Dose evaluation in the BWR reactor building with MAAP-DOSE

The dose assessment of the main control room has been required in order to show its residence under the present Japanese new regulatory standards. Furthermore, to assess the in-plant dose such as reactor building where a lot of operations are performed is considered to contribute to the evaluation of accessibility in the training scenario for future voluntary safety improvements. The severe accident analysis code MAAP (Modular Accident Analysis Program) has "DOSE" module that dose calculation and accident progress analysis can be implemented simultaneously. It is expected to contribute in the evaluation of more detailed accident scenario because temporal trend of the dose corresponding to FP (Fission Product) amount can be obtained for each accident sequence. This study intends to obtain the knowledge to handle the DOSE module of MAAP by performing a dose calculation of the reactor building of a BWR plant. The FP migration behavior and the dose rate in the reactor building and the main control room were evaluated against two accident sequences, the only SBO and the superimposition of SBO and LOCA, by using BWR4 Mark-I containment model which is distributed as a sample of MAAP. The dose calculation result reflecting the accident progress was obtained from the result such as the LOCA and SBO sequence showed relatively high doses because of the early pressure vessel damage. The interphase between the dose and the FP mass was good for each accident sequence, especially for the noble gas. The SBO sequence showed the overall higher dose and the FP amount flowing into the reactor building was also larger. Noble gases, CsOH and CsI were confirmed as FP nuclides with a high degree of influence on dose, and the contribution of noble gas with large release amount was particularly higher. Since the noble gas is released to the external environment along with the time, the contribution of CsOH and CsI became higher in the latter stage of the scenario. In addition, as one of the factors that have an impact on the dose rate in the building during a severe accident, drywell spray operation is considered. The effect of spray on dose can be considered as below, the capture of FP, cooling of the debris, and the shielding effect of the water. The effect of greatly reducing the dose in the SBO scenario was confirmed. In addition, it was confirmed that the cooling debris by spray had a large influence on the dose.