«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
Does it include the subcriticality, restoration of core cooling, protection of RCS integrity, treatment of combustible gases, protection of containment integrity and minimization of radioactivity release?
3.1.2 Do the selected strategies reflect the current international knowledge and practices?
3.1.3 Are there any specific generic (WOG) strategies that have not been considered in the plant-specific SAM? If so, what is the justification for such decisions?
3.1.4 Are the objectives and criteria clearly defined for each of the strategies considered in the plant-specific SAM?
3.1.5 What are the qualitative and quantitative goals of the severe accident management strategies implemented at Temelín?
State-of-the-art requirements and practices Selection of strategies and their descriptions is the first step in the development of SAMGs.
High-level WOG generic AM strategies cover all relevant functions (subcriticality, restoration of core cooling, protection of RCS integrity, treatment of combustible gases, protection of containment integrity, minimization of radioactivity release).
In the majority of the plants these high level strategies have been found applicable. Differences may be observed in the lower level strategies reflecting the availability of plant specific equipment and related instrumentation in a particular accident sequence. Specific plant features may also affect the priorities given to certain strategies versus other.
Current plant status NPP Temelín adopted the WOG SAMG approach. The WOG high level strategies have been selected based on the available information including EPRI SAMG Technical Basis report which defines 13 Candidate High level Actions, NRC reports: NUREG/CR 5474, -5780, -5856, IPE insights from several WOG utilities, Swedish BERGs, other plant PSAs and severe accident analyses, and plant specific information.
The high level strategies described in the WOG material [WOG 01] are applicable to Temelín NPP. The only procedure that is omitted in Temelín in comparison with the standard WOG SAMGs is SAG-8 “Flood Containment”, which is simply impossible to fulfil in view of the specific design of WWER 1000 containment. The lower level strategies have not been discussed in detail during the workshop. Information was limited to the specification of equipment that is to be applied in the implementation of strategies.
54 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues There are 7 Severe Accident Guidelines (SAG) that are entered first at onset of core damage and 4 Severe Challenge Guidelines (SCG) that are entered when there are severe challenges
to the containment integrity. The following strategies have been defined for Temelín NPP:
SAG-1 – Inject into the Steam Generators SAG-2 – Depressurize the RCS SAG-3 – Inject into the RCS SAG-4 – Inject into the containment SAG-5 – Reduce Fission product Releases SAG-6 – Control containment conditions SAG-7 – Reduce containment hydrogen SCG-1 – Mitigate Fission Product Releases SCG-2 – Depressurize Containment SCG-3 – Control Hydrogen Flammability SCG-4 – Control Containment Vacuum.
Evaluation The SAM strategies developed for Temelín are based on Westinghouse Owner Group (WOG) SAMGs, developed in close cooperation with Westinghouse, taking into account the specific features of the Temelín NPP. As the WOG SAMGs are well established and implemented successfully in many plants, the situation in Temelín is considered to be fully satisfactory in this respect.
3.2.2 Adequacy and Completeness of the Review of Plant Design Capabilities
VLI No. VLI title / description 3.2.1 Has systematic analysis been performed for identification of alternate equipment able to accomplish SAM related functions (pressure and flowrate for pumps, available water sources, power supply, etc.)?
3.2.2 Has systematic analysis been performed regarding the equipment performance outside its design range?
3.2.3 Has the influence of support system failures been properly evaluated (cooling water, power supply, etc.)?
3.2.4 Have potential design modifications of the existing plant equipment been identified in relation to SAM?
State-of-the-art requirements and practices Important step in the definition of plant specific SA defence-in-depth approach is a review of the plant-specific capabilities to perform basic severe accident recovery strategies (secondary side feed, RCS injection, RCS depressurisation, containment water addition and depressurisation, hydrogen control, etc.). The objective of review is to identify all possible means (systems, structures, components) for achieving safety objectives, even those involving the use of equipment outside its original design envelope. At the same time major equipment limitations (for example shutoff differential pressure for injection systems, maximum achievable flow rates, depressurisation efficiency and effectiveness, etc.) should be identified.
The definition of symptoms, and the associated plant process parameters, which must be monitored in order to detect and rank potential challenges and prioritize them for corrective actions is an important next step. It represents the formulation of the basic objectives of the ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 55 strategies in terms of the safety functions to be protected or restored, if perturbed (or challenges to be prevented). Strategies must then be developed which provide all practical means to protect the safety functions. During this phase it is important to clearly and unambiguously define specific criteria such as entry and exit conditions, diagnostic symptoms, etc. Efforts should also be made to demonstrate that the list of strategies developed is complete. An appropriate verification and validation programme must check the ‘correctness’ and ‘usability’ of the strategies.
Current plant status A Westinghouse team in close co-operation with the competent plant staff conducted the review of plant specific capabilities. All relevant steps as described above have been followed [Dessars 03, Sỷkora 03 b].
Evaluation WOG SAMG adaptation to Temelín NPP was similar to adaptation to any Westinghouse designed PWRs. The plant specific capabilities to implement the strategies have been identified and properly considered in the SAMGs. Symptoms that must be monitored and specific criteria used in the guidelines are clearly defined.
A team that has considerable experience in the area of SAMGs performed the adaptation to Temelín NPP.
3.2.3 External Information to Support the Development of SAM Programme VLI No. VLI title / description 3.3.1 Have all needs for the use of external equipment and related input been determined?
3.3.2 Was the external input made available for the development of SAM?
State-of-the-art requirements and practices The identification of possible requirements to bring in the equipment from outside the plant should also be addressed. In some areas such equipment can easily be used increasing the number of alternative AM options and reducing the related risk contribution. Information on the availability of such equipment and related time element as well as the equipment capabilities should be realistically assessed.
Current plant status No external equipment was explicitly mentioned in the adopted SAM strategies.
Evaluation The potential for using some equipment from outside the plant for SAM should be addressed in finalizing SAM strategies, and could be the subject of a discussion at a Czech-Austria nuclear issues bilateral meeting.
56 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues
3.2.4 Formal Definition and Documentation of SAM Strategies
VLI No. VLI title / description 3.4.1 Is basic information on the strategies completed within the AMP (including entry and exit conditions, equipment/systems used, diagnostic means and tools)?
3.4.2 Have the selected AM strategies been formally documented?
3.4.3 Has the effectiveness of the strategies been proven by specific accident analyses?
3.4.4 Has an independent review of the AM strategies been conducted?
Who performed the review and which documents were subject to the review?
What are conclusions/recommendations from the review?
State-of-the-art requirements and practices The selected strategies should be documented. Typically, this document is considered a part of SAMG-related background material. It provides comprehensive description of the strategy, its purpose/objectives, and identification of interfaces with the diagnosis tools and specific guideline (including entry and exit condition).
Plant specific equipment/systems used for achieving safety objectives should be discussed.
Basic characteristics of component/system and support systems needed for its operation should be provided. This discussion should include major equipment limitations (for example shutoff heads for injection systems, maximum achievable flow rates, depressurisation capacity, etc.) and survivability of equipment (see Section 4 for further discussion of this aspect). Possible negative impacts of implementing the strategy using available equipment should also be identified.
This background material should also provide information on the relevant plant specific instrumentation used as diagnostic means and tools for the specific strategies. Basic characteristics of instrumentation including the redundancy and measurements range should be provided. Survivability of instrumentation and reliability of the readings in severe accident conditions should also be discussed.
Current plant status The documentation of Temelín SAMGs has been described in the workshop presentations [Sỷkora 03 b]. It is composed of 6 Volumes. Volume 1 includes all guidelines used at the TSC (DFC, SAGs, SCST, SCGs, SAEGs, and CAs). Volumes 2 and 3 provide background material for DFC and SAGs (Volume 2) and for SCST, SCGs, SAEGs, and CAs (Volume 3).
Volume 4 includes general user’s guidelines, technical basis for setpoints, description of diagnostics and instrumentation, and description of SAMG strategies. Volume 5 provides information relating to interfaces with E-plan and EOPs, and a description of plant capabilities to be used in SAMGs. Volume 6 includes control room guidelines (SACRGs) with the related background.
Evaluation The SAMG documentation developed for the Temelín plant staff was found to be suitably structured. The subjects addressed in this documentation are those, which would typically be expected for SAMGs based on the Westinghouse Owners Group approach. Since the SAMG documentation was not made available for review, no assessment of the completeness of the information, the quality of presentation, and the usability of the whole SAMG package are possible.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 57
3.3 Performance of Equipment
Performance of equipment, which is involved in AM, is one of the relevant aspects that should be considered in the development of SAM programme and SAMGs. Related issues are discussed in the following subsections.
Sections 3.3.1 and 3.3.2 address general aspects related to the quality of safety equipment.
These general aspects are relevant for any accidents and included here to provide a broader safety perspective. Equipment performance in severe accident conditions is the key issue addressed in Section 3.3.3. Severe accident aspects relating to monitoring of the plant in severe accident conditions and related instrumentation are discussed in Section 3.3.4. Some aspects relating to initiation of equipment operation are addressed in Section 3.3.5. Provisions for preserving plant control capabilities in severe accident conditions are covered in Section 3.3.6.
3.3.1 Required Quality of Equipment Involved in Accident Management
VLI No. VLI title / description 4.1.1 Is there a systematic program at the plant to select reliable manufacturers?
4.1.2 Were the QA systems, used by equipment manufacturers, subject to verification?
4.1.3 Has the operational QA program at the plant been subject to independent peer review?
Who performed the review? Have the recommendations been implemented?
State-of-the-art requirements and practices A systematic programme should be established at the plant for controlling the procurement process. This is normally covered in the operational QA plant programme. Reference bases for the assessment of plant QA status can be found in the Code and in Safety Guide Q6 of the IAEA Safety Series 50-C/SG-Q [IAEA 88, IAEA 96]. Assessment of the plant status relating to this area is beyond the scope of this project. Only some general QA aspects that are relatively easy to verify can be addressed.
Specific interest should be given to a two-step approach to the procurement of safety related items that includes the qualification of the supplier and the evaluation/approval of any individual purchase of items or services. In this approach the supplier should be in an approved list, having a QA programme compliant to the QA requirements. QA system implemented by equipment manufacturers included in this list should be subject to independent verification.
Procurement of safety related items should be arranged using only the qualified suppliers.
Any individual purchase should be further evaluated and approved prior to placing a purchase order. The classification of items and services with regard to safety used at the plant is one of the related aspects.
Independent reviews of the operational QA programme at the plant should regularly be conducted. Recommendations from such reviews should be evaluated and appropriate actions implemented.
Current plant status Design and manufacturing quality and the operational care is specifically mentioned by the plant staff in the Prague workshop presentations as one of the important elements of the ‘defense-in-depth’ concept implemented at Temelín [Sỷkora 03 a]. However, no detailed information is available regarding the operational QA programme.
58 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues Evaluation No evaluation was possible with regard to this aspect. This aspect should be addressed within an independent review of QA programmes.