ML20236S006
| ML20236S006 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 11/16/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20236R998 | List: |
| References | |
| NUDOCS 8711240164 | |
| Download: ML20236S006 (3) | |
Text
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NUCLEAR REGULATORY COMMISSION y $%i.f'/e/
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i WASHINGTON, D. C. 20555
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ENCLOSURE 1 SAFETY EVALUATION FOR THE OFFICE OF NUCLEAR REACTOR REGULATION NORTHEAST NUCLEAR ENERGY COMPANY, ET AL.
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MILLSTONE NUCLEAR p0WER STATION, UNIT N0. 3 DOCKET NO. 50-473
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1.0 INTRODUCTION
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In a safety evaluation (SE) date'd January 71,1987, the staff evaluated and found acceptable the proposed modifications to the Millstone Unit 3 protec-J tion system proposed for 3-three loop operation. Also in that SE, the staff l
evaluated the design of the reactor coolant system loop isolation valve inter-
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locks for conformance to IEEE STD 779 based on Section 15.4.4 of the plant's final safety analysis report (FSAR) which stated that the interlocks were part of the reactor protection system and ensure that an accidental startup of an inactive loop with a lower temperature or boron concentration than the other active loops will be a relatively slow event. The staff found the design of i
the interlocks acceptable sub,iect to satisfactory resolution of the six open items discussed therein.
In meetings on February 12, 1987 and July 7, 1987 and in letters dated l
February 20, 1987, and July 31, 1987, the Northeast Nuclear Energy Company 1
(licensee) responded to the open items contained in the staff's SE. The l
licensee's position is that the reactor coolant system loop isolation valve interlocks are no longer required for plant safety and that these interlocks will probably be removed from the plant in the future (via 10 CFR 50.59).
This l
position is based on proposed administrative c(itrols (Technical Specification 3/4.4.1.6) which require verification of acceptable boron concentration and temperature in the isolated loop prior to opening the loop isolation valves in Mode 5 or 6.
In Modes 1 through 4 power is removed (per Technical Specification 3.4.1.5) from the loop isolation valve operators #or the isolated loop.
2.0 DISCUSSION i
The original FSAR evaluation of the startuo of an isolated loop took credit for interlocks that prevented opening the stop valves unless the temperature difference between the coolant in the isolated loop was within 20 degrees of that in the other loops and required that the cold leg stop valve remain closed for a fixed time (60 minutes) while coolant was circulated through a bypass around the stop valve at a low flow rate (less than 200 gpm).
A technical specification also required that the stop valves remain closed unless the boron concentration in the isolated 1000 wac creater than that in the RCS, Analyses were then performed to obtain the consequences of single failures or operator errors.
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The limited flow rate results in a mild transient even if the boron concentra-tion in the isolated loop is zero while the core concentration is at the high value present at beginning of cycle. The result of a mismatch in coolant temperature between the isolated loop and the core would be less limiting.
The presence of the mild transient would alert the operator and the loop stop l
I valves could be closed. More serious consequences could only occur if both a single equipment failure (in the delay circuity) and an operator error (failure to verify the isolated loop boron concentration) were to occur.
1 However, in testing the operation of the reactor coolant pump with throttled l
flow it was discovered that excessive vibration resulted. As a result the licensee proposed to alter the manner in which pump compliance with Specified Acceptable Fuel Design Limits (SAFDLs) is assured.
In brief, the licensee proposes to use revised interlocks and procedures (enforced via Technical Specifications) to preclude starting an isolated loop (i.e., opening stop valves) with low boron concentration or low temperature in the loop.
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j The staff reviewed the procedures and interlocks and concluded that insuffi-j l
cient information has been presented to permit the conclusions that sufficient protection was provided to prevent the violation of fuel design limits as the result of a single operator error. The licensee presented further information in a meeting with the staff on July 7, 1987 and documented the information in a letter dated July 31, 1987 3.0 EVALUATION The following interlocks and procedures are proposed:
1.
Restart of an isolated loop is permitted only during cold shutdown (Mode 5) or refueling (Mode 6).
Interlocks prevent opening the cold leg stop valve if the Reactor Coolant System (RCS) temperature is greater than 170*F.
2.
Interlocks prevent the opening of the cold leg stop valve unless the hot l
leg isolation valve is open and hot and cold leg temperature in the isolated loop are within 20*F of those in the other loops.
3.
The isolated loop is drained and refilled prior to restart.
Refill water is taken from the Volume Control Tank (VCT) which is part of the Chemical and Volume Control System and is the source of make up water for the RCS.
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. Prior to refilling the loop the boron concentrations in the VCT will be measured and confirmed to be greater than that in the RCS.
Immediately upon completion of the refill the boron concentration in the VCT will again be rneasured to confirm that it is still proper.
If not, the loop will be drained and refilled.
5.
The shutdown margin is determined to be at least 0.016 delta -k/k prior to opening cold leg loop valve (Technical Specification requirement).
The interlocks and procedures described above provide assurance that a single equipment failure or a single operator error will not produce unacceptable consequences, i.e... lead to violation of fuel design limits. The requirement for two measurements of the boron concentration in the fill water precludes the injection of low boron concentration water into the' core due to a single operator error. The use of the Volume Control Tank for filling the loop provides a further check of the boron concentration by way of the flow rates for the concentrated boric acid solution and clean water inputs.
The requirement (supported by interlocks) for low temperature difference among.
the various loops and the RCS insures that only a very mild transient can result from the startup of an idle loop. The transient analysis presented.in the FSAR bounds any transient that might occur.. The further requirement (enforcement by interlocks) that the RCS temperature be less than.170*F l
precludes the occurrence of a serious transient error in the event of the j
failure of the interlocks on temp 9rature differences.
4.0 CONCLUSION
Based on the above discussion the staff concludes that the proposed ~ procedures and interlocks are adequate to protect the Millstone Unit 3 plant from unacceptable consequences due to the startup of an idle loop.
Dated:
lKhf18 gg7 Principal Contributors:
W. Brooks F. Burrows l
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