ML20056B755

Ro:On 710709,two High Reactor Pressure Scram Switches Failed to Trip within Tech Spec Allowable Deviation.On 710618, Reactor Coolant Temp Increased by 115 F During 1 H Period

ML20056B755
Person / Time
Site:	Monticello
Issue date:	07/16/1971
From:	Duncanson R NORTHERN STATES POWER CO.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9102080505
Download: ML20056B755 (8)

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Y NORTHERN STATES 1

[y Minneapolis, Minne 2

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MGUULTORY Dr. Peter A. Morris, Director Matt SECTION h i

Division of Reactor Licensing g

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f United States Atomic Energy Comnission a

D Washington, D.C.

20545 a)

Dear Dr. Morris:

i MONTICELLO NUCLEAR GENERATING FLAll?

heks 'b. 50-263 License No. DPR-22 Reporting of Occurrences Three conditions have occurred recently at the Monticello Nuclear Generating Plant which we interpret to be reportable to your office in accordance with i

Section 6.6.B and 6.6.C of Appendix A, Technical Specifications, of the l

Provisional Operating License DPR-22. The three occurrences are:

i 1

1.

Failure of Two High Reactor Pressure Scram Switches to Trip Within the Technical Specification A11ov.ble Deviation.

j 2.

Reactor Coolent Temperature Increase of 115 F Daring a One Hour Period.

I 3.

Failure of #11 Standby Gas Treatment System Air Heater Control Circuitry.

Occurrence 1 is being repo - d under Section 6.6.B.3 of the Technical Specifications, Occurrence 2 is being reported under Section 6.6.C.4, and Occurrence 3 is being reported under Section 6.6,0.1.

The Region III Compliance Office has been previously notified of the occurrences.

The attached reports describe the occurrences and sumarize the actions taken to prevent future similar occurrences.

Yours very truly, WW g

R.O. Dancanson, Jr. P.E.

Gen. Supt. of Power Plants-Mechanical Chairman - Monticello Safety Audit Committee ROD /CEL/ car 3197 h020Go505710716 ADOCK 05000263 CF L)' j

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LONT ICELLO NUCLEAR GENERAT ING PLANT July 16, 1971

Subject:

Failure of Two High Reactor Pressure Scram Switches to Trip Within the Technical Specification Allowable Deviation g,gg gfh 7[

j 1.

Summerv Deseriofion of ihe O currenco On July 9,1971, while performing a regularly scheduled surveillance test, two of the four reactor high pressure scram trip switches were found to trip outside of the allowable deviation as stated in Section 3.2 of the Technical Speci fications.

Both pressure switches were immediately replaced with spare units.

In addition, the pin-type pressure snubbers installed in the sensing lines of the four switches were rernoved on the basis that dirty snubbers could have caused or contributed to the failures.

2.

Acenunt and Analysis of the Occurrence and Corrective Actions Taken i

On July 9th, while performing routine surveillance tests, two of the four reactor high pressure scram switches were found to exceed the Tech Spec allowable deviation.

The reactor high pressure scram protection was not 1

defeated, howevcr, since only one failure occurred in each of the two are connected in a "one of two twice" logic scheme.) pressure scram switches scram logic protection channels.

(The high reactor j

The first switch tested (PS 2-3-55A) tripped at a pressure of 1089 psig, exceeding the " Limiting Safety System Se tting" by 14 psig. The second i

switch test (d (PS 2-3-55B) initially tripped at 1188 psig, exceeding the

" Limiting Safety System SettinE" by 113 psi.

During several subsequent the switch tripped at 1071 psig, well within trips of twitch PS 2-3-55B 3

the " Limiting Safety System Setting" of 1075 psig. The two remaining j

high reactor pressure switches (PS 2-3-55C and D) were found to trip well within the Tech Spec allowable deviation.

These switches were each l

tripped several additional times with no measured changes in the setpoint.

Although the two suspect switches were tested successfully several times after recalibration, it was decided to replace the two switches.

In the process of replacing the switches, it was discovered that the pin-type pressure snubber installed in the sensing line of PS 2-3-55A was dirty.

j The remaining pre.ssure snubbers on the high reactor pressurt scram switches were immediately inspected.

No additional dirty snubbers were found.

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d(cision was then made to remove the snubber internals from all four pressure switches since it is believed that the snubbers are not essential to proper operation of the pressure switches.

Following completion of the pressure switch replacement and pressure snubber internals removal, the four switches were tested to verify operability.

On July 10, the two suspect pressure switches wt re visually inspected.

No abnormalities were r evealed. On July 12, attempt s were made to reproduce the l

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. failure of PS 2-3-55B with the switch connected in a circuit duplicating the protection system circuit. The failurc could not be rcproduccd.

un July 13, the pressure switch vendor was contacted in regard to the failures. The vendor could not explain the switch failures observed but requested that the failed switches be returned to the factory for testing.

3.

Prevent ion of Similar Occurrences As previously stated, the two most probable causes of the failures (the two suspect pressure switches and the sensing line pressure snubbers) have been removed from the system.

Also, the two replaced pressure switches have been returned to the manufacturer for additional testing. Results of the additional testing will be reviewed to determine if further action is required.

A study of critical pressure sensing instruments is being made to determine if presently installed snubbers are necessary.

Any pressure snubber that is not essential to the operation of critical instruments will be rrmoved to eliminate the possibility of defeating the instrument action through snubber plugging.

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1 MONTICELLO NUCLEAR GEt;ERATING PLANT assesees wAtr catg7-g-7/ July 16,1971

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Subject:

Reactor Coolant Tenperature increase of 115 F During a One Hour Period 1.

Summarv Descriot ion of the Occurrence Startup Test Procedure # performing a heatup rate test in conjunction with On June IP,1971, while 3, Reactor Vessel Tempcrature, the reactor coolant temperature was increased by 115 F (210 F to 325 F) during a one hour period.

2.

Account and Analysis of the-Occurrence j

A special heatup test wa being pgrformed on June 18, 1971 to obtain RPV temperatures during a 90 F to 100 F per hour heatup.

The data was being obtained to confirm thermal analysis models.

An increase in the reactor coolant icmperature of more than 100 F in one hour resulted because personnel directinE and performing the test failed to give proper attention to avail-able data.

It has been determined from a review of the test data that the 115 F coolant i

temperature rise in the one hour period created no unusual stresses on ihe vessel.

3.

Prevention of Future Occurrences i

During normal heatup and cooldown operations, reactor coolant temperature changes are controlled at a slower rate than was required for the special heatup test.

Although the gormal procedures should prevent the temperature from changing more than 100 F in z.n hour, a memo has been written to the operations personnel stressing that the coolant temperature changes must be watched more closely.

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l fAONT ICELLO IJUCLEAR GEi; ERAT l!S PLA!H Julv 16. 1971

Subject:

Failure of #11 Standby Gas Treatreent Sysicn Air Heater Control Circuitry 1.

Summarv Descriot ien of th~ Occurrences On June 19, 1971, a failure of the SGTS #11 air heater SCR power control circuit rendered the train inoperable.

The failed components in the control circuitry were replaced. The SGTS #11 was tested successfully and declared operational on June 24, 1971.

On July 5,1971, another failure of the SGTS #11 air hcater SCR power control circuit rendered the train inoperable.

The SCR power control circuitry was then replaced by a contactor which provides power to the air heater when flow through the SGTS is cstablished.

The SGTS #11 was tested successfully and declarcd operational on July 9,1971.

2.

De tail &d Descriot ion of t he Occurrences c.

Su r,arv of Condi 4 ione At the time of th( first control circuit failure, a plant startup was in progrest with the reactor at approximately 500 psig.

The recond failure was discovered during an operability test of the SGTS prior to a plant startup.

b.

Accouni and Analvsis of the Occurrence l

1 On June 19, 1971, at 0230 hours0.00266 days <br />0.0639 hours <br />3.80291e-4 weeks <br />8.7515e-5 months <br />, it was discovered that the air heater over temperature light for SGTS #11 was illuminated.

The problem was immediately investigated and it was found that the heater was operating even though the SGTS was shut down.

The heater was taken out of service by opening the circuit breaker to SGTS #11.

Subsequent investigation revealed that a silicon controlled rectifier (SCR j

1. 1, shown on attached figure) had developed a Icak allowing power to the air h(ater.

The leaking SCR caused overheating of the feedback transform er which in turn damaged the current limiting circuitry and the operational amplifier.

The SCR, current 1imiiing circuit, feedback iransformer and optrational amplifier were replaced.

The syttem was satisfactorily tested on June 24, 1971, and declared op(rational.

On July 5,1971, during an operability test of SGTS #11, it was discovered that the differential temperature controlle r for the air heater would provide only a maximum output signal of 60% even though there was a g

large deviation from the seipoint.

It was also notic(d that only a 3 i differential air tempe rature could be obtained after operation for one-half hour. An examination of the problem revealed that another SCR (SCR #3) had developed a leak causing damage similar to that which occurred on Junc 19, 1971.

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. The airheater SCR control circuitry was rmoved from SGTS #11 and l

replaced with contactors which energize the electric heaters when l

the proper minimtrn flow is detected by an existing flow switch.

The l

systm was f ested and declared operational on July 9,1971.

3.

Correctiva Action After the July Sih,1971 occurrence, it was proposed that the entire SGTS #11 air heater control circuit be replaced with contactors which will energize the heaters when minimtrn SGTS flow is detected.

The changc to the SGTS was reviewed and approved by the Operations f'ammittee. During the Operations Commitice review, it was shown that the SCR control circuit was not necessary and that the contacters would be adequate. Previous testing results show that full heater capacity was required to maintain adequate heating of the incoming gas for tFe operable range of system flows and that no overheating would occur with 'ull heater capacity at t h( minimum flow condi t ion.

The approved modification to the system was completed on July 9th.

A pre-operational type test was conducted on July 9,1971, to verify proper operation.f the newly installed air heaters controls.

The system was t hen declared opcrational.

Similar air hcater modifications are planned for SGTS #12.

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