National Report of The Slovak Republic , september 2001


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2.2.2.2 Accident analyses


Similarly as for NPP V-1 units, the approach towards the safety review of NPP V-2 has been based on internationally approved safety standards and guidelines, accounting for the relevant national regulations and recommendations.

The results of the accident analyses are part of chapter 15 of the SAR for NPP V-2. Initiating events defined by guideline RG 1.70 for pressurized water reactors (PWR) were analyzed, adjusted to the conditions of WWER-440/213, supplemented later on (following the IAEA guidelines IAEA-EBP-WWER-01) in accordance with the methodology of the latter, by the analysis of some additional casing cases of initiating events. In reviewing beyond design basis accidents, potential initiating events were chosen for the units based on an analysis of results of accident analyses conducted, under consideration of additional failures of systems and facilities.

The issues of beyond design basis accidents and/or severe accidents with respect to WWER-440/213 units were analyzed under several PHARE projects:

  • PH 4.2.7a “Beyond design basis accidents analyses and accident management in V-213 units“ - main beneficiary NPP V-2, additional - NPP Dukovany, NPP Paks
  • PH 2.06/94 “Analysis of alternatives for filtered venting of V-213 containment“ - main beneficiary NPP Paks
  • PH 2.07/94 “Handling of hydrogen in V-213 containment during severe accidents” main beneficiary NPP Paks

Validation and qualification of the MAAP4/WWER computer code was also performed under the PH 4.2.7 project. The code was found to be in compliance with other codes within uncertainties typical for severe accidents analyses , and the code was demonstrated to be of use for the applications considered. Situations and processes were identified that require analyses of additional codes.

The results of independent assessment of severe accident risk, performed at ÚJD, have been summarized in the report “A Regulatory Evaluation of Severe Accident Risk and Potential Impact of Selected Severe Accident Management Actions for Bohunice V-2 Nuclear Power Plant”. The evaluation was prepared in cooperation with the Swiss Inspectorate of Nuclear Safety (HSK).

Conclusions derived from PHARE projects:

  • large volumes of reactor coolant and secondary side water as well as the considerable temperature margins of the core provide sufficient time for preventive measures to be taken,
  • analyses performed in the field of severe accident phenomenology provided a good basis for the understanding of NPP type V-213 response,
  • spraying is a rather efficient method to reduce discharges of fission products, thanks to a lowered pressure and scrubbing,
  • vulnerable spots of the WWER-440/213 project were identified in relation to severe accidents, and subsequently classified into three levels of significance,
  • preventive measures were suggested that were used in preparing SBEOP, as well as measures to mitigate severe accidents,
  • a variety of configurations of filtered venting and containment spraying were suggested and verified in relation to radioactive substances discharge limits,
  • hydrogen distribution within containment during severe accidents was analyzed, and source term were computed for hydrogen, various combinations for the reduction of its production; the analyses suggested that mitigation of severe accidents will highly probably require the installation of a powerful system to reduce hydrogen concentration .

The results of the above mentioned projects will find applications in the development of SAMG, planned to start for NPP V-2 units and Mochovce units in 2001. Several supporting and preparatory activities are simultaneously under way, including a common project of SE, a. s., NPP Dukovany and NPP Paks supported by the European Union and coordinated by the Finnish company Fortum that operates WWER units.

The integrity of the bubble-condenser system, a category III safety issue according to the IAEA document EBP 03, was verified under the PHARE/TACIS project PH 2.13/95 “Bubble Condenser Experimental Qualification”. The study evaluated the bubble-condenser system as sufficiently functional under conditions of accidents, identified also some spots that require strengthening of the structure, such as between the first and the second tray, the area of tray twelve, and strengthening of the entrance control door.

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