ML20079R995
| ML20079R995 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 01/16/1984 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20079R984 | List: |
| References | |
| NUDOCS 8402030121 | |
| Download: ML20079R995 (4) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMEN 0 MENT N0. 20 TO FACILITY OPERATING L:CEhSE N0. DPR-22 NORTHERN STATES POWER C0fiPANY MONTICELLO NUCLEAR GENERATING PLANT DOCKET N0. 50-263 1.0 Introduction By letter dated September 30, 1982, Northern States Power Company (the licensee) proposed changes to the Technical Specifications (TS) of Facility.0perating License No. DPR-22 for the Monticello Nuclear Generating Plant.
The revisions-minimum count rate to fall below three counts per second (ge Monitors (SRM) to the Technical Specifications would allow the Source Ran cps), during full core discharge and subsequent reloading.
The SRMs monitor the core during periods of station shutdown and guide the operator during refueling operations and station startup.
Requiring a, minimum of 3 cps whenever criticality is possible provides assurance thct neutron flux is being monitored.
The licensee has proposed this change to allow full unloading of the fuel from the core.
In the process of removing all the fuel from the core, the count rate on the SRMs will drop below 3 cps, without supplemental neutron sources.
The proposed amendment is supported by a 3eptember 30, 1982 letter from General Electric, the reactor vendor.
2.0 Evaluation The proposed changes to tha TS would allow the count rate in the SRM channels to drop below 3 counts per second (cps) when both of the following conditions are met:
1.
No more than two fuel assemblies are present in the core quadrant associated with the SRM; and 2.
While in the core, these fuei assemblies are in locations adjacent to the SRM.
The present TS require that a count rate of at iaast 3 cps be maintained whenever one or more fuel assemblies are present in the core.
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 reasonable assurance that any approach to criticality would be detected in time to halt operations.
8402030121 840116 PDR ADOCK 05000263 P
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 minimnm due to the small number of neutrons being produced, and du.e 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" charbers to the SRM channels in place of the normal detectors, and to locate these detectors near the fuel.
Besides being operationally inconvenient, dunking chambers suffer from signal variations because of their lack of fixed geometry.
Moreover, the use of dunking chambers increases the risk of loose objects being dropped into the vessel.
The staff has reviewed the licensee's application and the associated letter from General Electric.
In a September 27, 1983 letter to the licensee, the staff requested additional information.
In a November 4,1983 letter, the licensee addressed the staff's concerns and provided information about the core and methods to be used in unloading and reloading the fuel intG the Core.
Subcriticality of the Intermediate Arrays The proposed Technical Specification would allow a modified spiral unloading and loading of the cure.
However, the 3 cps will be maintained unless the two previously stated conditions are met.
General Electric has done calculations that show that reactivity of the core decreases as each cell is removed.
The particular analysis was done for another reactor.
General Electric has stated that the analysis is applicable to Monticello.
Flux Monitoring Minimum 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, photofission and photo-disintegration of deuterium in the moderator.
This neutron production in exposed fuel is normally great enuugh to meet the 3 cps minimum for a full i
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. core after a refueling outage with the lumped neutron sources removed.
There is assurance that a sufficient flux level will be present as long as some exposed fuel is present. The proposed Technical Specification would require that.two diagonally adjacent fuel assemblies, be loaded into core positions next to each of the SRMs to provide a minimum of 3 cps before loading any other fuel. We therefore, find the proposed change to be acceptable from the point of view of minimum flux.
SRM Operability The Technical Specifications normally reouire a functional check of the SRM channels, including a check of neutron response, before making any alteration to the core and daily thereafter.
This would be sufficient for core unloading and reloading, except that the more extensive fuel handling operations involved imply a greater possibility of SRM failure.
The licensee has committed to load two assemblies diagonally adjacent to each SRM location before loading any other fuel. This should bring the count rate up to 3 cps and thus continucusly verify operability. We find this alternative to be acceptable.
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 remaining fuel. The unloading and loading pattern proposed by.the licensee is a spiral unloading or loading centered around one of the SRMs.
This approach will assure that there will be adequate detector sensitivity throughout core unloading and loading.
Summary We have examined the safety issues and found the proposed changes to the
. Technical Specifications to be acceptable.
3.0 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 insigni-ficant from the standpoint of environmental impact and pursuant to 10 CFR 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 amendment.
. 4.0 Conclusions 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 the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
M. Chatterton Dated:
January 16, 1984 e
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