ML20085C261
| ML20085C261 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 08/27/1991 |
| From: | Kovach T COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9109040026 | |
| Download: ML20085C261 (6) | |
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1400 Opus Place F
Downers Grove, Illinois 60515 August 27,1991 I
U.S. Nuclear Regulatory Commission Attn: NRR Document control Desk Washington, DC 20555
Subject:
Byron Station Units 1 and 2 Response to Unresolved item inspection Report Nos. 50 454/91011; 50 455/91010 NRG. Docket Nosm50:454.and 50 455
Reference:
(a) W. Shafer Letter to Cordell Reed dated June 28,1991 Reference provided the results of an Inspection conducted by Mr. W.J.
Kropp, et. al., from M 7 through June 18,1991 at Byron Station. The inspection identified an Unresolv d item re ated to the methodo!ogy used in the Interim 03erability Assessment of the Ultimate Heat Sink for April and May 1991. The attachmen; provides our response to the Unresolved item.
If there are any questions or comments regarding this response, please refer them to Perry Barnes, Compliance Supervisor at (708) 515 7278.
Respectfully, t
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.J.
Ovach Nuclear Li 'nsing Manager 1;
Attachment cc:
A.B. Davis, NRC Rlll Administrator i
W.J. Kropp, NRC Senior Resident inspector, Byron o-t v! f pr n
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i AllACKHENI CO4HONHEALTH EDISON COMPANY RESPONSE TO UNRESOLVED ITEM INSPECTION REPORT NO. 454/91011; 45S/91010 U kRES0LYL DJJ EMa 541910lR_455 /.91010du Af ter reviewing the preliminary operability assestment of the ESH Cooling Towers at Byron Station the NRC resident inspectors expressed the following concerns:
(1) The assessment did not account for the worst case wet bulb temperature as stated in the UFSAR.
NED utiltred wet bulb temperatures based on average high daily temperatures and average relative humidity.
The inspectors reviewed the 1987 UHS tower performance test data and determined that the wet bulb temperature used in the interim operability assessment for May (61 degrees f) was exceeded five of nine days during the May 1987 tower performance test.
The highest wet bulb tempereture recorded in those nine days for May was 78 degrees F compared to 61 degrees F used in the interim operability assessment.
The inspectors were concerned that NED used the average daily high temperature and average m3nthly relative humidity to calculate a wet bulb temperature rather than the worst case wet bulb temperature of 82 degrees F identified in Section 2 of the UfSAR.
(2) NED's assessment did not directly assess the non-conservative assumption of 33,000 gpm total SX flow. A graph was a/ailable from the 1987 UHS performance test to assess the decreased flow rate,-but was not used by NED.
Instead, NED qualitatively concluded that the lou r SX flow of 33,000
-gpm would result in a one degree increase in UHS basin temperature.
Etsponse_.to Item _1.
The operability determination process is used to ensure that a safety-related t
component or system can perform its design function under current operating conditions.
It is required when a deviation from accepted performance or the plant licensing basis is discovered. A positive assessment of operability concludes'that the deviation does not prohibit the performance of the components safety function, although it may reduce the safety margin. A positive assessment of operability allows the plant to continue operation, until the deviation is' corrected or the issue is resolved.
It is not intended to justify or change the plant licensing basis.. Should further analysis show an inoperable status, immediate action is to be taken in accordance with the Technical Specification Limiting Condition for Operation.
The CECO operability process requires that the Nuclear Engineering Department (NED) be notified of concerns regarding safety-related designs and equipment.
NED then performs a preliminary evaluation af the concern using procedure ZNLD/1153/1
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Unresolved itemTcontinued)
ENC-0E-40.1.
This procedure requires the cognizant engineer to determine all
~f functions of the equipment or system being evaluated, and determine if the concern affects the cperation of a safety-related system.
To permit a timely decision, this preliminary decision is based on the best information available at the time, which is confirmed through additional testing or evaluation as necessary.
The cognizant engineet is also required to identify compensatory or mitigating measures, where possible, which can be tafen to reduce the risk associated with the abnormal condition.
Once the preliminary evaluation is completed, it is reviewed by the NED supervisor who determines if additional testing, evaluation, or inspection is necessary to resolve the concern, and develops a schedule for completion based on the safety significance of the concern.
Operability of the Ultimate Heat Sink (UHS) at Byron Station is based on the ability of the Essential Service Hater (ESH) cooling towers to accommodate the heat load from both units simultaneously under both normal and emergency conditions.
Technical Specification (TS) 3/4.7.5 details the limiting conditions for operation for the VHS.
Performance of the cooling towers was validated by calculation in a January 1989 report, using the assumptions thown in Table 1 (Attached).
This analysis used a design wet-bulb temperature of 78 degrees f, which resulted in a maximum basin temperature of 98.2 degrees F.
On April 15, 1991, NED performed a preliminary operability assessment of the ESH Cooling Towers to evaluate the affect of increasing the estimated heat load for the non-LOCA unit and decreasing the total Essential Service Water (SX) flow for two pump operation.
The change in heat load and SX flow could potentially affect the stations ability to maintain the ESH pump diecharge temperature at or below 98 degrees i during a design basis accident.
The preliminary assessment was performed to ensure safe operation until a more cetailed evaluation would be completed on June 1, 1991.
To assess the impact of increasing the non-LOCA unit heat load, NED performed a sensitivity analysis on the 1989 analysis, which assumed a 78 degrees f wet-bulb temperature, to determine the impact of increasing the heat load.
This analysis indicated that the increased load froa the non-LOCA unit represented less than 9% of the original peak total load and would result in a 1 degree f increase in basin temperature.
The analysis was performed for the months of April and May, which are generally cooler, providing additional margin.
To quantify that margin, historical weather conditions were reviewed.
Using the historical maximum average daily temperatures and the average monthly relative humidity, as found in the Tables 2.3-11 & 2.3-44 of the Byron FSAR, the maximum expected wet-bulb temperatures for the months of April and May were calculated as 51 and 61 degrees f, respectively.
The corresponding basin temperatures were calculated as 86 degrees f for April and 90 degrees f for May.
This results in a 8 to 12 degrees f margin, when compared to the licensing basis (98 degrees f).
I ZNLD/1153/2
Based on the small increase in basin temperature and the additional margin provided by the existing weather conditions (12 degrees F in April and 8 degrees F in May), it was determined that the temperature limit would not be exceeded.
To ensure that the 98 degrees f Ilmit would not be exceeded, temporary proc 9 dural controls were implemented to provide an administrative limit of 88 degrees f for the maximum UHS basin and SX pump discharge water temperature.
In retrospect, compensatory actions should have been taken to monitor wet bulb temperature to ensure continued validity of the operability determination.
Response _to_ Item 2 The SX system at Byron Station is designed to provide cooling for the operation of safety-related equipment during normal and accident conditions.
Each cooling water pump was designed to provide 24,000 gallons per minute with one pump in operation.
The concern was raised that the actual SX system flowrate with two pumps in operation, is less than the 48,000 gpm assumed in the original calculations.
In the April 15, 1991 assessment, a qualltative approach was used because the actual flowrates to the individual tower cells were not expitcitly known, but were a function of the number and combination of tower cells in operation. As an added conservatism, only four of the eight cells were assumed to be in operation, which would reduce system performance.
Taking this conservative approach the cognizant engineer determined that the lower SX flowrate would result in a maximum jnc.rease of I degree F in the VHS basin temperature.
Based on these results, it was concluded that this concern did not affect the operability of the UHS.
On May 22, 1991, a more detailed calculation, NED-Q-MSD-2, was conpleted to support the April 15. 1991 operability assessment.
This cair.ulation, using the same assumptions as the April 15 qualitative approach, concluded that the decrease in SX flowrate would actually result in a 7 degree f decrease in the UHS basin temperature.
ZNLD/1153/3
In conclusion, the operability assessment was performed to determine whether
-there was a reasonable assurance that the ESH cooling towert could accomplish their intended safety function.
The preliminary assessment was prepared and documented in accordance with procedure [NC-Of.-40.1 justifying continued operation for the period of April 15 through May 31, 1991.
Operability determinations can rely on the known or expected conditions of a given situation or on the limiting conditivns of the design.
CECO's process does utilize known or expected conditions in its program as implemented by Procedure ENC-QE-40.1.
That procedure has recently completed its first year of use, and is currently being revised to provide additional guidance to ensure timeliness, adequacy and verification of compensatory measures, and distribution to other sites.
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ZNLD/1153/4 t
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r Table 1. Assumptions for the Operability of the Byron ESH Cooling Towers Parameter January 1989 April /May Assumptions Assumptions Initial B6$1n Temperature 88 F 88 f Admin 1.imit (78 f Alarm)
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Max. Het-Bulb Temperature 78 f 78 f LOCA Heat load 556 Million DTUS 556 Million B10s Non-LOCA Heat Load 24 Hillion 01Us 75 Million BTUS d
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