ML20084S367

AO 50-249/74-38:on 741031,excessive Power Peaking Observed in Lower Region of Core.Caused by Xenon Transient Combined W/Very Low Fuel Exposure at Core Bottom.Control Rod Withdrawn

ML20084S367
Person / Time
Site:	Dresden
Issue date:	01/17/1975
From:	Stephenson B COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
35-75, AO-50-249-74-38, NUDOCS 8306160706
Download: ML20084S367 (5)

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BBS 1.tr. #35-75 Dresden Nuclear Power Station R. R. #1 Morris, Illinois 60450 January 17, 1975 0)'

//g8 Mr. James G. Keppler, Regional Director Directorate of Regulatory Operations-Region III 4

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SUBJECT:

REPORT OF AN ABNOEIAL OCCURRD CE PER SECTION 6.6.A OF

w. k4ffCAL SP!IIFICATION OFF GAS HIGH RADIATION ON UNIT 3 Raferences:

1) Regulatory Guide 1.16 Rev. 1 Appendix A

2) Notification of Region III of A E Regulatory Operations Telephone:

P. Johnson, 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br /> on January 7, 1975 Telegram:

J. Keppler, 1545 hours0.0179 days <br />0.429 hours <br />0.00255 weeks <br />5.878725e-4 months <br /> on January 7, 1975 Ra.mrt F= ber: 50-2h9/7h-39 Report Date: January 17, 1975 Occurrence Date: October 31, 1974 Facility: Dresden Nuclear Power Station, Morris, Illinois IDFVPIFICATION 0F OCCURRE!CE Rapid local power changes causing an abnormal degradation of one of the several boundaries designed to conte.in radioactive materialo. No Technical Specification peaking limito were execeded.

CONDITIONS FRIOR TO OCCURRB;CE'

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On October 30, 1974 Dreeden 3 changed load for control rod movements to redistribute the neutron flux prior to preconditioning the fuel. Control rod position alterations began at 0248 hours0.00287 days <br />0.0689 hours <br />4.100529e-4 weeks <br />9.4364e-5 months <br /> on October 31, 1974 and were completed at 0500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br />. At that time, the power distribution was checked and found to be satiofactory. Electrical load was approximately 520 W e.

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Mr. Jam:s G. Kcppler January 17, 1975 O

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',DESCRIPPION OP OCCURRD;CE At 0624 hours0.00722 days <br />0.173 hours <br />0.00103 weeks <br />2.37432e-4 months <br />, excessive power peaking was observed in the lower region of the core. Control rod movements were made in an attempt to reduce the peaking, but observations showed that the peaking had increased. At this time, the high off gas radiation alam actuated. Control reds were imediately inserted to reduce peaking and power. The off gae raCation transient continued during the initial power reduction. The hi-hi offgas radiation alam tripped, starting the 15 minuto isolation timer. Additional control rods were inserted and at 0735 hours0.00851 days <br />0.204 hours <br />0.00122 weeks <br />2.796675e-4 months <br />, the hi-hi off gas radiation alarm cleared. A subsequent offgas sample analysis at 1421 hours0.0164 days <br />0.395 hours <br />0.00235 weeks <br />5.406905e-4 months <br /> showed a stack release rate of 81, 634 Mci /sec.

DESIGNATION 0P APPARENT CAUSE OF OCCURRANCE (Personnel Error)

The apparent cause of the Occurrence was that during a xenon transient which, ccmbined with a very low fuel exposure at the core bottom, rapid changes in local power low in the core resulted in fuel rod perforaticn.

ANALYSIS OF OCCURRC!CE Prior to control rod movement, power was approximately 440 ICle.

Power during control rod movements increased to approximately 520 ICle. After the off gas radiation hi alarms, ige power level uas reduced to about 370 ige.

During the transient, the maxitum total peaking factor recorded was 3.87.

For the core themal power of 1432 !Gth at the time of the transient, the maximum DiGH in any h1 md va a 12. 56 VU/ft 2n'i tho = :i=m.'."L'!CR

n-10 90 KW/ft. Neither valuo violatos technical specification limits.

The action level on maximum UICR at the existing 57% power condition is 9 92 KW/ft. This level was exceeded for less than one hour during which, corrective actions were being applied.

A conservative calculation of the minimum critical heat flux ratio (MChTd) yielded a ICHFR of 2 78 at the maximum peak. This is well within the unit tech spec limit of 1 90.

Unit 3 has had a flux and exposure distribution peaked toward the top of the core for at least two cycles. She net result of this distribution of exposure has been to leave the lower section of the core with relatively unexposed fuel. The unit was broar.ht down to about 50 percent power on October j], 1974 in an effort to ccrrvet this uituation by positioning control rod's such that the power distribution was peaked (within limits) toward the core bottom. Operaticn in thu modo fcr a cignificant period of time would have reculted in a more even axial ex;mure distribution.

Control rods were moved to change the axial power distribution, of the core at the same time as the xenon was peaking.

'4 hen xenon started burning out, the bottom peak, which had just been formed by control red novement, increased rapidly. Since the botto'n of the core haa relatively little exposure, the peaking problem was exaggerated.

(,, g, Jame,s G. K r,

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January 17, 1975

• An' attempt was made by the nuclear engineer to reduce peaking by spreading the peak over a larger volume. This action resulted in higher peaking and the peak shifting to segments of fuel rods which were previously controlled and operating at low LHGR's. Subsequently the off gas radiation monitor hi and hi-hi alarms occurred.

The maximum instantaneous stack offgas release rate that occurred during the transient is indeterminent, but is estimated to be in the vicinity of 300,000Aci/sec.

(The 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> Technical Specification maximum limit was exceeded for less than 10 minutes, well within the Tech Spec limit).

Since the time of this occurrence, Unit 3 operation has been accompanied by undesirably high off gas radiation levels which has resulted in a station imposed dorating of up to 50% power. Thus, it now appears that several fuel rods failed by rupture of their cladding during the event on October 31, 1974. This is an abnormal degradation in one of the several boundries designed to contain fission products and represents an Abnormal Occurrence. This occurrance was reclassified following Station Review of the original Deviation on January 7, 1975 The cause of these failures is evidently fuel-cladding interacticn, since no fuel safety limits were approached.

This mode of fuel failure has occurred previously in the Dresden Units and has occurred during normal reactor operation.

An isotopic analysis of the off gas composition yielded an approximately 75% " recoil" distribution. A recoil distribution implies little or no holdup in the pellet or fuel column for fission product gases, predicting that a few relatively large cladding perforat, ions occurred during the pcching tr:ncient.

There was no effect to the public health and safety. The off gas release rate for Unit 3 for the day of October 31, 1974 has been conservatively estimated to be an average of about 45,000 Mei/sec which is less than half the technical specification limit. Since the occurrance, the stack cas release rate has been about 24,000 Aci/sec at a derated load of 400 We.

CORRECTIVE ACTION The initial corrective action when the exccasive peaking occurred was to withdraw a control rod. This action was taken in an attempt to broaden the flux distribution and thus lower the peak. The peaking increased by about 10% in the area of the highest peak. Upon discovery of this fact, control rods were incerted in,a timely fashion to lower th,e power level and reduce the core peaking.

A thorough review of the occurrence by On-site Review has concluded that two actions are suspect. The first was the withdrawal of control rods to a given power shape during a xenon transient and the second was the initial response to the excessive power peaking.

Soveral corrective actions have been made with regard to the training of the nuclear engineers. These are

1) Training is being given all of the nuclear engineers i

at Dresdea on approximately a veckly basis.

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',Mr. James G. Keppler January 17, 1975

2) It is planned to send a nuclear engineer to the General Electric training course in Nuclear Engineering in 1975. This course is 3 months of intensive course work in all aspects of station nuclear engineering.

3) A directive from the Superintendent of Nuclear and Fossil systems of CECO has been received which details a review and approval procedure for bWR control rod acquences and planned changes thereto. This directive has been implemented at Dresden and a review process will be followed for control rod movements in the future.

4) A qualification program for station nuclear engineers is being developed for implementation by 1976.

FAILURE DATA A review of fuel failures at Dresden is inconclusive since no certain cause for most fuel failurcs cannot be assigned. The most comprehensive data available at Dresden is the fuel examination of Dresden 3 fuel during March and April, 1973 This examination was done prior to reconstituting some of the unit 3 fitel assemblies. Evidence frcm individual rod examinations has indicated that rou6hly 50% of all the fuel assembly failurcs (which n=bered 100) ucrc cau cd by fucl-cind interactic,a J. wing nunual opc41,1ww.

Based partly on this data, General Electric has recommended certain operational measures to reduce the number of fuel failures experienced decreasing normal operation. CECO has implemented these recommendations in a timely manner and continues to try to minimize fuel failures.in its nuclear units.

M. B. Stephenson Q

Superintendent BBS smp File /AEX:

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