ML20086H166
| ML20086H166 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 01/26/1973 |
| From: | Rosalyn Jones VERMONT YANKEE NUCLEAR POWER CORP. |
| To: | US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| AO-50-271-72-19, NUDOCS 8401160027 | |
| Download: ML20086H166 (5) | |
Text
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f VERMONT YANKEE NUCLEAR POWER CORPORATTON UCVENTY SCVEN OROVE STRCCT RUTLAND, Veitsoxr osvoi REPLY TO:
VYV-2398
- p. o. box 157 VcRNON. VERMONT 05354 January 26, 1973 Director R
Directorate of Licensing M
4 U.S. Atomic Energy Commission hf' l.
Washington, D.C.
20545
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REFERENCE:
Operating License DPR-28 k ; {uNEMI j[
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Abnormal Occurrence No. A0-72-19 0
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Dear Sir:
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As defined by Technical Specifications for the Vermont Yankee Nuclear Power Station, Section 6.7.B.d, we are reporting the following Abnornal 1
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Occurrence as A0-72-19.
At 1738 on December 27, 1972 an internal fault on the startup trans-former resulted in a loss of normal power (LNP) on site.
The reactor was operating steady state at approximately 63% power and was scrammed and isolated when the LNP cccurred deenergizing the reactor protection system motor generator sets.
(Refer to SEQUENCE OF SIGNIFICANT EVENTS, attached)
The initial pressure transient caused "A","C" and "D" relief valves to lift and "A" safety valve to lift or " chatter" on its seat.
(Reactor '
a vessel level decreased to -20" and the Iligh Pressure Coolant Injection (llPCI) System was started and utilized to restore water level to the normal band.) Both emergency diesel generators started and supplied their respective buses. The Residual llent Renoval and Core Spray Systems started but were not required to inject into the vessel. A plant cooldown was conducted utilizing IIPCI and RCIC to remove decay heat and maintain water Jevel, respectively.
Investigation and review of the incident han pointed up several areas that require further investigation and corrective action. A summary of the i
major items of concern and corrective action is provided below:
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COPY SENT REGION 8401160027 730126 e-g PDR ADOCK 0500027 r, g-f 5
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U VERMONT YANKEE NUCLEAR POWER CORPORATIOh Directorate of Licensing January 26, 1973 Page 2 Safety Valve "A" chattered, approximately 100 psig below thd set a.
pressure of 1230 psig. This valve was replaced with a spare valve and was disassenbled and inspected., Safety Valve "B" was verified
'not to h4ve lifted (as' expected)' based upo' thermocouple readings and a subsequent visual inspection of the sywell, b.
Relief Valve "B" did not lift when reactor pressure " spiked" to approximately 1130 psig. The valve was set to relieve at 1100 psig on 9/6/72 and pop pressure
's verified with nitrogen to be 1095 on 1/2/73.
Possible failure of this valve to lift is attributable to dif ferential expansion of components within the pilot valve. After discussions with the vendor regarding this problen, all relief valves have been set using nitroge,g 30 psig below their specified stean setpoint.,. Additionally, it is currently being planned to replace the appropriate internals of pilot valves to preclude the necessity of the 30 psi differential betueen nitrogen set pressure and steam lift pressure.
This replacement will connence after appropriate engineering review and when replacenent parts are available.
In cidition, the "B" relief discharge thernocouple was replaced and that temperature channel was ve'ified to respond r
properly from sensor to recorder.
The HPCI system apparently did not autostart when the drywell c.
pressure momentarily exceeded 2.0 psig. HPCI was innediately
. manually started and operated satisfactorily. HPCI, logic was verified completely satisfactory in a full logic check on 1/2/73.
Further confirmat' ion of HPCI. auto initiate capability was obtained during conduct of the LNP test performed as the Temporary Startup Tranaformer Loading Test.
d.
Replacement of the windings within the startup transforner will require approxinately four months tc conplete. A temporary set of replacement transformers (see Figure I) has been installed to provide. site power.
An electrical balance has been performed to check'the adequacy of the i
25 MVA replacement transforner (#3 Fig. I) which is supplying power for j
station service and the emergency buses. The plant auxiliary load is 20 j
Mi and the non-running safety load is 2.1 MW on each emergency bus.
With i
the safety load starting in-rush of five times running Icad (23.3 MVA) superinposed on the normal plant auxiliary load, calculations indicate Ii
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VERMONT' YANKEE NUCLEAR POWER CORPORATIC l
Dir'ectorate of Licensing January 26, 1973 Page 3 that the voltage dip at the transformer during the starting period is approxina.tely 6.5% as compared to the calcu1~ated voltage dip of 5.2%
using the original startup transformer.
Calculations also indicate the
- / v transform,cr uill be. capable'of.'darrying"this fnrush ar '. steady state-current. For the short period of time until running
. ant loads are automatically or manually tripped, the total running load is 26.7 MVA which is within the capability of the transformer.
Verification of electrical system capability *'s conducted to produce the inrush condition of all ECCS starting apd test the adequacy of the replacement transformer to start the ECCS loads.
Satisfactory conpletion of this testing proceeded the return to power operation.
The Plant Operations Review Connittee and Nuclear Safety Audit and Review Committee has reviewed the items outlined above,and concluded that a return to power operation could reasonably be conducted at the conpletion of the indicated testing and corrective action with this adequate assurance' that' no items affecting safet'y' had been lef t unresolved.
Very truly yours, VERMONT YANKEE NUCLEAR POWER CORPORATION r
, Robert F.
ones Plant Superintendent i
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i SEQUENCE OF SIGNIFICANT EVENTS December 27, 1972 1738 Total loss of Normal Power.
3 Automatic reactor scram followed imnediately by manual turbine trip, MSIV isolation, and immediate Relief Valve / Safety Valve Operation.
Relief / Safety valve max. discharge temp.
RV-A 350*F SV-A 280*F RV-B 220*F unchanged SV-B 170*F RV-C 335'F RV-D 375*F ECCS'and DG Auto Start Manual Start of HPCI High Drywell ?ressure' Alarm.- v.erifle.d spike.but alnost immediatie decrease'..
1745 Confirn Startup Transformer Differential Lockout.
Startup Transformer differential instantaneous and time delay flags, phases j'
B & C.
N Auto Transformer Differential Lockout.
.1 Auto Transformer Internal Pressure Alarm.
1815 MCC 68 on Bus 8 d
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1845 Main Turbine on Turning Gear.'
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1900 Closed 379T and 381T.on REMVEC orders.
1910 Reactor Pressure <600 #, reset scram j
Established Torus cooling using RHR pump "A" & RHRSW pump "A" l
1915 Start Turbine Generator Oil Pump, secure Emergency, Oil Pump
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1955 Closed K40 and Kl86 on'REMVEC Orders restoring 115 KV Bus.
2040 HPCI tripped on low steam pressure (%110 psi) RCIC continued to-feed vescel.
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2055 Closed 3T1 restoring power to busses 1 and 480 V #6,#7, and #11 vla DG-B.
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2112 Established shutdown cooling mode of RHR. Average cooldown rate %80*F/hr.
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2335 Closed '379 and 381 af t'er ' clearing Auto Transfor'mer lockout. '-
2346 Closed 4T2 restoring power to busses 2 and 10 via DG-A.
1 0220 Restored Reactor Building and Service' Building HVAC.
0310 Restored Turbine Building HVAC.
0324 Restarted Radwaste Supply Fan, RWS 1A and Radwaste Exhaust Fan, RWE 1A 0500 Auto Transformer restored to normal w
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