An analysis of reactor transient response for boiling water reactor ATWS events

Numerical simulations of BWR (boiling water reactor) dynamic response under ATWS (anticipated transient without scram) conditions are presented for the case where the reactor is operated at natural circulation conditions. In non-isolation events, reactor stability is strongly influenced by the degree of core inlet subcooling. At normal water level and pressure, instabilities develop if core-inlet subcooling exceeds a critical value of ∼21 Btu/lbm. A sensitivity study with regard to the steam separator pressure - loss coefficient, however, indicates that system stability is strongly dependent on the magnitude of this parameter which suggests a significant degree of uncertainty In the results. Under isolation conditions at rated pressure, stability is significantly enhanced by rapid pressure fluctuations generated through cycling of safety/relief valves. Large-amplitude instabilities develop, however, in depressurization events, and SRV cycling no longer stabilizes the system. In a simulated depressurization to ∼500 psia, prompt critical excursions occurred, and oscillation amplitudes reached 1000% of rated power. Implications of the Present Study: With the exception of guidance to avoid SRV cycling, these preliminary results have provided further support for the validity of this response strategy. SABRE calculations have shown that the reactor is probably slightly unstable in natural circulation operation, but relief valve cycling prevents the occurrence of instabilities at or near design pressure. Thus, unstable operation should not be a concern when boron injection and HPCI are available and depressurization is unnecessary. The reactor water level for injection flow corresponding to HPCI operation has been shown to be acceptable and consistent with earlier estimates based on the NSAC results; however, condensation effects have a significant influence on the equilibrium reactor water level. In addition, the SABRE results reinforce PP and L's concerns that depressurization will lead to mechanical clad damage. Even the mildest depressurization, sequence to 500 psia for use of a condensate pump, as would be required for loss of HPCI capability, has been shown to result in severe power excursions reaching 1000% of design power before model constraints terminated the calculation. The information obtained from these various simulation studies will be used to modify PP and L procedures for response to ATWS events in our current EOP revision activities. Further work is required to Investigate stability behavior with early and mid cycle core configurations. Equally important will be the investigation of reactor behavior for core configurations corresponding to manual rod insertion sequences to determine the degree to which stability margins are influenced by rod Insertion during depressurization events. (authors)