United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 87-05: Miswiring in a Westinghouse Rod Control System

                                                         SSINS NO.: 6835 
                                                           IN 87-05     
														     
                                UNITED STATES
                        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 
                              SYSTEM 

Addressees: 

All Westinghouse nuclear power reactor facilities holding an operating 
license or a construction permit. 

Purpose: 

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in manual, and all rods fully withdrawn, the operator inserted control rods 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 
two steps. 

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. 

Discussion: 

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 
does not 


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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
                    (301) 492-9428

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