ML19257D223

Forwards Response to IE Bulletin 79-26, Boron Loss from BWR Control Blades. Computer Program Developed to Track Control Rod History.Identified 81 Control Rods Which Require Replacement at End of Cycle

ML19257D223
Person / Time
Site:	Oyster Creek
Issue date:	01/04/1980
From:	Ross D JERSEY CENTRAL POWER & LIGHT CO.
To:	Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
IEB-79-26, NUDOCS 8002010546
Download: ML19257D223 (22)

Text

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Y.Ze-Jersey Central Power & Ught Company

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ff Madison Avenut, at Punch Bowl Road Morristown, New Jersey 07960 (201)455-8200 January 4, 1980 Mr. Boyce H. Grier, Director Office of Inspection and Enforcement United States Nuclear Regulatory Commission Region 1 631 Park Avenue King of Prussia, Pennsylvania 19406

Subject:

Oyster Creek Nuclear Generating Station Occket No. 50-219 IE Bulletin 79-26 The purpose of this letter to respond to the directions set forth in IE Bulletin 79-26 which is concerned with the loss of boron from BWR control blades. Our responses to Action items 1 through 4 are given in Attachment No. 1. Attachment No. 2 is included to provide information in support of the response to Item No. 2b.

Very truly yours,

,- O Donald A. Ross, Manager (

Generating Stations-Nuclear pk Attachments (2) cc: United States Nuclear Regulatory Commission Office of Inspection and Enforcement Division of Reactor Operations Washington, DC 20555 1857 034 ,

800201054-6 Jersey Central Power & Ught Company is a Memeer of the General Pubhc Ut.hties System

ATTACHMENT NO. 1 OYSTER CREEK NUCLEAR GENERATING STATION lE BULLETIN 79-26 Item 1: The operating history of the reactor is to be reviewed to establish a record of the current BI O depletion averaged over the upper one-fourth of the blade for every control blade; the record is to be maintained on a continuing basis. This action is required on all reactors whether shut down for refueling or operating.

Answer: A computer program has been developed by GPU/JCP&L to track control rod history over the Oyster Creek operating history. The code, RODEX, was modified to calculate exposure over the top quarter of the control blade and to include a correlation factor supplied by the General Electric Company (GE) to relate control blade exposure to %B-10 depletion. The code is being used to track control blade exposure and B-10 depletion for the current cycle.

The code was further modified to calculate projected control blade exposures based on exposure data from 3-D reactor simulator model and is used in the determination of requirements for control blade replacement.

Item 2: Identify any control blades predicted to have greater than 34% B10 depletion averaged over the upper one-fourth of the blade by next refueling outage.

a. Descrioe your plans for replacement of identified control blades.

b. Describe measures which you plan to take Justifying continued oper;tions until the next refueling specifically addressing (1) any blade with greater than 42% depletion averaged over the upper one-fourth of the blade; and (2) the condition where you find greater than 26% of the control blades calculated to have greater than 34% depletion averaged over the upper one-fourth of the blade.

Answer: 2a. Based upon the analytical approach described in 1, above, it was determined that 81 control blades will require replacement at the end of the current cycle. These 81 control blades include those that have exceeded their end of life criteria and those that are projected to exceed their end of life by the end of next cycle. However, due to storage limitations, only 75 of the 81 blades can be replaced. The six blades that cannot be replaced are projected te have between 34%

and 37% B-10 depletion by the end of next cycle. This will result in 4.4% of the control blades exceeding 34% B-10 depletion. GE has indicated that if no more than 26% of the control blades have exceeded 34% B-10 depletion, there is negligible effect on transient CPR reduction and MCPR effects. When the blades exceed 34% B-10 depletion during the cycle, the appropriate shutdown margin (SD:4) adder will be calculated and included in the minimum shutdown margin requirements.

1857 035

Oyster Creek Nuclear Generating Station Attachment No. 1 IE Bulletin 79-26 Page 2 2b. The GPU/JCP&L staff has been aware of the B-10 depletion problem in control blades since March 1979 when GE reported the results of its hot cell examination of an Oyster Creek control blade. At that time, a combined analytical and experimental program was established to determine the impact of B-10 depletion on the safety of operation. A report of this evaluation, describing the analyses performed and the results of the shutdown margin testing, was prepared and is included for information purposes (Attachment No. 2).

The analysis considered the impact of B-10 depletion on both shutdown margin and scram reactivity. GE calculated for Oyster Creek a shutd own margin adder to incorporate with minimum shutdown rargin requirements. GPUSC also calculated a shutdown margin adder to include control blades which have exceeded 42% B-10 depletion. The assumptions used in the GPUSC calculation were that blades having between 34% and 42% B-10 depletion had no boron in the top six inches and blades having greater than 42% B-10 depletion had no boron in the top three feet. These assumptions were more severe than have been observed in the destructive examination of control blades.

The difference in minimum shutdown margin between calculations with and without degraded control blades was the loss of SDM due to Bel 0 depletion. This loss of SDM was the SDM adder to be ir.cluded in minimum shutdown margin requirements. The GE adder was 2.24 mk while the GPUSC adder was 14.24 mk; the primary diffr2rence being in the treatment of rods having greater than 42% B-10 depletion.

The exposures used to determine which rods have exceeded their end of life were end-of-cycle projections; and therefore, the analysis would be valid for the entire cycle. Control blade exposure accumulation is frequently monitored to insure the projected exposures remain valid. Reanalysis wou!d be done for any change in the number or location of blades exceeding 34% or 42% B-10 depletion. Based on beginning-of-cycle (80C)

SDM measurements, there is sufficient margin to include the SDM adder. Further, SDM testing was performed during a forced outage in April 1979 as described in item 3 The effect of B-10 depletion on scram activity was analyzed using the end-of-cycle (EOC) scram reactivity. The analysis attempted to bound the consequences by using very conservative assumptions. The calculated E0C scram reactivity was assumed to be delayed by 0.2 of a second as a result of B-10 depletion and taking no credit for the stainless steel worth 'n the blade, it was shown that while this delay resulted in slightly increased ACRP and peak pressure in the transient analysis, it did not result in exceeding the Oyster Creek limiting ACPR transient 1857 036

Oyster Creek Nuclear Generating Station Attachnent No. 1 IE Bulletin 79-26 Page 3 (rod withdrawal error) and that peak pressure limitations were not exceeded. A second calculation was performed to calculate the end-of-cycle scram reactivity in which rods having greater than 42% B-10 depletion were assumed not to scram. There was minimal impact on the scram reactivity, and it remained well within the bounding curve utilized in the transient analyses. It was concluded that there is sufficient safety margin in the Cycle 8 core to operate safely with the assumption of degraded control blades.

Item 3: At the next cold shutdown or refueling outage, conduct shutdown margin tests to verify that:

a. Full withdrawal of any control blade from the cold xenon-free core will not result in criticality; and

b. Compliance with the shutdown margin requirement in a manner that accommodates the boron loss phenomena (i.e., by including a plant specific increment in the shutdown margin that takes the potential loss of baron from control blades identified from evaluation of item I into consideration). .

Answer: Oyster Creek shut down on April 2, 1979 due to a leak on a re-circulation pump seal. During this outage, shutdown margin testing was performed to insure there was sufficient shutdown margin to include the shutdown margin adder into the minimum shutdown margin requirements. The total required shutdowa margin was 18.84 mk (R + 2.5 mk SDM + 0.9 mk for 84C SETTLING

+ 14.24 mk SDM adder) with the highest worth rod fully withdrawn.

The R value at the time of shutdown was 1.2 mk. Under these conditions, the minimum measured shutdown margin was 20.04 mk.

Further shutdown margin testing was conducted in the vicinity of three control blades having greater than 42% B-10 depletion to further insure that the required margin was attained. Here, the measured SDM was 26.93 mk.

Item 4: Perform a destructive examination of the most highly exposed control blade at the end of the next cycle anci provide results of the examination within one calendar year after removal of the blade. The results to be reported should in:lude:

a. Tube number or identification.

b. The evaluation of each crack in the tubing.

c. The calculated B10 depletion versus eievation for each tuce.

d. The measured B10 loss versus elevation for each tube.

e. The maximum local depletion for tubes having no cracks.

f. The maximum local depletion for tubes having no loss of boron.

1857 037

Oyster Creek Nuclear Generating Station Attachment No. 1 IE Bulletin 79-26 Page 4 Altu.,ately, the resu!*s of a destructive examination of a blade of similar f . sir, and operational history may be provided within ona year :1 the date of issuance of this Bulletin. . he h:ghc_t local B10 depletion is less than

'i0%, this e' r,ination can be deferred until the next fueling.

Answer: 4 destructis examination of an Oyster Creek control blade has been performed by GE and is the basis for their discussion with the NRC and l&E Bulletin 79-26.

1857 038 January 4, 1980

i IE Bul1etin 79-26 i

r fs ~b b".2* Q TDR NO. 042 REVISION NO. O L f A .2if , _

395IN PAGE 1 OF 17 TECHNICAL DATA REPORT PROJECT NO.

Systems Engineering /

PROJECT:

^ ' #3 AMC OC-1 Control Rod Evaluation RELEASE DATE _7 /?n479 REVISION DATE DOCUMENT TITLE:

B-10 Depletion in Oyster Creek Control Blades ORIGINATOR SIGNATURE DATE APPROVAL (S) SIGNATURE. DATE

_ /7 R. V. FuriaG, \/, 'M 7/go/77 G. R. Bond /

/. 8 M 7-z r-77 l

DATE APPROVAL FOR EXfERN L D\STRIBUTION R.

F. Wilson %% 7{ty

• DISTR 130 TION It was determined that a number of Oyster Creek control blades have exceeded the end of life D. A. Ross l criteria for B-10 Depletion.

J. T. Carroll, Jt.

K.O . E . Fickeis s en ' A series of calculations and low power physics J.Knubel tcsting to evaluate the impact of B-10 depletion R. B. Lee on the safety of operation were performed.

M. Zukor R. W. Keaten ,

It was concluded that the continued operation of R. L. Williams Oyster Creek for the remainder of cycle 8 will not compromise the safety of operation.

It is recommended that a program to follow control blade exposure history be implemented to determine annual requirements for control blade replacement.

I 1857 039 eCOVER PAGE ONLY A000 0030

.s 2 CONTENTS Section Title Page 1 Introduction and Summary 3 2 Methods 7 3 Evaluation 10 4 Results 13 5 Conclusions 14 6 Recommendations 15 7 References 16 1857 0A0

1. INTRODUCTION AND SUMMARi The General Electric Company (GE) had requested and recoived from Oyster Creek Nuclear Generating Station a control blade which was removed from the core at the end of cycle (EOC) 5 to perform a de-tailed destructive examination. GPU was notified by GE just prior to cycle 8 startup that cracking was found in the absorber tubes of the control blade, but the extent of B4 C depletion had not been determined. The plant staff, in addition to conducting shutdown margin testing prior to cycle 8 operation, performed low power physics tests to determine if changes in control blade performance due to B 4 C depletion was evident. The results of the low power physics tests (reference 1) and the shutdown margin (SDM) measure-ments (reference 2) presented nr evidence of B-10 depletion.

GE, based upon the analysis of the Oyster Creek Control blade exam-ination d'ta and one done by Kernkraftwerk RWE-Bayernwerk GmbH (KRB), revised its end of life (EOL) criteria for control blades from 42% to 34% B-10 depletion. The B-10 depletion is estimated in the top three feet of the control blade. They also calculated, as a result of the analysis, a conversion factor to relate Oyster Creek control rod exposure to B-10 depletion. It was then deter-mined that a number of Oyster Creek control blades have exceeded the EOL criteria (Figure 1) . This determination was 'de follow-ing the cycle 8 startup when the analysis was completed.

GE has visited with GPU (March 15, 1979) and other Utilities that were affected by the results of the examinations and to discuss their recommendations prior to issuing a Service Information 1857 041

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1857.042

Letter (reference 3) on the subject. It was then determined by GE that B-10 depletion will have its largest impact on SDM and a lessor impact on scram reactivity. Although control blades have exceeded EOL criteria, GE has recommended to Utilities to continue operations until replacement of control rods could take place during routine maintenance / refueling 'utages, provided the affected core has adequate SDM. This would also allow time for the manufacture of control blades.

This report describes the analysis and testing performed to assure the safe operation of Oyster Creek for the remainder of cycle 8.

For control blades that have exceeded 34% B-10 depletion, GE has recommended using an SDM adder to account for the control rod worth. The adder would be incorporated in the minimum SDM require-ment for cycle operation. The adder extends only to ;ontrol blades which have greater than 34% but less than 42% B-10 depletion. The Oyster Creek plant has 10 control blades that have exceeded 42%

B-10 depletion. GPU requested GE to perform an SDM adder calculation (reference 4) and performed an inhouse calculation (reference 5) similar to GE's to include control blades that have exceeded 42%

B-10 depletion. Additional calculations (reference 6) to review the impact on scram reactivity were also performed.

Soon after the discussions with GE, Oyster Creek was required to shutdown due to a recirculation pump seal leak. The opportunity was utilized to perform a series of criticals (references 7 and

8) to determine if B-10 depletion was present and what effects 1857 043

it had on SDM. With the information obtained from GE, a more thorough set of criticals was proposed (reference

8) specifically aimed at blades which exceeded 42% B-10 depletion.

It was concluded from the analysis and testing that no safety aspect of Oyster Creek operation will be compromised by continued operation of the cycle 8 core.

e 1857 044

7

2. METHODS 2.1 Low Power Physics Testing The series of criticals performed during the Oyster Creek shutdown (reference 7) were designed to measure minimum SDM and detect signs of B-10 depletion.

There have been five control blades replaced (reference

9) at Oyster Creek. One control blade is in a four rod group in which the others have exceeded 42% B-10 depletion.

The blade that was replaced has 30% B-10 depletion which is still balow the EOL criteria. A method to detect boren de-pletion was to have a series of symmetric criticals set up such tnat the control blade of interest would be the one with which criticality was achieved. It was proposed that any difference in the critical position of the control blade would be due to boron depletion.

Anether method was to pull criticals in areas where three control rods have B-10 depletion greater than 42%. These blades remained inserted and the control blades adjacent to these were withdrawn (figure 2). It was assumed that a change in bias for this critical from one in which the blades having greater than 42% were withdrawn would be attributable to boron depletion. These criticals were to determine an accumulative effect of boron depletion. The SDM for this area of the core was also calculated.

i857 045

FIGURE 2 8 EAST

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• Blade was at Notch Six 1857 046 cORs w awaz e n (ngICL rrTT.9

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9 2.2 Calculations Calculations were performed to evaluate the impact on SDM and scram reactivity of continued operation of

_ Oyster Creek with control blades that have lost boron.

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The method used to determine the loss of SDM is similar to the calculation performed by GE to determine SDM adder, but makes additional assumptions to include rods with greater than 42% B-10 depletion. The calculation described in reference 5 used the 3-D XTRA code. Code inputs were adjusted to simulate loss of all B 4 C from the top six inches of a control blade with greater than 34% and less than 42%

BC 4 depletion. Control blades with greater than 42% B4C

. depletion were simulated to lose all B 4 C in the top 3 feet of the blade.

The difference in K gfe between the minimum SDM calculated with depleted control blades and the minimum SDM calculated with non-depleted control blades was the loss of SDM. This loss then becomes the adder to minimum SDM requirements.

The method to er*imate the effect on scram reactivity was to delay scram by C.2 seconds. This was to simulate a complete loss of B4 C (and stainless steel) in the top six inches of all control blades and thereby delay the insertion of negative reactivity. Another calculation was to have 12 blades, having greater than 42% B-10 depletion, fail to scram. In both cases the method used was to bound the problem and be able to assure safe operation rather than calculate a more realistic loss of reactivity.

1857 047

10 3 EVALUATION The low power physics testing performed in April during the recirc pump maintenance outage and at the BOC 8 indicates the considerable SDM present in the Oyster Creek cycle 8 loading. The testing further indicates that GE calculated SDM adder (reference 10) is sufficient to cover any loss of SDM attributed to B 4 C loss including control blades exceeding 42% B-10 depletion. The calculation performed by GE and the one by GPU used EOC 8 exposures projected for the control blades to determine how many blades exceeded 34% B-10 de-pletion. The control blades which have exceeded 42% B-10 depletion have already experienced the majority of exposure they will receive in cycle 8 prior to the April SDM measure-ment in the Al sequence. The remaining control blade patterns (reference 11) will have these rods out of core (B1 ant 32 requences) and as shallow rods (A2 requence) at the end of cycle. Since GE has established a direct correlation between exposure and B4C depletion the likelihood for a reduction in SDM beyond tne R value and the GE SDM adder is small.

The GPU calculation (reference 5) resulted in a 14.24 mk SDM adder as opposed to the GE result of 2.24 mk. In the GE cal-culation all rods exceeding 34% B-10 depletion were treated as being at 42% B-10 depletion (including rods that have ex-ceeded 42% B-10 depletion). As indicated above, the GE adder is sufficient to cover any expected loss of B 4C. The GPU cal-culation attempts to treat rods with greater than 42% as having a worse case of B-10 depletion and thereby bound the consequences for B-10 depletion. The complete loss of B 4 C in the top three 1857 048

11 feet of any rod that excceded 423 B-10 depletion is taken as the worse case. This would be supported by the control blade exoosure profiles. After the first 3 feet the control blade exposures are below the 34% B-10 depletion value. If the B4 C loss extends below 3 ft in the outer boron pins, the total B 4 C loss would still be ('Ising average exposure values across the blade) less than losing the top 3 ft. completely.

The SDM adder calculated by GPU when added to other tech spec SDM requirements is within the measured minimum SDM. A compari-son in the calculated loss of A K due to B-10 depletion between GE and GPU for various control rods is presented in table 1.

1857 049

12 TABLE 1 CALCULATED 6 K DUE TO B-10 DEPLETION Control Rod Location GE A K* GPU A K*

06-35 -0.00224 -0.010956 10-39 -0.00316 -0.014018 10-35 -0.00317 -0.004754 14-31 -0.00827 -0.011884 22-43 -0.00777 -0.016883 14-35 -0.01148 -0.024325 22-31 -0.01014 -0.015243 14-27 -0.01639 -0.019295 GE A SDM = -0.00224 GPU 6 SDM = -0.01424

• Difference in K eff Between the Cycle 8 Core with No Boron Depleted Rods and the Cycle 8 Core with Depleted Rods 1857 050

13

4. RESULTS 4.1 Low Power Physics Testing The results of the low power physics tests are as follows:

a) Minimum SDM based. on XTRA code normalization to measure-ments is 20.04 mk for rod 24-07 b) The measured differences !etween a control rod with 30% B-10 depletion and one with 42% is approximately 0.63 mk.

c) The accumulative effect of B-10 depletion on snutdown margin is approximately 1.2 mk. The SDM in the area of the 3 control rods with greater than 42% B-10 depletion is 26.93 mk.

d) Control rods with less than 30% B-10 depletion do not appear to show any signs of B 4C loss.

4.2 Calculations The SDM adder calculated by GE is 2.24 mk and the one cal-culated by GPU is 14.24 mk. The total SDM required to operate safely is 6.84 using the GE adder and 18.84 mk using the GPU adder (R + 2.5 mk SDM + 0.9 mk for B 4 C settling +

SDM adder). The minimum SDM required is determined from core exposure data at the time of shutdown (182 GWD) for the recirc pump maintenance outage with an R value of 1.2 mk occurring at 250 GWD.

1357 051 The bounding calculations for the effect of B4 C depletion on scram reactivity resulted in a maximum A CPR of 0.1392 and peak pressure of 1202 psia in the turbine trip without bypass transient. These values are within the technical specifications for Oyster Creek.

14

5. CONCLUSIONS There is evidence of B-10 depletion from the Oyster Creek Control blades. However, the current level of B-10 depletion in the control blades has a minimal effect on shutdown margin and there is suffi-cient shutdown margin to insure that any further B-10 depletion will not exceed minimum shutdown margin requirements for the re-mainder of cycle 8.

The operation of the Oyster Creek plant ith control blades ex-ceeding their EOL criteria will not compromise the safety of operation for the remainder of cycle 8.

1857 052

6. RECOMMENDATIONS 6.1 The following recommended actions from reference 3 should be implemented:

a) Maintain records on individual blade exposures using the RODEX code to allow a blade managerent program.

b) In the future if it is expected that control blades will exceed 34% B-10 depletion before the completion of the next cycle, plans should be made to repl ce control blades at the upcoming refueling / maintenance outage.

c) As an interim measure for cycle 8 use the shutdown margin adder.

6.2 The control blade replacement schedule presented in re-ference 12 should be met which will satisfy (b) above.

6.3 The control blade exposure history should be followed closely for the remainder of cycle 8 to insure the projected blades exposure used in the analysis are cor-rect. If any other blades than the ones projected exeed either 34% or 42% B-10 depletion the analysis should be redone with the new data.

1857 053

7. REFERENCES

1. Memorandum to G. R. Bond from R. V. Furia, NF-496,

" Oyster Creek Control Rod Depletion", January 24, 1979

2. JCP&L mLmorandum to G. R. Bond, K.O.E. Fickeissen from R. V. Fu'ria, " Oyster Creek BOC 8 Shutdown Margin Calcu-lation and XTRA Cold Bias Results," November 30, 1978.

3. General Electric BWR Services Information Letter #157 supplement 1, " Control Blade Lifetime," March 1979.

4. Letter to G. C. Nelson (GE) from G. R. Bond OGPU), NF-573, " Calculation of Shutdown Margin Adder," March 21, 1979

5. Calculation C-395IN-321-001, "The effect on Shutdown Margin of B 4 C depletion in Oyster Creek Control Rods,"

April 5, 1979.

6) Calculation C-395IN-321-002, "The Effect on Transient Analysis of B 4 C Depletion in Oyster Creek Control Rods,"

April 4, 1979

• / ) Calculation C-395IN-321-003, " Low Power Physics Testing to determine the Effect of B4 C Depletion in Oyster Creek Control Rods," April 4, 1979

8) Memorandum to K.O.E.Fickeisen from R. V. Furia, NF-579, "Criticals for Control Rod Evaluation" March 30, 1979.

1857 054

17

9. Memorandum to G. R. Bond from R. V. Furia, NF-479, " Oyster Creek Control Blade Replacement," January 4, 1979.

10. Letter to G. R. Bond (GPU) from G. C. Nelson, (GE) " Oyster Creek Cycle 8 Shutdown Margia Analysis at 250 GWD", April 17, 1979

11. JCP&L Memorandum to A. H. Rene from R. J. Thompson, Jr. and F. A. Saksa, " Cycle 8 Exposure Predictions" October 15, 1978.

12. Memorandum to G. R. Bond from R. B. Lee, NF-579, "CC-1 Deter-mination of % B-10 Depletion" April 4, 1979 I

1857 055 i

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