MANKAMO Tuomas (Avaplan Oy), MARTTILA Jouko, REPONEN Heikki. Experiences from the LNPP-P&DSA review. Lessons learned from RBMK safety studies. STUK-YTO-TR 168. Helsinki 2000. 43 pp.
ISBN 951-712-412-0
ISSN 0785-9325
Keywords: Leningrad nuclear power plant, LNPP, LAES, Sosnovy Bor, RBMK, probabilistic safety assessment, PSA, peer review
RBMK is the Russian acronym for "Channelized Large Power Reactor". The Soviet-designed RBMK plants deviate substantially from typical Western BWR or PWR plants. The safety of the RBMK plants has raised severe concerns since the major accident at Chernobyl Unit 4 in 1986. In addition, a fire destroyed the turbine hall of Chernobyl Unit 2 in 1991 resulting in a near-accident: the rector cooling could only be maintained through improvised measures. Another well-known fire event is the control cable room fire at Ignalina Unit 2 in 1989, which led to a partial loss of the main control room functions.
After the collapse of Soviet Union several multilateral safety programs were started to evaluate and improve the safety of the RBMK plants. A Probabilistic and Deterministic Safety Assessment (P&DSA) of the Leningrad Nuclear Power Plant (LNPP) Unit 2 was started in 1996. Phase 2 of the project was completed in January 1999. A Peer Review was performed by Russian and Western experts.
This report describes the insights from the RBMK risk studies, especially from the LNPP P&DSA with emphasis on the deeper understanding of the risk-important design factors and identification of possible ways to increase safety. LNPP P&DSA has meant a significant progress in this respect. Despite of its certain limitations P&DSA Phase 2 could point out short-term measures, which substantially reduced the risk of identified weaknesses, mostly related to the reliability of the emergency feedwater function and its support systems.
The findings of LNPP P&DSA and the review recommendations emphasize the extensions needed to the analysis scope. The spreading and other influences of fires and floods between connected spaces should be analyzed because of incomplete separation and protection in these regards in the 1st generation RBMK plants. High priority should be given to the analysis of external hazards, which were found important at the Loviisa NPP on the Northern side of the Finnish Gulf, e.g. seismic events, mass emerge of algae, frazil ice, high wind and snow storm. An in-depth analysis is needed to evaluate the reliability of the control and protection systems to improve the simplified approach of P&DSA Phase 2. The reliability of the primary circuit piping should be evaluated based on the results of the in-service inspections, which recently have revealed many severe defects at the RBMK plants.
The review recommendations have been taken into account in planning the LNPP In-depth Safety Assessment (LISA), which includes PSA Phase 3. The work thus far adds to the expertise of the PSA earlier started for Ignalina NPP. It is highly recommended that the PSA activity is also launched at the other RBMK plants to support the identification and prioritization of cost-effective safety improvements and to facilitate tackling the generic safety weaknesses of this type of reactor and plant.
Whole report (PDF-file, 300 KB)