Analyses of Instability Events in the Peach Bottom-2 BWR Using Thermal-Hydraulic and 3D Neutron Kinetic Coupled Codes Technique

Antonella Lombardi Costa,Francesco D'Auria,Walter Ambrosini,Alessandro Petruzzi

doi:10.1155/2008/423175

Abstract

Boiling water reactor (BWR) instabilities may occur when, starting from a stable operating condition, changes in system parameters bring the reactor towards an unstable region. In order to design more stable and safer core configurations, experimental and theoretical studies about BWR stability have been performed to characterise the phenomenon and to predict the conditions for its occurrence. In this work, contributions to the study of BWR instability phenomena are presented. The RELAP5/MOD3.3 thermal-hydraulic (TH) system code and the PARCS-2.4 3D neutron kinetic (NK) code were coupled to simulate BWR transients. Different algorithms were used to calculate the decay ratio (DR) and the natural frequency (NF) from the power oscillation predicted by the transient calculations as two typical parameters used to provide a quantitative description of instabilities. The validation of the code model set up for the Peach Bottom Unit 2 BWR plant is performed against low-flow stability tests (LFSTs). The four series of LFST have been performed during the first quarter of 1977 at the end of cycle 2 in Pennsylvania. The tests were intended to measure the reactor core stability margins at the limiting conditions used in design and safety analyses.

Highlights

In the last four decades, the nuclear power industry has been upgrading and developing light water reactor technology, and preparing to meet the future demand for energy
The stability of Boiling water reactor (BWR) systems has been of great concern from the safety and the design point of view at the beginning of the nuclear era; nowadays, the design of reactors having appropriate stability margins, the adoption of operating procedures avoiding possible unstable regions, and the development of mitigation strategies to cope with inadvertent instability occurrences have strongly limited safety concerns in this regard
Data from experimental low flow stability tests (LFSTs) have been compared with results obtained with coupled 3D simulations

Summary

Introduction

In the last four decades, the nuclear power industry has been upgrading and developing light water reactor technology, and preparing to meet the future demand for energy. The stability of BWR systems has been of great concern from the safety and the design point of view at the beginning of the nuclear era; nowadays, the design of reactors having appropriate stability margins, the adoption of operating procedures avoiding possible unstable regions, and the development of mitigation strategies to cope with inadvertent instability occurrences have strongly limited safety concerns in this regard. This is a direct consequence of the large operating experience gained with BWRs and of the increased knowledge of instability phenomena obtained from both experimental and computational activities aimed at simulating reactor behaviour. State variables identifying the reactor working conditions are observed to oscillate in different ways depending on the modalities of the departure from the stable operating point

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