Feasibility of risk-informed regulation for Generation-IV reactors

Abstract

With the advent of new and innovative Generation-IV reactor designs, new regulations must be developed to assure the safety of these plants. In the past a purely deterministic way of developing design basis accidents was prevalent, however this is felt to not be satisfactory, since this leaves insufficient safety restrictions in certain areas, while being overly restrictive in others, not being able to optimize where the safety constraints are truly needed. Currently the USNRC is investigating how one might go about this approach, but no method is finalized. In this paper, a methodology for creating risk-informed design basis accidents is developed. This not only incorporates the Surrogate Risk Guidelines developed by the USNRC for each overall accident initiator the plant may experience, but helps to select which sequences are the most important to that initiator, and from that develop a set of risk-informed assumptions that form the basis of the design basis accident. This method was applied to the test case of the Massachusetts Institute of 'Technology' s Gas-Fast Reactor (GFR) design, as considerable risk-informed design work has been carried out on various initiators for this design (including Turbine Trip, Loss-of-Coolant Accident, and Loss-of-Offsite Power).

(cont.) The turbine trip was chosen for extensive investigation. It was found that the CDF of this event for the GFR (7.098E-6 / RY) did not pass the overall NRC Surrogate Risk Guideline (1E-6 / RY). The method identified the dominating sequence, which was dominated itself by the failure of the passive shutdown cooling system for the GFR design. It was determined that the designers could in fact develop a risk-informed DBA by developing a set of assumptions to ensure success in the Passive SCS. This process showed how risk-informed DBAs could be developed for various new reactor designs.