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LER 83-052/03L-0:on 830525,axial Flux Difference Target Band Limit Exceeded.Caused by Axial Burnup Distribution,Large Negative Moderator Temp Coefficient & Considerable Rod Withdrawal Required to Overcome Xenon Buildup
	ML20024B071
Person / Time
Site:	Cook
Issue date:	06/22/1983
From:	Palmer R; INDIANA MICHIGAN POWER CO. (FORMERLY INDIANA & MICHIG
To:	; NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML20024B062	List: ML20024B061; ML20024B066; ML20024B071;
References
	LER-83-052-03L, LER-83-52-3L, NUDOCS 8307050068
	Download: ML20024B071 (5)
	v • d • e

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EVENT CESCRIPTION AND PRCSAGLE CON!ECUENCES h toi.ii DURING ROUTINE STARTUP, FOLLOWING A RAPID POWER REDUCTION TO APPR0XIMATELY 2% FOR STEAMI io i 21 i GENERATOR STOP VALVE SURVEILLANCE TESTING. THE AXIAL FLUX DIFFERENCE TARGET BAND LIMIT I tniaiI WAS EXCEEDED. THIS EVENT WAS NON-CONSERVATIVE WITH RESPECT TO TECHNICAL SPECIFICATION f in i-J l 3.2.1. THE ACTION REQUIREMENT WAS MET. PUBLIC HEALTH AND SAFETY WERE NOT AFFECTED. I iot~i LJHIS WAS THE FIRST OCCURRENCE OF THIS TYPE. (SEE ATTACHED SUPPLEMENT) l Ia i e 1 I l

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I I AFTER A RAPID POWER REDUCTION (AS EXPERIENCED ON 5-25-83) IS EXPECTED. THIS IS CAUSED i, , !I BY THE AXIAL BURNUP DISTRIBUTION, LARGE NEGATIVE MODERATOR TEMPERTAURE COEFFICIENT i

. . , ;i l AND THE CONSIDERABLE R0D WITHDRAWAL REQUIRED TO OVERCOME THE XEN0N BUILDUP. (SEE l 1 i . i 1 ATTACHED SUPPLEMENT) 8 9 I 80 ST *'. PC'.'. E R OTHER STATUS 500 RV DISOOVERY OESORIPTION I NA i i * *! Lc_.@ ! O I 1 I 5' l@l l lB!@l OPERATOR OBSERVATION I ACTiv.Tv Co'.lT E N T

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ATTACHMENT TO LER# 83- 052/03L-0 SUPPLEMENT TO EVENT DESCRIPTION On May 25,1983, Unit 1 power level was reduced from 100% RTP to approximately 2% RTP to perform Steam Generator Stop Valve surveillance tests. The power reduction was accomplished through rod insertion and RCS boration from 100% RTP to 40% RTP then by rod insertion and RCS dilution from 40% RTP to 2% RTP. During the power reduction, the Axial Flux Difference, AFD, did not exceed the AFD 5% Target Band Limit, see attached Figure 1. While at the reduced power level, maxi-mum dilution and considerable rod withdrawal were required to compen-sate for the negative reactivity, due to xenon buildup, to maintain the RCS temperature and reactor power level .

After the completion of the Steam Generator Stop Valve testing, reactor power escalation commenced. By 1745 hours0.0202 days 0.485 hours 0.00289 weeks 6.639725e-4 months , the power level had increased to 15% RTP. The AFD at this time was approximately +9%,

greater than 4% outside of the target band positive limit of +4.7%. As the AFD was outside the target band, the AFD penalty deviation began to accumulate time. This event was logged by Operations personnel at 1745 hours0.0202 days 0.485 hours 0.00289 weeks 6.639725e-4 months on May 25, 1983. The power ascension was subsequently continued until a reactor power level of approximately 48% was achieved.

During the time the AFD was out of the target band (until 0800 hours0.00926 days 0.222 hours 0.00132 weeks 3.044e-4 months on May 26, 1983) and for a twenty-two (22) hour period after the AFD was returned to_the target band (0610 hours0.00706 days 0.169 hours 0.00101 weeks 2.32105e-4 months on May 27,1983) the reactor power level was maintained below 50% RTP per Technical Specification Action Item 3.2.lc.

l ATTACHEMENT TO LER# 83-052 /03L-o SUPPLEMENT TO CAUSE DESCRIPTION i,

The three important contributing factors inducing the large positive AFD are

. summarized below:

I. The' Moderator Temperature Coefficient temperature feedback, which causes the flux level at the top of core to be relatively higher than that produced at the bottom of core, when compared to HFP condition, during a power reduction.

II. The axial burnup distribution, see attached Figure 2.

1. Due to burnup the axial offset is large at low power levels at the end of cycle.

2. Due to the axial core power sharing prior to the unit's power

reduction, an axial Xenon oscillation develops.

III. The rod withdrawal required to keep the unit critical and begin power escalation. This rod withdrawal develops an axial Xenon oscillation.

A large positive AFD swing after a rapid power reduction (as experienced on May 25,1983)1or unit trip, is expected at the end of core life. Experience has shown that rod movement is not sufficiently effective,because of the above items, to completely dampen the positive AFD swing.

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ML20024B071

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