Resolution of Unresolved Safety Issue A-17, "Systems Interactions in Nuclear Power Plants"(Generic Letter 89-18)

September 6, 1989



This generic letter informs licensees and applicants of the final resolution 
of USI A-17, "Systems Interactions in Nuclear Power Plants."  There are two 
enclosures which are provided for information.

Enclosure 1 outlines the bases for resolution of USI A-17.  

Enclosure 2 provides a grouping of five general lessons learned from the 
review of the overall systems interaction issue.  The review of this 
information will give licensees additional appreciation of the kinds of 
adverse systems interaction which have appeared in operating experience and 
can aid them in continuing evaluation of operating experience.

No specific action or written response is required by this letter.  If you 
have any question about this matter, please contact the technical contact 
listed below or the Regional Administrator at the appropriate regional office.


                                        James G. Partlow
                                        Associate Director for Projects
                                        Office of Nuclear Reactor Regulation

Technical Contacts:
   D. Thatcher, RES
  (301) 492-3935

1.   Bases for Resolution of Unresolved Safety Issue A-17
2.   Summary Information Relevant to Operating Experience Evaluations
3.   List of Recently Issued NRC Generic Letters

.                                                               Enclosure 1 



The U.S. Nuclear Regulatory Commission (NRC) has concluded its resolution of 
Unresolved Safety Issue (USI) A-17, "Systems Interactions in Nuclear Power 
Plants."  This document provides a summary of that resolution.  More detailed 
background information is provided in References 1 and 2.  

Adverse systems interactions (ASIs) involve subtle and often very complicated 
plant-specific dependencies between components and systems, possibly 
compounded by inducing erroneous human intervention.  The staff has identified 
actions to be taken by the NRC to resolve USI A-17, and has made the judgment 
that these actions, together with other ongoing activities, should reduce the 
risk from adverse systems interactions. 

The staff's judgment is not based on the assertion that all adverse systems 
interactions have been identified, but rather that the A-17 actions plus other 
activities by the licensees and staff, as discussed further below, give 
reasonable assurance that the more risk-significant interactions will be 
recognized and appropriate action taken.  


(1)  Ongoing Actions by Licensees

     (a)  Water Intrusion and Flooding From Internal Sources

     As part of the resolution of USI A-17, the staff has identified that 
     water intrusion and flooding of equipment from internal plant sources may 
     result in a risk-significant adverse systems interaction.  Such events 
     could cause a transient and could also disable the equipment needed to 
     mitigate the consequences of the event.  The appendix to NUREG-1174 
     (reference 1) provides insights regarding plant vulnerabilities to 
     flooding and water intrusion from internal plant sources.  It is expected 
     that these insights will be considered in implementing Generic Letter 
     88-20 [Individual Plant Examinations (IPE)] which includes an assessment 
     of internal flooding. 

     (b)  Review of Events at Nuclear Power Plants

     Licensees are expected to continue to review information on events at 
     operating nuclear power plants in accordance with the requirements of 
     Item I.C.5 of NUREG-0737.  Such information is disseminated by the NRC in 
     the form of information notices, bulletins, and other reports; by 
     individual licensees in the form of licensee event reports; and by 
     industry groups such as the Institute of Nuclear Power Operations (INPO).
     The NRC has an aggressive program of reviewing events at nuclear power 
     plants.  Each licensee is required to notify the NRC staff rapidly by 
     telephone of any event that meets or exceeds the threshold defined in 
.10 CFR 50.72 and to file a written licensee event report for those events that 
meet or exceed the threshold defined in 10 CFR 50.73.  Also, the NRC regional 
offices report events of significance every day.  This information is reviewed 
daily by members of the NRC staff and followup efforts are assigned for events 
that appear to be potentially risk significant and/or are judged to be a 
possible precursor to a more severe event.  A weekly meeting is held to brief 
NRC management on those events of significance.  This ongoing process provides 
a great deal of assurance that any potentially significant event is brought to 
the attention of the appropriate NRC staff and management.  Depending on the 
significance, further action may be taken to notify licensees or to impose 
additional requirements.  The total process offers a high degree of assurance 
that precursors to potentially significant events, including those involving 
adverse systems interactions, are treated expeditiously.  Attachment 2 
summarizes the A-17 information relevant to these ongoing operating experience 

(2)  Actions by the NRC Related to Adverse Systems Interactions

     (a)  Integration of Specific, Ongoing, Generic Issues Related to A-17

     The NRC is considering certain aspects of potential interactions as part 
     of the resolution of identified generic issues. 

.         USI A-46, "Seismic Qualification of Equipment

          Actions to resolve this issue have been sent to the licensees.  The 
          NRC and industry are working on detailed procedures that will be 
          used to implement the requirements on a plant-specific basis.  These 
          implementation procedures will include walkdowns of individual 
          plants to ensure that the systems needed to shut down the plant and 
          maintain it in a safe condition for 72 hours can withstand a 
          design-basis seismic event.  The scope includes not only the systems 
          needed to control reactivity and remove decay heat, but also the 
          supporting power supplies, controls, instrumentation, and 
          environmental control subsystems needed by those systems.  The plant 
          walkdown reviews include seismic systems interactions.

.         Generic Issue 128, "Electric Power Reliability"

          The USI A-17 review of operating experience reemphasized the 
          potential interactions stemming from the electric power system and, 
          in particular, instrumentation and control (I&C) power supply 
          failures.  I&C power loss can cause significant transients and can 
          simultaneously affect the operator's ability to proceed with 
          recovery by disabling portions of the indications and the equipment 
          needed for recovery.  The events that have occurred were mostly 
          limited to a single electrical division and therefore not strictly 
          adverse systems interactions by the definition in the USI A-17 
          program.  In addition, actions have already been taken by licensees 
          to improve the operator's ability to cope with such events.  As a 
          separate activity, a number of generic issues involving electrical 
          power supplies were integrated into one generic issue.  This issue 
          became GI 128, "Electric Power Reliability," and consists of the 
          following specific electric issues: 

.         -    GI-48, "LCO for Class 1E Vital Instrument Buses in Operating 
          -    GI-49, "Interlocks and LCOs for Redundant Class 1E Tie 

          -    GI-A-30, "Adequacy of Safety Related DC Power Supplies"

     It was concluded that the additional information developed on USI A-17, 
     (NUREG/CR-4470) should be used as an input to the GI-128 program.  
     Therefore, that information was communicated to GI-128 for possible 

     (b)  Define and Prioritize Other Issues

     The Advisory Committee for Reactor Safeguards (ACRS) and other groups 
     have identified concerns in the context of systems interactions.  In many 
     cases, the concerns are not considered to be within the scope of systems 
     interactions as defined in the USI A-17 Task Action Plan.  In some cases, 
     these concerns have not been described specifically enough to permit the 
     risk to be estimated.  The NRC has undertaken a program [referred to as 
     the Multiple System Responses Program (MSRP)] with Oak Ridge National 
     Laboratory (ORNL) to define these concerns in sufficient detail so that 
     they may be prioritized in accordance with NRC procedures.  
     Examples of concerns involve potential coupling of postulated plant 
     events such as seismically induced fires and seismically induced 
     flooding, and the attendant potential for multiple, simultaneous, adverse 
     systems responses.  These concerns are beyond the defined scope of USI 
     A-17.  If the definition, priority determination, and peer review 
     processes identify one or more issues as having high or medium priority, 
     the issue(s) will be assigned to the appropriate organization for resolu-

     (c)  Probabilistic Risk Analyses or Other Systematic Plant Reviews

     .    Existing Plants

     The Commission's Severe Accident Policy, 50 FR 32128 (August 8, 1985), 
     calls for all existing plants to perform a plant-specific search for 
     vulnerabilities.  Such searches, referred to as individual plant 
     examinations (IPEs), involve a systematic plant review (which could be a 
     PRA-type analysis).  NRC is issuing guidance for performing such reviews.
     One subject area to be treated by the IPEs is common-cause failures (or 
     dependent failures).  USI A-17 recognizes that ASIs are a subset of this 
     broader subject area and, therefore, is providing for the dissemination 
     of the insights gained in the A-17 program for use in the IPE work.

     .    Future Plants

     The Commission's regulations (10CFR50.52) require all future plants to 
     perform a probabilistic risk assessment (PRA).  NRC is issuing guidance 
     on the content of PRA submittals for future light-water reactors (LWRs).  
     As part of that guidance, A-17 is providing the insights gained in the 
     A-17 program for the treatment of plant dependencies.

.     (d)  Additional Considerations for Future Plants

     The above actions acknowledge the fact that future plants will perform 
     probabilistic risk assessments, and that such studies can uncover ASIs.  
     The staff also recognizes that the continual review of operating 
     experience will identify systems interactions, some of which may be ASIs.
     Further prioritization of issues defined by the MSRP may result in 
     additional generic issues whose resolution may lead to requirements 
     applicable to future plants. 
     Therefore, future plants should keep current on lessons learned from 
     operating experience and continue to monitor the ongoing NRC process of 
     developing, prioritizing, and resolving generic issues. 
     In addition, the staff plans to develop a standard review plan (SRP) for 
     future plants.  The SRP would include specific guidance regarding 
     protection from internal flooding and water intrusion events.

     Staff Findings

     On the basis of the technical findings reported in NUREG-1174 and the 
     regulatory analysis reported in NUREG-1229 the staff has concluded that 
     these actions can further reduce the risk from ASIs.  The staff does not 
     recommend further broad searches for ASIs because such searches have not 
     proved to be cost-effective, and in any case, there is no guarantee after 
     such a study is performed that all ASIs have been uncovered.  Although 
     these actions complete the staff's work under the Task Action Plan for 
     USI A-17, and constitute technical resolution of the issue as defined 
     therein, the potential for systems interactions remains an important 
     consideration in the design and operation of nuclear power plants.


1.   U.S. Nuclear Regulatory Commission, NUREG-1174, "Evaluation of Systems 
     Interactions in Nuclear Power Plants."

2.   ---, NUREG-1229, "Regulatory Analysis for Resolution of USI A-17."

.                                                            Enclosure 2 

                         SUMMARY INFORMATION RELEVANT TO


The U.S. Nuclear Regulatory Commission (NRC) has concluded its technical 
resolution of Unresolved Safety Issue (USI) A-17, "Systems Interactions in 
Nuclear Power Plants."  This summary presents a portion of the results of that 
technical resolution for use in operating experience evaluations.  More 
detailed background information is provided in References 1 and 2. 

Because of the complex, interdependent network of systems, structures, and 
components that constitute a nuclear power plant, the scenario of almost any 
significant event can be characterized as a "systems interaction."  As a 
result, the staff recognized that if the term 'systems interaction' was to be 
interpreted in a very broad sense, it became an unmanageable safety issue.  
Focusing was required to address perceived safety concerns.  It is recognized 
that by the very nature of such a focusing effort, all concerns that one may 
characterize as systems interactions may not be addressed.  It is, therefore, 
extremely important that the scope and boundary of the focused program be 
clearly defined and understood.  Then, if other concerns still exist after 
completion of the program, they can be addressed as part of separate efforts 
as deemed necessary. 

The information presented in this attachment is based on the following 

(1)  Systems Interaction (SI)

     Actions or inactions (not necessarily failures) of various systems 
     (subsystems, divisions, trains), components, or structures resulting from 
     a single credible failure within one system, component, or structure and 
     propagation to other systems, components, or structures by inconspicuous 
     or unanticipated interdependencies.  The major difference between this 
     type of event and a classic single-failure event is in those aspects of 
     the initiating failure and/or its propagation that are not obvious (i.e., 
     that are hidden or unanticipated). 

(2)  Adverse Systems Interaction (ASI)

     A systems interaction that produces an undesirable result.

(3)  Undesirable Result (Produced by Systems Interaction)

     This was defined by a list of the types of events that were to be 
     considered in USI A-17: 
     (a)  Degradation of redundant portions of a safety system, including 
          consideration of all auxiliary support functions.  Redundant 
.         portions are those considered to be independent in the design and 
          accident analysis (Chapter 15) of the Final Safety Analysis Report (FSAR)
          of the plant.  (Note:  This would violate the single-failure criterion.) 
     (b)  Degradation of a safety system by a non-safety system.  (Note:  This 
          result would demonstrate a breakdown in presumed "isolation.")

     (c)  Initiation of an "accident" (e.g., LOCA, MSLB) and (i) the 
          degradation of at least one redundant portion of any one of the 
          safety systems required to mitigate the event (Chapter 15, FSAR 
          analyses); or (ii) degradation of critical operator information 
          sufficient to cause the operator to perform unanalyzed, unassumed, 
          or incorrect actions.  (Note:  This includes failure to perform 
          correct actions because of incorrect information.)

     (d)  Initiation of a "transient" (including reactor trip) and (i) the 
          degradation of at least one redundant portion of any one of the 
          safety systems required to mitigate the event (Chapter 15, FSAR 
          analyses); or (ii) degradation of critical operator information 
          sufficient to cause the operator to perform unanalyzed, unassumed, 
          or incorrect actions.  (Note:  This includes failure to perform 
          correct actions because of incorrect information.)

     (e)  Initiation of an event that requires plant operators to act in areas 
          outside the control room (Perhaps because the control room is being 
          evacuated or the plant is being shut down) and disruption of the 
          access to these areas (for example, by disruption of the security 
          system or isolation of an area when fire doors are closed or when a 
          suppression system is actuated).

The intersystem dependencies (or systems interactions) have been divided into 
three classes based on the way they propagate: 

(1)  Functionally Coupled: 

     Those SIs that result from sharing of common systems/components; or 
     physical connections between systems, including electrical, hydraulic, 
     pneumatic, or mechanical.

(2)  Spatially Coupled: 

     Those SIs that result from sharing or proximity of structures/locations, 
     equipment, or components or by spatial inter-ties such as HVAC and drain 

(3)  Induced Human-Intervention Coupled: 

     Those SIs in which a plant malfunction (such as failed indication) 
     inappropriately induces an operator action, or a malfunction inhibits an 
     operator's ability to respond.  As analyzed in the A-17 program, these 
     SIs are considered another example of functionally coupled ASIs.  
     (Induced human-intervention-coupled systems interactions exclude random 
     human errors and acts of sabotage.)

.As a result of the staff's studies of adverse systems interactions (ASIs) 
undertaken as part of A-17 and reported in Reference 1, the staff has 
concluded the following: 

(1)  To address a subject area such as "systems interactions" in its broadest 
     sense tends to be an unmanageable task incapable of resolution.  Some 
     bounds and limitations are crucial to proceeding toward a resolution.  
     Considering this, the A-17 program utilized a set of working definitions 
     to limit the issue.  It is recognized that such an approach may leave 
     some concerns unaddressed.

(2)  The occurrence of an actual ASI or the existence of a potential ASI is 
     very much a function of an individual plant's design and operational 
     features (such as its detailed design and layout, allowed operating 
     modes, procedures, and tests and maintenance practices).  Furthermore, 
     the potential overall safety impact (such as loss of all cooling, loss of 
     all electric power, or core melt) is similarly a function of those plant 
     features that remain unaffected by the ASI .  In other words, the results 
     of an ASI depend on the availability of other independent equipment and 
     the operator's response capabilities.

(3)  Although each ASI (and its safety impact) is unique to an individual 
     plant, there appear to be some characteristics common to a number of the 

(4)  Methods are available (and some are under development) for searching out 
     SIs on a plant-specific basis.  Studies conducted by utilities and 
     national laboratories indicate that a full-scope plant search takes 
     considerable time and money.  Even then, there is not a high degree of 
     assurance all, or even most, ASIs will be discovered. 
(5)  Functionally coupled ASIs have occurred at a number of plants, but 
     improved operator information and training (instituted since the accident 
     at Three Mile Island) should greatly aid in recovery actions during 
     future events.

(6)  Induced human-intervention-coupled interactions as defined in A-17 are a 
     subset of the broader class of functionally coupled SIs.  As stated for 
     functionally coupled SIs, improvements in both operator information and 
     operator training will greatly improve recovery from such events.

(7)  As a class, spatially coupled SIs may be the most significant because of 
     the potential for the loss of equipment which is damaged beyond repair.  
     In many cases, these ASIs are less likely to occur because of the lower 
     probability of initiating failure (e.g., earthquake, pipe rupture) and 
     the less-than-certain coupling mechanisms involved.  However, past 
     operating experience highlighted a number of flooding and water intrusion 
     events and more recent operating experience indicates that these types of 
     events are continuing to occur. 
(8)  Probabilistic risk assessments or other systematic plant-specific reviews 
     can provide a framework for identifying and addressing ASIs.

.(9)  Because of the nature of ASIs (they are introduced into plants by design 
     errors and/or by overlooking subtle or hidden dependencies), they will 
     probably continue to happen.  In their evaluations of operating 
     experience, NRC and the nuclear power industry can provide an effective 
     method for addressing ASIs.

(10) For existing plants, a properly focused, systematic plant search for 
     certain types of spatially coupled ASIs and functionally coupled ASIs 
     (and correction of the deficiencies found) should improve safety.

(11) The area of electric power, and particularly instrumentation and control 
     power supplies, was highlighted as being vulnerable to relatively 
     significant ASIs.  Further investigation showed that this area remains 
     the subject of a number of separate issues and studies.  A concentrated 
     effort to coordinate these activities and to include power supply 
     interactions should prove an effective approach in this area. 
(12) For future plants, additional guidance regarding ASIs could benefit 

(13) The concerns raised by the Advisory Committee on Reactor Safeguards 
     (ACRS), on A-17, but which have not been addressed in the Staff's study 
     of A-17, should be considered as candidate generic issues, separate from 
     USI A-17. 
It should be noted that the staff has concluded that adverse systems 
interactions (ASIs) involve subtle, and often very complicated, dependencies.  
Therefore, total elimination of ASIs is unachievable.  For these reasons, the 
staff is not recommending that each plant undertake a large, comprehensive 
study to uncover ASIs.  Instead, the staff is recommending other, more cost-
effective actions for reducing the frequency and impact of ASIs.  Although 
these actions complete the staff's work under the task action plan for USI 
A-17, and constitute technical resolution of the issue as defined therein, the 
potential for ASIs remains an important consideration in the design and 
operation of nuclear power plants.  The staff has, therefore, acknowledged the 
continuing importance of ongoing activities such as probabilistic risk 
assessments or other systematic plant evaluations and the continuing review 
and evaluation of the industry's operating experience. 

The regulatory analysis (Reference 2) considered a number of alternatives for 
resolution, and based on that analysis, the staff has concluded that certain 
actions should be taken by NRC to resolve USI A-17.  These actions are: 

(1)  Send a generic letter to all plants outlining the resolution of USI A-17 
     and providing information developed during the resolution of A-17.

(2)  Consider the insights developed in the resolution of USI A-17 for 
     flooding and water intrusion from internal sources in the Individual 
     Plant Examinations (IPE).

(3)  Consider systems interactions involving the electrical power systems in 
     the integrated program on electrical power reliability.

(4)  Provide information for use in future PRAs.

.(5)  Provide a framework for addressing those other concerns related to 
     systems interactions which are not covered by the USI A-17 program.

(6)  Acknowledge that the resolution of USI A-46 addresses aspects of systems 

(7)  Develop a standard review plan for future plants to address protection 
     from internal flooding and water intrusion.

The following discussion addresses the first action.  The second action is 
addressed in the IPE guidance documents.  The remaining five actions involve 
staff actions. 


A.   Background 

The adverse systems interactions  (ASIs) sorted from the survey of experience 
appeared to be due to two general causes.  Some of the ASIs resulted from 
obvious errors or failures to meet clearly specified design requirements 
and/or guidance.  Others arose from more subtle causes such as the lack of 
sufficient consideration, or analysis, of all the significant failure 
mechanisms or modes and the associated event combinations and/or sequences. 

In the case of older plants, the causes often are related to the fact that 
less design guidance and associated analyses were available and/or required 
when the plants were licensed. 

Although no specific licensee actions are required, the staff concluded that 
it should communicate to industry certain highlighted concerns identified in 
the A-17 studies.  The insights gained from this information should be 
beneficial to industry in their ongoing evaluations of operating experience. 

B.   Highlighted Concerns * 

As part of the effort to provide a more focused approach for the resolution of 
A-17, a set of tasks was defined to accomplish a search of operating 
experience to accumulate a data bank on the types of common-cause events of 
concern.  The major portion of this work was performed by the Oak Ridge 
National Laboratory (ORNL), and a summary of ORNL's findings is included in 
Reference 3. 

The search emphasized events included in the LER (licensee event report) files 
and involved a screening of those events based on the task action plan 
definition.  On the basis of the characteristics or attributes of the systems 
interaction events, a group of general categories of SI events was developed.  
The results of the ORNL experience review indicate 23 general categories of 
events (see Table 1) which have involved systems interactions. 

*More details on the highlighted concerns and other ASIs are provided in Ref-
 erences 1, 3, and 4, and those documents should be consulted for additional 

.             Table 1 Event categories involving systems interactions

Category                                                         No. of 
   No.     Title                                                  events 

     1    Adverse interactions between normal or offsite         34
          power systems and emergency power systems
     2    Degradation of safety-related systems by vapor         15
          or gas intrusion

     3    Degradation of safety-related components by fire       10

          protection systems
     4    Plant drain systems allow flooding of safety-           8
          related equipment

     5    Loss of charging pumps due to volume control tank       6
          level instrumentation failures
     6    Inadvertent ECCS/RHR pump suction transfer              4

     7    HPSI/charging pumps overheat on low flow during         6
          safety injection
     8    Level instrumentation degraded by HELB conditions      21

     9    Loss of containment integrity from LOCA conditions     10
     10   HELB conditions degrading control systems               3

     11   Auxiliary feedwater pump runout under steamline 
          break conditions                                        2
     12   Waterhammer events                                      4

     13   Common support systems or cross-connects               18
     14   Instrument power failures affecting safety systems      5

     15   Inadequate cable separation                             8
     16   Safety-related cables unprotected from missiles         3
          generated from HVAC fans

     17   Suppression pool swell                                  3
     18   Scram discharge volume degradation                      2

     19   Induced human interactions                              4
     20   Functional dependencies from failures during            5
          seismic events

     21   Spatial dependencies from failures during seismic      13
     22   Other functional dependencies                          21 

     23   Other spatial dependencies                             30

.Review of these 23 general categories led to the identification of five areas 
of highlighted concerns.  These are discussed below: 

Electric Power System 

The electric power system includes the offsite sources, the switchyard, the 
power distribution buses and breakers, onsite generating equipment, and the 
control power and logic to operate the breakers and start and load the diesel 
generators.  Some of the lower voltage (typically 120-V ac and 125-V dc) power 
supply portion of the system is also dealt with under the "Instrumentation and 
Control Power Supplies" heading below. 

As outlined in References 3 and 4, concerns were highlighted in the area of 
electric power systems in Categories 1 and 13 (Table 1).  Three important 
factors appear to contribute to the possible significance of this area:  

(1)  It is one of the most (if not the most) extensive support systems in a 
     plant.  Power is supplied from various sources including the offsite 
     network, the main plant turbine-generator and, in certain situations, the 
     safety-related diesel generators.  Power is then distributed to various 
     items of equipment for normal plant control which is not related to 
     safety, various engineered safety feature equipment which is safety 
     related, and various items of equipment for shutdown and decay heat 

(2)  Given these system demands, the power system is therefore an inherently 
     complex system.  A large number of normal operating modes at the plant, 
     as well as transient and accident situations, must be accommodated.  
     Interfaces are created between redundant safety-related equipment.  In 
     addition, the power system itself relies on a number of other support 
     systems such as HVAC and cooling water. 
(3)  Because of individual plant requirements and situations (a number of 
     significant events occur when the system is in any abnormal temporary 
     alignment), each power system tends to have some unique aspects.  Very 
     few specific ASIs can be stated to be generically applicable; however, 
     the staff believes that general classes of electric power events can be 
     potentially generic.  

ORNL (References 3 and 4) categorized the electric power system concerns into 
four areas: 

.    load sequencing/load shedding
.    diesel generator failures caused by specific operating modes
.    breaker failures due to loss of dc power
.    failures that propagate between the safety-related portion and the non-
     safety-related portion of the power systems

With respect to these four areas of concern, the staff noted that although 
regulatory practice has allowed non-safety-related equipment to be powered 
from safety-related buses, this practice has created the potential for a 
number of undesirable interactions.  In such situations, the isolation devices 
protect the safety-related equipment.  These isolation devices have been the 

.subject of much concern, both in the main power supply area (such as breakers 
that open on fault current or "accident" signals) and in the instrumentation 
and control power supply area (such as isolation transformers and other 
devices).  In some cases, the "isolation" devices do not isolate the full 
range of undesirable events.  In addition, the A-17 investigation has focused 
on another concern.  Specifically, some ASIs involve scenarios in which a 
non-safety-related load is supplied by a safety-related bus and is adequately 
isolated.  The non-safety load is part of the normal plant operation and/or 
control.  A failure in the safety-related portion can propagate and create a 
situation in which a plant transient occurs as a result of non-safety loads 
supplied by the safety-related bus and, simultaneously, significant 
safety-related equipment is unavailable because of the same failure. 

The most significant events of this type appear to be those that involve the 
instrumentation and control power systems.  As stated below in the discussion 
of these specific power supplies, the staff believes that current activities 
in the area of instrumentation and control power supplies should be integrated 
and should address this type of concern specifically.  Accordingly, the staff 
has initiated an integrated program to review these issues. 

Plant Support Systems 

Although relatively few events of note were identified from the operating 
experience (Categories 13, 14, 18, and 22 of Table 1 and References 3 and 4), 
PRAs have consistently shown the potential importance of support systems.  
(Note:  The electric power system, also a support system, was dealt with 
separately above.)  This category includes other support systems such as 
component cooling water; service water; heating, ventilating, and air 
conditioning; lube oil; and compressed air. 

As is the case for the electric power system, these support systems are often 
extensive and may be unique.  These support systems can affect multiple 
frontline safety systems and can often affect systems not related to safety.  
As a result, failures in support systems can potentially initiate a transient 
and also can degrade other systems, some of which may have been designed to 
mitigate that very same event. 

The support systems of concern often have interconnections between redundant 
divisions for operational flexibility or they may have interconnections to 
non-safety-related equipment.  In some cases, single failures such as headers, 
drain lines, and vents are designed into the systems because the probability 
of a passive failure in conjunction with the need for the system is assumed to 
be low. 

If the support system failure and the initiation of an event are coupled, a 
risk-significant situation could result from the failure of the support system 
(depending on other plant mitigating features). 

Less attention may have been paid to the design and review of plant support 
systems than was paid to some of the frontline systems such as the ECCS.  The 

.safety significance of event initiation coupled with limiting the capability 
for mitigation may not have been recognized. 

Incorrect Reliance on Failsafe Design Principles 

Protection systems at nuclear powers plant rely on the design principle of 
"failsafe" to varying degrees.  There have been instances (see Category 18 in 
Table 1 and References 3 and 4) in which some failure modes were 
insufficiently analyzed because someone relied too much on the concept of 

The events to date have involved the scram system and its related support 
functions such as the air system and electric power system.  Specifically, it 
was discovered that water could be in the scram discharge volume (SDV) of a 
BWR as a result of poor drainage or an air supply failure.  Water in the SDV 
would inhibit the insertion of control rods.  The failure involving the air 
system was of particular concern because it involved a system that had been 
considered a portion of the reactor protection system not related to safety.  
Action was taken at all boiling-water reactors to correct this problem. 

This type of ASI may have resulted from the use of a design approach that 
actually requires of a number of non-safety-related features to function and, 
therefore, does not truly rely on failsafe principles.  In the case of the air 
system, the system was assumed to fail safe, i.e., bleed off, and, as a 
result, a partial failure went unanalyzed.  It was also noted that the 
electric supply system to this scram system had been modified previously 
because of a similar type of concern.  Specifically, the electric power was 
originally assumed to fail safe (i.e., voltage going to zero) and, as a 
result, partial failure (such as low voltage or high voltage) went unanalyzed 
for a time. 

The problems appear to have been created when portions of the systems were 
allowed to be classified as not related to safety because they were assumed to 
always fail safe. 

Automated Safety-Related Actions With No Preferred Failure Mode 

Another area of adverse systems interactions that was highlighted involved the 
inadvertent actuation of an engineered safety feature (ESF) (Category 6, 
"Inadvertent ECCS/RHR pump suction transfer").  The most significant 
characteristic of this area appears to be that, unlike a reactor trip, such a 
function does not have an "always preferred" failure mode.  As a result, extra 
precautions may be needed to avoid (a) a failure to actuate when needed and 
(b) a failure that actuates the system when not required (i.e., 
inadvertently).  The area of automatic ECCS switch to recirculation is the 
subject of a separate generic issue, Generic Issue 24. 

Although the reported events involved only the automatic switchover to the 
sump in PWRs, some concern exists that individual plants may have other 
functions with the same characteristic.  Some possible other functions 

.    containment isolation functions
.    logic that selects a faulted steam generator to isolate it
.    low-pressure-to-high-pressure system interlocks in the RHR system

.Of particular note is the possibility that these types of functions will 
actuate inadvertently during testing or maintenance.  It is a fairly common 
practice to put portions of the actuation logic in a trip or actuated state 
and to assume then that the plant is in a "safe" condition.  Although this may 
be true for functions that have a preferred failure mode, it may not be a 
conservative assumption for functions that do not have an always preferred 
failure mode. 

Instrumentation and Control Power Supplies 

The ORNL review (NRC, NUREG/CR-3922) highlighted several events related to 
instrumentation and control (I&C) power supplies (Category 14).  The events at 
all plants, and specifically at B&W plants, have already received significant 
attention as outlined in the ORNL assessment.  Some residual concern was 
expressed that the potential for a significant event related to I&C power 
supply interactions may still exist.  Because of this concern, further review 
work at ORNL was identified. 

ORNL completed this work (reported in Reference 5).  A significant number of 
I&C power supply events were noted, some of which involve ASIs.  Although 
there is concern about the area of I&C power supplies, a significant amount of 
work (both at NRC and in the industry) has addressed this area.  The A-17 
resolution has not recommended any specific actions to deal with this area at 
this time, but has concluded that the existing efforts at NRC be coordinated 
to ensure that this critical area receives the proper emphasis.  This is being 
done under Generic Issue 128, "Electric Power Reliability." 

C.   Recommendations 

Ongoing industry reviews and evaluations of operating experience should 
consider the above types of events.  It is further recommended that where 
utilities determine that specific evaluations (e.g., plant walkdowns, limited-
scope accident safety analyses, or probabilistic risk assessments) are needed 
to address other safety concerns, awareness and recognition of potential 
adverse systems interactions such as highlighted above should be included in 
these evaluations. 

D.   References 

1.   U.S. Nuclear Regulatory Commission, NUREG-1174, "Evaluation of Systems 
     Interactions in Nuclear Power Plants." 
2.   ---, NUREG-1229, "Regulatory Analysis for Resolution of USI A-17." 
3.   ---, NUREG/CR-3922, "Survey and Evaluation of System Interaction Events 
     and Sources," January 1985.

4.   ---, NUREG/CR-4261, "Assessment of System Interaction Experience in 
     Nuclear Power Plants," June 1986.

5.   ---, NUREG/CR-4470, "Survey and Evaluation of Vital Instrumentation and 
     Control Power Supply Events," August 1986.


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