4) SAFETY ANALYSES
Safety analyses make an
essential part of the safety documentation for every nuclear installation and serve as a
means of demonstrating and passing judgements on nuclear safety. They give an answer to
how the nuclear installations are secured for the case when a failure or accident occur,
and predict consequences of such failures and accidents. The results of safety analyses
have to comply with the generally accepted criteria.
In 2001 the activity in the field of safety analyses was focused on verification of the
safety analyses reports for NPP V-2 Bohunice and NPP Mochovce concerning the new profiled
fuel and probabilistic safety assessment study for NPP Mochovce. The calculation safety
analyses were performed and expert reviews for the internal UJD needs were elaborated. An
important part of work was performed also in solving of scientific and technical tasks
appointed within bilateral projects of co-operation between UJD and its international
partnership organisations as well as within international projects ordered and financed by
the European Commission. All these activities served as an independent support for UJD in
its deterministic and probabilistic safety assessment of nuclear installations.
Verification of safety reports for NPP V-2 Bohunice and NPP Mochovce concerning the new
profiled fuel was focused in the part of the safety analyses especially on checking and
passing judgement on the completeness of the considered initiating events, safety
criteria, input data, adequacy of the used calculation models and also on the overall
quality of the submitted documentation. Suitability of the used methodology and the
calculation programs, achieved level of their verification, correctness and interpretation
of the results were assessed. The results of the analyses were evaluated against the
determined safety criteria and used in passing judgement on safety of both NPP V-2
Bohunice and NPP Mochovce in the licensing process. The performed review has shown that
the checked safety analyses were performed in compliance with the internationally accepted
practice, recommendations of UJD and the IAEA. The required level of safety of NPP V-2
Bohunice and NPP Mochovce for the new profiled fuel has been approved. The documents with
the results and all findings of the performed review have been prepared. The listed
questions and comments were sent to the safety reports submitters.
A special attention was paid to a review of probabilistic safety assessment study of level
1 for NPP Mochovce. The probabilistic safety analysis of NPP related to the full power
operation was elaborated in the study and a contribution of the technical and operational
improvements to the risk decreasing was quantified. A core damage frequency of the reactor
was calculated and the dominant initiating events and accident sequences with the major
contribution to the risk were determined. The target of the review was to determine the
acceptance of the sources of input information, assumptions, models, data, analyses and
obtained results, so that the probabilistic model could give a real picture of the NPP.
The review of the study was performed in co-operation of UJD with the IAEA (IPSART
mission) as well as with other external organisations, which were not involved in the
elaboration of the reviewed document and probabilistic model of NPP. The review was made
in accordance with the IAEA guidelines and methodical documents of UJD and US NRC.
In the field of calculation safety analyses the UJD activity was focused on the analysis
of an operational event, analyses of the selected accident scenarios for NPP V-1 Bohunice
and on review of the impact of the modelling of selected components to the results of
calculation safety analysis (a sensitivity study for NPP Mochovce).
An important part of the activities throughout 2001 belonged to the elaboration of
independent expert reviews for the internal UJD needs within reviewing of technical
documents in the decision making process. The set of innovated limits and conditions for
the operation of the 2nd unit of the NPP V-1 Bohunice, reliability of the selected systems
in the NPP V-1 Bohunice and Mochovce and progress of the science and technique in the
field of nuclear safety in Slovakia were judged.
In 2001 the project of expert co-operation between UJD and its partnership German
organisation Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) focused on the support
of UJD in safety assessment of NPPs in Slovakia was terminated. The employees of UJD have
been trained to work with German thermo-hydraulic computer codes within the project.
Thanks to this project UJD has obtained another analytical tools for independent nuclear
safety assessment. A part of the tasks of the project was to perform series of independent
calculation analyses for supporting of UJD decision making. The results of the calculation
analyses are documented in the technical reports of GRS.
In 2001 UJD joined a new European project Alternative Approaches to the Safety Performance
Indicators. The project is aimed at the information collecting and determining of
approaches and recommendations for implementation of the risk oriented indicators,
identification of the impact of the safety culture level and organisational culture on
safety and applying of indicators to the needs of regulators and operators. In frame of
the PHARE project UJD participated in the task focused on severe accident mitigation for
nuclear power plants with VVER-440/V213 units. The main results of the analyses of nuclear
power plants responses to severe accidents were summarised and the state of their
analytical base performed in the past was evaluated within the project. Possible severe
accident mitigation and preventative measures were proposed and their applicability for
the nuclear power plants with VVER-440/V213 was investigated. The obtained results will be
used in assessment activities and accident management of UJD.
Participants of the CENS workshop organized by
UJD has been involved also
in EVITA project which makes a part of the 5th EC Framework Programme. The project aims at
validation of the European computer code ASTEC dedicated for severe accidents modelling.
In 2001 the ASTEC computer code was tested on different platforms. The results of the
testing are summarised in the technical report of EC issued in September 2001. Further
activities within this project were focused on performing of selected accident scenarios
analyses and comparison of the obtained results with the analyses realised with the help
of other computer codes. The work on the project will continue in 2002.
In 2001 a groundwork on establishing the Centre for Nuclear Safety in Central and Eastern
Europe (CENS), the seat of which is going to be in Bratislava, has continued. The
employees of UJD have been preparing the bases for decisions and operation of the working
group of the CENS and participated in organisation and arrangements of the international
pilot course on Regulatory Safety Assessment. The course was positively evaluated by all
its local and foreign participants.