Information Notice No. 96-69: Operator Actions Affecting Reactivity
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
December 20, 1996
NRC INFORMATION NOTICE 96-69: OPERATOR ACTIONS AFFECTING REACTIVITY
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 operating events that have affected reactivity. 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 are not NRC
requirements; therefore, no specific action or written response is required.
Generic Letter 85-05, "Inadvertent Boron Dilution Events," dated January 31,
1985, was used to indicate the staff's position that resulted from the evaluation
of Generic Issue 22, "Inadvertent Boron Dilution Events." The generic letter
considers an unmitigated boron dilution event as a serious breakdown in the
licensee's ability to control its plant and strongly urges each licensee to
assure itself that adequate protection against boron dilution events exists in
its plants. However, the consequences are not severe enough to warrant
backfitting requirements for boron dilution events at operating reactors.
In the past several years, this year in particular, there have been numerous
events where operator actions inappropriately affected reactivity. This
information notice highlights several recent events in which poor command and
control during reactivity evolutions have led to unanticipated conditions.
Description of Circumstances
Byron Unit 1
On June 12, 1996, the licensee made four dilutions of the reactor coolant system.
Only the first dilution was calculated in advance. At the time, Byron Unit 1 was
in cold shutdown for a refueling outage. Fuel had been reloaded into the core,
and the reactor head was in position. The reactor coolant loops were isolated
to support steam generator tube inspection and repair. The reactor coolant
system (RCS) boron concentration was 1,984 parts per million (ppm). The RCS
silica concentration was elevated at 4 ppm.
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A series of dilutions was planned to reduce silica levels and bring the RCS boron
concentration closer to the 1,600 ppm starting point for the planned dilution to
criticality. The target RCS boron concentration was 1,700 ppm.
The operations staff added approximately 7,600 liters [2,000 gallons] of pure,
unborated water from the primary water storage tank through a feed-and-bleed
dilution. A reactor operator calculated the expected boron concentration after
this dilution to be no less than 1,837 ppm. The subsequent chemistry sample
results indicated a boron concentration of 1,942 ppm.
On the basis of the chemistry sample result, the reactor operators performed a
second dilution of 7,600 liters [2,000 gallons] without conducting formal
calculations expecting to achieve a boron concentration of around 1,800 ppm. The
chemistry sample after the second dilution indicated a boron concentration of
1,877 ppm. Based on this and a subsequent chemistry sample, but without formal
calculations, the reactor operators made two additional dilutions of 15,200
liters [4,000 gallons] each, expecting a final boron concentration of greater
than 1,700 ppm. The chemistry sample results after the fourth dilution indicated
a boron concentration of 1,521 ppm. The reactor operators added borated water
to increase the boron concentration to about 1,585 ppm to ensure adequate
shutdown margin. The licensee's Technical Specifications require a 1.3-percent
shutdown margin, which the licensee indicated was about 1,164 ppm boron.
The licensee determined that the sample line was not adequately purged before the
first three samples were obtained. However, the sample valve was left open for
about 1 hour before the fourth sample was taken, which allowed the line to be
adequately purged; thus a representative sample was obtained.
Washington Nuclear Project No. 2
On June 27, 1996, the reactor achieved criticality at Step 8-3 in the rod pull
sequence. Criticality was expected at Step 12-18 of the rod pull sequence, with
an acceptable range of achieving criticality (+/- 10 mkeffective) between Steps
11-12 and 14-20. Achieving criticality at Step 8-3 was outside the licensee�s
self-imposed acceptable range of values and was approximately 16 mkeffective before
the calculated estimated critical position. In accordance with plant procedures,
operators manually inserted control rods to shut down the reactor.
The estimated critical position calculated for the startup was performed using
an inappropriate parameter for the plant conditions. The nuclear engineer
selected an incorrect parameter for xenon dependence. The reactor was shutdown
for a short period of time and xenon did not completely decay and was incorrectly
accounted for in the calculation.
During the approach to criticality, members of the control room staff were
involved with activities related to shift turnover, this may have distracted
personnel involved with the startup.
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December 20, 1996
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St. Lucie Unit 1
On January 22, 1996, while performing a routine manual boron dilution of the
reactor coolant system, the board reactor controls operator (RCO) was distracted
leading to an over dilution with reactor power reaching 101 percent. During the
evolution the RCO responded to a secondary plant annunciator and lost track of
the routine dilution. He then requested to be relieved by the desk RCO while he
prepared his lunch. During the turnover, there was no discussion of the dilution
in progress which continued for seven minutes until the board RCO returned and
realized his error. The operators took prompt corrective action of stopping the
dilution and initiating manual boration.
At Byron, an inadequate sampling procedure and inadequate calculations of boron
concentration led to an unexpected dilution of 179 ppm below the target boron
concentration of 1,700 ppm. The licensee determined the chemistry sample
procedure to be deficient. This deficiency was originally noted during review
of procedures for post-accident sampling; however, the chemistry staff failed to
recognize the implications on routine sampling. The licensee's dilution
procedure was deficient, also, in that it did not have provisions for dilutions
with the loop stop isolation valves closed. The operators calculated the reduced
volume for the dilution calculations and attributed the differences in expected
and sample boron concentrations to the conservative reactor coolant system volume
used in the calculation. The reactor operators continued with successive
dilutions based on the original calculation and the sample concentrations but
failed to adequately question the higher than expected sample values and to
perform acceptable calculations between dilutions in order to determine the
additional dilution amounts.
At Washington Nuclear Project No. 2, Shift Nuclear Engineers, because of
inadequate training on a recent software modification, incorrectly selected a
parameter which resulted in the wrong estimated critical position. These
engineers and operators suspected a problem with the estimated critical position
but did not effectively resolve their concerns or express them to higher
management. The engineers did perform an independent verification which
confirmed the estimated value; however, they used the same software and input
During the startup, the control room staff realized that the reactor would go
critical outside their self-imposed +/- 10 mkeffective reactivity band; however,
they continued the startup because of their interpretation of a poorly written
startup procedure. The likelihood of achieving early criticality was not
communicated to upper management, either. When criticality was achieved,
operators then acted conservatively and manually shut down the reactor.
At St. Lucie Unit 1, the board RCO exhibited inattentiveness to a routine
evolution affecting reactivity. The RCO initiated the dilution without notifying
other control room personnel and failed to discuss the evolution in progress with
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December 20, 1996
Page 4 of 4
control room. As a result, the senior reactor operator and the other operators
were unaware that a reactivity addition was taking place. Upon returning to the
control room, the RCO noted an alarm which was due to increasing reactor coolant
system pressure, realized his error, and took prompt corrective actions.
Both the Byron and the Washington Nuclear Project events involved a lack of
questioning attitude that would have allowed the operators to suspend the ongoing
evolutions affecting reactivity until they had an understanding of the unexpected
plant indications. Furthermore, all three events contained inappropriate command
and control over activities associated with reactivity manipulations.
Additional details of these events can be found in the following inspection
reports: Byron Unit 1 IR 50-454/96-05; 50-455/96-05 dated July 31, 1996
; Washington Nuclear Project No. 2 IR 50-397/96-16 dated September
12, 1996 ; and St. Lucie IR 50-247/96-03 dated February 22, 1996
This information notice requires no specific action or written response. If you
have any questions about the 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 D.B. Matthews
Thomas T. Martin, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: N. D. Hilton, RIII M. S. Miller, RII
(815) 234-5451 (407) 464-7822
E-mail: firstname.lastname@example.org E-mail: email@example.com
R. C. Barr, RIV S. S. Koenick, NRR
(509) 377-2627 (301) 415-2841
E-mail: firstname.lastname@example.org E-mail: email@example.com
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