ML20046C759
| ML20046C759 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 10/26/1992 |
| From: | Tucker H DUKE POWER CO. |
| To: | NRC OFFICE OF THE SECRETARY (SECY) |
| Shared Package | |
| ML20046C749 | List: |
| References | |
| RTR-NUREG-1465 NUDOCS 9308120065 | |
| Download: ML20046C759 (7) | |
Text
11 16 Duke (buer Company thi B Tucate PQ Rar1006 Senior Vice President Charloce, NC 23:011006 Nuclear Generatlon (704)382-4531 DUKEPOWER October 26, 1992 Secretary of the Commission U.
S.
Nuclear Regulatory Commission Washington, D.
C.
20555 Attention: Docketing and Service Branch
Subject:
Duke Power Company Response to Request for Comments on Draft NUREG-1465, " Accident Source Terms for Light-Water Nuclear Power Plants" Duke Power Company is pleased to take this opportunity to respond to the NRC's request for comment on the subject draft NUREG.
As a general comment, it is obvious that a great deal of effort was invested in the development of the proposed source terms.
The NUREG has attempted to provide a more realistic prediction of the timing of fission product releases, and a better analysis of the chemical form of the released iodine.
Ultimately, however, the source term remains more conservative than necessary; this is particularly true with respect to the undue emphasis placed on ex-vessel releases for source term applications. As a result, the revised source terms are unlikely to provide any significant relief in dose calculations; the costs associated with redoing the analyses will not be justified by any commensurate benefit.
Specific comments on the NUREG are contained in the attachment.
If there are any questions, or you would like to discuss these comments further, please call Scott Gewehr at (704) 382-7581.
l Very truly yours,_,
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Hal B. Tucker
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N1465/ sag 9308120065 930722 PDR NUREG 1465 C PDR
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.s U.
S. Nuclear Regulatory Commission October 26, 1992 Page 2 cc:
Mr.
T. A. Reed, Project Manager Office of Nuclear Reactor Regulation U.
S. Nuclear Regulatory Commission Mail Stop 14H25, OWFN Washington, D.
C.
20555 Mr.
L. A. Wiens, Project Manager Office of Nuclear Reactor Regulation U.
S.
Nuclear Regulatory Commission Mail Stop 14H25, OWFN Washington, D. C.. 20555 Mr.
R.
E. Martin, Project Manager Office of Nuclear Reactor Regulation U.
S.
Nuclear Regulatory Commission Mail Stop 14H25, OWFN Washington, D.
C.
20555 Mr.
S.
D.
Ebneter-Regional Administrator U.S.
Nuclear Regulatory Commission - Region II 101 Marietta Street, NW - Suite 2900 Atlanta, Georgia 30323 Mr.
P.
K. Van Doorn Senior Resident Inspector McGuire Nuclear Station Mr. W.
T.
Orders Senior Resident Inspector Catawba Nuclear Station Mr.
P.
E. Harmon Senior Resident Inspector Oconee Nuclear Station
l Comments on NUREG-1465
\\
i i
l NUREG-1465, " Accident Source Terms for Light-Water Nuclear Power Plants," proposes a source term to replace the source term developed in~ TID-14844, " Calculation of Distance Factors for Power and Test Reactors."
Due to the many uses to the TID-14844 source term (e.g., Reg. Guides 1.3 and 1.4), NUREG-1465 will have significant implications to future Light Water Reactors.
NUREG-2465 indicates that current licensees may apply the source term to currently-licensed facilities.
The NRC will review any application of j
NUREG-1465 to existing plants.
It is in the context of existing plants that the following. comments are provided.
1.
Section 2.2 " Accidents Considered" describes the basis for the proposed source term.
The accident considered in NUREG-1465 is a severe accident with melting of the core, failure of the reactor vessel, and core-concrete interaction.
There are four release phases consisting of a gap release, in-vessel release, ex-vessel release, i
and late in-vessel release.
l Current understanding of reactor accidents suggests that less severe accidents -- those accidents that are I
recovered before vessel failure -- are more likely to occur than accidents resulting in core melt with vessel failure.
Although less likely to occur, a complete core melt accident with vessel failure and ex-vessel release constitutes the basis of the proposed source term.
This basis places a disproportionate emphasis on i
ex-vessel releases for source term applications.
For application of the source term, a weighted average of release fractions from accidents that terminate before vessel failure and accidents that include vessel failure with ex-vessel release should be considered.
The estimated probability of a type of accident may be used to weight the accident's fission product release.
I Using a weighted average, one can define a group of accidents that only considers the gap activity, a group that considers a small fraction of in-vessel core melt activity in addition to gap activity, and finally a third group that considers an appropriate combination of gap activity, in-vessel activity, and ex-vessel activity.
The probability of each accident group can be used to weight each group.
For example, if the objective of NUREG-1465 is to provide a source term for Final Safety Analysis Report (FSAR) safety analysis, then the gap activity group should be given the most
5 weight with little weight applied to the core melt group.
The ex-vessel fission product group should not be considered for the FSAR analysis.
If the source term is to be used for siting, then all three groups should be considered and given an appropriate weight.
2.
Tables 3.9 and 3.12 include a value for the gap fraction (i.e.,
5% of noble gases, iodines and cesiums) that is higher than existing drata would suggest.
If NUREG-1465 is to be considered the compendium of source term assumptions for effluent calculations, then source term implications for other accidents resulting in only gap release should be addressed with the same rigor.
For example, reactor coolant pump locked rotor accident for existing LWRs may involve breach of fuel pins such that the value cited for gap fraction is significant for this accident.
Evidence exists to demonstrate that 5% is an unnecessarily conservative value (NUREG/CR-0722 & NUREG/CR-1288).
In addition, industry guidance, such a ANSI /ANS-5.4, is available that provides methods to calculate the gap activity based on operating parameters.
A more mechanistic approach to the activity in the fuel pin plenum is appropriate.
3.
Within NUREG-1465 there are sections that require additional discussion in order to apply the source term to existing plants.
This information needs to be addressed prior to application of the source term.
A.
Section 5 "In Containment Removal Mechanisms" does not provide a discussion of the removal of aerosols with ice condensers.
This section should be expanded to include a discussion of acrosol removal rates with ice condensers.
B.
In section 5.3 a question concerning the effect of hydrogen burns on the chemical form of iodine in containment is raised.
Also the necessity of charcoal filters to remove elemental iodine is questioned.
These questions are posed in NUREG-1465 but no further discussion of the topics follows.
C.
NUREG-1465 describes the chemical form of iodine as mainly CsI and HI in the containment atmosphere but does not address the reactor coolant system or the containment sump and corresponding iodine partition factors.
Iodine partition fractions affect the source term from ESF component leakage.
Additional guidelines need to be developed to reflect the current understanding of behavior of iodine and iodine partitioning factors.
5 D.
Do. tables 3.9 and 3.12 of NUREG-1465 apply only to the containment atmosphere?
The definition of "in-containment source terms" on page 7 on NUREG-1465 seems to imply that the values provided in table 3.12 are for containment atmosphere and available for release directly from containmerL.
No breakdown of fractions of the source term '9r sump activity is provided in NUREG-1465.
The liquid source becomes significant for post-accident shielding and effluent calculations in which ESF component leakage carries with it the source term in the containment sump.
While this can also be a significant contributor to off-site dose, it is not addressed in NUREG-1465.
Also, it is not clear from NUREG-1465 whether the sump radionuclide inventory should.be modeled versus time due to washout of radionuclides in the containment atmosphere as well as decontamination (scrubbing) of particulates by the sump water (see pages 22 and 23 of NUREG-1465).
Modeling of radionuclide inventory of the sump effluent versus time could become a quite complex but integral part of post-accident shielding calculations with the adoption of NUREG-1465.
Additionally, depending upon the interpretation as to exactly-what constitutes the post-accident shielding source term, the impact on post-accident shielding with the values outlined in NUREG-1465 could be greater than the off-site dose considerations, especially if the ex-vessel release is to be 1
included (see item #1).
Additional guidance concerning liquid source terms and post-accident shielding should be provided.
E.
NUREG-1465 indicates that.a significant portion of the iodine source term is considered to be CsI.
On the basis of this information, increased filtration credit may be assumed for ESF filtration systems.
However, in section 5.3 of NUREG-1465, the following statement appears:
"Although such filtration systems are not designed-to handle the large mass loadings expected as a i
result of ex-vessel releases, when they are used in conjunction with large water sources such as BWR suppression pools or significant water depths overlaying core debris, the water sources will reduce the aerosol mass loadings on the filter system significantly, making such filter systems effective in mitigation of a large spectrum of accident sequences."
This statement implies that significant credit is granted in NUREG-1465 for
I' q
J l
pool decontamination.
The scrubbing factors provided in NUREG-1465 do not appear to agree with-the statement.
Further, it is not clear whether the above statement is meant to be a qualitative assertion of the ability of HEPA filters to maintain efficiency and viability, or to require the licensee to model filter loading and demonstrate that the filters are indeed viable.
If the later is the correct interpretation, modeling methodology should be provided to accomplish the required task.
F.
Significant conservatism seems to be embodied within the decontamination factors credited in Section 5.4 of NUREG-1465.
This section as well as figure 1 minimizes the scrubbing credit allowed by open pools.
Evidence exists that increased credit beyond the values allowed by NUREG-1465 may be appropriate.
Further discussion should be included to define those conditions under which a large as opposed to a small aerosol diameter would be obtained, and allow credit for increased scrubbing if it can be substantiated.
G.
Tables 5.5 and 5.6 provide containment atmosphere removal rates.
Are the removal rates in addition to containment sprays?
H.
The timing and release rates for the release phases should be clarified.
Currently, this information is dispersed throughout sections 3.3 and 3.6.
A summary of this information should be provided that includes the start time (from~the start of the accident) for each release phase, the duration, and the release rate (linear over the duration of the release phase).
This information could be added to tables 3.11 and 3.12.
4.
The following comments concern applications of the-source term.
A.
The proposed source term is more conservative than necessary for design basis considerations.
The source term does not consider engineered safety features designed to prevent or mitigate a reactor accident.
Applications of the source term should give credit to those features designed to prevent or mitigate a reactor accident.
For accident analysis the source term should be primarily based on the gap release.
These analyses indicate that departure
l from nucleate boiling may occur resulting in some release of.the gap activity.
A frequency weighted source term for accident analysis, as described in item #1, would provide an appropriate accounting of the significant events resulting in fuel damage.
-l 1
B Has the cost penalty associated with the use of the source term proposed in NUREG-1465, including fuel management and equipment design constraints, versus the benefits gained from the large conservatism been considered?
The source term proposed in NUREG-1465 appears to be associated with extremely low probability events.
Use of the conservative source term in lieu of more reasonable source terms may not_significantly 1
improve overall plant risk.
Accordingly, the benefits gained may not justify the cost.
C.
Current regulations contain dose limits for the thyroid and the whole body.
Will the addition of other isotopes result in dose limits for organs other than the thyroid and whole body?
1 o
t n
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