Extension of the SIMMER cross-section processing scheme for core degradation analyses of thermal systems

The SIMMER multi-physics code system was initially developed for safety analyses of liquid metal-cooled fast reactors. For these reactors material homogenization was considered to be an acceptable approximation in the neutronics part of the code. In order to increase the application range of SIMMER for analyses of transient phenomena in thermal reactors, the SIMMER cross-section processing scheme and the related data libraries have been extended. New options allow taking into account heterogeneity effects in e.g. water-cooled systems with pin-type or plate-type fuel, where these effects may play an important role. By now, the new extension was checked for intact core geometries. Accident initiators in light water reactors, e.g. reactivity induced accidents (RIAs), may lead to severe core degradation. Therefore, the fuel sub-assembly geometry may dramatically change during an accidental transient. In this paper we investigate the capability of the extended SIMMER cross-section processing scheme to take into account heterogeneity effects for distorted fuel sub-assembly configurations in water-cooled systems. The new SIMMER option improves the accuracy of reactivity calculations if parameters for heterogeneity treatment are evaluated and implemented for degraded configurations in the cross-section processing scheme. It is shown in the paper that only a few distorted configurations may be considered and the parameters can be obtained by interpolation in intermediate cases.

► Extension of the SIMMER code for the analysis of reactivity induced accidents in water-cooled systems. ► Degraded fuel-plate sub-assembly models investigated. ► Good agreement with the reference neutronics ECCO and MCNP codes. ► A simple interpolation scheme can be applied to the pre-calculated parameters during the transient.