Three-dimensional neutronic / thermal hydraulic core model with boron feedback

The advancements in numerical methods and computer technology together with the continuing increase in operational experience has been moved in the last few years toward the investigation of best estimate simulations of complex scenarios in nuclear power plants with different coupled codes. The University of Pisa has, so far, experienced several activities in the field of coupled code calculations using 3D neutron kinetic, structural mechanic, containment, and thermal-hydraulic system codes. In addition, an uncertainty methodology has been developed in order to assess code calculations errors. This paper, part of the broader activity carried out at the Pisa University dealing with various software couplings, suitable for safety analysis in nuclear technology, briefly describes the coupled 3D NKC / THSC core model developed to investigate the deboration issue in PWR. The core has been tested to perform a full transient simulation of a Small Break Loss of Coolant Accident (SBLOCA) followed by a MCP restart, which in turn causes a deborated water plug flowing through the core and causing a Reactivity Insertion Accident (RIA). This activity has been carried out in cooperation with the University of Valencia. The reference plant is the TMI-1, 2772 MWe, 2 loops PWR. Although the analysis has been performed for a B and W reactor, for which the transient is more threatening, the generic issue is applicable to all PWRs (e.g. W and CE), since deborated water can fill the loop seals. All phases, from the break up to the primary system deboration and to the loop restart, with the possible recriticality and core damage, have been simulated with RELAP5-3D computer code, in order to assess the capability of the code to represent such phenomenon. Furthermore, the 3D neutron kinetic model allows the assessment of the energy released to the fuel, which is the most significant parameter during RIAs [9]. (author)