Beneficiary Organisation details
Main Beneficiary:
Paks NPP (Hungary)
www.npp.hu
Co-Beneficiaries:
Bohunice NPP (Slovak Republic)
http://www.seas.sk
Dukovany NPP (Czech Republic)
http://www.cez.cz
Project Aims

• The purpose of this project was to assess, using probabilistic safety assessment (PSA), boron dilution faults which could occur in a VVER;
• The scope of work of this project covered a review of the full range of potential initiators, in particular:
  • Mechanical failure within operating systems that might lead to the introduction of low-borated water to the RCS;
  • Inadvertent actuation of systems connected to the RCS but not operating initially, and containing low-borated water;
  • Operating procedures that allow addition of low-borated water without sufficient controls;
  • Failure by the operators to follow operating procedures correctly, such that low-borated water is added to the core;
  • Build up of slugs of low-borated water following other faults (such as LOCA, PRISE, ATWS or loss of RHR).
• In each case a PSA approach was used to identify the most important faults and their frequencies. The scope was not limited to the above scenarios and considered all potential initiators in all plant operating states.

Project Results

• The verification and validation of all the codes used for the PSA and transient analysis were checked. The status was found to be satisfactory for the various code applications in this project;
• The PSA showed that at Paks NPP the contribution of the dilution initiators to the total core damage frequency was about 1.1%. The number of scenarios with probability of occurrence higher than the overall screening probability (1.0E- 7) was five. Their overall contribution to the total core damage probability was less than 1 %;
• Transient analysis demonstrated that for four of the dominant scenarios (homogeneous dilution) the time intervals for reaching a critical state and DNB were very long for the plant operational states (POSs) considered. Best estimate calculations showed the limits of the acceptance criteria were not exceeded. A remaining dominant scenario was not amenable to transient analysis. Although none were dominant, inherent dilution scenarios following other initiating events were evaluated to improve the estimate of overall core damage risk;
• The results of the PSA study were used to recommend design changes and changes to operating procedures to limit the likelihood and severity of the most onerous boron dilution events identified;
• The total contribution of the five dominant scenarios to the core damage risk originating from the shutdown states was 1.5% and this can be considered acceptable even without introducing modifications;
• The implications for the VVER-213 plants at Dukovany in the Czech Republic and Bohunice in the Slovak Republic were also examined.

Further Project Results
Further information on the project results could be sought from the beneficiary organisations.