05000528/FIN-2009008-01
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Finding | |
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| Title | Calculations To Support The Graded Voltage Relay Setpoint |
| Description | The team identified an unresolved item (URI) with two aspects, relating to the degraded voltage protection scheme. The first aspect involved the time delay for the degraded voltage protection scheme. Specifically, the time delay of 35 seconds for transfer of safety buses to the onsite power supplies may be too long to prevent core damage in case of a sustained degraded voltage condition concurrent with an accident. The second aspect involves the lack of calculations to support the degraded voltage relay voltage setpoint. Description. There has been a series of correspondence between the NRC and Palo Verde regarding degraded voltage relays and the team noted during the inspection that item 32 of NRC letter dated December 12, 1977 provided guidance on complying with NRC requirements for the capacity and capability of power sources. This letter required the installation of automatic voltage monitors to detect the presence of a sustained degraded voltage condition. Position 1.a of item 32 stated that the selection of voltage and time setpoints shall be determined from an analysis of the voltage requirements of the safety-related loads at all onsite system distribution levels, and position 1.C(1) stated, The time delay selected shall not exceed the maximum time delay that is assumed in the FSAR accident analysis. The licensee\'s proposals for meeting these requirements were accepted by the NRC in NUREG-0857, Supplement 5, dated November 1983. In 1992, the NRC Electrical Distribution System Functional Inspection (EDSFI) team identified a lack of calculations to support the setpoints of degraded voltage relays. Palo Verde updated the voltage calculations, and identified several deficient conditions involving the degraded voltage scheme in LER 93-011 and its supplements. During the period 1993 to 1999, Palo Verde and the NRC exchanged several communications, including two license amendment requests, which discussed these issues. License amendment request dated December 16, 1998, proposed technical specifications changes involving administrative controls aimed at preventing the spurious separation of safety buses during an accident. Specifically, Technical Specification 3.8.1, Condition G provided an 1-hour action statement for restoring the capability of an offsite power source that was determined not to be capable, or to transfer of the affected safety buses to the onsite emergency diesel generators. The technical specification bases were revised to provide guidance and criteria for determining the capability of an offsite source. The first issue identified by the team involved the degraded voltage relay time delay . The time delay for the degraded voltage relay is set at ~ 28.6 seconds and s 35 seconds. This time delay could result in a delay in supplying water to the core in case of an accident concurrent with degraded voltage, due to the inability of electrical equipment to respond as required during the timeout period. Final Safety Analysis Report 6.3.3.3.2 states that the accident analysis assumes 30 seconds from a safety injection actuation signal (SIAS) to the time that safety injection flow is delivered to the core. As part of a December 16,1998 submittal, Palo Verde included a revision to Final Safety Analysis Report Section 8.3.1.1 .3.13 to delete criteria associated with the degraded voltage relay time delay taken from the December 12,1977, letter, item 32, position 1.C(1), that is quoted above. This was done without explanation or any apparent relationship to the primary objective of the amendment request. The NRC questioned this deletion in item 13 of a letter dated June 14, 1999. The Palo Verde response provided in a letter dated July 16, 1999, stated that the degraded voltage relays were not capable of providing a protective function, or required to function during a design basis accident, so that the time delay had no effect on the accident analysis. The safety evaluation report dated December 29, 1999, did not comment on this response and there was no indication that the NRC had accepted the assertion that a degraded voltage condition concurrent with an accident was not credible. The team also noted that the degraded voltage relay delay setpoints are still included in Final Safety Analysis Report Table 7.3-1 B, Engineered Safety Features Response Times, and Table 7.3-11A, NSSS Engineered Safety Features Actuation System Setpoints and Margins to Actuation. They are also included in Technical Specification 3.3.7, Diesel Generator (DG) - Loss of Voltage Start (LOVS). The team questioned the rationale included in the July 1999 licensee response, as follows: The basis for the original time delay of s; 35 seconds did not appear to be based on the correct criteria. It was based on the time required for starting a reactor coolant pump motor rather than the much shorter time required for starting a large emergency core cooling system motor. A shorter time delay for accident conditions can be implemented without increasing the risk of spurious separation for momentary voltage excursions, or for longer voltage excursion during normal operation. A shorter time delay will not delay the time required to provide water to the core, but will actually improve it. e The licensing basis for degraded voltage concurrent with an accident was not properly characterized. The licensee did not provide any information indicating that a shorter time delay would increase the probability of double sequencing, or worsen its affects. The second issue identified by the team is in regards to the calculations for the degraded voltage relay setpoints. In the Technical Specification 3.3.7, Diesel Generator (DG)Loss of Voltage Start (LOVS) a degraded voltage relay trip setpoint of ~ 3697 V and $3786 V is specified and the team noted that the current calculations of record do not establish the adequacy of this setpoint. The load flow calculation 01-EC-MA-0221, AC DistributionJ was used as an input to other calculations which analyzed voltage available to safety-related loads based on the minimum voltage afforded by administrative controls described in Technical Specification 3.8.1 J Condition G which provided results that were higher than that afforded by the automatic protection of the degraded voltage relays by up to approximately 2.5 percent. The lack of calculations to support the setpoints was previously documented by the licensee in Palo Verde Action Request (PVAR) 3253612 and CRDR 3256729. In the operability determination PVAR, it stated that this was a paper nonconformance that does not affect operability because it is not credible to have switchyard voltage low enough to cause degraded voltage due to technical specification limitations provided for in Technical Specifications 3.8.1 Condition G. The team performed a review of the record relating to the original licensing of the degraded voltage relays in the early 1980s and to the license amendment request dated December 16, 1998, relating to the Technical Specification 3.8.1 change that implemented administrative controls to limit the vulnerability to spurious grid separation. The licensee had offered the proposition that degraded voltage concurrent with an accident was not credible, but the team could not find evidence that the NRC had accepted this position, or that the degraded voltage relays were no longer required to perform a protective function during accidents. The team believes that the licensee\'s position in the operability determination was further refuted by the continued inclusion of the degraded voltage relay setpoint in FSAR Table 7.3-11A, and Technical Specification 3.3.7. Although the licensee disputed the need to perform calculations based on the degraded voltage relay setpoint, preliminary calculations were performed by the licensee to assess the adequacy of voltage based on the settings of the degraded voltage relays. These calculations showed that motor control center circuits would generally have adequate margin to operate at the lowest voltages afforded by the degraded voltage relay setpoint, although available voltage margins for several devices were reduced by approximately 75 percent. Evaluation of MOV circuits was ongoing at the close of the inspection. Because the licensee has taken the position that formal calculations to support the design basis for the degraded voltage relays need not be performed, this item is considered an URI pending further review by NRR and is identified as : URI 05000528; 05000529;05000530/2009008-01, Failure to Perform Adequate Calculations for Degraded Voltage Relay Setpoints |
| Site: | Palo Verde  |
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| Report | IR 05000528/2009008 Section 1R21 |
| Date counted | Sep 30, 2009 (2009Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | B Parks D Reinert D You J Watkins M Hay S Makor T Brownc Baron G Skinner S Alferink M Young M Runyan R Kellar S Mako |
| INPO aspect | |
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Finding - Palo Verde - IR 05000528/2009008 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Palo Verde) @ 2009Q3
Self-Identified List (Palo Verde)
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