ML20086H360

Ao:On 720818,partial Insertion of Six Out of 15 Withdrawn Control Rods Occurred Because of Two Overlapping Half Scrams.Caused by Spurious Electronic Noise in Monitor Channels.Rods Manually Inserted & Tested

ML20086H360
Person / Time
Site:	Vermont Yankee
Issue date:	10/27/1972
From:	Rosalyn Jones VERMONT YANKEE NUCLEAR POWER CORP.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
Shared Package
ML20086H363	List: ML20086H360
References
VYV-2248, NUDOCS 8401170069
Download: ML20086H360 (3)

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,i VERMONT YANKEE NUCLEAR POWER CORPORATION SCVENTY ScVEN OROVE STREET RuTLAxo, VERMONT 03701 REPLY TO:

VYV-2248 p. O. sox i57 VERNON, VERMONT 05354 I

October 27, 1972 i

i United States Atomic Energy Commission ~

i Directorate of Regulatory Operations, Region I 970 Broad Street Newark, New Jersey 07102 4

REFERENCE:

Operating License DPR-28 Docket No. 50-271 1

Dear Sir:

' This letter is to provide you with infornation concerning certain activities and events at the Vermont Yankee Nuclear Power Station which 1

we feel might be of interest to you in your long tern assessment of the l overall dependability of nuclear power plants.

On August 18, 1972, with the reactor critical, in the startup mode I

and at a power level corresponding to Range 7 on the Intermediat'e Range

, Monito.s (IRM), a partial insertion of six out of fifteen withdrawn

control rods occurred because of two overlapping one half scrams that were caused by spurious electronic noise in the IRM channels.

l A Channel B half scram of the Reactor Protection System (RPS) occurred when electronic noise induced a false signal to IRM Channel D causing it to trip on a high level signal of 120/125 of full scale. Due to the short duration of the noise, the readout of IRM Channel D returned to its previous and correct level. At that time the Control Room Operator reset the half scram condition for the rods in Groups I and IV first and then reset the scran for rods in Groups II and III. Between the time he reset the half scram for Groups I and IV and while he was rotating the reset switch to the Groups II and III position, a noise spike was experienced by IRM Channel A causing a half scram in RPS Channel A. There are two solenoid scram pilot valves in the air line that supplies instrument air to the insert and exhaust scram valves. In a scram condition, the pilot valves bleed 80 air from the scram valves causing them to open.' The pilot valves are arranged that a half scram in Channel A will open one pilot valve and a half scram in Channel B will open the other. Opening of both pilot valves is required to open the scram valves and therefore scram the control rods.

8401170069 721027 ,

b%d9.ll PDR ADOCK 05000271 S PDE M "9 -

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O O VERMONT YANKEE NUCLEAR POWER CORPORATIO i

United States Atomic Energy Commission October 27, 1972 I

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i When the operator reset the half scram in Channel B for rods in

. Group II and III, their respective scram pilot valves started to close.

The scram pilot valve associated with Channel A started to open when it received a half scram signal and the result was that both pilot valves on the hydraulic control units for the rods in Groups II and III were open at the same time for a short period of time causing the scram valves to open partially and start to scran the control rods in Group III. However, before the rods were fully scrammed the pilot valve associated with scran Channel B closed fully thereby restoring air pressure to the scram valves, closing them and interrupting the scram.

Each rod has its own hydraulic control unit and therefore its own set of scram pilot and scram valves and therefore only the rods in Groups II and III were in a condition of having both scram pilot valves open simultaneously for a short period of time. The six rods that partially scrammed were in Groups II and III. The attached figure shows the positions of the affected rods before and af ter the . incident.

This incident in no way constitutes a failure of a safety system.

, The result of the partial scram was to insert negative reactivity when plant conditions did not require it. Immediately following the incident all of the withdrawn rods were manually inserted to the full in position.

All si:: teen of the rods were exercised and scram tested to prove their operability prior to continuing reactor operations. All of the rods operated normally and their scram times were well within the maximum time limit.

If you have any questions concerning this incident, please contact me.

Very truly yours, VERMONT YANKEE NUCLEAR' POWER CORPORATION lf!i,ft l<N[' s~

Robert F. Jones Plant Superintendent WPM /rph 9

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, 1 VE UO.h YANKEE 43 39 - 48 48 35 .._ .

31 48 28 48 "'

27 23 48 48 48 48 48 19 15 48 48 28

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07 48 48 . Y,.:

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02 06 10 14 18 22 26 30 34' 38 '42 .

Rod pattern just prior to rod insertion.

All other rods in the full in position.

VE.ddONT YANKEE 43 39 48 48 i

35 31 30 12 30 27

23. 48 48 48 48 .48 19

15 30 30 19

.11 07 48 48 l . 02' 06 10 14 18 22 26 30 34 38 42 Rod pattern just af ter rod insertion.

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