Part 21 Report - 1995-186
ACCESSION #: 9508080129
BW
B&W NUCLEAR TECHNOLOGIES JHT/95-79 3315 Old Forest Road
July 31, 1995 P.O. Box 10935
Lynchburg, VA 24506-0935
Telephone: 804-832-3000
Telecopy: 804-832-3663
Document Control Desk
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
Subject: Potential Safety Concern on Post SBLOCA Core Recriticality
following Moderator Dilution by RC Pump Bump
Gentlemen:
The purpose of this letter is provide you with preliminary information on
a Potential Safety Concern. The concern relates to the potential for
post SBLOCA core recriticality caused by a reactivity insertion due to
moderator dilution following an RC pump bump or restart of natural
circulation for the B&W-designed operating plants.
Discussions for resolution of this concern are being held with the B&W
operating plant owners. However, BWNT and the B&W Owners Group believe
it is prudent, at this time, to provide the NRC with information on the
nature of the concern. This letter is not a notification of a
substantial safety hazard under 10CFR21.
The concern is that during recovery from SBLOCA the plant may spend some
time in boiler/condenser mode of cooling and may accumulate substantial
deborated water in the RC pump suction piping. In addition, the boron
concentration of the coolant in the reactor vessel downcomer and lower
head region may decrease as steam flow through the reactor vessel vent
valve is condensed on the ECCS injection flow. If, after this
accumulation, an RC pump is started or bumped or natural circulation is
reinitiated, the boron concentration of water entering the core may not
be sufficient to assure a continued subcritical condition. Preliminary
calculations of core boron concentration indicated that, under 10CFR50.46
Appendix K conditions, a return to critical or even prompt criticality is
possible.
BWNT is currently performing preliminary calculations that will predict
the reactivity change due to the moderator dilution under realistic plant
initial conditions; 1979 ANS decay heat, non-infinite operation or Xenon
buildup, full ECCS availability, natural circulation flow rates, and
appropriate feedback. It is expected that reasonable results will be
obtained for reactivity insertion rates under natural circulations
conditions. Further studies will be performed considering pump bump and
pump restart conditions. Attached is a more detailed description of the
concern, including an evaluation which explains why no real safety
concern is believed to exist. As described in that evaluation, this
conclusion is preliminary and requires further analytical work to
document this conclusion.
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BWNT will provide information to the NRC concerning the evaluation and
resolution schedule for this concern at a later time.
If you have any questions concerning this matter, please contact the
undersigned at 804-832-2817 or Robert Schomaker at 804-832-2917.
Very truly yours,
J. H. Taylor, Manager
Licensing Services
RJS/bcc
Attachment
c: R. C. Jones/NRC
A. C. Attard/NRC
R. J. Schomaker/OF57
Evaluation of Safely Significance
Moderator Boron Dilution Core Recriticality
Preliminary Safety Concern
1. Problem Statement
During SBLOCA and other events that evolve to boiler/condenser
operation, deborated water can accumulate at the RC pump suction.
If, after sufficient accumulation, an RC pump is started or bumped
or natural circulation is reinitiated, deborated water may enter the
core, possibly leading to a criticality incident. The concern was
initially identified by BWNT as PSC 8-81, which was resolved in
1985. The calculations used to resolve PSC 8-81, however,
incorporated assumptions that are no longer valid for the B&W-
designed NSS. Therefore, it is necessary to revalidate that a
severe core criticality incident will not follow the return of
circulation following an extended period of boiler/condenser
operation.
2. Detailed Description
During the course of a small break LOCA and any other transients
that involve the partial loss of reactor coolant system (RCS)
inventory, the steam generators may provide an energy sink for part
or all of the core decay heat via boiler/condenser operation. In
this mode, steam generated in the core through boiling is passed
through the hot legs to the steam generators and condensed. The
condensate is returned to the core through the cold legs. Because
boron volatizes with the steam production at a concentration
substantially below that of the source, the concentration of boron
carried with the steam is greatly reduced with the result that the
boron concentration downstream from the steam generator is gradually
reduced. Evaluation of the boron carryover fraction with the MULTI-
Q code shows that the fraction is temperature dependent and limited
to ten percent for the conditions of interest.
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Additionally, if the ECCS HPI system is in operation, steam
accumulating in the reactor vessel upper head and upper downcomer
(vent valve flow) will be condensed in the downcomer or cold leg.
At high pressures, the amount of condensate approximates the ECCS
injection, but the condensate carries only a fraction of the boron
concentration. Thus, the downcomer boron concentration can be
substantially below (perhaps one-half) the borated water storage
tank concentration.
With aggressive core designs, the critical boron concentrations at
reduced coolant temperatures can range from 1000 to 1500 ppm. For
such a design, a rapid flooding of the core with coolant of a boron
concentration of 600 or 700 ppm could lead to a criticality
incident. So long as the RCS is in a separated condition, boiling
with steam flowing through the hot legs to the steam generators, the
core is concentrating boron and whatever low boron concentration
makeup is provided mixes with highly concentrated liquid in the core
and poses no problem. However, if bulk circulation of the RCS is
reinitiated, the highly concentrated core liquid may be displaced by
downcomer or steam generator liquid of a low boron concentration and
a reactivity event initiated. Such a possibility exists if natural
or forced circulation is reinitiated.
The potential for a criticality concern depends on the rate of
concentration in the core, the rate of condensation in the steam
generators, the rate of downcomer deboration, and the amount of
mixing between concentrated and low boration coolant during
circulation initiation. Theoretically, it is possible for low
boration levels to accumulate in the steam generators. However, an
examination of existing SBLOCA calculations shows that the
accumulation process may take one to two hours. Within that time,
core concentrations will have elevated substantially. Natural
convection currents within the core are expected to provide
substantial mixing to assist in mitigating the consequence of a
circulation restart. Even if the core does experience a reactivity
insertion sufficient to become critical, unless core boiling has
been suppressed, thermal effects will be likely to produce
sufficient reactivity feedback to control the power excursion.
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The first step in the evaluation of this concern by the B&W Owners
Group was to evaluate selected boundary conditions in an effort to
demonstrate that the conditions under which a deboration criticality
concern exists would not be generated by SBLOCA or the other
possible events. To this end, several important parameters of the
concern have been determined. The carryover of boron in the steam
flow is one. However, because the possible scenarios are highly
dependent on operator response to the accident, it is possible that
the plant will evolve to a condition under which a substantial
amount of low boration coolant is present in the steam generators,
cold legs, and downcomer. Since unfavorable initial conditions can
exist, an evaluation has been initiated to quantify the effects of
limiting conditions during a restart of natural circulation. The
expected result of this evaluation is that thermal reactivity
feedback during the return to power limits the power level to
reasonable values and that a restart of natural circulation does not
pose a concern for the safety of the plant or the eventual cooldown
and recovery of stable plant conditions.
3. Evaluation of Safety Significance
Deboration events are an active issue under consideration by the
NRC. NUREG/CR-6266 presents an evaluation of two potential
deboration events caused by continued injection of boron-free
coolant. Although the evaluation results were problematic, they
were limited to a determination of possible reactivity insertions
based on simplistic neutronics. This, in combination with the low
probability of the initiating events, indicates a need for continued
study which, it is believed, will demonstrate that these events are
not of consequence.
The deboration events postulated by this safety concern are a subset
of the possible events being considered by the NRC. As presented,
this concern elevates to one of safety significance only if
reactivity insertions are sufficient to create prompt critical
conditions
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for a significant duration. That result is not expected. The B&W
Owners Group position is that this PSC will not evolve to a safety
issue. However, more evaluations are considered necessary prior to
disposition of the concern.
Calculations performed in support of the CE System 80**+ SAR and
accepted in the NRC's SER of System 80**+ are similar to those being
conducted by the B&W Owners Group and show that the power excursion
would be limited. The CE calculations are described starting on
page 15-27 of the SER. The event is essentially the same as
postulated herein except that the plant is an RSG PWR. Two
conclusions are evident:
1) The excursion as modelled by CE (highly conservative approach)
allowed the plant to return to critical and even prompt
critical during natural circulation restart.
2) With the simple reactivity model (only doppler feedback) used
by CE, a stable power fraction of 10 percent was computed. The
use of thermal feedback and transient boron concentrations
should substantially reduce the predicted power and should
demonstrate a fairly rapid return to subcritical conditions.
Thus, the B&W Owners Group is confident that there will be no
significant safety hazard demonstrated as a result of the
evaluations of this preliminary concern. However, there is a
possibility that operational considerations will be involved.
Because of the need to provide operations with the best possible
advice and guidance, the evaluations being conducted by the B&W
Owners Group are based on best estimate conditions.
4. Conclusions
A Preliminary Safety Concern on the possible buildup of coolant with
a low boron concentration in the RCS is under evaluation by the B&W
Owners Group. To date, it has not been possible to eliminate
conclusively the evolution of the postulated unfavorable
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initiating conditions. Evaluations performed by industry and
accepted by the NRC have shown that the consequences of the
postulated event is limited. The concern is not expected to evolve
to one of safety significance, but does require continued evaluation
to establish a basis for this expectation and to develop operational
guidance for dealing with the possibility of these conditions
developing.
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