ML19305B751
| ML19305B751 | |
| Person / Time | |
|---|---|
| Site: | Big Rock Point File:Consumers Energy icon.png |
| Issue date: | 02/06/1976 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19305B745 | List: |
| References | |
| NUDOCS 8003200253 | |
| Download: ML19305B751 (4) | |
Text
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l' eJ o J et SAFETY EVALUATION BY TrE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING ACCEPTM:CE OF CASK DROP ACCIDENT ANALYSIS i
CONSUMERS POWER COMPANY BIG RCCK POINT PLANT j
DOCKET NO. 50-155 INTRODUCTION By letter dated February 4,1974, we requested Consumers Power Company (CPCo) to provide an analysis and relevant information required to determine the potential consequences of a fuel cask drop caused by a system failure at the Big Rock Point Plant.
Both the fuel transfer cask and the fuel shipping cask w'ere to be considered in this analysis.
We further requested that if the evaluaticn indicated changes were necessary to protect plant structures, systems, or components important to safety or to prevent damage to irradiated fuel, information on the plant modifications should be provided as well as the schedule for design, fabrication and installation of any modifications.
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In response to our February 4,1974 letter, a study perfomed by Bechtel Corporation for CPCo was filed by CPCo on July 1,1974, with supplemental information filed on August 25, 1975, in response to our request dated June 30, 1975.
Additional information was provided in CPCo's letter of January 22, 1976. This Safety Evaluation presents the results of our review of the infor=ation provided relative to the postulated fuel cask drop accidents at Big Rock Point.
DISCUSSION /EVALUATICN Areas of particular concern which we asked to be included in the CPCo evaluation and which we have reviewed are as follows:
1.
Potential effect on scent fuel storace pool - The licensee was asked to determine whetner cropping a cask over the storage pool could damage the pool floor to the extent that adequate =akeup water capability could not be assured or resultant flooding could cause critical systems to become inoperable.
As analy:ed by CPCo 8 0 032 00;;L63
4 a free fall of the fuel shipping cask or the fuel transfer cask could cause a crack of the spent fuel pool bottom slab and the j
pool liner plate may be perforated.
Slab penetration is not considered possible.
In such an event, the rate of pool drainage into the 585' - 6" level of containment could exceed the pool makeup rate.
Se proposed modification to prevent structural failure of the spent fuel pool base slab in this area is the installation of a cellular energy absorption assembly. We have reviewed the proposed modifi-cation and have determined that it constitutes an acceptable method for eliminating this potential failure.
CPCo stated in its letter of July 1,1974, that the proposed modifi-cations to the spent fuel pool would be completed within one year from the date of our concurrence with the proposed modifications.
J We consider this schedule to be acceptable.
2.
Potential effect on suent fuel - he licensee was asked to determine whether a cask can be positioned over or near spent fuel so that a
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cask could drop on the spent fuel or be deflected onto spent fuel.
As analy:ed by CPCo in its July 1,1974 submittal, the. potential exists for fuel damage from a direct drop of the transfer cask or the shipping cask onto fuel asse:blies located in the fuel pool storage racks.
Similar damage could occur as the result of either f3
( m) of these casks if dropped to strike the edge of the spent fuel pool and then be deflected onto the fuel racks.
CPCo proposed relocation of fuel racks and a different cask routing scheme to limit cask movement over the spent fuel pool.
The fuel transfer cask is the only cask. allowed over the fuel racks by new procedures. Rese changes greatly reduce the probability of a cask drop onto the spent fuel. Although the possibility exists for the transfer cask dropping on the spent fuel stored in the pool, the probability is very low.
In addition, safety slings added to the transfer cask as a result of a previous cask drop analysis will prevent dropping the cr.sk if a crane failure should occur.
Even so, we have calculated the potential magnitude of offsite radiation doses for the accident. We assumed that 28 fuel assemblies (1/3 of the core) in the spent fuel pool were damaged by a cask drop accident.
Regulatory Guide 1 6,ological assumptions and the models given in Based on the meteor. 5, we concluded that the site boundary doses at Big Rock Point would be within the guidelines of 10 CFR Part 100 for a design basis cask drop accident.
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he site bounda2y doses would be within acceptable limits with the containment ventilation system operating. CPCo advised in its January 22, 1975 letter that procedures would require operator action to shut the isolation valves on receipt of a containment area high radiation alarm in the control room.
However, to provide an additional level of safety, the need for operator action will be eliminated by installation of an automatic contain=ent purge system isolation feature.
CPCo has agreed to f
complete these modifications within the same time fr=e as the modifications to the crane and insta11atior cf the impact pad./
We have reviewed the proposed relocation of fuel racks within the pool and the proposed restrictions on cask movements over the pool Both proposals will reduce the probability of a cask drop on the spent fuel and are acceptable 3.
Potential effect on critical systems and eauipment - The licensee was asked to deter =ine whether the casks were moved over syste=s or equip =ent important to safety which could be damaged by a cask drop.
They were asked to consider the capability of the floors to protect equip =ent or syste=s important to safety which are located j
below the floor.
CPCo's analysis showed that certain safety related systems might be dcaged if their existing cask routing and pro-cedures were used and a cask dropped.
For these reasons they pro-posed cask routing changes, crane modifications and procedural changes to minimize the accident potential.
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We reviewed the licensee's July 1,1974 submittal and determined i
that additional information was necessary to co=plete our review.
CPCo was requested to show that the safety margins for the spent fuel pool floor slabs, beams and protective structures for Category I equipment are of the same order of magnitude as required for Category I structures outside of containment.
We also requested more information relating to the energy absorbing impact pads pro-posed for the floor area of the spent fuel pool.
Our review of CPCo's July 1,1974 submittal and the answers provided by their letter of August 25, 1975, in response to our questions is complete.
We find that the design criteria and design methods used are those of the Bechtel Topical Report " Design of Structures for Missile Impact" BC-TOP-9-A, Revision 2,1974 previously reviewed and approved by NRC.
The design of the energy absorbing impact pad. is acceptable.
Based on the considerations discussed above we conclude that the potential effect of cask drop accidents on critical systems and equipmt;nt is minimal and acceptable.
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Integrity of handling eculement for the scent fuel cask - ne licensee was asxed to evaluate the existing crane facilities as
.they relate to fuel cask handling and movement.
ne CPCo's submittal of July 1,1974, preposed codifications to upgrade the cranes with regard to brakes, li=it switches, ther:::a1 over-loads and redundant controls.
Sey proposed to rigidly control travel paths and handling procedures which would restrict cask and crane move =ents.
In addition, CPCo has a program of opera-tional tests and inspections of the cranes and associated handling equipment..This program will give added assurance that the fuel handling equipment is reliable.
We conclude that the preposed modifications and program of opera-tional tests and inspections provide adequate assurance of the integrity of the handling equipment.
CONC 1.USION Based on our review of the analysis, the descriptive information pro-vided and CPCo's response to our questions, we have concluded that the provisions for preventing a postulated fuel transfer cask or spent fuel shipping cask accident at the Big Rock Point Plant are acceptable when the proposed changes are co=pleted.
Date: February 6,1976 f.
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