05000298/FIN-2007011-07
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Finding | |
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Title | Results of Modeling of the Onset of Vortexing Results Are Pending |
Description | The team identified an unresolved item concerning the vortex calculation of the ECST and the available margins of the required volumes for switchover from ECST to suppression pool (torus) scenario and the 8-hour shutdown scenario. Specifically, the team identified that the licensee used an incorrect area for the calculation of the velocity and, therefore, the submergence Froude number, which would have an impact on the prediction to vortexing. Moreover, the available margins for the required volumes for switchover and 8-hour shutdown were minimal. The team reviewed Calculation NEDC 98-001, Vortex Limit for the Emergency Condensate Storage Tanks A & B, Revision 1. The purpose of the calculation was to determine the minimum tank level required to prevent the formation of stable, air-core vortices during the switchover of high pressure coolant injection and reactor core isolation cooling from ECST to suppression pool. The team noted that the ECST to suppression pool switchover scenario is described in Sub-Section 7.0, Reactor Core Isolation Cooling System of Section IV, Reactor Coolant System of the USAR. Calculation NEDC 98-001 stated that the required submergence of the ECST suction piping is based on Electric Power Research Institute Report TR-106266, Inverted Draft Tubes to Improve Suction Performance of Vertical Pumps. A review of the formulas used in Calculation NEDC 98-001 and Electric Power Research Institute Report TR-106266 revealed that the statement in the calculation is incorrect. Electric Power Research Institute Report TR-106266 used pipe cross-sectional area in the calculation of the velocity ( i.e., the same as used in NUREG/CR-2772, Hydraulic Performance of Pump Suction Inlets for Emergency Core Cooling Systems in Boiling Water Reactors). However, Calculation NEDC 98-001, incorrectly used the non conservative approach area, which is larger than the pipe cross sectional area, thus, resulting in lower velocity and, therefore, smaller Froude number. The team reviewed Calculation NEDC 01-072, ECCS [emergency core cooling system] Pump NPSH/Vortex Limit with Suction from CST [condensate storage tank], Revision 0. This calculation determines the minimum condensate storage tanks water level required when the emergency core cooling system pump suctions are aligned to the CST during shutdown conditions. The team verified that, in contrast to Calculation NEDC 98-001, calculation NEDC 01-072 used the correct pipe cross sectional area for the calculation of the Froude number. The incorrectly calculated Froude numbers in Calculation NEDC 98-001 were lower than 0.8, a cutoff limit for vortex creation derived from tests in NUREG/CR-2772 for a suction pipe equipped with strainer, and therefore vortexing was not identified in Calculation NEDC 98-001. However, approximate calculations performed by the team based on the ECST suction cross-sectional pipe cross-sectional area, rather than approach area, indicate that the use of approach area vs. pipe area results in a minimum submergence that is substantially lower than the minimum submergence calculated based on the pipe cross-sectional area. That is, vortexing could have occurred at a higher elevation submergence (i.e., earlier condensate depletion) had the pipe area been used. The review of the results of Calculation NEDC 98-001 indicated that a Froude number as high as 0.7934 was calculated (with 0.8 being the cutoff number for vortex creation). Moreover, for a suction pipe with no strainer (as the case in Calculation NEDC 98-001), NUREG/CR-2772 recommends 0.7 as the cutoff Froude number. This suggests that vortexing would have occurred if this lower Froude number was used. The licensee justified the use of a larger cutoff Froude number of 0.8 because the suction from the condensate is an inverted 90-degree elbow, which should help retard vortex formation. A second concern related to impact of the high pressure coolant injection and reactor core isolation cooling pump suction motor-operated valve stroke times on the switchover transient. During this transient there is a potential of air entrainment as the available volume is reduced to very low levels. The transient is largely affected by the assumed closing time of the high pressure coolant injection and reactor core isolation cooling pump suction motor-operated valves. The licensee used stroke-time results from in-service tests as part of the input to the design calculation. The stroke times used in Calculation NEDC 98-001 were chosen to conservatively bound the in-service test results. The calculation stated that Within the limits of a hand-calculation, the results indicate very little margin exists to accommodate any unexpected increase in the stroke time of either high pressure coolant injection or reactor core isolation cooling pump suction valves. In fact the current operability limit for Motor-Operated Valve HPCI-MOV-MO58 should be lowered by 3 to 5 seconds. This is further discussed in Condition Report CNS-2006-09585, which states, Note that the stroke time is within limits for the current NEDC 96-039 Revision 2, Status 1 (as-built) calc version. If the Revision 3, Status 2 For Information Only version of NEDC 96-039 were used, the vortexing calculation limits in NEDC 98-001 would need to be re-evaluated. Condition Report CNS-2006-09585 had been generated by the licensee in response to Information Notice 2006-21, Operating Experience Regarding Entrainment of Air into Emergency Core Cooling and Containment Spray Systems. With respect to current operability the team inquired whether the licensee had identified and evaluated the appropriate design basis scenarios for ECST drawdown to the low-level set-point for switchover of the high pressure safety injection/reactor core isolation cooling suction valves to the torus. The licensee generated Condition Report CNS-2007-07402 to address this issue and their evaluation demonstrated that there are no immediate operability concerns. The team reviewed Engineering Evaluation 01-090, Revision 1, which addressed condensate requirements for an 8-hour shutdown (Calculation NEDC 01-064, Revision 1) and station blackout (Calculation NEDC 89-1886, Revision 2). As part of the licensee operability evaluation, the licensee has stated emergency procedure bases provide guidance to maintain the ECST level between 60 and 120 inches by refilling from the condensate storage tank and that 60 inches minimum is above the ECST low level set-point (24.5 inches) for switchover to the torus but the Updates Safety Analysis Report does not allow the ECST to suppression pool (torus) switchover input to the emergency core cooling system to be over-ridden. The team reviewed set-points in Calculation NEDC 92-050K, HPCI-LS-074A/B and HPCI-LS- 75A/B Set points, Revision 2. The calculation used an analytical limit of 21.71 inches and refers to GE Calculation NEDC-32676P as its basis. Based on this analytical limit, Calculation NEDC 98-001 calculates the volume of the available water from both tanks is 9,918 gallons. The licensee takes credit of the approximately 400 gallons water in the ECST suction piping to meet the at least 10,000 gallons requirement stated in the Sub-Section 7.0, Reactor Core Isolation Cooling System of Section IV, Reactor Coolant System of the USAR. The minimum levels for the ECSTs were found in Calculation NEDC 01-064, Revision 1, references the requirement in Cooper Nuclear Station Procedure 6.LOG.601, Attachment 11, ECST Level, Revision 90. The stated ECST minimum is 14.8 ft for each tank. Using 282 gallons per inch, a level of 14.8 ft equates to 50,083 gallons per tank or 100,166 gallons for the total from both tanks. The ECSTs consist of two separate tanks, which are cross-connected. The required volume of 86,072 gallons from both tanks is stated in Calculation NEDC 01-064, Revision 1. However, after depletion of 86,072 gallons the remaining volume is 14,094 gallons (i.e.,100,166 gallons minus 86,072 gallons) for two tanks, or 7,047 gallons for one tank, which corresponds to an ECST tank level height of 25 inches from the bottom of each tank. This is only 0.5 inch over the switchover setpoint limit of 24.5 inches stated in Calculation NEDC 01-064. Based on this calculation, the licensee contends there will not be an unintended switchover to the torus, but the team was concerned about the minimal margin. Given the low margin to switchover, the potential for air entrainment during this transient becomes important and must be considered. The team noted that NUREG/CR-2772 does not address submergence at the low end of the ECST drawdown when the suction valves are swapping over. Therefore, analytical evaluations of air entrainment in this region are not very reliable. Calculation NEDC 98-001 did not indicate any vortexing throughout the transient. Other sites, however, have seen test results indicating that vortexing would start at about 4.5 inches above the top of the pipe. The licensee planned to perform a scaled test of the ECST switchover transient and assess the potential for vortex formation in the suction flow. Testing was in progress during the team inspection. Prior to leaving the site, the team was verbally informed of some preliminary results, which indicated a potential for air entrainment during the end of switchover transient. The team will review the modeling results to confirm assumptions on their operability and corrective actions and assess the impact on the remaining margin and any potential for margin erosion. This issue is unresolved item pending completion of this review. (URI)05000298/2007011-07 |
Site: | Cooper ![]() |
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Report | IR 05000298/2007011 Section 4OA5 |
Date counted | Dec 31, 2007 (2007Q4) |
Type: | URI: |
cornerstone | No Cornerstone |
Identified by: | NRC identified |
Inspection Procedure: | |
Inspectors (proximate) | T Farnholtz S Garchow M Young M Yeminy N Greca N Okonkwol Ellershaws Graves R Latta |
INPO aspect | |
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Finding - Cooper - IR 05000298/2007011 | |||||||||||||||||||||||||||||||||
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Finding List (Cooper) @ 2007Q4
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