TMI-1 MSLB coupled 3-D neutronics/thermal hydraulics analysis: application of RELAP5-3D and comparison with different codes

A comprehensive analysis of the double ended Main Steam Line Break (MSLB) accident assumed to occur in the Babcock and Wilcox nuclear power plant of Three Miles Island Unit 1 (TMI-1) has been carried out of the University of Pisa in co-operation with the University of Zagreb and the Texas A and M University. The overall activity has been completed within the framework of the participation in the OECD-CSNI/NSC (Committee on the Safety of Nuclear Installations - Nuclear Science Committee) 'PWR MSLB Benchmark'. Different code versions have been adopted in the analysis. Results from the following codes (or code versions) are described in this paper: RELAP5/MOD3.2.2, beta version, coupled with the 3-D neutron kinetics Parcs code; RELAP5/MOD3.2.2, gamma version, coupled with the 3-D neutron kinetics Quabbox code; RELAP5/3D, coupled with the 3-D neutron kinetics Nestle code. Boundary and initial conditions of the system including those relevant to the fuel status, have been supplied by Pensilvania State University that had a co-operation GPU (the utility, owner of TMI) and NRC (US Nuclear Regulatory Commission). The capability of the control rods to recover the accident has been demonstrated in all the cases as well as the capability of all the codes to predict the time evolution of the assigned transient. However, one stuck control rod caused some 're-criticality' or 'return-to-power' whose magnitude is largely affected by boundary and initial conditions. The comparison among the results obtained by adopting the same thermalhydraulic nodalization and the different 'coupled' code version is discussed in the present document. (author)