United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-21: Main Steam Isolation Valve Closure Logic

                                                         SSINS No.:  6835 
                                                            IN 85-21  

                               UNITED STATES 
                          WASHINGTON, D. C. 20555 

                               March 18, 1985 



All nuclear power pressurized water reactor (PWR) facilities holding an 
operating license (OL) or construction permit (CP). 


This information notice is provided as a notification of a potentially 
significant problem pertaining to logic for closure of main steam isolation 
valves (MSIVs). It is expected that recipients will review the information 
for applicability to their facilities and consider actions, if appropriate, 
to preclude a similar problem from 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: 

In early 1985 the licensee for Robinson 2 discovered and reported a logic 
fault with respect to MSIV closure. The MSIVs at Robinson are operated by 
air pressure admitted to the top or bottom area of a piston controlling the 
MSIV clapper (see Figure 1). The air pressure is controlled by two parallel 
"closing" solenoid valves on the top of the piston and two parallel 
"opening" solenoid valves on the bottom of the piston. For each MSIV, a 
closure signal from either of the redundant ESF logic trains (A or B) should 
be able to pressurize the top of the piston and to depressurize the bottom 
of the piston to provide positive and rapid closure of the MSIV. 

At Robinson, which is a three loop plant, the "B" and "C" MSIVs had both 
opening solenoid valves controlled by the B train. In the event of a single 
failure of the B logic train, it is not clear what the response of the B and
C MSIVs would be. While one closing solenoid valve would open to admit air 
pressure to the top of the piston, both the opening solenoid valves would 
remain in position to pressurize the bottom of the piston. The test solenoid
valve connected to the space above the piston would remain in position to 
continue slowly venting the top of the piston through an orifice. The 
resulting difference in pressure above and below the piston is judged to be 
nearly zero. The only closing force remaining would be provided by the 
spring, which is designed to move the clapper only partly closed for testing
purposes. Although steam flow may close the MSIV under accident conditions, 
this is not how the valves were designed or qualified to close and rapidly 
terminate the blowdown. 


                                                           IN 85-21 
                                                           March 21, 1985 
                                                           Page 2 of 2    

The licensee is correcting this situation by modifying the control circuitry
to ensure that either a Train A or a Train B closure signal will cause 
positive closure of all three MSIVs. 


In the event of a steamline break accident, rapid closure of the MSIVs pro-
tects against positive reactivity excursions from rapid cooling and against 
loss of heat sink. At most, the design basis contemplates that one MSIV 
might fail to close due to failure of a single active component. In this 
instance, two MSIVs might have failed to close due to a single failure of 
the B closure signal. Licensees may wish to review their MSIV control logic 
in the light of this discovery. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate NRC regional office or this 

                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  C. V. Hodge, IE 


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