ML20052A125
| ML20052A125 | |
| Person / Time | |
|---|---|
| Site: | MIT Nuclear Research Reactor |
| Issue date: | 03/29/1982 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20052A118 | List: |
| References | |
| NUDOCS 8204270129 | |
| Download: ML20052A125 (6) | |
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NUCLEAR REGULATORY COMMISSION g
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WASHINGTON, D. C. 20S55 g
SAFETY EVALUATION REPORT FOR MASSACHUSETTS INSTITUTE OF TECHNOLOGY AMENDMENT TO USE THIN CLAD FUEL Background _
In a letter dated March 22, 1982, Massachusetts Institute of Technology [MIT]
submitted a request to amend Section 5.2 of the Technical Specifications entitled " Reactor Core." MIT wishes to use state of the art technology that maintains the cladding as thin as possible while still ensuring its integrity to contain fission products.
MIT refers to a report concerning the extensive experience at the Advanced Testing Reactor facility with the use of this thin cladding fuel. The conclusion reached by the authors of that report is that cladding integrity is ensured if the thickness is maintained above 0.008 inches [0.2mm] [Ref.1].
Cladding Fabrication Experience Authors J. M. Beeston, et. al. in the referenced article " Development and Irradiation Perfomance of Uranium Aluminide Fuels in Test \\eactors" [Ref.1]
indicate that with state of the art fabrication technology and the use of an ultrasonic inspection device called a " min-clad gage", thin claddings can be effectively inspected to ensure minimum cladding cover and to identify spots, blisters or other imperfections that may cause cladding failure.
Fabrication techniques are based upon the well established picture-frame process in use for more than 25 years and modified to accomodate vaporous
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fission products and to minimize swelling and dogboning.
Over 1700 plate-type uranium aluminide fuel elements have been operated in the INEL test reactors [ATR, ETR, MTR] since 1970. Favorable burnups of 2.3x10-21 fiss/cm3 of core meat are now utilized with fuels that have passed inspections using the " min-clad gage". MIT burnup is limited by 2
the Technical Specifications to 2x10 0 fiss/cm.
Growth and Swelling The authors indicate from extensive data that swelling is proportional to the fission density and that less than 0.5% change in volume is expected 20 fiss/cm (i.e., MIT Tech Spec for a fission density of less than 2x10 limit).
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Axial Buckling
" Buckling due to axial compressive loads developed from themal stresses or irradiation growth stresses has not been detected in the ATR fuel elements or plates." 1f Release of Gas Because of the thinness of the clad, the authors investigated diffusion in the fuel with increased irradiation. They concluded that irradiation does not increase the diffusion of fission gases in the fuel materials.
Fission Product Release The MIT reactor utilizes a helium sweep over the @0 surface to inhibit D 0 2
contamination and for a Q0 vapor sweep to the ho recombiner. The sweep gas flows past a continuous monitor which monitors radiation levels. Nomal readings over the past year have been in the range of 1% - 5". of MPC. Changes from this value are investigated by additional sampling of the helium sweep line.
Operation of this helium activity monitor, was dramatically illustrated during the fuel element failure event that occurred June 22, 1979. [
Reference:
Preliminary Notification of Event PNO-79-188 8 MIT letter report of Fuel Element Failure dated July 2,1979]. The monitor readings jumped to approximately 75% of MPC. The reactor was shutdown and the suspect element was identified and removed. MIT procedure is to shutdown the reactor if readings of this monitor indicate radiation levels greater than 10% of MPC.
Conclusion It is the staff's opinion that the newly designed elements use state-of-the art technology and quality control and that the design of these elements have been proven by extensive use in the ATR facility.
In addition, the MIT failed fuel element detection instrumentation, con-tainment, ventilation and filtration system are an adequate backup in event of a fuel failure.
The staff concludes that the new fuel clad design can be safely used in the MIT reactor. Accordingly, we have concluded, based on the considerations discussed above that:
(1) because the amendment does not involve a significant increase in the probability or consequences of accidents previously considered and does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission'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.
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. Reference 1/ Beeston, J. M., R. R. Hobbins, G. W. Gibson, and W. F. Francis, " Development and Irradiation Performance of Uranium Aluminide Fuels in Test Reactors",
Nuclear Technology, Vol. 49 p.136-149 (June 1980).
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Dated: March 29,1982
ATTACHMENT i
TECHNICAL SPECIFICATION AMENDMENT 5.2 Reactor Core Applicability This specification applies to the design of the reactor core.
Objective To assure compatibility of the reactor core with the present safety analysis.
Specifications 1.
The reactor core may consist of up to 27 fuel elements approxi-mately 2-3/8" on a side. The fuel shall be plates of uranium in the form of VA1 alloy or UA1x with a maximum of 50 w/o uranium in the fuel matrix clad by a layer of aluminun metal incorpora' ting fins on the surface that enhance the heat transfer and having a nominal clad thickness not,less than 0.008 inches at the base of the groove between the fins.
2.
Design of in-core sample assemblies shall conform to the following criteria:
a.
they shall be positively secured in the core to prevent movement during reactor operation.
b.
materials of construction shall be radiation resistant and compatible with those used in the reactor core and primary coolant system.
c.
sufficient cooling shall be provided to insure structural integrity of the assembly and to preclude any boiling of the primary coolant.
d.
the size of the irradiation thimble shall be less than 16 square inches in cross section.
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Basis The thermal design analysis in the SAR and the power distributions on which the analysis was based assumed fuel elements of the type specified in item 1.
Any change in this design would require re-evaluation of the heat transfer and flow characteristics of the element.
The nominal clad thickness of 0.008 is based upon standard practice for MTR type elements with clad of similar thickness. Reference 5.2-1 states (p.148) that the release of radioactive fission gas to the primary cooling water appears to be adequately prevented by cladding of uniform
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thickness (>0.2mm, >0.008 in). This corresponds to the minimum nominal cladding permitted for ATR fuel, where the term " uniform" implies a provision for minor manufacturing deviations (e.g., scratches, identations, etc.) from the uniform thickness. Ref. 5.2-2, (p. 672) shows that a thick clad increases the delay time for heat removal in event of a fast transient. Therefore, the clad should be as thin as possible while still remaining compatible with fission product retention requirements.
In-core sample assemblies which satisfy Specification 2a cannot be credibly ejected during operation and are therefore considered part of the reactor structure.
Specification 2b and 2c insure the structural integrity of the assembly and prevent chemical interactions with the core and primary coolant system.
Specification 2b limits the size of the irradiation area as required by 10 CFR 50.2(r).
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. References Ref. 5.2-1 Beeston, J. M., R. R. Hobbins, G. W. Gibson, and W. F. Francis
" Development and Irradiation Perfonnance of Uranium Aluminide i
Fuels in Test Reactors", Nuclear Technology, Vol. 49 p.136-149 (June 1980).
Ref. 5.2-2 Thompson, T. J., and J. G. Beckerly (eds.) The Technology of Nuclear Reactor Safety, Vol. I, the MIT Press, Camb. MA (1964).
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Dated: March 29,1982
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