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5. SAFETY ANALYSES

Safety analyses make an essential part of the safety documentation for every nuclear installation and serve as a means to demonstrate and pass judgements on nuclear safety. They give an answer to how the nuclear installations are secured for the case that a failure or accident occurs, and predict consequences of such failures and accidents. The results of safety analyses have to comply with the generally accepted criteria.
In 2000 the safety analyses were focused on verification of the safety analyses report and probabilistic safety assessment study for NPP V-1 Bohunice after the reconstruction, reviewing of the suggested changes of the Limits and Conditions for NPP V-2 Bohunice and on the assessment of operational events. 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 partnerships’ organisations, i.e. within international PHARE programme as well as the 5th framework of the European Commission.


Reactor hall of NPP V-1 Bohunice after reconstruction

Verification of safety analyses part of the safety report for NPP V-1 Bohunice after the gradual reconstruction was focused 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 programmes, achieved level of their verification, correctness and interpretation of the results were assessed. The used data and means, i.e. steady-state parameters, initial and boundary conditions, neutron-kinetic and material characteristics, configuration of safety systems, single failure criterion, intervention of the operator and mathematical models, were verified. Selected representative accident scenarios were recalculated at UJD. The results of the analyses were evaluated against the determined safety criteria and used in passing judgement on NPP V-1 Bohunice safety 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-1 Bohunice has been approved. The document with the results and all the findings of the performed review has been prepared. It includes the details of the performed independent calculations, their results and comparison with the results given in the safety report.
A special attention was paid to a review of probabilistic safety assessment study of level 1 for NPP Bohunice V1 after its gradual reconstruction. The probabilistic safety analysis of NPP in full power operation was elaborated in the study and the impact of the gradual reconstruction 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 objective of the review was to determine the acceptance of the sources of input information, assumptions, models, data, analyses and obtained results, in order that the probabilistic model gave a real picture of the NPP. The review of the study was performed by UJD and in co-operation with external organisations, which were not included in elaboration of the reviewed document and the probabilistic model of NPP. The review was made in accordance with the IAEA guidelines. The methodical documents of UJD, IAEA and US NRC were applied as well. The results of the study and its review were utilised to pass judgement on safety of NPP V-1 Bohunice in the approval process.
An independent judgement of the permitted period of unavailability and testing intervals for the systems of NPP V-2 Bohunice, which were submitted to UJD by the operator, has been performed. The proposed changes concerning the Limits and Conditions were calculated on the basis of the probabilistic approach. Within the framework of the review the completeness, clearness and well-arrangement of the submitted documentation was evaluated. The methodology of the optimisation and probabilistic safety analysis, criteria and their application for NPP V-2 Bohunice, completeness of optimised Limits and Conditions, their wording and interpretation were checked. The results of the independent review were used for accreditation of the suggested changes of the Limits and Conditions.
In 2000 the expert co-operation of UJD and its German partnership organisation Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) continued. In the frame of the project of scientific and technical co-operation in application and validation of German thermo-hydraulic computer codes, the employees of the Department of Safety Analyses and Technical Support have been trained to work with a computer code ATHLET dedicated for modelling the neutron-physic and thermo-hydraulic of the anticipated transients and postulated accidents in NPPs. By this UJD has obtained another analytical tool for independent nuclear safety assessment with the help of computer codes.
The main tasks of the project was to perform check calculations - analyses of various modifications of the design basis accident for nuclear power plants with VVER-440/V213 and simulation of an operational event within the framework of validation and verification process of the computer code and the used calculation model of the nuclear power plant. Another task was to simulate the test of the Hungarian experimental device PMK-2 which had been performed and documented within the PHARE project. The knowledge and experience obtained during the project have a great contribution for further work of the Department of Safety Analyses and Technical Support in the field of independent judgements of nuclear safety of nuclear power plants in Slovakia.
In the frame of the international Swiss-Slovak-Ukrainian project SWISSUP sponsored by the Swiss government the activities aimed to three main tasks were performed:

  • exchange of information and sharing of expertise in the field of nuclear power plant inspection practices and accident analyses;
  • development of Event Reporting Requirements and associated Database software;
  • formulation of Safety Performance Indicators for use by the regulatory authority and introduction of corresponding software for processing of input information and calculation of safety indicators.

The elaborated technical reports include a summary of the differences in the safety analyses report content, regulatory acceptance criteria and other review requirements between the Slovak and Ukrainian authorities, as well as comparison of the performance indicators used for safety assessment of nuclear power plants operation in Ukraine and Slovakia.
In the PHARE project led by the English AEA Technology company UJD has participated in the task focused on severe accident mitigation for nuclear power plants with VVER-440/V213. The main results of the previous analyses of nuclear power plants responses to severe accidents were summarised and the state of their analytical base performed in the past was evaluated in the year 2000 within the project. Possible severe accident mitigation and preventive measures were proposed and their applicability for the nuclear power plants with VVER-440/V213 was investigated. After finishing the project the obtained results will be used in assessment activities and accident management of 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 2000 the familiarisation with the computer code and some check calculations has been started. The work on the project will continue also in 2001.

 

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