ML18141A176
| ML18141A176 | |
| Person / Time | |
|---|---|
| Site: | Surry, North Anna, 05000000 |
| Issue date: | 10/19/1983 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML18141A175 | List: |
| References | |
| GL-81-21, NUDOCS 8311020508 | |
| Download: ML18141A176 (8) | |
Text
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Safety Evaluation Report for North Anna 1 and 2, Surry 1 and 2 Regarding Generic Letter 81-21, Natural Circulation Cooldown
Background
On June lll 1980, St. Lucie Unit 1 experienced 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 Generic 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 events. This assessment should include:
(1)
A demonstration (e.g., analysis and/or test) that controlled natural circulation cooldown from operating conditions to cold 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 (3) A description of their training program and the revisions to their procedures.
The licensee responded to this request in the reference 2 letter. The following is our evaluation of the licensee's response.
Evaluation In its submittal, the licensee refers to a study performed by
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e Westinghouse for the Westinghouse Owners Group.
This study*evaluates the potential for steam formation in Westinghouse NSSS 1s and recommends modifications to the operator guidelines.
The results of the W~stinghouse report 1 W-OG-57 (RefPren~e 3); ~re bounding in that they ~re a~plicable 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 Thot and Tcold plants.
Tcold plants are those which during normal reactor coolant pump operation have sufficient flow between the downcomer and the upper head I
such that the temperature of the upper head is approximately the same as the cold leg temperature. Thot plants have an upper head tr:~perature between the hot leg and cold leg temperatures. This SER will deal with the Thot analysis because the Surry and North Anna plants are considered to be Thot 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.
The analysis assumes an inverted top hat upper support plate design since it results in a large upper head volume and hence conservatively large total heat in the upper head region.
The initial upper head e
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 ana1ysis is also done which accounts for the effect of the Control Rod Drive Mechanism (CROM) 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 4QO*psia when the primary system temperature is 350°F.
These conditions will permit the Residual Heat Removal System (RHRS) to be used to continue plant cooldown.
However, RHR entry conditions vary somewhat from plant to plant.
The analysis without the I
CROM fans shows that upper head voiding will occur unless the depressurization 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_ point. The reference report 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 25°F/hr cooldown rate and approximately 27-hours for a 50°F/hr cooldown rate.
An additional analysis includes the effect of the CROM cooling fans and results in a significant increase in the rate of cooldown of the upper head.
Per the reference report the CROM fan cooling system removes
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e 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 600°F.
Assuming that the cooldown rate is proportional to the temperature difference between coo1 the ur~Pr head at a rate of 17°F/hr when the upper head fluid is 350°F.
Based on these analyses the Westinghouse report makes the following recommendations for operator guidelines:
J.'
- 1.
If the CROM cooling effect is available the ~perator 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 **
- 2.
If the CROM fans are not available the operator should commence a 25°F/hr cooldown ar;d 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 temperature reaches 350°F, a 20-hour holding period should be allowed before depressurization to RHRS entry conditions.
Although the above recommendations were based on best estimate analyses, these analyses were conducted for a worst case plant i.e., a 12" thick e
e inverted top hat upper support plate with upper head region volume of 847 ft 3
- 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 the Westinghouse emergency response guidelines ERGs (ES-0.2) *.
The North Anna 2nd Surry plants have a 5 11 thick flat 1.,1pper support plate design with upper head region volume of 580 ft 3
- The Westinghouse ERGs recommend a 200°F subcooling margin and a 9 hour1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> cooloff period at 1200 psig for this type of plant.
The licensee stated that the North Anna plants fully implement the ERGs' recommendations.
However, the Surry plants can only implement 150°F
. subcooling due to their pressure-temperature technical specification limitations.
To account for the decreased subcooling margin the licensee is implementing an interim cooloff period of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> until NRC approval of the ERGs, Rev. 1.
Fur:hermore, the applicant stated that, although, the CROM fans on their plants are powered from emergency buses and will provide upper head cooling even during a loss of offsite power, they did not take credit for their operation. The licensee committed to adopt revision 1 of the ERGs after NRC approval. These ERGs provide for natural circulation cooldown with or without a reactor e
e vessel head bubble using the reactor vessel level indication system. The licensee states that North Anna's onsite condensate storage backed by.
the Service Water Reservior, a~d Lake Anna, provide a virtually unlimited supply of cooling water.
The *onstte--conderrs-ate--sto-ra-ge--------
includes a 110,000 gallons seismic Category I Emergency Condensate Storage Tank (CST) and a 300,000 gallons non-seismic CST.
The Surry supplies include a 110,000 gallon emergency water storage tank with a technical specification minimum of 96,000 gallons. Additionally, there is 100,000 gallons underground tank that is always maintained full although there is no technical specification limit. There is a 300,000 gallons CST that is normally greater than 50% full. There is also a
- 250,000 gallons non condensate grade tank that could be used if necessary.
The staff conducted a check analysis and concludes that North Anna and Surry plants have adequate water supplies.
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.l, Generic Review of Vendor Guidelines (Reference 4).
The staff concludes that if the licensee appropriately implements the generic NRC-approved emergency guidelines into their plant-specific procedures, adequate procedures will be available for the operator to safely conduct a controlled natural circulation cooldown.
We have confirmed that the licensee's submittal has addressed the training requirements needed to assure that the operators adequately understand the st.
Lucie 1 event and consequences.
Conclusion Upper head voiding, in itself, does not present any safety concerns provided that the operator has adequate training and procedures to recognize and react to the situation.
Voiding in the upper head makes RCS pre~5ure rontrol more difficult and therefore, if the situation warrants, natural circulation cooldown should be done without voiding.
i]
The Westinghouse analyses prov)de the length of the holdi~g period necessary to cool down the upper head region on natural.circulation without void formation when the CROM fans are not available.
Natural Circulation tests are planned for Diablo Canyon.
These tests will provide experimental verification of the upper head cooling rate calculations.
The staff concludes the licensee has verified it has sufficient condensate supplies.
We conclude that the licensee has adequately addressed training requirements.
the staff finds that upon acceptable implementation of the I
NRC-approved Westinghouse Owners Group Emergency Response ~uidelines with appropriate plant specific modicications, the licensee's procedures will be adequate to perform a safe natur?l circulation cooldown. ~(l.
(1)
Generic Letter 81-21, "Natural Circulation Cooldbwn", May 5, 1981.
(2) Leasburg, R.H. to H.R. Denton, "Natural Circulation Cooldown,"
Serial No. 309, November 30, 1981.
(3) Jurgensen, R.W. to P.S. Check, ~St. Lucie Cooldown Event Report,"
W-OG-57, April 20, 1981.
(4) Generic Letter 83-22, "SER of Emergency Response Guidelines,"June 31, 1983.