Modelling study of the potential release of contaminated water from storage tanks at the Fukushima Dai-ichi NPP into marine environment

<p>The accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2011 led to the uncontrolled release of a significant amount of radioactive materials into the marine environment. To prevent the further release of highly contaminated water, which was used for cooling the overheated nuclear reactor cores, and groundwater, which was continuously pumped out the reactor buildings, a large number of tanks was installed in the area around NPP to collect all this water. However, at the moment the capacity of tanks is almost exhausted. The contaminated water was processed to decrease the activity stored in the tanks, but any decontamination system cannot remove all radionuclides from the water. According to TEPCO (2020) data, about 1.2 million m<sup>3</sup> of contaminated water were stored in tanks in March 2020 containing radionuclides with long and moderate half-life, among which 10 radioisotopes (H-3, C-14, Co-60, Sr-90, Tc-99, Ru-106, Sb-125, I-129, Cs-134, Cs-137) are dominant (Buesseler, 2020). Therefore, it is important to estimate the impact on human health of potential release of contaminated water from tanks to the ocean. This impact significantly depends on the ability of radionuclides to concentrate in the marine organisms, which are in the human diet, and the values of dose coefficient. The compartment model POSEIDON-R was applied for calculation the concentration of activity in the water, bottom sediments and biota at different distances from the FDNPP. The area of interest was covered by the system of compartments with specification around FDNPP. The exchanges of activity between compartments were governed by average currents in the region. The maximal concentrations and doses were conservatively estimated for coastal box 4x4 km around the FDNPP. Accumulation of activity in the organisms was calculated by dynamical model taking into account chemical properties of the element, its role in metabolic processes and the positions of organisms in the pelagic and benthic food webs. The potential individual doses of radiation were estimated using average consumption rates of marine products in Japan based only on domestic production. The conservative scenario, when a whole volume of contaminated water will be released into the marine environment at a constant rate during 10 years, was chosen. According to results of modelling for 50 years, the obtained dose even in the coastal box turned out to be significantly lower than the maximum annual effective dose commitment for the public equal to 1 mSv (IAEA, 2011). The main contribution into the dose is expected from I-129 and C-14. Although the activity of tritium (H-3) far exceeds activities of other radionuclides in tanks, its contribution to the total dose is only third due to low ability to concentrate in organisms and low dose coefficient. The dose factors and activity factors for 10 radionuclides at different distances from the FDNPP were obtained to be used for estimation of doses to human and concentration of activities in marine organisms for any long-lasting release scenario.</p>

Estimation of radiation dose from ingested tritium in humans by administration of deuterium-labelled compounds and food

Radiation doses from organically bound tritium (OBT) in foods have been a major concern near nuclear facilities. The current dose coefficient for OBT is calculated using a standard model from the International Commission on Radiological Protection, in which some biokinetic values are not based on human metabolic data. Here, the biokinetics of ingested OBT, and radiation doses from them, were estimated by administering labelled compounds and foods to volunteers, using a deuterium (D) tracer as a substitute for tritium. After the administration of D-labelled glucose, alanine, palmitic acid, or soybean, the D/H ratios in urine were measured for up to 119 days, and the biokinetic parameter values were determined for OBT metabolism. The slow degradation rates of OBT could not be obtained, in many volunteers administered glucose and alanine. The estimated committed effective dose for 1 Bq of tritium in palmitic acid varied from 3.2 × 10–11 to 3.5 × 10–10 Sv Bq−1 among volunteers and, for those administered soybean, it varied from 1.9 × 10–11 to 1.8 × 10–10 Sv Bq−1. These results suggest that OBT, present in some ingested ingredients, gives higher doses than the current dose coefficient value of 4.2 × 10–11 Sv Bq−1.

ICRP recommendations on radon

The International Commission on Radiological Protection (ICRP) publishes guidance on protection from radon in homes and workplaces, and dose coefficients for use in assessments of exposure for protection purposes. ICRP Publication 126 recommends an upper reference level for exposures in homes and workplaces of 300 Bq m−3. In general, protection can be optimised using measurements of air concentrations directly, without considering radiation doses. However, dose estimates are required for workers when radon is considered as an occupational exposure (e.g. in mines), and for higher exposures in other workplaces (e.g. offices) when the reference level is exceeded persistently. ICRP Publication 137 recommends a dose coefficient of 3 mSv per mJ h m−3 (approximately 10 mSv per working level month) for most circumstances of exposure in workplaces, equivalent to 6.7 nSv per Bq h m−3 using an equilibrium factor of 0.4. Using this dose coefficient, annual exposure of workers to 300 Bq m−3 corresponds to 4 mSv. For comparison, using the same coefficient for exposures in homes, 300 Bq m−3 corresponds to 14 mSv. If circumstances of occupational exposure warrant more detailed consideration and reliable alternative data are available, site-specific doses can be assessed using methodology provided in ICRP Publication 137.

Application of the ICRP Database for Calculation of the Dose Coefficient for Aerosol Having Multimodal Particle Size Distribution

Purpose: Development of a method for calculating radioactive aerosol dose coefficient when the aerosol particle size measurements resulted in a multimodal radionuclide activity distribution by particle diameters. Material and methods: The physical prerequisite for the proposed method is that the multimodal distribution may be caused by the presence of several sources of aerosols with different particle sizes. In the ICRP database to each value of the aerosol dose coefficient there corresponds one of ten functions of log-normal (unimodal) distribution with specified parameters. In the developed method the result of the aerosol particle size measurement is approximated by the sum of said standard functions with weight factors of each of the functions defined such that the best least squares approximation is obtained. Then the dose coefficient of the aerosol under study is calculated based on the dose value additivity property, i.e. each weight factor is multiplied by a respective value of the dose coefficient from the ICRP database, and the obtained products are added up. Results: There was carried out a series of numerical experiments, in each of which “experimental” points were simply plotted on a graph of a certain cumulative distribution function. Coordinates of the points are used as input for the programme implementing the developed algorithm. The calculated dose coefficient value is compared with the true value and/or the value obtained with the linear interpolation method using the AMAD. Conclusion: Physical prerequisites and results of numerical experiments confirm the validity of the developed method.