ML20127J284

Order Modifying License DPR-22 to Require Submittal of Reevaluation of ECCS Cooling Performance Calculated in GE Evaluation Model Approved by NRC & Corrected for Errors

ML20127J284
Person / Time
Site:	Monticello
Issue date:	03/11/1977
From:	Rusche B Office of Nuclear Reactor Regulation
To:	NORTHERN STATES POWER CO.
Shared Package
ML20127J276	List: ML20127J273 ML20127J284
References
NUDOCS 9211190259
Download: ML20127J284 (7)

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- UllITED STATES OF-AliERICA t!UCLEAR REGULATORY C0!illISS10H f'. .

In the liatter of- )

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Northern States Power Company ) Docket No. 50-263 .

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' lionticello fluclear Generating )

. Plant )

! ORDER FOR 11001FICATION OF LICENSE I.

l The llorthern States Power Company (the licensee), is the holder of Provisional Operating License fio. DPR-22 which authorizes the operation of. the nuclear i

! power reactor known as Monticello Muclea Generating Plant (the facility) i i at steady state reactor power levels not in excess of 1670 megawatts thermal (rated power). The facility consists of a boiling water reactor (BUR)

{ located at the licensee's site in Uright County, Minnesota.

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j In conformance with evaluations of the performanc of the. Emergency Core 4 Cooling Systen (ECCS) of the facility submitted by the licensee on July 9,.

1975, and supplements thereto dated August 4,1975, and April 23, 1976, the

Technical Specifications issued for the facility on October 30,1975, and June 18,1976, limit the Average Planar lleat

Generation Rates to the values shown on Technical Specifications Figures 3.ll.1-A through 3.11.1-D.

The ECCS-performance evaluation submitted by the licensee was based upon a' previously approved ECCS evaluation model developed by General Electric-1 .

' Company (General Electric), the-designer of the facility. This model has L

been found'to conform to the requirements of the Commission's ECCS l 921119o2s9 77o323 ADOCK 05000263

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2-Acceptance Criteria,10 CFR Part 50 % 50.46 and Appe'ndix K. The evaluation indicated that with the average planar linear heat generation rate limited ,

as set forth above, and with the other limits set forth in the facility's Technical Specifications, the ECCS cooling performance for the facility would conform with the criteria contained in 10 CFR l 50.46(b) which govern calculated peak clad temperature, maximum cladding oxidation, maximum hydrogen generation, coolable geometry and long term cooling.

Recently, the NRC staff was informed by General Electric that several errors had been discovered in the computer codes used to calculate peak clad temperature and the clad oxidation percentage in the General Electric evaluation model. These errors have been discovered by General Electric during a continuing internal Quality Assurance audit of their LOCA evaluation model codes.

This audit is still under way and the errors reported reflect those found to date. The additional effort expanded by the vendor to enhance the assurance of the quality of its evaluation model, the staff believes, was prudent and desirable. Identification of additional errors of a ninor nature may still develop during the ongoing QA checks. Nonetheless, the staff believes it appropriate to order the correction of tnos< uncovered thus far. While some of- these errors discussed herein have either no significant effect or a conservative effect on the evaluation results, one or more of the errors ,

included in the Monticello ECCS evaluation leads to non-conservative values.

Based on a preliminary assessment, including information and supportive calculations by General Electric, the HRC staff has determined that the l

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.3 3-combined effect of the following code errors would, when corrected, result

, in an ECCS evaluation requiring no reduction in operating limits for 11onticell o.

(1) Pressere Rule

, The LAMB code is used to calculate system pressure during the LOCA. This 4

calculated pressure is then used as an input to the REFLOOD code which calculates the water level vs time relationship in the core. General 4

Electric used an approxination of the pressure response of the LN1B code i that was thought, at the time of approval, to be an acceptable represen-tation of the physical phenomena involved. Later application of this approximation to certain cases showed it to be non-conservative. General Electric proposes to correct this nonconservatism by utilizing a conservative i

! approximation to the pressure rule for input into REFLOOD. This correction increases reflood time by 0 to 50 seconds and decreases MAPLHGR by 0 to 5%.

i-l (2) Bundle Vanorization General Electric has used incorrect coefficients in the calculation of l the amount of vaporization occurring during core spray. The vapor formation in the- bundle is a prime determinant of the amount of spray

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water that can get through the upper tie plate and reflood the core.

The vapor formation was under-calculated by approximately 4% resulting in. a 20-second increase in reflooding time and about a 2% decrease .in the MAPLHGR.

- (3) Discharoe Break Modeling

- General- Electric proposes to take credit for an approved model for suction line friction (from the vessel nozzle to the discharge side of the

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recirculation punp) that improves re'iooding time fo'r the- discharge break -

by approximately 15 seconds. This increases the f1APLHGR for discharge break, limited plants by about 1.5%. This error does not apply to tionticello.

4 (4) Structural Absorption of Gamma Heat General Electric has erroneously taken double credit for power generation l

i in non-fuel structural material. Correction of this error results in i approximately a 4% decrease in the MAPLl!GR for certain plants. - This error j does not apply to Monticello.

(5) Increased Counter Current Flow Linitino (CCFL) Differential Pressure 4

Some experimental evidence exists that the differential pressure in a fuel assembly during periods of CCFL nay be higher than previously assumed.

This could cause a delay in reflood time. Correction of this error reduces the Monticello MAPLHGR by 1 to 2%.

i (6) Others i

Several small changes of inputs to the evaluation codes were identified

! as being necessary to correct errors. They included:

l (a) The use of actual plant specific break areas for the LOCA; (b) A reduced core plate weight; j (c) An increase in the peripheral bypass area used in the counter current flooding calculations; (d) The correction of a decimal point error in the assumed guide -

tube thickness; and (e) Credit is no longer-. assumed for recirculation loop discharge

! valve closure during blowdown.

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Due to the errors in the ECCS analysis currently approved by URC for flonticello, the staff requested the licensee to submit an estimate of the .

impact of these errors on the peak clad temperature that would result from the worst break, if the errors were corrected. The revised ECCS calculations indicated that the liAPLHGR is conservative by approximately 2% considering the cumulative effect of these errors. Although GE model changes under review by the staff generically show that fiAPLHGR limits even higher than those presently set forth in the Technical Specifications for flonticello would still satisfy ECCS liaits, no credit for such increase was considered by the t'RC.

The staff expects that when final revised. calculations for the facility are submitted using the revised and corrected model, they will demonstrate that operation with the linear heat generation rates set forth in this Order will conform to the Criteria of 10 CFR' f 50.46(b). Such revised calculations fully conforming to the requirements of 10 CFR f 50.46 are to be provided for the facility as soon as possible.

i As discussed herein, the present flAPLHGR limits for this facility are such that they assure that the ECCS will conform 'to the performance requirements of 10 CFR f 50.46. Accordingly, such limits provide reasonable assurance that the public_ health and safety will not be endangered.

Upon notification by the !!RC staff on February 14, 1977, the licensee committed to submit a re-evaluation of the ECCS performance of flonticello on a timely basis. The staff believes that the licensee's action, under the circumstances, is appropriate and that this action should be confirmed i by NRC Order.

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III. I Copies of the following documents are available for inspection at the ,

l Commission's Public Decument Room at 1717 H Street, Washington, D. C.

20555 and are being placed in the Commission's local public document i i

room at the Environmental Conservation Library, Minneapolis Public Library, l

300 Nicollet Mall, Minneapolis, Minnesota 55401.

(1) Letters from General Electric to URC dated February 14, 1977, and January 26, 1977; (2) Letters from Northern States Power Company to the Director of Nuclear Reactor Regulation dated January 13, 1977, and February 18,1977; (3) Letter dated July 9,1975, from Morthern States Power Company to NRC and supplements thereto dated August 4,1975, and April 23, 1976; (4) This Order for Modification of License in the matter of Northern States Power Company (Monticello !!uclear Generating Plant).

Accordingly, pursuant to the Atomic Energy Act of 1954, as amended, and I the Commission's Rules and Regulations in 10 CFR Parts 2 and 50, IT IS i

ORDERED THAT Provisional Operating License No. DPR-22 is hereby amended by adding the following new provision:

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7 (1) As soon as possible, the licensee shall submit a re-evaluation of ECCS cooling performance calculated in accordance with General. .

Electric Company's Evaluation flodel approved by the !!RC staff and corrected for the errors described herein and any other e

corrections in the iiodel of which the licensee is aware at the time the calculations are performed.

FOR TijE t'UCLEAR REGULATORY COMi11SS10t1 l f.. ,Y,/'tederiq

/p.3'6m.e l

t Ben C. Rusche, Director Office of fluclear Reactor Regulation

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Dated in Bethesda, l'aryland this lith day of March, 1977.

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