Level 3 PRA computation and its integration to level 2

This report studies level 3 probabilistic risk analysis (PRA) and the integration of PRA levels 2 and 3. Level 3 PRA analyses the consequences that radioactive release from a nuclear power plant can have on population and environment. The major parts of level 3 evaluations are the atmospheric dispersion computation of the releases and prediction of various consequences caused by radiation. Here the focus was to assess collective dose. Calculations are performed using ARANO software, which is a simple and fast straight line dispersion model. Level 2 PRA analyses the magnitude and likelihood of a release. FinPSA level 2 software is used for level 2 calculations in this report. Level 2 results are used as an input for level 3. However, they consist of large number of simulation data and estimated statistical parameters. It is not clear what numbers from level 2 should be used in level 3. In this report, a moderately large number of level 3 calculations are presented to study the integration of the levels 2 and 3. It was found out that the release categorisation used in the analysed containment event tree of level 2 does correspond to level 3 results quite well, but not perfectly. It is recommended to consider release categorisation from level 3 point of view when performing new level 3 analyses. Uncertainties were also propagated from level 2 to level 3. Uncertainties are so high that at least limited uncertainty analyses should be performed on level 3. The results also indicate that the magnitude of the collective dose is mainly determined by the amounts of radionuclides, and other parameters of the source term have quite small effect.