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«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»

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The monitoring process conducted so far within the framework of the Temelín Roadmap in the area of severe accidents helped to clarify a number of relevant issues. It was demonstrated that a comprehensive process directed towards accomplishing the comprehensive SAM and mitigation of SA consequences is in place at Temelín NPP. However, this process is ongoing and the Specialist’s Team at the present can only follow a number of views and expectations on the SAMs final implementation as expressed by the Czech side.

Based on the recognition that the pertinent Czech-Austrian Bilateral Agreement is the appropriate framework giving the opportunity for further discussion and sharing additional information on these issues, the Specialist’s Team would appreciate if the major findings could be revisited in the further monitoring process of the SAM. The following areas were identified as those,

where additional information would be most valuable to consolidate the Monitoring results:

• The supporting severe accident analysis and PSA as well as their use in the verification of SAM strategies and the related procedures,

• SAMG implementation activities including procedural framework, SAMG validation, and SAM related staff training,

• Identification of the permissible degree of non-uniformities in the hydrogen distribution in the atmosphere,

• Implementation of plant changes to enhance the technical measures for SAM.

More detailed discussion of the proposed monitoring issues in these areas is provided below.

Calculations to be made by the operator/regulator are recommended on the technical level to be monitored jointly in the framework of the pertinent bilateral Agreement between Austria

and the Czech Republic:

The calculations will be performed using the MELCORE 1.8.5 Version and are supposed to

provide results on the following topics:

- Confirmation of capability of PORV to reliably depressurize the RCS (if its reliability at low pressure is lower than that announced during the Prague meeting, the credit taken for EGRS operation should be checked).

- Effectiveness of PAR recombiners under various severe accident conditions and related final states to be reached with SAM strategies planned in Temelín.

- Additional use of filtered venting for mitigation of radioactive releases.

- Filtered venting system operation, in particular the resistance of filters to heating due to the accumulation of fission products in those filters.

- Completeness of the analyses of corium attack on the basemat (the calculations should be carried out up to the moment of final basemat failure and include radiological consequences of this scenario).

ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 135 SAMG implementation activities at Temelín should be further monitored to confirm that the remaining steps of the implementation process are successfully completed. Important items

that need further monitoring/verification include:

- Confirmation that the revised procedural framework is implemented (E-plan, Emergency Implementation Procedures, and Emergency Operating Procedures).

- Confirmation that the responsibilities and authorities for intentional release of radioactive effluents in emergency conditions (as one of the SAM strategies) are clearly defined and understood within the overall ER organisation.

- Extent and the results of the SAMG validation process including observations on the sufficiency of staffing and organisational structure of the TSC (SAMG Evaluation Group) and the related feedback.

- Extent / scope of staff training process in the area of SAM and the related feedback.

- The recommendations from any independent review of SAM and their resolution, if made available by the plant.

Technical measures needed for prevention and mitigation of risk significant scenarios should be monitored to demonstrate that appropriate plant arrangements are in place (both procedures and hardware measures). Due attention should be given to SA situations that are most relevant from safety point of view such as basemat penetration in case of molten corium release from the RPV and station blackout. The following aspects are worth to be mentioned in

this context:

- Timely opening the rector cavity door before the RPV failure,

- Installation of removable shielding walls to restrict molten corium pool area and protect the containment liner and other barriers against molten corium and missiles hazards,

- Protection of containment penetrations and reactor cavity instrumentation channels against molten corium penetration.

- Increasing capacity of batteries so that they would be available throughout the blackout accident at least until RPV failure

- Assuring possibility of fire truck water deliveries in case of total blackout on the site.

- Evaluation of possibilities to stop the MCCI,

- Estimates of possibilities of hydrogen burn in case of basemat failure and of resulting revolatilization of fission products deposited inside the containment

- Effectiveness of possible reduction of leakages through the rooms under the containment basemat.

136 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues

–  –  –

Maruyama 99 MARUYAMA, Y., et al., "Experimental Study on In-vessel Debris Coolability in ALPHA Program," Nuclear Engineering and Design, 187, (1999).

Miasnikov 03 MIASNIKOV, A., “Regulatory Approach to Accident Management”, State Office for Nuclear Safety (SUJB), presented at the Workshop on Severe Accident Related Issues, SUJB, Prague, Czech Republic, June 17 and18, 2003.

Mlady 01 MLADY, O., KUCERA, L., “PSA Insights for EPZ scenarios selection”, in:

Technical Basis for Temelín Emergency Planning Zones, Presentation to the Expert Mission with Trilateral Participation on the Temelín NPP Unit 1, Prague, 4 April 2001 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 139 Mlady 03 a MLADY, O., “Temelín NPP PSA Update Results Level 1 and Level 2”, ČEZ, Temelín NPP, presented at the Workshop on Severe Accident Related Issues, SUJB, Prague, Czech Republic, June 17 and18, 2003.

Mlady 03 b MLADY, O., “Existing BDB Analyses for Temelín NPP – PSA Insights for Analyses Selection”, ČEZ, Temelín NPP, presented at the Workshop on Severe Accident Related Issues, SUJB, Prague, Czech Republic, June 17 and18, 2003.

Morozov 03 Morozov V. et al.: Development of PSA-2 for NPPs with WWER 1000, (in Russian), Atomenergoproekt, Arch. No. 85.1, Moscov 2003 (Proprietary).

NEA 96 NUCLEAR ENERGY AGENCY, Implementing Severe Accident Management in Nuclear Power Plants, NEA-0252, Paris, January 1996.

NEA 2000-19 NUCLEAR ENERGY AGENCY, “PWG4 Perspective on Ex-Vessel Hydrogen Sources”, Report No. NEA/CSNI/R(2000)19, 7 July 2000, NEA 2001-15 NUCLEAR ENERGY AGENCY, “In-Vessel and Ex-Vessel Hydrogen Sources”, Report No. NEA/CSNI/R(2001)15, 1 Oct. 2001.

NRC 02 US NUCLEAR REGULATORY COMMISSION, “Combustible Gas Control in Containment, Proposed Rules”, Federal Register, August 2, 2002.

NRC 03 US NUCLEAR REGULATORY COMMISSION, Final Rulemaking – Risk-Informed 50.44, ‘Combustible Gas Control in Containment’, 10CFR50, 18 September 2003. RIN 3150-AG76 NRC 88 US NUCLEAR REGULATORY COMMISSION, “Individual Plant Examination for Severe Accident Vulnerabilities”, Generic Letter 88-20, 10CFR 50.54(f), November 1988.

NRC 90 US NUCLEAR REGULATORY COMMISSION, “Accident Management Strategies for Consideration in the IPE Process”, Generic Letter 88-20, Supplement 2, April 1990.

NRC 93 US NUCLEAR REGULATORY COMMISSION, “Evaluation of severe accident risks”, Report NUREG/CR-4551, US NRC, Washington D.C., USA (1993) NRC PRA 2000 US NUCLEAR REGULATORY COMMISSION, Probabilistic Risk Analysis Branch, Feasibility Study for a Risk-Informed Alternative to 10CFR 50.44 ”Standards for Combustible Gas Control System in LWR Power Reactors”, Attachment 2 to SECY-00-0198, August 2000 NUREG-0654 US NUCLEAR REGULATORY COMMISSION, “Criteria for Preparation and Evaluation of Radiological Emergency Response Plans and Preparedness in Support of Nuclear Power Plants”, NUREG-0654.

NUREG-0696 US NUCLEAR REGULATORY COMMISSION, “Functional Criteria for Emergency Response Facilities”, NUREG-0696.

NUREG-0818 US NUCLEAR REGULATORY COMMISSION, “Emergency Action Levels for Light Water Reactors”, NUREG-0818.

Pazdera 03 PAZDERA, F., et al., “WWER operational safety improvements:

lessons learnt from European co-operation and future research needs”, Nuclear Eng. and Design, 221, (2003), pp. 193-204.

Pilch 94 PILCH, M.M., et al., “The Probability of Containment Failure by Direct Containment Heating in Zion”, NUREG/CR-6075, 1994.

POSAR “Pre-operational Safety Analysis Report”, Temelín NPP RSK 94 Bekanntmachung von Empfehlungen der Reaktor-Sicherheitskommission Vom

15.Juni 1994, RSK-284-E1.

140 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues

–  –  –

Werner 94 Werner W.F., Insights from the Comparison of the Level-2 Results of recent PSAs.

International Meeting on PSA/PRA and Severe Accidents '94, Ljubljana, Slovenia, 1994.

WESE 96 WESTINGHOUSE ENERGY SYSTEMS EUROPE S.A., “VVER 440-213 Beyond Design Basis Accident Analysis ans Accident Management”, Project 4.2.7.a, WENX-96-18, June 1996 (Proprietary).

WESE 97 WESTINGHOUSE ENERGY SYSTEMS EUROPE S.A., “VVER 440-213 Bohunice V2 Reference Plant Analysis of BDBA and Severe Accidents Without Operators Actions” Design Basis Accident Analysis and Accident Management”, Project 4.2.7.a/93, WENX-97-25, September 1997 (Proprietary).

WOG 94 WESTINGHOUSE ELECTRIC CORPORATION, “Severe Accident Management Guidelines”, Rev. 0, Vols 1-3, Background documents, WOG Program, MUHP-2310, June 1994 (Proprietary).

WOG 01 WESTINGHOUSE ELECTRIC CORPORATION, “Severe Accident Management Guidelines”, Rev. 1, Vols 1-3, Background documents, WOG Program, MUHPProprietary).

142 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues

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VLI Verifiable Line Item WANO World Organization of Nuclear Operators WEC Westinghouse Electric Corporation WESE Westinghouse Energy Systems Europe S.A.

WOG Westinghouse Owners Group WWER Russian design of PWR 146 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues

–  –  –

In order to collect in a brief overview the rather heterogeneous AM approaches embarked on by the various countries operating NPPs we summarise here the main aspects of the stateof-the-art in severe accident management at NPPs in Western Europe. This should enable also to conclude about the European position of the SAM approach at Temelín NPP.

For this purpose the statements associated with the individual monitoring topics above will be taken into consideration in the first place and then the overall position versus the European environment will be judged as it is described here in brief.

Many of the SAM practices in Western Europe are based on the earlier developments in the United States, where the four groups of owners of nuclear power plants each developed and implemented their proprietary SAMG approach. Inasmuch as Temelín has selected the Westinghouse Owner's Group (WOG) SAMG methodology, the discussion that follows will focus on that aspect. Comments on other approaches will be made as necessary.

History of SAMG development

In 1985, the USNRC issued a "Severe Accident Policy Statement" in which, based on thenavailable information, it judged that the existing NPPs were acceptably safe in their thencurrent layout and configurations [NRC 85]. Nevertheless, the NRC set in motion a fourpronged "integration plan for closure of severe accident issues" that identified areas where

action by nuclear utilities was required [NRC 88 a]:

- Individual Plant Examination (IPE) for severe accident vulnerabilities

- Containment performance improvements (CPI)

- IPE of External Events (IPEEE) for severe accident vulnerabilities

- Accident Management (AM) The IPE and IPEEE programs were initiated in 1988 and 1991 [NRC 88 b, NRC 91]. All NPPs were required to perform IPE evaluations (for internal events) and IPEEE evaluations (for external events). (Level 1 PSA largely met the IPE & IPEEE program requirements with a containment performance evaluation or Level 2 PSA; some events, especially seismic, were analyzed by a margins approach without quantifying the core damage frequency contribution.) The CPI program was defined in 1990 [NRC 90 b]. For large dry containment PWRs, it was concluded that utilities with such plants should evaluate containment and equipment vulnerabilities to localized hydrogen combustion and the potential need for improvements (including accident management procedures) as part of the IPE program.

Concerning AM, after the NRC informed the utilities that it expected a certain level of severe accident management at their plants, the nuclear industry committed itself in November 1994 to SAM programmes consistent with industry guidance [NEI 94]. The NRC accepted the industry initiative on accident management on 9 January 1995 (via a letter to the Nuclear Energy Institute). Implementation of SAM programmes in the US was completed between June 1997 and December 1998. The US SAM guidance approach established by the industry and accepted by the NRC was focused on the establishment of SAM guidance that made maximum use of existing plant capabilities, and with only minor modifications. This was consistent with NRC's earlier finding in the Severe Accident Policy Statement in 1985 concerning the acceptability of the NPPs in their then-current configuration and layout [NRC 85].

150 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues

Implementation of US SAMGs

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