Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 85-21
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
March 18, 1985
Information Notice No. 85-21: MAIN STEAM ISOLATION VALVE CLOSURE LOGIC
Addressees:
All nuclear power pressurized water reactor (PWR) facilities holding an
operating license (OL) or construction permit (CP).
Purpose:
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.
8503140033
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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.
Discussion:
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
office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: C. V. Hodge, IE
301-492-7275
Attachments:
1. Figure 1
2. List of Recently Issued IE Information Notices
