On the consequences of axial midplane failures during hypothetical transient overpower accidents

It is generally accepted that coherent fuel pin failures at the axial midplane during hypothetical transient overpower accidents can cause a superprompt-critical excursion leading to an energetic core disassembly. Two approaches can be taken to resolve the concern of axial midplane failures. The first approach is to determine the distribution of fuel pin failure locations throughout the core. The second approach is to demonstrate that even if all the fuel pin failures occurred at the axial midplane, there are physical mechanisms and phenomena which will mitigate the consequences. This latter approach was taken in this paper to demonstrate that inter- and intra-assembly incoherence effects are likely sufficient to prevent an energetic accident consequence. Of the several sources of intra-assembly incoherence within the fuel bundle, one of these sources, the power and flow variations within the fuel bundle, was used to predict the time incoherence of fuel pin failures. The intra-assembly failure incoherence results were then used in a whole-core analysis of a 10c/s hypothetical transient overpower accident in the 2550 MWt LDP heterogeneous core. Taking into account intra-assembly failure incoherence, a superprompt-critical excursion was avoided and neutronic shutdown was obtained along with the prospect for adequate post-accident cooling of the damaged assemblies