Best-Estimate Evaluation of Large-Break Loss-of-Coolant Accident for Advanced Natural Circulation Nuclear Reactor

This paper presents the methodology, which will be adopted, for quantifying the effect of uncertainties on the peak clad temperature of an advanced natural circulation nuclear reactor. The method relies on probabilistic analysis, treating uncertain parameters as random variables. The paper will cover a case of a loss-of-coolant accident due to a 200% (that is, double ended) break of the largest pipe with partial unavailability of the low-pressure emergency core cooling system. The break has been postulated at the inlet header, which is the largest pipe in the main heat transport system. For this assessment a two-step procedure has been adopted. In the first step the probability of the peak clad temperature exceeding 800°C has been evaluated using the response surface, generated from the results of thermal-hydraulic analyses. One of the fuel failure criteria for this reactor is the peak clad temperature exceeding 800°C. Such a high temperature is expected during typical large-break loss-of-coolant accident conditions. The thermal-hydraulic analyses, using the computer code RELAP5/MOD3.2, were done for several cases involving different combinations of six selected uncertain parameters. The probabilistic analysis was carried out using Monte Carlo and first-order reliability methods. The first step results in conditional probability of the peak clad temperature exceeding the criteria subject to the condition of a 200% break in the inlet header. The probability of a 200% break is calculated in the second step. The probability of an inlet header pipe weld rupture has been evaluated based on probabilistic fracture assessment. The pipe break analysis considers the uncertainties in strength, fracture, and stress corrosion properties and initial crack/flaw sizes produced during fabrication or welding. It also accounts pre-service and in-service inspection, inspection quality, and different damage mechanisms such as fatigue and intergranular stress corrosion cracking. The combined results of both these steps give the overall probability of the peak clad temperature exceeding 800°C.