NUREG/CR -4 537 SAND86 - 045 1 RV I * Printed September 1986 e - . / Summary Report: Electrical Equipment Performance Under Severe Accident Conditions (BWR/Mark I Plant Analysis) P. R. Bennett, A. M. Kolaczkowski, G. T. Medford Prepared by Sandia National Laboratories Albuquerque, New Mexico 87 185 and Livermore. California 94550 for the United States Department of Energy under Contract DE-AC04-76DP00789 Prepared for U. S. NUCLEAR REGULATORY COMMISSION NOTICE This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, or any of their em- ployees, makes any warranty, expressed or implied, or assumes any legal liability or res onsibility for any third party’s use, or the results of such use, ora ny information, apparatus product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights. Available from GPO Sales Program Division of Technical Information and Document Control US. Nuclear Regulatory Commission Washington, D.C. 20555 and National Technical Information Service Springfield, Virginia 22161 NUREG/CR-4537 SAND86-0451 RV SUMMARY REPORT: ELECTRICAL EQUIPMENT PERFORMANCE UNDER SEVERE ACCIDENT CONDITIONS (BWR/MARK I PLANT ANALYSIS) P. R. Bennett Sandia National Laboratories A. M. Kolaczkowski G. T. Medford Science Applications International Corporation September 1986 Sandia National Laboratories Albuquerque, NM 87185 Operated by Sandia Corporation for the U.S. Department of Energy Prepared for Division of Engineering Technology Electrical Engineering and Instrumentation Control Branch Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington, DC 20555 Under Memorandum of Understanding DOE 40-550-75 NRC FIN No. A1382 . ABSTRACT The purpose of the Performance Evaluation of Electrical Equipment during Severe Accident States Program is to deter- mine the performance of electrical equipment, important to safety, under severe accident conditions. In FY85, a method was devised to identify important electrical equipment and the severe accident environments in which the equipment was likely to fail. This method was used to evaluate the equip- ment and severe accident environments for Browns Ferry Unit 1, a BWR/Mark I. Following this work, a test plan was written in FY86 to experimentally determine the performance of one selected component to two severe accident environ- ments. Specifically, equipment important to safety for a BWR was identified--equipment which could mitigate a severe accident or provide monitoring information on plant status. Of this list of equipment, only that located in the primary contain- ment or reactor vessel of Browns Ferry Unit 1 was analyzed further. For five selected BWR severe accident sequences (TB, TC, TW, TQUV, and AE), environmental conditions within containment reached temperatures and pressures exceeding the current equipment qualification testing requirements prior to or during the time the equipment was needed. The results of this analysis suggest the need for testing equipment important to safety to assess performance under severe acci- dent conditions. In particular, the performance of the pneu- matic control manifold assembly (part of the main steam isolation valve equipment assembly) should be tested in the severe accident environments resulting from the TC and TW accident sequences. In addition to writing a test plan for the pneumatic control manifold assembly, a number of important insights are dis- cussed in the areas of accident management, emergency plan- ning, probabilistic risk assessments, probability and risk reduction, and current equipment qualification requirements. These insights help illustrate how the environmentally- induced failure of certain equipment during a severe accident may adversely impact the ability of a nuclear power plant to cope with severe-accident conditions. However, without test- ing to confirm the actual limits of equipment survivability, the safety importance of the insights cannot be assessed or addressed, -iii/iv- - - - CONTENTS Paqe . . . . . . . . . . . . . . . . . . . Executive Summary 1 . . . . . . . . . . . . . . . . . . . . 1 Introduction 6 . . . . . . . . 1.1 Purpose of the PEEESAS Program 6 1.2 Prior Efforts. PY85 Work. and Plant . . . . . . . . . . . . . . . . . . . . Choice 7 . . . . . . . . . . . . . . . . . 2 Accident Sequences 9 . . . . . . . . . 2.1 Accident Sequence Selection 9 2.2 Likely Scenarios for the Five Accident . . . . . . . . . . . . . . . . . . Sequences 9 . . . . . . . . . . . . . . . . 3 Electrical Components 13 . . . . . . 4 Environmental Profiles for Each Scenario 15 . . . . . 4.1 Environmental Parameters Considered 15 4.2 Pressure and Temperature Profiles for Each . . . . . . . . . . . . . . . . . . . Scenario 16 . . . . . . . . . . . . . 4.2.1 TB Short Term 16 . . . . . . . . . . . . . 4.2.2 TB Long Term 17 . . . . . . . . . . . . . . . . . . 4.2.3 Tw 17 . . . . . . . . . . . . . . . . . . 4.2.4 TC 18 . . . . . . . . . . . . . . . 4.2.5 TQW . . 19 . . . . . . . . . . . . . . . . . . 4.2.6 AE 19 . . . 4.3 Typical Equipment Qualification Profiles 20 4.4 Comparison of the Pressure and Temperature Profiles for Each Scenario to the Typical . . . . . . . Equipment Qualification Profile 20 . 5 Reduction of Equipment and Environments to Select . . . . . . . . . . . . . . . . . Test Candidates 23 . . . . . . . . . . . 5.1 Screening and Ranking 23 5.1.1 Time Equipment Demanded and Environments Exceeding Qualification . . . . . . . . . . . . . . . Levels 23 . . . . . . . . 5.1.2 Functional Importance 25 . . . . . . . . . . . . . . . 5.1.3 Ranking 28 5.2 Importance of Selected Test Candidates to . . . . Probabilistic Risk Assessments (PRA) 28 5.2.1 Specific Effects of Environmentally- Induced Equipment Failure on Accident . . . . . . . . . . . . . . Sequences 32 5.2.2 Effect of Environmentally-Induced Equipment Failures on the Total . . . . . . . . Core Melt Probability 33 5.2.3 Resulting Test Candidates and . . . . . . . . . . . . Environments 33 5.2.4 PRA and Emergency Preparedness . . . . . . . . . . . . . . Insights 34 -v- .I----.-. ......l_. ..... I.. ....... ......................... ..-.-___. ___- . CONTENTS (Continued) Paqe . . . . . . . . . . . . . . . . . . . 6 Test Plan Input 35 . . . . . . . . . 6.1 Choosing the Test Candidate 35 . . . . . . . . . . . . 6.2 Expected Failure Modes 35 . . . . . . . . . . . . . . 7 Summary of the Test Plan 38 . . . . . . . 7.1 Sample Description and Mounting 38 . . . . . . . . . . . . . . . . 7.2 Test Strategy 38 . . . . . . . . . . . 7.2.1 Aging Simulation 38 . . . . . . . . . . 7.2.2 Accident Simulation 40 . . . . . . . . . . . . . 7.3 Acceptance Criteria 45 . . . . . . . . . . . . . . . 7.4 Test Facilities 45 . . . . . . . . . . . . . . 8 Conclusions and Insights 46 . . . . . . . . . . . . . . . . . 8.1 Conclusions 46 . . . . . . . . . . . . . . . . . . . 8.2 Insights 46 . . . . . . . . . . . . . . . . . . . . . 9 References 48 Appendix A: Likely Scenarios for Five Accident Sequences Appendix B: Identification of Electrical Equipment Appendix C: Generation of Environmental Profiles for Selected Accident Sequences Appendix D: Data Analysis and Final Test Recommendations Appendix E: Test Plan for MSIV Pneumatic Control Manifold Assembly -Vi- LIST OF FIGURES Figure Paae . . . . . . . . . 1 Typical Mounting Configuration 39 . 2 Drywell Temperature Profile for TC (MSIV Open) 41 . 3 Containment Pressure Profile for TC (MSIV Open) 42 . . . . . . . 4 Drywell Temperature Profile for TW 43 . . . . . . . 5 Containment Pressure Profile for TW 44 LIST OF TABLES Table Paqe . . . . . . . . 1 Summary of BWR Accident Sequences 10 . . . . . . . . . . . . 2 Likely Accident Scenarios 12 3 Components Recommended for Further . . . . . . . . . . . . . . . . . . . Examination 14 4 Comparison of Severe Accident Profiles to . . . . Typical Equipment Qualification Profiles 22 5 Profiles Where Equipment Must Operate Under the . . . . . . . . . . . . . Most Severe Conditions 24 . . . . . . . 6 Ranking of Environmental Parameters 26 7 Environmental Importance: Tabulated Results . . . . . . . . . . . for Each Piece of Equipment 27 . . . . . . . . . . . 8 Functional Ranking Results 29 9 Functional/Environmental Ranking . . . . . . . . . . . . . . . . . . . Comparisons 30 . . . . . . 10 Failure Modes for the Test Candidate 36 EXECUTIVE SUMMARY The purpose of the Performance Evaluation of Electrical Equipment during Severe Accident States (PEEESAS) Program is to determine the performance of important electrical equip- ment under severe accident conditions. Important electrical equipment is defined as electrical equipment that is impor- tant to safety. This includes equipment used to mitigate an accident or provide information on the status of the plant. Specifically, this program will 1. Devise a method to identify important electrical compo- nents and the severe accident environments in which they are likely to fail, 2. Use the method to analyze equipment performance for nuclear power plants, 3. Test the performance of selected components to the severe accident environments to determine performance, and 4. Provide the results of equipment performance to opera- tors, emergency planning teams, probabilistic risk assessment analysts, and the Nuclear Regulatory Commis- sion to influence actions and decisions. During FY85 and FY86, a method was devised to answer ques- tions on the performance of electrical equipment under severe accident conditions. This method provided the means to identify important electrical components and the severe accident environments in which they are likely to fail. Also during FY85, Browns Ferry Unit 1 (BWR/Mark I) was chosen to be the first nuclear power plant analyzed. For this plant, the following areas were investigated: (1) acci- dent sequences (including operator actions that are likely to occur during those sequences), (2) important electrical equipment in the primary containment, (3) environmental profiles, (4) important electrical equipment that will be subjected to environments beyond their current qualification levels, and (5) test plan for the selected equipment. Accident Sequences and Likely Scenarios The five accident sequences chosen for this study are as follows: TB (station blackout including loss of all AC power), TC (anticipated transient without scram), TW (transient with loss of long-term heat removal), TQW (transient with early loss of core cooling), and AE (large- break LOCAs with early loss of core cooling). The selection of BWR/Mark I accident sequences was based on the following criteria: (1) sequences with high probability, (2) sequences with high risk, (3) sequences with the potential for extreme environments, and (4) sequences with operator action required. Likely scenarios are series of events that are most likely to happen during an accident sequence based on operator actions, timing of system failures, and automatic system actuation. These likely scenarios were used to identify (1) failed equipment by accident sequence definition, (2) equipment assessed to succeed and additional equipment needed to mitigate or provide plant status, and (3) boundary conditions for determining the environmental profile for each accident sequence. Fourteen likely scenarios resulted from the five selected accident sequences. The likely scenarios included the following variations: operator depressurization of the reactor pressure vessel, no operator action, and stuck-open relief valve. Equipment From an initial list of BWR equipment important to safety and from a review of the Browns Ferry Unit 1 design, equip- ment was identified that was important to safety and was located in the primary containment or reactor vessel. (This equipment is generally in the most severe environment.) From qualitative arguments considering the possible impor- tance of the equipment in mitigating or assessing the status of the plant for each selected accident sequence, the fol- lowing equipment was identified: inboard main steam isola- tion valves (MSIV); inboard high pressure coolant injection (HPCI) and reactor core isolation cooling (RCIC) isolation valves; safety relief valve (SRV) pilot and service air solenoid valves; residual heat removal (RHR) shutdown cooling valve; in-core and reactor vessel surface thermocouples; drywell temperature element (RTD); drywell pressure monitor; and drywell hydrogen and radiation monitors. Environmental Profiles Environmental profiles of the primary containment were developed for each selected accident sequence. The follow- ing parameters were considered: humidity, submersion, spray, radiation/aerosols. vibration, pressure, and tem- perature. Pressures and temperatures were determined, for each likely scenario, from MARCH and LTAS computer codes. These severe accident environmental profiles were compared to typical equipment qualification profiles to identify areas where the severe accident environmental profile exceeded the equipment qualification profile of IEEE 323-1974. Appendix A. -2- Reduction of Equipment and Environments The list of equipment and environments was reduced in two steps. Step 1 identified the time that equipment was demanded and whether the severe accident environments exceeded the equipment qualification levels prior to or during that time. Step 2 determined the relative functional importance of the equipment. In Step 1, equipment was eliminated if the severe accident environments were below that of the typical qualification environment. Then the severe accident environments were further reduced by retaining only those profiles with (1) maximum pressure or temperature or (2) maximum time above the maximum pressure or temperature for the typical equipment qualification profile. (These results represent profiles where the equipment must operate under the most severe conditions for the five selected accident sequences.) In Step 2, the relative functional importance of the equipment was based on the following criteria: redundancy, backup systems, non- complexity, electrical independence, fail-safe position appropriate, plant status indication only, and separation. The equipment is less functionally important if the equipment meets these criteria. The equipment remaining after Steps 1 and 2 is the main steam isolation valves (MSIV) and the safety relief valves (SRV). This equipment was required to operate during the TC and TW accident sequences. To choose the first test candidate, the effect of an environmentally-induced failure of the MSIV or SRV (for the TC and TW accident sequences) on probabilisric risk assessments was determined. From a PRA perspective, both the MSIV and the SRV are good test candidates. But for the first test candidate, the MSIV equipment assembly was chosen because (1) failure of the MSIV may increase the core melt probability as well as increase the risk and (2) the performance of the MSIV may be tested in more than one accident environment. Several pieces of equipment are associated with the MSIV equipment assembly. The pneumatic control manifold assembly was chosen to be the first test candidate because it is required to operate the MSIV globe valve, it provides a large heat rejection path, and it is a complex electrical component. Testinq The performance of the pneumatic manifold assemblies will be evaluated for both the TC (with the MSIV initially open) and the TW accident sequences. (Two manifold assemblies will be tested--one for each accident profile.) Moisture intrusion, due to a combination of moisture and high temperature or pressure, is the dominant failure mechanism for the manifold assembly. -3-