ML17202U899

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SE Re Evaluation of Responses to NRC Bulletin 90-002. Responses Acceptable
ML17202U899
Person / Time
Site: Dresden 
Issue date: 11/29/1990
From:
Office of Nuclear Reactor Regulation
To:
Shared Package
ML17202U897 List:
References
IEB-90-002, IEB-90-2, NUDOCS 9012130339
Download: ML17202U899 (5)
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UNITED STATES NUCLEAR REGU.LATORY COMMISSION

. WASHINGTON, D. C. 20555 SAFETY EVALUATION BY THE-OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO EVALUATION OF RESPONSE TO NRC BULLETIN NO. 90-02 COMMONWEALTH EDISON -COMPANY DRESDEN NUCLEAR POWER STATION, UNIT 2 DOCKET NO. 50-237 1.0. INTRODUCTION.

ENCLOSURE The licensee responded to.NRC Bulletin No~ 90-02, "(~ss.of Thermai Margin Caused by Channel Box Bow," in a letter dated April 26, 1990, and indicated*

tha~ 303 channel boxes were being used for a second fuel bundle lifetime in the

  • Dresden_ Unit 2 reactor* core (Reference 1). -The response described the number and disposition of these channel boxes in the core, and described the actions taken.to assure compliance with the technical specification thermal limits.

The initial 'NRC staff-review of the April 26, 1990 submittal (response to NRC Bulletin 90-02), found the actions described in Attachment B of Reference 1

  • acceptable since the four bundle **array used in the analysis of. the channel box bow effect on thermal limits provided ~n_appropriate limiting configuration (Reference 2). This approval only applied tb the procedure Dresden Unit 2-took

-to circumvent the collection-and analysis of bow data, with. respect to Cycle 12.

It did not approve the methodology or sanction the future reuse of used.channel boxes *. This safety evaluation covers the staff review of the Commonwealth Edison Company strategy for reuse of channel boxes for Cycle 13 and.future cycles.

2.0 EVALUATION

_2.1 Background - Dresden Unit 2, Cycle 12 Dresden Unit 2 currently ha~ a signiftcant-number of reused channel boxes as a result of Commonwealth.Edison's previous channel box management strategy.

During the Cycle 9 and Cycle 10 refueling outages, fresh reload fuel assemblies received channel.boxes with one or two prior cycles of irradiation.

Magnitudes of-Channel Box Bow Channel box exposures were compiled and projected to the end of Cycle 12. Of the 303 reused channels, 194 will exceed 40 GWd/MTU exposure and 28 will surpass 50 GWd/MTU prior to shutdown for refueling scheduled for September 1990.

Essentially all of the reused channel boxes, 297 out of 303, were manufactured by Carpenter Technology.

Due to relatively high channel box exposure, the licensee had requested Advanced Nuclear Fuels (ANF) to perform a cycle (Cycle 12) specific analysis for Dresden Unit 2.

ANF ~ethodology consisted of assuming a limiting four 9012130339 901129 I \\

PDR ADOCK 05000237 I

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. -- bundle cell in the core (Reference 3).

Bow data base-d on three channel boxes

  • at approximately 52 GWd/MTU exposure adjacent to a limiting fresh channel box at about 12 GWd/MTU exposure were used *. The bow data at this exposure showed very good agreement when compared to inhouse NRC correlated bow data. The bounding channel box bow data used for this configuration was greater than the actual narrow gap width in Dresden Unit 2.

Channel Box Bow Effects on 1Local Peaking Factors Channel box bow data was also used to make detailed physics calculations of a

. single bundle lattice to determine local peaking factors/effects due to channel box bow.

These local peaking factors were determined using the computer co~e CASM0-3G.

The.limiting bundle '(in terms of thermal margin) in the core.was assumed to reside next to assemblies contained.in channel b.oxes with high exposure. A conservative set-of local power distributions as a function of exposure for the limiting lattice type in the cqre was subsequently determ.ined and used as input in the thenn~l margin calculations (Reference 4).

Channel Box Bow Effects on Critical Power Ratio (CPR)

ANF's generic channel box bow methodology statistically accounts.for the effects of channel box bow by an adjustment to the Mimimum Critical Power Ratio.

(MCPR) safety limit; however, an equivalent adjustment to the MCPR operating limit provides the same level_ of thermal margin protection. The thermal margin, reduction was calculated using ANF (Revision 1) ther'1'!al _hydraulic codes and _.

methods (Reference 5)~. The critical power ratio was determined from analysis*

of th~ base local peaking distributions when the bowed local distributions would result in a bundle *CPR dryout. This.analysis also simulated the core monitoring code to determine the value of the CPR that the POWERPLEX CMSS could predict that wou_ld insure that the dryout does not occur for a bundle* next to*

highly bowed channel boxes.* This re.sulted in a CPR penalty of approximately 0.15. This penalty was offset by known conservatisms in the XN-3 critical heat flux correlation (greater than 0.09).

ANF discussed this conservatism with the_

-NRC staff, pnd submitted documentation to that effect (Reference 6). This conservatism reduced the net CPR p~nalty to 0.06. This net CPR adjustment was incorpo.rated into the MCPR operating limit_ for.Dresden Unit*2, Cycle 12.

The results of Dresden Unit 2 channel box bow analysis for Cycle 12.are summarized below:*

(MCPR Operating Limit)

+

(CPR Bow Penalty) 1.39.

0.15

= (New MCPR Operating Limits) 1.45 (XN-3 Cons~rvatism) 0.09 2.2 Effects of Channel Box Reuse on Thermal Margin - Dresden Unit 2, Cycle 13 The licensee stated in their September 21, 1990 submittal titled, "Dresden Nuclear Power Station Unit 2, Fuel Channel Evaluation for Dresden Unit 2, Cycle 13" (Reference 7), that approximately 50 percent of the reused channel

-~ '

- ~*-

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boxes will be discharged from Dresden Unit 2 prior to start of Cycle 13.

Specifically, 183 of these fuel assemblies are scheduled for use in Dresden Unit 2, Cycle 13. Also Conmonwealth Edison will be placing new*channel boxes on 16 of the 183 fuel assemblies; therefore, there will be 167 assemblies with reused channel boxes in Dresden Unit 2, Cycle 13.

The 16 fuel assemblies which will receive new channel boxes will be located in a limiting location in the central region of the core. Replacement of these channel boxes will minimize the effect of channel box bow on thermal margins for Dresden Unit 2, Cycle 13 since all o.ther assemblies with reused channels will be.loaded into low power core locations, which are located within three rows of the core's* periphe.ry.

The low assembly power associated.with these reused channels will provide a large degree of MCPR_operating limit margin in these core locations~

Altho1.i'gh fuel assemblies located in low power peripheral regions of the core normally operate with substantial thermal margin, collected data and analysis shows that they are subject to greater channel box bowing because of the flux difference between the interior and exterior sides of the channel box.

Analysis by the licensee shpws that the greater thermal margin available is sufficient to offset the la.rger CPR penalty needed for expected channel box bow.

Figure A-1 of Reference 7 shows the projected end of Cycle 13 channel box exposure for those locations with reused channel boxes.

T~e 11,1aximum projected end of Cyc1e*13 channel box exposure is 53.8 GMd/MTU in a peripheral location. Figure A-1 also illustrates control blade locations to indicate in which direction the channel boxes will tend to bow. The licensee pointed out that those channel boxes with a water face :will tend to bow away from the water face due to the fltix gradients across the channel.* Therefore, the bow is likely to be restricted. by the narrow gap width of 374 mills in Dresden Unit 2~

Bowing of a channel box causes an increase in the wide-wide water gip between

  • assemblies which are adjacent to one another on either side of a control blade. For this reason,.the assemblies most affected by channel box bow are the assembly with the channel box having the* high.exposure, and the adjacent.

assemblies across the wide-wide water gap.

For Cycle 13, the assenblies with the reused channel boxes are not limiting due to their low power, as these *

  • assemblies are located near the core periphery and ar~ typically on th~ir
  • fourth cycle of. irradiation..

Assemblies loaded adjac'ent.to reused channel boxes were evaluated to determine the most *limiting location. This evaluati~n showed that assemblies loaded at Position (13, 4) of Figure A-1 (Reference 7) and the octant symmetric partners of this location are the most limiting* of a 11 assemblies affected by the peripheral reused channe 1 boxes. These assemblies are once-burne*d and are located closest to the core center (relative to all other assemblies which are

    • .loaded adjacent to assemblies with reu~ed channel boxes).

J Analysis by the licensee indicated that for Cycle 13, Dresden Unit 2 will be operating with substantial margin to the operating limit. *The analysis showed that the MCPR margin on the most limiting assemblies which would be affected by the peripheral reus~d channel boxes have a minimum of 27 percent margin to the

  • operating limit. This minimum, which occurs during the middle of the cycle at

-. peak reactivity is significantly greater than the maxi111.1m degradation (4%) due to extreme channel bow as determined by the licensee. Therefore, the reuse of channel boxes as proposed will not have a MCPR safety margin impact.

The licensee conducted in-depth analysis.of the effects of excessive channel*

box bow on the MCPR Safety Limit and the Linear Heat Generation Rate (LHGR).

Previous cycle specific sensitivity analyses were carried out for the licensee by Advanced Nuclear Fuel (ANF) ~sing the ANFB methodology.

The analyses.

showed that channel bow magnitudes similar to those projected for Dresden Unit 2, Cycle_13, could result fo an increase.to the MCPR Safety Lim;t of approximately 0.08 *. This is within the XN-3 critical power correlation conservatism of 0.09.

There.fore, the potential increase in the MCPR safety limit due to* reused channels is bounded by. the conservatism of the XN-3 correl_ation *. -

The licensee also addressed the issue of the LHGR Safety Limit. Analysis by ANF for the licensee_, showed that a sufficient operating margin (at least a

.-factor of two) to the ANF transient d~sign limit exists to ensure that the fuel design safety* limits wnl not be,approached during Cycle 13, should an overpower event occur.

MAP~HGR analysis by the licensee showed that those assemblies without the effect of channel box bow bound th~ MAPLHGR limit of those assemblies adj~cent to and wjth a second lifetime channel.

.. Conse_quently, operati_on within MAPLHGR limits for unbowed channels assures that MAPLHGR limits are*not*.exceeded,for

  • bowed channel cases.

3~b CONCLUSION.

  • Based on the above evaluation the NRC staff has concluded that the licensee's Cycle_ 13 reload core _.design with used channel boxes and the methods used to account for channel box bow has an acceptable impact on core operating limits
  • in the upcoming cycle (Cycle 13) because the data and methodology provide reasonable assurance.that the thermal margin to the CPR safety limit is main-

,tained. The Dresden Unit 2, Cycle 13 submittal has two sign_ificant conmit-

_ments which must be prese~ved.

(1)

Reused fuel channel boxes distribution (current Cycle 13*Jocations) must not be changed without NRC approval.

(2) All 167 reused channel boxes must be replaced_~t the end of Cycle 13.

If in future cycles chann*e l box reuse is considered by the 1i censee, further review and prior approva 1 by the NRC s_taff will be required.

Principal Contributor: T. Attard Dated:

November 29, 1990,

4.0 REFERENCES

1.
  • Le.tter from M. H. Richter, Commonweal th Edi son, to NRC, Response to NRC Bulletin 90-02, "Channel Bow Analysis for Dresden 2," April 26, 1990 *.
2.

Letter from NRC to Thomas J. Kovach, Commonwealth Edison Company, June 5, 1990.

3.

ANF-524(P}, Revision 2, Supplement 1, "Advanced Nuclear Fuels Corporation Critical Power Methodology for Boiling Water Reactors - Methodology for

  • Analysis of Assembly Cha11nel Bowing Effects," November 1989 *

. 4.

  • Studsvik/NFA-86/8,, "CASM0-3:

A Fuel Assembly Burnup Program (Methodology),"

Studsvik Energitekn.i k AB, Nykoping, Sweden, November 1986.

5.
  • ANF-524(P), Revision 1, "Exxon Nuclear.Critical Power Methodology for Boiling Water Reactors," Exxon Nuclear Corporation, November 1983 *

. 6.

Letter R. A. Copeland (ANF}, to R. C. Jones (USNRC), "Loss of Thermal Margin Caused by Channel*Box Bow," April 9, 1990.

7.

Letter from M. *H. Richter, Commonwealth Edison, to'NRC, "Effect of Channel

  • Reuse on Thermal Margins Dresden Unit 2 Cycle 13," dated September 21, 1990~

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