ML19312D368
| ML19312D368 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 02/29/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19312D364 | List: |
| References | |
| NUDOCS 8003240280 | |
| Download: ML19312D368 (6) | |
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UNITED STATES g
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,a WASHINGTON. D. C. 20655
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO. 61 TO LICENSE N0. DPR-46 NEBRASKA PUBLIC POWER DISTRICT DOCKET NO. 50-298 COOPER NUCLEAR STATI0t{
l.0 Introduction By letter dated January 14, 1980,II) Nebraska Public Power District (the licensee) has requested an amendment to-the Technical Specifi-cations for the Cooper Nuclear Station. The effect of the amendment would be to allow the count rate in the Source Range Manitor (SRM) channels to drop below 3 counts per second (cps) when the entire reactor core is being removed or replaced. The present Technical Specifications require that a count rate of at least 3 cps be main-tained whenever one or more fuel assemblies are present in the core.
2.0 Discussion _
Cooper Technical Specifications require that all control blades be inserted _ into the core during fuel loading. This is no problem during normal refueling and control blade drive maintenance since only one core cell (defined as a control blade plus the four adjacent fuel assemblies).is worked on~at any given time. However, a removal of the entire core would require all the fuel to be removed before any control blade was removed. This is not possible unless the plant has a full complement of control blade guides. These guides are needed to provide lateral support to control blades in defueled cells.
Cooper Technical Specifications, in addition to the control blade insertion requirement, require that the SRM count rates be above 3 cps whenever more than one control blade is removed.
The safety issues are two-fold if control blades are to be removed with fuel still in the core. First, the intermediate fuel and con-strol blade arrays must be subcritical at all times, even if the highest worth blade is withdrawn. Second, there must be adequate monitoring of neutron flux levels during the core alterations.
During any core alteration, and especially during core loading, it is necessary to monitor flux levels.
In this manner, even in the highly unlikely event of multiple operator, errors, there is reason-able assurance that any approach lto criticality would.be detected in time to halt oper'ations.
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. The minimum count rate requirement in the' Technical Specifications accomplishes; three safety functions:
(1) it assures the presence of some' neutrons in the core, (2) it provides assurance. that the analog portion of. the SRM channels is-operable, and (3) it provides assurance that the SRM detectors are close enough to the array of fuel assemblies to monitor core flux levels.
Unloading and reloading of the entire -core leads to some difficulty with this minimum count ~ rate-requirement. When only a small number of assemblies are present within the core, the SRM count rate will drop below the minimum due to the small number of neutrons being produced, and: due to attenuation of these neutrons in the water and control blades separating the fuel from the SRM detectors. Past practice has been to connect temporary " dunking" chambers to the SRM channels in place of the normal detectors, and to locate these detectors near the fuel.
Besides being operationally inconvenient, dunking chembers ' suffer.
from signal variations due to their lack of fixed geometry. More-
.over, the.use of dunking chambers increases the risk of loose objects being dropped into the vessel.
3.0 Evalua' tion 3.1 Subcriticality of the Intemediate Arrays The proposed Technical Specifications would only allow spiral unloading and reloading of the core.
In such a program, only cells on the edge of the array are unloaded or reloaded.
No imbedded cavities or major peripheral concavities would be permitted.
In such a case, the neutron multiplication factor of the intermediate arrays must be less than or equal to that of the fully. loaded core.
-Since the shutdown margin of the fully loaded core is well assured by other specifications,'we find the proposed change to be acceptable from the point of view of shutdown margin.
.By telephone discussion,_the.lic.ensee ha_s. modified his request to l
L include changing Specification 4.10.A.2 to facilitate maintenance The change in Specification 4.10.A.2 allous
- of single control rods.
maintenanc'e.o'f'a single control rod to be performed provided (a) all other rods are ful.ly inserted with ~ their refueling interlocks oper-L
.able and (b) the required shutdown margin of 0.38 percent has been Ldemonstrated for the current core configuration. This restriction l-assures' that no more than one control' rod will be withdrawn during
' single control rod maintenance and that the core will remain sub-critical by a sufficient: margin during such maintenance. We find
.this change,.also, to be acceptable from the point of view of shutdown margin;
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.3.2 Flux Monitoring
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Min'imum Flux in the Core A multiplying medium'with no neutrons present, forms the_ basis for
- an accident; scenario in which reactivity is gradually but inadvertently added. until the-medium is highly supercritical. No neutron flux'will be evident since there are no neutrons present to be multiplied. The introduction of some neutrons at-this point would cause the core to undergo a sudden power burst, rather than a gradual startup, with no warning from the nuclear instrumentation.
This scenario is of great concern when loading fresh fuel, but is of lesser concern for exposed fuel.
Exposed fuel continuously produces neutrons by spontaneous fission of. certain plutonium isotopes, photo-fission, and.photodisintegration of deuterium in the moderator. This neutron production in exposed fuel is nonnally great enough to meet the 3 cps minimum for a full core after a refueling outage with the lumped neutron sources removed.
Thus, there is assurance that a minimum flux level will be present as long as some exposed fuel is present. We therefore find the proposed amendment to be acceptable from the point of v_iew of minimum flux pro-vided the words ". spiral re_ load" are interpr_et.e.d to mean " reload.of i
the core with fuel, at least 50% of which has previously accumulated a minimum exposure of 1000 MWD /T."
We have modified the licensee's Techni_ cal Specification to include this definition ~ in this amendment, and he has agreed to this change. We do not find the amendment to be applicable to the loading of a new core containing only fresh fuel.
-Such a loading must use lumped neutron sources and dunking chambers to meet the normal-3 cps minimum count rate.
3.2.2 SRM Operability The_ Technical Specifications normally require a functional check of the SRM channels, including a check of neutron response, prior to g
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making any alteration to the core and ' daily thereafter. This would f
_.be sufficient for core unloading and reloading, except that the more-extensive fuel handling operations involved imply a greater possibility of _SRM mechanical failure. The Cooper Technical Specifications have -
been modified to increase the functional; check frequency from once/ day to once/12 hours while using the' spiral reload technique-to compensate.
Testing the neutron response is accomplished by using an external source or.by loading two fuel assemblies in different cells contain-ing control blades:around each SRM to obtain the required 3 cps.
This should being the count rate up to 3 cps and thus continuously
._ verify operability.- Therefore, we find this alternative to be T acceptable.c
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,4, 3.2.31 Flux-Attenuation The four SRM detectors are located..one per quadrant, roughly half a core radius from the center. Although these are incore detectors and -thus 'very sensitive when the reactor is fully loaded, they lose some of their effectiveness when the reactor is partially defueled and the detectors are located some distance from the array of remain-
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.ing fuel.
GE's ' spent fuel' pool studies have shown(2) that 16 or more fuel
-assemblies (i.e., four.or more control cells) must be loaded together before criticality is possible. Moreover,'in a spiral reloading, the cell centered on the central blade must.be 'oaded first. Given this, we have examined the Cooper core layout and deter-mined that the worst case configuration fer monitoring purposes consists of four loaded cells,~ centered on blades 22-27, 22-31, 26-27 and 26-31. This array is two control cells (i.e., about two feet) away from the nearest SRM detectors.
(All other arrays con-
-taining 16 or more assemblies are closer to a detector.) We have previous examined the sensitivity loss in such a case on another docket,(
and found it to be at most one decade of sensitivity
~(i.e., about one fif th of the SRMs logarithmic scale). As in Reference 3, we find this to'be acceptable.
4.0 Summary We have-examined the safety issues and found the proposed amendment to be acceptable provided that.the spiral reload consists of 50% -
previously exposed fuel as discussed herein (Section 3.2.1).
5.0 Environmental Considerations,
We have determined that the amendment does not authorize a change in effluent types or. total amounts nor an increase in-power level and will:not result in any significant environmental impact. Having made this determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact, and pursuant to 10 CFR Section 51. 5(d)(4) that an environmental impact statement, or negative declaration and environmental impact appraisal need not be prepared in~ connection with the issuance of the amendnent.
6.0 Conclusion We have concluded, based on the considerations discussed above, that:
- (1) because the amendment does ~not-involve a significant increase in -
the prob' ability or consequences of accidents'previously considered and does not' involve a significant decrease in a safety margin, the amend-mentidoes'not involve a significant hazards consideration, (2) there -
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is ~ reasonable. assurance ' that' the ' health and safety of the public -
will not be endangered by operation in. the proposed manner,cand~
- (3) such activities will be conducted in compliance with the Com-
' mission's regulations and the-issuance-of this-amendment will not-be inimical to the common defense and security or to the health
- and safety of.the public.
Dated:
February 29, 1980 l
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- 6, References 1
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Letter,;J. M.~ Pilant (Nebraska Public Power District) to T. A.
1 Ippolitol.(NRC),datedJanuary 14, 1980.
- 2. - General Electric Standard Safety A1alysis Report,-251-GESSAR, Section 4.3.2.7, pg. 4.3-27.
3.- " Safety Evaluation by the Office of Nuclear Reactor Regulation Support-ing Amendment No. j27 to Facility 0perating License No. DPR-63,"
- Docket No. 50-220, enclosed with. letter, T. A. Ippolito (NRC) to D.
P.Dise(NiagaraMohawkPowerCorporation),.datedMarch2,1979.
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