Sampling Based Uncertainty Analysis of 10 % Hot Leg Break Loca in Large Scale Test Facility

Thermal hydraulic system codes are complex tools developed to estimate the transient behaviour of nuclear power plants (NPPs) during off-normal conditions. The evaluation of safety margins, the operator training, the optimization of the plant design and related emergency procedures are some of the major applications of these codes. Best estimate computer codes are used to calculate postulated accidents and transients in a realistic way and not in a conservative way. Currently worldwide there is an increasing interest in computational reactor safety analysis to replace conservative evaluation model calculations with best estimate calculations supplemented by quantitative uncertainty analysis. The need to quantify uncertainty in predictions of the best estimate codes comes from the unavoidable approximations embedded in the development and application processes of computational tools including inadequate knowledge of a number of input parameter values. The performance assessment and validation of these codes and their uncertainty evaluation are among the major objectives of international research programs. As a part of a similar coordinated research project, uncertainty analysis was carried out to quantify uncertainty in predictions of best estimate code RELAP5/MOD3.2 for 10% hot leg break loss of coolant accident (LOCA) simulation experiment at the Large Scale Test Facility (LSTF). Total failure of high pressure injection system and auxiliary feed water as well as loss of off-site power concurrent with scram is assumed as per experimental boundary conditions. The primary pressure decreases fast and core dry out takes place. The accumulator coolant injection occurs twice. Long term core cooling takes place by the actuation of the low pressure injection system. This paper mainly describes the post test analysis with uncertainty evaluation of the test (10% hot leg break LOCA) carried out at the LSTF/ROSA IV, Japan. Test facility description and details of the transient, including transient data for the test and a sample input deck, were Sampling based uncertainty analysis was carried out to quantify uncertainty in predictions of best estimate code RELAP5/MOD3.2 for a thermal hydraulic test (10% hot leg break LOCA) performed in the Large Scale Test Facility (LSTF) as a part of an IAEA coordinated research project. The nodalisation of the test facility was qualified for both steady state and transient level by systematically applying the procedures led by uncertainty methodology based on accuracy extrapolation (UMAE); uncertainty analysis was carried out using the Latin hypercube sampling (LHS) method to evaluate uncertainty for ten input parameters. Sixteen output parameters were selected for uncertainty evaluation and uncertainty band between 5 and 95 percentile of the output parameters were evaluated. It was observed that the uncertainty band for the primary pressure during two phase blowdown is larger than that of the remaining period. Similarly, a larger uncertainty band is observed relating to accumulator injection flow during reflood phase. Importance analysis was also carried out and standard rank regression coefficients were computed to quantify the effect of each individual input parameter on output parameters. It was observed that the break discharge coefficient is the most important uncertain parameter relating to the prediction of all the primary side parameters and that the steam generator (SG) relief pressure setting is the most important parameter in predicting the SG secondary pressure.