«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
28 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues The Federal Ministry of Agriculture, Forestry, Environment and Water Management entrusted the Umweltbundesamt (Federal Environment Agency Ltd. with the general management of the implementation of the “Roadmap”. Each entry to the “Roadmap” corresponds to a specific technical project. [see ANNEX C].
The objective of the Roadmap process covered by the item 7 as stated in Annex I of the “Brussels Agreement” is: "Effective prevention and mitigation of consequences of beyond design basis accidents (severe accidents)”.
Annex I provides the following statements regarding the “Present Status and specific Actions
“A set of preventive and mitigative measures is, at present, applied in NPP Temelín with respect to beyond design basis accidents. These include software and hardware measures, among others, e.g. Symptom Based Emergency Operating Procedures, Technical Support Centre, Post Accident Monitoring System, Emergency Preparedness.
For the purpose of emergency preparedness, the PSA was employed with the aim to identify and group events with different initiating occurrences, but with similar end-effects. On the basis of this assessment the relative risk was estimated for specific events in order to select those, which will serve for the determination of emergency response activities (pre-planned, reactive).
Severe Accidents Management Guidelines (SAMG) as a state-of the-art tool will complete the whole system of mitigation measures with respect to the beyond design basis accident management. The project for SAMG development is scheduled to be finished by end 2002 to be followed by validation.
To foster mutual understanding two lines of activities will be followed within the framework of
the bilateral agreement:
a) A Working Group on comparison of calculations regarding the radiological consequences of BDBA with a view to harmonise the basis for emergency preparedness will be established.
b) The exchange of information related to SAMG will include discussion on the analytical basis as well as on corresponding software and hardware measures."
The issue (a) has been covered by a separate project PN1 [see ANNEX C], the issue (b) is covered by this project.
Refering to Chapter IV of the “Brussels Agreement” and the principles of the “Roadmap”, a number of issues identified in the “trialogue” of the Melk Process are found suitable to be followed-up in the framework of the Bilateral Agreement. The following seven issues are closely
related to the topic of item No. 7 (b) and are therefore also covered in this project:
• Issue No. 1 Containment bypass and preliminary-to-secondary (PRISE) leakage accidents
• Issue No. 4 Containment Design and Arrangement
• Issue No. 5 Probabilistic Safety Assessment and Severe Accidents
• Issue No. 6 Emergency Operating Procedures EOPs & Severe Accident Management Guidelines (SAMGs)
• Issue No. 16 Hydrogen Control
• Issue No. 26 Beyond Design Bases Accident Analysis
• Issue No. 29 Technical Basis for Temelín Emergency Planning Zones (EPZs) ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 29 The Roadmap specified that the related Specialists’ Workshop would be held in the 1st half of 2003 to discuss this issue. This workshop on the “Roadmap” item No. 7 was conducted in Prague on 17 and 18 June 2003 according to Article 7 (4) of the Bilateral Agreement of the Exchange of Information on Nuclear Safety. This workshop was the key element in the monitoring process. The analysis of information made available there played a significant role in the development of the basis for the Preliminary Monitoring Report.
A Specialists’ Team of international experts was committed by the Umweltbundesamt (Federal Environment Agency) on behalf of the Austrian Government to provide technical support for the monitoring on the technical level of the implementation of the SAMGs Issue as listed in Annex I of the Conclusions of the Melk Process and Follow-up. This specific technical project is referred to as project PN7 comprising altogether seven predefined “project milestones” (PMs).
In a series of presentations the outline of the technical approach to the Severe Accidents Management Guidance (SAMG) item was described by Czech experts, including the legal framework for the issue and the information provided to the Licensing Authority about the technical approach.
The Czech presentations at the Specialists’ Workshop covered a broad scope of aspects related to the development and implementation of Symptom Oriented Emergency Operating
Procedures (EOPs) and SAMGs. The following presentations were provided:
- Regulatory Approach to Accident Management
- National Projects and International Programmes in Accident Management
- The Temelín Accident Management Programme
- Temelín Probabilistic Safety Assessment (PSA) Update Results
- VVER-1000 PSA Studies Comparison
- Progress in Analytical Tools for Severe Accident Analysis
- Existing Severe Accident Analyses for Temelín
- Temelín EOPs - Severe Accident Prevention
- Westinghouse Owners Group (WOG) Approach to SAMGs
- Temelín SAMGs Development & Implementation
- Concluding Positions of Temelín NPP and SÚJB The approach to Severe Accident Management of the Temelín Nuclear Power Plant is to rely on a systematic process, which has been established and used by the Westinghouse Owners Group (WOG) for the development, implementation and maintenance of Severe Accident Guidelines.
The presentations provided insight into the extensive work accomplished by the plant operator and its technical support organisations to consolidate the SAMGs in the framework of the implementation of emergency procedures.
The descriptions identified the approach taken, but as overviews they provided only limited insight into the results and how these were obtained. A number of the questions posed by the Specialists’ Team were considered by the Czech side to exceed the level of detail or the scope of the Roadmap Workshop activities. Consequently, both sides agreed that the pertinent Czech-Austrian Bilateral Agreement is the appropriate framework giving the opportunity for further discussion and sharing additional information on these issues.
From technical point of view the assessment of SAM addresses all the elements and aspects, which are recognized important in the preparation and development of SAM program.
These include supporting accident analysis, assessment of plant vulnerabilities, selection of AM strategies, evaluation of plant equipment and instrumentation, development of accident management procedures and guidelines (SAMGs), and their implementation (including the SAMG verification and validation, related staffing and qualification, training, and co-ordination with emergency preparedness plan).
30 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues The project includes all related activities such as the identification of information sources for plant specific data, which are needed for the assessment, analysis of the reference material provided by the plant, and evaluation of the current plant status against the state-of-the-art practice.
Gathering appropriate information on the plant status with regard to the above-indicated areas is an essential part of the project. The main concept implemented in the project was to break down the overall subject into the line items, which could then be verified for completeness and compliance with the accepted international practice. They are further called Verifiable Line Items (VLIs).
The first step of the project (project milestones 1) focused on the definition of VLIs. This task was the “road map” for the whole project. The VLIs were identified considering both the state-of-the-art practice in the subject and the available knowledge on the plant status. Information on the plant status was gathered from the technical documents and publications on the Temelín NPP, previous studies conducted within the framework of ‘Melk Process’, and the results of accident calculations conducted by the project team for the Temelín NPP as well as the available results for similar plants.
The second step (project milestones 2), the Specifique Information request (SIR) considered to contain the kind of information required to provide profound answers to the VLIs.
The third step (project milestones 3) was intended to complete all the preparatory activities for the workshop (PM 3). This included the benchmarking of information/documents provided by the Czech side during the workshop against the state-of-the-art consolidated practice.
VLIs formulated in the Task 1 were used for this purpose. The scope of this task also included the development of briefing material and preparation of the briefing session for the Austrian delegation.
Project milestones includes the preparation of a preliminary monitoring report (PMR) on the status of SAM at Temelín (PM 4). This task was conducted based on the results of the Prague workshop. This report is also intended to identify SAM related issues for further monitoring by Austrian Government.
At the time of the Specialists Workshop, the SAMGs were in the process of translation by the operator for training and implementation in 2004. Further monitoring should therefore focus in some detail on the SAMG implementation process, including further attention to some specific plant design changes (changes to primary coolant system depressurization options, core debris cooling procedures, core debris spill management, etc.), announced at the Specialists Workshop, as part of SAMG implementation.
Further tasks (project milestones 5 – 7) will concentrate on the consolidation of findings and their presentation in the form of final reports (Final Monitoring Report and Summary Monitoring Report).
1.2 Technical Background
The Temelín nuclear power plant (NPP) is a two-unit facility originally designed as WWERpressurised water reactors according to the standards of the former Soviet Union.
Following the "Velvet Revolution", the plant design was upgraded (including implementation of fuel and instrumentation & control equipment delivered by Westinghouse) and placed into operation beginning with Unit 1, which began startup testing in 2001.
NPPs are designed based on the occurrence of certain initiating events such as loss of coolant accidents (LOCAs), transient events (such as loss of offsite power or loss of feedwater), and external man-made and natural phenomena hazards (such as fires, earthquakes, flooding, and the like). In plant safety analyses reports (SARs), the plant response to such "design ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 31 basis accidents" (DBAs) is evaluated assuming a single active failure in the safety system response, and the performance of the plant is evaluated to ensure that basic safety criteria are met. Such SAR assessments are performed with a substantial measure of conservatism, including the assumption of a degree of core damage that is far out of proportion to the actual circumstances, which are most likely to exist in reality. Prior to the end of the 1970s, it was widely believed that the likelihood of a severe accident involving actual core damage in commercial NPPs was extremely low (i.e., less than one in a milion per year - that is, a frequency less than 10-6 per year).
Although severe accidents had been studied for some time already (including the first quantitative risk assessment of severe accidents in the 1975 "Reactor Safety Study", WASH-1400), the issue of severe accidents gained increased international significance following the severe accidents at the Three Mile Island Unit 2 reactor in 1979 and the Chornobyl Unit 4 reactor in
1986. As a consequence of these accidents, increased application of probabilistic safety assessment (PSA), and the results of international research and development programmes, the issue of accident management (AM) gained prominence in the 1990s.
Severe accident management (SAM) has a formal, internationally recognized definition and framework [NEA 96]: "SAM consists of those actions that are taken by the plant staff during the course of an accident to prevent core damage, terminate progress of core damage and retain the core within the vessel, maintain containment integrity, and minimize off-site releases. SAM also involves pre-planning and preparatory measures for SAM guidance and procedures, equipment modifications to facilities procedure implementation, and severe accident [management] training. The overall objective is to further reduce the risks of large releases. It is the responsibility of the licensees to develop and implement a SAM program."
SAM plays an important role in the defence-in-depth concept for accidents, which exceed the design basis. SAM and so-called "complementary measures" define Level 4 of defence-indepth, which represents an attempt to bring severe plant conditons under control and/or to mitigate a radioactive effluents release which may nonetheless occur despite the capabilities of plant systems and structures and the best efforts of the plant staff. "Complementary measures" are defined as plant systems, structures, or components (SSCs) which have been added to the design of the plant beyond the normal complement of SSCs required to prevent or mitigate design basis accidents (DBAs), and whose function is related to severe accident prevention or to the mitigation of the consequences of severe accidents (that is, related to accident management).
The last level of defence-in-depth, Level 5, consists of the implementation of pre-planned offsite emergency response measures to reduce the consequences of severe accidents.
In the context of the Temelín Road Map, the issue of severe accidents and severe accident management was raised during the Melk Protocol process. When the Czech Republic and the Republic of Austria jointly issued the Melk Concluding Statement and the Road Map, the issue of severe accidents and their management was specified for further technical exchange.
1.3 Technical Approach – The Horizontal and Vertical Approaches