Technical and regulatory concerns in the use of best estimate methodologies in an LBLOCA analysis licensing process

Based on the acquired experience of the Brazilian Nuclear Regulatory Body (CNEN), reviewing and assessing the Angra 2 Nuclear Power Plant (NPP) Large- Break Loss-of-Coolant Accident (LBLOCA) analysis, performed with a realistic evaluation methodology, this paper describes technical and philosophical questions identified during the licensing process and the adopted regulatory approach to deal with the main safety issues. CNEN adopted the American rule 10 CFR 50.46, September 1988 revision, that allows the use of realistic evaluation models to calculate the performance of the Emergency Core Cooling System. The LOCA analysis shall in such cases fulfil the requirement of identifying and evaluating the uncertainty in the analysis methods and inputs and this uncertainty must be considered when comparing the calculated results with the acceptance criteria. There is a lack of an established set of specific regulatory requirements applied to the acceptance of a realistic evaluation model used to analyze the LBLOCA. The Advisory Committee on Reactor Safeguard (ACRS) of US Nuclear Regulatory Commission emphasizes in an ACRS Letter Report [1], "We perceive a need for the staff to be more specific about what are acceptable methods of deriving and expressing the uncertainties in codes and how these methods are to be used in the regulatory context ... . The staff should reevaluate the design specifications for the outputs of codes and their relationship to present and anticipated regulatory requirements". It is also recognized that in several countries there are initiatives in developing these requirements and in redefining the LBLOCA [2-5]. It is within this scenario that the Angra 2 realistic LBLOCA analysis was submitted, based on the Code Scaling, Applicability, and Uncertainty (CSAU) methodology [6] to evaluate the uncertainty. Aiming at performing a consistent safety assessment of this analysis, the Brazilian regulatory body relied upon two international consultants, GRS (Gesellschaft fur Anlagen und Reaktorsicherheit) and University of Pisa. The LBLOCA analysis presented in the Final Safety Analysis Report (FSAR) was reviewed by CNEN staff taking into account these two independent reviews which led to a request for additional information, with a total of 27 questions to the applicant, each one classified according to their significance to safety [7]. Together with CNEN staff, the University of Pisa consultant performed an independent calculation [8,9]. This includes the independent LBLOCA calculation with Relap5/Mod3.2.2Gamma code and the independent uncertainty evaluation with the CIAU method (Code with capability of Internal Assessment of Uncertainty) [10]. Based on its conclusions, three requests for additional information were issued to the applicant.