Potential consequences of hypothetical nuclear power plant accidents in Finland

Abstract

An assessment of the environmental consequences of radioactive releases from severe reactor accidents was performed for Finnish nuclear power plants (NPP) in operation (Loviisa 1 and 2, and Olkiluoto 1 and 2), in commissioning (Olkiluoto 3) and in the preparatory phase (Hanhikivi 1). Three hypothetical radioactive releases (Basic, Large and Very Large cases) with different magnitudes were considered. Basic case represents a release corresponding to the limit set in requirements for severe reactor accidents, and thus, the other two represent extremely unlikely situations where severe accident management has failed. The magnitude of Very Large case is similar to the Fukushima NPP accident and roughly one order of magnitude smaller than the Chernobyl NPP accident, while the magnitude of Large case is roughly one order of magnitude smaller. The environmental assessment on the consequences of the releases was performed using a probabilistic approach. The aim is to estimate radiation doses to the population around the NPP and land contamination resulting from the release of radioactive material. The results of the assessment were compared to the protective action criteria for radioactive release from an NPP, and further used to evaluate the sufficiency of the Precautionary Action Zone (PAZ) and Emergency Planning Zone (EPZ) given in Finnish regulation. The probability of exceeding different operational intervention levels (OIL) and dose criteria set for external dose rate, effective dose, thyroid dose, deposition of strong gamma emitters, deposition of cesium, and concentration of gamma emitters in the air were estimated for distances up to 300 km from each site.

Dispersion and deposition of releases were modelled with the dispersion model SILAM of the Finnish Meteorological Institute. The modelling applied historical weather forecast data from the years 2012–2015 retrieved with operative weather forecast models AROME and HARMONIE. Dose rates and doses were determined from the radionuclide air concentrations and depositions with the threat assessment tool TIUKU. The assessment was preformed using 2920 dispersion realizations for each reactor type and release scenario. For the assessment, the 95th percentile of the modelled dispersion, deposition and dose data was selected.

The assessment showed that it is unlikely that any OILs or dose criteria would be exceeded outside PAZ and EPZ in a Basic case. In a Large case, the OILs may be exceeded up to about 110 km from the NPP sites, while it could be up to 170 km for dose criteria for the recommendation to limit being outside and iodine prophylaxis for children and pregnant females. The assessment indicated that it is unlikely that sheltering indoors or evacuation are needed outside the PAZ and EPZ during the early phase of the emergency due to the release in a Large case. In a Very Large case, extensive protective actions including permanent or temporary relocation are needed outside EPZ and less restrictive protective actions (i.e., recommendation to limit being outside and iodine prophylaxis for children and pregnant females) are needed beyond the area considered in this study (> 300 km). It was shown, however, that the protective actions outside the PAZ and EPZ are strongly dependent on prevailing weather conditions.

It was observed that the dose criteria for an adult’s effective and thyroid dose were exceeded in larger areas than suggested by the criteria based on the external dose rate. Furthermore, dose criteria for a child’s thyroid dose were exceeded in larger areas than the dose criteria for the recommendation to limit being outside, even though the iodine prophylaxis for children and pregnant females is set as a complementary protective action for the recommendation to limit being outside. The observation demonstrates that the content of the radioactive release must be carefully considered in a severe NPP accident as the amount of released iodine isotopes affect the thyroid dose, while other nuclides make only a small contribution to it. It was also observed that precipitation may cause the deposition of radioactive substances such that in large areas the criteria for protective action based on deposition may be exceeded, even when protective actions would not be warranted according to dose criteria for effective doses. In Large and Very Large cases, the deposition may lead to the need for decontamination of large areas, and temporary or even permanent relocation of people. These findings demonstrate the importance of primarily using dose criteria in decision making if dose calculations can be performed reliably during the actual nuclear or radiological emergency. The OILs shall be used only if measures with dose criteria cannot be determined reliably e.g., due to a lack of information. Based on the results, it was concluded that further study is needed to set an Extended Planning Distance (EPD) and Ingestion and Commodities Planning Distance (ICPD). In such a study, a level 2 Probabilistic Risk Assessments (PRA) considering the probability of atmospheric releases should be better linked with environmental consequence assessment. The International Atomic Energy Agency (IAEA) recommends the EPD and ICPD to facilitate preparedness planning and the execution of protective actions beyond the PAZ and EPZ. The existence of an EPD and ICPD could also improve the effective co-operation of different authorities and emergency workers in a nuclear emergency.

In Large and Very Large cases, the seasonal variations between summer and winter were found to be significant especially when considering deposition of cesium and strong gamma emitters. It was observed that precipitation during the winter season causes more wet deposition than in the summer. In a Very Large case, the seasonal variations were also significant in other measures.