The paper presents results of certification of the ATHLET/ BIPR-VVER software system (version 1.0) in the Russian Federation. The following stages of the software system certification are described: preparation of a validation report; work with appointed experts; performance of additional studies to increase the level of the software system validation; and preparation of a software system certificate. Examples of validation computations simulating experiments in operating power units and thermal-hydraulic experimental facilities are given. The ATHLET/BIPR-VVER software system (version 1.0) was certified by the Expert Council on software certification under Rostechnadzor on 24.10.2018 (Certificate No. 455).

Unauthorized dilution of boric acid in the reactor coolant due to improper operation of the chemical and volume control system (KBA) is a project mode with the violation of normal operation. The operator is obliged to identify unauthorized dilution within half an hour and stop feeding with pure condensate. The power unit has automation, which stops feeding on EP (emergency protection) and ATWS (Anticipated transient without scram) signals. The situation without the intervention of the operator and the failure of automation to stop dilution can be considered as a beyond design basis accident. The article presents an analysis of unauthorized boric acid dilution process in the primary circuit coolant of the NPP-2006 power unit up to the controlled state, either considering the operator's intervention after a 30-minute period of process, i. e. with a forced cessation of dilution, or without the operator's intervention until the tanks with pure condensate are completely emptied.

The method developed by the Kurchatov Institute's WWER reactor department to detect local coolant boiling in the reactor core on the basis of the neutron noise analysis is currently being implemented at some WWER-1000 units within their regular in-core noise diagnostic systems. Given that WWER-1000 power uprating – as well as new designs being developed, such as AES-2006 and WWER-TOI characterized with higher core power and coolant temperature – would increase the probability of coolant boiling, the efficiency of the available boiling diagnostics method was to be re-evaluated. On the basis of relevant computations, it was suggested to modify the diagnostic model and introduce new parameters to improve the reliability of local boiling detection. Computations simulating in-core boiling were thus performed and showed that the improved method was quite efficient. This paper discusses the possibilities to detect – using the improved method – various in-core coolant boiling cases anticipated for WWERs.

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