Investigation of Strategies for Mitigating Radiological Releases in Severe Accidents

196 Pages·2012·2.17 MB·English

Investigation of Strategies for Mitigating Radiological
Releases in Severe Accidents
BWR Mark I and Mark II Studies
2012 TECHNICAL REPORT
Electric Power Research Institute
3420 Hillview Avenue, Palo Alto, California 94304-1338 • PO Box 10412, Palo Alto, California 94303-0813 USA
800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

Investigation of Strategies for
Mitigating Radiological
Releases in Severe Accidents
BWR Mark I and Mark II Studies

This document does NOT meet the requirements of
10CFR50 Appendix B, 10CFR Part 21, ANSI
N45.2-1977 and/or the intent of ISO-9001 (1994).

EPRI Project Managers
R. Wachowiak
K. Canavan

3420 Hillview Avenue
Palo Alto, CA 94304-1338
USA

PO Box 10412
Palo Alto, CA 94303-0813
USA

800.313.3774
650.855.2121
1026539
[email protected]
Final Report, September 2012
www.epri.com
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ERIN Engineering & Research, Inc.

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Acknowledgments
The following organization, under contract to the Electric Power
Research Institute (EPRI), prepared this report:
ERIN Engineering and Research, Inc., an SKF Group Company
158 West Gay Street, Suite 400
West Chester, PA 19380
Principal Investigators
J. Gabor
G. Teagarden
D. True
This report describes research sponsored by EPRI.
This report was developed using a substantial body of work that was
initially created by the BWR Owners Group (BWROG) and PWR
Owners Group (PWROG). These contributions were extremely
valuable in generating this report.
Owners Groups Team
F. Emerson BWROG
K. Kiper PWROG, NextEra Energy
G. Krueger BWROG, Exelon
R. Linthicum PWROG, Exelon
B. Lutz PWROG, Westinghouse

 iii 

Abstract
The Fukushima Dai-ichi accident highlights the need to reduce the
magnitude of radioactive fission product releases from BWR Mark I
and II containments following beyond-design-basis events. There is
no evidence that such accidents have a long-term effect on public
health and safety; however, the Fukushima Dai-ichi accident did
result in widespread contamination of surrounding areas, both on-
site and off-site. This report assesses various strategies that can be
used to maintain BWR Mark I and II containment systems as
effective barriers and to retain fission products in containment
following a beyond-design-basis accident.
The best way to avoid radiological release and potential land
contamination is to prevent an accident from occurring by improving
and augmenting the strategies for preventing core damage. These
improvements are treated in other assessments and are outside the
scope of this report. This report focuses on severe accidents in which
the containment vessel provides the chief barrier to fission product
release. Various studies have shown that accidents involving long-
term loss of electric power and subsequent loss of core cooling
capability can significantly challenge the containment barrier.
This report evaluates strategies that maintain or enhance the
containment function in scenarios involving long-term loss of electric
power. The strategies evaluated include water injection (by flooding
or spraying), alternative containment heat removal, venting,
controlled venting, filtered venting, and combinations. Key insights
from the analysis include the following:

No single strategy is effective. No single strategy is optimal in
retaining radioactive fission products in the containment system.
The most effective strategies involve combinations of active
debris cooling strategies and containment venting. Even
containment vent filters are rendered ineffective when active
debris cooling strategies are not used.

Active core debris cooling is required. Core debris cooling is an
important element of a robust strategy for mitigating releases. If
debris cooling is not provided through water injection or spray
into the drywell, containment failure or bypass is likely. Without
core debris cooling, the containment can be challenged in several
ways. Molten debris can come into direct contact with the
containment wall, melting the liner and providing a release path
 v 
to the environment. Elevated drywell temperatures in the
containment atmosphere can cause seals and other containment
penetrations to fail, leading to containment bypass. Finally, core–
concrete interactions can generate large quantities of
noncondensable gases that increase containment pressure and
also can accelerate concrete erosion that could challenge
containment integrity over time.

Existing severe accident management guidelines (SAMGs)
strategies provide substantial benefit. The strategies defined in
existing SAMGs provide benefit in reducing radiological
releases. For example, active debris cooling with containment
flooding or spraying is addressed in the SAMGs and represents
an effective element of release reduction strategies for BWR
Mark I and II containments. Containment venting under severe
accident conditions is also addressed in the SAMGs and is
important in providing steam and heat removal in addition to
release mitigation.

Spraying the containment atmosphere is beneficial. Spraying the
drywell atmosphere reduces the airborne fission products in
containment. Current and previous analyses confirm that the
amount of fission products removed using a particular strategy
(as measured by the decontamination factor [DF]) is higher
when sprays are used. Sensitivity studies conducted for this
analysis also confirm that an effective spray pattern can increase
the overall containment DF by a factor of two, as compared to a
containment flooding case.

Venting prevents uncontrolled release and manages hydrogen.
The severe accident scenarios evaluated in this report assume that
core debris is discharged into the containment. As previously
noted, water is needed to cool the debris The quenching of the
debris is beneficial; however, it produces a large amount of
steam.Unless active heat removal systems are available to remove
the steam, pressurization will continue beyond containment
design pressure to the point of containment failure. Therefore,
even if water is available to cool the core debris, containment
venting is required to avoid containment failure. Venting also
helps manage the buildup of hydrogen and other noncondensable
gases generated during the core melting process. Up to 20% of
the pressure inside containment can be the result of hydrogen
and other noncondensable gases. Venting maintains the
containment pressure below the design pressure and removes
hydrogen and other gases from containment.

 vi 

Control of the vent provides benefit. The key to controlling the
amount of radioactive material released to the environment is
minimizing the amount of contaminants that are airborne in
containment during venting. Opening and closing the vent at the
most appropriate times is essential. Such controlled venting
strategies could be beneficial, but additional analysis is needed to
more fully understand this option and ensure coordination with
the plant’s emergency procedures.

Low-efficiency filters can further reduce radionuclide releases.
The analyses conducted for this research indicate that several of
the combined strategies could reduce radiological releases
significantly, with DFs greater than 1000. These combined
strategies could potentially be enhanced by adding a low-
efficiency filter to the vent path to provide additional fission
product capture. However, the aerosol remaining after using the
strategies would be composed of much smaller particles, and the
efficiency of the removal of these very small particles has not
been demonstrated with current filter designs. Additional
research is needed to assess the efficacy of current filter designs
when used in combination with the combined strategies to
evaluate whether new filter designs significantly change
radiological releases.
The research shows that viable methods exist for reducing
radiological releases and the potential for land contamination from
postulated severe core damage accidents in BWR Mark I and II
containments. Many of the strategies would likely be applicable to
other nuclear reactor types, as well. No single strategy provides
assurance of reduced releases across the potential spectrum of severe
accident scenarios. In fact, combinations of debris cooling strategies
requiring active systems and various containment venting strategies
appear to provide the greatest benefit. Plant-specific analyses would
be necessary to optimize implementation.
Keywords
Containment filter
Land contamination
Radiological release
Severe accident

 vii 