An evaluation methodology development of boric acid precipitation for LOCA reflecting system effects

摘要

Emergency Core Cooling System (ECCS) should be equipped with sufficient capability to maintain core temperature at a low level being acceptable at any during long-term cooling (LTC) phase of a LOCA to demonstrate the compliance with the 10 CFR 50.46 requirements. To assure this, the occurrence of boric acid precipitation must be avoided throughout the long-term cooling phase to prevent the core channel blockage by the precipitant. Until now, a simple evaluation methodology has been applied to the post-LOCA boric acid precipitation evaluation. This model treats the core as one node with fixed mixing volume, and assumes that the coolant and the boric acid are uniformly mixed within that volume. Recently, the USNRC has identified several non-conservative assumptions in the previously approved analysis methodology regarding the boric acid build-up in the reactor vessel and requested that the licensees respond to the safety concerns addressed in the USNRC letters on August 1, 2005. The letter also specified that the use of existing evaluation methodology is not allowed until the NRC staffs safety concerns are sufficiently resolved. A new evaluation methodology for boric acid precipitation has been developed to resolve the safety issues by modifying the current licensing code, BORON. In this paper, the code modification reflecting system effect such as time-varied mixing volume due to the variation of the core mixture level and void fraction is discussed herein. The issue on the system effect is the most influential safety concern raised by the USNRC. RELAP5/MOD3.1 was used to extract a time-varied mixing volume to implement system effects into the BORON code. Newly developed methodology is also capable of resolving the USNRC concerns regarding void fraction within the core in addition to system effects of mixing volume. The evaluation shows that the mixing volume for boric acid precipitation can possibly be reduced if the system effects are considered using system code simulation for LOCA. If the same boron precipitation limit is applied, the faster increase in the boric acid concentration necessitates the earlier initiation timing of the simultaneous hot and cold leg injection specified in the emergency operation procedure. However, the favorable effect of containment spray additive on the precipitation limit was evaluated to be able to compensate the adverse effect of reduced mixing volume. No change in the timing of the simultaneous hot and cold leg safety injection implemented in the existing emergency procedure guideline is required according to the results of newly developed boric acid methodology and the favorable effect of containment spray additive on the precipitation limit.