ML20005D713
| ML20005D713 | |
| Person / Time | |
|---|---|
| Site: | Washington State University |
| Issue date: | 09/06/1989 |
| From: | WASHINGTON UNIV., ST. LOUIS, MO |
| To: | |
| Shared Package | |
| ML20005D712 | List: |
| References | |
| NUDOCS 8912140320 | |
| Download: ML20005D713 (1) | |
Text
e 3.3 Pulse Mode Ooeration Aeolicability: This specification applies to the peak fuel temperature in thc reactor as a result of a pulse insertion of reactivity.
Obiective: The objective is to ensure that fuel element damage does not occur in any fuel rod during pulsing.
Soecification: The maximum reactivity inserted during pulse mode operation shall be such that the, peak fuel temperature in any fuel rod in the core does not exceed 830 0C. ,
safe allowable reactivity insertion shall be The maximum calculated annually for an existing core and prior to pulsing a new or modified core arrangement.
Basis: TRIGA fuel is fabricated with a nominal hydrogen to zirconium ratio of 1.6 for FLIP fuel and 1.65 for Standard.
This yields delta phase zirconium hydride which has a high creep strength and undergoes no phase changes at temperatures over 1000 0C. However, after extensive steady state operation at 1 Mw, the nydrogen will redistribute due to migration from the central high temperature regions of the fuel to the cooler outer regions. When the fuel is pulsed, the instantaneous temperature distribution is such that the highest values occur at the surface of the element and the lowest values occur at the center. The higher temperatures in the outer regions occur in fuel with a hydrogen to zirconium ratio that has now substantially increased above the nominal value. This produces hydrogen gas pressures considerably in excess of that expected for ZrH 1 If the pulse insertion is such that the temperature of tIie.6 fuel exceeds 874 C, then the pressure will be sufficient to cause expansion of microscopic holes in the fuel that grow larger with each pulse. The expansion of the fuel stresses turn, and distorts the fuel rod material which, can cause in overall swelling and distortion of the cladding and entire fuel rod. The pulsing limit of 830 0 C is obtained by examining the equilibrium hydrogen pressure of zirconium hydride as a function 0
of0 temperature. The decrease in temperature from 874 C to 830 C reduces hydrogen pressure by a factor of two, which provides an acceptable safety factor. This phenomenon does not alter the steady state safety limit since the total hydrogen in a fuel element does not change. Thus, the pressure exerted on the clad will not be significantly affected by the distribution of hydrogen within the element.
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