Page 1 of 13

DOE Review Release 11/03/2020

Calculation of Dose from Intakes of Special 

Tritium Compounds

ORAUT-OTIB-0066 Rev. 01

Effective Date: 10/15/2020

Supersedes: Revision 00

Subject Expert(s): Thomas R. LaBone

Document Owner 

Approval: Signature on File

Elizabeth M. Brackett, Document Owner

Approval Date: 09/02/2020

Concurrence: Signature on File

Scott R. Siebert, Objective 3 Manager

Concurrence Date: 09/02/2020

Concurrence: Vickie S. Short Signature on File for

Kate Kimpan, Project Director

Concurrence Date: 09/02/2020

Approval: Signature on File 

Timothy D. Taulbee, Associate Director for Science

Approval Date: 10/15/2020

FOR DOCUMENTS MARKED AS A TOTAL REWRITE, REVISION, OR PAGE CHANGE, REPLACE THE PRIOR 

REVISION AND DISCARD / DESTROY ALL COPIES OF THE PRIOR REVISION.

New Total Rewrite Revision Page Change
Document No. ORAUT-OTIB-0066 Revision No. 01 Effective Date: 10/15/2020 Page 2 of 13

PUBLICATION RECORD

EFFECTIVE 

DATE

REVISION 

NUMBER DESCRIPTION

04/26/2007 00 Approved new document to provide guidance for the calculation of 

best estimates of the annual organ doses for intakes of tritium bound 

to organic compounds (organically bound tritium; OBT) and tritium in 

a metal matrix (stable metal tritide; SMT). Incorporates formal 

internal and NIOSH review comments. There is no change to the 

assigned dose and no PER is required. Training required: As 

determined by the Task Manager. Initiated by Thomas R. LaBone. 

10/15/2020 01 Removed recommendation to use ORAUT-OTIB-0011 for assessing 

OBT. Added discussion on practical interpretation of urinalysis 

results following an intake of an SMT. Incorporates formal internal 

review comments. Constitutes a total rewrite of the document. 

Training required: As determined by the Objective Manager. Initiated 

by Scott Siebert.
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TABLE OF CONTENTS

SECTION TITLE PAGE

Acronyms and Abbreviations ............................................................................................................... 4

1.0 Introduction .............................................................................................................................. 5

2.0 Purpose ................................................................................................................................... 5

3.0 Biokinetic Models for Tritium Compounds ................................................................................ 5

4.0 Dose Calculations .................................................................................................................... 8

5.0 Recommendations ................................................................................................................. 10

References ........................................................................................................................................ 11

ATTACHMENT A USING IMBA TO EVALUATE INTAKES OF HTO........................................... 12

LIST OF TABLES

TABLE TITLE PAGE

4-1 Comparison of calculated doses for a 1-pCi acute intake of tritium compounds ..................... 10

LIST OF FIGURES

FIGURE TITLE PAGE

3-1 Biokinetic model for HTO ......................................................................................................... 6

3-2 Biokinetic model for SMTs........................................................................................................ 6

3-3 Biokinetic model for OBT.......................................................................................................... 7

3-4 Concentration of tritium in the urine after 1-pCi intakes of various tritium compounds.............. 7

3-5 Concentration of tritium in the urine from Figure 3-4 normalized to the same peak 

concentration ........................................................................................................................... 8

4-1 Diagram from ORAUT-OTIB-0011 that shows the types of methods used to determine 

the area under an excretion curve............................................................................................ 9

A-1 IMBA urine bioassay model for inorganic tritium..................................................................... 12
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ACRONYMS AND ABBREVIATIONS

AWE atomic weapons employer

d day

DOE U.S. Department of Energy

HTO tritiated water

ICRP International Commission on Radiological Protection

IMBA Interactive Modules for Bioassay Analysis

L liter

mCi millicurie

mrem millirem

NIOSH National Institute for Occupational Safety and Health

OBT organically bound tritium

ORAU Oak Ridge Associated Universities

ORAUT ORAU Team

pCi picocurie

PER program evaluation report

SMT stable metal tritide

SRDB Ref ID Site Research Database Reference Identification (number)

TIB technical information bulletin

USC United States Code

μCi microcurie
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1.0 INTRODUCTION

Technical information bulletins (TIBs) are not official determinations made by the National Institute for 

Occupational Safety and Health (NIOSH) but are rather general working documents that provide 

historical background information and guidance to assist in the preparation of dose reconstructions at 

particular sites or categories of sites. They will be revised in the event additional relevant information 

is obtained about the affected site(s), such as changing scientific understanding of operations, 

processes, or procedures involving radioactive materials. TIBs may be used to assist NIOSH staff in 

the completion of individual dose reconstructions.

In this document the word “facility” is used to refer to an area, building, or group of buildings that 

served a specific purpose at a U.S. Department of Energy (DOE) or Atomic Weapons Employer 

(AWE) facility. It does not mean, nor should it be equated to, an “AWE facility” or a “DOE facility.” The 

terms AWE and DOE facility are defined in 42 United States Code (USC) 7384l(5) and (12) of the 

Energy Employees Occupational Illness Compensation Program Act of 2000, respectively.

2.0 PURPOSE

Stable metal tritides (SMTs) are a class of tritium compounds that cannot be detected by urine 

bioassay as easily as tritium oxide. “Stable” is used to indicate that the tritium is not easily separated 

from the metal matrix in which it is bound. This material is more strongly retained in the lung, resulting 

in much smaller fractions of the intake excreted in urine. Therefore, a relatively small amount of tritium 

in a urine sample can indicate a large intake of an SMT. Ideally, workplace information, in the form of 

air monitoring, surface contamination activity, and process knowledge are used to assign potential 

intakes of this material, which is addressed in a site profile when possible.

In the absence of other available monitoring data, urinalysis can be used to provide a best estimate of 

an intake. The purpose of this TIB is to provide guidance on how to use urine bioassay data to 

calculate best estimates of the annual organ doses for intakes of tritium in a metal matrix.

3.0 BIOKINETIC MODELS FOR TRITIUM COMPOUNDS

Figure 3-1 shows the biokinetic model for tritiated water (HTO) as implemented in the Interactive 

Modules for Bioassay Analysis (IMBA) software. The only difference between this model and the 

International Commission on Radiological Protection (ICRP) HTO model in Publication 67 [ICRP 

1994a] is that the IMBA model specifically includes the urinary bladder whereas the ICRP model does 

not. The water compartment has a biological half-life of 10 d, and it is assumed that 1.4/3 = 0.47 of 

the HTO in this compartment goes to urine.

The organically bound tritium (OBT) compartment represents tritium that is incorporated into carbon 

compounds and is retained with a half-life of 40 d as described in the ICRP carbon biokinetic model 

[ICRP 1994a]. All of the tritium in this compartment is assumed to be excreted in urine. By default, 

IMBA does not have a urinary excretion function for HTO, which prevents modeling of that excretion. 

Instructions on how to enable IMBA to evaluate HTO urinary excretion data are given elsewhere 

[DOE 2006].

SMTs are metals such as titanium that absorb and store tritium (i.e., H-3) atoms in the crystalline 

structure of the metal. When an SMT is inhaled, the lungs retain the material and slowly release the 

tritium. This release of tritium occurs as the particle of the SMT dissolves and the tritium diffuses out 

of the particle. Tritium released from the SMT particle is assumed to be converted to HTO, which 

subsequently behaves according to the HTO biokinetic model. The SMT biokinetic model shown in

Figure 3-2 is therefore the HTO biokinetic model with an ICRP Publication 66 human respiratory tract 

model feed compartment [ICRP 1994b].
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Figure 3-1. Biokinetic model for HTO. Figure 3-2. Biokinetic model for SMTs. 

The solubility of tritides in the lung is a function of the metal substrate (e.g., titanium versus hafnium) 

and the size and shape of the particles. Of the SMTs discussed in the literature, titanium and 

zirconium appear to be best described as type M, whereas hafnium is best described as type S 

[Mound 2001]. Other tritides that consist of more reactive metals such as lithium and uranium are 

expected to be best described as type F. [Note that lung dose from the uranium in a uranium tritide 

will be much larger than the lung dose from tritium in the uranium tritide.]

Tritium can be taken into the body already bound to carbon compounds. Such materials are referred 

to as OBT compounds as opposed to the OBT compartment in the biokinetic model. The ICRP model 

assumes that 50% of the OBT compounds taken into the body are immediately converted to HTO and 

that the other 50% remain OBT.

Figure 3-3 shows the biokinetic model for OBT compounds as implemented in IMBA. The only 

difference between this model and the HTO model is the 50/50 split between HTO and OBT versus 

the 97/3 split for water.

Figure 3-4 shows the predicted urinary excretion curves after acute 1-pCi intakes of various tritium 

compounds, and Figure 3-5 shows these excretion curves normalized to the same peak excretion 

rate.
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Figure 3-3. Biokinetic model for OBT. 

Figure 3-4. Concentration of tritium in the urine after 1-pCi intakes of various tritium 

compounds. 
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Figure 3-5. Concentration of tritium in the urine from Figure 3-4 normalized to the same 

peak concentration. 

4.0 DOSE CALCULATIONS

Intakes of HTO are usually evaluated using an isotopic dilution technique [Health Physics Society

1994], which is based on the following assumptions:

1. HTO is metabolized like H2O.

2. The metabolic space of H2O is the 42 L of body water.

3. The concentration of HTO in the urine is the same as the concentration of HTO in the body 

water.

These assumptions are part of the ICRP Publication 30 HTO biokinetic model [ICRP 1979]. Given that 

these assumptions are valid, (1) the quantity of HTO in the body at any given time is simply the 

concentration of HTO in the urine times the volume of body water and (2) the dose to the soft tissues 

of the body is proportional to the area under the excretion curve.

The standard method for calculating the dose from intakes of HTO given in ORAUT-OTIB-0011, 

Tritium Calculated and Missed Dose Estimates [Oak Ridge Associated Universities (ORAU) Team 

(ORAUT) 2004], uses three different techniques to calculate the area under the excretion curve. As 

shown in Figure 4-1, a Type 1 tritium dose calculation is a linear interpolation that is performed when 

two urinary excretion measurements are less than 40 d apart. This is the classic connect-the-dots 

area calculation. A Type 2 tritium dose calculation is an exponential extrapolation from the first point 

to another that is performed when the two measurements are more than 40 d apart. A Type 2 

calculation, which gives an area that is smaller than a Type 1 calculation, is used because it is 

assumed that the person did not work with tritium if there was no sampling for more than 40 d. Finally, 

a Type 3 tritium dose calculation is an exponential extrapolation to t = infinity that is performed to 

account for the “tail” after the last measurement.

The introduction of the OBT compartment into the ICRP HTO biokinetic model technically invalidates 

this approach because the tritium in the OBT compartment is not uniformly distributed in the bodywater space. However, the introduced error in dose is slight (about 10% is often mentioned in the 

literature) because only 3% of the HTO goes to the OBT compartment. The method in 
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ORAUT-OTIB-0011 compensates for this slight underestimate by adjusting the dose conversion factor 

upward, which makes the dose to systemic organs from a Type 1 calculation slightly conservative 

(about 5% high) for intakes of HTO [ORAUT 2004]. Because SMTs in the lung are essentially in a 

longer term feed compartment for HTO going to the systemic organs, a Type 1 calculation is also 

slightly conservative for systemic organ dose from these materials. On the other hand, a Type 1 

calculation underestimates systemic dose by approximately 30% for intakes of OBT because half of 

the intake is not uniformly distributed in the body-water space. In practice, the error is smaller 

because occupational sources of OBT (e.g., pump oil) tend to have a significant HTO component.

Figure 4-1. Diagram from ORAUT-OTIB-0011 that shows the types of 

methods used to determine the area under an excretion curve

[ORAUT 2004]. 

Types 2 and 3 calculations make an exponential extrapolation of the urine concentration with the 

assumption of a 10-d half-life. As shown in Figure 3-5, portions of the urinary excretion curves have 

half-lives that are significantly longer than 10 d. Given a urine concentration, the dose to systemic 

organs (the area under the urinary excretion curve) from OBT and SMTs will be underestimated by 

Type 2 and 3 calculations because they assume a 10-d half-life when the actual half-life is 

significantly longer. As an example of a worst-case scenario, if a single urine result is evaluated using 

a Type 3 calculation (i.e., assuming it is HTO) and it is in fact from an intake of type S tritide, the 

systemic dose for the tail (not the total dose) will be underestimated by a factor of about 6.

The degree to which the total systemic dose is underestimated for OBT and SMTs depends on the 

fraction of the systemic dose from Types 2 and 3 calculations. For example, if the entire systemic 

dose up to the time of diagnosis is determined with Type 1 calculations, the systemic dose can be 

taken to be accurate regardless of the material. On the other hand, if most of the systemic dose is 

determined with Type 2 or 3 calculations and the material is a type S SMT, the systemic dose could 

be significantly underestimated.

The dose to the lung from intakes of SMTs can be underestimated by orders of magnitude if ORAUTOTIB-0011 calculations are used to calculate the lung dose from urinary excretion measurements 

[ORAUT 2004]. For example, consider the doses in Table 4-1 that result from a 1-pCi acute intake of 

the material. 
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Table 4-1. Comparison of calculated doses for a 1-pCi acute intake of 

tritium compounds (mrem). 

Section 

ORAUT-OTIB-0011

soft tissue dose

IMBA

soft tissue dose

IMBA

lung dose

Type S SMT 6.58E−10 6.30E−10 3.86E−06

Type M SMT 7.28E−09 6.93E−09 5.52E−07

HTO 7.11E−08 6.76E−08 6.76E−08

OBT 1.08E−07 1.52E−07 1.52E−07

5.0 RECOMMENDATIONS

1. In the vast majority of occupational exposures to tritium it is not possible to identify the tritium

compound taken into the body based on the observed urinary excretion. Therefore, the

selection of the appropriate tritium compound in an intake evaluation must usually be based on

process knowledge of the source terms in the workplace.

2. The method in ORAUT-OTIB-0011 [ORAUT 2004] can be used without modification to

calculate doses from intakes of HTO to all organs and tissues.

3. Because the ORAUT-OTIB-0011 [ORAUT 2004] method underestimates the dose from an

OBT intake by about 30%, it cannot be used for assessment of OBT. IMBA must be used for

intake assessment when based on urine bioassay and IMBA or Web CAD used for dose

assessment.

4. If the observed urinary excretion of tritium is deemed to be the result of intakes of SMTs,

ORAUT-OTIB-0011 [ORAUT 2004] can be used to calculate systemic dose if the majority of

the dose is calculated with the Type 1 method. This will typically be the case if there are a

relatively large number of urine samples over a relatively long period. On the other hand, if the

majority of the systemic dose is calculated with Type 2 or 3 methods, IMBA should be used to

calculate the dose rather than ORAUT-OTIB-0011 (see next recommendation).

5. If the observed urinary excretion of tritium is deemed to be the result of intakes of SMTs and

the dose to the lung or gastrointestinal tract is needed, the urinary excretion data must be

evaluated with IMBA in the same fashion as any other radionuclide (see Attachment A). Note

that the urinary excretion that results from a small intake of HTO (which gives a small lung

dose) translates into a large SMT intake (which gives a large lung dose) if the observed

urinary excretion is assumed to be the result of an SMT intake. This is important because an

SMT would be expected to be associated with varying amounts of HTO in an occupational

setting.

6. If the metal substrate of the SMT is not known, type S solubility should be assumed. However,

fairly modest tritium urine concentrations can imply extremely large type S SMT exposures

that might be quite implausible. For example, 1 μCi/L of tritium in the urine that is assumed to

be the result of an intake of a type S SMT 30 d earlier implies an unencapsulated source term

in excess of 300 mCi. This assumes that the fraction of an accidental release inhaled is

1 × 10-6.
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REFERENCES

DOE [2006]. Guidance on use of IMBA software for DOE safety applications. Washington, DC: U.S. 

Department of Energy, Office of Environment, Safety and Health. DOE-HS-0002, December. [SRDB 

Ref ID: 33212]

Health Physics Society [1994]. HPS N13.14-1994 An American national standard - internal dosimetry 

programs for tritium exposure-minimum requirements. April 13. [SRDB Ref ID: 172505]

ICRP [1979]. Limits for intakes of radionuclides by workers. ICRP Publication 30, Part 1. Ann ICRP 

2(3/4). [SRDB Ref ID: 29220]

ICRP [1994a]. Age-dependent doses to members of the public from intake of radionuclides: part 2 

ingestion dose coefficients. ICRP Publication 67. Ann ICRP 23(3/4). [SRDB Ref ID: 176884]

ICRP [1994b]. Human respiratory tract model for radiological protection. ICRP Publication 66. Ann 

ICRP 24(1-3). [SRDB Ref ID: 22732]

ICRP [1996]. Age-dependent doses to members of the public from intake of radionuclides: part 4 

inhalation dose coefficients. ICRP Publication 71. Ann ICRP 25(3/4). [SRDB Ref ID: 22729]

Mound [2001]. Mound technical basis document for stable tritiated particulate and organically bound 

tritium. Mound Site, Miamisburg, OH. MD-10516, April 23. [SRDB Ref ID: 32921]

ORAUT [2004]. Tritium calculated and missed dose estimates. Oak Ridge, TN: Oak Ridge Associated 

Universities Team. ORAUT-OTIB-0011 Rev. 00, June 29. [SRDB Ref ID: 19430]
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ATTACHMENT A 

USING IMBA TO EVALUATE INTAKES OF HTO

Arguably, the most common form of tritium to which workers are exposed is HTO, which is called 

inorganic tritium in IMBA. IMBA is not typically used to evaluate tritium urine bioassay data resulting 

from intakes of HTO because there are simple approaches that allow complex intake patterns to be 

readily evaluated (i.e., the method described in ORAUT-OTIB-0011 [ORAUT 2004]). However, the 

ORAUT-OTIB-0011 approach is frequently not applicable to urine bioassay data resulting from intakes 

of SMTs. In these cases the urine data must be evaluated with IMBA in the same fashion as urine 

bioassay data from other radionuclides. The problem is that while IMBA has a default urine bioassay 

model for organic tritium, it does not offer a default urine bioassay function for inorganic tritium. This 

shortcoming can be easily rectified with the following procedure:

• Select the inorganic tritium model.

• Go to the Bioassay Model panel (Figure A-1).

Figure A-1. IMBA urine bioassay model for inorganic tritium. 

• Select Urine as the Bioassay Function.

• Click User Defined Mode in the Select section. 
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ATTACHMENT A

USING IMBA TO EVALUATE INTAKES OF HTO (continued)

• Enter the coefficients and rate constants from the following table:

A(i) 

coefficient

Lam(i) 

rate constant

9.639E−03 1.200E+01

3.237E−02 6.931E−02

5.239E−04 1.733E−02

−4.253E−02 2.773E+00

• Enter 10-7 for the blood half-time.

Urine bioassay data in units of activity per day can now be used to directly evaluate intakes of 

inorganic tritium in the form of SMTs.

The intake retention function calculated from this user-defined urine bioassay function is the 24-hour 

incremental urinary excretion with units of activity per day rather than per liter. This means that tritium 

urine bioassay results, which are typically in units such as μCi/L, must be multiplied by 1.4 L/d to 

obtain μCi/d before being entered into IMBA.

IMBA files for the inhalation of types M and S SMT aerosols are provided at Q:\DR 

Information\Internal Dosimetry\IMBA\Metal tritide files.

These templates have the urine bioassay function preloaded along with the parameters for SMT 

aerosols from ICRP Publication 71 [ICRP 1996]. It is recommended that these templates be used as 

the starting point for all evaluations of SMT intakes. Note that reloading the default parameters in 

these files (i.e., clicking LOAD ICRP DEFAULTS on the IMBA main screen) will delete the HTO 

urinary excretion model.