Analysis of a Pressurized Water Reactor Natural Circulation Transient at Beginning of Life

A pressurized water reactor (PWR) with a positive moderator temperature coefficient of reactivity is potentially susceptible to a severe overheating transient. This study identifies a scenario in which such a transient could occur and is similar in some respects to the accident at Chernobyl Unit 4. The scenario so identified is a natural circulation test at beginning of life under the assumption that all scrams are disabled. The results obtained demonstrate that a runaway power excursion does not occur and that the domestically designed PWR that was analyzed displays inherently safe behavior for the chosen scenario.The analysis is performed using two codes in tandem over three sequential stages of the analysis. Since the transient is assumed to be quasi-steady-state, steadystate calculations first are performed with the advanced three-dimensional core simulation code ARROTTA to generate a moderator reactivity table for input to the point kinetics model in the RETRAN-02 systems transient code. The entries in this table are taken directly from ARROTTA results, so they implicitly contain three-dimensional reactivity effects. Next, a RETRAN-02 transient analysis is performed for the system as a whole. This analysis serves two purposes: It predicts the overall behavior of the plant during the transient, and it also produces time-dependent forcing functions for ARROTTA. Finally, ARROTTA is run in a transient mode, providing a detailed description of the behavior of the core during the transient. The high level of consistency produced by the two transient calculations validates the initial assumption of quasisteady-state behavior. A methodology that may be applied to the analysis of this and similar transients is developed and tested as part of this study.