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
- 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
- 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.