United Nations Scientific Committee on the Effects of Atomic Radiation

ELECTRONIC ATTACHMENT 4SENSITIVITY STUDIES IN SUPPORT OF THE
METHODOLOGY DEVELOPMENTUNSCEAR 2016 Report, Annex A,
Methodology for estimating public exposures due to radioactive discharges

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ELECTRONIC ATTACHMENT 4: SENSITIVITY STUDIES IN SUPPORT OF THE METHODOLOGY DEVELOPMENT

3Contents
I. ATMOSPHERIC DISPERSION MODELLING ................................................................... 3II. ASSESSMENT OF DOSES DUE TO INGESTION............................................................. 5
A. Study of the effect of regional differences in diet ............................................................ 5
B. Study of the effect of regional differences in uptake characteristics of foods ................. 6REFERENCES ............................................................................................................................. 71.
As noted in the main text, some limited sensitivity studies were carried out in supportof the decisions made by the Committee in developing the current methodology. This
attachment outlines the studies and gives the key results. Two sets of sensitivity studies arepresented: those relating to atmospheric dispersion modelling and those related to theassessment of doses due to the ingestion of terrestrial foods.

I.

ATMOSPHERIC DISPERSION MODELLING

2.
The activity concentration of a radionuclide in air as a function of distance depends on
the height of the discharge. There are many other factors that also affect the degree ofdispersion in atmosphere and hence the concentration in air. For the purposes of this sensitivity
analysis, a standard atmospheric dispersion model and approach was used with just the heightof the discharge being altered and all other parameter values kept the same. The activity
concentrations in air (at ground level) resulting from a continuous discharge of a radionuclide
as a function of distance were calculated using a simple Gaussian model [S1], and assuming auniform wind rose and a range of atmospheric conditions with Pasquill Category D for 65% of
the time. The results are shown in figure I for a range of discharge heights from 0 (groundlevel) to 200 m.
3.
This figure demonstrates that, for continuous discharges, the differences in thecalculated concentrations in air with discharge height become smaller with increasing distancefrom the discharge point. A distance of 5 km is used for estimating characteristic individual
doses and long distances (100 km or more) are relevant for the estimation of collective doses.
At a distance of 5 km, the difference between the concentrations in air for a 30 m dischargeheight compared to discharge heights of 0, 100 and 200 m is about a factor of around 1.6, 2.3
and 6, respectively. At a distance of 100 km, the differences in the calculated concentrations in
air between a discharge height of 30 m and discharge heights of 0, 100 and 200 m falls tofactors of around 1.4, 1.3 and 2.7, respectively. This indicates that it would be reasonable tosimplify the methodology by adopting a single discharge height. A stack height of 30 m istherefore applied for all sites. For many electricity generation sites, the effective dischargeheight is between 0 and 100 m and therefore the uncertainty associated with using a single
discharge height of 30 m is around a factor of 2 or less, even at 5 km distance. However, somecoal-fired power stations discharge effluents from tall stacks with an effective heightapproaching 200 m; in these cases, the estimated individual doses may be overestimated byaround a factor of 6 (at a distance of 5 km) but the effect on collective doses is much smaller(less than a factor of 3). Similar results are obtained if different percentages of Pasquill
Stability categories are assumed.

ELECTRONIC ATTACHMENT 4: SENSITIVITY STUDIES IN SUPPORT OF THE METHODOLOGY DEVELOPMENT4Figure I. Illustration of the variation in time-integrated activity concentration in air as a functionof distance for a range of discharge heights

4.
Mill and mine tailings tend to be spread over areas of tens of hectares and therefore are
not strictly point sources in the way that stacks are. In order to investigate the influence of thedifference between a point and an area source, the Numerical Atmospheric-dispersion
Modelling Environment (NAME) model was used to model the discharge of particles from apoint and an area source [J1]. A discharge height of 30 m was assumed for the point source andthe area source was assumed to have an area at the surface of 100 hectares (1 106 m2). A

ELECTRONIC ATTACHMENT 4: SENSITIVITY STUDIES IN SUPPORT OF THE METHODOLOGY DEVELOPMENT

5discharge rate of 500,000 particles per hour was assumed and the time-integrated activity
concentrations in air were calculated for various distances from the discharge point. The results
are presented in table 1.
5.
At a distance of 5 km, there is a 10% difference between the values for the point andarea sources, while at 10 km the difference is around 5%. At greater distances, there is no
significant difference between the activity concentrations associated with point and areasources. Thus, as these differences are small compared with the uncertainties associated withthe methodology and, given the larger distances involved in assessing collective doses, theCommittee considered it reasonable to apply the same approach to assess the dispersion ofradionuclides from mine and mill tailings as from other facilities. An effective discharge heightof 30 m for a point source is therefore also used for the calculation of doses from mine and milltailings in this methodology.
Table 1. Comparison of time-integrated air concentrations arising from an area and point source
as a function of distance
Distance from release
point (km)0151020304050II.

Time-integrated air concentration (Bq s)/m3Area source

Point source

Ratio of area/point source

1.4 × 10−1

2.2 × 10−1

0.64−1−1

1.4 × 10
3.4 × 10
1.5 × 10
6.2 × 10
3.6 × 10
2.4 × 10
1.7 × 10−2−2−3−3−3−3

2.1 × 10
3.7 × 10
1.6 × 10
6.2 × 10
3.6 × 10
2.4 × 10
1.8 × 10−2−2−3−3−3−30.670.90
0.96
0.99
1.00
1.0
1.0

ASSESSMENT OF DOSES DUE TO INGESTION

6.
The Committee agreed that some additional regional data should be included in thecurrent methodology, particularly for estimating individual doses. There were two main aspects
that were considered: the use of region-specific consumption rates for different foods; and theuse of region-specific information in determining the transfer of radionuclides to terrestrialfoods. A limited sensitivity analysis was carried out as part of the development of the current
methodology considering these two aspects.
A.

Study of the effect of regional differences in diet

7.
Generic factors relating deposition of radionuclides from atmosphere to the levels invarious terrestrial foods were used with the different regional diets given in table 7 of the main
text to estimate doses from ingestion for each region. Also, the effect of changes in the uptakeof radionuclides from soil to plants was determined. This analysis suggested that, among theradionuclides considered, differences between the transfer of radionuclides from soil todifferent terrestrial foods would lead to a variation in dose of around a factor of two greaterthan that from differences in the quantity of foods consumed alone.
8.
These results also indicated that the calculated doses were relatively insensitive tovariations in the composition of the diet, although the influence of differences in the total

ELECTRONIC ATTACHMENT 4: SENSITIVITY STUDIES IN SUPPORT OF THE METHODOLOGY DEVELOPMENT

6

quantities of food consumed were apparent (notably from the higher mass of food consumed inNorth America). The analysis also indicated that it would be reasonable to simplify the foodand regional groupings (e.g. for the comparison of the radiological impact of different energy
sources).
9.
Given the importance of 14C in the calculation of collective dose from discharges toatmosphere, the possible variations in the uptake of 14C in foods arising from the different
composition of diets in different regions was considered. For the purposes of illustration, thedifferences between the components of a European (United Kingdom) and Asian (Japan) dietwere considered.
10.
This analysis indicated that the average daily intake of carbon for the two diets wasvery similar (at around 420 g/d), although the foods from which the carbon was derived varied.
In Japan, the intake is primarily associated with starchy products (such as rice, wheat and root
vegetables), while in the United Kingdom, the intake is associated with sugar, milk and milk
products, meats, oils and fats. As the dose from 14C is related to the intake of stable carbon, thisindicates that the doses would be very similar in the two regions despite the differences in diet.
B.

Study of the effect of regional differences in uptake characteristics offoods

11.
There are possible differences in the transfer of radionuclides to foods grown in
different regions of the world owing to differences in the foods produced, the soil andagricultural practices. As an input to the Committee’s assessment of doses arising from theaccident at the Fukushima-Daiichi nuclear power station, the FARMLAND model [B1] wasmodified to allow uptakes of radionuclides in the main components of the Japanese diet to be
estimated. The results of using the FARMLAND model with modified soil–plant concentration
ratios were compared with the standard output from FARMLAND for the United Kingdomdiet. Activity concentrations were calculated for unit deposition of a range of radionuclides,
including 90Sr, 137Cs, 210Po and 226Ra. In most cases, the differences between the two sets ofresults were small compared with the uncertainties associated with the methodology as awhole. The greatest differences between the estimated activity concentrations for the United
Kingdom and Japanese foods were found for unit discharge of 90Sr. These results are presented
in figure II. The differences in the results for the transfer of a range of radionuclides to cereals
are illustrated in figure III.
12.
In summary, for the radionuclides considered, there are only relatively small
differences between the activity concentrations in terrestrial foods arising from the use ofdifferent transfer parameters (particularly soil–plant concentration ratios) appropriate forconditions in the United Kingdom and Japan. For 90Sr, some differences were identified, as
shown above, most notably for beef, liver and milk, where a difference of up to about a factor
of seven in the activity concentrations between Japan and the United Kingdom were found.
However, for the radionuclides that are likely to give rise to the greatest contribution to doses
for routine discharges, differences in uptake do not significantly affect the results, as
demonstrated by the similarity in estimated activity concentrations for key radionuclides incereals (see figure III).

ELECTRONIC ATTACHMENT 4: SENSITIVITY STUDIES IN SUPPORT OF THE METHODOLOGY DEVELOPMENT

790

Figure II. Activity concentrations of Sr in foods from a continuous deposition rate at21 (Bq/s)/m using transfer parameters for United Kingdom and Japan

Figure III. Estimated activity concentrations of radionuclides in cereals from continuous
2deposition rate at 1 (Bq/s)/m using transfer parameters for the United Kingdom and Japan

References
B1J1

S1Brown, J. and J.R. Simmonds. FARMLAND a dynamic model for the transfer of radionuclides throughterrestrial foodchains. NRPB-R273. National Radiological Protection Board, Chilton, Didcot, 1995.
Jones, A., D. Thomson, M. Hort et al. The U.K. Met Office's Next-Generation Atmospheric Dispersion
Model, NAME III. pp.580-589 in: Air Pollution Modeling and Its Application XVII. (C. Borrego and A.L. Norman, eds.). Springer, U.S., 2007.
Smith, J.G. and J.R. Simmonds. The methodology for assessing the radiological consequences of routine
releases of radionuclides to the environment used in PC-CREAM 08. HPA-RPD-058. Health Protection
Agency, Chilton, Didcot, 2009.