Improvement of an advanced system code for loop-type lMFBRs, SSC-L; Modeling of reactivity feedback effects

Ohshima, Hiroyuki; not registered; Ninokata, Hisashi

In the safety analysis of ATWS (Anticipated Transient Without Scram) sequences, emphasis is placed on the thermohydraulics in reactor core and interactions between core and heat transport system (HTS) are not considered. However, if progress of the sequence is not so fast, thermohydraulics in reactor core and HTS should be calculated at the same time. A whole plant system code, such as SSC-L, is available for this purpose. Since SSC-L has not been applied to the ATWS analysis so far, reactivity feedback model in this code has something to be improved. If uncertainty in parameters used in the analysis is large, conservative assumptions are employed. Therefore, the results of simulations are conservative and have large uncertainty in general. In order to evaluate ATWS sequences, it is desirable to take reactor core and HTS interaction into consideration and to improve accuracy of reactivity feedback model in SSC-L as well as decreasing uncertainty of the input data. Grace period available for mitigating the ATWS by the operator recovery action can be also evaluated from the whole plant thermohydraulics. Therefore, reactivity calculation module in SSC-L has been modified and improved in this study. Thus the reactivity feedback effects calculated in SSC-L are as follows: (1)Fuel doppler, (2)Sodium density and void (3)Fuel axial expansion, (4)Thermal expansion of the core internal structure, and (5)Thermal expansion of the core support structure. For the purpose of checking performance of the new model, SSC-L has been applied to the simulations of ULOHS (Unprotected Loss of Heat Sink) accidents and the results are consistent in our perception. This model should be validated by experiments and SSC-L is to be extensively applied to the safety analysis of LMFBR plants in future.