ML20210C505
| ML20210C505 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 09/29/1983 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML16340C148 | List:
|
| References | |
| FOIA-86-151 GL-81-21, TAC-51638, NUDOCS 8609180347 | |
| Download: ML20210C505 (8) | |
Text
r, Safety Evaluation Report for Diablo Canyon Units'1 and 2 i
Regarding Generic Letter 81-21, Natural Circulation Cooldown
Background
On June 11, 1980, St. Lucie Unit 1 experie'nced a natural circulation cooldown event which resulted in the formation of a steam bubble in the upper head region of the reactor vessel.
Consequently the NRC Gen.eric Letter dated May 5,1981 was sent to all PWR licensees.
Per that letter the licensees were asked to provide an assessment of the ability of their facility's procedures and training program to properly manage similar event's.
This assessment should include:
i (1) A demonstration (e.g., analysis and/or test) that controlled r1tural circulation cooldown from operating conditions to cold t
shutdown conditions, conducted in accordance with their procedures, should not result in reactor vessel voiding (2)
Verification that supplies of condensate grade auxiliary feedwater are sufficient to support their cooldown method, and l
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(3) A description of.their training program and the revisions to their procedures.
'l The licensee responded to this request in the reference 2 letter.
The following is our evaluation of the. licensee's response.
8609180347 860908 PDR FOIA HOLMESB6-151 PDR
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9 Evaluation In its submittal, the licensee refers to a study perfonned by Westinghouse for the Westinghouse Owners Group. This study evaluates the potential for steam formation in Westinghouse NSSS's and reconsnends modifications to the operator guidelines.
The results of the Westinghouse report, W-0G-57 (Reference 3), are bounding in that they are applicable to all 2, 3, and 4 loop Westinghouse plants. The report concludes that in previous analyses for operating guidelines and safety analyses, void formation in the upper head is explicitly accounted for if it is calculated to occur. These previous analyses indicate that voiding is not a safety concern because the voids will collapse when they come in contact with the subcooled region of the vessel.
The present analysis differentiates between T and Teold plants.
hot Tcold plants are those which during normal reactor coolant pump operation, have sufficient flow between the downcomer and the upper head such that the temperature of the upper head is approximately the same as the cold leg temperature.
T plants have an upper head temperature hot between the hot leg and cold leg temperatures.
This SER will deal with the T analysis because the Diablo Canyon plants are conside, red to be hot Thret plants.
The analysis is done using the WFLASH code with a best estimate model.
The WFLASH code has 2-phase capability and can track void propagation.
SER:Diablo Canyon. -.
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The analysis assumes an inverted top hat upper support plate design N
since it' results in a large upper head volume and hence conservatively large total heat in the upper head region.
The initial upper head temperature is conservatively set equal to the hot leg temperature.
Metal heat addition to the upper head area from.the vessel and internals is taken into account.
It is assumed that the reactor coolant pumps are stopped at the beginning of the transient.
The analysis is done for two cooldown rates 25'F/hr and 50*F/hr.
An 4
analysis is also done which accounts for the effect of the Contro1gRod Drive Mechanism-(CRDM)' cooling fans.
These fans blow containment air across the vessel head and provide cooling of the upper head and the CRDMs.
One of the conditions that must be met during a cooldown is that the primary system pressure be 400 psia when the primary system temperature is 350'F.
These conditions will permit the Residual Heat Removal System 7
(RHRS) to be used to continue plant cooldown.
However, RHR entry conditions vary somewhat from plant to plant.
The analysis without the CRDM fans shows that upper head voiding will occur unless the depressuri.ation is halted at 1200 psia and cooldown continued to a hot leg temperature of 350*F and the upper head is allowed time to cool off before depressurization to the RHRS p.idnt. The reference report i
calculates this cool-off period to be{approximately 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> for a g?S*F/hr cooldown rate and approximately 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> for a 50'F/hr cooldown 7
hate.
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'm An additional analysis includes the effect of the CRDM cooling fans and results in a significant increase in the rate of cooldown of the upper head.
Per the reference report the CRDM fan cooling system removes 780KW (12 Kw/ drive train times 65 drive trains for the analyzed plant) at full power.
This energy removal is equal to an upper head cooldown rate-of 32'F/hr when the upper head temperature is 500*F. Assuining that the cooldown rate is proportional to the temperature difference between the upper head metal and the containment atmosphere, the CRDM fans would cool the upper head at a rat'e of 17'F/hr when the upper head fluid is 350*F.
Based on these analyses the Westinghouse report makes the following recomendations for operator guidelines:
1.
If the CRDM cooling effect is available the operator can reach shutdown cooling entry conditions without void formation if a 25'F/hr cooldown rate is used and a 50*F subcooling at the hot leg _is maintained..
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2.
If the CRDM fans are not available the operator should comence a 25*F/hr cooldown and depressurize at a rate which maintains 50*F subcooling until the system reaches 1900 psia.
At this point the depressurization rate should be changed so that a 200*F subcooling margin is maintained until the system reaches 1200 psia. At this time the depressurization should be stopped, but the cooldown continued.
When the hot leg temp.trature reaches 350*F, a 20-hour holding period should be SER:Diablo Canyon -.
allowed before depressurization to RHRS entry conditions.
J Although the above re'comendations were based on best estimate analyses, these analyses were conducted for a worst case plant i.e., a 12" thick inverted top hat upper support plate with upper head region volume of 847 ft3 Recognizing that not all plants fit that description Westing-house conducted another set of analyses that account.for the variations in the upper head internal design, i.e., whether the. upper support plate is of the top hat, flat, or the inverted top hat design.
The upper support plate design determines the rate of heat conduction and the upper head water volume which~must be adequately cooled before depres-surization to the RHRS conditions is attempted. This additional set of analyses was presented by Westinghouse in the background information for i
the West.inghouse emergency response guidelines ERGS (ES-0.2).
The Diablo Canyon plants have a 5" thick top hat upper support plate design with upper head region volume of 508 ft3 The Westinghouse ERGS recomend a 200*F subcooling margin and an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> cooloff period at 1200 psig for this type of plant.
The licensee has revised it's natural circulation cooldown procedures in accordance with the Westinghouse ERGS.
The plant's procedures EPOP-23, natural circulation cooldown, Rev. 3, July 1983 specifies a cooldown rate of no more than 25'F/hr, a subcooling margin of 200*F (if CRDM t
cooling fans are not operating), and a cooloff period of 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> at 1200 psig before the RCS can be depressurized to the RHRS entry pressure of 425 psig. The licensee staed that the Diablo Canyon Technical SER:Diablo Canyon __. _
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Specifications require that a minimum of 448,006 gallons be maintained at either unit.
The above water source is made up of a minimum of 178,000 gallons in one condensate storage tank per unit plus a minimum of 270,000 gallons in a common fire water storage tank.
This source of water is backed by the plant's ultimate heat sinke, the Pacific Ocean.
The staff emphasizes the importance of procedures and operator training in resolving this issue. The review of generic guidelines is part of TMI Action Item I.C.1, Generic Review of Vendor Guidelines.
1 The staff concludes that if the licensee appropriately implements the generic NRC-approved emergency guidelines into their plant-specific
. procedures, a equate proce ures will be avai,lable for the operator to d
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safely conduct a controlled natural ~ cire,ulation cooldown.
i The Reactor Systems Branch (RSB) has not reviewed the licensee's l
l training program.
Conclusion Upper head voiding, in itself, does not present any safety concerns provided that the operator has edequate training and procedures to recognize and react to the situation.
Voiding in the upper head makes RCS pressure control more difficult and therefore, if the situation warrants, natural circulation cooldown should be done without voiding.
The Westinghouse analyses provide the length of the holding period necessary to cooldown the upper head region on natural circulation SER:Diablo Canyon,
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without void formation when the CRDM fans are not available.
Natural Circulation cooldown tests are planned for Diablo Canyon. These tests will provide experimental verification of the upper head cooling rate calculations.
The staff c6ncludes that the licensee has verified it has sufficient condensate supplies. This SER did not attempt a review of operator training and operating procedures.
The staff finds that upon acceptable implementation of the NRC-approved Westinghouse Owners Group Emergency Response Guidelines with appropriate plant. specific modifications, the licensee's procedures will be adequate to perform a safe natural circulation cooldown.
l SER:Diablo Canyon f
References (1)
Generic Letter 81-21, " Natural Circulation Cooldown", May 5, 1981.
(2)
P. A. Crane Jr., Pacific Gas & Electric Co., to F. J. Miraglia, Jr., NRC dated December 7,1981.
(3) Jurgensen, R.W. to P.S. Check, "St. Lucie Cooldown Event Report "
W-0G-57, April 20 1981.
1 SER:Diablo Canyon i
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