ML20151A663
| ML20151A663 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 01/03/1962 |
| From: | Coe R YANKEE ATOMIC ELECTRIC CO. |
| To: | US ATOMIC ENERGY COMMISSION (AEC) |
| Shared Package | |
| ML20151A662 | List: |
| References | |
| NUDOCS 8011030827 | |
| Download: ML20151A663 (3) | |
Text
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YANKEE ATOMIC ELECTRIC COMPANY
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Y 441 STUART STREET, BOSTON 16, MASSACHUSETTS January 3, 1962 U.S. Atomic Energy Commission Washington 25, D. C.
Attention: Division of Licensing and Regulation
Reference:
License No. DPR-3 (Docket No. 50-29)
Dear Sirst We have a proble.m in connection with the design and fabrication of Core III for the Yankee reactor on which we need Commission guidance and advice.
It concer.ns the use of a core having somewhot higher enrichment than at present and the use of soluble poison in the main coolant ~'during the early part of core life to offset the increased reactivitv. *here would be e
no change in the mechanical design of the fuel assemblies.
The initial core for the Yankee reactor is of the single region type"having 76 fuel assemblies, of stainless steel clad, uranium dioxide fuel elements, uniformly enriched to 3.4%.
Core II, because of the lead time required for its fabrication, was ordered. in November 1960 before any data was available from power operation of Core I.
Because the decision had to be made at such an early date, Core II _is an exact duplicate of Core I.
The life of these early cores depends on the amount of excess reactivity that can be provided in the new clean core, and this amount is limited by the ability of the control system to shut down the reactor by a safe margin at the beginning of core life.
In the case of Cores I and II, the 24 control rods when fully inserted provide a shutdown margin of approx-imately 3.5% at operating temperature and, supplemented by the addition of 1150 ppm of boron as boric acid in the main coolant, provide a shutdown margin of approximate 1y'5% at room temperature. The expected minimum life of Cores I and II is 8,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> at 392 iMt with a net electrical production per core of 920 Million KWH.
In an effort to substantially lengthen core life and thus lower fuel costs, we should like to work toward a fuel cycle, starting with Core III, which would involve cyclic leading of 2 region cores, adding half cores of approximately 4.1% enrichment at each refueling according to an "out-in" loading scheme. With such a fuel cycle, we estimate a nominal life of 14,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> at 455 JGt for a complete core (2 cycles) which is equivalent to a net electrical prcduction of approximately 2 billion KWH. The effect of this greatly increated production per core on fuel cycle costs is obvious.
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--2 U.S. Atomic Energy Commission Attn.: Div. of Lic, and Ron.-
January 3, 1962 To attain this steady state fuel cycle, we shall have to go through a transition situation, starting with a partial loading of Core III at the cocond refueling of the. reactor, tentatively scheduled for the first quarter of 1963. The initial excess reactivity of a 2 region core, composed of 36 new assemblies of 4.1% enrichment in the outer region of the core, and 40 of.the least irradiated fuel assemblies from Core II relocated' to the inner region of the core, would be about 2.5% higher ~ than is the' case for Cores I and II. This'is roughly equal to the reactivity worth of equil-ibrium xenon af ter the first 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> of power operation. Some additional control would have to be provided during this period to ensure having an adequate hot shutdown margin.- This requirement'could be met by leaving boron, having.the.same reactivity worth as equilibrium xenon, in the main coolant for the first 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> of power operation.
It was for this reason that we sought and obtained AEC auth'oriza-tion last September to make a test of reactor operation.at power with boron in the main coolant. The test was conducted during a one week period, Sep-tember 13 to 20, and a report on the test was submitted as an Appendix to Operation Report No. 10 for the, month of October, 1961.
l A principal concern regarding the use of boron in.the coolant of water reactors at power has.been the possibility'that due to the action of-the neutron flux, boron representing sizeable amounts of reactivity would be depoeited or plated out in the core, and then could be suddenly released.
One of the purposes of the test performed by Yankee in September was to de-termine if any such ef fects did in fact occur,and whether or not they could s
be detected. The results of the test were not wh'olly conclusive on this point.
Unexplained reactivity losses did occur during the test and subsequently were largely regained. On the other hand similar effects have been noted both be-fore and after the test when no boron was present in the main coolant.
Our conclusions with respect to the boron test are stated in Section 4.2 of the report.
In our opinion, reactivity variations such as those experienced during and after the test, even if attributed to boron deposition and release, were of such small magnitude and occurred at such slow rates that they represent no.significant operating problem.
It is our-view that the results of the test are sufficient to demonstrate that no safety problems of consequence.tre involved in the use of boric acid during operation at power.
A commitment has already been made for the design and fabrication of Core III. A complete core of 76 fuel essemblies has been ordered, although, as already mentioned, our plan is to load only one half of these assembliesin the outer regions of the core.
If, for any reason, it appears to be undesirable to subject Core II i
fuel assemblies to another term of residence in the reactor, we would plan to load an entire new core of this higher enrichment, and reduce its initial ex-cess reactivity to approximately the same value as for the 2 region core by
lx U.S. Atomic Energy Commission Attn.: Div. of Lic. and Reg.
January 3, 1962 the use of fixed absorber shim rodo in the eight outer positions of the core.
In either case, we would need to use boron'as described above to hold down the additional excess reactivity until equilibrium xenon is reached and pro-vide the minimum hot shutdown margin of 3% now required by the Technical Specifications under Paragraph D.2.a (3).
Our reason for seeking guidance concerning the use of boron at this time,when the need for its use is at least one year away, is that to meet the delivery schedule of Core III we must set the enrichment by mid-February, 1962. The enrichment that we set, of course, depends on whether or not we will be permitted to use boron in the manner described above. The date mentioned for setting enrichment provides for a period of notice to the AEC for scheduling withdrawals of special nuclear material and also allows for the various intervals of time needed for conversion and pelletization of special nuclea. material, procurement of other materials and fabrication of the core itself.
We realize, of course, that before Core III is loaded we must submit complete design information on the core and make a full safety analysis of its operation. This work is now in progress and the results will be sub-mitted in ample time for the Commission to make the necessary review and safety determination of the modified core. Nevertheless, some assurance by the Commission now that the use of boron will be permitted, provided that othei aspects of the core design are acceptable from the safety standpoint, would be of great assistance to us in going forward with design and fabrica-tion of the core. We therefore request that the staff review this matter and also obtain the advice of the Advisory Committee on Reactor Safeguards who, we understand, are meeting in early February. Our engineers will be happy to meet with the staff or appear before the ACRS as you may suggest.
We should appreciate receiving the Commission's guidance and advice by February lb, 1962 at the latest.
Very trulv yours, YANKEE ATOMIC ELECTRIC COMPANY
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