ML20024G793

Discusses Planned Reactor Operation from 2,000 Mwd/T to End of Cycle 2

ML20024G793
Person / Time
Site:	Monticello
Issue date:	08/21/1973
From:	Mayer L NORTHERN STATES POWER CO.
To:	Oleary J US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9104300421
Download: ML20024G793 (7)

Text

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MSR2 NORTHERN STATES POWER COMPANY MIN N E A PO LI S. M t N N E S OT A 89400

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N ', C d 1 Mr. J F O' leary, Director Directorate of Licensing

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Dear Mr. O' Leary:

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MONTICELLO FOCLEAR GENERATING P1 ANT Docket No.54-263 License No. DPR-22 Planned I.cactor Operation From 2000 MWD /T to the End of Cycle 2 I. Introduction We are herein stating our plans for operation of the Monticello reactor during the period from the limiting exposure threshold to the end of cycle 2 as requested in the July 2,1973 1ctter from Mr. D J Skovholt. Previous letters have discussed recent transient reanalyses done for Monticelin end of cycle conditions. See references 1 to 6. Analyses show that late in cycle 2 following a turbine trip without bypass, the limiting transient used for relief valve sizing, the peak vessel pressure will fall within the 25 psi design margin to the lowest safety valve set point. Operation can safely continue to the end of cycle 2 if a power limitation is imposed late in the cycle; no changes in the Technical Specification are required.

However, changes to plant equipment and Technical Specificatiots are being evaluated which will minimize or eliminate the power restriction.

II. ,End of Cycle Cot. sideration Calculations show that operations can safely proceed throughout an initial portion of cycle 2, but beyond which special restrictions must be imple-mented. As control rods are continually being withdrawn to compensate for fuel depletion, the negative scram reactivity available decreases. At the same time the natural power shape of a BWR depictes the bottom and middle sections of the core with respect to the top of the core; this further y shifts the scram reactivity curve. The effect of the shif t is to delay

/ t.egative reactivity insertion on scram.

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NORT. .RN CTATEC POWER Coh ANY e ~ ~2-The most limiting transient with respect to governing criteria is the turbine trip without bypass. The General Electric recommended design criteria has been that the peak vessel pressure must not come within l The scram reactivity re-25 psi of the lowest safety valve set point.

quired to meet this requirements is then determined. The control rod pattern for rated power as a function of exposure is then compared to the required scram reactivity to determine the exposure threshold to which full power operation can safely continue. If power is reduced 1 af ter reaching the threshold exposure, the transients are less severe and operation can proceed safely. The most 10miting condition comes at the end of cycle when all control rods are fully withdrawn. Through a series of scoping calculations, the power icvel is determined at which the 25 psi margin is maintained for the turbine trip without bypass transient. The exposure threshold for full power operation and the all-rods-out exposure and power pointa fall on a locus of allowabic operating cor.ditions shown in the attached figure. Operation below and to the left of the locus is acceptable.

As shown in the figure, one allowabic option is to operate to the cal-culated exposure threshold at rated power. At that point, one could maintain a constant control rod pattern and coast down in power until reaching the all-rods-out power threshold af ter which additional control rods could be withdrawn until reaching the all-rods-out condi-tion. To meet system power requirements, the next refueling outage is not scheduled until late in February,1974; we plan to coast down the all-rods-out curve until that date.

Calculations are done using the same assumptions, techniques and accep-tance criteria presented in the FSAR and in references 2 and 6. It should be noted that these analyses incorporate conservative multiplying factors on the void and Doppler coefficients and apply a conservative reduction factor on the scram worth.

III. Present Evaluations Evaluations of the locus of allowable end of cycle conditions center around safety valve set point changes, relief valve modifications and improved scram times. Relief valve mcdifications will reduce peak vessel pressure follov*rg transients for the end of cycle 2 as well as subsequent cycles. Safety valve setting increases will maintain or bnprove the margin between vessel pressure and valve set points. A modified Technical Specification scram tLme is being evaluated as an interim step for the remainder of cycle 2 until more major changes, such as installation of additional relief valves can be implemented.

The nature of near term improvements are as follows:

NORT. .RN CTATE'3 POWER Coh ANY 3

A. Safety Valve Set Point Change The end of cycle concern is only that of peak vessel pressure ap-proaching safety valve set points, not over-pressurization of the reactor vessel. One solution is to raise all safety valves above the present 1210 and 1220 psi settings. The criteria used for safety valve sizing is the MSIV closure event with failure of direct scram initiation on MSIV position but an indirect scram on high neutron flux. Analyses show that the allowable peak vessel pressure will not be exceeded if this unlikely event were to occur from rated power at the end of cycle 2 with all-rods-out and all safety v:1ves set at 1240 psi. With this higher safety valve set point, there is less potential for lif ting a safety valve during operational transients such as the turbine trip without bypass and therefore a power level nearer rated power can be attained in the all-rods-out condition. Since this set point change requires a cold shutdown , it is planned for a fall outage.

A safety evaluation and proposed Technical Specification changes are presently in preparation and review.

B. Relief Valve Openinn Time Reference 6 states that the time required for initial opening of the relief valves is approximately 0.8 sec rather than the 0.2 sec design opening time. As reported, the slower than expected opera-tion results from steam condensation in the area above the main piston. A modification to alleviate this problem has been tested and found acceptable. The modification consists of machining a small groove in the valve sleeve to provide a gravity drain w* ere the condensate collects. A second groove is machined to introduce steam at a high point in the chamber to drive the steam out the drain port. This modification also requires a cold shutdown and is planned for a short fall outage.

C. Scram Times Transient analyses and scram reactivity calculations are based on the Technical Specification control rod scram times. The measured scram times over hundreds of data points have consistently been fas-ter than Technical Specification scram times. We are, therefore, studying the ef fects of shorter scram times on transient analyses.

If analyses show significantly improved results, we may propose interim Technical Specification changes to that effect. While we do not consider faster scram times to be attainabic for the life of the plant, we believe the past performance is indicative of at Icast the remainder of cycle 2 and, therefore, a reasonable basis for

NORT. ;RN CTATE'3 POWER Coh ANY consideration of an interim Technical Specification for the remainder of cycle 2. We have recently modified our control rod drive system to operate at the GE '67 product line scram valve air supply pres-sure which is known to improve scram times. While we have not had time to collect sufficient data following this modification upon which to support an additional decrease in scram time, it will give us assurance of repeating or Lmproving the data observed in the past, thereby increasing the conservatism in the transient analysis.

IV. Present Need For Technical Specification Changes The need for changes to the Technical Specifications has been evaluated.

Assuming no changes are made to the safety valves, relief valves or scram times, the plant can operate to a conservatively calculated full power exposure threshold of 2000 MWD /T af ter which the control rod pattern would remain fixed while coasting down to 84% power. Power operation would continue at 84% until reaching the all-rods-out condition. This power restriction would be administratively controlled in the same way operation not exceeding 100% power is presently controlled. The need for reducing trip settings has been evaluated. It was found that events initiated from the restricted operating icvels will be no more severe than previously ar.alyzed.

V. Future Alternatives With tne safety valve set points at 1240 psi, relief valve delay tLaes close to their design value and modified control rod scram times, the effects of the limiting transients are expected to be at or near the conservative General Electric recommended 25 psi margin to the safety valve set point for the remainder of the cycle. With such changes, a less restrictive locus for allowable operating power levels exists which lies above and to the right of that shown in the attached figure.

Additional larormation including Technical Specification changes will be forthcoming to address these areas. Future alternatives to eliminate the end of cycle scram reactivity effects for the present and future cycles are being formulated and will be reported as they are established.

Yours very truly, GP k L 0 Mayer, PE /

Director of Nuc1 car Support Services LOM/MHV/br cc: B H Grier G Charnoff Minnesota Pollution Control Agency 1

l Attn. Ken Dzugan i

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I REFERFNCES

1. L 0 Mayer (NSP) to A Ciambusso (USAEC), " Report of a Change in the Transient Analysia as Described in the FSAR," dated August 14, 1972

2. L 0 Mayer (NSP) to A Ciambusso (USAEC), " Supplemental Report of a Change in the Transient Analysis as Described in the FSAR," dated February 13, 1973

3. L 0 Mayer (NSP) to J F O' Leary (USAEC), " Change Request Dated June 1, 1973," dated June 1, 1973

4. D J Skovholt (USAEC) to L 0 Mayer (NSP), (Issuance of Technical Specification Change No. 8), dated July 2,1973

5. L 0 Mayer (NSP) to J F O' Leary (USAEC), " Submittal of Cycle 2 Startup Report," dated July 12, 1973

6. L 0 Mayer (NSP) to J F O' Leary (USAEC), " Observed Relief Valve Opening Times Different Than Those Assumed in the Transient Analysis," dated August 1, 1973

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• BUTION FOR P ART 50 DOCKET MATT **' ~'L (TEMPORARY FORM) CONTROL NO:_6458 FILE:

FROM: DATE OF DOC DATE REC'D LTR MEMO RPT OTHER Northern States Power Company Minneapolis, M!nnesots 55401 6-21-73 8-24-73 x L. O. Mayer TT): ORIG CC OTHER SENT AEC PDR X John F. O' Leary 1 signed 39 SENT LOCAL PDR X CLASS UNCLASS PROP INFO INPUT NO CIS REC'D DOCKET NO:

XXX 40 50-263 DESCRIPTION: ENCLOSURES:

Ltr te our 8-2-73 Itr... furnishing planned reacto peration from 2000 MWD /T to end of g DO NOT REMOVE PLANT NAME: Monticello FOR ACTION /INFORMATION 8-25-73 fod BUTLER (L) SCHWENCER(L) ./ LID 4 ANN (L) REGAN(E)

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