ML20032C611
| ML20032C611 | |
| Person / Time | |
|---|---|
| Site: | 05000139 |
| Issue date: | 11/02/1981 |
| From: | Chalk W WASHINGTON, UNIV. OF, SEATTLE, WA |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8111100644 | |
| Download: ML20032C611 (2) | |
Text
t UNIVERSITY OF WASHINGTON SEATTLE, WASilINGTON 98195 College of Engineering Department of Nuclear Engineering Nuclear Reactor Laboratory, FD-10 2 November 1981 Director g
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Division of Reactor Licensing e.
U.S. Nuclear Regulatory Commission k
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Washington, D.C.
20555 g
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Dear Sir:
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Re: Docket No. 50-139
,f License R-73
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.9 In accordance with Section K:1 of the Technical (Specificat!ons for the
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University of Washington Nuclear Reactor this letter is being transmitted.
Section K:1 requires a telephone call within one working day to Region V office and a written report in ten days to the Director of Reactor Licensing.
This item is for abnormal occurrences as defined in Section J:3 and specifi-cally J:3:b, an uncontrolled or unanticipated change in reactivity.
As defined in J:3:b we may have an unanticipated reactivity change al-though the explanation seems logical.
In summary, during September and October of this year we changed the fuel-box gaskets. This required removal of the fuel, core graphite and the fuel boxes. During reassembly the fuel elements were returned to the same boxes and positions they occupied originally.
On October 23, 1981 the reactor was taken critical and the excess re-activity was calculated to be 1.052% Ak/k. On October 27 and 29 the calcu-lated excess reactivities were 1.046% Ak/k and 1.055% Ak/k. These values were then compared with original measurements and conditions (no samples).
On August 18 and 21 the excess reactivity was 1.182% Ak/k and 1.64% Ak/k.
The net changc as a result of the unloading and reloading the fuel is there-fore approximately -0.12% Ak/k.
Earlier experiments show that a change in the small gap between fuel bundles can cause a change in reactivity. Data taken during tbe original start-up of the reactor in April 1961 shows a reactivity difference between,
- 1. all fuel bundles wedged toward the central graphite island (minimum spacing between bundles) and 2. the maximum gap between fuel bundles to be 0.54% Ak/k.
In the U. of W. reactor, fuel bundles are separated at the top by an aluminum wedge which maintains maximum separation in one direction at the top of the fuel box.
However, there is no positive separation at the bottom except the QO bolts in the bundles which give a minimum separation of 0.25 inch between Offljf2
%~ con Hall, BF-10 / Telephone: (206) 543-2751 PDg
Director Division of Reactor Licensing 2 November 1981 Page Two.
plates on adjacent bundles.
The maximum separation between outside plates from adjacent bundles could be 0.55 inch.
In the other direction the fuel bundles are separated by a plate support at the bottom of the bundles.
With the wedges in place at the top of the bundles, the bundles are forced to the edge of the box and maintained there. This prevents the bundles from movement within the boxes. From prior operating experience there is no evidence to indicate any movement of the bundles once the wedges at the top are in place.
There are other possibilities to explain this reactivity change. One is that there could be some water in the graphite, which we do not believe is the case. The second is a neutron absorber in the primary water and we do not think this is the case either. The primary water was retained in the dump 6
tank and the specific resistivity is about 10 ohm-cm (same as prior operating condition).
The change in reactivity can be accounted for by the possible difference in the positioning of the fuel bundles at the bottom in that they may be closer together in some cases than before. We believe that this is the reason for the unanticipated reactivity change and feel that our operations will be essentially unaffected by the small change.
Another observation should be mentioned. As a result of the water spill in December 1979 we found some corrosion on the outside of the fuel boxes be-tween the aluminum boxes and graphite moderator interfaces where water made contact. This corrosion is the result of a water-graphite-aluminum reaction.
Some minor pitting was observed on the outside of the boxes during cleaning; however, none of the boxes showed any evidence of leaking during a static-head leak-test prior to re-installation.
l We called Region V on October 23, 1981 and will be meeting soon with the l
University of Washington Safety Advisory Committee.
l Sincerely yours, l
b 44&&
I W.S. Chalk Director I
l WSC:mt cc: Region V Safety Advisory Comnittee i
l
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