Information Notice No. 97-62: Unrecognized Reactivity Addition During Plant Shutdown

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                          WASHINGTON, D.C. 20555-0001

                                August 6, 1997

                               PLANT SHUTDOWN

All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a recent incident in which a licensed reactor
operator continuously inserted control rods to bring the reactor to the
subcritical stage and then promptly withdrew the rods in order to take the
reactor to the critical stage and return power to the point of adding heat
(POAH).  These actions constituted an unauthorized attempt to start up the
reactor.  It is expected that recipients will review this information for
applicability to their facilities and consider actions, as appropriate. 
However, suggestions contained in this information notice are not NRC
requirements; therefore, no specific action or written response is required.

Description of Circumstances

On February 19, 1997, the licensee observed that the containment spray (CS)
pump 1C took an abnormally long time to start during a surveillance test and
at 10:40 a.m. declared the pump inoperable.  The Technical Specifications (TS)
required that the CS pump be restored to an operable status within the next 48
hours or that Unit 1 be in hot shutdown within the following 4 hours.  In the
early morning of February 21, the licensee completed corrective maintenance on
the CS pump 1C and initiated post-maintenance testing.  However, the pump did
not start within the required time and the licensee initiated additional
troubleshooting efforts.  Later that morning, a new operating crew began their
first day of shift duty after 3 days off.  Although the 48-hour limiting
condition for operations (LCO) would expire in less than 4 hours, requiring
entry into a 4-hour shutdown in accordance with the TS action statement, no
plan had been developed to support an orderly and controlled shutdown of
Unit 1.

At 10:40 a.m. on February 21, the 48-hour LCO expired and the licensee entered
a 4-hour shutdown action statement for Unit 1.  At approximately 11:00 a.m.,
plant management, operations management, and operations supervisors held an
informal discussion regarding the status of the CS pump 1C and when to
initiate the Unit 1 shutdown.  Ten minutes later, the shift engineer (SE)
conducted a formal shutdown briefing.

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At 12:09 p.m., control room operators began reducing power at 0.25-percent per
minute.  At approximately 12:55 p.m., the licensee noted through a review of
surveillance testing records that the CS pump 1C should have been declared
inoperable at 10:20 a.m. rather than at 10:40 a.m. on February 19th.  As a
result, Unit 1 was required to be in hot shutdown by 2:20 p.m. instead of 2:40
p.m. in order to comply with the TS.  The licensee subsequently increased the
rate of power reduction to 0.5-percent per minute.

At approximately 2:05 p.m., with Unit 1 at 7-percent power, the SE directed
the Unit Supervisor (US) to maintain the reactor critical because he expected
the CS pump to be returned to service within a few minutes.  The US and the
primary nuclear station operator (NSO) reviewed the steps in the plant
shutdown procedure for taking the turbine off-line and inserting control rods
to establish power at or below the POAH, defined in the procedure as 0.025-
percent power.  The review consisted of the US's reading the relevant step
aloud and requesting the primary NSO to locate the point on the intermediate
power range monitor corresponding to 0.025-percent power.  The primary NSO
identified this point and then asked the US if he wanted him to drive control
rods in.  The US responded by re-reading the procedure step aloud, which
states, "HOLD `#363, ROD MOTION CONTROL' switch IN to minimize dumping steam
and establish power at or less than the Point of Adding Heat (2.5x10-2% IR)."

At 2:07 p.m., the main turbine was tripped and the primary NSO inserted
control rods continuously for 3 minutes and 48 seconds (232 steps) until power
indicated 0.025-percent.  Reactor power continued to decrease as a result of
the negative reactivity associated with the control rod insertion and when
power reached 0.01 percent, the primary NSO informed the US that he intended
to withdraw control rods to stabilize power at 0.025 percent.  The primary NSO
then proceeded to withdraw control rods continuously for 1 minute and 45
seconds (84 steps) until he was directed to trip the reactor by the US.  The
SE directed the US to trip the reactor because the CS pump had not been
restored to service and Unit 1 was required to be in hot shutdown within the
next 6 minutes to comply with the TS.


The NRC dispatched an Augmented Inspection Team (AIT) to review the facts
surrounding the February 21 event at Zion Nuclear Plant.  The findings and
conclusions of this inspection are documented in NRC Inspection Report 50-

The AIT concluded that the US and SE did not exercise their respective
responsibilities for ensuring that shift activities were conducted in a
controlled manner and became focused on CS pump restoration activities and
balance-of-plant problems.  The shutdown briefing was informal, poorly
planned, and ineffective.  Operations supervisors did not provide any
direction to the operating crew during the briefing regarding the decision
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to hot shutdown.  The SE did not provide clear direction to the US regarding
his intent to keep the reactor critical after the main turbine had been
tripped.  Operations supervisors also failed to inform the operating crew of
the intent to keep the reactor critical.

Despite a number of communications and control room indications such as "low
rod insertion limit" annunciator, the "low-low rod insertion limit"
annunciator, audible clicking of the group rod position step counter, control
banks "C" rod bottom lights, and control bank C and D rod position indication,
the US and the SE were unaware that the primary NSO had continuously inserted
control rods a total of 232 steps, which placed the reactor in a substantially
subcritical condition, and then continuously withdrew control rods 84 steps in
an attempt to re-establish power at the POAH.

Upon noting that the control bank "C" rod bottom lights were illuminated, the
qualified nuclear engineer (QNE) assigned to monitor the shutdown evolution
asked the primary NSO "why control rods had been driven in so far."  The
primary NSO replied, "This doesn't look right, but I am just following
procedures."  Approximately 7 minutes later, the QNE observed that the primary
NSO was withdrawing control rods, approached the operator, and stated that he
"did not like what the operator was doing."  The primary NSO responded that he
was uncomfortable with what he was doing as well.  However, neither the QNE
nor the primary NSO informed the US of their concerns about control rod

The actions of the primary NSO in continuously withdrawing control rods with
the intent of taking the reactor to critical stage in order to re-establish
power at the POAH reflected a significant lack of understanding of proper
control rod manipulations for a controlled approach to criticality.  Although
he was concerned with the actions directed by a specific step in the shutdown
procedure, the primary NSO did not adequately communicate his concerns to the
US.  The US also did not clarify the intent of the procedural step to the
primary NSO.

Although the actual event did not pose a risk to the health and safety of the
public, the event was considered safety significant from a human performance
perspective.  With the Unit 1 reactor substantially subcritical, a licensed
reactor operator withdrew control rods continuously in an attempt to take the
reactor to the critical stage, disregarding established procedural controls
for conducting a safe reactor startup.  The rod manipulations were conducted
without the knowledge of operations supervisors.  Proper manipulation of
control rods during reactor shutdowns and startups is fundamental to
operational safety.

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This information notice requires no specific action or written response.  If
you have any questions about information in this notice, please contact one of
the technical contacts listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.

                                            signed by S.H. Weiss for

                                       Marylee M. Slosson, Acting Director
                                       Division of Reactor Program Management
                                       Office of Nuclear Reactor Regulation

Technical Contacts: Thomas Koshy, NRR

                    Marc Dapas, RIII
                    (630) 829-9601

Page Last Reviewed/Updated Wednesday, March 24, 2021