ML20056B686

Forwards Rept of Significant Variation in Core Reactivity Calculation,Per Tech Spec 6.6.B.1,app a

ML20056B686
Person / Time
Site:	Monticello
Issue date:	02/03/1972
From:	Mayer L NORTHERN STATES POWER CO.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9102080352
Download: ML20056B686 (3)

Text

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i NSR3 NORTHERN STATES POWER COMPANY MIN N E A PO LI S. M I N N E S OTA 88401 9

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Dear Dr. Morris:

MONTICELIO NUCLEAR GENERATING PLAKI' Docket No. 50-263 License No. DPR-22 Reporting of Significant Variation in Core Reactivity Calculation 1

A condition occurred at the Monticello Nuclear Generating Plant recently which we are reporting to your office in accordance with provisions of Section 6.6.B.1 of Appendix A, Technical Specifications of the Provision-j

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al Operating License DPR-22.

Tbe Region III Compliance Office has been notified of the occurrence in accordance with Technical Specification 6.6.A.2.

On January 24, 1972, the reactor was being made critical after an extended 3

outage.

The reactor became critical after 23 control rods were fully with-dravn. As a supplemental operatir4;; procedure, prior to restarts, the crit-ical rod pattern is analyzed in light of the temperature, exposure, and poison fission product inventory at the time of criticality. Tbe 23 r 1 criticality was not anticipated because during initial criticality, the clean core vent critical with 30 rods fully withdrawn and figure 3-3-2 of the FSAR, which has been used in the predicted critical procedure, shows that, with one stuck rod, at no time during the life of the core is the reactivity expected to be greater than that of the clean core. All rods were fully inserted and analysis of the situation commenced.

The reactivity increase observed on January 24, 1972, was felt to be the result of poison curtain depletion.

Core average exposure at the time of the occurrence was 1760 WD/STU; the previous criticals were at low expo-sures and therefore the effect of curtain depletion was less noticeable.

The situation was analyzed by a number of parallel paths. Ihta taken dur-ing past criticals along with the data point at 1760 WD/S1U vere reanalyzed.

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NC,.<THERN OTATED POWER L )MPANY Dr. P A Morris February 3,1972 A curve was fit to the data to show changes in reactivity as a function of exposure. Three independent consultants also performed theoretical calcu-lations to determine changes in reactivity as a function of exposum.

Gen-eral Electric was also asked to review the data supplied for the FSAR.

They have found that the computer code used in the initial core design tended to underpredict the degree of flux peaking in the water gaps where the curtains are located.

As a result, curtain worth and the effect of curtain depletion on co m reactivity was underestimated.

The code also predicted too large a negative temperature coefficient. General Electric has recalculated the reactivity of our core with one stuck rod using improved codes and this data is supported by the other independent calculations. A revised FSAR figure 3-3-2 is attached.

A test was conducted to measure the temperature coefficient at the present exposure and this demonstrated that the temperature coefficient is less negative than that which we have been using.

Data from past criticals were reanalyzed in light of the measured temperature coefficient to secure more refined data.

All calculated and empirical data are consistent in predict-ing (1) that the core is and should be more reactive at the present time than the cold, clean core and (2) that the core vill continue to become more reactive.

Best figures show that the peak core k-eff with one stuck rod vill be 0 980 at approximately 2500 WD/STU. A shutdown mrgin test demon-strated that the margin exceeds that required by Technical Specification 4.3.A.1.

The above investigations and use of NSP's recently completed core analysis program should further enhance our predictive capabilities.

A surveillance check for reactivity anomalies hrs been done regularly at operating power conditions in accordance with Technical Specification 4 3.E.

This test uses data supplied by another more recent General Electric computer model.

A continuous gain in reactivity has been observed during power oper-ation.

Test results at operating powr conditions have continuously been well within the acceptance criteria of the Technical Specification; bovever, comparison of hot operating data to one rod out data would not have revealed that figure 3-3-2 of the FSAR was incorrect.

A Significant Operating Event report with extensive core analysis detail vill be available at the site.

Division of Compliance representatives have been at the plant and all of this data has been mde available to them.

Yours very truly,

. h.

L 0 Mayer, P..

Director-Nuclear Support Services LOM/br cc: B H Grier Attachment

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