United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-109: Diaphragm Failure in Scram Outlet Valve Causing Rod Insertion

                                                            SSINS No.:  6835
                                                            IN 86-109 

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                              December 29, 1986

                                   CAUSING ROD INSERTION 


All boiling water reactor facilities holding an operating license or a 
construction permit. 


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 

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. 


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. 


                                                         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
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