The 2019-2020 EURADOS WG10 and RENEB Field Test of Retrospective Dosimetry Methods in a Small-Scale Incident Involving Ionizing Radiation

With the use of ionizing radiation comes the risk of accidents and malevolent misuse. When unplanned exposures occur, there are several methods which can be used to retrospectively reconstruct individual radiation exposures; biological methods include analysis of aberrations and damage of chromosomes and DNA, while physical methods rely on luminescence (TL/OSL) or EPR signals. To ensure the quality and dependability of these methods, they should be evaluated under realistic exposure conditions. In 2019, EURADOS Working Group 10 and RENEB organized a field test with the purpose of evaluating retrospective dosimetry methods as carried out in potential real-life exposure scenarios. A 1.36 TBq 192Ir source was used to irradiate anthropomorphic phantoms in different geometries at doses of several Gy in an outdoor open-air geometry. Materials intended for accident dosimetry (including mobile phones and blood) were placed on the phantoms together with reference dosimeters (LiF, NaCl, glass). The objective was to estimate radiation exposures received by individuals as measured using blood and fortuitous materials, and to evaluate these methods by comparing the estimated doses to reference measurements and Monte Carlo simulations. Herein we describe the overall planning, goals, execution and preliminary outcomes of the 2019 field test. Such field tests are essential for the development of new and existing methods. The outputs from this field test include useful experience in terms of planning and execution of future exercises, with respect to time management, radiation protection, and reference dosimetry to be considered to obtain relevant data for analysis.

Internal dose assessment of 148Gd using isotope ratios of gamma-emitting 146Gd or 153Gd in accidently released spallation target particles

The pure alpha emitter 148Gd may have a significant radiological impact in terms of internal dose to exposed humans in case of accidental releases from a spallation source using a tungsten target, such as the one to be used in the European Spallation Source (ESS). In this work we aim to present an approach to indirectly estimate the whole-body burden of 148Gd and the associated committed effective dose in exposed humans, by means of high-resolution gamma spectrometry of the gamma-emitting radiogadolinium isotopes 146Gd and 153Gd that are accompanied by 148Gd generated from the operation of the tungsten target. Theoretical minimum detectable whole-body activity (MDA) and associated internal doses from 148Gd are calculated using a combination of existing biokinetic models and recent computer simulation studies on the generated isotope ratios of 146Gd/148Gd and 153Gd/148Gd in the ESS target. Of the two gamma-emitting gadolinium isotopes, 146Gd is initially the most sensitive indicator of the presence of 148Gd if whole-body counting is performed within a month after the release, using the twin photo peaks of 146Gd centered at 115.4 keV (MDA < 1 Bq for ingested 148Gd, and < 25 Bq for inhaled 148Gd). The corresponding minimum detectable committed effective doses will be less than 1 µSv for ingested 148Gd, but substantially higher for inhaled 148Gd (up to 0.3 mSv), depending on operation time of the target prior to the release. However, a few months after an atmospheric release, 153Gd becomes a much more sensitive indicator of body burdens of 148Gd, with a minimum detectable committed effective doses ranging from 18 to 77 µSv for chronic ingestion and between 0.65 to 2.7 mSv for acute inhalation in connection to the release. The main issue with this indirect method for 148Gd internal dose estimation, is whether the primary photon peaks from 146 and 153Gd can be detected undisturbed. Preliminary simulations show that nuclides such as 182Ta may potentially create perturbations that could impair this evaluation method, and which impact needs to be further studied in future safety assessments of accidental target releases.

Insights into the Pu isotopic composition (239Pu, 240Pu, and 241Pu) and 236U in marshland samples from Madagascar

This work provides new insights into the presence of ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, and ²³⁶U in the Southern Hemisphere through the study of peat bog cores from marshlands in Madagascar (19°S). ²¹⁰Pb, ²³⁸Pu and ^239+240Pu activities were characterized by alpha spectrometry in previous studies. Here, Pu from alpha-spectrometry discs corresponding to 10 peat-bog cores (85 samples) was reassessed for the aim of completing its isotopic composition (²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu) by Accelerator Mass Spectrometry. In addition, ²³⁶U was studied in a single core exhibiting unusually low ²⁴⁰Pu/²³⁹Pu ratios. Integrated ²⁴⁰Pu/²³⁹Pu atom ratios in the single cores ranged above and below the (0-30°S) fallout average ratio, 0.173 ± 0.027, from 0.126 ± 0.003 to 0.206 ± 0.002, without a regional pattern, thereby demonstrating the heterogeneous distribution of the ²³⁹Pu and ²⁴⁰Pu signal. However, such a variability was not observed for ²⁴¹Pu/²³⁹Pu, ranging from (6 ± 1) · 10^-4 to (11 ± 1) · 10^-4 and consistently below the (0-30°S) fallout ratio of (9.7 ± 0.3) · 10^-4 (2012). The integrated ²³⁶U/²³⁹Pu atom ratio in the studied core, 0.147 ± 0.005, was also significantly lower than the values reported for the global fallout in the Northern Hemisphere, in the 0.20-0.23 range. Our results point out to stratospheric fallout as the main source of both ²³⁶U and ²⁴¹Pu at the studied site, whereas ²³⁹Pu and ²⁴⁰Pu signals show the influence of tropospheric fallout from the low-yield tests conducted in Australia (1952-1958) by United Kingdom and in French Polynesia (1966-1975) by France despite the long relative distances (i.e. about 15,000 and 8500 km). It was also demonstrated that a representative number of samples is necessary in order to assess Pu contamination and its various origins in a specific region in the Southern Hemisphere due to the heterogeneous distribution, and results based on single sample analysis should be interpreted with caution.

Averting cumulative lifetime attributable risk (LAR) of cancer by decontamination of residential areas affected by a large-scale nuclear power plant fallout: time aspects of radiological benefits for newborns and adults

The averted cumulative lifetime attributable risk (LAR), the residual dose and highest ground deposition of ¹³⁷Cs complying with a reference dose level of 20 mSv yr^-1 to an individual returning after one year to an area contaminated by unfiltered releases of fission products from a nuclear power plant (NPP) were evaluated by applying an existing exposure model designed to compute age- and gender-dependent time-integrated LAR. The model was applied to four types of nuclear fallout scenarios, partly based on data from the Chernobyl and Fukushima releases and from theoretical source terms from Swedish NPPs. For rapid decontamination measures that achieve a 50% relative reduction in external dose rate within 1 year, compliance with the reference level 20 mSv yr^-1 can be attained for an initial ¹³⁷Cs ground deposition of up to 2 MBq m^-2 with relaxed food restrictions. This compliance can be attained at even higher ground deposition (up to 4.5 MBq m^-2) if using the strict food restrictions employed in Japan after 2011. Considering longer than 1 year return times it was also found that the benefit of implementing decontamination decreases rapidly with time (2-3 years half-time), especially if the fallout has a high initial ¹³⁴Cs to ¹³⁷Cs activity ratio and if the ecological half-time of the external dose rate is short (<5 years). Depending on fallout scenario the averted cumulative LAR for newborn girls by decontamination that is achieved after 5 years is only between 6% and 11% of that obtained by evacuation alone during the same time, indicating a rather limited radiological benefit of decontamination if delayed more than a few years. We conclude that decision makers and emergency response planners need to consider that protracted decontamination measures may have limited radiological benefit compared with evacuation in terms of averted future cancer cases, albeit it may have other societal benefits.

Introducing the concept of the isodose for optimisation of decontamination activities in a radioactive fallout scenario

In the recovery phase after a radioactive release incident, it is important to be able to focus decontamination operations on the areas that contribute most to the radiation dose. Monte Carlo simulations were applied to determine the shielding effect of a building against radiation from various directions, also giving information on the dose contributions at various locations inside the building from specific areas outside. The concept of the isodose was developed to optimise decontamination activities, and was applied as isodose lines to define the smallest areas that lead to a certain dose reduction through decontamination of areas surrounding the building. The shape and position of the isodose lines depend on the building's geometry, wall thickness, and material, and on the observation point inside the building. Calculations have been made with a surface resolution of 1 m² for four observation points in a modular building, assuming depositions of ¹³⁷Cs and ⁶⁰Co on the ground surface and on the roof, as well as 1 cm below the ground surface to represent ground penetration. For example, a ten times as large area would have to be decontaminated to increase the dose reduction from 10% to 30%, if it is assumed that all the contamination is located at a depth of 1 cm.

Measurements of long-term external and internal radiation exposure of inhabitants of some villages of the Bryansk region of Russia after the Chernobyl accident

A Nordic-Soviet programme was initiated in 1990 to evaluate the external and internal radiation exposure of the inhabitants of several villages in the Bryansk region of Russia. This area was one of the number of areas particularly affected by the nuclear accident at the Chernobyl Nuclear Power Plant in 1986. Measurements were carried out yearly until 1998 and after that more irregularly; in 2000, 2006 and 2008 respectively. The effective dose estimates were based on individual thermoluminescent dosemeters and on in vivo measurements of the whole body content of (137)Cs (and (134)Cs during the first years of the programme). The decrease in total effective dose during the almost 2 decade follow-up was due to a continuous decrease in the dominating external exposure and a less decreasing but highly variable exposure from internal irradiation. In 2008, the observed average effective dose (i.e. the sum of external and internal exposure) from Chernobyl (137)Cs to the residents was estimated to be 0.3mSv y(-1). This corresponds to 8% of the estimated annual dose in 1990 and to 1% of the estimated annual dose in 1986. As a mean for the population group and for the period of the present study (2006-2008), the average yearly effective dose from Chernobyl cesium was comparable to the absorbed dose obtained annually from external exposure to cosmic radiation plus internal exposure to naturally occurring radionuclides in the human body. Our data indicate that the effective dose from internal exposure is becoming increasingly important as the body burdens of Chernobyl (137)Cs are decreasing more slowly than the external exposure. However, over the years there have been large individual variations in both the external and internal effective doses, as well as differences between the villages investigated. These variations and differences are presented and discussed in this paper.