ML20137B265
| ML20137B265 | |
| Person / Time | |
|---|---|
| Issue date: | 11/19/1985 |
| From: | Rossi C NRC OFFICE OF INSPECTION & ENFORCEMENT (IE) |
| To: | Stromback G GENERAL ELECTRIC CO. |
| References | |
| NUDOCS 8511260222 | |
| Download: ML20137B265 (5) | |
Text
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NOV 19 IINE Mr. George B. Stramback M/C 682 General Electric Company 175 Curtner Avenue San Jose, CA 95123
Dear Mr. Strambadk:
SUBJECT:
IE DRAFT INFORMATION NOTICE:
LEAK OF REACTOR WATER TO REACTOR BUILDING CAUSED B( SCRAM SOLEN 0ID VALVE PROBLEM The draft IE information notice is enclosed for your review per the November 18, 1985, telephone conversation between you and Eric Weiss of my staff.
The draft has been reviewed within the NRC and is ready for issuance.
If you have any comments, please send them to me as soon as possible because we expect to issue the notice soon.
If you have any questions regarding this matter, please feel free to contact me or Mr. Weiss.
I can be reached at (301) 492-4193 and Mr. Weiss at (301) 492-9005.
Sincerely,
$W signed by Charles E. Rossi, Chief Events Analysis Branch Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement
Enclosure:
IE draft information notice Distribution:
DEPER R/F EAB R/F CERossi EWWeiss SASchwartz ELJordan RHVollmer E
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8511260222 851119 PDR I&E GENERAL PDR
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6835 IN 85-UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C.
20555 November 1985 IE INFORMATION NOTICE N0. 85- :
LEAK OF REACTOR WATER TO REACTOR BUILDING CAUSED BY SCRAM SOLEN 0ID VALVE PROBLEM Addressees:
All boiling water reactor (BWR) facilities holding an operating license (0L) or a construction permit (CP).
Purpose:
This information notice is to alert addressees of the potential for leakage of reactor water to the reactor building caused by problems associated with the scram pilot air solenoid valves.
Recipients are expected to review the information for applicability to their facilities and consider actions, if appropriate, to preclude similar problems occurring at their facilities.
However, suggestions contained in this information notice do not constitute NRC requirements; therefore, no specific action or written response is required.
Description of Circumstances:
On September 1,1985, Dresden Unit 3 experienced leakage of reactor water into the reactor building because of what appears to be a generic problem associated with the scram pilot air solenoid valves used in most BWRs.
The event began when the reactor scrammed from 83% power on high average power range monitor (APRM) level caused by a pressure spike resulting from an inadvertent closure of the turbine control valves.
During the scram recovery, a problem with the reactor mode switch prevented resetting channel "B" of the reactor protection system (RPS).
During this half reset condition, steam entered the reactor building and radiation levels were elevated in the first three floors of the reactor building.
(A half reset condition is equivalent to a half scram immediately following a full scram.)
Although it was not immediately recognized, the leakage was caused by the scram outlet valves being open when they should have closed as soon as RPS channel "A" was reset.
For about 23 minutes, the leaking reactor water was flashing to steam and causing contamination of the first three levels of the reactor building. The leakage path was from the scram outlet valves, to the scram discharge volume (SDV) vent and drain valves, and then to the reactor building equipment drain tank (RBEDT) and the reactor building atmosphere.
When RPS channel "A" was reset, air header pressure (to the scram valves) increased to about only 38 psig instead of the normal 83 psig.
The air pressure was sufficient to open the SDV vent and drain valves but was insufficient to close the scram outlet valves.
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IN 85-November, 1985 Page 2 of 3 Discussion:
This event,is still being investigated.
However, the current understanding of this event suggests taat it could occur at most other BWRs if the reactor is first scrammed, bringing the air header pressure to zero, and then the "A" channel of the RPS is reset and the "B" channel is left tripped.
With low air pressure, some of the scram pilot air solenoid valves tend to vent air preventing the air header pressure from rising to the proper value.
In this condition, the scram outlet valves will likely remain open providing a path for leakage to the reactor buildin'g.
Although the mode switch was part of the initiating sequence for the event at Dresden Unit 3, other possible causes of half scrams, such as misapplied maintenance or surveillances, could start this type of event provided that a full scram occurred first.
The problem with the mode switch is believed to be that it was inadvertently left in between positions.
Testing in place and on the workbench sh6ws t. hut the leakage of the scram pilot air solenoid valves only occurs when the "A" channel valve (number 305-117) is reset and the "B" channel valve (number 305-118) is tripped.
(See the attached drawing of the scram pilot air solenoid valves.) The problem does not occur in the reverse situation when the "B" channel is reset and the "A" channel is tripped.
The problem is believed to be associated with the low pressure operating characteristics of the valve and with back pressure on the exhaust diaphragm "E2" which causes that diaphragm to leak under the described circumstances.
The problem should not adversely affect the ability of any plant to scram.
The problem should not occur in BWRs with fast-scram drives because they have solenoid valves of a different design.
Operators may be misled as they were during this event, when there were false indications that the reactor water cleanup system relief valve was causing the leakage to the RBEDT.
If trained, operators should be able to recognize the real condition.
The full core display contains a blue light for each control rod indicating that the scram outlet valve is open, and the SDV vent and drain valvesalso have position indication in the control room.
In response to this event, the licensee has adopted corrective actions that deal with the mode switch and with the SDV vent and drain valves.
In regard to the mode switch, the corrective actions include visual inspection for any impairment.
Another mode switch related corrective action includes revision of the scram procedure so that the reactor operator will be directed to move the mode switch to the " shutdown" position after any scram occurs.
This will prevent any future mis positions of the mode switch.
If the mode switch is replaced in the future with a more reliable type, this instruction may be removed.
(The reactor operator had been placing the mode switch in " refuel" position in order to obtain a white light single rod permissive that indicated all rods were in following a scram.
Because some rods usually bounce out to position 02 following most scrams, the white light permissive has not been that useful.)
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IN 85-
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November
, 1985 Page 3 of 3 In regard to the SDV vent and drain valves, corrective actions include two changes to the scrain procedure.
One change is that the reactor operator will be directed to close the SDV vent and drain valves using the individual control switches in the control room before resetting any scram.
This will prevent any possible steam releases in the future if the scram air header pressure were to become degraded.
The other change is that a caution statement will be added to the procedure.
If the SDV vent and drain valves will not close during any half scram condition, following a full reactor scram reset, the reactor operator will be instructed to manually scram the reset RPS channel.
(A full scram will cause the SDV vent and drain valves to close.)
No specific action or written response is required by this information notice.
If you have questions about this matter, please contact the Regional Adminis-trator of the appropriate NRC regional office or this office.
4
$dT ShWS$
Edward L. Jordan, Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical
Contact:
Eric Weiss, IE (301) 492-9005 Attachments: 1.
Figure of Scram Pilot Valve Configuration 2.
List of Recently Issued IE Information Notices hh /Y s
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