05000282/FIN-2012005-06
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Finding | |
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| Title | Concerns with Analytical Methods Used for Predicting Void Transport Behavior |
| Description | The inspectors identified a URI regarding the analytical methods used by the licensee for evaluating and predicting void transport behavior. Specifically, some methods used by the licensee relied on the use of computer software and test results which were previously questioned by the NRC staff. As a result, the inspectors questioned the acceptability of the analytical methods. Description: On January 11, 2008, the NRC requested each addressee of GL 2008-01 to evaluate its Emergency Core Cooling System (ECCS), Decay Heat Removal, and Containment Spray (CS) systems licensing basis, design, testing, and corrective actions to ensure that gas accumulation (voids) was maintained less than the amount that would challenge the operability of these systems. Licensees were instructed to take appropriate actions when conditions adverse to quality were identified. As part of this effort, Prairie Island developed analytical methods for evaluating identified voids in the subject systems. During a subsequent onsite inspection, the inspectors noted concerns with respect to the licensees void assessment methodologies. Specifically, the inspectors noted the licensee relied on the use of computer codes to evaluate the acceptability of some voids. Specifically, the licensee used a combination of PIPER Q2.05, SYSFLO Q3.08, and AIRDST codes in their evaluations. The code, PIPER Q2.05, was used to generate a mathematical model of the piping in the form of control volumes and connectors. The control volumes represented the mass and energy of the fluid while the connectors represented the inertia of the fluid and the hydraulic resistance of the flow path. The SYSFLO Q3.08 code used this model to solve the mass, energy, and momentum conservation equations to obtain the pressure, temperature, and flow rate information. The AIRDST program used these results to simulate transport of air in the flow. The inspectors noted instances where the basis of this void assessment analysis tool was not well supported. Specifically, the licensee used WCAP-17271-P, Air Water Transport in Large Diameter Piping Systems, Analysis and Evaluation of Large Diameter Testing Performed at Purdue, to show that the AIRDST code could acceptably predict quantitative void transport behavior. The inspectors noted the test configuration and conditions used in the WCAP-17271-P report differed from actual plant configuration and conditions, and questioned whether the application of some of the test results was acceptable. For example: FnThe difference between test and plant pressures was not considered in assessing void decrease in the vertical test section. The pressure range used during the test was significantly lower than the typical range in nuclear power plants. Therefore, 41 Enclosure the inspectors questioned if the void fraction change observed during testing would be analogous in a nuclear power plant. FnTwo phase fluid flow test data typically exhibited significant scatter. This was addressed by running many duplicate tests and carefully examining the test results. However, as documented in, Forthcoming Meeting with the Nuclear Energy Institute to Discuss NRC Generic Letter 2008-01, (ML090150637), the NRC stated this effort was not fully successful and some of the conclusions were not adequately supported by the test data due to data scatter. Specifically, this effort did not address allowances for uncertainty and the effect of actual plant pressures in contrast to test pressures. FnThe inspectors questioned whether the test report adequately considered a water fall effect (also known as hydraulic jump) when the upper part of the vertical pipe was voided. Specifically, the inspectors questioned whether the pipe length used for the test was representative of the limiting conditions of a plant. The inspectors were concerned if such an effect could propel air further down in the pipe than would be predicted using a single dimensional Froude number and would be of concern if the vertical pipe length was significantly less than the pipe used for the test. The inspectors also noted the evaluation which validated the use of AIRDST, Calculation 1067-1106-0038-00, Comparison of Purdue Experimental Results to SYSFLO and AIRDST Program Predictions, stated the repeatability of some of the test results was questionable. Specifically, the evaluation stated multiple readings did not always match with each other with the differences being significant. The evaluation also noted the AIRDST Program over-predicted and under-predicted void fractions depending on the conditions in the piping. The inspectors discussed these observations with individuals from NRR. It was determined these observations required further evaluation by NRR to better understand the acceptability of the application of the test results contained in the WCAP-17271-P report to void assessment analysis. The inspectors also noted that the licensee was unable to remove several voids which currently existed in the suction piping for the residual heat removal (RHR) and CS systems. The licensee justified the void acceptability using the above mentioned computer codes. Because of the inspectors questions associated with these computer codes, the licensee re-evaluated these voids using the conventional methods contained in Guidance to NRC/NRR/DSS/SRXB Reviewers for Writing TI Suggestions for the Region Inspections (ML103400347), and confirmed the voids met the acceptance criteria with the exception of the voids located between the containment sump B isolation valves. These voids were procedurally created in order to alleviate pressurelocking concerns on these valves. Based on the information currently available, the licensee determined that these voids did not impact operability. The licensee was also evaluating potential modifications to address the voids. Similarly, the inspectors noted the licensee had relied on these computer codes to justify the acceptability of previously identified voids (that no longer exist). The licensee also confirmed that these voids did not challenge system operability using NRRs conventional method with two exceptions. Specifically, voids found at locations 2CS-06 and 1RH-03 were determined to exceed NRRs acceptance criteria when using the conventional method. The licensee used the simplified method contained in the WCAP-17276-P, Investigation of Simplified Equation for Gas Transport, report and concluded the voids were acceptable. However, the inspectors noted the void at location 1RH-03 was acceptable per the simplified equation method; however, the void at location 2CS-06 did not meet the limitations of the simplified equation method. The inspectors consulted with NRR on the acceptability of this methodology and determined this methodology was also based on the same tests used to validate the computer codes. Because a void did not currently exist at locations 2CS-06 or 1RH-03, the inspectors determined the past operability of the CS and RHR systems would be addressed when NRR concluded their reviews on the use of computer software and the simplified equation methodology. This issues discussed above were determined to be unresolved pending further evaluation of the licensees analytical methods. The NRR staff will evaluate the matter and provide a determination on the acceptability of: (1) applying the test results contained in the WCAP-17271-P report to void assessment analysis; (2) the use of computer software for void transport analysis of the sump voids; and (3) using the simplified method contained in the WCAP-17276-P report for locations 1RH-03 and 2CS-06 |
| Site: | Prairie Island  |
|---|---|
| Report | IR 05000282/2012005 Section 4OA5 |
| Date counted | Dec 31, 2012 (2012Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | |
| Inspectors (proximate) | D Mcneil K Riemer T Bilik C Moore M Phalen R Baker K Stoedter D Passehl J Laughlin K Barclay M Jones M Learn J Beavers D Oliver |
| INPO aspect | |
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Finding - Prairie Island - IR 05000282/2012005 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Prairie Island) @ 2012Q4
Self-Identified List (Prairie Island)
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