ML20199K715

Corrected Safety Evaluation Supporting Amend 163 Issued to FOL DPR-28.Pages 2 & 3 Required Correction & Clarification

ML20199K715
Person / Time
Site:	Vermont Yankee
Issue date:	01/21/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20199K713	List: ML20199K708 ML20199K715
References
NUDOCS 9901260492
Download: ML20199K715 (2)

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l l 2.0 EVALUATION As a basis for the temperature limit on torus water temperature the licensee stated that a suppression pool temperature of 90 'F was used as an input for the containment analyses with l acceptable results. An initial suppression pool temperature of 90 'F results in a peak post l accident torus water temperature of less than 185 'F. The containment analyses were done for l loss-of-coolant-accident (LOCAs) and events involving safety relief valve (SRV) discharges to the suppression pool. The licensee stated that environmentally qualified electrical equipment was determined to be qualified for the expected temperatures, including 185 *F for the torus i

temperature. In addition, instrument accuracy and ECCS pipe stress were also evaluated for the effect of a 185 'F torus temperature with no adverse effects. The licensee stated that all safety analysis requirements are met with a normal operating limit of 90 'F.

The licensee's analysis described the methodology used to calculate the maximum suppression pool temperature during the analyzed accident scenarios which reject heat to the suppression pool. The TS suppression pool temperature limits were derived from RELAP5YA-B1 A and GOTHIC 5.0e accident analysis of the suppression pool heatup following various heat rejection load accident scenarios. These scenarios used conservative assumptions and methods for a design basis accident LOCA (DBA-LOCA) with limiting single failures to maximize the heat rejection load to the suppression pool following the accident. Additionally, individual sensitivity runs to increase the short- and long-term peak pool temperature were performed. These sensitivity studies utilized case-specific conservative assumptions to investigate the effect of varying different parameters during the limiting single failure cases. The new DBA-LOCA analysis performed to determine pool heatup incorporated the ANS 5.1 1979 decay heat model, increased residual heat removal (RHR) heat exchanger fouling in the RHR model, and included additional heat addition from the feedwater system. The ANS 5.1 1979 decay heat model has been found to be acceptably conservative when used with a 2 sigma adder. The two other models added additional conservatism to the analysis in comparison to the assumptions used in the original design basis by increasing the heat addition to the pool and decreasing the heat removal rate by the RHR. The most limiting short- and long-term peak pool temperatures were obtained from the RHR heat exchanger failure in combination with maximum ECCS injection flow or increased feed flow rate sensitivi'y studies, respectively.

NRC review of the proposed change focused on the containment response methodology, codes used, benchmarking of the computer codes, resulting containment response, effect on fuel clad integrity, and adequacy of net positive suction head (NPSH) for appropriate pumps.

2.1. Methodoloov. Codes. Benchmarkina. and Resultina Containment Response The licensee performed a detailed validation and benchmarking of the two computer codes to demonstrate the applicability of the approach. The containment response methodology consists of two distinct elements identifiable by the two computer codes used. They are:

l a. LOCA mass and energy calculations using RELAP5YA-B1 A code. A plant model was derived from the current NRC-approved 10 CFR 50 Appendix K LOCA licensing analysis. This calculation modified the NRC-approved VY LOCA Appendix K model with inputs chosen to conservatively calculate suppression pool temperature and wet-l well pressure and meets many of the requirements detailed in the SRP. Changes from r

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3 the Appendix K analyses include maximizing the vsssel inventory by bounding the initial water level in the downcomer to the maximum expected normal operating level, setting '

the power level at 1625 Mwt, basing the mass and energy release on 107 percent flow, l performing a sensitivity study to assure that all ECCS flows are conservatively established, and the use of the ANS 5.1 1979 decay heat model with a 2 sigma adder.

The staff has reviewed this methodology and agrees with the licensee that performing the mass and energy releases in this way is appropriate for containment analyses. The l staff has concluded that the methodology as proposed for establishing the mass and energy release profiles to support the proposed TS changes is acceptable with feedwater addition considered in a conservative manner as 6scussed below,

b. Containment calculation using the GOTHIC 5.0e code. GOTHIC is used in this calculation to perform the dynamic mass and energy balance on the containment. It.

has been validated against a selected matrix of separate effects and integral tests to evaluate the available modeling choices. More importantly, benchmarking is included in this calculation for the purpose of demonstrating a direct comparison of results to similar results previously found to be acceptable by the NRC.

The NRC concluded that the basic methodology was reasonable based on its use of a modified Appendix K methodology and the benchmarking described later in this evaluation.

The overall shutdown process was also reviewed. It was found that the consideration of feedwater addition was essential in establishing a conservative analysis. A review found that after a large-break LOCA or main steamline break, the operators were likely to use continued feedwater in order to assist in mitigation and recovery of the accident. As a result, the developed methodology used in the calculations assumes conservative feedwater injection from the perspective of maximizing suppression pool temperature. Continued feedwater addition until the incoming water temperature is less than the expected peak suppression pool temperature is considered conservative. For this purpose a 175 *F value was picked to terminate feedwater flow. The selection of 175 *F was based on the assumption that this represents the approximate peak suppression pool temperature. However, the most recent reanalyses were showing results higher than the 175 *F selected value. Therefore, a sensitivity study was performed. It showed that an increase above 175 *F, but below 185 *F, would have a negligible impact on the resulting peak suppression pool maximum temperature. Based on this result, the licensee concluded that an iteration was unnecessary and that the 175 *F value remained an adequate feedwater shutoff temperature for analytical purposes. The staff agrees with this assessment since it results in a negligible impact on the resulting peak suppression pool maximum temperature.

The initial mass and energy release profile was mechanistically calculated using a detailed RELAP5YA model of the reactor vessel with a coupled feedwater system model for the initial

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