ML19340A922
| ML19340A922 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 02/01/1963 |
| From: | Bryan R US ATOMIC ENERGY COMMISSION (AEC) |
| To: | |
| Shared Package | |
| ML19340A921 | List: |
| References | |
| NUDOCS 8009080621 | |
| Download: ML19340A922 (3) | |
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I HAZARDS ANALYSIS BY THE RESEARCH AND POWER REACTOR SAFETY BRANCH IN THE MATTER OF COMMONWEALTH EDISON COMPANY PROPOSED CHANGE NO. 2 Introduction The Commonwealth Edison Company has requested, by letter dated November 15, 1962, a relaxation of the requirements in the technical specifica-tions appended to License No. DPR-2, as amended,which pertain to a
" cocked rod criterion" as applied to the removal of irradiated fuel from the reactor to the fuel storage pool and to the temporary removal and subsequent replacement of a single bundle of irradiated fuel in the reactor for testing purposes.
This change, designated as Proposed Change No. 2, has been requested to allow operations involving the removal or replacement of fuel bundles during the current refueling outage when the control rod drive mechaniscs have been deactivated and the core is being unloaded.
3 Evaluation During refueling operations the core is to be unloaded and a number of
'" maintenance" operations performed. These include inspection of the fuel bundles that are suspected of leaking, and inspections of the lower core grid support, the control blades, and the control rod drive mechanisms.
In addition, a minimum critical will be constructed in part of the core for physics testing. Thus, while these inspections are being performed a large part of the core will bt unicaded or contain only scattered ele-ments. At such times all control blades, except one that may be under examination, will be fully inserted in the core.
Since refueling repre-sents a time relatively late in core life, the reactor is expected to be well shutdewn.
The " cocked rod criterion" expressed in the Technical Specifications, namely, that one rod shall be cocked and available for rapid shutdown if required, is in some instances very desirable from a safety standpoint.
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Having such a control rod readily available for use provides a means of rapidly reducing reactivity and rendering the reactor suberitical-in event that some movement of core components or o,ther changes in the core should cause the reactor to accidentally attain criticality. The action of a cocked rod would, for example, terminate a transient in neutron flux in a reactor which accidentally attains supercriticality during the insertion of a fuel element into the reactor.
A prerequisite of effectiva protection through the use of cocked rods is the presence of nuclear instrumentation of suitable sensitivity and design to actuate a scram.
It is an implicit requirement of the pre-sent Technical Specifications that such instrumentation be operative whenever a rod is cocked.
It must be recognized, however, that the cocked rod procedure is completely effective only in those circumstances in which the accidental addition of reactivity is limited in magnitude to the worth of the cocked rod.
The antithesis of the cocked rod procedure can also be shown to be advantageous in some circumstances. Allowing a rod to remain in its uncocked inserted position provides, without further action, all of the shutdown capability attainable by scramming the rod, had it been previously cocked.
In addition to having superior reliability because of its passive nature, the uncocked rod would prevent the accidental attainment of criticality, while the cocked rod only terminates the accident after it occurs.
In some cases such accidental criticality could be injurious to personnel even though it is terminated almost immediately.
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A weak point in the uncocked rod procedure 'is its dependence on the I
operator having previously accurately observed neutron multiplication and subsequently making only those changes in the core which would not be expected to allow a close approach to criticality.
(All planned
. changes in core configuration which would be expected to enhance re-
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activity to a significant degree, or which would result 1n a new con-figuration, are generally made by a critical experiment type of procedure.)
i-Furthermore, if criticality were accidentally achieved with no rods cocked, the operator would have to reduce reactivity by reversing the procedure by which criticality was attained, which could be a slow process.
While Co=monwealth Edison in proposing the change in procedure does not indicate why it is believed that the proposed procedure is better than the original procedure, we nevertheless have concluded that cocking of rods for the operations described is not necessary for safety.
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be recognized, however, that these and all operations involving the man-ipulation of core components in such a way so as to enhance reactivity must be safeguarded by effective surveillance of neutron multiplications through nuclear instrumentation, unless the reactivity increase can be predicted with the utmost confidence.
We believe, therefore, that, provided the core manipulations are restricted to only those involving the removal and re-placement of single elements in their original locations, the proposed movement of fuel may be safely i.
conducted without a cocked rod.
Existing requirements of the Technical Specification, provide for adequate instrumentation for the proposed operation.
Conclusions Based upon our review of the request submitted, we have determined that operation of the reactor in accordance with the changes generally pro-posed by Commonwealth Edison does not involve significant hazards con-siderations not described or implicit in the hazards summary report as amended and that there is reasonable assurance that the health and safety of the public will not be endangered, provided that the Technical Specifications are modified as indicated herein.
Accordingly, we believe that the cocked rod criterion in section D.5.b of the Technical Specifications dated August 6, 1962, to License No. DPR-2 as amended, should be revised as follows:
"' Cocked Rod' Criterion: The. reactor must be sub :ritical by at least 0.01a k with at least one control rod fully withdrawn in the region of the alteration and available for rapid scram in-sertion. Exceptions to this requirement may be taken for re-moval of irrac.iated fuel to the fuel storage pool during refuel-ing and for teuporary removal and subsequent re-placement in the core location from which it came of a single bundle of irradiated fuel for testing purposes; provided that, while such a fuel bun-die is out of the core, no manipulation or movement of other com-ponents or equipment in or near the core shall be made, nor shall the state of the reactor be otherwise changed in such a way that reactivity would be increased."
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Robert h. Bryan, Chief Research & Power Reactor Safety Branch Division of Licensing and Regulation Date:
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