ML15264A302
| ML15264A302 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 06/05/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML15264A301 | List: |
| References | |
| GL-81-21, NUDOCS 8506190531 | |
| Download: ML15264A302 (6) | |
Text
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION GENERIC LETTER 81-21., NATURAL CIRCULATION COOLDOWN OCONEE NUCLEAR STATION, UNITS 1, 2 AND 3 DOCKETS NOS. 50-269, 50-270 AND 50-287
Background
On June 11, 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.
Subsequently, an NRC Generic Letter dated May 5, 1981 (Reference 1) was sent to all PWR licensees.
The licensees were asked to provide an assessment of the ability of their facility's procedure and training program to properly manage similar events. The assessment was to 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 revision to their procedures.
The licensee, Duke Power Company, responded to this request in reference 2.
Additional information, in response to staff requests, was provided in references 3 and 4. The following is our evaluation of the licensee 's responses to Generic Letter 81-21.
8506195318 O PDR ADOCK 05000269 PDR
-2 Evaluation In its initial submittals, references 2 and 3, the licensee stated that a natural circulation cooldown mode is not expected to be undertaken at the Oconee Nuclear Station. Operating procedures recommend that hot shutdown be maintained until those systems required for forced circulation could be restored. This assessment was based upon the fact that the emergency power sources, the Keowee hydro units, at Oconee have a large capacity and can be utilized for powering a reactor coolant pump if offsite power was lost.
However, the licensee did decide to develop and implement a procedure for a natural circulation cooldown.
The natural circulation cooldown procedure, which was attached to reference 3, was not based upon any analysis. Rather, the procedure was based upon a nominal cooldown rate of 50oF/hr and specified operator actions to cope with a vessel head void if one developed. The licensee's opinion was that a vessel head void was a condition which could be managed without undue difficulty.
In reference 5, we reiterated that it was the staff's position that the natural circulation cooldown procedure should be developed to prevent upper head voiding to the extent possible. In addition, we requested that the licensee provide the demonstration (analysis and/or test) we believed was necessary to develop a natural circulation procedure.
The licensee responded, in reference 4, with a revised natural circulation cooldown method. The licensee now intends to continuously vent the reactor
-3 coolant through the reactor vessel upper head and out the upper head high point vent will keep the upper head cool enough to prevent flashing of the upper head as the reactor coolant system is depressurized. This method allows the operators to perform a 50'F/hr cooldown to the Decay Heat Removal System (DHRS) conditions. Guidelines for this approach were attached to reference 4.
Reference 4 also contained a supporting calculation to demonstrate that the proposed natural circulation cooldown guidelines will prevent reactor vessel upper head void formation. The RETRANO2 computer code, reference 6, was used to analyze the thermal response of the Oconee reactor vessel upper head during the cooldown. An eight-volume model was utilized for the upper head. A time dependent volume was utilized to simulate the reactor vessel upper plenum fluid temperatures and pressures based upon the operator guidelines. The upper head metal was simulated in the model.
No credit was taken for flow of the reactor coolant loop fluid through the top portion of the upper head with the exception of the flow through the high point vent.
The model was initialized assuming a system pressure of 2155 psig and upper head metal and fluid temperatures of 600*F. The reactor vessel high point vent was then opened and a 500F/hr primary system cooldown was initiated.
System pressure was maintained at 2155 psig until the hot leg temperature reached 450 0F.
System pressure was then decreased to maintain a 150OF hot leg subcooling margin throughout the remainder of the cooldown. DHRS actuation conditions were o
In i iLtIe mure hhan n hours.
ihe redLUr vessel uppe head remained subcooled throughout the cooldown.
-4 We have reviewed the licensee's analyses. The RETRANO2 code has previously been reviewed by the staff and found acceptable (Reference 7).
We have reviewed the code application and noding utilized and find it appropriate for this simulation. The assumptions utilized were consistent with the guidelines in reference 4.
Based on our review, we have concluded that the natural circulation cooldown guidelines of reference 4 provide reasonable assurance that the cooldown can be performed without formation of a reactor vessel head void. We note, however, that the guidelines in reference 4 do not provide guidance in the event that a reactor head bubble does form during the cooldown. We recommend that the licensee provide this guidance.
Operating procedures which implement the natural circulation cooldown guidelines should be developed by October 1985 per the reference 8 Order.
The licensee has verified that there is sufficient supply of condensate grade auxiliary feedwater to support a natural circulation cooldown. Reference 9 states that the Standby Shutdown Facility Auxiliary Service Water System is capable of providing sufficient secondary side cooling for over 3k days.
Since a natural circulation cooldown can be achieved in less than eight hours, we conclude there is ample auxiliary feedwater to support the cooldown.
The licensee has stated that it has implemented training following the St.
Lucie 1 event about natural circulation cooldown.
-5 Conclusion The staff concludes that upper head voiding is not a safety concern provided the operator is equipped with proper training and procedures. However, upper head voiding makes reactor coolant system pressure control and, therefore, if the situation warrants, natural circulation cooldown should be done without voiding.
The licensee proposed a continuous venting of the reactor vesel head during a natural circulation cooldown. Based upon the calculations which have been performed, we have concluded that a natural circulation cooldown to the DHRS conditions can be performed in approximately seven hours without formation of a reactor vessel head void.
We recommend that the licensee provide guidance in the procedures to cope with a reactor vessel head void should one form. These procedures should be implemented by October 1985 per the reference 8 Order.
The staff concludes the licensee has sufficient condensate supplies to perform a natural circulation cooldown.
Dated:
Principal Contributor: R. Jones
References
- 1. Generic Letter 81-21, "Natural Circulation Cooldown," May 5, 1981.
- 2.
Letter, W. 0. Parker, Jr. (Duke Power Company), to H. R. Denton (NRC),
November 5, 1981.
- 3.
Letter, H. B. Tucker (Duke Power Company) to H. R. Denton (NRC), July 19, 1983.
- 4.
Letter, H. B. Tucker (Duke Power Company) to H. R. Denton (NRC), December 12, 1984.
- 5.
Letter, J. F. Stolz (NRC) to H. B. Tucker (Duke Power Company), "Response to Generic Letter 81-21, Natural Circulation Cooldown," April 23, 1984.
- 6.
EPRI NP-1850-CCM, RETRAN-02 Computer Code Manual, May 1981.
- 7.
Letter, C. 0. Thomas (NRC) to T. W. Schnatz (Utility Group for Regulatory Licensing), "Acceptance for Referencing Licensing Topical Reports EPRI CCM-5 and EPRI NP-1850-CCM," September 4, 1984.
- 8.
Order, J. F. Stolz (NRC) to H. B. Tucker (Duke Power Company), July 6, 1984.
- 9.
Letter, W. 0. Parker (Duke Power Company) to H. R. Denton (NRC), April 3, 1981.