LOCA with Consequential or Delayed LOOP: Modeling of Accident Sequences and Associated Core Damage Frequency

• Published in: Nuclear technology 2000-09, Vol.131 (3), p.297-318
• Main Authors: Martinez-Guridi, Gerardo, Samanta, Pranab, Chu, Tsong-Lun, Yang, Ji-Wu
• Format: Article
• Language: English
• Subjects: Applied sciences; COOLANTS; DAMAGE; ELECTRIC POWER; Energy; Energy. Thermal use of fuels; ENGINEERING; EVALUATION; Exact sciences and technology; FAULT TREE ANALYSIS; FERMILAB COLLIDER DETECTOR; Fission nuclear power plants; GENERAL STUDIES OF NUCLEAR REACTORS; Installations for energy generation and conversion: thermal and electrical energy; LOSS OF COOLANT; NUCLEAR POWER PLANTS; PROBABILISTIC ESTIMATION; REACTOR SAFETY; RISK ASSESSMENT; SIMULATION
• ISSN: 0029-5450; 1943-7471
• DOI: 10.13182/NT00-A3118

Following a loss-of-coolant accident (LOCA) in a nuclear power plant (NPP), the loss of electric-power generation, as might be precipitated by the unit tripping, may cause switchyard- and grid-instability with a subsequent loss-of-off-site power (LOOP). The LOOP usually is delayed by a few seconds or longer. This accident is called a LOCA with consequential LOOP, or a LOCA with delayed LOOP (abbreviated as LOCA/LOOP). NPPs are designed to cope with simultaneous LOCA and LOOP. The U.S. Nuclear Regulatory Commission (NRC) identified this issue as generic safety issue (GSI) 171, "Engineered Safety Feature Failure from a Loss-Of-Off-Site Power Subsequent to a Loss-of-Coolant Accident." NRC subsequently dropped GSI-171 and considers it resolved. We present the probabilistic risk analysis of the LOCA/LOOP scenario that was conducted as part of NRC's resolution of GSI-171. We analyze and quantify the core damage frequency (CDF) associated with this accident. Event/fault trees are developed covering the progression of the accident to core damage. We used engineering evaluations and judgments to estimate probabilities for the conditions identified in a LOCA/LOOP scenario and to obtain a bounding evaluation of the CDF. We show that the contribution of such an accident to CDF depends on electrical-load sequencing and shedding capabilities; plants with adequate capabilities incur a minimal additional contribution to risk. No single plant design is known to be vulnerable to all the conditions; only some of the conditions may apply to some plants.