Uncertainty analyses of hot channel calculations - Determination of the number of failed fuel rods

In safety analyses, the final goal is to investigate the fulfillment of the acceptance criteria and the evaluation is usually based on separate hot channel thermal hydraulic/fuel behavior calculations using the results of the system (nodal) calculations. In the conservative methodology the envelopes of the statistical distributions of the respective input parameters - partly leading to the most adverse consequences, partly covering the uncertainties of methodological origin - are used simultaneously. In order to get more realistic results of hot channel calculations of Reactivity Initiated Accident (RIA) and Anticipated Transient Without Scram (ATWS) events, a new uncertainty analysis methodology was elaborated by combining the one sided tolerance limits method of Wilks with the response surface method. The paper shows the elaborated hot channel calculation methodology by analyzing an ATWS event of a Russian designed pressurized water reactor (VVER-440) and the results of conservative vs. uncertainty analysis are compared with regard to the number of failed fuel rods, the maximum clad surface temperature and the maximum fuel temperature.