ML20137A602
| ML20137A602 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 12/31/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20137A573 | List: |
| References | |
| NUDOCS 8601140425 | |
| Download: ML20137A602 (5) | |
Text
.
out
+
UNITED STATES
[
~g%
NUCLEAR REGULATORY COMMISSION E
WASHINGTON, D. C. 20555
,o*
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.91 TO FACILITY OPERATING LICENSE N0. DPR-39 AND AMENDMENT NO.
81 TO FACILITY OPERATING LICENSE N0. DPR-48 COMMONWEALTH EDISON COMPANY ZION NUCLEAR POWER STATION, UNITS 1 AND 2 DOCKET NOS. 50-295 AND 50-304 (the licensee) is the holder of Operating The Comonwealth Edison Company (the licenses) which authorize operation of License Nos. DPR-39 and DPR-48 the Zion Station, Units Nos. I and 2 located in Zion, Illinois, at steady state core power levels not in excess of 3250 megawatts thermal (rated power).
Due to the relatively high population density surrounding the Zion site as compared to other nuclear plant sites, NRC Staff (the staff) perception at the time of the Confirmatory Order (Reference 1) was that the Zion site presented a disproportionately high contribution to the societal risks from reactor accidents. Based upon this perception, the staff identified a number of interim measures thought at that time to significantly increase the level of safety at the Zion station and thereby further reduce the probability of a severe reactor accident.
The licensee comitted to these measures in a letter dated February 22, 1980.
The licensee's comitments were confinned by the Order dated February 29, 1980.
Among the actions committed to by the licensee were Items A.1 and A.2 of the Order which states:
"A.
The licensee shall:
1.
Maintain reactor power level as necessary such that calculated fuel peak clad temperature does not exceed 2050 F under large break LOCA conditions per the 10 CFR 50 Appendix K analysis submitted on October 22, 1979.
2.
Revise plant operating procedures as necessary to require a base load mode type of operation only, without load following except for power reduction required to maintain i
system load for grid stability, required for maintenance l
(e.g., containment entry, main feed pump repairs, etc.) or as required by Technical Specifications."
8601140425 851231 PDR ADOCK 05000295 P
. In reference 2, (see Attachment), the licensee proposed amendment to the operating licenses to rescind Items A.1 and A.2 of the Confirmatory Order.
Provided in reference 2 was justification for this request, along with the licensee's determination that no significant hazards consideration is involved with this request.
We have reviewed the information provided by the licensee and, as discussed further below, have concluded that the requirements of Item A.1 and A.2 of the Confirmatory Order can be rescinded from the Zion Units 1 and 2 operating licenses.
Since the issuance of the Confirmatory Order, the licenseo has provided a probabilistic risk assessment (PRA), documented in reference 3, for Zion Units 1 and 2.
The PRA concluded that the contribution to core damage from large and medium break LOCAs is about 2% of total core damage occurrences.
The small contribution of large and medium LOCA to core melt frequency is due to a limited number of occurrences and the influence of protective features which mitigate the consequences of a LOCA.
Several failures are necessary in addition to the LOCA for significant core damage to occur.
The Regulations (10 CFR 50.46 and 10 CFR 50 Appendix K) allow only minimal core damage under LOCA conditions with the additional requirements that only safety-related equipment be considered in LOCA mitigation, that the single most damaging protective equipmer.t failure occur, and that the calculation be accomplished by application of conservative modeling.
The LOCA is typically assumed at a location which maximizes the loss of mitigation equipment, and it is assumed to cause a perturbation to the offsite electrical power grid which results in loss of offsite power.
Then the single most damaging failure is taken as the loss of a diesel which power the emergency electrical trains, with a corresponding loss of 2
emergency coolant injection pumps which take power from that electrical train.
In addition, the diesel is selected to be the one with the maximum mitigation capability, which usually is associated with pumps that are unaffected by the LOCA. The regulatory limit set out in 10 CFR 50.46(b)(1) of 2200*F is generally found to be the most stringent of the specified acceptance criteria.
The other conservatisms in the calculation provide an additional safety margin.
The overall result is that at least one, and generally more than one, additional failure, beyond the single failure criteria, in critical equipment must occur before significant core damage is encountered. The difference between the 2050*F interim fuel peak clad temperature criteria and the 2200'F criteria of 10 CFR 50.46 is perceived to have no measurable impact on risk for the Zion units.
This is because occurrence of multiple equipment failures, not a difference in the fuel peak temperature, are necessary to result in significant core damage.
Therefore, the requirement of Item A.1 of the Order is not deemed necessary to protect the public health and safety.
In addition to changing the fuel peak clad temperature, elimination of Item A.1 of the Order will result in future large break LOCA analyses for the ir 4
9
Zion units being performed with the currently approved Westinghouse ECCS evaluation model.
The ECCS evaluation model used in the October 22, 1979 analysis referred to in the Order was subsequently approved by the Staff in 1981. As part of this approval, the October 22, 1979 model was modified, notably by requiring the incorporation of the clad swelling and rupture models of NUREG-0630.
Additionally, the current Westinghouse ECCS evaluation model includes the BART computer program, which is a mechanistic core heat transfer model. These changes frcm the October 22, 1979 evaluation model constitute an approved ECCS evaluation model and represent significant improvements in the large break LOCA analysis technology. We have concluded that the regulatory requirements of 10 CFR 50.46 and Appendix K to utilize an approved ECCS evaluation model is appropriate for the Zion units, and there accordingly will be no measurable impact on plant risk.
In reference 2, the licensee notes that a revised ECCS analysis (reference 4) has been perfonned for the Zion units.
The new analysis is based upon the currently approved Westinghouse ECCS evaluation model.
In addition, the new analysis assumes that 10% of the steam generator tubes are plugged; the 1979 analysis assumed a tube plugging level of 1%. The increased tube plugging in the new analysis accounts for the current average plugging level of 4.7% in Unit I and therefore is more appropriate.
The staff has reviewed and approved applicants' revised analysis in reference 4.
Based on the foregoing, the staff has concluded that Item A.1 of the Confirmatory Order is no longer required to assure the public health and safety.
Therefore, we find the licensee's proposal to rescind Item A.]
acceptable.
With respect to Item A.2, the licensee notes in reference 2 that, although base load operation was required, approximately 103 load changes to 10% or greater were performed above 50% of rated power during the time period of January 1,1982 through January 31, 1985. These load-follow maneuvers were in compliance with Item A.2 of the Order. The licensee also noted that between January 1,1980 and December 31, 1984 there were 69 reactor trips t
l for both Zion units.
None of these trips were the result of load changes above 50% of rated power. Based upon these data, the licensee has concluded that load-follow maneuvers have been demonstrated to be reliable and unrelated to any decrease in reactor safety.
The staff has reviewed the licensee's conclusions with respect to Item A.2 l
and agrees.that operational experience demonstrates that load-following t
maneuvers will not significantly reduce reactor safety. Thus, we also find the licensee's proposal to rescind Item A.2 to be acceptable.
Environmental Consideration These amendments involve a change in the installation or use of the facilities' components located within the restricted areas as defined in 10 CFR 20. The staff has determined that these amendments involve no l
l l
l
. significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite and that there is no significant increase in individual or cumulative occupational radiation exposure.
The Commission has previously issued a proposed finding that these amendments involve no significant hazards consideration and there has been no public comment on such finding.
Accordingly, these amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of these amendments.
Conclusion We have concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such activities will be conducted in compliance with the Commission's regulations and the issuance of these amendments will not be inimical to the common defense and security or to the health and safety of the public.
Dated:
December 31, 1985 Principal Contributor:
R. C. Jones s
a
w~
l 5-References 1.
Letter, H. R. Denton (NRC) to D. L. Peoples (Commonwealth Edison),
Docket Nos. 50-295/50-304, Confirmatory Order, February 29, 1980.
2.
Letter, P. C. LeBlond (Commonwealth Edison) to H. R. Denton (NRC),
Subject:
Proposed Amendment to Facility Operating License Nos. DPR-39 and DPR-48 NRC Docket Nos. 50-295 and 50-304, June 25, 1985.
3.
" Zion Probabilistic Safety Study," Commonwealth Edison Company, September, 1981.
4.
Memorandum, R. Wayne Houston to Gus C. Lainas,
Subject:
Zion Units 1 and 2 Revised ECCS Analysis, May 17, 1985.
9 V
wY
-