United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-21: Inadvertent Criticality Events at Oskarshamn and at U.S. Nuclear Power Plants

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

                                   May 9, 1988


Information Notice No. 88-21:  INADVERTENT CRITICALITY EVENTS AT
                                   OSKARSHAMN AND AT U.S. NUCLEAR 
                                   POWER PLANTS

Addressees:

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

Purpose:

This information notice is being provided to alert addressees to undesirable 
procedural practices that could lead to inadvertent criticality events in 
nuclear power plants.  It is expected that recipients will review the infor-
mation for applicability to their facilities and consider actions, as appro-
priate, 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:

On July 30, 1987, an unplanned criticality event occurred at Oskarshamn Unit 
3, a boiling water reactor (BWR) in Sweden, during routine control rod 
shutdown margin testing.  A night shift team, consisting of a shift 
supervisor, a physicist, and an operator, had decided to proceed with shutdown 
margin testing, even though they knew that the fast-acting hydraulic scram 
system was inoperable.  A slower acting electric rod insertion system and the 
boron injection system remained operational.  

Upon partial withdrawal of the first control rod, the core unexpectedly went 
critical.  Although the flux rise was indicated on the instrument panels, the 
team was not immediately aware that the reactor was critical.  However, the 
control logic for the electric system was initiated by the high flux signal, 
blocking further withdrawal and reinserting the control rod.  

The team then reset the electric control system and continued the test on a 
second control rod without further incident.  The night shift was in the 
process of testing a third control rod when they were relieved by the day 
shift.  The night shift apparently failed to inform the day shift of the 






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inoperable fast-acting hydraulic scram system.  The day shift had completed 
testing the third rod and had started testing the fourth rod when they dis-
covered that the hydraulic scram was inoperable.  They then stopped further 
testing and reported the situation to their supervisors.  

Several unplanned criticality events have occurred at nuclear power plants in 
the United States, although none have been reported in which the crew deliber-
ately withdrew rods with an inoperable scram system.  

On November 7, 1973, an inadvertent criticality event occurred at the Vermont 
Yankee nuclear power plant when an operator withdrew a control rod with the 
adjacent rod already in the fully withdrawn position.  At the time of the 
event, the reactor was shut down with the reactor vessel and primary contain-
ment heads removed, and the refueling cavity above the reactor vessel flooded.  
Control rod friction tests and core verification procedures were in progress 
simultaneously.  To allow traversing of the television camera mounted on the 
fuel grapple while rods were being withdrawn for the friction tests, the 
operators used jumpers to defeat the refueling interlock of the manual control 
system for the control rods.  Contrary to the normal refueling condition, this 
action permitted the withdrawal of more than one rod at a time.

As the rod testing progressed, the rod in position 30-23 was inadvertently 
left in the fully withdrawn position.  Meanwhile, the core verification pro-
cedure was completed, but the interlock jumpers were not immediately removed.  
The reactor operator conducting the rod testing failed to observe that rod 
30-23 was still withdrawn and withdrew an adjacent rod in position 26-23.  At 
about rod notch position 16, the reactor went critical.  Somewhere between 
notch positions 20 and 26, the operator saw the power rising on the nuclear 
instruments and attempted to insert the rod.  However, a full scram was 
initiated by the high-high flux signals on the intermediate-range monitors.  

Because the scram system remained operational and terminated the power rise, 
the event did not cause any serious consequences.  The dosimeter readings of 
the personnel working on the refueling floor were normal.  Five fuel 
assemblies in the affected area were removed for inspection and testing, and 
no damage was found. 

On November 12, 1976, an inadvertent criticality event occurred at Millstone 
Unit 1 when an operator withdrew the wrong control rod during a shutdown 
margin test on a partially loaded core.  The operator was supposed to withdraw 
two diagonally adjacent control rods, in positions 46-23 and 42-19, as part of 
a shutdown margin test during a core-loading procedure.  The operator had 
positioned rod 46-23 at notch 10, but then erroneously selected the rod at 
position 46-19, which was directly adjacent to the first rod, and withdrew it 
to notch 10 also.  He then continued to withdraw rod 46-23 in steps.  When rod 
46-23 was 
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withdrawn from notch 14 to notch 16, the reactor went critical and scrammed a 
few seconds later.  A few minutes later, the operator made the same error, 
withdrawing both rods 46-23 and 46-19 to notch 10 and then attempted to 
withdraw rod 46-23 further.  This time the operator saw that the startup-range 
monitor was increasing and inserted the rods before another scram occurred.

Once again, the presence of an operational scram system prevented any serious 
consequences from this event.  Personnel exposures were normal.  The four fuel 
bundles surrounding rod 46-23 were removed, partially disassembled, and 
examined.  No damage was found.

Discussion:

These events highlight the importance of maintaining an operable fast-acting 
scram capability whenever any coupled control rods are withdrawn from a 
reactor core.  Licensees are encouraged to review their procedures and 
training programs to ensure there is no ambiguity on this point. 

These events also highlight the importance, during control rod manipulations, 
of following procedures and staying alert to the relevant instrumentation, 
even when the reactor is not expected to become critical.  In each of the 
three cases described, procedures were violated.  At Oskarshamn, withdrawing 
control rods with the fast-acting scram inoperable was a violation of the 
plant procedures.  At Vermont Yankee, the "Lifted Lead Log" procedure that was 
required to be used for jumper installation was not adhered to.  The jumper 
installation was not recorded in the general plant log and consequently 
operating personnel were not adequately informed of the jumpered interlock 
status.  In addition, the jumpers were not removed immediately after core 
verification.  At Millstone, the operator who was performing the shutdown 
margin test reselected the incorrect control rod and tried to withdraw it a 
second time without determining the cause of the initial reactor scram.

In each of the three cases, the operators failed to observe indications on the 
instruments that could have prevented or mitigated the event.  At Oskarshamn, 
when the core unexpectedly went critical, the flux rise was indicated on the 
instrument panels.  However, the operators were not immediately aware that the 
reactor was critical.  At Vermont Yankee, the operator failed to observe that 
rod 30-23 was mistakenly left in the withdrawn position, though it was later 
proved that the rod's digital position display was functioning properly.  At 
Millstone, the operator observed that the startup-range monitor was increasing 
during the second erroneous rod withdrawal and managed to prevent a second 
reactor trip.  If the operator had observed that the startup-range monitor was 
increasing the first time, both the initial criticality and the subsequent 
repetition of the error might have been prevented.

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No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact 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:  Robert J. Giardina, NRR
                     (301) 492-1188

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


Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment
                                                            IN 88-21 
                                                            May 9, 1988 
                                                            Page 1 of 1

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

88-20          Unauthorized Individuals      5/5/88         All holders of OLs
               Manipulating Controls and                    or CPs for nuclear
               Performing Control Room                      power, test and 
               Activities                                   research reactors,
                                                            and all licensed 
                                                            operators. 

88-19          Questionable Certification    4/26/88        All holders of OLs
               of Class 1E Components                       or CPs for nuclear
                                                            power reactors. 

88-18          Malfunction of Lockbox on     4/25/88        All NRC licensees 
               Radiography Device                           authorized to 
                                                            manufacture, 
                                                            distribute, and/or 
                                                            operate radio-
                                                            graphic exposure 
                                                            devices. 

88-17          Summary of Responses to NRC   4/22/88        All holders of OLs
               Bulletin 87-01, "Thinning of                 or CPs for nuclear
               Pipe Walls in Nuclear Power                  power reactors. 
               Plants" 

88-16          Identifying Waste Generators  4/22/88        Radioactive waste 
               in Shipments of Low-Level                    collection and 
               Waste to Land Disposal                       service company 
               Facilities                                   licensees handling
                                                            prepackaged waste, 
                                                            and licensees 
                                                            operating 
                                                            low-level waste 
                                                            disposal 
                                                            facilities. 

88-15          Availability of U.S. Food     4/18/88        Medical, Academic,
               and Drug Administration                      and Commercial 
               (FDA)-Approved Potassium                     licensees who 
               Iodide for Use in Emergencies                possess 
               Involving Radioactive Iodine                 radioactive 
                                                            iodine. 

88-14          Potential Problems with       4/18/88        All holders of OLs
               Electrical Relays                            or CPs for nuclear
                                                            power reactors. 
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013