ML20197G517

SER Re Generic Ltr 81-21, Natural Circulation Cooldown. Safety Concerns Adequately Addressed

ML20197G517
Person / Time
Site:	Three Mile Island
Issue date:	06/05/1984
From:	NRC
To:
Shared Package
ML20197G509	List: ML20197G503 ML20197G517
References
GL-81-21, NUDOCS 8406150281
Download: ML20197G517 (7)

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Safety Evaluation Report for Three Mile Island Unit 1 Regarding Generic Letter 81-21 Natural Circulation Cooldown Backerau"d On J me 11, 1980, St. Lucie Unit 1 experienced a natural circulation coi t:3wn event which resulted in the formation of a steam bubble in the uppdr head region of the reactor vessel. This resulted in the generation of NPC deneric Letter 81-21, dated May 5,1981, to all PWR licensees. The ifcansees were to provide an assessment *of the ability of their facility's procedures and training program to properly manage similar event.s. This ass'esiment was to include:

4 (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.

l (2) (Nrification 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 pro-cedures.

The licensee responded to this request in references 1 and 2. The following is our evaluation of the licensee's response to the concerns outlined above. l 8406150281 840605 l PDR ADOCK 05000289 P_ FDR ~

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Evaluation To prevent reactor vessel upper head void formation during a natural circula-tion 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 2, an analysis of the reactor vessel upper head temperature during a natural circulation cooldown.

This analysis was utilized by the licensee to identify improvements to the natural circulation cooling 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 performed using the HEATING 6 (Reference 3) computer code. HEATING 6 is a multi-dimensional, generalized heat conduction code. The reactor vessel upper head was modeled in two di-mensions, R-Z geometry, based on symetry about the center control rod drive.

Ninety-one regions and 2033 nodes were used to describe the upper head in detail. The primary ccmponents of the model are the plenum cover, upper head water nass, the vessel wall, the vessel head, the vessel insulation, and the control rod drive leadscrews, guide tubes and nozzles.

In performing the analysis, the initial temperatures for the upper head

, fluid and metal were assumed to be 6040F 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. l l

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! Natural convection heat transfer correlations were utilized at all metal-water interfaces in the upper head. Thermal miking, as a result of natural convection within the upper head, was simulated via an effective thermal conductivity for the water.

j The analysis covered the natural circulation cocidown from 6040F to the decay

heat removal system (DHRS) c.utin point. To allow operation of the DHRS, RCS pressure and temperature must be reduced to 325 psig and 30G0F, respec-tively. To prevent void formation in the reactor vessel upper head t

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f at the time of DHRS cutin, the upper head fluid temperature must be reduced to less than 4290F. This temperature correspo,nds to the saturation temperature at 325 psig.

J The RCS cooldown was simulated in three steps. First, a cooldown from 6040F to 3750F, at a rate of 100F/hr, was used. A ' soak' or holding period of 15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> at 3750F was then simelated to allow the reactor vessel upper l head to cool . The RCS was then cooled to 3000F at 100F/hr.

Based on this cooldown scenario, the results showed that the reactor vessel upper head cooled at an average rate of 40F/hi. At the end of the simula-j tion, the upper head temperature had been reduced below 4290F, thereby per-

, mitting operation of the DHRS. Thus, the licensee concludes that a natural 4

circulation cooldown can be performed in 46 hours5.324074e-4 days <br />0.0128 hours <br />7.60582e-5 weeks <br />1.7503e-5 months <br /> without resulting' in re-f actor vessel upper head void formation.

1 p The staff has reviewed the licensee's analysis and finds it acceptable. .

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Based upon the analysis results, the licensee stated, in reference 2, that the TMI-1 Natural Circulation Cooling Procedure (0P1102-16) will be revised, in June 1984, to include a curve which provides the minimum pressure necessary to prevent reactor vessel upper head void formation. An example of this curve was provided in reference 2. However, the licensee intends to perform additional analysis, utilizing the same techniques described above, for various cooldown rates in order to further refine the minimum pressure curev. The staff finds this approach acceptable. In addition, the staff concludes that appropriate implementation of the minimum pressure curve into the plant-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, via reference 4, 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 identifed 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 allow for bubble collapse and thereby returning pressure control to the pressurizer. In addition, the procedure requires that a subcooling margin of at least 25'F be maintained in order to prevent void formation in the hot leg which could potentially lead to an interruption of natural circulation. The licensee has committed to improve the procedures OP1102-16 by including instructions for collapsing a steam bubble in the vessel head and L

for re-establishing natural circulation if it should be interrupted. The staff finds the guidance and commitments discussed above to be acceptable.

The licensee judged, in reference 1, that the TMI-1 condensate-grade auxiliary feedwater supplies are sufficient to support a natural circulation cooldown. TMI-1 has two condensate storage tanks each normally containing 250,000 gallons, with a Technical Specification minimum of 150,000 gallons per tank. In addition, the licensee identified two other potential sources:

a one million gallon capacity demineralized water storage tank; and about 24,000 gallons in the condenser hotwell. Thus,, TMI-1 has at least 1,300,000 gallons of condensate-grade EFW supply. We estimate that this is sufficient inventory to support a cooldown time in excess of 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br />. Since the licensee estimates a cooldown time of approximately 46 hours5.324074e-4 days <br />0.0128 hours <br />7.60582e-5 weeks <br />1.7503e-5 months <br />, we judge that TMI-1 has adequate condensate-grade AFW feedwater supply.

The licensee also provided 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 the licensee's training program adequately addresses upper head voiding during a natural circulation cooldown.

Conclusion Upper head voiding, in itself, does not present any safety concerns provitted the operater has adequate training and procedures to recognize and react to L _

l the situation. Voiding in the upper head makes RCS pressure control more difficult and therefore if the situation warrants, natural circulation

! cooldown should be performed without voiding.

The licensee's analysis showed that a 46 hour5.324074e-4 days <br />0.0128 hours <br />7.60582e-5 weeks <br />1.7503e-5 months <br /> cooldown, which includes a 15 l hour holding period to allow the reactor vessel upper head to cool, is I

necessary to prevent upper head void formation during ja natural circulation cooldown. The staff concludes that the' licensee has demonstrated its ability to cooldown without 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 CoolingProcedure(0P1102-16). The staff finds that upon appropriate implementation of these changes, the licensee's procedures will be adequate to perform a safe natural circulation cooldown.

4 Dated:

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! This Safety Evaluation was prepared by Robert Jones, Reactor Systems Branch.

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References:

1. Letter from H. D. Hukill (GPU) to J. F. Stolz (NRC), Natural Circulation Cooldown (Response to Generic Letter 81-22), December 7,1981.

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2. Letter from H. D. Hukill (GPUN) to J. F. Stolz (NRC), Na cural Circulation Cooldown, (GL81-21), April 4,1984 t

3. HEATIN G6: A Multi-Dimensional Heat Conductfon Analysis with the Finite -

Difference Formulation, RSIC # RSR-199.

4 NRC Letter, J. F. Stolz (NRC) to H. D. Hukill (GPUN), Request for Addi-tional Information, Natural Circulation Co61'down (GL81-21), July 20,1983.

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