

Desertic

F

FA

A

A

A

AVP

0.91 ± 0.09

1.0 ± 0.3 0.33 ± 0.23

1.2 ± 0.3 0.54 ± 0.16 0.18 ± 0.06

0.62 ± 0.17

0.56 ± 0.16

Texas

Plains (3) Central (3) Coastal (3)

Chernozemic

Chernozemic

Latosolic

A

A

A

0.41 ± 0.12

0.20 ± 0.06

0.23 ± 0.07

.Numbers in parentheses give number of measurements. Two sites. A-accumulator; VP-vertical profile; F-flow (see Section IV.C.l.).

Wilkening, et a1 . [WCS 72] noted that "wide variations occur in radon flux density measured at

different sites, at different times at a given site, and by different methods employed simultaneously

at the same site." They listed 6 factors that influence the flux density at a given location:

(i) the radium concentration in the soil and rocks (see Sections II. B. and III. A.);

(ii) the emanating power of mineral grains or soil particles - i.e., the ability of the radon

atoms formed by the decay of radium to escape from the grains (see Section II. A.);

(iii) the porosity and equivalent depth (diffusion length) of the soil which determines the transport characteristics of radon in the interstitial pore spaces of the soil (see Chapter I);

49
(iv) the state of vegetative cover;

(v) water and or ice involvement (see Sections II. C. and III.C.l.); and

(vi) variations in atmospheric characteristics that influence transport processes between the soil

gas and lower layers of the atmosphere (see Chapter I and Section III.C).

Radon exhalation from "technologically-enhanced" materials is much more variable. Bernhardt,

et al . [BJK 75], for example, reported values for radon flux density from uranium mill tailings at four

widely separated areas. The values range, as shown in Table 3.4, over three orders of magnitude.

Table 3.4

Radon Flux Density from Uranium Mill Tailings [BJK 75]

Location of Uranium

Mill Tailings^

Range of Values for Radon Flux Density

(pCi-m"^'s'^)

Shiprock, NM (8) Salt Lake City, UT (20)

Lakeview, OR (10)

Boulder City, NV (4)

440 - 2200

110 - 780 0.76 - 31 0.77 - 11

Numbers in parentheses give number of samples.

Norton [Hor 79] similarly found the radon flux density from inactive uranium mill tailings piles and

from phosphogypsum piles to vary over several orders of magnitude (Table 3.5).

Table 3.5

Typical Radon Flux Density from Inactive Uranium Mill Tailings and

Phosphogypsum Piles [Hor 79]

Range of Values for Source Radon Flux Density

(pCi-m'^-s"^)

Inactive uranium mill tailings piles 60 - 1200

Phosphogypsum piles 2 - 120

Few data for radon flux densities or exhalation rates from building materials are available. The

1977 UNSCEAR report [UN 77], for example, cites only four references. Table 3.6, taken from UNSCEAR

[UN 77], summarizes the values for various materials. Because of the large variability in radium

content even within a given type of material, the most useful data are those which relate the radon

50
flux density or exhalation rate to the radium concentration of the material (as in Table 3.6). This

type of information is not always available, however, and the utility of the data is thereby more

limited.

Table 3.6

Radon Flux Density Normalized to Radium Concentration

for Various Materials [UN 77]

Material Comments

222

Rn Flux Density per Unit Concentration

of 226r3

-2 -1 -1 (pCi-m 'S per pCi-g )

Phosphogypsum

Phosphogypsum

Uranium mill tailings

Uranium mill tailings

Soil

Concrete

Light Concrete

Heavy Concrete

internal walls, 7.6-cm

thick

ceilings, 1.3-cm thick

10-cm thick

"infinite" thickness

"infinite" thickness

10-cm thick

20-cm thick

8-cm thick

0.01

0.001

0.2

1.6

0.5

0.005'

0.02

0.01

Two more recent studies of radon exhalation from building materials have been reported by Jonassen

and McLaughlin [JoM 76; JoM 78] and by Pensko and Stpiczynska [PeS 78]. In the first study, Jonassen

and McLaughlin measured the exhalation rates from 14 different types of materials, and reported the

values on surface area, volume, and mass bases. They noted that in some cases the material should be

characterized in terms of the exhalation rate per unit surface area, (i.e., flux density) while in

other cases it appears to be more relevant to describe the material in terms of the exhalation rate per

unit mass or exhalation rate per unit volume. They also indicated that it is not possible to predict

a priori "which if any of these exhalation units is appropriate to use in scaling up the results obtained

from samples to a room with walls made of the same material [JoM 76]." Table 3.7 tabulates the radon

flux density for the 14 materials studied in their work. For illustrative purposes, it may be useful

to compare the flux density from some of the concretes to that expected from soil. For soil having a -2 -1 flux density of 0.4 pCi*m -s below a 10-cm concrete slab floor, the contribution to the flux density

-2 -1 at the surface of the floor could be 0.01 pCi-m -s (assuming a 40-fold diminution by the floor).

Thus, the flux density from any of the first six concretes listed in Table 3.7 is within a factor of 5

and nearly comparable to the flux density through the slab from the underlying soil.

51
Table 3.7

Radon Flux Density from Various Building Materials [JoM 78]

Material

???

Rn Flux Density

(pCi-m'^-s'-^)

1. Ordinary concrete, gravel and sand from

the sea, Danish deposits 0.0095

2. Ordinary concrete, gravel and sand from

pits, Danish deposits 0.0076

3. Ordinary concrete, unknown composition 0.0084

4. Ordinary concrete, unknown composition 0.0104

5. Light weight concrete, Swedish origin

alum shale, old type 0.232

6. Light weight concrete, Swedish origin

alum shale, new type 0.082

7. Light weight concrete, Danish origin, clay based 0.0018

8. Light weight concrete, Danish origin, clay based 0.0012

9. Expanded clay concrete, LECA 0.0012

10. Bricks, solid type 0.0013

11. Bricks, cavity type 0.000055

12. Chipboard 0.000019

13. Fiberboard 0.000030

14. Gypsum board 0.000035

In the second study, Pensko and Stpiczynska [PeS 78] evaluated several types of by-products that

may be utilized in building materials as well as some conventional building materials. Their results

for both the radium concentrations and emanation powers are provided in Table 3.8.

Table 3.8

Emanation Power of Various Bu" Iding Materials [PeS 78]

Number

"ope Ra Concentration Emanation Power

of Samples(pCi-g"^) {%)

Material

Range Mean Range Mean

Fly-ash 33 1.7-3.9 2.6 0.20-1.16 0.54

Slag 11 1.0-3.6 1.8 0.24-1.53 0.70

Phosphogypsum 3 0.7-19 11.8 3.5-19.7 12.0

Red brick 1 0.54 5.1

Silica brick 1 0.18 16.3

Portland Cement 1 0.65 4.1

Soil 5 0.11-0.51 0.27 12.1-50.1 28.9

52
Very recently, Nero et a1 . [NBH 80] at Lawrence Berkeley Laboratory, reported results for the

exhalation rate per unit mass from about 100 ordinary concrete samples which originated from 10 different locations throughout the U.S. Values ranged between 0.2 and 2.0 pCi*kg -h

In addition to these studies, Culot et al . [COS 76] at Grand Junction and Oil worth, et al . [DSM 78]

at Elliot Lake measured the radon flux density from concrete slabs overlying "technologically-enhanced"

-2 -1 materials. Culot, et al . obtained a flux -density ranging from 0.21 to 0.25 pCi-m -s through a 20-cm

thick slab adjacent to uranium mill tailings having a radon concentration of 40,000 pCi-L In the_

work at Elliot Lake, the radon concentration in the underlying soil was approximately two orders of

magnitude less than that at Grand Junction, but the radon flux density from the concrete slab was only

a factor of two less. In the Elliot Lake Study, it was assumed that a substantial fraction of the flux

density was due to contributions from the concrete itself.

III.C. Meteorological Factors Affecting Radon Exhalation

The influence of meteorological factors on the exhalation of radon is an important consideration

that should not be overlooked in any predictive model. Although nearly 60 papers which address the

effects of external factors on radon exhalation have been surveyed, very few contain sufficient quantitative data that are directly usable. This review focuses exclusively on those few studies which do

provide quantitative results.

In one of the most comprehensive studies, Steinhausler [Ste 75] evaluated the dependence of 24

meteorological variables on the indoor radon concentrations in private and public buildings. He found

large daily and seasonal fluctuations even under constant ventilation conditions. Using a multipleregr^ession analysis technique, he was able to identify those variables most strongly correlated to

changes in the indoor concentrations. These were: the barometric pressure; the soil temperature;

daily mean atmospheric temperature; daily mean wind speed; the relative humidity; the daily atmospheric

temperature range; and vertical temperature gradient.

Although meteorological effects may be important, it must be emphasized that the internal characteristics of materials (e.g., radium concentration, porosity, water content, etc.) are usually more

significant. Styro [Sty 68], for example, concluded that the influence of meteorological variables on

radon exhalation from soils "has only a minor role and that the radon flux (density) is predominantly

determined by the characteristics of the soil" (see also, Wilkening, et al. [WCS 72]). Auxier [Aux 73]

noted that this was also the case for concrete materials, and concluded that the most important parameter

53
influencing radon exhalation from building materials (for a given radium concentration) is the moisture

content of the material (see also Kovach [Kov 46], and Wilkening and Hand [WiH 60]).

III.C.l. Moisture Content of Materials and Humidity Effects

Although moisture effects, particularly the influence of precipitation on radon exhalation from

soils, have been discussed in a number of studies [Oka 56; JaH 58; EKS 62; Pea 65; PeJ 65; Pea 67; IsH

70; SNS 70; MeM 73], quantitative results obtained with other factors held constant are scant.

Cox, et al . [CBK 70] examined the radon concentrations in ground-level air at Cincinnati over a

7-year period, and derived two alternative 2-parameter equations for predicting outdoor radon concentrations. The two parameters used to obtain the average monthly morning radon concentration (C), in

units of pCi*m , were the atmospheric thermal stability (S), in °F/27 m, and the total moisture dete

tion in soil (TMD), in units of mm*. The two equations obtained were:

C = 139 (S) + 93,000(TMD)"-'- - 14 (3.1)

and

In C = 0.332 (S) - 0.00384(TMD) + 6.877 (3.2)

2 both of which had a squared correlation coefficient of r =0.89. As shown in figure 3.1, the radon

concentrations, which varied over a factor of about 6, were highly correlated with the TMD. In fact,

the radon concentration C could be predicted almost as well using only the TMD in a 1-parameter equation,

C = -44 + 114,000 (TMD)'-'- , (3.3)

shown as the solid curve in figure 3.1. Equation (3.3) which has a squared correlation coefficient of

2

r =0.84 has an overall predictive power nearly equal to eqs. (3.1) and (3.2). The relative significance

of the moisture content of soil can be readily seen by differentiating eq. (3.3). At a typical TMD

value of 200 mm,

AC = -2.85 A(TMD)

which indicates that a 20% change in TMD gives rise to nearly a 60% change in C. At a TMD value of 100

mm, the effect is even greater; a 20% change in TMD results in a change in C of over 200%.

*Atmospheric thermal stability is defined as the difference between temperatures at elevations of 30

and 3 meters. Total moisture detention is a measure of the soil moisture content that accounts for

precipitation, evapotranspiration, run-off, storage of water in the ground and the retention of additional

water as snow and ice (see Cox, et al. [CBK 70] and references therein).

54
1600

200 300 400

TMD (mm)

500 600

Figure 3.

1

. Average monthly radon concentration in ground-level air

as a function of total moisture detention (TMD) in soil. Taken from

Cox, et al . [CBK 70].

Auxier [Aux 73] investigated the effect of moisture content on concrete blocks. He measured radon

exhalation from two blocks--one with low (2 to 4% by weight) and one with a higher (6-8%) moisture

content. The moisture content was determined from the mass difference between the "moist" blocks and

after baking them. The exhalation results showed a "strong dependence on the moisture content of

the concrete." Differences on the order of 30% were observed with higher radon exhalation resulting

from higher moisture content which was obtained by placing concrete blocks in atmospheres with varying

water vapor pressures.

Auxier [Aux 73] also investigated the influence of humidity on radon exhalation from concrete. He

found that with increasing relative humidity from 60% to 80%, the exhalation decreased by approximately

30%. This work is the only known quantitative study of the effect of humidity on radon exhalation from

building materials.

III.C.2. Pressure Effects

The dependence of radon exhalation on pressure has been noted many times [Zup 34; Kov 45; Oka 56;

Ros 57; Tan 59; KSE 64; SEK 66; PoP 69; Bak 71; CIW 74; Jon 75; UN 77; Tan 78; SBU 79], and there is

general agreement that the exhalation increases as underpressure increases. Several reports by Bates,

€t al . (see [EdB 80] and references therein) have treated exhalation from mine walls. Perhaps the only

55
significant study of the systematlcs of the underpressure effect in building materials was performed by

Jonassen and McLaughlin [JoM 77; McJ 78] who examined concrete. They found the radon flux density to

Increase linearly with underpressure as shown in figure 3.2. Their results agree with the frequently

quoted value for soil: a 1% difference in barometric pressure results in a 60% change In the exhalation

rate [Har 78]. It should be emphasized that the exhalation rate is essentially independent of the

value for the pressure P, and that it is pressure differences between pore air and outside air, AP,

that cause significant changes in exhalation. Such differences between the pressure within the interstitial

pore spaces of a material and the external pressure can conceivably exist since the relaxation time for

the pore spaces to equilibrate in pressure with the outside air is of the order of one hour (see Section

I.D.7). It is not unusual for there to be a 1 or 2% pressure change during such a period of time, so

that the variability in the radon flux density from a material, based on the results shown in figure

3.2, could be as much as a factor of 2.

3.0

10 15

AP (mmHg)

Figure 3.2 . Radon flux density from a concrete as a function of

the under pressure AP. Taken from Jonassen and McLaughlin [JoM 77].

III.C.3. Temperature Effects

The influence of temperature changes is apparently quite complex as evidenced by the substantial

number of temperature-related meteorological variables found by Steinhausler [Ste 75] to be correlated

with radon exhalation. Two general types of behavior in soils have been noted in the literature.

56
The first type is the widely observed diurnal variations [Zup 34; Kov 46; Ros 57; WiH 60; HOS 66;

Kir 70; SNS 70; WCS 72] that are associated with the competition between two factors: (i) changes in

convective flow due to temperature differences in the soil from day to night; and (ii) turbulent mixing

in the atmosphere which leads to an increase in exhalation in the daytime and a reduction at night [UN

77].

The second type of behavior is the temperature effect arising from the direct heating of soil

samples. Shashkin and Prutkina [ShP 70] report a sharp decrease in emanation power after roasting

samples at 100 to 600 °C. They reported that there is a partial restoration in the emanation power

after keeping the roasted samples under normal conditions or moistening them. The authors offer a

number of hypotheses relating to the internal effects (e.g., radon adsorption onto internal surfaces

and the effects of capillary water films) to explain the results. Similar considerations are also

discussed by Tanner [Tan 78] (see also Chapter II.).

One of the more amusing temperature "effects" was observed by Okabe [Oka 56] who measured the

diurnal variation of atmospheric temperature and radon concentration. The results, which are partially

illustrated in figure 3.3, clearly indicate a strong correlation between temperature and radon concentrationthis change in temperature coming after the change in radon concentration! It should be noted that, in

this case, should a correlation analysis be performed, a strong association between the variables would

be found (e.g., as indicated by the square of the correlation coefficient). This certainly does not

imply that a change in the radon concentration caused a variation in the temperature, or vice versa.

It is probably a good example that calls into question the validity of drawing conclusions from mere

searches for associations or correlations for the purpose of identifying the variables that influence

radon exhalation.

RADON CONC. . . o

z z • TEMPERATURE o o IJ

cc 3

••. ^^••• ••

•

.5 5

1-<

> z

- UJ

UJ o

2

1 '^\l>

'^' \

•• •

• y^

'0«

Q. 5 S

Ul K O 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 i 1 1 1—

12 18 DAY I6 12 18 DAY 2

6 12 DAY 3

Figure 3.3 . Diurnal variation of atmospheric temperature and atmospheric radon concentration. Individual data points, shown in the

original from Okabe [Oka 56], have been omitted.

57
Only two quantitative studies of temperature effects on building materials are known, and these

are contradictory. Gabrysh and Davis [GaD 55] found roughly a 20X increase in radon exhalation from a

concrete sample when it was heated from 20 to 59 °C. Auxier [Aux 73], in contradistinction, concluded

that there is a "negligible temperature effect on the exhalation of radon from concrete between 23 and

43 °C. Considering the number of unknown internal variables which may influence these results (e.g.,

moisture content or porosity of the materials), this discordance may be discomforting, but not surprising. For example. Tanner, in an unpublished study of a sample core of natural sandstone which had

the appearance of a uniform sand- aggregate concrete, observed a very irregular, but steady-state decrease

in radon exhalation with increasing temperature. He inferred that temperature influenced the exhalation

of radon indirectly through its effect on the water distribution in the sample material [Tan 80].

III.C.4. Other Factors

Other factors that have been reported to affect radon exhalation are wind speed, seasonal changes,

soil disturbances, and ground cover [Oka 56; EKS 62; KSE 64; Pea 65; PeJ 65; Pea 67; GLF 70; IsH 70;

SNS 70; BaK 71; MeM 73; UN 77; UN 80]. Most of this information is qualitative in nature, however, and

of little value in estimating or predicting radon exhalation. Tanner [Tan 78] also describes possible

internal effects such as the role of radon adsorption onto surfaces, but these are not well understood.

III.D. Conclusions and Recommendations

Much of the data presented in the preceding three sections is summarized in tabular form in Table

3.9. The first category (A) presents ranges for typical values of radium concentrations in common

building materials, and estimates the corresponding expected variation in an exhalation rate or radon

flux density, E, due to the indicated natural variation of the radium content of these materials. The

variation in E is given as the expected maximum value divided by the expected minimum value. It must

be emphasized, however, that the radon flux density from a material may not necessarily scale proportionally with the radium concentration. The second category (B) merely tabulates the range and ratio

of typical values for the radon flux density from materials. The third category (C) summarizes the

known variations in E for given values or changes in the meteorological variables. In general, the

variations in E, as reflected in the values of E „/E ., are much less affected by meteorological

max min

variables than they are by the choice of materials. It must also be emphasized that the values in

Table 3.9 are only meant to be representative. They are very selective and are based on only a very

limited set of results.

58
Based on this review, several general conclusions, and hence some recommendations, can be made.

The first conclusion is that existing data are not sufficient to quantitatively predict or estimate

radon flux density or exhalation rates from building materials. Although the flux density from a

material such as concrete may be estimated a priori within one or two orders of magnitude, more quantitative estimates are not possible because the exhalation process is exceedingly complex and very

dependent on the specific characteristics of the material, and because many of the causal internal and

external variables are only qualitatively understood.

Table 3.9

Variable

Range of Typical Values

or Change in Variable

/Emax\

Win/

A. Radium Concentration

concrete

concrete block

brick

phosphogypsum

0.6-2.0 pCi-g ^ 0.2-4. 0.2-1.7

17-33

320

9

2

B. Radon Flux Density

soils concretes phosphogypsum

uranium mill tailings

0.1-1.4 pCi-m'^-s'-'-

0.001-0.2

2.-120

0.8-2200

10 200

60 3000

C. Meteorological Effects

moisture content

relative humidity pressure change

temperature

temperature

2-4% to 6-8% (by weight)

60% to 80%

1-2%

23 to 43 °C 20 to 59 °C

61.3

2 negligible [Aux

1.2 [Oka 56]

The second conclusion is that only "technologically enhanced" soil and building material exhalation

rates are normally of concern as major contributors to indoor radon concentrations. If we establish a

"concern level" at 10% of 3 pCi-L (corresponding to 10% of 0.015 WL, assuming an equilibrium factor

of 0.5), then in a single-room structure (with a height of 2.5 m and a ventilation rate of 0.5 hr ) -2 -1 the corresponding radon flux density from the floor surface would be approximately 0.1 pCTm -s .

This is above the radon flux density from most building materials (see Table 3.7). Further, if we

assume that a concrete slab floor provides a 25- to 50-fold diminution in the radon flux density from

the underlying soil, then the "concern level" for the radon flux density from soils is in the range of

-2 -1 about 2 to 5 pCi-m -s . This is also above the radon flux density from naturally-occurring soils

(see Table 3.3). If, however, this diminution is not achieved or if the radon enters the structure

through cracks or openings in the foundation, then the radon flux density from soils may approach or

exceed the "concern level," and may be a substantial contributor to the indoor radon.

59
The above considerations do not apply to buildings with very low ventilation rates, or if other

air-dynamic mechanisms (e.g., the "stack effect" suggested by Nazaroff—see introduction of Chapter

III) are operative. Nor do they apply to "technologically-enhanced" soils or materials. Such materials

would certainly include uranium mill tailings and phosphate slags which have maximum observed radon

flux densities above the minimum "levels of concern." Their incorporation into buildings or near

building sites may significantly contribute to indoor radon concentrations. Hence, the third conclusion

is that the use of "technologically-enhanced" raw materials in building components needs more investigation.

Exhalation rates from uranium and phosphate mill tailings vary over two or three orders of magnitude

with the lower end of the ranges approximating values for the more radioactive soils found in the

natural environment (see Tables 3.4 and 3.5). Considering the high radium content of phosphogypsum and

the wide variety of potential uses in building materials, exhalation rates from these materials should

be carefully monitored. Similarly, the use of slags and fly ash as concrete fillers should be evaluated.

Local variabilities in these materials may be expected to be extremely important.

The fourth conclusion is that meteorological effects due to pressure changes, temperature, humidity,

and even moisture content do not seem to be of great importance individually in causing significant

changes in radon exhalation. There is, however, very little quantitative information and more thorough

studies of these effects may deserve attention.

The fifth conclusion is that there remain some unanswerable questions and uncertainties concerning

radon exhalation from building materials and within buildings. For example, there is a large variability

in the exhalation rate from soil possibly due to moisture detention which suggests that there may be

corresponding large variabilities in radon exhalation from wet concrete basements. The magnitude of

this effect is unknown and further investigations with water-soaked concrete blocks may be revealing.

Similarly, it appears that an integral 10-cm concrete slab provides a substantial reduction in radon

flux from soils, but there is no indication in the literature as to whether this is typical or that

factors of 100 or even 1000 are attainable by judicious selection and treatment of materials.

60
CHAPTER IV

ASSESSMENT OF MEASUREMENT METHODOLOGIES

Most of what is known about radon transport through or exhalation from building materials is based

upon observations and measurements in laboratory or in situ (field) settings. Therefore, an understanding of these previously employed measurement methodologies is necessary in order to evaluate the applicability and validity of existing transport and exhalation data, as well as to plan future laboratory

transport and exhalation experiments.

This review considers and summarizes only those laboratory- and field-based measurement methods

that are most directly applicable to studies of radon transport and exhalation from building materials.

Four major areas are considered:

A. assay of gaseous radon samples;

B. laboratory measurements of radon exhalation;

C. in situ measurements of radon flux density and concentrations in soil gas and pore spaces;

and

D. in situ measurements of radon concentration in indoor air.

Although other types of measurement methods (e.g., for individual radon progeny concentrations in air

or working levels) may be indirectly applicable, they are not as central or directly relevant to

transport and exhalation studies. For the four areas outlined above, the various measurement methods

will be briefly described; and to the extent possible will be evaluated (1) by considering their applicability for measuring the quantity of interest, (2) by identifying some of the limitations, underlying

assumptions of the methods, and the most serious sources of inaccuracy, (3) by estimating their sensitivity, precision, and accuracy, and (4) by identifying some of the calibration requirements. Often,

it will only be possible to identify what is unknown or undocumented. It will also be apparent in some

cases, that a more critical assessment and detailed method evaluation should be performed, but this

type of effort is outside the scope of this report.

*The wide variety of techniques used for the measurement of radon and related quantities (progeny

concentrations, working levels, etc.) have been summarized by Budnitz [Bud 74], and more recently by

Breslin [Bre 80] and by George [Geo 80].

This need for critical assessments of measurement methodologies has also been recently identified by

the Radiation Policy Council Task Force on Radon in Structures [RPC 80].

61
IV. A. Assay of Gaseous Radon Samples

The assay of gaseous radon samples is a common element in many of the methods described in Section

IV. B. (laboratory measurement of radon exhalation), in Section IV. C. ( in situ measurement of radon flux

and soil-gas concentration), and in Section IV. D. ( in situ measurement of radon concentration in indoor

air). The assay methods can be subdivided into two classes: internal gas counting techniques, and

scintillation counting techniques.

IV. A. 1. Internal Gas Counting Techniques

For these techniques, the radon is introduced, together with a suitable counting gas, directly

into a chamber or "counter" whose characteristics primarily depend on the operating voltage of the

chamber (i.e., in the ionization, proportional or Geiger-Muller regions). Although a variety of shapes

(parallel -pi ate, cylindrical, spherical) are possible, such counters usually consist of a cylindrical

cathode with a wire anode (typically 0.02-0.03 mm in diameter) stretched along the axis of the cylinder.

The physics and operating characteristics of these types of counters are described in a number of

elementary texts and other references [Pri 64; MAG 79; NCRP 78].

Irrespective of the type of counter which is used for the radon assay, an associated gas-handling

system is usually necessary. This system is used for separating and purifying the radon sample, mixing

it with the counter filling gas, and transferring the gas into the counter. These systems are commonly

226 associated with methods for assaying Ra, and have been extensively developed for many applications

by Evans [Eva 35; Eva 37], by Curtiss and Davis [CuD 43], and by Harding et al . [HSS 58]. Similar

radon gas-handling systems have been described by Blanchard [Bla 64], by Harley [Har 72], and by Krieger

and Jacobs [KrJ 78].

Ionization chambers, both direct-current and pulse (see, for example, [NCRP 78] for a discussion

of these types), are the most commonly employed types of internal gas counters for the assay of radon.

Any of the counters, however, are subject to certain limitations associated with the nature of the gas

admitted into the counter [DaH 52]. The operation of current ionization chambers is very adversely

affected by the presence of ions and condensable vapors in the gas sample. Pulse ionization chambers,

proportional counters, and G-M tubes are also affected by these factors, as well as by the presence of

electronegative gases (such as oxygen). These limitations necessitate the ancillary gas-handling and

purification system. Proportional counters and G-M tubes are also extremely sensitive to changes in

counter-gas mixture and pressure.

62
Internal gas counting techniques are primarily laboratory-based, and have limited applicability

for in situ field measurements. Because it is less convenient than other methods, it is used for field

measurements only when great sensitivity is required. Although other sample collection schemes can be

employed in the field (see Sections IV. D.), normally a grab sample of air is collected with an evacuated

glass flask (typically one liter volume) and then returned to the laboratory for processing and assay

[ABL 77; ISC 72; Bre 80; Bud 74].

Internal gas counting techniques are very sensitive, and conceptually can be very accurate.

Typical chambers have overall detection efficiencies of 85%. With one-liter air samples, sensitivities

of 0.01 to 0.05 pCi-L for pulse ionization chambers have been reported [Bre 80; Geo 80]. An overall

accuracy (more correctly, inaccuracy) of plus or minus several percent is achievable, but may be limited

in low-level samples to the imprecision ("counting error") arising from the nature of the radioactive

decay process itself. In assaying radon, it is necessary to use counting intervals that are long 222 222 214 compared to the halflife of the individual members of the Rn subseries ( Rn through Po).

Consequently, the approximations normally used for estimating the statistical "counting error" are not

valid [LuW 64]. The random uncertainty associated with the radioactive decay process is given by the

expression S 2; VJI where I is either the mean number of observed events ("counts") for counting operations, or is the product Qq of the total charge (Q) collected in an ionization chamber in the counting

2 interval and the average charge (q) released per interacting particle, and where J = a /(j, the variance

222

in the number of observed events from the radon subseries (i.e., Rn parent plus progeny) divided by

the mean number of observed events for the subseries. The methodology for this problem has been solved

222 by Lucas and Woodward [LuW 64], and they have tabulated values of J for the Rn subseries for a

variety of combinations of counting interval and detection efficiency. For typical low-level samples,

the average uncertainties attributable to the "counting statistics" alone may be of the order of 2-8%

[Bar 75].

However, uncertainties of this magnitude can be achieved only if the gas sample is carefully and

quantitatively processed and transferred to the chamber, and if the chamber is accurately calibrated.

Internal ionization chambers are used chiefly for comparative measurements [NCRP 78], i.e., the assay

is performed by comparing the chamber output from the unknown radon sample with that from a sample,

prepared in the same way and containing a known amount of radon activity. Many of the better laboratories calibrate their ionization chambers with radon sources from standard radium solutions (e.g., NBS

226

Ra solution Standard Reference Materials) utilizing a widely- employed "bubbler" arrangement (see,

for example, [WaH 69; Har 72; KrJ 78; Geo 80; Bee 75]). The major sources of inaccuracy arise from

errors in the gas-handling procedures used during the sample processing and in the chamber calibration.

These gas-handling procedures and subsequent counting techniques are generally a very complicated

63
sequence of many individual unit operations or steps. It must be recognized that virtually every step

in this sequence introduces a conceivable source of inaccuracy.

IV. A. 2. Scintillation Counting Techniques

For the scintillation-counting techniques, the radon sample is introduced into a suitable vessel

(chamber, flask, tube, etc.) containing a silver-activated zinc sulfide phosphor which emits light that

222 can be detected with photomultiplier tubes when the a particles from decay of the Rn and progeny

interact with the phosphor. The basic physics and operating characteristics of scintillation detection

devices are also described in a number of elementary texts and other references [Pri 64; MAG 79; NCRP

78]. Two distinct techniques, one utilizing electrostatic collection, are employed.

In the electrostatic-collection technique, the initially positively-charged recoiling daughter

atoms from the decay of radon are collected by means of an electric field, on to a ZnS(Ag)-coated

electrode within the chamber containing the radon sample [BrM 52]. Normally, the electrode (usually a

screen), may be viewed from outside the chamber by a phototube [MaS 58; RoD 66; NCRP 78], but the electrode may be removed after a suitable collection period and placed in an external a particle detector

[KRH 76].

In the second technique, the gas sample is contained in a vessel whose walls are coated with a

ZnS(Ag) phosphor [DaH 52; VDT 55] and viewed by one [Luc 57] or two [KSL 64] phototubes. The scintillation flask, popularly known as the Lucas cell [Luc57], is a widely employed radon assay method used

both in the laboratory and in the field [Luc 57; Har 72; Bre 80]. Scintillation flasks both commercial

and home-made, come in a variety of sizes (nominally 50 mL up to 650 mL), are constructed of either

glass or plastic walls, and are equipped with one or two valves. The flasks are usually pre-evacuated

for collection of a sample of whole air, and then mounted on the phototube assembly for counting,

usually after a delay of a few hours to allow the radon decay products to come into radioactive equilibrium with the radon. For field use, the flasks may be returned to a laboratory for counting, or may

be measured entirely in the field with the use of portable, battery-powered photomultiplier assemblies

[Bre 80]. The DOE Environmental Measurements Laboratory has also developed a continuous scintillation

flask monitor in which air is continuously drawn through the flask (see Section IV. D. 3.) [HLW 57;

CoT 77; Tho 77b; Geo 76; Bre 80].

Unlike internal gas counting techniques, scintillation flasks are relatively insensitive to the

gas mixture and the presence of ions, dust, or condensable vapors [DaH 52; Luc 57]. In the laboratory,

however, they can be and frequently are employed with the same type of gas-handling systems which are

used for separating, purifying, and transferring radon samples.

64
In routine use, the sensitivity of scintillation counting techniques is about one-tenth that for

the internal gas counting techniques. The sensitivity may be increased by a factor of two to four by

using larger flasks and increasing the counting interval [Luc 57; Geo 76; Bre 80], or by greater factors

by preconcentrating the samples prior to filling the scintillation cells. The sensitivity is often

controlled by the background counting rate of the scintillation vessel and phosphor. Although early

scintillation tubes were reported to have backgrounds as large as 5 alphas per minute primarily due to

the phosphor [DaH 52], the background counting rates for most scintillation flask assemblies are now

typically 0.1 counts per minute [Luc 57; Geo 76].

The accuracy considerations and calibration requirements are similar to those described for the

Internal gas counting techniques. Perhaps because scintillation flasks are commercially available and

more widely employed, improper flask calibrations are more frequently encountered. George [Geo 80]

reported that in an interlaboratory calibration exercise in which 12 groups took part, the reported

values of half of the participants (6 of them) deviated by more than 20% from the expected value.

Every scintillation flask must be individually calibrated and must be used with the same counting

equipment with which it was calibrated. Flasks counted on different equipment can be substantially in

error because of different operating characteristics of the photomultiplier tubes [Geo 80]. Other

frequently encountered sources of inaccuracy are diffusion of the radon into the phosphor and walls of

the flask, and leakage from the flask [Geo 76; Geo 80]. Internal field effects occurring with electrically-insulating internal chamber surfaces may result in a consequent variation in the detection

response due to the migration of radon progeny. Lucas [Luc 57] reported that with an insulating window,

trace impurities produced localized internal fields which created a variation in the distribution of

the radon progeny within the flask. Kraner, et al . [KSL64] also observed inconsistent counting rates

due to the presence of an electrostatic field within the flask that was induced by the applied voltage.

Lucas corrected the variations by making the transparent internal surface of the flask window electrically conducting. Kraner eliminated the problem by providing grounded external electrodes and electrostatic fields. The Environmental Measurement Laboratory, and other laboratories, have eliminated

effects of this type by coating all of the internal surfaces of the flask with ZnS(Ag) phosphor and by

grounding the cathode of the photomultiplier tube [Geo 76].

IV. B. Laboratory Measurement of Radon Exhalation

The literature on laboratory measurements of exhalation of radon from materials is extensive and

dates back to the 1930s (see, for example, [Hah 36; Hai 64]). Most of this work has been of a microscopic nature such as to characterize the emanating power of crystals, minerals and rocks, or used to

study the adsorption characteristics and surface properties of materials. These were mainly performed

65
for either geological interest or for various physico-chemical applications. Studies specifically

related to the measurement of radon exhalation from macroscopic quantities of building materials,

however, are quite limited. The measurement methods used in these studies can, in large part, be

characterized as follows:

i) All of the studies are similar in principle and are based on the same general methodology,

ii) The specific apparatus and detailed operating procedures for each study are unique.

The general methodology consists of placing the sample of building material into a sealed vessel

which is operated under controlled conditions; allowing the radon to escape (exhale) from the sample

and build up inside the vessel; collecting and/or sampling and transferring the radon in the vessel

atmosphere; and assaying the radon.

Many different types and sizes of vessels to contain the samples have been used. These include

various laboratory "de-emanation" flasks [Bar 73; Bar 75], chambers [Naz 79], and vacuum-tight desiccators [PeS 78]. In an early study of the effect of radiant heating on radon exhalation from concrete,

Gabrysh and Davis [GaD 55] used a simple glass tube (approx. 5 cm diam. x 30 cm) molded around a smaller

heating tube. Jonassen and McLaughlin [JoM 76; JoM 78] used two different types of containers: a 200

L cast iron pressure chamber, and simple sheet metal barrels with an internal volume of approximately

120 L. Similar types of metal cylinders were also used by Culot, et al . [COS 73; COS 76]. Auxier and

coworkers [Aux 73; ASK 74] employed a large chamber constructed of polyethylene supported by a wood

frame.

To a large extent, the degree to which the experimental conditions in the vessel are controlled

determines the scope of the inferences that may be drawn from the measurement results obtained. In

many reports, the influencing variable under study (e.g., temperature or pressure) is controlled, but

no mention is made of the control placed on other factors that may affect the results. Therefore, the

results obtained may be of only limited validity.

The length of time that the radon is allowed to build up within the vessel may range from a few

hours or less [JoM 76] to many days [GaD 55; Bar 75]. It depends mainly on the exhalation rate of the

material under study and on the sensitivity of the sampling and assay procedure that is utilized.

One of the more common operating procedures consists of performing a single sampling and assaying

of the radon in the vessel at the end of the buildup period. The exhalation rate is then determined

from the total amount of radon in the vessel (or from an aliquant of the total) and the buildup time

interval. Frequently, the vessel is purged with radon-free air or other gases prior to sealing to

remove any initial radon from the vessel's atmosphere. This type of procedure suffers in that only a

66
final determination of the amount of radon released from the material is made, and any potential problems (e.g., loss of radon by leakage from the vessel, or "back diffusion" into the sample) will be

masked.

An alternative technique is to study the buildup of radon in the vessel over time by either continuous monitoring or periodic sampling [JoM 76; JoM 78]. This procedure is similar in principle to the

accumulation method (see Section IV. C) used to determine radon flux density or the exhalation of radon

from soils [KSE 64; Wil 74; WCS 72; BJK 75]. The exhalation is determined from the initial slope of

the growth curve, and in this way obviates, to an extent, the possible problems of leakage and back

diffusion [BJK 75; JoM 76; JoM 78; Jon 80].

Possible variations in the sampling and collecting procedures include collecting the radon in the

vessel at the end of the buildup period by gas-sweeping [Naz 79], vacuum-transfer (see, for example,

[PeS 78]) and volume-sharing transfer [Bar 71; Bar 75] techniques, or by periodic sampling of only a

small portion of the vessel atmosphere with an evacuated flask (perhaps directly into the chamber used

to assay the radon) [JoM 76]. Obviously, in order to measure only the naturally escaping fraction of

radon produced within the sample material, the material should be disturbed as little as possible

during sampling and collecting. Therefore, the volume-sharing transfer method is less susceptible to

this possible problem than the gas-sweeping and vacuum-transfer techniques. The grab sampling techniques introduce periodic underpressures in the vessel which may perturb the exhalation rate, and may

result in inhomogeneous samples that are not truly representative of the radon concentration in the

entire vessel. Jonassen has reported that, through a variety of experiments, it appears that these two

possible problems are not significant [Jon 80]. The radon sample from the vessel may also be filtered

to exclude the radon-progeny radionuclides from the samples (see, for example, [PeS 78]). Further

variations include collecting the radon in a charcoal trap [Naz 79] or in a low-temperature condensation

flask [JoM 76] for subsequent preconcentration and/or purification of the radon sample (see Section

IV. A). Auxier and coworkers [Aux 73; ASK 74] used a closed-loop flow system to collect the radon

progeny on filters which were subsequently alpha counted with surface-barrier detectors. Although they

utilized a vibrator inside their vessel in an attempt to ensure a constant distribution of dust particles,

it is unclear how the concentration of condensation nuclei was controlled, or how they accounted for

plate out of the radon-progeny radionuclides.

In laboratory measurements of radon exhalation, the radon samples are most frequently assayed by

scintillation counting or internal gas-counting techniques (see Section IV. A). The samples are either

introduced directly into the counting chambers or purified with an associated gas-handling system prior

67
to assay. The latter is not as prevalent a practice. Although several of the studies [JoM 76; Bar 73;

COS 73] include a description of the calibration procedures which were used for the radon assay (it is

important to include the radon collection efficiency), the number of studies in which they are not

reported is dismaying. One can only conjecture whether they failed to report the calibration procedure,

or failed to calibrate!

As indicated, the reported methods of laboratory measurement of radon exhalation from building

materials all utilize the same general methodology, but offer a nearly semi-infinite variety of possible

variations in the specific apparatus and operating procedures. Because each study is somewhat unique

to itself, it is difficult, if not impossible, to unequivocally evaluate and state the adequacy and

reliability of any one of these possible sets of variations. One can, however, summarize the most

common failings or possible sources of inaccuracy which may occur. Most of these have been mentioned,

and they include: (1) uncontrolled experimental conditions; (2) unaccountable losses or leakage of

radon from the vessel; (3) "back diffusion" of radon into the sample material; (4) adsorption of radon

onto surfaces in the vessel or inhomogeneous radon concentrations in the vessel resulting in unrepresentative samples; (5) perturbations of the exhalation rate during sampling and collection; (6) sampling and collection losses such as adsorption of the radon in the transfer system; and (7) errors in the

assay and calibration procedures.

IV. C. In Situ Measurement of Radon Flux Density and Concentration in Soil Gas and Pore Spaces

In situ measurements of radon flux density and radon concentrations in soil gas and in pore spaces

are the two classes of measurements that are most applicable to radon transport and exhalation studies

of building materials. Measurements of radon flux density, for example, are often used to evaluate the

sources and pathways of radon into a building [DSM 78; GHA 80]. Similarly, soil gas concentrations of

radon are useful in evaluating the diffusion and transport of radon through soils near building foundations [Sco 80], and for determining diffusion coefficients through building materials (e.g., building

foundations). Scott [Sco 80], however, has cautioned that although measurements of radon flux density

and soil gas concentration should be valuable in determining radon transport in soils, the large variability seen in both of these measurements leads one to suspect that the radon concentration in the

upper soil layer is so greatly influenced by atmospheric effects that short-term measurements may not

be very helpful. Similarly, in those porous soils where radon transport may be significant, it may be

very important to not disturb the surface pressure distribution, or else the radon concentration near

the surface may be changed by the presence of the measuring equipment.

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IV.C.l. Radon Flux Density

Radon flux density is just the exhalation rate of radon from a given surface area. Although most

of the methods for measuring radon flux density were developed for studying radon exhalation from the

earth's surface (see, for example, [WCS 72]), the methods may, with slight variation, be applied to

measure the flux density from any type of surface. Six main categories of methods can be recognized:

(i) Accumulation Method

(ii) Induction Method

(iii) Flow Method

(iv) Adsorption Method

(v) Vertical Profile Method

(vi) Soil Concentration Gradient Method

The accumulation method (i) involves the direct accumulation of radon in an open-faced vessel

which is inverted and placed (and possibly sealed) on the surface. In this way, the vessel and surface

forms a closed container for the accumulation of exhaled radon. On soil surfaces, the vessel is normally embedded several centimeters into the ground [Cul 46; KSE 64], or sealed to the ground with, for

example, wetted bentonite [BJK 75]. On other surfaces, such as building materials, the vessels are

sealed with epoxy resins or other caulking agents [JoM 76; JoM 78]. Many different types and sizes of

accumulators have been used including large barrel drums [KSE 64; BJK 75], large surface area parallelepipeds [GLF70] and disc cylinders [WiH 60].

Although early deployments of the method [Zup 34] relied on a long accumulation period followed by

a single sampling and assay of the radon, the method is now commonly performed by periodic sampling and

assaying to follow the growth of radon concentration in the accumulator [KSE 64; WCS 72; BJK 75; JoM

76]. This is done because with materials of high porosity, such as most soils, the radon concentration

in the accumulator will rapidly become comparable to the concentration of radon in the pores of the

soil. "Back diffusion" of radon into the pores will then occur, resulting in marked departures from

the growth curve which is governed by the simple radioactive growth equation. The exhalation rate is

determined from the initial slope of the observed radon concentration growth curve using a well-known

and developed methodology [KSE 64; WCS 72; BJK 75; JoM 78].

Wilkening, et al . [WCS 72] reported that the accumulation method, as commonly employed, is based

on several assumptions: (1) the accumulation time is short compared to the radon halflife (radioactive

decay corrections can, of course, be made); (2) the concentration of radon in the accumulator is significantly below the soil gas concentration to prevent appreciable back diffusion; and (3) the presence of

the accumulator does not seriously perturb the exhalation process. With respect to the second assumption,

69
Wilkening, et a1 . [WCS 72] recommends, as noted by Kraner, et a1 . [KSE 64], that the accumulated radon

concentration be kept below 10% of the soil gas concentration at a depth of 13 cm. The third assumption

is more troublesome and more difficult to verify. Other than by, perhaps, comparing the results obtained

with the accumulation method with that obtained by other methods (e.g., the flow method)--under identical

conditions— it is not clear how the third assumption can be validated.

A number of efforts have been made to control the conditions within the accumulator. This includes

the use of fans to mix the accumulator atmosphere and insure homogeneous sampling [Cul 46; WiH 60; KSE

64; WCS 72; BJK 75], maintaining constant pressures in the accumulator by drawing air in during sampling

[Cul 46] or using pressure-equalizing orifices [WCS 72], and monitoring and controlling internal temperatures [BJK75].

Sampling and assay of the radon from the accumulator is similar to the techniques used for laboratory measurement of radon exhalation (see Section IV. A). The techniques employed, however, are usually

those more compatible to the demands of field deployment. As a result, the most common variation is

the collection of the radon grab samples directly into evacuated scintillation flasks [BJK 75; WCS 72;

Cle 74], although other sampling, transfer, collection and gas-counting techniques have been employed

[Zup 34; Cul 46; WiH 60; KSE 64].

The most extensive evaluation and verification of the accumulation method using large approximately

100 L and 200 L barrel accumulators and scintillation-counting procedures was performed by Bernhardt,

et al . [BJK 75]. They reported that the "counting errors" for the radon assays were generally less

than 5% for each sample, and that the overall precision in the exhalation rate estimated from replicate

measurements (both sequential measurements using one accumulator, and measurements using accumulators

of different sizes) was within the estimated uncertainty of the growth curve regression analyses. The

-2 -1 precision (actually imprecision)was typically 20% for radon fluxes of approximately 10 fCi'Cm -s , -2 -1 and increased to 50-100% for radon fluxes of less than 1 fCi'cm -s They also reported that the use

of a common calibration factor for the scintillation flasks (rather than individual calibrations for

each flask) introduced on additional analytical uncertainty of 6%.

The accumulation method has also been modified and used for measuring the exhalation of thoron and

thoron flux density [Cro 69; GLF 70].

The induction method (ii) may be viewed as a specialized form of the accumulator method (i) which

utilizes an electrostatic-collection technique. Although various forms of the induction method have

been employed since the early 1900s, Rosen [Ros 56; Ros 57] is primarily responsible for adapting the

70
method to the measurement of radon exhalation from the earth. For this method, a large surface area

accumulator, containing a negatively-charged collector wire, is placed over the ground. The method

utilizes the fact that some of the radioactive decay products of radon when first formed are positively

charged, and these ions are collected by means of the electric field about the collector wire. The

radioactive deposit on the collector is then assayed. Rosen [Ros 56] used a relatively large metal

accumulator tank (approx. 3m x Im x Im hgt. ) in which the radon decay products were measured by an

internally located gauze cylindrical ionization chamber situated around the wire. Other than Rosen's

work [Ros 56; Ros 57], the induction method has not been widely employed and therefore is only of

limited interest.

The flow method (iii) for measuring radon flux density was principally developed by Pearson and

coworkers [Pea 65; PeJ 65; Pea 67] in an effort to more nearly simulate natural conditions at the

sampling site. The method uses a long, shallow collector in which the exhaled atoms of radon are

entrained in a closed-loop stream of air flowing across the ground-air interface. The entrained radon

in the air stream is continuously collected in a dry-ice cooled charcoal trap after first passing

through drying tubes and water traps. A by-pass flow route containing another charcoal trap is usually

provided to flush radon from the system prior to sample collection.

Pearson [Pea 67] outlined the principal requirements for the collecting device: (1) it should

minimally disturb the soil during the placement and operation of the collector; (2) it should be constructed so that it can be sealed to the soil surface and thereby prevent both leakage from the collector and admission of external air; (3) the flow within the collector should simulate natural air

movements on the free, unobstructed ground-air interface; (4) it should have a well-defined collection

area in order to determine the flux density (exhalation rate per unit area); and (5) it should be

durable, inexpensive and easily fabricated. The collector Pearson, et al . [Pea 67; PeJ 65] designed

was a straight tunnel which permitted a reasonably constant flow across the entire surface being sampled.

They found that a collection flow rate of approximately 4 liters per minute approximates average wind

speed and gustiness, although the flow can be adjusted at the sampling site to duplicate, as nearly as

possible, the meteorological conditions over the area being sampled.

In Pearson's work, the charcoal traps containing the radon samples were returned to the laboratory

where the radon samples were desorbed by heating, flushed and transferred into scintillation counting

flasks for assay (see Section IV. A). They have made a rather comprehensive evaluation of the method.

Including various assumptions and some possible sources of inaccuracy [Pea 65]. In addition, they

reported a coefficient of variation (standard deviation divided by the mean) of 30% for a set of 8

replicate determinations of the radon flux (taken close together and within a 1-hour interval) [Pea 67].

71
The flow method has also been used by Israel, et a1 . [IHL 68] in which an ionization chamber (instead of charcoal traps and scintillation counting) was incorporated into the flow system. A similar

system using a Pearson collector and ionization chamber was adopted by Styro, et al . [SNS 70] to measure

thoron exhalation.

A portable and automatic instrument which directly measures radon exhalation from a mine wall has

also been developed [Gre 74]. This instrument, which is based on the flow method, consists of a metal

2 collecting cylinder (with a surface area of approximately 0.1 m ) that is attached to the surface of a

mine wall. A stream of carbon dioxide inside the collector is used to continuously sweep the mine

wall, and the exit stream is filtered to remove radon progeny radionuclides. Alpha particles from the

decay of the radon entrained in the exit stream are detected with an external, flow-through gas proportional counter. A prototype of the instrument was tested, and the efficiency of the proportional

counter for radon in the exit stream was calibrated against an ionization chamber. The sensitivity of

the instrument has not been reported, and it may not be sufficiently sensitive for most non-mine applications because of the low detection efficiency for the external proportional counter.

In the adsorption method (iv) , the surface to be measured is covered directly with granular activated charcoal, and the exhaled radon is adsorbed onto the charcoal and collected. The quantity of

radon collected is then determined by directly assaying the charcoal for the radon progeny radionuclides

by gamma- ray spectrometry .

Megumi and Mamuro [MeM 72] used the adsorption method to determine the exhalation rate of both

radon and thoron from soils. In their work, they used plastic frames 35 x 70 cm in area and 1.5 cm

high to contain the charcoal. The soil surface surrounded by the frame was first covered with a sheet

of gauze, then with a bed of charcoal uniformly distributed over the gauze, and sealed with a polyvinyl

chloride film and a 5-cm thick sheet of polystyrene foam. Usually, they simultaneously employed four

frames, each containing approximately 210 grams of charcoal, for collection periods of 5 hours. The

charcoal was then removed from the four frames and sealed in polyvinyl chloride cases which were 17 cm

in diameter and 8 cm high. Interestingly, the authors noted that they used a "windscreen" to prevent

the gases adsorbed on the charcoal from being carried away by the wind!

The sample cases containing the charcoal were then returned to the laboratory and measured with a

5 X 5-inch Nal(Tl) scintillation detector. Thoron and radon were determined from measurement of the

583-keV vrays of ^°®T1 , and 609-keV y-rays of ^'^Bi , respectively. Because the 583- and 609-keV

214 photopeaks in the y-ray spectrum are not completely resolved, the Bi (and hence radon) determination

208

is not made until after the Tl has sufficiently decayed. The assay procedure was standardized with

72
a calibration source prepared by mixing a standard radon solution with charcoal and sealing the mixture

in a similar sample case.

In separate experiments using a laboratory chamber, Megumi and Mamuro [MeM 72] evaluated the radon

adsorption efficiency as a function of temperature and humidity. They found that in the temperature

range of 4-35 °C, the adsorption efficiency was 85-90%. The efficiency gradually decreased as the

temperature increased until it reached 60% at 70 °C. The adsorption efficiency was independent of

humidity in the relative humidity range of 40-90%. They also evaluated the amount of charcoal needed

to effectively adsorb the radon and thoron exhaling from a given surface area. Using frames of seven

different sizes, each containing the same amount of charcoal (100 g), they found that the amount of

2 adsorbed thoron was proportional to the frame area for areas between 500 and 1500 cm , and concluded

-2 that more than 0.067 g*cm of charcoal should be used. They postulated that the proportionality above

2 1600 cm was lost because of the decrease in the probability that thoron will be adsorbed by the

charcoal before it decays." This explanation does not seem plausible, however, since one might expect

oig 212

the Po (0.15 s) daughter or Pb (10.6 hr) granddaughter to be more readily adsorbed than the

thoron.

A more widely used variation of the adsorption method is the charcoal canister technique developed

by Countess [Cou 76; Cou 77a; Cou 77b]. It has been used by many others to determine the radon flux

density from a variety of surfaces both indoors and outdoors, including building materials, soils, and

phosphate gypsum and uranium mill tailings piles. The technique has similarities to other charcoal

adsorption methods for collecting radon, but as noted by Countess [Cou 77b], it avoids the possible

radon losses and problems inherent in either transferring the radon from the charcoal as in the Pearson

flow method [Pea 67] or transferring the charcoal as in the Megumi and Mamuro [MeM 72] method.

Countess used canisters from conventional personal respirators (U.S. Army Mil gas canister) which

were modified by removing the metal casing from one face. The canisters have a cross-sectional area of

2 87 cm , are 6 cm high, and contain approximately 150 g of activated charcoal. They are sealed to the

soil surface by simply embedding the rim of an outer protective plastic casing into the surface of the

soil to a depth of about 1 cm, or sealed to other surfaces with either a clay-like material or pliable

weather stripping materials. A canister is normally deployed for a period of one day or longer, and

returned to the laboratory for analysis.

Similar to the method used by Megumi and Mamuro [MeM 72], the adsorbed radon in the canister is

assayed with a gamma-ray spectrometer. Countess used an 8 x 8-cm Nal(Tl) scintillation detector. The

exposed side of the canister was placed on the detector, and counted after the early progeny radionuclides

73
reached radioactive equilibrium with the adsorbed radon (about 5 hours after the end of the exposure

period). Analysis is performed by measuring either three or four y rays from Pb and Bi (242-,

294-, 352- and 609- keV). In his first work. Countess calibrated the method by drawing measured volumes

of filtered air having a known radon concentration (determined by the two-filter method, see Section

IV.D.l) through the canisters [Cou 76]. Subsequent calibrations, however, were performed by direct

comparisons with the more traditional accumulation method [Cou 77a; Cou 77b]. Canisters and accumulators

were exposed simultaneously to several test surfaces (and positions were alternated to average spatial

and temporal variations in radon flux), and used to obtain a calibration factor for the canisters. The

radon flux and hence the amount of radon adsorbed by the canisters was calculated from the accumulator

measurements. The calibration factor was found to be independent of exposure time for periods of

several hours to several days [Cou 77b].

Countess investigated the adsorption efficiency, and reported that the canisters quantitatively

adsorbed radon [Cou 77a]. Possible losses of radon from the canisters due to temperature, reduced

pressure, and humidity effects during exposure and transit were also evaluated. There were losses in

all cases, but for normal temperature and pressure ranges the losses were usually less than 1%. The

humidity dependence was much greater, but Countess concluded that "canisters exposed to wet soil for a

period of several days will not adsorb enough water to reduce the capacity of the canisters for radon

by more than 10-15%" [Cou 77a].

By making multiple measurements on several test surfaces in which the canisters were deployed

simultaneously in a relatively small area, Countess (Cou 77a) reported that the precision of the canister

method ranged from 6% (coefficient of variation) for an outdoor location in the high radon flux density

-2 -1 -2 -1 phosphate region of Florida (10 pCi*m -s ) to 15% for a measurement on soil in New Jersey (1 pCi-m -s ).

Wide variations in radon flux obtained by the charcoal canister method have sometimes been seen over

even small areas. A two- laboratory intercomparison study was recently performed [Hor 79] to determine

if the actual analysis of the canister is the major contributor to this wide variation. Fifty canisters

exposed to radon flux densities ranging from approximately 2 x 10 to over 10 Ci-m -s were

assayed in one laboratory and then shipped to and assayed in the second laboratory. The two assays on

individual canisters typically differed by 10 to 30%, but the 50-canister average difference between

the two assays was 16%.

The charcoal canister method is not as sensitive as some of the other radon flux density methods.

Countess reported a typical lower limit of detection of 70 pCi which corresponds to the radon accumulated

-2 -1

by a canister in a four-day exposure at a flux density of 0.03 pCi-m -s [Cou 77a]. To achieve

adequate sensitivity, the canisters usually must be deployed for at least a day or longer. As a result,

74 .
the canister method provides only an average (time- integrated) flux density rather than the series of

more "instantaneous" values which are possible by the accumulator method.

The charcoal canister method is probably now the most widely used method for determining radon

flux density. In large part, this is because it is relatively simple, inexpensive, and much less labor

intensive than the other methods. Several types of commercial canisters have been used for a variety

of purposes by many different groups. The method has been used indoors to locate and measure radon

sources and routes of entry into buildings. Various adapters have been employed for mounting canisters

in corners, across wall-wall or wall-floor joints and over cracks in concrete foundations [Cou 77a;

Sco 80]. The original development and employment of the method by Countess, and subsequently continued

by the DOE Environmental Measurements Laboratory [Bre 80], has been admirable. Unfortunately, not all

users of this method are as careful in performing the method, evaluating their use of it, and calibrating

it. On inquiry with several users, it has been learned that some merely use Countess' original calibration

factor, and that substantial losses of radon from the canister are accounted for by using the concept

of an "effective halflife" (which assumes, of course, that the desorption and loss of radon from the

canister over the entire collection, transit and assay time periods is a first-order kinetic processwhich is not likely).

It is not clear to what extent the adsorption method, by the direct placement of the charcoal on

the exhaling surface, perturbs the exhalation rate. Of the radon flux density methods, it probably

most poorly simulates natural conditions. Furthermore, perhaps there should be some concern over the

relatively small area sampled by charcoal canisters. Since the popular adsorption method using charcoal

canisters is primarily calibrated against the accumulation method, both of these effects will be partially

masked, and an independent assessment of them (e.g., by comparing the adsorption method to other methods)

is not possible.

The vertical profile (v) and soil concentration gradient (vi) methods are indirect methods whereby

an average radon flux density is inferred from measurements of the vertical concentration gradients of

radon in either the atmosphere or the soil. Both methods are based on a large number of assumptions,

and therefore have limited utility for the direct measurement of radon flux density from surfaces.

In the vertical profile method, the vertical distribution of radon in the atmosphere is determined

by measuring the radon concentration at ground level through the mixing layers by means of captive

balloons or aircraft [Kir 70; WCS 72]. The radon flux density from the soil is then estimated by

summing the vertical radon concentration distribution (taking into account the structure of the atmosphere). The method neglects advective effects (localized changes in the radon concentration gradient

75
caused by mixing and air movements in the gradient), and is based on the assumption that the total

radon content of a vertical column of air (extending into the atmosphere) must balance with the radon

flux into the base of that column.

The soil concentration gradient method is based on assuming a particular transport model (see

Section I.D.)- Radon flux density and soil gas concentration are known to be correlated [WCS 72].

With a given transport model, the radon flux density at the surface of a material can be related to the

profile of the concentration gradient of radon in the interstitial pore volume [CIW 74; JoM 77]. This

method was used by Senko [Sen 68] to estimate radon flux density from soils (and an effective diffusion

coefficient) by measuring soil radon concentrations at two depths (see Section IV. C. 2) and assuming

that the radon concentration in the soil was exponentially distributed with depth. More recently,

Mochizuki and Sekikawa (MoS 78) using silicon surface-barrier detectors measured the concentration of 218 the radon daughter Po in soil air at various depths to infer radon flux density. Inasmuch as the

measurement of radon flux density from materials may actually be used to validate models of radon

transport in the materials, a method that is based on an a priori transport model assumption cannot be

used.

Very few efforts have been made to directly intercompare the various methods for measuring radon

flux density. As indicated earlier, the widely used adsorption method (iv) using charcoal canisters is

commonly calibrated against the accumulation method (i). Wilkening, et a1 . [WCS 72] reviewed various

measurements by the accumulation (i), flow (iii), direct charcoal adsorption (iv) and vertical profile

(v) methods. On comparing the accumulation, flow and vertical profile methods, they concluded that the

accumulation and flow methods generally agree (usually within 30%), but that the vertical profile

measurements "tend to yield higher values when compared at the same location" (WCS 72). They also

noted that the vertical profile method (unlike the accumulation and flow methods) has the advantage of

not disturbing the surface and of integrating over large areas, but that its assumption of no advective

flow is extremely restrictive and exceedingly difficult to correct for. In contradistinction, Senko

[Sen 68] compared the accumulation (i), Pearson flow (iii) and soil concentration gradient (vi) methods

and reported that radon flux densities measured by the flow method averaged 3.2 times higher than

values obtained by the accumulation method. He stated that the "reasons for this discrepancy are not

clear" [Sen 68]. He also found that the soil concentration gradient method (with its exponential radon

concentration profile assumption) was in better agreement with the flow method under "convective conditions", but was closer to the accumulation method under "inversion conditions" [Sen 68]. Clements

and Wilkening [CIW 74] used a more sophisticated transport model to compare the accumulation and concentration gradient methods. (The experiment was performed, of course, to test the radon-transport model.)

76
In a laboratory experiment using a column of radium-bearing porous material in which they could induce

interstitial gas flow, they compared the direct measurement of radon flux density by the accumulation

method to a predicted flux density calculated from the measurement of the radon concentration profile

in the column. The agreement was exceptionally good.

Some of the above discrepancies may, qualitatively at least, be rationalized. The vertical profile method (which is only useful for geographical surveying of large areas) may give higher values for

the radon flux density since its use does not disturb the surface. The extent to which the actual

measurement of the radon flux density perturbs the flux density itself is, of course, a serious concern.

Similarly, one may expect the flow method to give higher values than the accumulation method because

the former attempts to simulate natural air movements on the unobstructed surface. This type of argument

may also explain Senko's [Sen 68] finding that the flow and concentration gradient methods better

agreed under maximum convective flow, but not under inversion conditions.

Wilkening, et al . [WCS 72] have noted that "wide variations occur in radon flux (density) measured

at different sites, at different times at a given site, and by different methods employed simultaneously

at the same site." One would only hope that the differences in the method of measurement could be

eliminated, so that the real spatial and temporal variations could be better understood.

IV. C. 2. Radon Concentration in Soil Gas and Pore Spaces

The measurement of radon concentration in the pore space of soils has been widely used for uranium

exploration [IAEA 76], as well as for understanding the transport of radon through porous materials

[CIW 74; JoM 76; JoM 78].

Traditionally, sampling holes are prepared by drilling or driving various types of pipes, rods,

cylinders and sampling tubes into the ground which are then covered [BoW 32; Tan 59; NMS 52; ElO 60;

Sco 80]. The holes are typically 1 to 10 cm in diameter, and precautions are normally taken to prevent

compacting the soil. The soil gas in the sampling hole is then sampled and assayed for radon. Sampling

may be performed by vacuum-collecting, aspirating, or pumping the gas into collection systems or directly

into scintillation or ionization chambers [BoW 32; NMS 52; Tan 59; SIO 60]. Variations on this theme

include flow-through and recirculation systems [IAEA 76], and several techniques involve the insertion

of the detection devices directly into the hole [Dye 69; MoS 78].

77
The basic premise of the method is that the concentration of radon in the sampling hole equilibrates with the concentration in the interstitial pores, and that the meteorological influences on the

radon concentration are the same in the pores as in the sampling hole [Kov 45; Kov 46; Tan 59]. Scott

[Sco 80] has noted that unless the soil gas is drawn into the sampling tube by suction, then it is

unlikely that the radon concentration in the hole equilibrates with that in the soil gas. He has

reported that measurements of radon concentration in open holes are often ten times less than soil gas

concentrations measured by suction.

The only known measurements of the radon concentration in the pore volume of a building material

were made by Jonassen and McLaughlin [JoM 77; JoM 78]. They drilled narrow bore holes to different

depths in a 30-cm thick concrete wall to obtain a pore air radon concentration profile. Sampling tubes

were sealed in the holes using an epoxy resin, and the wall surface surrounding each sampling tube was

also sealed with epoxy. Samples were vacuum collected from the sampling tubes using one liter evacuated flasks, and assayed by scintillation counting techniques (see Section IV. A. 2). The effect of the

epoxy resin on perturbing the exhalation of radon into the pore air is unknown. On the basis of leak

testing the sampling procedure, they concluded that the major part of the sampled air came from wall

pores, although they acknowledged that "the air in any sample comes from an undefined volume of pores

surrounding the inner end of the sampling tube" [JoM 77].

IV.D. In Situ Measurement of Radon Concentration in Indoor Air

The methods used to measure indoor radon concentrations may be subdivided, as done of Breslin [Bre

80], into three groups:

1. "Instantaneous" radon concentration methods in which an air sample is obtained nearly instantaneously or over only a few minute sampling period;

2. Time-integral radon concentrations methods which provide a single, average concentration for

an extended period of time ranging from a few days to a week or longer; and

3. Continuous radon monitoring methods.

The methods within each group may be further categorized as shown in Table 3.1.

78
Table 3.1

Methods for Measuring Radon Concentration in Indoor Air

INSTANTANEOUS METHODS

° grab-sample collection/laboratory assay techniques

° grab-sample/scintillation flask techniques (Lucas)

° two-filter tube method

TIME INTEGRAL METHODS

° large-volume sample collection/laboratory assay techniques

° TLD-based diffusion-electrostatic monitor ("PERM")

** track-etch techniques

CONTINUOUS METHODS

° flow-through scintillation-cell monitor

° flow-through ionization chambers

° automated two-filter tube method

° diffusion-electrostatic techniques (Wrenn)

IV.D.l. Instantaneous Methods

Some of the "instantaneous" sampling/laboratory assay techniques for radon concentration were

briefly described in Section IV. A. Normally, a grab sample of air is collected with an evacuated flask

and returned to the laboratory for processing and assay by either internal gas-counting or scintillationcounting techniques. Alternatively, the air sample may be collected directly in an evacuated scintillation

flask (Lucas cell).

Another important technique for the instantaneous measurement of radon concentrations in indoor

air is by the two-filter tube method [Fon 64; Jac 65; TaL 66; ThL 70]. The method consists of drawing

air through a long cylindrical tube fitted with filters on both ends. The front filter removes all

particulates including radon progeny radionuclides from the air. Some of the radon entrained in the 218 air stream within the tube decays in-flight, and a fraction of the Po daughter atoms that are produced

are collected on the second filter. Typical sampling periods range from 5 to 10 minutes. Immediately

following the sampling period, the second filter is removed and counted for several minutes with a

portable alpha-particle detector. In addition to the tube, field measurements require a portable

alpha-particle counter and an air sampling pump.

Although the radon concentration can be calculated from a simple equation that considers the tube

dimensions, sampling volume, and sampling and counting time intervals, it must include a correction

218 factor for the loss of Po in the tube by plate out and diffusion [ThL 70]. A direct calibration.

79
such as performed by Thomas and LeClare [ThL 70], with a known radon concentration is preferable, but

this is not always possible for all users. In the absence of a direct calibration, at a minimum, the

a-particle detector and ancillary electronics should be calibrated with an a-particle emission-rate

standard that has the same geometric configuration as the filter. The standard should also match, as 218 closely as possible, the Po a-particle energy (6 MeV) and the typical emission rates of the filters.

The sampling flow rate should be metered, and it also requires calibration. A correction for the 218

self- absorption of the Po a-particles in the filter may also be necessary.

In a laboratory experiment based on 17 replicate determinations at a radon concentration of

570 pCi'L , Thomas and LeClare [ThL 70] reported a precision of approximately 1.4% (coefficient of

variation). If the precision scales with the inverse square root of the radon concentration, one might

expect coefficients of variation of approximately 11% at 10 pCi-L and 35% at 1 pCi-L (under identical

conditions).

George [Geo 80] identified several common sources of inaccuracy in the two-filter method. Air

leakage around the front filter will increase the radioactivity on the second filter and result in an

overestimate of the radon concentration. This may be avoided by pushing rather than pulling air through

the tube. The front filter should be essentially 100% efficient for removing radon progeny radionuclides; and the second filter should have surface characteristics favorable for the collection and

218 assay of Po. The two-filter method is known to have a strong humidity dependence at low relative

humidities, but in the range 20-90% the dependence is less critical (ThL 70). Before the two-filter

tube is deployed for in situ measurements, it should be evaluated under different conditions of flow

rate, collection time, and radon concentration to test the linearity of the system. Calibrations

should also be conducted under these same conditions.

The sensitivity of the two-filter tube method depends primarily on the tube volume, sampling flow

rate, and sampling and counting times. With a 1-liter tube and typical flow rates (0.1-0.2 L*s ),

sampling times (5-10 minutes) and counting times (1-6 minutes), radon concentrations on the order of 5

to 10 pCi-L can be measured. With larger tubes, larger flow rates, and longer times, sensitivities

as low as 0.1 to 0.01 pCi-L' have been reported [Bre 80; Geo 80].

IV. D. 2. Time- Integral Methods

Time- integrated radon concentration measurements can be performed by large-volume sample collection or preconcentration techniques, followed by laboratory radon assays (see Section IV. A). One of

the simplest large-volume sample collection methods was developed by Sill [Sil 69]. In this method,

80
the air sample is collected over two days by pumping air into a Mylar bag at a very slow rate (typically

10 mL per minute). The field equipment consists of only a 40-L Mylar bag and a storage battery powered

aquarium aerator pump with a flow regulator. The radon concentration is assayed in the laboratory by

pumping the sample from the bag through a cold charcoal trap, and then transferring the radon from the

charcoal to a suitable scintillation or internal gas counting device. The method is reported to be

very sensitive, capable of measuring radon concentrations in the range of 0.01-0.1 pCi-L [Bre 80;

Geo 80]. Alternative large-volume preconcentration sampling methods include collecting the air sample

by pumping the air through cooled charcoal traps [Bud 74; ISC 72], and other cryogenic techniques [HaP

80; PDH 80]. In these cases, the preconcentrated air samples are also returned to the laboratory for

radon assay. The accuracy of these methods is primarily dependent on the accuracy with which the flow

rate or sample volume can be measured, and the losses of radon during collection and transfer. George

[Geo 80] recently summarized some of the possible sources of inaccuracy and calibration requirements

for the measurement of sample volumes and flow rates.

The completely passive track-etch film techniques, which have been used for many years in uranium

exploration [IAEA 76], can also be used to determine time- integrated radon concentration [Cou 76b;

FAF 72; FrB 77]. Although they have been used for this latter application, the method is probably more

suitable for estimating working levels and total alpha-particle incidence [Lov 69; RLN 69; Cou 76b; FrB

77]. For this method, a small piece (nominally 1 cm ) of a suitable plastic film such as cellulose

nitrate, is exposed for a long period of time in the air to be monitored. Alpha particles, incident on

the film, damage the surface. By etching the film in suitable alkaline solutions (e.g., sodium hydroxide),

each point of impact can be enlarged. The resultant tracks and pits are then counted with a microscope

or by other electro-optical means [FrB 77] to provide a measure of the total number of alpha particles

incident on the film during the exposure period. Various types of membranes and filters have been used

with the films to achieve different hold-up times that can be used to exclude thoron, and to filter the

particulate radon progeny radionuclides [Alt 80]. Specific track-etch methodologies for radon dosimetry

have recently been reviewed by Frank and Benton [FrB 77] and will not be detailed here. In general,

direct calibrations to relate track density to the quantity of interest (radon concentration, working

level, etc.) must be performed for each specific application [Cou 76b]. Because of its simplicity and

low cost, track-etch methods offer great potential for wide-scale deployment. Breslin [Bre 80] noted,

however, that it has not been widely used because of its inaccuracy and poor sensitivity. At typical

Indoor radon concentrations, year-long exposure periods are needed to obtain sufficient track density;

and at that concentration, the precision is poor (about ±50%). Other than unpublished claims by a

commercial vendor, very few efforts [Cou 76b; FrB 77] have been made to evaluate the method for typical

Indoor radon concentrations.

81
Another passive method to measure time-integrated radon concentrations uses an instrument commonly

known as a "passive environmental radon monitor" (PERM) which was developed at the DOE Environmental

Measurements Laboratory [Geo 77; GeB 77; Bre 80; Geo 80]. The instrument operates on an electrostaticcollection principle which was used in previous monitors [CTD 69; WSC 75]. These earlier versions have

since evolved into continuous monitors (see Section IV. D. 3) and are commonly known as "Wrenn meters".

The PERMS, however, are completely passive time-integrating devices which rely on a thermoluminescent

detector (TLD).

For comparison, the operation of a PERM will be described in relation to the "Wrenn meter" (see

Section IV. D. 3). The PERM developed by the Environmental Measurements Laboratory consists of a large

1.5-L metal funnel with a paper filter sealed to the large opening (20 cm diam. at the base) and a

stopper (containing a hollow electrode) inserted in the narrow opening. The "Wrenn meter" uses a more

efficient hemispherical design. The PERM samples the ambient air by molecular diffusion, although some

versions utilize an electric fan to draw the air in. Air enters the device and diffuses through a bed

of desiccant (to eliminate a serious humidity dependence in the instrument's performance [GeB77]), and

enters the sensitive volume by diffusing through a porous barrier (used to exclude the particulate

radon progeny radionuclides). The PERM uses a large filter paper (Whatman #41); the "Wrenn meter" uses

a polyurethane foam medium. The radon concentration in the sensitive volume gradually equilibrates

with the ambient radon concentration in the surrounding atmosphere. An electrostatic field created by

application of a negative potential on the hollow electrode is used to attract and collect the positively218 charged Po ions that form within the sensitive volume by decay of the radon. The hollow electrode

also serves as an exit for the air in the sensitive volume. In a PERM, an alpha-particle sensitive

218 lithium fluoride TLD chip is located at the negative electrode to detect the a-particle emitting Po

radon daughters that accumulate on the electrode. The "Wrenn meter" uses a ZnS(Ag) scintillator to

detect the daughters. Early models used photographic film (read by conventional densitometry) to

obtain similar time- integrated radon concentrations [CTD 69]. More recent versions of the "Wrenn

meter" operate in a continuous manner by using a photomultiplier tube coupled to the scintillator, and

pulse counting [WSC 75]. For the PERM, the TLD chip is removed after a suitable period of exposure

(usually one or two weeks), and read on a thermoluminescent detector. The thermoluminescence of the

chip (which must be corrected for gamma- radiation background with a control chip located outside the

PERM) is directly proportional to, and calibrated in terms of, a time-integrated radon concentration.

The PERM has a response lag of about 5 hours [GeB 77] due to the length of time needed for air to

diffuse through the desiccant, but this is short in comparison to the usual length of the monitoring

period.

82
The sensitivity of the PERM is reported to be 0.03 pCi-L in a one-week exposure period [GeB 77]

which is considerably less than that obtained with a continuous reading "Wrenn meter". The difference

may be due to the greater detection efficiency of the ZnS(Ag) scintillator and the more efficient

hemispherical shape of the "Wrenn meter" sensitive volume.

The PERM must be directly calibrated in atmospheres having known radon concentrations [GeB 77;

Geo 77], and under conditions simulating field atmospheres. This can be accomplished by placing the

device in a chamber containing radon under controlled conditions. The chamber atmospheres can then be

sampled, and the radon concentration measured by methods of known accuracy (e.g., with scintillation

flasks or pulse ionization chambers). Alternatively, the chamber could be filled with a known amount

226 of radon which is obtained from an equilibrated Ra standard. Standard procedures for the performance,

use, and calibration of TLD systems when used to measure environmental gamma-radiation are available

[ANSI 75; PEG 76; GJP 80], but George has noted that no recommended procedures for the use of TLDs for

radon monitoring are available [Geo 80]. He has also suggested, however, that the same performance

specifications and testing procedures that are used for gamma- radiation applications could equally well

be applied for measurements of alpha particles. In this same paper, George [Geo 80] also addressed

some of the sources of inaccuracy and practical problems encountered in the use of TLDs for radon.

Measurement precision for the PERM developed by the Environmental Measurements Laboratory was found

during calibration experiments to be about 5% to 10% (coefficient of variation). At typical ambient

levels (say 1 pCi-L ), one can probably expect the monitor to be less precise. Considering the large

correction for the gamma-radiation background and the uncertainty in presently available thermoluminescent

dosimetry (TLD) techniques, the accuracy, even with a good calibration, is probably no better than 20%

to 30%.

A commercial instrument similar to the PERM is available, but is reported to be much less sensitive than the original Environmental Measurement Laboratory design and is humidity dependent [Bre 80].

IV. D. 3. Continuous Methods

Radon concentrations in indoor air continuously change, and, as a result, measurements based on a

single grab sample are rarely sufficient [Bre 80]. For this reason, continuous methods are probably

more useful. Obviously, any of the "instantaneous" methods described in Section IV.D.l. could be

performed repeatedly to obtain a continuous record, but methods designed specifically for continuous

monitoring are considerably more convenient. Many of them are adaptations of previously described

methods that were modified for use as continuously- reading monitors. The three most common are:

continuously-reading flow-through devices using either an ionization chamber [WaH 69] or a scintillation

83
flask [HLW 57] (see Sections IV. A. and IV.D.l.); an automated two-filter tube method [Tho 77a] (see

Section IV.D.l.); and the diffusion-electrostatic "Wrenn meter" [WSC 75] (see Section IV. D. 2.).

Flow-through ionization chambers have been used for many years to measure radon concentrations in

uranium-mine atmospheres, but their use indoors has been very limited. A typical detector consists of

a gamma- radiation compensated, flow-through ionization chamber, of approximately 1-L volume, preceded

by a filter and electrostatic precipitator. All attached and unattached radon progeny radionuclides

are removed from the air stream entering the chamber so that the ionization current is due only to

radon decay. The units are also equipped with a pump to draw air through the filter, precipitator and

chamber; an electrometer to measure the ionization current; and some suitable recording device. A

ope calibration procedure for this type of instrument (using an NBS Ra solution standard as a source of

radon) was described by Waters and Howard [WaH 69]. They reported a full-scale sensitivity of 66

pCi-L on the most sensitive range of the electrometer, and an ability to measure radon concentrations

in the 5-10 pCi'L range. The estimated precision of the measurements at full-scale (i.e., 66 pCi-L )

was approximately 10% [WaH 69].

Flow-through scintillation flasks were first developed in the mid-1950s by the former Health and

Safety Laboratory (now Environmental Measurements Laboratory) [HLW 57], and have become much more

popular than ionization chambers for measuring indoor radon concentrations. Other groups have since

reported the development of similar instruments, and a commercial version is available. Breslin [Bre 80]

and George [Geo 80] reported that the approximate sensitivity for this type of instrument is typically

0.5 pCi'L , but is dependent on the size of the scintillation flask, the flow rate, and the counting

time interval.

The instrument consists of a Lucas-type scintillation flask (see Sections IV. A. 2. and IV.D.l.)

with two ports that allow air to be continuously drawn through the flask. It is permanently coupled to

a photomultiplier tube and pulse counter, and thus can continuously monitor changes in the radon concentration in the air drawn through the flask. As noted by Breslin [Bre 80], the fallacy in this arrangement is that even if the inlet stream is filtered, radon progeny radionuclides produced within the

flask "plate out" and deposit within the detecting volume. As a result, the count rate is affected by

the plated-out progeny that arise from radon sampled in the previous several hours [Tho 77a; BVP 79].

This problem persisted for many years until a procedure, first proposed by Thomas [Tho 77b], was developed by Countess and Thomas [CoT 77] to correct for the previously-deposited radon progeny. The procedure consists of an experimental determination of a "deposition constant" for the flask, and radioactive

decay corrections for the previously deposited progeny radionuclides.

84
The continuous scintillation-flask monitor must be calibrated under the same prevailing conditions

and in the same manner as it is used for field measurements [GeO 80], and must include the correction

for the previously deposited radon progeny. The precision of the calibration factors for individual

flasks is about ±5% [Tho 77a], presumably due to the variability in the scintillator coating on the

walls. The overall precision, including "counting errors", is estimated to be about ±20% (coefficient

of variation) for a radon concentration of 1 pCi-L [Tho77a].

Another type of instrument for continuously measuring indoor radon concentrations is an automated

two-filter tube monitor [Tho 77a]. This instrument consists of a two-filter tube (see Section IV.D.l.)

with a scintillation detector positioned in front of the exit filter to continuously detect the radon

progeny. Like the continuous scintillation-flask monitor, this instrument is also susceptible to

interference from previously deposited radon progeny. Similar corrections for the radionuclides deposited during previous sampling intervals must be made. Another source of inaccuracy with continuous

two-filter tubes arises from the collection of the longer-lived thoron progeny radionuclides on the

exit filter. This has been eliminated from some instruments by passing the air through two large delay

chambers (850 L each) located upstream from the two-filter tube [Tho 77a; Geo 80]. At a flow rate of 5

L'S , the chambers provide a delay time of about six minutes which is sufficient to allow decay of the

short-lived thoron (55 s halflife) [Tho 77a]. These conditions have little effect on the radon, but

remove about 98% of the thoron. Use of delay chambers is recommended whenever the thoron concentration

is suspected to be approximately equal to or greater than the radon concentration [Tho 77a].

A more recent modification of the two-filter tube method uses an automatic drive that changes the

exit filter every hour and transports the filter to a built-in scintillation detector [Neg 79]. This

modification eliminates the accumulation of thoron progeny (thus obviating the need for delay chambers),

and the need for corrections for previously-deposited progeny.

As with continuous scintillation flasks, automatic two-filter tubes must be directly calibrated in

the same manner and for similar conditions used in field measurements [Tho 77a; Geo 80]. The overall

precision at 1 pCi-L is reported to be similar (i.e., an approximate coefficient of variation of

about ±20%) [Tho 77a]. Negro [Neg 79] directly compared results from the automatic-drive two-filter

tube monitor to the average of grab samples assayed with pulse ionization chambers (see Section IV. A. 1.).

For radon concentrations in the range 0.1-0.2 pCi-L , the differences between the monitor reading and

grab-sample averages were less than 10%. The sensitivity of continuous two-filter tube monitors depends

on the volume of the tube, and sensitivities of 0.01 pCi-L have been reported [Bre 80]. Very large

volume tubes are required to achieve this sensitivity, however, and sensitivities on the order of a few

tenths of a picocurie per liter are more typical for portable units.

85
Most continuous monitors based on a diffusion and electrostatic-collection technique are adaptations of an instrument that is popularly called a "Wrenn Meter" [WSC 75; PDH 80]. As described in

Section IV. D. 2., the instrument is based on the diffusion of radon through a porous barrier into a 218 hemispherical sensitive volume, and the attraction and collection of the Po radon daughters by an

electrostatic field applied within the sensitive volume (typically one liter). In the original design

by Wrenn, et al . [WSC 75], the central electrode consisted of a hemispherical lucite light pipe painted

with a ZnS(Ag) scintillator coating which was then covered with aluminized mylar to obtain an electrically conductive surface. Scintillations produced in the ZnS(Ag) coating are detected by a photomultiplier tube which is optically coupled to the light pipe.

As in the other continuous monitors, interferences from previously deposited radon progeny partially

mask changes in the ambient radon concentration. Although corrections similar to those used for other

monitors could, in principle, be applied, it does not appear that such corrections are presently being

made [Bre 80; Geo 80]. Their absence seriously compromises the utility of these instruments for accurate

measurements of radon concentrations. Another limiting deficiency may be the humidity dependence of

these instruments. Although Wrenn, et al . [WSC 75] reported that the collection efficiency "appears to

be unaffected by normal fluctuations of relative humidity", Breslin [Bre 80] and George [Geo 80] suggested

that these instruments have a serious humidity dependence. Tests at the Environmental Measurements

Laboratory with similar monitors showed a 6% decrease in sensitivity for a 10% increase in relative

humidity [New 78]. This apparent discrepancy may be due, in part, to changes in the shape and size of

the original "Wrenn meter".

As for other continuous monitors, instruments based on the electrostatic-collection technique must

also be directly calibrated by exposure to known radon concentrations [WSC 75; NeW 78; Geo 80]. The

sensitivity of continuous "Wrenn meters" is in the range of 0.1-0.5 pCi-L [WSC 75; PDH 80].

A similar ten-liter instrument which uses a solid-state detector rather than a scintillation

detector to detect the alpha particles from the radon progeny has been investigated [NeW 78]. This 218 214 instrument can be used to separately resolve the Po and Po alpha particles. It is reported that,

218

by detecting only the alpha particles from the shorter-lived Po daughters, the instrument's response

to changes in radon concentration is ten times faster than other instruments [NeW 78].

IV. E. Conclusions and Recommendations

As suggested throughout this chapter, there is a plethoric number and variety of measurement

methodologies which have been used for both laboratory and in situ studies of radon transport and

86
exhalation. Although some of these measurement methods, particularly the laboratory-based ones, have

survived the test of time, others have never undergone a detailed assessment and evaluation. The need

to evaluate the suitability of the track-etch technique for measuring indoor radon concentrations is

perhaps the most representative example. Although several fragmentary studies of the adequacy of the

track-etch techniques have been or are being performed, a major unequivocal assessment of the suitability and reliability of the method needs to be performed. Similarly, some of the radon flux density and

continuous monitoring methods would also benefit from more broad-based, critical assessments and evaluations. As noted earlier, this need for critical assessments of measurement methodologies has also

been recently identified by the Radiation Policy Council Task Force on Radon in Structures [RPC 80].

In fact, the need to give more consideration to the evaluation of methods prior to their wide-spread

deployment is pleaded whenever three or more "radon experts" gather in the same room!

Equally important, is the continuing and often cited need for the development of uniform measurement procedures, the development of calibration standards and facilities, and the development of mechanisms for measurement assurance and interlaboratory comparison programs that can be related to national

measurement standards. At present, the only national standards that are available for any radon226 measurement-related quantity are NBS Ra solution standard reference materials which are used for

radon analyses. Although some laboratories, such as the DOE Environmental Measurements Laboratory,

have over many years striven to relate all of their radon-related measurements to this standard, not

all users can be expected to have either the resources or capability to accomplish this. Many users

now need and will continue to need readily available measurement standards, calibration services, and

measurement assurance mechanisms.

The importance of measurements to understanding radon transport and exhalation from building

materials, and to future remedial corrections and controls of this source of indoor radon cannot be

overstated. Yet, this importance is overshadowed by the absence of some of the most basic measurement

support services. Inasmuch as NBS is the only national organization which has both the responsibility

and long-term commitment to insuring the quality of measurements, it is recommended that NBS be encouraged and supported in undertaking studies to assess and evaluate the adequacy of the more important

measurement methodologies, and in developing measurement standards, calibration facilities and measurement assurance mechanisms that can be disseminated to the entire radon-measurement community.

87

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and Effects of Ionizing Radiation, 1977 Report to the General Assembly, United Nations,

1977.

UN 80 United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), Sources

and Effect of Ionizing Radiation , Environmental Behavior and Dosimetry of Radionuclides:

2. Radon, draft preprint (15 July 1980).

VDT 55 M. A. Van Dilla and D. H. Taysum, Scintillation Counter for Assay of Radon Gas, Nucleonics,

13, No. 2. 68-69 (1955).

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VeH 69 G. J. Verbeck and R. H. Helmuth, Structures and Physical Properties of Cement Paste, in Proceedings of the Fifth International Symposium on the Chemistry of Cement , Tokyo, Japan,

1969, Vol. in, pp. 1-32.

WaH 69 J. R. Waters and B. Y. Howard, Calibration of Instrument for Measurement of Radon

Concentrations in Mine Atmospheres, Health Phys. , 16, 657-659 (1969).

WCS 72 M. H. Wilkening, W. E. Clements, and D. Stanley, Radon-222 Flux Measurements in Widely

Separated Regions, in J. A. S. Adams, W. M. Lowder, and T. F. Gesell (eds.) The Natural Radiation Environement II , U.S. Energy Research and Development Administration Report

C0NF-720805-P2, National Technical Information Service, Springfield, Virginia, 1972, pp. 717-730.

WiH 60 M. H. Wilkening and J. E. Hand, Radon Flux at the Earth-Air Interface, J. Geophys. Res.,

65, 3367-3370 (1960).

Wil 73 R. B. Williamson, Solidification of Portland Cement, in B. Chalmers, J. W. Christian,

and T. B. Massalski (eds.) Progress i_n Materials Science , 15, 189-286 (1973).

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97
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS IN 1139

2. Performing Organ. Report No 3. Publication Date September 1981

4. TITLE AND SUBTITLE

Radon Transport Through and Exhalation From Building Materials: A Review

and Assessment

5. AUTHOR(S)

R. Colle, R.J. Rubin, L.I. Knab, and J.M.R. Hutchinson

6. PERFORMING ORGANIZATION (/f jo/nt or other tJion NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. AD-13-F-0-133-0

8. Type of Report & Period Covered

Interim

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, Stote, ZIP)

Office of Radiation Programs

U.S. Environmental Protection Agency

Washington, DC 20460

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This report was prepared, at the request of the U.S. Environmental Protection Agency,

for the purpose of reviewing, assessing, and summarizing what is currently known about

radon transport through and exhalation from building materials. In four chapters, the

report 1) considers the routes of entry of radon into buildings, describes the basic

models for radon transport through building materials, critically reviews the small

number of existing values for the necessary transport coefficients, and summarizes the

solutions of both steady-state and time-dependent transport cases; 2) reviews and

considers how the microstructural properties and internal characteristics of building

materials may affect the transport and exhalation of radon; 3) considers the exhalation

process from a more macroscopic, phenomenological viewpoint, and summarizes selected

experimental data on radium concentrations in building materials, radon flux and

exhalation from soils and building materials, and the effects of meteorological variables on radon exhalation; and 4) reviews and assesses various measurement methodologies

that are used for laboratory and in situ studies of radon transport and exhalation.

Needs for further research in each area are also recommended.

12. KEY WORDS (S/x to twelve entries; olphabet/co/ order; capitalize only proper names; and separate key words by semicolons)

buildings; concrete; diffusion; exhalation; materials; measurement; permeability;

radon; transport

13. AVAILABILITY

Pn Unlimited

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PRINTED PAGES

101

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$4.50

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