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PostPosted: Aug 12, 2009 10:37 pm 
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Joined: May 14, 2009 12:21 am
Posts: 41
Location: southwest corner of the US of A
It is interesting that we can change k and dk/dT with the graphite to fuel ratio. We are depending on the relative expansion of fuel vs the expansion of moderator to provide a stable (negative) k. That bothers me a bit. I'd like to operate the core at the peak of the fuel/moderator ratio. My reasoning is that a mechanical core upset would necessarily decrease reactivity if the original fuel/moderator configuration is optimized. This raises a concern since the optimized configuration does not have the most negative dk/dT. We are trading one type of stability for another. A core configuration that is under fueled (over moderated) would have a more negative dk/dT as the fuel expands (faster than the moderator). Unfortunately, being over moderated allows a local increase in reactivity if a fuel channel becomes larger while another channel becomes smaller as the core shifts. Doppler broadening has to provide the -dk/dT if the fuel/moderator ratio is optimized.

Is this an accurate description of the trade off, or am I missing a key factor. Perhaps we would sacrifice local absolute thermo-mechanical stability for a more negative dk/dT over the entire core.

Thanks in advance for your thoughts.
Rob

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