United States Nuclear Regulatory Commission - Protecting People and the Environment

Inadequate Core Cooling Instrumentation System (Generic Letter No. 82-28)

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                              December 10, 1982




On November 4, 1982, the Commission determined that an instrumentation 
system for detection of inadequate core cooling (ICC) consisting of upgraded
subcooling margin monitors, core-exit thermocouples, and a reactor coolant 
inventory tracking system is required for the operation of pressurized water
reactor facilities. 

On the basis of analysis of information provided by licensees, meetings with
industry groups and independent studies by the NRC Staff, the Commission has
found that during a small LOCA, there is a period of time before the core 
has boiled dry (indicated by core exit thermocouples) when the operators 
have insufficient information to clearly indicate a void formation in the 
reactor vessel head or to track the inventory of coolant in the vessel and 
primary system. The Subcooling Margin Monitor gives early indication of a 
problem but does not indicate whether the condition is getting worse or 

The addition if a reactor coolant inventory system will improve the 
reliability of plant operators in diagnosing the approach of ICC and in 
assessing the adequacy of responses taken to restore core cooling. The 
benefit will be preventive in nature in that the instrumentation will assist
the operator in avoidance of ICC when voids in the reactor coolant system 
and saturation conditions result from over cooling events, steam generator 
tubes ruptures, and small break loss of coolant events. The addition of a 
reactor coolant inventory system, coupled with upgraded in-core thermocouple
instruments and a subcooling margin monitor, provides an ICC instrumentation
package which could significantly reduce the likelihood of human 
misdiagnosis and errors for events such as steam generator tube ruptures, 
loss of instruments bus or control system upsets, pump seal failures, or 
overcooling events originating from disturbances in the secondary coolant 
side of the plant. For less frequent events, involving coincidental multiple 
faults or more rapidly developing small break LOCA conditions, the ICC could 
also reduce the probability of human misdiagnosis and subsequent errors 
leading to ICC. 



The Nuclear Regulatory Commission has completed its review of several 
generic reactor level or inventory system instrumentation systems which have 
been proposed for the detection if ICC in PRWs. The Combustion Engineering 
Heated Junction Thermocouple (HJTC) system and the Westinghouse REactor 
Vessel Level Instrumentation System (RVLIS) are acceptable for tracking 
reactor coolant system inventory and provided an enhanced ICC instrument 
package when used in conjunction with core exit thermocouple systems and 
subcooling margin monitors designed in accordance with NURGE-0737 and 
operated within approved Emergency Operating Procedure Guidelines. The 
details of the NRC staff review of these generic systems are reported in 
NURGE/CR-2627 and NUREG/CR-2628 for the Combustion Engineering and 
Westinghouse systems, respectively. 

Other differential pressure (d/p) measurement techniques for reactor coolant
system inventory tracking are acceptable provided that they meet NURGE-0737 
design requirements and monitor the coolant inventory over the range from 
the vessel upper head top the bottom of the hot leg as a minimum. 

In order for the Commission to complete its review of your ICC system to 
assure that an acceptable system is installed as soon as practicable, the 
NRC requires additional information. 

Accordingly, in order to determine whether your license should be modified, 
you are required to submit to the Director, Division of Licensing, NRR, the 
following information in writing and under oath or affirmation pursuant to 
Section 182 of the Atomic Energy Act and 10 CFR 50.54 (f) of the 
Commission's regulations. 

1.   Within 90 days of the date of this letter, identify to the Director, 
     Division of Licensing, the design for the reactor coolant inventory 
     system selected and submit to the Director, Division of Licensing, 
     detailed schedules for its engineering, procurement and installation. 
     References to generic design descriptions and to prior submittals 
     containing the required information, where applicable, are acceptable. 

2.   Within 90 days of the date of this letter review the status of 
     conformance of all components of the ICC instrumentation system, 
     including subcooling margin monitors, core-exit thermocouples, and the 
     reactor coolant inventory tracking system, with NUREG-0737, Item II.F.2
     and submit a report on the status of such conformance. 


3.   The installation if the ICC instrumentation system shall be completed 
     during the earliest refueling shutdown consistent with the existing 
     status of the plant and practical design and procurement 
     considerations. It has become apparent, though discussions with owners' 
     groups and individual licensees, that schedules must adequately 
     consider the integration of these requirements with other TMI related 
     activities. In recognition of this and the difficulty in implementing 
     generic deadlines, the Commission has adopted a plan to establish 
     realistic plant-specific schedules that take into account the unique 
     aspects of the work at each plant. Each licensee is to develop and 
     submit its own plant-specific schedule which will be reviewed by the 
     assigned NRC Project Manger. The NRC Project Manager and licensee will 
     reach an agreement on the final schedule and in this manner provide for 
     prompt implementation of these important improvements while optimizing 
     the use of utility and NRC resources. 

Licensees who have completed installation if an approved generic ICC 
instrumentation system, are authorized to make their system operable prior 
to final NRC approval for purposes of operator training and familiarization.
However, the ICC instrumentation system should not be turned on until the 
licensee has completed the task analysis portion of the control room design 
review, and should be used with prudence in relation to any operator actions
or decisions until the plant specific design and installation has been 
approved y the staff and instructions in its use and operation have been 
incorporated in accordance with the Emergency Operating Procedure Guidelines
into approved emergency operating procedures. 

For your convenience in preforming the status review (Item 2) of your 
conformance with NURGE-0737, a check list of the nine items of documentation
cited on pp II.F.2-3 and 4 of that document is provided in an appendix to 
this letter. Even though you may have provided much if the information 
required for our review, we have not yet received all of the information 
required to complete our review of plant specific installations for any 
licensee. In addition, some licensees have modified their positions during 
the period when NRC was re-reviewing the II.F.2 requirements. Therefore, 
your status report should review for completeness and reference those 
earlier submittals, including generic submittals, which remain valid in 
response to documentation items on the check list. In addition, you should 
include a proposed schedule for the remaining submittals. Information items 
to be addressed in the submittal regarding your review of core exit 
thermocouples for conformance to NURGE-0737,II.F.2, Attachment 1, and your 
review of the ICC instrumentation for conformance to NURGE-0737, Appendix B, 
are also listed in the appendix to this letter. The staff review will focus 
on deviations from the design criteria. 



This request for information was approved by OMB under clearance number 
3150-0065 which expires May 31,1983. Comments on burden and duplication may 
be directed to the Office of Management and Budget, Reports and Management, 
Room 3208, New Executive Office Building, Washington, D.C. 


                                   Darrell G. Eisenhut, Director 
                                   Division of Licensing 

As stated

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