ML19345C830
| ML19345C830 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 11/26/1980 |
| From: | Withers B PORTLAND GENERAL ELECTRIC CO. |
| To: | Clark R Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8012080461 | |
| Download: ML19345C830 (5) | |
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Pcrtland General E!ectnc Cctrpany
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[Cb November 26, 1980 Trojan Nuclear Plant Docket 50-344 License NPF-1 Director of Nuclear Reactor Regulation ATTN:
Mr. Robert A. Clark, Chief Operating Reactors Branch No. 3 Division of Licensing U. S. Nuclear Regulatory Commission Washington, D. C.
20555
Dear Sir:
Attached is our response to actions requested in the NRC letter dated September 30, 1980 concerning inadvertent boron dilution. We have analyzed this potential problem for the Trojan Nuclear Plant and conclude that return to criticality due to a boron dilution event during operational Modes 4 and 5 will not occur. Current shutdown margins available will preclude this from happening. Consequently, no revisions to plant procedures are necessary. However, since Technical Specification require-ments alone will not guarantee an adequate shutdown margin during Mode 5, we will submit a Technical Specificat.on revision on or before March 1, 1981 to ensure minimum shutdown requirements.
Please do not hesitate to cantact me if you have any questions.
Sincerely, M t2 Bart D. Withers
" ice President Nuclear Attachment c:
Mr. Lynn Frank, Director State of Oregon Department of Energy Mr. R. H. Engelken Director NRC, Region V
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TROJAN NUCLEAR PLANT RESPONSE TO NRC CONCERNS ON INADVERTEhT BORON DILUTION DURING OPERATIONAL MODES 4 AND 5 NRC Request 1:
Recalculate the boron dilution event for the following plant conditions using the reduced RCS volume and 15-minute criteria containtd in SRP Section 15.4.6:
Mode 5 (cold shutdown) with the Reactor Coolant System a.
drained to the " half pipe" level (reduced volume).
b.
Mode 4 (hot shutdown) with no reactor coolant pumps running (effective reduced RCS volume).
PGE Response:
PCE has completed a plant-specific evaluation of the boron dilution event during operational Modes 4 and 5 for the Trojan Nuclear Plant. These calculations are based on the Westinghouse procedure outlined in Appendix A of their letter NS-THA-2273 to the NRC, dated July 8, 1980 (Westing-house's calculations appear unduly conservative relative to RCS drained system volume and/or dilution fronts for a large 4 loop plant such as Trojan).
Figure 1 depicts the drained-down reduced RCS volume assumed for our analysis. The total active volume included this active portion of the RCS plus connected Residual Heat Removal (RHR) piping, heat exchangers, and pumps associated with the primary coolant flow path for decay heat removal. Maximum dilution capability was determined to be limited by the design flow of the Primary Makeup Water pumps totaling a maximum of 300 gpm with two pumps running. The time for operator action of 15 min.
allows two dilution fronts to pass through the core with one RHR pump operating and three dilution fronts for two pumps operating.
Minimum shutdown margin requirements during Modes 4 and 5 are based on the Trojan Technical Specifications that assume the most reactive control rod is stuck out of the core, plus the stuck rod vorth. It is fair to assume that the reactivity equivalent of the stuck rod has been compensated for by boration or subsequent dropping of the stuck rod before cooldown.
In practice, Trojan procedures have always required 100 ppm boron more than that needed for the nominal margin required by the Technical Specifi-cations. This additional shutdown margin required by Trojan Operating Instructions is used to determine the available shutdown margins. These values are plotted in Figure 2.
The most limiting conditions with regard to reduction of shutdown margin occur for the case of one RHR pump running and full dilution flow for both Modes 4 and 5.
The results are plotted in Figure 2 showing minimum shutdown margins necessary to avoid recriticality conditions and are calculated over a range of initial RCS boron concentrations.
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The conclusion reached is that adequate shutdown margin has always been available, and recriticality at maximum dilution flow does not occur within the 15-min. criteria for a boron dilution event during Modes 4 and 5.
As shown in Figure 2, the minimum shutdown margin available is always larger than the predicted reactivity insertion due to the worst case boron dilution, event. However, minimum Technical Specification requirements for Mode 5 do not guarantee an adequate shutdown margin during periods of RCS boron concentration greater than approximately 1250 ppa.
NRC Request 2:
i Provide a commitment to revise appropriate plant procedures as necessary j
to ensure the correct shutdown margin is maintained.
PCE Response:
Based on the conclusions reached in the analysis, and since Trojan has always maintained 100 ppm boron additional shutdown margin over Technical Specification requirements in Modes 4 an 5., PGE,has determined that no revisions to procedures are necessary for Trojan.
NRC Request 3:
Propose revised Technical' Specification changes as necessary.
PGE Response:
Based on the conclusion reached in the analysis, PGE has determined that although the shutdown margins available are satisfactory, a Technical Specification change is desirable to ensure that the minimum shutdown margin during Mode 5 operation is adequate without the additional margin Trojan procedures require. Therefore, we will submit a proposed change to Trojan Technical Specification 3.1.1.2 prior to March 1, 1981 changing the minimum shutdown margin from 1.0%Ak to 1.6%Ak.
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300 O.96 Shutdown Margin in Terms of Initial k gg Shutdown Margin Requirements and Availability for a Boron Dilution Accident at Trojan Figure 2 i
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