National Report of the Slovak Republic - Radiation and Nuclear Safety , September 1998

5.3.5 Analysis of Events at Nuclear Installations

Operators as well as regulatory authorities pay significant attention to prevention and analyses of events at nuclear installations (NIE) because of their significant impact on safe and reliable operation of nuclear installations.

Throughout the operation of nuclear installation, operator continuously investigates, analyzes, reports to the regulatory Authorities and records events at nuclear installations, and takes corresponding measures.

QA guidelines on "Management of Events at Nuclear Installations" have been developed to raise the effectivity of the entire process as well as to express the support of the management to the analysis of reasons of events at nuclear installations and to investigate them in an objective manner. They define procedures and the organization of investigations of NIE causes aimed at taking remedial measures at the individual power plants.

The objective is to

The entire process connected with the investigation, reporting, analysis and archiving of NIE as well as remedial measures is the responsibility of the Feedback Group (SSV) which is a part of Nuclear Safety Divisions at the individual power plants. The training of the Feedback Group staff on the use of HPES methodology (introduced by the US organization INPO) was organized by the company Nuclear Electric from the United Kingdom. Repeated training sessions were organized in the framework of PHARE program and delivered, among others, by the Spanish companies Union Fenosa and Tecnatom.

  1. Definition and Classification of Events at Nuclear Installation
  2. An event at nuclear installation(NIE) is a deviation from normal operating conditions as defined in the operating regulation or project documentation, due to the occurrence or continued presence of a single or several partial breakdowns on the equipment or due to inappropriate interventions by the maintenance staff or deficiencies in regulations.

    Two categories of NIE are distinguished depending on their actual or potential impacts on nuclear and/or technical safety, and also depending on the human factor contribution. Every NIE category has its own way of management. For NIE which meet the criteria defined by the regulation, a comprehensive analysis must be conducted using one of the methods for the identification of the root cause.

    NIE of Category II the reasons for which are known are investigated aimed at taking and implementing of effective remedial measures.

    The administrative mechanism of the investigations with respect to both NIE categories is the responsibility of the feedback group.

    Category I. includes all safety significant NIE which have negative impact on the safety (nuclear, radiation or technical), NIE with non-design course during nuclear safety significant transient conditions, which have negative impact on the safety NIE with a significant share of the human factor (including sabotage), significant deficiencies identified during controls (tests) as well as serious deficiencies concerning operating regulations.

    Also, this group includes breakdowns of the availability of safety systems identified upon reviews in the framework of general overhauls or in-service inspections.

    Category II. includes non-safety significant NIE, breakdowns of isolated systems with smaller impact on the technological process, reductions of output for standard repairs of statistical breaks or planned repairs, reduction of output due to safety insignificant statistical breaks of the installation, and identified non-category I operation procedures deficiencies.

    The guideline "Management of Events at Nuclear Installations" defines criteria for NIE classification as category I or II.

  3. Documentation and Analysis of Events at Nuclear Installation
  4. The guideline includes a flow diagram of the investigation procedure of NIE including notification of Regulatory Authority. For both NIE categories, the shift supervisor completes a standard reporting form, and also attaches comments by the respective staff.

    To analyze of Category I NIE is the responsibility of the feedback group who develops it based on positions by professional divisions and on own analyses and/or conclusions arrived at by Working Groups.

    For NIE which meet the criteria for the investigation of the root cause, the feedback group conducts, together with the corresponding divisions, a comprehensive analysis using one of the following methodologies (or their combination):

    The analysis of Category NIE I is completed by presenting Operating Event Report which is submitted for review by Breakdown Commissions which are a collective advisory body to directors of nuclear power plants for NIE management. Breakdown Commissions meet once a month to approve conclusions from analyses and to instruct to implement remedial measures which are mandatory to all employees. NIE reports are sent to the Regulatory Authorities the representative of which has the right to attend the Breakdown Commission sessions.

    The implementation of remedial measures is documented by the corresponding responsible division via computer network where it is filed. Breakdown Commissions check the fulfillment progress.

    The investigation of Category II NIE is coordinated by the feedback group which works closely together with professional divisions of the operation, maintenance, technical support and safety department.

    The entire agenda of NIE investigations and analyses is kept in the computer network; all network users have access to this agenda. Any network user may contribute his/her comments or relevant observations to the statements concerning operation-related events. Only feedback group members involved in the investigation of nuclear installation are however authorized to make changes in the agenda.

    Extraordinary Breakdown Commission

    Sessions of the Extraordinary Breakdown Commission are convoked by the Deputy Director, Operation, according to the urgency of the situation so as to draft and submit to the Regulatory Authorities, within 72 hours, minutes of the session. The session of the Commission is convoked within 24 hours of the identification of NIE for those NIE with respect to which such procedure is required by the regulation. The task of the Extraordinary Breakdown Commission is to identify direct cause of inappropriate functioning of systems or inaccurate interventions by the maintenance staff, and to define immediate remedial measures. To the extent the cause of the failure has been fully clarified and the correct functioning of safety significant systems is has been restored, and the Deputy Director, Operation, considers that restarting of the unit is safe, no session of Extraordinary Breakdown Commission needs to be convoked prior to unit start-up. Completed "Surveillance Program" is submitted to the Extraordinary Breakdown Commission session for post-accident review.

    Minutes from the Extraordinary Breakdown Commission sessions represent the official preliminary report on the event required by the Regulatory Authorities for safety significant events, in accordance with OLC. Subsequently, the root cause is analyzed, and final analysis is conducted as standard report on category I NIE including remedial measures. It is then reviewed and approved by the Extraordinary Breakdown Commission.



    Independent NIE Review

    Reports on category I NIE are also sent to the Research Institute VJE Trnava, and a representative of the Institute participates in the Breakdown Commission's sessions where he/she can present his/her positions and suggest measures. In addition, he/she performs independent reviews of safety significant NIE on a quarterly and annual basis, and suggests remedial measures if needed. The report is made available to the operator.

    Event Reporting

    The operator is responsible to inform State authorities on selected NIE within set deadlines. The information has to contain preliminary assessment evaluation of the respective NIE according to the INES scale. It is prepared by the Shift Supervisor of the shift during which the respective event occurred, if there is any change in the preliminary assessment of the event (e.g. as a consequence of the event development), such change is notified to the Regulatory Authorities by Deputy Director, Technical Support and Safety. The final assessment of the event from operator's viewpoint is the responsibility of the feedback group. The report is approved by Breakdown Commission. A guideline defines the format, contents and time sequence of the report provided to the Regulatory Authorities.


    Feedback from Nuclear Installation Events at other NPPs

    The operator uses international information systems on operation-related experience in the nuclear power sector (WANO and IAEA) to implement measures on own units based on analyses of breakdowns experienced by operators in other countries, as well as to share his own experience with other operators. The objective of such activities is to prevent similar breakdowns by implementation of preventive measures, as well as avoiding of double safety analyses and non - standard approaches to solutions. QA Guideline "NIE Feedback from NPP Abroad" develops details of processing and utilization of information from other NPP events.

    Efficiency Assessment of Implemented Remedial Measures

    The main parameter of feedback efficiency from own NIE is the trend of the occurrence of breakdowns showing identical mechanism of failure. Once a year, the feedback group prepares a summary statistical evaluation of the occurrence of repeated events, and assesses the efficiency of related implemented measures.

    The control of the efficiency of remedial measures depends on the type of measures, and is being dealt with in the framework of quality assurance:

    Information Flows on NIE within SE

    Heads of the respective departments and divisions are obliged to:

    Any employee involved is obliged to be familiar with the results of NIE analyses, in particular those he/she was a part of. Any backgrounds or results of analysis which contradict his/her observations or understanding of the event make him/her entitled to ask the head of the Operation Conditions Department to further deal with the respective NIE or to explain the discrepancies.

  5. Statistical Evaluation of NIE, Development and Trends
  6. Below we present data on the occurrence of NIE in 1997, along with development and trends for the recent period of time.

    The Number of NIE in 1997 at NPP Bohunice units has been the lowest in the history of the units operation. A single NIE event occurred assessed as class 1 INES. Also of importance is the clearly decreasing trend of NIE since 1993, even the more that extensive gradual reconstruction works are going on at the units during extended general overhauls. The following diagram illustrates the development of breakdown rates and of safety significant NIE at NPP Bohunice within 1990-1997.










    Total NIE









    Out-of-Scale NIE INES









    Below-Scale INES (0)









    NIE INES 1









    NIE INES 2 +











    Causes of nuclear installation events at SE EBO

    1997, comparison with 1995 and 1996

























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