Resolution of Generic Safety Issues: Issue 159: Qualification of Safety-Related Pumps While Running on Minimum Flow (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
This issue was identified1481 by NRR and addressed concerns involving centrifugal pumps operating at some point below the best efficiency point (BEP) on their characteristic curves that could cause hydraulic instability or flow recirculation inside the pumps. This unstable flow phenomenon becomes progressively more pronounced as the flow is further decreased and could result in pump damage from vibration and recirculation cavitation (voids created by the recirculating current).
On May 24, 1986, Susquehanna Unit 1 experienced an event (LER 86-021-OO) involving pump damage due to erosion caused by recirculation cavitation during partial flow operation of an ESW pump. An inspection of all other ESW pumps revealed similar cavitation damage. The licensee estimated that cavitation occurred when the pumps ran at a flow rate significantly lower than the design rate. Because they were of similar design, the RHR service water pumps were also inspected and cavitation damage was found on the impeller liners.
In a search of an operational data base for the period from 1983 to 1988, four additional events involving either pump failure or potential for pump degradation resulting from low flow operation were found. These events occurred at Vermont Yankee, H. B. Robinson 2, Turkey Point 3, and Haddam Neck, and were reported in AEOD/E807.1488
In 1988, W issued notices to all of its NSSS users regarding minimum flows for thermal protection (thermal) and long-term protection (mechanical minimum flow) in all postulated scenarios (including design basis accident conditions) anticipated during the lifetime of all safety-related pumps including the RHR, HPSI, Core Spray, and AFW pumps. Based on the operating events and the W notification, the NRC issued Bulletin No. 88-041489 requesting licensees to take actions to deal with the concerns involving centrifugal pumps operating at low flow conditions. Two areas of concern were discussed in the bulletin:
(1) When two centrifugal pumps operate in parallel and one of the pumps is stronger than the other (i.e., it has a higher developed head for the same flow), the weaker pump may be dead-headed when the pumps are operating in the minimum flow mode. The phenomenon is manifested at low flow rates because of the flatness of the pump characteristic curve in this range. The head difference is not a problem at moderate to high flow conditions because of the shape of the pump characteristic curve in these regions.
(2) In the past, the required minimum flow for these pumps was established solely from the temperature rise in the pumped fluid. However, it became generally understood that temperature rise is the only factor that can influence safe, continuous, minimum flow operation. Centrifugal pumps will demonstrate a flow condition that is described as hydraulic instability or impeller recirculation at some point below the BEP on their characteristic curves. These unsteady flow phenomena become progressively more pronounced as the flow is further decreased and can result in pump damage from vibration, excessive forces on the impeller, and cavitation.
The above first concern pertaining to dead-heading of pumps was not included in the NRR memorandum.1481 This concern was considered resolved by actions licensees took to either: (1) install an additional bypass line; (2) replace the existing line with a larger bypass line; or (3) change the operational procedures to prevent the potential of a pump being dead-headed. However, licensee responses dealing with the second concern were not completely satisfactory to NRR.1481
This issue was concerned with the operability of pumps in all safety-related systems that are addressed in GDC 34 (RHR Heat Removal), GDC 35 (Emergency Core Cooling), GDC 38 (Containment Cooling), and GDC 44 (Cooling Water Supply) of Appendix A to 10 CFR 50; these cooling functions are vital for reactor or containment protection.
Bulletin No. 88-041489 identified several action items and associated general guidance. Specifically, Action Item 3 requested each licensee to evaluate the potential pump damage caused by low flow operation and testing. The guidance for conducting the evaluation included consideration of the effects of cumulative operation during the postulated accident scenario and testing during the plant life, the use of test data and field experience data, and verification of the pump supplier's recommended minimum flow rate. Action Item 4 requested licensees to identify, if needed, the short-term and long-term modifications to plant operating procedures or hardware changes to mitigate the problems.
Licensee responses to Bulletin No. 88-041489 were reviewed by ORNL and the results were summarized in NUREG/CR-5706.1490 ORNL indicated that the level of information provided in the licensee responses varied substantially. Some plants provided fairly detailed discussions of original and existing minimum flow recommendations and system configurations, as well as an identification of specific design, procedural, or other changes made in response to the bulletin. However, a number of responses provided only an indication that the concerns had been reviewed, with little or no system/pump-specific information provided.
There were 29 PWRs (at 18 sites) that identified a total of 56 procedure changes and 38 design modifications made in response to Bulletin No. 88-04.1489 There were 8 BWRs (at 6 sites) that identified a total of 17 procedure changes and 6 design modifications. In addition to design or procedure changes, special tests and analyses performed in conjunction with the bulletin were identified. The ORNL findings were: (1) there were no generic guidelines for determining the acceptability of a pump's mission time for various modes of operation; and (2) some licensee responses did not adequately address the low flow concern. Therefore, ORNL recommended that the industry be encouraged to develop pump qualification criteria and new diagnostic techniques for pump testing.
NRR reviewed some responses to Bulletin No. 88-041489 to determine if additional guidance was needed to supplement the bulletin. With respect to the concern of pump mission time for all anticipated operational modes, NRR concluded that "... Rather than performing detailed accident analyses to determine the longest possible time which a pump could be called upon to run, licensees have modified procedures or made physical modifications to preclude low flow operation outside the range recommended by the vendor." Some licensees have relied on this approach to deal with the low flow problems and the staff found this approach reasonable and acceptable.1491
Following the issuance of Bulletin No. 88-04,1489 NRC Temporary Instruction 2515/105 was used to conduct inspections to ensure adequate licensee response to the bulletin and to ensure that safety-related pumps would not be damaged during minimum flow operation or testing. Twelve plants (PWRs and BWRs) were selected for the inspection that was completed in September 1991. Although there were some weaknesses in the licensee responses, the inspection found them adequate, except for a few plant-specific open items that were expected to be resolved before the next refueling outage at each of the selected plants. MPA-X804 was established by NRR to track licensee implementation of Bulletin No. 88-04.1489
The Nuclear Plant Aging Research Program,1492 which included the pumps of concern in this issue, was expected to investigate the cumulative effect of low flow operation and testing on pump aging and provide additional insights for future considerations.
The conclusions1491 of the staff and the ORNL review1490 of responses to Bulletin 88-041489 indicated that, in general, the concerns involving pump operation under low flow conditions had been addressed by most licensees. For the areas that remained to be resolved, the ORNL insights and recommendations were expected to be accommodated within the context of the bulletin. The follow-up inspection of 12 plant sites with 20 units turned up no apparent damage to the pumps inspected. The actions taken by the licensees in response to the bulletin were expected to improve the reliability of pumps under low flow operation. Therefore, the safety concerns of this issue were previously addressed by the staff with the issuance of Bulletin No. 88-041489 and the issue was DROPPED from further consideration as a new and separate issue. In an RES evaluation,1564 it was concluded that consideration of a 20-year license renewal period did not change the priority of the issue.