Information Notice No. 87-05: Miswiring in a Westinghouse Rod Control System
SSINS NO.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
February 2, 1987
IE INFORMATION NO. 87-05: MISWIRING IN A WESTINGHOUSE ROD CONTROL
All Westinghouse nuclear power reactor facilities holding an operating
license or a construction permit.
This notice is to alert addressees to the miswiring of circuit card
interconnections in the Westinghouse-supplied rod control system of a
Westinghouse reactor. 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:
Beaver Valley Unit 1
On December 1 1986, with the plant at full power, the bank selector switch
in manual, and all rods fully withdrawn, the operator inserted control rods
consistent with a minor power redution. Although control bank 'D' rods were
expected to move, the operator observed that control bank 'A' rods inserted
Subsequent investigation revealed that two circuit boards in the bank
overlap unit (BOU) of the rod control system had been wired together
improperly. This miswiring was determined to have existed since initial
installation of the rod control system when the plant was new. Rod control
systems of this design have been supplied for approximately 60 other
Westinghouse nuclear units.
With the rod control system in manual or automatic, the BOU automatically
selects the sequence and overlap of the control banks. To keep track of the
process, each in or out step of the control banks is recorded by a counter
in the BOU. The BOU count increases from 000 to 999 as control bank rods are
stepped out. Shutdown bank motion does not affect BOU count. To support
special evolutions such as troubleshooting and repair, the BOU is fitted
with "+1", "-1" and "reset" push buttons which permit advancing, retarding,
or zeroing, respectively, the BOU count. Depressing any of these buttons
February 2, 1987
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result in rod motion, but only in an adjustment of the BOU count memory. The
nature of the incorrect wiring was such that depressing the +1 button caused
the BOU count to reset to 000, as would depressing the reset button. With
the BOU count at or near 000, and the bank selector switch in manual or
automatic, any demand for rod motion would result in movement of bank 'A'
rods, as was observed in this case. This is a violation of technical
specification rod bank insertion limits which require all control bank 'A'
rods to be fully withdrawn when the plant is operating in the power range.
This is to ensure that adequate negative reactivity due to rods is available
to safely shut down the reactor in the event of a reactor trip.
After testing eliminated the possibility of a failed BOU electronics card,
it was concluded that the BOU counter had been zeroed by operation of the
BOU +1 button. In a properly wired BOU, depressing the +1 and -1 buttons an
equal number of times would have no net effect on BOU count, i.e. , the
final count would be the same as it had been before manipulating the
buttons. In the miswired configuration, the BOU count used to supervise rod
motion was reduced to 000. Investigation showed, however, that the visual
display of the counter incremented and decremented in step with the push
buttons. Thus, although the internal counter which controls rod sequence and
overlap was reset to 000, the display indicated normal function. In
addition, since the BOU count automatically resets to 000 on a scram signal,
any existing counter misalignments are erased at that time. These factors
may have masked the problem.
This problem was rectified at Beaver Valley Unit 1 by rewiring the BOU
circuit board interconnections as prescribed by the original drawings, which
were correct. The unit remained at power during repairs. The Beaver Valley
Unit 2 rod control system was inspected and found not to be affected.
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 regional office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Kevin Wolley, IE
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