United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-36: Possible Sudden Loss Of RCS Inventory During Low Coolant Level Operation

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

                                  June 8, 1988


Information Notice No. 88-36:  POSSIBLE SUDDEN LOSS OF RCS INVENTORY
                                   DURING LOW COOLANT LEVEL OPERATION


Addressees:

All holders of operating licenses or construction permits for pressurized 
water reactors (PWRs).

Purpose:

This information notice is being provided to alert addressees to the potential 
for a sudden loss of reactor coolant system inventory while conducting steam 
generator tube inspections and modifications with hot leg nozzle dams in 
place.  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 do not constitute NRC requirements; therefore, no specific action or 
written response is required.  

Description of Circumstances:

During the second refueling of Diablo Canyon Unit 1, in the spring of 1988, 
deficiencies in the procedures to be used during the steam generator tube 
inspections were identified that could significantly increase the probability 
of a sudden ejection of reactor coolant followed by core uncovery.

In order for the steam generator tubes to be inspected at Diablo Canyon, they 
were drained, by drawing air through reactor and pressurizer vents, until the 
reactor coolant inventory was drained down to the mid-level of the hot leg 
piping (see Figure 1).  Lowering the reactor coolant to this level also un-
covers the steam generator primary side manways so that they can be removed to 
gain access to the steam generator hot and cold leg plenums and their re-
spective hot and cold leg nozzles.  Nozzle dams are then placed in these steam 
generator plenum nozzles so that the reactor coolant level can be raised to 
increase the net positive suction head to the decay heat removal pumps without 
refilling the steam generators.

If the hot leg nozzle dams were all installed before all of the cold leg 
nozzle dams were in place, a small increase in reactor vessel pressure would 
cause reactor coolant to be rapidly expelled from the open cold leg manways.  
This would occur because the increased pressure, unable to vent through the 
dammed-up hot legs, would force the coolant down in the vessel, through the 
cold legs, and out of the manways.  A pressure increase of only 2-1/2 psig in 
the vessel 



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would lower the coolant level to the point where the top of the fuel would 
begin to be uncovered, with the level of the remaining coolant in the open 
steam generator located at the bottom of the cold leg plenum manway.

Similar mechanisms have been identified at San Onofre Units 2 and 3 in their 
response to Generic Letter 87-12 (Reference 1), and by the Westinghouse Owners 
Group in an ongoing analysis of reactor behavior during the shutdown 
condition. 

The possibility of ejecting coolant by this mechanism can be eliminated by 
ensuring that a steam generator hot leg plenum manway and its associated hot 
leg pipe are kept open to provide an adequate vent path whenever any cold leg 
openings are made.  This can be accomplished by ensuring that a hot leg manway 
is the first manway to be opened, and a hot leg nozzle dam is the last dam to 
be installed.  In addition, not installing the last hot leg nozzle dam until a 
sufficient vent path is established in the reactor vessel or pressurizer will 
reduce the possibility of developing a pressure differential which could eject 
a dam.

Discussion:

On April 10, 1987, the Diablo Canyon Unit 2 reactor vessel became pressurized 
to approximately 7 to 10 psig when the residual heat removal flow was lost for 
a period of 1-1/2 hours (Reference 2).  Fortunately, during this event the 
man-ways, although loosened, were still in place and the nozzle dams had not 
yet been installed.  Operating a reactor coolant system that has been drained 
to a low level often involves unusual problems that have a significant 
probability of causing a loss of residual heat removal unless special care is 
taken.  NUREG-1269 (Reference 3), the report of the NRC investigation into the 
Diablo Canyon event, discusses a number of these problems.  These include the 
following:

     The level which is established for draining the steam generator tubes is 
     frequently only slightly above the level which will provide an adequate 
     suction head for the residual heat removal pumps.  This marginal suction 
     head can lead to air entrainment due to vortexing at the suction point, 
     which may cause a loss of pump suction.

     The temporary reactor vessel level measurement system necessary for this 
     type of operation tends to be inaccurate because of the long lengths of 
     tubing normally used.  The possible air entrainment and the surface level 
     variations due to fluid flow at this low level provide additional 
     mechanisms that cause error in the level measurement.

The NRC has documented many instances where residual heat removal has been 
lost, because of loss of pump suction, while the plant was being operated at 
reduced reactor coolant water levels.  Generic Letter 87-12 (Reference 1) 
lists 37 loss-of-decay-heat-removal events, occurring from 1977 to 1987, that 
were attributed to inadequate reactor coolant system level.  In four cases, 
including the 1987 Diablo Canyon event, boiling is known to have occurred 
before residual heat removal could be reestablished.

Although small vents are normally established in the reactor vessel head and 
in the pressurizer before the coolant level is drained down, these are far too 
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                                                                 June 8, 1988
                                                                 Page 3 of 4


small to prevent pressurization of the reactor coolant system after the 
boiling point is reached.  For the recent steam generator inspection at Diablo 
Canyon, which was initiated 10 days after shutdown, the reactor was producing 
5 MW of decay heat.  This is sufficient to produce 5 lb of steam per second, 
which would require a vent area greater than 12 square inches in order to hold 
the pressure rise to less than 25 psi.  During the 1987 Diablo Canyon event, 
the reactor, which had been shut down for seven days, reached the boiling 
point about 1/2 hour after decay heat removal capability was lost.  The 
pressure increased to the 7-to-10-psig maximum value a short time later even 
though small vents were available in the vessel head and pressurizer.  

With the hot leg nozzle dams in place the pressure rise would be quite rapid.  
Generation of a small amount of steam would be sufficient to produce the 
partial pressure of 2-1/2 psi necessary to uncover the core by ejecting the 
coolant through the open cold leg plenum manway.  This amount of steam could 
be produced in less than a minute.  However, the actual time to produce this 
pressure would depend on the time to heat the reactor coolant to the higher 
boiling point and on the rate of energy deposition in the cold materials in 
the upper part of the reactor vessel and, to a lesser extent, in the 
pressurizer.  The time required for this to occur would likely be only a few 
minutes. 

Loss of residual heat removal capability after the nozzle dams are installed 
and before the vessel level is raised would still result in a hazardous situ-
ation, however, more time would be available for operator action before loss 
of coolant occurred.  The nozzle dams used at Diablo Canyon are designed to 
withstand about 50 psi of differential pressure.  Approximately 1/2 hour of 
additional time would be available before the reactor coolant heated up to the 
approximately 300� F necessary to boil at this higher pressure.  However, if a 
cold leg dam were to be expelled at this point, coolant ejection through the 
affected steam generator manway followed by core uncovery would be very rapid.
For this reason, it is prudent to provide a means of venting the vessel with 
the dams installed.  At Diablo Canyon, the schedule for detensioning the 
reactor vessel head was advanced so that this would be done before the reactor 
was drained for the steam generator inspection.  Although the pressure neces-
sary to lift the detensioned vessel head, in order to vent the vessel, is less 
than the pressure required to eject the nozzle dams, this pressure is greater 
than that which would be required to uncover the top of the fuel by expelling 
coolant through an undammed steam generator cold leg nozzle and the associated 
manway.  Therefore, even with the head detensioned, the hot leg nozzles should 
be left open until all cold leg openings are closed.  

Generic Letter 87-12 also identified a comparable mechanism for uncovering the 
core by pressurization during low coolant level operation.  An opening in a 
cold leg, such as one caused by the opening of a reactor coolant system pump 
or a loop isolation valve (in some plants), would vent the space of the af-
fected cold leg, maintaining this space at atmospheric pressure.  Any pressure 
increase, such as would be caused by boiling in the reactor vessel, would be 
propagated throughout the remainder of the reactor coolant system, including 
both hot and cold sides of steam generator primary spaces.  This differential 
pressure would force the coolant levels in the vessel down while the displaced 
coolant would be forced up and out of the affected cold leg opening.  As with 
the mechanism already discussed, only about 2 1/2 psi would be required to 
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expel the water down to the top of the core with the coolant in the affected 
cold leg at the level of a pump opening.  Although in this case some steam 
condensation may occur in the steam generators, as the 1987 Diablo Canyon 
event showed, this will not prevent pressurization.  Note that this mechanism, 
involving coolant expulsion through a cold leg opening, does not require 
plugging the steam generator nozzles.  As with the previous mechanism, this 
hazard might be eliminated by venting the reactor vessel through a large 
opening, such as a hot leg steam generator plenum manway or pressurizer 
opening, before opening the cold leg.  

The loss of residual heat removal capability during low reactor coolant level 
operation has proven to be a frequent occurrence; leading in several cases to 
boiling in the reactor vessel.  If this should occur, pressurization of the 
reactor vessel can lead to sudden core uncovery by the expulsion of coolant 
through any opening in the cold leg side of the reactor coolant system.  This 
hazard can be eliminated by providing a large vent for the reactor vessel 
space before opening the cold leg.

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the 
technical contacts listed below or the Regional Administrator of the 
appropriate regional office.  




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  Paul P. Narbut, RV
                     (805) 595-2354

                     Donald C. Kirkpatrick, NRR
                     (301) 492-1152

                     Warren Lyon, NRR
                     (301) 492-0891 

Attachments:  1.  Figure 1 - Reactor Coolant System
              2.  List of Recently Issued NRC Information Notices

References:

1.   Generic Letter 87-12, "Loss of Residual Heat Removal While the Reactor 
     Coolant System is Partially Filled," July 9, 1987.

2.   IN 87-23, "Loss of Decay Heat Removal During Low Reactor Coolant Level 
     Operation."

3.   NUREG-1269, "Loss of Residual Heat Removal System, Diablo Canyon, 
     Unit 2," April 10, 1987.
.                                                            Attachment 2
                                                            IN 88-36 
                                                            June 8, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
_____________________________________________________________________________
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-35          Inadequate Licensee Performed 6/3/88         All holders of OLs
               Vendor Audits                                or CPs for nuclear
                                                            power reactors. 

88-34          Nuclear Material Control      5/31/88        All holders of OLs
               and Accountability of                        or CPs for nuclear
               Non-Fuel Special Nuclear                     power reactors. 
               Material at Power Reactors 

87-61,         Failure of Westinghouse       5/31/88        All holders of OLs
Supplement 1   W-2-Type Circuit Breaker                     or CPs for nuclear
               Cell Switches                                power reactors. 

88-33          Recent Problems Involving     5/27/88        All Agreement 
               the Model Spec 2-T                           States and NRC 
               Radiographic Exposure                        licensees 
               Device                                       authorized to 
                                                            manufacture, 
                                                            distribute or 
                                                            operate radio-
                                                            graphic exposure 
                                                            devices and source
                                                            changers. 

88-32          Promptly Reporting to         5/25/88        All NRC material 
               NRC of Significant                           licensees. 
               Incidents Involving 
               Radioactive Material 

88-31          Steam Generator Tube          5/25/88        All holders of OLs
               Rupture Analysis                             or CPs for 
               Deficiency                                   Westinghouse and 
                                                            Combustion 
                                                            Engineering 
                                                            designed nuclear 
                                                            power plants. 

88-30          Target Rock Two-Stage         5/25/88        All holders of OLs
               SRV Setpoint Drift                           or CPs for nuclear
               Update                                       power reactors. 

88-29          Deficiencies in Primary       5/24/88        All holders of OLs
               Containment Low-Voltage                      or CPs for nuclear
               Electrical Penetration                       power reactors. 
               Assemblies 
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013