Information Notice No. 86-109: Diaphragm Failure in Scram Outlet Valve Causing Rod Insertion
SSINS No.: 6835
IN 86-109
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
December 29, 1986
Information Notice No. 86-109: DIAPHRAGM FAILURE IN SCRAM OUTLET VALVE
CAUSING ROD INSERTION
Addressees:
All boiling water reactor facilities holding an operating license or a
construction permit.
Purpose:
This notice alerts addressees to the potential for a failure of the
diaphragm in the scram inlet or outlet valve operator on the control rod
drive hydraulic control units. This could result in a single control rod
scram while at power. This event highlights the importance of preventive
maintenance programs. It, is suggested that recipients 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:
During routine full-power operations on September 8, 1986, the Nine Mile
Point Unit 1 Nuclear Power Plant experienced a single control rod scram
(insertion) when the diaphragm in the air operator of the scram outlet valve
failed. The hydraulic control unit was isolated, the failed diaphragm was
replaced, and the control rod was returned to its normal operating position
in the core.
Investigation revealed that the diaphragm failed because of an aging process
that resulted in a radial crack in the rubber (buna-n and nylon material).
The licensee initiated a review of its spare parts inventory and past
operating experience and contacted the General Electric Company to determine
the need for a diaphragm replacement program and to discuss the generic
implications of this event.
Discussion:
The related central issues that need to be evaluated are: (1) whether
programs to ensure that component shelf life and service life have properly
included this diaphragm and other similar components susceptible to aging
and (2) whether a failure of the diaphragm on the inlet valve alone (with
the outlet valve closed) could cause damage to the control rod drive and
possibly stick the control rod in an intermediate position.
8612230093
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IN 86-109
December 29, 1986
Page 2 of 2
The licensee (Niagara Mohawk Power Corporation) had previously changed out
the diaphragms in approximately one half of the control rod drive units in
1975-1976, and about 10 more since then. The remaining installed diaphragms
are original plant equipment that was purchased prior to 1970. Although
spare diaphragm packages contained markings of an "estimated" shelf life of
7 years, preventive maintenance practices and a specific in service or shelf
life were apparently not specified for this part when the event occurred.
Subsequently, the licensee is in the process of determining a shelf life and
service life for the diaphragm.
Damage to a control rod drive resulting in a stuck control rod was
previously reported in NRC Information Notice (IN) No. 86-68, "Stuck Control
Rod," dated August 15, 1986. That issue involved a closed manual outlet
valve that resulted in internal damage to drives at Clinton and Perry due to
excessively high internal pressures. In these two cases, the preoperational
tests were conducted with a depressurized, or low, reactor coolant system
pressure which may have resulted in a high differential pressure across the
cylinder tube.
The failure at Nine Mile Point Unit 1 is being reviewed to determine whether
an event similar to that discussed in IN 86-68 would be possible as a result
of a failed scram inlet valve diaphragm. During a reactor protection system
actuation, the inlet and outlet scram valves are opened nearly
simultaneously because of the de-energization of the scram pilot valves and
venting of the air header. However, in the case of a ruptured diaphragm on
the scram inlet valve it is not certain whether a simultaneous opening of
the scram outlet valve would occur. The licensee has contracted for an
analysis of this problem but the results are not yet available.
No specific action or written response is required by this information
notice. If you have questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contacts: Eric Weiss, IE
(301) 492-9005
Charles S. Marschall,
Resident Inspector, Nine Mile Point NPP
(315) 342-4041
Attachment: List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021