Dose Calculation

Contact: Gabriele Mraz

Radiation doses were derived from the results of the atmospheric dispersion calculations by use of a dose model. These doses were compared to dose levels and limits and other indicators for health risks.

Selection of nuclides

Accidental releases from nuclear installations contain many radionuclides, but not all of them contribute equally to the dose received by the population. In order to make a first assessment of the radionuclides of interest, some test runs with PC Cosyma 2.01 were performed. The tests were performed with two different release scenarios from US EPR final safety report (AREVA 2009) and reactor inventories described in SSK (2003). The test runs were evaluated with respect to the relative contributions of all major radionuclides to different 7-day organ doses, and in one case also the 1-year doses, at distances of 28 and 155 km from the release point.

According to these calculations, the following 20 radionuclides contributed 98% of the doses and were therefore chosen for flexRISK calculations: Cs-134, Cs-136, Cs-137, I-131, I-132, I-133, I-135, Kr-87, Kr-88, Rb-88, Ru-103, Ru-105, Ru-106, Sr-89, Sr-90, Sr-91, Te-131m, Te-132, Xe-133, Xe-135. Furthermore, some decay chains are considered, which lead to the addition of the following nuclides contained in the progeny: Y-91, Y-91m, Rh-105, Te-131, Xe-133m, Xe-135m.

Definition of doses

The absorbed dose is the measure for energy absorbed by the body. Unit: Gray (Gy).

The effective dose is the sum of all equivalent organ doses multiplied with a tissue weighting factor. This tissue weighting factor represents the contribution of the equivalent organ dose to the total dose – the sum of all tissue weighting factors is 1. Unit: Sievert (Sv).

The committed dose (equivalent or effective) is the sum of all dose contributions of an intake of radioactivity that will result over an integration period up to 70 years.

Pathways and coefficients

Pathways considered in dose calculation are ground-shine, cloud-shine (submersion) and inhalation for different contamination periods, nuclides and organs depending on the intended endpoints. Resuspension was not included because the resulting dose was very small as was confirmed by the test runs with PC-COSYMA.

The ingestion pathway was not considered because the necessary modelling would have been too complex for the scope of this project.

For calculation of doses from deposition and concentration in air, dose coefficients were used. For internal radiation, these coefficients were taken from ICRP (1996) and for external radiation from Eckerman and Legett (1996) and Health Canada (1999). Because radiation has different effects on children and adults, doses were calculated for children from 0-1 years and for adults.

Dose reduction factors

Doses can be reduced by shielding. People stay indoors for a substantial part of time (80% indoors assumed in flexRISK). For longer-term dose calculation so-called location factors are used to describe the resulting dose reduction. These location factors are taken from Müller et al. (2003), they depend on the population density, assuming better shielding in areas with higher population density. Population data on raster of about 1 km2 (30") cell size from CIESIN (1995) have been used to calculate average dose reduction factors for each model grid cell. For short-term doses (7 d and 30 d) a location factor of 1 is assumed.

Radionuclides migrate into the soil over time resulting in an additional reduction of dose from ground-shine. Such ground-shine correction factors lie in a range of 0.5-0.8 for 1 a- or lifetime doses. For short-term doses no correction factor is applied.

Dose levels and limits

It seems sensible to use intervention dose levels to describe some of the effects on a country or a population group that regulated by law. For Austria, these are primarily dose levels of the Intervention Regulation (IntV 2007). Read more about intervention levels

Furthermore, effective doses were compared to the current dose limit proposed by Council Directive 96/29/Euratom for members of the public. This dose limit is set to 1 mSv/year from artificial radioactive sources. A higher dose may be authorized in a single year if the average over five consecutive years does not exceed 1 mSv/year.

This dose limit is not necessarily applied under all circumstances. According to Art. 48 of the Directive, in case of a radiological intervention Member States should define special intervention dose levels (see also below in detail). This would most probably apply immediately following an accident. Thus the International Commission on Radiological Protection (ICRP), after the accident in Fukushima (ICRP 2011), recommended applying a yearly dose limit of 20-100 mSv for the public, with the intention of reducing it quickly back to 1 mSv/a.

Because of the missing ingestion pathway, doses calculated within flexRISK will considerably underestimate real doses in the case of an accident, even if otherwise being conservative. In Austria after Chernobyl, babies, especially those fed with normal milk, achieved more than half of their total dose after the Chernobyl accident through ingestion (58%); for adults the percentage was even higher (75%) in the first year.

Results will be expressed as probability of exceedance of the following levels and limits:

Endpoints of dose calculation Limit / level in mSv for adults Limit / level in mSv children (< 18a) Type of dose Nuclides Pathways Integration period for dose calculation Contamination Period (start with first non-zero contamination)
Intervention level for sheltering 10 1 Effective dose All Inhalation, ground-shine, cloud-shine Inhalation: Lifetime
Ground-shine, cloud-shine: 7d
Intervention level for iodine prophylaxis 100 10 Thyroid dose Iodines Inhalation Lifetime 7d*
Intervention level for temporary relocation 30 30 Effective dose All Ground-shine 30d 30d
Dose limit for members of the public/average per year 1 1 Effective dose All Inhalation, ground-shine, cloud-shine Inhalation: Lifetime
Ground-shine, cloud-shine: 1a

* 7 d and 30 d doses were calculated for the 7 d (30 d) starting from the arrival of the first non-zero value of air or ground contamination, and then at three-hourly intervals until the end of the dispersion calculations. Among these values, the one with the highest dose is presented in the results. This is not always the first one.

A comparison with the former Austrian intervention levels (Rahmenempfehlungen 1992) was made by assessing the probability of exceedance of effective doses of 2.5, 25 and 250 mSv in the first year.

Furthermore, the frequency for exceedance of levels of ground contamination with Cs-137 was considered. The levels defining contaminated areas where long-term measures were necessary after Chernobyl in the former Soviet Union were used.

Deposition thresholds in areas contaminated by the Chernobyl accident (National Report Belarus 2006)

Zone Effective dose in mSv per year Cs-137 in kBq/m2 Sr-90 in kBq/m2 Pu-238, Pu-239, Pu-240 in kBq/m2
Zone of regular radiation control <1 37-185 5.55-18.5 0.37-0.74
Zone with the right to resettlement 1-5 185-555 18.5-74 0.74-1.85
Zone of subsequent resettlement >5 555-1,480 74-111 1.85-3.7
Zone of primary resettlement >5 >1,480 >111 >3.7
Zone of evacuation (exclusion zone) Territory around Chernobyl NPP, from which population was evacuated in 1986      

Results for exceedance of these contamination levels can also be used to compare flexRISK results with those of the predecessor project RISKMAP, where a threshold of 185 kBq Cs-137/m2 was used. This level corresponds roughly to a possible exceedance of the dose limit for the public of 1 mSv per year.

Dose results

Results are presented as sets of comprehensive maps and box-plots of the main endpoints defined in the project. These results are produced with flexible programmes and scripts (using Fortran, python, bash). This enables automatic calculations and graphs for different, selectable criteria based on dispersion and dose calculations, for example country-based screening of the results. Read more about dose results

Future flexRISK options

Contamination of milk

Milk that is contaminated with radioactive iodine can contribute considerably to the thyroid dose. From the results of deposition of I-131 as reference nuclide, the contamination of locally produced milk can be calculated with the use of transfer factors. By estimating the total consumption of milk in a given time period, the resulting ingestion dose can be assessed. Comparisons with maximum permitted levels for radioactive contamination of food in the European Community after a nuclear accident that affects Europe and of levels that were established in the aftermath of Chernobyl would be of special interest. More information about food levels

Collective doses and health consequences

As gridded population data are available, it will not be difficult to calculate collective doses for the pathways considered, and resulting health effects. However, applicable dose periods and dose coefficients need further attention.