ML20206G212
| ML20206G212 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 05/15/1986 |
| From: | Stolz J Office of Nuclear Reactor Regulation |
| To: | Rodriquez R SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| References | |
| GL-81-21, TAC-47170, NUDOCS 8606250119 | |
| Download: ML20206G212 (10) | |
Text
... -
D N Ob
' n May 15,1986 Docket No. 50-312 j
i Mr. Ronald J. Rodriguez Assistant General Manager, Nuclear Sacramento Municipal Utility District L
6201 S Street P. O. Box 15830 Sacramento, California 95813
Dear Mr. Rodriguez:
SUBJECT:
Rancho Seco Nuclear Generating Station - Natural Circulation Cooldown Generic Letter 81-21 (MPA B-66) 1 i
References:
1.
Letter, J. J. Mattimore (SMUD) t6 J. F. Stolz (NRC), " Natural j
Circulation Cooldown, Generic letter No.'81-21," Nov. 16, 1981.
2.
Letter, W. K. Latham (SMUD) to J. F. Stolz (NRC), " Natural Circulation Cooldown Information," September 21, 1983.
3.
Letter, R. J. Rodriguez (SMUD) to H. L. Thompson (NRC), " Generic Letter 81-21, Natural Circulation Cooldown," September 19, 1985.
4.
Letter, W. K. Latham (SMUD) to H. L. Thompson (NRC), " Generic Letter 81-21, Natural Circulation Cooldown," November 27, 1985.
5.
HEATING 6: A Multi-Dimensional Heat Conduction Analysis with the Finite-Difference Formulation, RSIC #RCR-199.
We have completed our review of the information you provided in References 1-4 for Rancho Seco in response to Generic letter 81-21 on natural circulation cooldown.
From our review, we have concluded that you have demonstrated your ability to cool down the reactor without upper head voiding and have shown that you have sufficient condensate supply to support such a cooldown. Our Safety Evaluation Report (SE-) is enclosed for your information.
Based on the results of your analysis of the reactor vessel upper head cooling during natural circulation cooldown, you committed to revise your natural circulation cooling procedure prior to riartup from the next refueling outage.
In addition, you stated that natura? c:.culation cooldown and the potential for upper head voiding has been ir hsd. d in your training. program.
We find this acceptable.
8606250119 860515 PDR ADOCK 05000312 P
\\
U'
n s
Mr. Ronald J. Rodriguez During a telephone conversation on May 2,1986, your staff stated that procedure B.4, natural circulation cooldown, was revised and approved and the training was completed this past-fall (fall 1985). Since upper head voiding is not a safety concern, provided that the operation is equipped with adequate training and procedures, this MPA B-66 is complete for Rancho Seco.
Sincerely,
!.huGI.un sigm g
'd M ?. 320 g =..
John F. Stolz, Director PWR Project Directorate #6 Division of PWR Licensing-B
Enclosure:
Safety Evaluation cc w/ enclosure:
See next page QISTRIBUT'ON ACRS-10 Dan _ket Fi' e 3 BGrimes NRC POR JPartlow L PDR SMiner PBD-6 Rdg RIngram FMiraglia Gray File-OELD EBrach EJordan H0rnstein-gnw PBD-6 PB.f PBD-6 A
6 SMiner-G alman RWeller JS
/
5// /
5//7/86 5/D786
-5/
86
Mr. R. J. R'odriguez Rancho Seco Nuclear Generating Sacramento Municipal Utility District Station I
cc:
Mr. David S. Kaplan, Secretary Sacramento County and General Counsel Board of Supervisors Sacramenta Municipal Utility 827 7th Street, Room 424 District Sacramento, California 95814 6201 S Street t
P. O. Box 15830 Ms. Helen Hubbard Sacramento, California 95813 P. O. Box 63 Sunol, California 94586 Thomas Baxter, Esq.
Shaw, Pittman, Potts & Trowbridge 1800 M Street, N.W.
Washington, D.C.
20036 Mr. Robert B. Borsum Babcock & Wilcox Nuclear Power Generation Division Suite 220, 7910 Woodmont Avenue Bethesda, Maryland 20814 Resident Inspector / Rancho Seco c/o U. S. N. R. C.
14410 Twin Cities Road Herald, California 95638 Regional Administrator, Region V U.S. Nuclear Regulatory Commission 1450 Maria Lane, Suite 210 Walnut Creek, California 94596 Director Energy Facilities Siting Division Energy Resources Conservation &
Development Commission 1516 - 9th Street Sacramento, California 95814 Mr. Joseph 0. Ward, Chief Radiological Health Branch State Department of Health Services 714 P Street, Office Building #8 Sacramento, California 95814
=,r,.
Safety Evaluation Report for Rancho Seco IJnit 1
)
Regarding Generic Letter 81-?1 l
Natural Circulation Cooldown
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. This resulted in the generation of NRC Generic Letter 81-21, dated May 5, 1981, to all PWR licensees. The licensees were to provide an assessment of the ability of their facility's procedures and training program to properly manage similar events. This assessment was to include:
(1) A demonstration (e.g., analysis and/or test) that controlled natural circulation cooldown from operating conditions to cold shutdown condi-tions 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 references 1 through 4.
The following is our evaluation of the licensee's responses to the concerns outlined above.
l l
_2_
Evaluation To prevent reactor vessel upper head void formation during a natural circulation cooldown, the reactor coolant system (RCS) pressure must be maintained above the saturation pressure corresponding to the reactor vessel upper head fluid temperature. The licensee provided, in reference 3, an analysis of the reactor vessel upper head temperature during a natural circ:;1ation cooldown. The analysis was performed by GPU for the TMI-1 Nuclear Generating Station. Since the TMI-1 and Rancho Seco reactor vessels are virtually identical, the licensee concluded that the results are appropriate for Rancho Seco. This analysis,was utilized by the licensee to identify improvements to the natural. circulation cooldown procedure needed to assure that voids will not form in the reactor vessel upper head during the cooldown.
The analysis of the upper head cooldown was perfonned using the HEATING 6 (Reference 5) computer code. HEATING 6 is a multi-dimensional, generalized heat conduction code. The reactor vessel head was modeled in two dimensions, R-Z geometry, based on symmetry about the center control rod drive. The primary components of the model are the plenum cover, upper head water mass, the vessel wall, the vessel head, the vessel insulation, and the control rod drive leadscrews, guide tubes and nozzles.
l In performing the analysis, the initial temperatures for the upper head fluid and metal were assumed to be 604 F which corresponds to the hot leg temperature at 100% power. The reactor coolant pumps were tripped at the start of the analysis and a flow coastdown to a natural circulation flow of 3% was used.
Coolant flow through the control rod drive guide tubes was assumed to be 8%
of the system loop flow. The guide tube flow was assumed to mix only in the first 20.5 inches above the plenum cover. Natural convection heat transfer coefficients were utilized at all metal-water interfaces in the upper head.
l Thermal mixing, as a result of natural convection within the upper head, was simulated via an effective thermal conductivity for the water.
The analysis covered the natural circulation cooldown from 604 F to the decay heat removal system (DHRS) cutin point for Rancho Seco. To allow operation of the DHRS, RCS pressure and temperature must be reduced to 250 psig and 280*F, respectively. To prevent void formation in the reactor vessel upper head at the time of DHRS cutin, the upper head fluid temperature must he reduced to less than 406*F. This temperature corresponds to the saturation temperature at 250 psig.
Two analyses were performed to calculate the thermal response of the reactor vessel head as a function of cooldown rate in the RCS. The cooldown rates imposed on the RCS were 10 F/hr and 50 F/hr and were continued until the RCS temperature reached 204 F.
Using an RCS cooldown rate of 10 F/hr, the licensee concluded that it would take approximately 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> before the head temperature was reduced below 406 F.
At a 50 F/hr RCS cooldown rate, the head was cooled below 406 F in approximately 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />.
The staff reviewed the approach utilized by the licensee. We find the methods utilized and the assumptions made to be reasonable. Therefore, we find the cooldown analyses acceptable.
i i
l l
. Based upon the analysis results, the licensee committed to revise its natural l
circulation cooldown procedure to reflect a cooldown rate of greater than 10 F/hr but less than 50 F/hr.
In addition, the data from the 50 F/hr cooldown analysis will be included in the pressure / temperature limits for a natural circulation cooldown. This case was found to yield the highest reactor vessel head temperatures, and thereby the highest system pressure needed to prevent upper head flashing, for a given RCS temperature. These modifications to the procedure will be fully implemented prior to startup from the next refueling outage.
We conclude that appropriate implementation of these modifications into the pl6nt-specific procedures will be adequate for the operator to safely conduct a natural circulation cooldown without upper head void formation.
While Generic Letter 81-21 requested that the licensee demonstrate that a natural circulation cooldown could be performed without upper head void formation, the staff also requested that the licensee demonstrate that the procedures also provide guidance to the operator to recognize and respond to an upper head void should one occur. The licensee identified specific portions of its Natural Circulation Cooling Procedure which includes guidance on recognizing void formation and actions to be taken should a void form. The procedure states that a reactor vessel head void can be recognized by a large, rapid increase in pressurizer level while reducing RCS pressure. Should this occur, the procedure prescribes increasing RCS pressure to the maximum pressure allowed by the applicable cooldown curve to allow for bubble collapse and I
l
. thereby returning pressure control to the pressurizer.
In addition, the procedure requires that a subcooling margin of at least 50*F be maintained in order to prevent void formation in the hot leg which could potentially lead to an interruption of natural circulation. The staff finds the guidance to be acceptable.
The licensee judged, in reference 2, that the Rancho Seco condensate-grade auxiliary feedwater supplies are sufficient to support a natural circulation cooldown. Ranch Seco has a minimum 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> supply of condensate water from the condensate storage tank.
In addition, the licensee identified two other potential sources: The Folsom South Canal pumps and the plant reservoir.
Since the licensee's analyses show a maximum cooldown time of approximately 32 hours3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br />, we judge that Rancho Seco has adequate condensate-grade AFW supply.
The licensee also provided in reference 1 a description of its training program dealing with reactor vessel upper head-voiding. The licensee stated that its operators have been trained on the St. Lucie Unit 1 event.
In addition, the operators are trained on the use of the Natural Circulation Cooling Procedure, including recognition and mitigation of an upper head void. We conclude that 4
the licensee's training program adequately addresses upper head voiding during a natural circulation cooldown.
4
. Conclusion i
Upper head voiding, in itself, does not present any safety concerns provided the operator has adequate 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 ciruclation cooldown should be performed without voiding.
The licensee's analysis showed it would take less than 32 hours3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br /> to allow the reactor vessel upper head to cool sufficiently to prevent upper head void formation during a natural circulation cooldown. The staff concludes that the licensee has demonstrated its ability to cool downwithout voiding and has shown it has sufficient condensate supply to support such a cooldown.
The licensee identified changes to be made in the natural circulation cooling l
procedure. The staff finds that upon appropriate implementation of these J
changes, the licensee's procedures will be adequate to perform a safe natural circulation cooldown.
References 1.
Letter, J. J. Mattimore (SMUD) to J. F. Stolz (NRC), " Natural Circulation Cooldown, Generic Letter No. 81-21," Nov. 16, 1981.
2.
Letter, W. K. Latham (SMUD) to J. F. Stolz (NRC), " Natural Circulation Conidown Information," September 21, 1983.
3.
Letter, R. J. Rodriguez (SMUD) tq H. L. Thompson, (NRC), " Generic Letter 81-21, Natural Circulation Cooldown," September 19, 1985.
4 Letter, W. K. Latham (SMUD) to H. L. Thompson (NRC), " Generic Letter 81-21, Natural Circulation Cooldown," November 27, 1985.
5.
HEATING 6: A Multi-Dimensional Heat Conduction Analysis with the Finite-Difference Formulation, RSIC #RCR-199.
1
-,e
,.m-.--,----
.,