RELAP5/MOD2 Analysis of N Reactor Cold-Leg Manifold Break Transient Without Scram

Abstract

A modified version of the RELAP5/MOD2 thermal-hydraulic computer code is used to perform anticipated transient without scram (ATWS) calculations for the N Reactor. The ATWS calculations are performed for a spectrum of transients to determine the accident end state in support of the Level 2/3 probabilistic risk assessment. The predicted N Reactor response to a most severe, but highly unlikely accident, due to the postulated double-ended guillotine break of the cold-leg manifold combined with the failure of scram systems, is described. The calculated core melt frequency for the N Reactor due to such an event is <10⁻¹⁰/yr. The transient response for this event is predicted using a single-loop, eight-level core RELAP5/MOD2 model of the N Reactor. The reactor power behavior is modeled using point-reactor kinetics. The kinetics model includes the contributions to the reactivity from the feedback effects of core void, fuel, graphite, and water temperature variations. To verify the basic response of the model, the RELAP5 analysis results for the scram transient are compared with a twodimensional neutronics code (TWIGL) calculation. The excess reactivity results for the loss-of-coolant accident, combined with simultaneous failure of scram systems, compare favorably with three-dimensional neutronics code (3DN) computations. The analysis also shows that the fuel temperatures during the transient have increased sufficiently in the top 30% of the core leading to fuel failure. The fuel temperatures are predicted assuming a constant power profile. However, the neutronics code calculations show that the normalized power varies from 100% at the core bottom to <5% at the top for 50% voided core. Therefore, the analysis provides a very conservative estimate of fuel temperatures for the transient.