Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
