Large early release frequency

Safety Monitor is an interactive computer program (Stamm, 1996) that performs real-time assessments of configuration-specific plant accident risk. Originally used at Southern California Edison s San Onofre station in 1994, further development was sponsored by three nuclear utilities and EPRI, to include shutdown operation and expanded user features. It will be enhanced to calculate large early release frequency (a Level 2 risk). 

The current principles of risk-informed decisionmaking have been developed considering many parameters including the currently usual core damage frequencies (CDF) and the currently usual large early release frequencies (LERF). 

The large early release frequency (LERF) is a risk measure, which shows, how possible the accident in a nuclear power plant is, i.e. large release of the radioactivity into the environment of the nuclear power plant. 

Figure 1. Acceptance guidelines combining two risk measures (core damage frequency and large early release frequency).

A plant-specific PRA model provides quantitative results in the form of point estimates and probability distributions that postulated risk events may occur in a nuclear power plant. Two figures of merit are commonly used by the PRA analysts to demonstrate how a nuclear power plant meets the safety goals put forth by the NRC, namely the core damage frequency (CDF) and the large early release frequency (LERF). These figures of merit are applicable to all modes of operation of a nuclear power plant. For the shutdown mode, other figures of merit are equally important, such as the time to core uncovery and time to core boiling. 

The assessment of core damage and large early release frequencies for M034 NPP demonstrates that safety targets defined in BNS 1.4.2 are fulfilled. Additionally, in accordance to general international guidelines (IAEA SSR 2/1, SSG-3, SSG-4), the PSA results show that ... 

From those, a criterion closer to a direct evaluation of the probabilistic safety assessment results was derived, which states that the expected frequency of the large early releases from a NPP should be less than lOE-6/year. In addition, the core damage frequency should be proved to be below lE-4/year. 

The large release frequency is dominated by containment failures or bypasses due to steam generator tube rupture, and high primary pressure core damage sequences. The remaining containment failures ate dominated by an early containment failure due to reactor cavity flooding failure. 

Compared with the unchanged conditions of the plant, modifications of measures, equipment or the operating mode of the plant shall neither result in an increase of the average Core Damage Frequency (CDF) and/or the average frequency of large and early releases (LRF/LERF), for power operation nor for low power and shutdown states, considering all plant internal events as well as all... 

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