05000456/FIN-2012005-04
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Finding | |
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| Title | Operability Evaluation of Block Walls for High Energy Line Break Loads |
| Description | Safety-related auxiliary building block walls at Braidwood were previously evaluated for seismic loads in accordance with the Interim Criteria for Safety-Related Masonry Wall Evaluation, provided in NUREG-0800 (Standard Review Plan), Attachment A, Section 3.8.4, Other Category I Structures. The subject walls, consisting of 12-inch thick unreinforced hollow masonry units, are divisional separation walls in the auxiliary building required to maintain the fire and ventilation barrier function while not failing in a manner that would adversely affect safety-related equipment. These walls were assumed to span horizontally in the evaluations and steel columns were provided for additional support along the length of the walls as needed to limit the spans. The HELB analyses at the time did not identify any pressure loading on the walls resulting from postulated pipe breaks. During a turbine building HELB design basis reconstitution effort, the licensee identified the existence of differential pressure on certain auxiliary building safety-related block walls following postulated pipe break events. The HELB scenario involved a pipe break in the turbine building that would initially communicate with the auxiliary building rooms through open fire dampers. After a period of time, as the temperature rises, the fire damper in one room closes while the fire damper in the other room fails to close (single active failure assumption), which would result in a buildup of a differential pressure across the block wall separating the two rooms containing safety-related equipment. The licensee performed an operability evaluation, documented as Braidwood Operability Evaluation 12-004, HELB Load Not Considered in Structural Calculations, for the subject walls and concluded that the walls remained operable, but were non-conforming. The inspectors noted that the functionality evaluation did not consider seismic loads acting concurrently with pressure loads due to the pipe break. The Standard Review Plan, Attachment A, Section 3.8.4, which was consistent with the design basis described in the Braidwood UFSAR, required consideration of the following load combinations: 6. D + L + 1.5 Pa 7. D + L + 1.25 Pa + 1.25 E 8. D + L + Pa + E (D = dead load, L = Live loads, E = operating basis earthquake, E = safe shutdown earthquake, Pa = Pipe break pressure load; terms not applicable are omitted in the above load combinations) In their evaluation, the licensee assumed that the seismic and HELB events started at the same time. The pressure buildup would not start until 200 seconds after the event, at the time when one fire damper closes. Since the seismic event was not assumed to last for more than 200 seconds, the licensee concluded that it was not necessary to aggregate the effects of seismic and HELB loads. In addition, the licensee also concluded that it was not necessary to consider any load factors for the operability evaluation. Consequently, the three load combinations noted above were reduced to the following governing combinations: 1. Initial 30 seconds: Seismic activity ends within 30 seconds, pressure negligible o D + L + E - condition previously evaluated in the original calculations 2. After 200 seconds: No seismic activity, HELB pressure starts to build o D + L + Pa - the operability evaluation addressed this condition The inspectors reviewed the Braidwood design basis documents; OP-AA-108-115, Operability Determinations; the Standard Review Plan; and NRC Inspection Manual Part 9900, Technical Guidance for Operability and Functionality Determinations, but did not find any basis for using a time sequence of events in combining the effects of the two loadings. Additionally, the inspectors could not find any basis to conclude that the load factors for the HELB pressure were specified for additional safety margin only and were not required to account for the other considerations, such as the inherent uncertainty of the calculated values based on methodologies and inputs involved in such calculations. The inspectors further noted that the masonry allowable stress used in the licensees evaluation was equal to the MOR value based on test data documented in the UFSAR, which was about 65 percent higher than the allowable stress for the design basis Safe Shutdown Earthquake load combination. By omitting the load factor and not considering combined effects of seismic and HELB, the licensee also significantly reduced the design basis loads. The licensees current evaluation documented an overall very small margin, suggesting that applying a load factor of 1.25 or 1.5 to the pipe break pressure load, or combining a seismic event of much smaller intensity than a Safe Shutdown Earthquake or an Operational Basis Earthquake, could result in the walls exceeding the operability acceptance criteria. While the licensees analysis may be reasonable based on assumptions regarding the sequencing of HELB and seismic events, there may be other scenarios with a slightly different sequence of events that may not be bounded by the current evaluation. Specifically, the possibility of a seismic event or a seismic aftershock occurring after the HELB while a differential pressure across the block wall was present would subject the wall to a combined seismic and HELB effect. The inspectors further noted that while the probability of such occurrence could be very low, the current staff guidance precludes the use of probabilities in operability considerations. Specifically, Section C.6 of Inspection Manual Part 9900 Technical Guidance states, in part, the definition of operability is that the SSC must be capable of performing its specified safety function or functions, which inherently assumes that the event occurs and that the safety function or functions can be performed. Therefore, the use of Probabilistic Risk Assessment or probabilities of occurrence of accidents or external events is not consistent with the assumption that the event occurs, and is not acceptable for making operability decisions. The inspectors also reviewed the licensees procedure for operability determinations and found that it was consistent with Section C.6 of the NRCs Part 9900 Technical Guidance. Based on the above, the inspectors questioned whether the effect of the load factors and that of concurrent application of the seismic and the HELB must be considered to be consistent with the Braidwood design basis and the operability determination procedure requirements, and whether the licensee provided an adequate justification for not including these factors in their operability evaluation. The inspectors were unable to determine during the inspection whether the licensees justification was acceptable and therefore this issue will be considered an URI pending further NRC review. |
| Site: | Braidwood  |
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| Report | IR 05000456/2012005 Section 1R15 |
| Date counted | Dec 31, 2012 (2012Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.15 |
| Inspectors (proximate) | M Bielby M Holmberg M Perry R Baker R Ng T Go V Meghani A Garmoe E Duncan J Benjamin J Robbins J Steffesm Holmberg M Perry N Feliz_Adorno R Ng T Go V Meghani A Garmoe B Bartlett C Moore C Zoia D Szwarc E Duncan J Benjamin J Jandovitz J Laughlin J Robbin |
| INPO aspect | |
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Finding - Braidwood - IR 05000456/2012005 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Braidwood) @ 2012Q4
Self-Identified List (Braidwood)
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