ML20056B797
| ML20056B797 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 02/29/1972 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Morris P US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 9102110341 | |
| Download: ML20056B797 (3) | |
Text
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NSP NORTHERN STATES POWER COMPANY mi~~eu.ou..ui~~e.
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Dr. Peter A Morris, Director mal=l 0 />l d' /la e; Division of Reactor Licensing United States Atomic Energy Commission l!y g
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Dear Dr. Marris:
MONTICELLO NUCLEAR GENERATING TLANT N
ibterv Docket No. 50-263 License No. DPR-22 Supplemental Information on the Variation in Core Reactivity Calculation A letter reporting a variation in the core reactivity calculation was trans-mitted on February 3,1972, in compliance with the 10-day reportire require-ment of Section 6.6.B.1 of Appendix A, Technical Specifications. The follow-ing informtion is to supplement that provided in the initial report.
The attached figure shows the current sequence of withdrawal of the first three rod groups, each grcup consisting of 15 rods.
It can be seen that groups #1 and y2 each require fully withdrawing rods starting near the core center and spiral-ing out to the periphery. Utilizing this sequence for initial criticality, the cold, clean core became critical with 29 rods fully withdrawn and the 30th rod partially withdrawn.
This rod pattern, as shown by the shading in the figure, creates a uniform 1-rod-in-4 pattern throughout the core.
Qualitatively, one can see that the 25th through the 30th rods merely add to the size of the core without adding significantly to the k-effective.
In addition,-because of the loose coupling of large cores, the 22nd through the 24th rods add very little to the k-effective of the 20 rod lattice.
On January 24, 1972, the core was made critical with 23 rods fully withdrawn and the 24th rod partially withdrawn. Although this is a difference of six complete rods, it must be understood that all but one of the six rods lie on the core periphery. Therefore the reactivity difference between the cold, clean critical I
core and the January 24, 1972, critical core is very small.
The rod withdrawal sequence suggested for the startup test program started group
- 3 withdrawal in the core center and spiraled out, just as for groups #1 and f2.
As can be seen in the figure, the first group f3 rods in a.ch a scheme (numbered 45 and 4h) have a large effect on the core k-effective.
Early in the startup test program it was recognized that criticality would occur on these high worth 92021i0341 720229 CF ADOCK 05000263 CF
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Nt THERN STATES POWER JMPANY Dr. P A Morris February 29, 1972 rods during hot startups.
For this reason gmup #3 was redefined to start on the core periphery and spiral in.
Similar reactors having slight differences in core design did not experience initial criticality until the first or second group #3 centrally located rods were withdrawn.
Therefore the difference of six peripherally located mds at Monticello, as discussed above, may be equiva-lent to a few notches on a centrally located group #3 rod at a similar facility.
Utilizing NSP's recently co=pleted core analysis program to perform 3-dimensional core calculations, we have calculated the core reactivity at 1760 M4D/T for 23 rods withdrawn, 24 rods withdrawn, and 30 rods withdrawn. Tbe calculated vorth of the 24th rod is 0.13% a k; the total calculated worth of the 25th through 30th rods is also 0.13% o k.
Therefore, the net difference in the calculated core k-effective from a cold, clean critical core to a critical core at 1760 MJD/T is 0.26% a k.
A revised FSAR figure 3-2-2 for the " Stuck Rod M1rgin As a Function of Exposure" was transmitted with our February 3,1972, letter.
The revised curve shows a net increase in reactivity from a cold, clean core to the point of peak reactiv-ity to be 1.2% a k with one rod out.
Our core analysis program predicts a 1.1%
& k increase, thereby establishing confidence in understanding the present core performance.
On the basis of revised FSAR figure 3-2-2 which shows that the exposed core is more reactive than the clean core and that it will become slightly more reactive later in life, a shutdown margin test was performed. Technical Specification 4 3. A.1 requires that the core can be made suberitical by a margin of (0.25 + R)%
A k with one rod fully withdrawn. At the time of the demonstration, the compos-ite value of R was 0.65% A k which included effects of the existing moderator temperature, further curtain depletion and non-peak samarium conditions. The required margin was therefore 0 90% 6 k.
The test on January 27, 1972, demon-strated that the shutdown margin with the strongest rod withdrawn exceeded 1.h8% 8 k.
Yours very truly, L 0 Mayer, P. E.
Director-Nuclear Support Services LOM/MHV/br cc: B H Grier Attachment
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