ML20041B013

RO 82-001/01T-0:on 820107,six RCS Resistance Temp Detectors Did Not Meet Tech Spec Time Constant.Caused by Inadequate Coupling Between Detectors & Thermal Wall Due to Dried Out Couplant Compound.Couplant Replaced

ML20041B013
Person / Time
Site:	Arkansas Nuclear
Issue date:	01/29/1982
From:	John Marshall ARKANSAS POWER & LIGHT CO.
To:	Jay Collins NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
References
2CAN018209, 2CAN18209, LER-82-1-1T, RO-82-001-OIT-0, RO-82-1-OIT, NUDOCS 8202230158
Download: ML20041B013 (8)

Text

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ARKANSAS POWER & LIGHT COMPANY POST OFFICE BOX 551 LITTLE ROCK ARKANSAS 72203 (501) 371-4000 January 29, 1982 s- *1 2CAN018209 9 9

RECEIVED 9 g,-

FEB 2 219825 Mr. John Collins amu mumm 'i Regional Administrator V Mugymru Office of Inspection & Enforcement O

Region IV fu U.S. Nuclear Regulatory Commission N 611 Ryan Plaza Drive, Suite 1000 Arlington, Texas 76011

Subject:

Arkansas Nuclear One - Unit 2 Docket No. 50-368 License No. NPF-6 Reportable Occurrence 50-368/82-001/0IT-0 (File: 2-0520)

Gentlemen:

At 0920 hours0.0106 days <br />0.256 hours <br />0.00152 weeks <br />3.5006e-4 months <br /> on January 7, 1982, a determination was made that six (6)

Reactor Coolant System (RCS) resistance temperature detectors (RTD's) which are inputs to the Reactor Protection System (RPS) did not meet the 6.0 second time constant required by Technical Specification (T.S.)

I Table 3.3-2 per T.S. 4.3.1.1.3 surveillance requirements. The 6 degraded RTO's are inputs to the Core Protection Calculators (CPC's) and affected all RPS Channels. At 0940 hours0.0109 days <br />0.261 hours <br />0.00155 weeks <br />3.5767e-4 months <br /> on January 7, 1982, a shutdown was initiated per T.S. 3.0.3. This occurrence was determined to be reportable per T.S. 6.9.1.8.e. Further details were reported in Licensee E.ent Report (LER) 50-368/82-001/0IT-0.

l The cause of this occurrence appears to be that the couplant compound

('Never-Seez') used in the RTD wells dried out during operation leaving a powdery residue. This caused inadequate coupling between the RTD and the thermowell wall. 'Never-Seez' was provided by the NSSS vendor to improve heat transfer between the thermowell and sensor (RTD).

I

' 1 8202230158 820129 PDR ADOCK 05000368 e eor _ _ __ e - _

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-RTD time constant testing using the Loop Current Step Response method and data analysis was contracted with AMS Corporation of Knoxville, Tennessee. A total of 32 RTD's were tested of which 24 are inputs to the RPS. The test results were received by AP&L on 1/7/82 at which time 6 of 24 RTDs which are RPS inputs did not meet the required time constant per T.S. 4.3.1.1.3.

Although several RTD tin., constants were degraded beyond the T.S.

required value and at least one degraded RTD was an input to each of the four RPS channels, the. occurrence did not necessarily lead to non-conservatisms in the CPC trip calculations. This is because of the actual usage of the signals within the calculators. Each CPC has four hot leg temperature sensors which are averaged for use in the CPC calculations, thus the time constant (although not a direct algebraic average)_of the average signal is also an effective average. Each CPC has two cold leg temperature sensors which are auctioneered for use within the CPC, for additional conservatism, thus the effective time constant of the CPC cold leg temperature signal is the lower time constant of the two. Attachment 1 presents the measurement results recieved January 7, 1982, and as can be seen from the effective time constant column, none of the CPC calculations were actually non-conservative due to degraded time constants. However, from a plot of average time constants.(see Attachment 2) taken during three tests performed following the spring 1981 refueling, a steady rate of degradation was occurring which would have soon resulted in non-conservative CPC calculations.

Corrective actions began at 0940 on 1/7/82 when a shutdown was initiated per T.S. 3.0.3. The 32 RCS RTD's were removed and cleaned, and their thermowells were visually inspected and cleaned. New couplant was installed using a procedure to control the amount of "Never-Seez" applied, and the RTD's were reinstalled into the thermowells. Response time testing was performed on January 20, 1982, and acceptable results were verified prior to criticality. The final results of the Reactor Coolant System RTD response time tests are given in Attachment 3.

AP&L believes that the degradation of RTD response times will ccntinue to occur but that they are reasonably predictable. Review of the design bases of the CPC shows that the following analyzed events are potcatially affected by degraded RTD time constants:

Single CEA Withdrawal Single CEA Drop Excess Load Loss of Feedwater Loss of Load The single CEA events are affected because it cannot be assured that all the excore neutron flux detectors will "see" a single CEA effect on power which occurs in a localized area of the core. It should be pointed out, however, that any CEA deviation would be detected by the CEA calculators L i

l and a penalty factor will be applied to the calculated DNBR and LPD valves which should be adequate to ensure fuel protection. The RPS high  !

pressurizer trip also provides backup protection for CEA withdrawal l events and the low pressurizer pressure trip provided backup protection i for CEA drops.

The secondary system transients; excess load, loss of feedwater and loss of load are also potentially impacted by degraded RTD response time. The overcooling events also have backup protection provided by the RPS Low Steam Generator Pressure and Low Pressurizer Pressure, whereas the undercooling events have backup protection provided by the High Pressurizer Pressure trip and Low Steam Generator Water Level trip.

AP&L's intention is to monitor the RTD time constant degradation by performing monthly measurements for at least the first 3 months of operation in Mode 3 or above with evaluation of measurement results to be completed within approximately one week after the measurements. Test results from the first measurement following startup will be provided to NRC within 2 weeks of the completion of the first measurement. After performing the first three monthly tests, the measurement interval will be reevaluated. If justified, the measurement interval will then be adjusted on once per two months of Mode 3 or above operation.

Preliminary analysis by Combustion Engineering indicates that a penalty factor can be determined which can be applied to the CPC calculations via addressable constants which will compensate for degraded RTD time constants. Upon completion of this analysis, AP&L plans to submit a Technical Specification change for NRC approval which would require the needed penalty to be applied should be RTD time constants degrade beyond 6 seconds in the future. We expect submittal of this Technical Specification change request by March 15, 1982. If this change is acceptable to NRC, we would propose to reduce the time constant measurement interval to that needed to insure conservatism is always maintained.

In the longer term, AP&L will continue to investigate methods of correcting the degradation of RTD time constants. These will include evaluation of different couplants, RTD modifications to improve heat transfer, as well as RTD and thermowell changeout.

Prior to our submittal of the Technical Specification we propose that we meet with you to provide the technical basis of the Technical Specification change. In addition, we can provide you with details on our long term plans at that time.

L

The NRC will be appraised as developments. occur. If there are any questions, please feel free to call.

Very truly yours, J

zL2 ahn R. Marshall Manager, Licensing JRM:nak _

cc: W. Cavanaugh, III w/a T. H. Cogburn L. J. Dugger J. M. Griffin

'J. N. Levine D. A. Rueter INP0 Resident NRC Inspectors NRR Division of NRC AN0-DCC 4

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ATTACHMENT 1 RTD Time Effective Signal Type RCS Sensor Constant Time Constant Channel (T H rT)C Loop No. (sec.) (sec.) #

A TE 4610-1 4.5 4.6 Tg TE 4635-1 4.7 5.33 A B TE 4710-1 5.9 6.05 TE 4735-1 6.2 A TE 4611-1 5.3 T

C B TE 4711-1 5.1 A TE 4610-2 4.7 4.85 T

H TE 4635 5.0 4.85 B TE 4710-2 5.0 4.85 B TE 4735 4.7 A TE 4611-2 5.1 T

C B TE 4711 8.7 5.1 A TE 4610-3 4.7 4.9 TE 4635-3 5.1 5.95 T

H B TE 4710-3 7.0 7.0 TE 4735-3 7.0 C A TE 4611-3 5.1 T

C B TE 4711-3 4.7 A TE 4610-4 4.2 5.4 TE 4635-4 6.6 5.6 T B TE 4710-4 5.6 5.8 D H

TE 4735-4 6.0 A TE 4611-4 5.3 5.3 1 T

C B TE 4711-4 7.2 1

u_.

ATTACHMENT TO AN0-82-0042 Page 1 of 2 ATTACHMENT 3 Response time tests were performed on all 32 primary system HTDs at Arkansas Nuclear One Unit 2 on January.20, 1982. The results are given

.in Table 1. The Loop Current Step Response (LCSR) method as endorsed by the USNRC was used to perform the response time tests.

The time constants listed in Table 1 are upper limit values except for the ones marked "**". The values marked "**" have an accuracy of i 10 percent.

The upper limit time constants include a safety factor of 1.5. When only a single time constant can be identified from the LCSR data, the USNRC requires that the single time constant be multiplied by 1.5 to give a conservative estimate for the overall time constant of the sensor. If more than a single time constant can be identified from the LCSR data, a safety factor is not required but the total time constant is subject to an accuracy of i 10 percent.

)

ATTACHMENT TO ANO-82-2-0042 Page 2 of 2 ATTACHMENT 3 TABLE 1 TIME CONSTANTS OF THE PRIMARY SYSTEM RTDs (ARKANSAS NUCLEAR ONE UNIT 2)

JANUARY, 1982 Time RPS RTD Element Constant Channel Element No. Number THTC or Seconds Remark A 4710-1 1 Tg 4.8 A 4610 1 T H

3.8 A 4611-1 1 T 4.8 **

A 4711-1 1 T 5.0 C

A 4635-1 1 T 3.9 H

A 4735-1 1 T 5. 0 H

B 4710-2 1 Tg 4.8 B 4610-2 1 T H

44 8 4711-2 1 T C

4.5 B 4611-2 1 T 4*4 C

B 4635-2 1 T H

4.2 B 4735-2 1 T H

3.9

, C 4710-3 1 T 4.2

• 4710-3 2 T H

5.0 **

H **

C 4610-3 1 T 3.3 3.2 **

4610-3 2 T H

C 4611-3 1 T 4.0 4611-3 2 3.9 l

C 4711-3 1 Th T 3.9

• C 4711-3 2 T 3.9 C

C 4635-3 1 T H

4.8 C 4735-3 1 T 5.3 H

D 4710-4 1 T 4.5 4.8 **

4710-4 2 T D 4610-4 1 T N

4.7 4610-4 2 T 5.0 **

D 4711-4 1 T 5.0 4711-4 2 Th 5.0 0 4611-4 1 T 3.8

• 4611-4 2 3.6 D 4635-4 1 Th T 4*1 H

D 4735-4 1 T H

5.0 Not an RPS input.

• • The time constants market "**" have an accuracy of i 10 percent.

The ree' are the upper limit time constants. l

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