United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-11: Single Failure Vulnerability of Engineered Safety Features Actuation Systems

                          UNITED STATES
                  NUCLEAR REGULATORY COMMISSION
              OFFICE OF NUCLEAR REACTOR REGULATION
                     WASHINGTON, D.C.  20555

                        February 4, 1993


NRC INFORMATION NOTICE 93-11:  SINGLE FAILURE VULNERABILITY OF ENGINEERED    
                               SAFETY FEATURES ACTUATION SYSTEMS

Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this notice to alert
addressees to potential single failure vulnerabilities in engineered safety
features actuation systems.  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 July 6, 1992, during a planned outage at the Millstone Nuclear Power
Station, Unit 2, with the core off loaded to the spent fuel pool, the
licensee, the Northeast Nuclear Utilities Company, was preparing to replace
two vital inverters.  Millstone Unit 2 uses four inverters, two on each vital
dc bus, to power two trains of engineered safety feature actuation comprised
of four sensor cabinets and two actuation cabinets.  Operators removed power
from one actuation train, which caused a false loss of normal power signal and
a false start signal for the emergency core cooling system.  The effect of
this action was similar in consequence to the complete loss of one of the two
vital dc buses.  

One emergency diesel generator (EDG) started and tied onto the bus.  The
second EDG did not start because it was out of service for maintenance.  
After the one EDG started, the safety loads failed to sequence onto the bus
because of a continuous false load shed signal.  Operators recovered from the
event by stopping the EDG and restoring power to one of the sensor cabinets. 
This action removed the false loss of power signal and thus the load shed
signal.  

The licensee reviewed the event and concluded that an unblocking feature of
the automatic test insertion (ATI) system had caused the continuous load
shedding signal.  The ATI system, a continuous, on-line, logic tester that is
common for both trains, was still energized and permitted the spurious loss of
power signal to continue to shed the loads.  The ATI system applies 
2-millisecond unblocking pulses to the input of the actuation logic modules

9301290025.

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and checks the module outputs for proper operation.  The 2-millisecond pulses
are too brief to actuate relays and start equipment.  In 1978, the licensee  
added a feature to permit ATI testing of the loss of normal power logic.
To test the logic, the licensee determined that the ATI system needed to
provide an unblocking of the loss of power signal for 500 milliseconds.  In
the actual event, the false signal generated by the lack of control power was
continuously present during the 500 ms ATI unblocking signal.  This caused a
recurring load shed signal to be generated even though the EDG was ready to
accept loads; therefore, the EDG load breakers never closed.

In reviewing the event, the licensee determined that the engineered safety
feature actuation system could also cause other unintended actions under
certain power supply failure conditions.  These automatic actions are not
related to the ATI modification.

(1)  If power is lost to either one of the two dc vital buses, both the safety
     injection actuation signal and sump recirculation actuation signal would
     be simultaneously initiated.  The recirculation actuation signal would
     result in tripping all low pressure injection pumps.  Also, the spurious
     sump recirculation actuation signal would cause one of the containment
     sump outlet valves to open.  

(2)  If power was lost only to the sensor cabinets in one actuation train,
     both containment sump outlet valves would open.  If this occurred during
     a loss-of-coolant accident, high pressure in containment could shut both
     refueling water storage tank check valves, inhibiting flow to all
     emergency coolant injection pumps.
 
(3)  The loss of all dc power to one actuation train would cause a power
     operated relief valve in the other train to open.  In addition, when
     control power alone is lost to only the sensor cabinets in a single
     actuation train, spurious high pressurizer pressure signals would cause
     the relief valves in both trains to open.  Both cases would result in a
     loss of primary coolant.

Discussion

The design deficiency in the on-line testing feature could have prevented both
emergency diesels from accepting emergency loads under certain single failure
conditions.  The licensee investigated this event at Millstone Unit 2 and
found several single failure vulnerabilities related to loss of a vital dc bus
which may apply to engineered safety features actuation systems at other
plants.  Although the described event resulted from an ATI modification, the
other vulnerabilities are inherent in the actuation system design and its
power supplies.

Millstone Unit 2 uses two-out-of-four logic supplied by Consolidated Controls
Incorporated to actuate automatically a number of safety features.  In the
actuation system, a sensor, and subsequent interposing electronic logic,
condition the signal for use by the actuation logic.  Upon loss of power, the
interposing logic generates a signal to perform the safety function.  The
problems discussed above result from having a two-out-of-four logic powered by.

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only two safety-related power sources coupled with a lack of coherence in
specifying the preferred failure mode for automated safety-related actions,
given a loss of power.

The licensee is preparing modifications to correct these problems and is
reviewing the design of Unit 2 for other similar problems.  

In NRC Bulletin 79-27, "Loss of Non-Class 1E Instrumentation and Control Power
System Bus During Operation," the NRC requested licensees to evaluate the
effects of a loss of power to 1E and Non-1E instrument and control systems. 
In addition, in NRC Generic Letter 89-18, "Systems Interactions in Nuclear
Power Plants," the NRC highlighted concerns regarding actuation system designs
which may have automated safety-related actions with no preferred failure
modes.

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 (NRR) project manager.


                                       ORIGINAL SIGNED BY


                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Ram S. Bhatia, Region I
                     (215) 337-5262

                     Thomas Koshy, NRR
                     (301) 504-1176

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