Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
June 10, 1993
NRC INFORMATION NOTICE 93-46: POTENTIAL PROBLEM WITH WESTINGHOUSE ROD
CONTROL SYSTEM AND INADVERTENT WITHDRAWAL
OF A SINGLE ROD CONTROL CLUSTER ASSEMBLY
Addressees
All holders of operating licenses or construction permits for
Westinghouse (W)-designed nuclear power reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potential problem with the Westinghouse rod
control system that can cause an inadvertent withdrawal of one or more control
rod cluster assemblies in a selected bank. It is expected that recipients
will review the information for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems. However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances
On May 27, 1993, operators at the Salem Nuclear Generating Station, Unit 2,
experienced problems with the rod control system. During an attempt to
withdraw Shutdown Bank A, the operator observed that the Analog Rod Position
Indicator (ARPI) did not indicate that the control rods were being withdrawn.
The operator stopped attempting to withdraw rods at 20 steps as indicated on
the Group Demand Indicator. The Group Demand Indicator tells the operator the
position the rods should have moved to based on the demand from the rod
control system. The ARPI provides the actual position of each rod. The
operator then attempted to insert Shutdown Bank A. However, one control rod
(1SA3) withdrew to 8 steps while the Group Demand Indicator counted down from
20 steps to 6 steps. The operator continued to try to insert the Shutdown
Bank A control rods until the Group Demand Indicator showed a rod position of
zero. The operator observed that the indicated position on the ARPI for
control rod 1SA3 was 15 steps. Public Service Electric & Gas (the licensee)
removed the power from the rod by pulling fuses and rod 1SA3 dropped to the
0 step position as indicated by ARPI.
The licensee initiated troubleshooting activities on the Salem, Unit 2, rod
control system. An NRC Augmented Inspection Team (AIT) has been sent to
Salem, Unit 2, to evaluate this issue and observe the investigation of this
event by the licensee. Westinghouse Electric Corporation personnel are
providing technical assistance to the licensee.
9306090459.
IN 93-46
June 10, 1993
Page 2 of 3
Discussion
During a refueling outage this spring, the licensee and Westinghouse performed
extensive maintenance work on the solid state electronic rod control system
for Salem, Unit 2. On May 26, 1993, the licensee initiated the startup of
Salem, Unit 2, from the refueling outage. From May 26, 1993 to June 3, 1993,
the licensee experienced a series of failures in the rod control system.
Following each failure, the licensee located the failed components in the
system, performed repairs and retests, and returned the rod control system to
operation. On June 4, 1993, the licensee shut down Salem, Unit 2, pending the
results of an investigation into the rod control system failures. None of the
failures in the rod control system interfered with the operation of the
reactor scram function.
The licensee, in response to NRC questions in consultation with Westinghouse,
has postulated that, for the event that occurred on May 27, 1993, a single
failure in the rod control system caused a single rod to withdraw from the
core 15 steps while the operator was applying a rod insertion signal. The
failure, an integrated circuit on a slave cycler decoder card, disrupted the
normal sequence of pulses that the rod control system sends to the rods in the
selected bank. Normally on insert demand, the pulses are staggered in a
sequence that leads to rod insertion. With the failure, the rod control
system periodically sent simultaneous pulses to the movable gripper coil, lift
coil, and stationary coil for each of the rods in the selected bank. Under
these conditions, based on the preliminary investigation, each rod in the bank
may either remain where it is or withdraw from the core when a rod movement
demand occurs. When the rod control system is in the automatic mode of
operation, a rod movement demand is generated automatically in response to
changes in turbine load and changes in the average reactor coolant
temperature. Rod movement then occurs without any operator action until the
demand is satisfied. When the rod control system is in the manual mode of
operation, a rod movement demand is generated only in response to operator
manipulation of the IN-HOLD-OUT switch, given no failures in the demand
circuit.
The Updated Final Safety Analysis Report (UFSAR) for Salem, Unit 2, states
that multiple failures would have to be present in order for an inadvertent
single rod withdrawal event to occur. The event on May 27, 1993, indicates
that the present design for Salem, Unit 2, appears to violate this statement.
The licensee issued a standing order for the operators at Salem, Unit 1, which
was operating at 100 percent power at the time. The standing order required
(1) placing the rod control system in the manual mode of operation,
(2) maintaining the control rods at or near the top of the core, and
(3) manually tripping the reactor if the control operator and supervisor judge
that safety system setpoints are being challenged. With the rod control
system in the manual mode of operation, two failures would be required to
cause an inadvertent rod withdrawal; a failure in the rod control system in
combination with an inadvertent rod movement demand. After the standing order
was issued, Salem, Unit 1, experienced a scram due to a clogged intake
structure. The operators were unable to prevent the scram by manual turbine .
IN 93-46
June 10, 1993
Page 3 of 3
or reactor runback. The licensee had provided operator training and prepared
an engineering evaluation of the event on May 27, 1993. The licensee had also
identified a periodic Technical Specification Surveillance Requirement that
would detect the presence of the postulated failure.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation project manager.
ORIGINAL SIGNED BY
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: Evangelos Marinos, NRR Edward Wenzinger, RI
(301) 504-2911 (215) 337-5225
Margaret Chatterton, NRR Eugene Imbro, RI
(301) 504-2889 (215) 337-5080
Attachment:
List of Recently Issued NRC Information Notices
.
