An innovative method for nuclear emergency source term evaluation based on pressure vessel water level

In the event of severe accidents in nuclear power plants (NPPs), timely and accurate evaluation of the source term released from the reactor can provide technical support for a nuclear emergency response. Currently, the method for source term evaluation during a nuclear emergency in NPP is mainly based on the core damage assessment technique. The first step in the evaluation process is core damage assessment; and the second step is source term evaluation, based on core damage state and NUREG-1465 reference guide. In this paper, a nuclear emergency source term evaluation method (RVLST) based on measured reactor pressure vessel water level is presented. The single-step evaluation process combines pressure vessel water level monitoring system, core degradation model, and fission product release model. The feasibility, computational speed and accuracy of RVLST are compared with the methods in IAEA-TECDOC-955/NUREG-1465, IAEA-TECDOC-1127 and MELCOR using source term released into the containment in a large break loss of coolant accident (LBLOCA) in the AP1000 nuclear power plant, and the results obtained by the four methods were comparatively analyzed. The results show that the release fraction of noble gases calculated by RVLST is close to the one calculated by the IAEA-TECDOC-955/NUREG-1465 method and MELCOR. Moreover, compared with the results for Cesium (Cs) calculated with the IAEA-TECDOC-955/NUREG-1465 method, the calculations obtained from the RVLST are closer to those derived from MELCOR. Also, more conservative calculations were obtained from RVLST than those calculated with IAEA-TECDOC-955/NUREG-1465 and MELCOR for halogen Iodine (I) concentration.