United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-105: Potential for Loss of Reactor Trip Capability at Intermediate Power Levels

                                                            SSINS No.:  6835
                                                            IN 86-105 

                                UNITED STATES
                            WASHINGTON, DC 20555

                              December 19, 1986

Information Notice No. 86-105:  POTENTIAL FOR LOSS OF REACTOR TRIP      
                                   CAPABILITY AT INTERMEDIATE POWER LEVELS 

All holders of an operating license or a construction permit for pressurized
water reactors (PWR) or boiling water reactors (BWR). 


This notice is intended to alert licensees operating Westinghouse reactors 
of the potential for loss of some reactor trip functions when operating 
below 10 percent of full power as a result of failure of the P-10 interlock 
circuitry. It is expected that recipients will review this information for 
applicability to their reactor facilities and will consider actions, if 
appropriate, to preclude occurrence of this problem or similar problems. 
Suggestions contained in this notice do not constitute NRC requirements; 
therefore, no specific action or written response is required. 

Description of Circumstances: 

The reactor protection system for Westinghouse reactors includes source, 
intermediate, and power range channels for monitoring neutron flux during 
startup, operation at power, and shutdown of the reactor. During startup, 
trip points at the upper end of the intermediate range channels and at the 
lower end of the power range channels will automatically shut the reactor 
down if an unanticipated increase in power of sufficient magnitude occurs. 
Typically, these trip points on the overlapping intermediate and power range
channels are set at 25 percent of full power. So that reactor power can be 
raised above 25 percent, the P-10 permissive interlock is enabled, typically
at 10 percent of full power. Among other things, this interlock, when 
enabled, permits the operator to manually block the intermediate flux trips. 
When power is decreased below 10 percent, the P-10 permissive interlock 
automatically reinstates the intermediate flux trip logic. 

Circuitry for the P-10 permissive interlock includes one solid-state 
bistable switch on each of the four power range channels. During power 
ascension, when the indicated power level for each power range channel 
exceeds 10 percent, its associated bistable switch trips. When the second of 
the four bistable switches trips, the interlock is enabled. After the flux 
trips at 25 percent are blocked, protection against high neutron flux 
continues to be provided by the power 


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range trips that are set typically at 109 percent of full power. When power 
is decreased below 10 percent, three of the four bistable switches must 
reset; otherwise, the intermediate flux trips will not be automatically 

By letters dated February 27 and April 29, 1986, Westinghouse Electric 
Corporation advised the Office of Inspection and Enforcement that, under 
certain circumstances, the intermediate trips would not be reinstated when 
reactor power is decreased below 10 percent. For example, if. one power 
range channel is taken out of service when reactor power is above 10 
percent, as is permitted by technical specification, and if the bistable 
switch for another power range channel should fail such that it does not 
reset, then the three-out-of-four logic would not be satisfied when reactor 
power is decreased below 10 percent. Thus, a single failure could prevent 
reinstatement of the intermediate trips when a power range channel is out of 
service with reactor power below 10 percent. 


The P-10 permissive interlock performs two functions. First, when at least 
two of the power range channels are above the P-10 set point and their P-10 
bistable switches are tripped, the P-10 interlock provides one of the 
redundant inputs (with P-13) to the P-7 interlock which enables other 
reactor trips, automatically disables the source range channels and the 
reactor trip signals that they provide, and permits manual blocking of the 
trips at the upper end of the intermediate range channels and the lower end 
of the power range channels. Second, when at least three of the four power 
range channels are below the P-10 set point and their P-10 bistable switches 
have reset, the P-10 interlock enables the trips at the upper end of the 
intermediate range channels and the lower end of the power range channels 
which were previously disabled and, if P-13 has reset, automatically 
disables several other reactor trips. Additionally, the P-10 interlock 
enables the source range channels by connecting high voltage to their 
detectors, and it enables the reactor trips at the high end of the source 
range channels when P-6 is reset at the lower end of the intermediate range 
channels. If P-10 fails, indication in the con-trol room of the flux level 
from the source range channels would be lost. This is particularly important 
for plants that depend on this instrumentation to initiate automatic or 
manual actions to protect the reactor under certain conditions, including 
flux doubling or high flux which could be indicative of boron dilution. If 
the P-10 interlock does not function properly when the reactor is below 10 
percent of full power, a substantial amount of protection would be lost. 

Accident analyses in safety analysis reports which take credit for the 
intermediate trips are (a) the uncontrolled boron dilution accident, (b) the 
uncontrolled withdrawal of a control rod bank from subcriticality, (c) the 
control rod ejection accident, and (d) the excessive feedwater accident from 
subcritical. Evaluation of the first three of these analyses by Westinghouse 
has led them to conclude that the margin for safety as defined in the bases 
for technical specifications might be reduced under certain circumstances 
for accidents (a) and (b) if the intermediate trip is not available below 10 
percent of full power. Westinghouse states that, while fuel failure would 
not be expected, the departure from nucleate boiling ratio (DNBR) might be 
lower than the design criterion. For accident (c), Westinghouse indicates 
that the  

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                                                       Page 3 of 3 

margin of safety would be essentially the same with or without the 
intermediate trip. For accident (d), Westinghouse does not address the 
impact on margin of safety, but does imply that corrective action would be 

The Westinghouse letter of February 27, 1986, suggested several actions that
licensees could consider pending resolution of this problem. After 
discussion with the NRC staff, Westinghouse submitted a clarifying letter on 
April 29, 1986. In summary, Westinghouse has recommended to licensees that 
they monitor the status lights for the P-10 bistable switches and for the 
P-10 permissive interlocks in both trains of the reactor protection system 
when power is being reduced below 10 percent. In the event that a licensee 
cannot confirm proper performance of the P-10,interlock, Westinghouse 
recommends that the licensee consider either (a) using jumpers as 
appropriate to enable any lost trips and alarms or (b) completing an orderly 
shutdown, opening the reactor trip breakers to preclude withdrawal of a 
control rod bank, and closing valves as necessary to preclude boron dilution 
and excessive feedwater flow. 

It is suggested that licensees with Westinghouse reactors consider informing
their reactor operators of this potential problem and consider the need to 
revise operating procedures to guide operators in diagnosing and correcting 
8the problem in a timely way if it occurs. If corrective actions involve the
use of jumpers or lifted leads, Information Notice No. 84-37, "Use of Lifted 
Leads and Jumpers During Maintenance and Surveillance Testing," provides 
further suggestions. 

This notice requires no specific action or written response. If you have any
questions regarding 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 Contacts:  Frederick H. Burrows, NRR 
                     (301) 492-9789 

                     Roger W. Woodruff, IE 
                     (301) 492-7205 

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