Each participant filled out an elicitation questionnaire dealing with age related health issues. The results from this exercise were reviewed with the meeting participants on Wednesday morning. As part of this exercise, Lee Abramson indicated that it is usually easier to determine relative rates versus absolute rates. Various absolute and relative questions were posed in order to demonstrate this concept.

Definition of Terms for Elicitation

Terms used during the elicitation must be commonly understood by the group in order to foster discussion, issue development, and subsequent elicitation. Certain key terms must be defined.

Rob Tregoning indicated that, for this exercise, all definitions should be kept generic, not plant specific. The first term to be defined is LOCA. Rob Tregoning presented the NUREG/CR-5750, Appendix J definition as a starting point. This report defines a LOCA as “an unisolable breach of the Reactor Coolant Primary Boundary (RCPB) requiring ECCS initiation.”

The group felt that the term “unisolable” was not appropriate because the main point is to limit the scope to Class 1 piping. Also, the merits of the phase ECCS initiation were debated because the ECCS response in some plants requires use of normally operating plant equipment. Therefore, some plants might require a large leak before implementation of standby ECCS systems. There was also a discussion on the merits of using break instead of breach, but the term breach was determine to be more generic than break. The addition of the term “sudden breach” instead of just “breach” was also neglected because of the vagueness of the word sudden.

The group agreed to a definition of a general LOCA as follows. A LOCA is “a breech of the reactor coolant pressure boundary which results in a leak rate beyond the normal makeup capacity of the plant”.

The next definitions are required to determine the size classifications of LOCAs. Once again, Rob Tregoning presented the definitions used in NUREG/CR-5750, Appendix J as a starting point. These definitions were also used in NUREG-1150 and form the basis of plant PRA event trees. This document defined three LOCA size categories: SB, MB, and LB. The NUREG/CR-5750 definitions are as follows:

• SB LOCA — A break that does not depressurize the reactor quickly enough for the low pressure systems to automatically inject and provide sufficient core cooling to prevent core damage. However, low capability systems (i. e., 100 to 1,500 gpm [380 to 5,700 lpm]) are sufficient to make up the inventory completion. For a BWR, this translates to a pipe in the primary system boundary with a break size less than 0.004 ft2 (370 mm2), or a 1 inch (25 mm) equivalent inside pipe diameter, for liquid, and less than 0.05 ft2 (4,600 mm2), or an approximately 4 inch (100 mm) inside diameter pipe equivalent, for steam. For a PWR, this equates to a pipe break in the primary system boundary with an inside diameter between Уг to 2 inches (13 to 50 mm).

• MB LOCA — A break that does not depressurize the reactor quickly enough for the low pressure systems to automatically inject and provide sufficient core cooling to prevent core damage. However, the loss from the break is such that high capability systems (i. e., 1,500 to 5,000 gpm [5,700 to 19,000 lpm]) are needed to makeup the inventory depletion. For a BWR, this translates to a pipe in the primary system boundary with a break size between 0.004 to 0.1 ft2 (370 to 9,300 mm2), or an approximately 1 to 5 inches (25 to 125 mm ) inside diameter pipe equivalent, for liquid, and between 0.05 to 0.1 ft2 (4,600 to 9,300 mm2), or an approximately 4 to 5 inches (100 to 125 mm) inside pipe diameter equivalent, for steam. For a PWR, this equates to a pipe break in the primary system boundary with an inside diameter between 2 to 6 inches (50 to 150 mm).

• LB LOCA — A break that depressurizes the reactor to the point where the low pressure system injection automatically provides sufficient core cooling to prevent core damage. For a BWR, this translates to a pipe in the primary system boundary with a break size greater than 0.1 ft2 (9,300 mm2), or an approximately 5 inch (125 mm) inside diameter pipe equivalent, for liquid and steam. For a PWR, this equates to a pipe break in the primary system boundary with an inside diameter greater than 6 inches (150 mm).

The elicitation panel questioned the basis of the equivalent pipe diameter relationships to break size provided in the NUREG/CR-5750 Appendix J. Bill Galyean thought that they could be traced back to NUREG 1150 and possibly WASH-1400. It was quickly determined that “break” should be replaced by “breech” everywhere for consistency with the general LOCA definition. Also, the group decided that the formal definitions should be based on leak rate, and not equivalent break area or size.

At this point, the need for additional LOCA size classification was revisited. This request was promulgated by Bruce Bishop based on discussions with the Westinghouse Owner’s Group (WOG). The System response and mitigation procedures for a 5,000 gpm (19,000 lpm) LOCA (lower limit LB LOCA within NUREG/CR-5750, Appendix J) and a DEGB of the largest class 1 pipe (flow rate up to 860,000 gpm [3,250,000 lpm] according to WOG) are significantly different. Because, the elicitation results will used in existing PRAs, it was also stressed that the original leak rate classifications in NUREG/CR-5750, Appendix J should also be maintained. While the group also agreed that the leak rate threshold should ideally be based on the equipment needed to mitigate a specific event, this information is highly plant specific and could not be approximated generically.

For the reasons stated in the above paragraph, the group decided to keep the NUREG/CR-5750, Appendix J leak thresholds of 100 gpm (380 lpm), 1,500 gpm (5,700 lpm), and 5,000 gpm (19,000 lpm), but to add several leak rate categories above 5,000 gpm (19,000 lpm). The highest category was set at 500,000 gpm (1,900,000 lpm) to capture the DEGB events of the largest primary system pipes. Additional ranges of 25,000 gpm (95,000 lpm) and 100,000 gpm (380,000 lpm) were chosen to span the range from 5,000 gpm (19,000 lpm) to 500,000 gpm (1,900,000 lpm) in roughly equivalent magnifications. These leak rate categories were also chosen because they tend to group DEGBs by primary system functionality.

The LOCA size classification thresholds adopted by the group are summarized in Table B.1.1[2]. A category 1 LOCA is defined as “a breach of the reactor coolant pressure boundary which results in a leak rate which is greater than 100 gpm (380 lpm). Similarly, a category 6 LOCA is a breach of the RCPB which results in a leak rate which is greater than 500,000 gpm (1,900,000 lpm). It should be stressed that category 1 LOCAs include contributions from all categories. The group preferred the threshold classification of LOCA sizes instead of partitioning the sizes into ranges as in NUREG/CR-5750, Appendix J. Care will be needed during the elicitation to ensure that these definitions are understood.

It was determined by the group that these leak rates should be roughly correlated to breach area, and converted into an equivalent pipe diameter so that the UB leak rates for various piping systems could be determined. There was some concern about the feasibility of developing generic estimates. It was suggested that equivalent pipe sizes could be based on 250 gpm/in2 (1.47 lpm/mm2) for liquid PWR lines and 175 gpm/in2 (1.03 lpm/mm2) for liquid BWR lines. However, these estimates did not agree with the Westinghouse equivalent pipe diameter estimates.