Innovative probabilistic risk assessment applications: barrier impairments and fracture toughness. 2. Demolition Debris and Tornado Missile Hazard During Decommissioning

During their operating lives, nuclear power plants typically maintain a high level of control over the amount of debris that is allowed to accumulate at the plant site. Although primarily intended to reduce the potential for fire damage, some plants also rely on these controls to limit the damage that could be caused during a tornado from missiles generated from loose debris. Demolition work associated with power plant decommissioning inevitably increases the quantity of debris. When bulk commodities such as piping and electrical distribution components are demolished, they are subject to various staging, handling, and storage processes before they can be released from the site. The demolition of plant structures dramatically increases the quantity of loose steel and concrete debris. For the foreseeable future, all plants that undertake decommissioning will have spent-fuel assemblies present on the plant site during the demolition project whether the spent fuel remains stored in a spent-fuel pool or is transferred to an independent spent-fuel storage installation (ISFSI). Under present regulations, protection from tornado missiles would be required for both types of spent-fuel storage. In addition, a small proportion of decommissioning plants will have operating units in close proximity. Licensing commitments for tornado missile protection may mandate controls on the production or storage of demolition debris. This paper presents a case study of the San Onofre Nuclear Generating Station (Fig. 1). Tornado missile protection licensing commitments from three types of facilities will be in force during the decommissioning of San Onofre Unit 1 (Unit 1): 1. Unit 1, under a possession-only license; 2. an ISFSI that will eventually store spent fuel from Unit 1; 3. San Onofre Operating Unit 2 (Unit 2) and San Onofre Operating Unit 3 (Unit 3). Together, these three facilities illustrate the range of impacts that licensing commitments designed for tornado protection may impose on decommissioning projects. Unit 1 began operation in 1968. Because of the age of Unit 1's design and the low frequency of tornadoes in California, the original plant design did not provide any protection from tornado hazards. Tornado protection requirements were later imposed as a back-fit; however, the approved license change was based on a probabilistic risk assessment that defined Unit 1's tornado missile damage acceptance limit in terms of reactor core damage frequency. When several Unit 1 buildings have been demolished, construction will begin on an ISFSI for Unit 1's spent fuel. The ISFSI design incorporates tornado missile barrier features into the storage canister and transfer cask. These design provisions will alleviate any need to manage tornado missile hazards. Units 2 and 3 share a design basis for tornado missile protection that closely follows the U.S. Nuclear Regulatory Commission's Standard Review Plan (NUREG 0800), Revision 1. Critical components are identified that are required to be functional following design-basis tornadoes. Missile barriers protect most critical components; however, some critical components are allowed to be exposed to tornado missiles provided the aggregate annual probability of damage to all critical components is <10-7 per unit. According to the analysis that established this probability, it is directly proportional to the inventory of unrestrained objects within a missile pickup/transport area that includes the entire site. To determine the increased probability of damage due to demolition work, the quantity of loose debris was estimated for several discrete time intervals of the decommissioning process. This intermediate result showed that debris controls would be necessary to protect critical components in Units 2 and 3 during the demolition of Unit 1. Several different methods for controlling debris were evaluated for efficacy, feasibility, and cost-effectiveness. Unit 1 decommissioning work will increase the number of potential tornado missiles by ∼500%. Unless controlled, this increased missile inventory would raise the aggregate annual probability of damage to critical components in Units 2 and 3 to a value greater than their licensed acceptance limit. The following controls on storage of Unit 1 demolition debris have been developed to reduce the probability of damage to an acceptable level: 1. Minimize debris storage within 110 m (350 ft) of critical components in Units 2 and 3. 2. Minimize debris storage at elevations >7.5 m (25 ft) above grade level. 3. Store long or flat materials with their long dimensions in a horizontal position. (authors)