A comprehensive analysis of the double ended MSLB (Main Steam Line Break) accident assumed to occur in the B & W (Babcock & Wilcox) nuclear power plant of TMI-1 (Three Miles Island, unit No. 1) has been carried out at the Dipartimento di Costruzioni Meccaniche e Nucleari (DCMN) of the University of Pisa (Italy) in cooperation with the University of Zagreb (Croatia). This has been done in the framework of the participation in the OECD-CSNI/NSC (OECD Committee on the Safety of Nuclear Installations -Nuclear Science Committee) "PWR MSLB Benchmark". The Relap5/mod3.2 code is being used during the three phases of the research dealing with the use of point neutron kinetic (phase 1), 3-D neutron kinetics for the core (phase 2) and 3-D neutron kinetics coupled to the entire system model (phase 3). Boundary and initial conditions of the system including those relevant to the fuel status, have been supplied by PSU (Pennsylvania State University) that had cooperation with GPU (the utility, owner of TMI) and NRC (US Nuclear Regulatory Commission). The present paper deals with the documentation of the phase 1, also mentioning the results obtained by other participants to the Benchmark. It has been found that the Engineered Safety Features that are part of the current design of the TMI plant are capable to cope with the considered accident, though in some cases, a smooth core power increase due to moderator cooling following the scram was predicted. The suitability of the Relap5 code and of the developed nodalisation was also demonstrated, as far as reasonable.
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