Development and testing of RBMK-1500 model for BDBA analysis employing RELAP/SCDAPSIM code

The processes that might occur during beyond design basis accidents are investigated for all types of nuclear reactors in the world regardless of low probability of occurrence because the consequences of such accident could be catastrophic. Most researches are performed for the prevailing vessel-type light water reactors, such as PWRs and BWRs. For the analysis of such processes in reactor core and cooling system the codes RELAP5/SCDAP, ATHLET-CD, ASTEC and other are used. Smaller amount of researches are performed for the channel-type reactors, such as CANDUs and RBMKs, as they are operated only in a few countries. Up to now the phenomena that could occur in case of a severe accident in RBMK reactors were not analyzed in detail and only few publications are available on this topic. On the other hand, the understanding of these physical phenomena is necessary for developing strategies for the management of these accidents. This article discuses the approach of modelling the severe accident phenomena in RBMK type reactors using RELAP/SCDAPSIM code. The developed model was tested against RELAP5 calculations and available data regarding steam–zirconium oxidation and fuel channel rupture in Leningrad NPP. The analysis of typical BDBA in RBMK – break of the largest diameter pipeline with failure of emergency reactor cooling system – is presented as an example, which provides information regarding code acceptability for the BDBA analysis in RBMK-1500 reactor. The analysis is performed for the case without any operator intervention and assuming possible operator’s actions. The results of the analysis were used during the development of severe accident management guidelines for RBMK-1500 at Ignalina NPP.