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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.

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Participants of the CENS workshop organized by the UJD

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.


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