Sunday, October 23, 2005

Improved Nuclear Fuel

Today's nuclear power reactors overwhelmingly use fuels based on uranium oxide. It's a technology that works well, but it could use improvement. Power density is limited by thermal conductivity of the oxide -- if the power is too high, the temperature at the center of the UO2 fuel pellets becomes too high, and they can be damaged or even melt. This also affects fuel performance in accidents, making it easier to keep the pellets from being damaged when cooling is impaired.

Now, researchers at Purdue have come up with an improved oxide fuel with thermal conductivity 50% higher than before. The trick is to make a ceramic composite in which the uranium oxide is mixed with beryllium oxide, which has a thermal conductivity six times that of UO2 at 1000 C. The BeO forms a network that efficiently carries heat out of the pellet.

Burnup in oxide fuels is limited by degradation of thermal conductivity with time. If the separate BeO phase does not suffer the same degradation (and remember, the fission isn't occuring in that phase), then it's possible that fuel could be left in a reactor for much longer times before being replaced, reducing both cost and waste volume.


Blogger Iain McClatchie said...

I thought nuclear fuel was burned to (nearly) zero excess reactivity. Once there is no excess reactivity, it won't support a chain reaction, and they take it out.

The obvious thing to do to get higher burnup is to put the zero excess reactivity fuel around the reactor core periphery. Even with a reactivity of 0.9, 1 neutron leads to an average of 10 fissions before the chain dies out. At the limit, you ought to be able to burn fuel down to something like 0.6 to 0.7 reactivity, about twice the burnup possible if you stop at 1.0. Since this is obvious, I expected that it was current practice, but I haven't been able to verify that.

It also seems to me that fuel pellet conductivity limits the power density of the reactor, but not necessarily the burnup. Is the cost of a reactor strongly tied to the core volume? If not, just make the core bigger for a fixed thermal output, and that will allow the fuel pellets' thermal resistance to rise.

3:50 PM  
Blogger Seo Link Master said...

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9:08 PM  

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