Originally, ORNL designed the Aircraft Reactor Experiment as a Na-cooled, BeO-moderated, solid UO₂ fueled reactor. At the high power densities required to propel aircraft, they realized there would be a serious positive temperature reactivity effect caused by the Xenon-135 capture cross section. - ThreadSky

whatisnuclear.com • 31 days ago

Originally, ORNL designed the Aircraft Reactor Experiment as a Na-cooled, BeO-moderated, solid UO₂ fueled reactor. At the high power densities required to propel aircraft, they realized there would be a serious positive temperature reactivity effect caused by the Xenon-135 capture cross section.

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whatisnuclear.com•31 days ago

To operate in this condition, they'd need a fast-acting, reliable automated control system. Instead, they decided to switch the design to tubes of stagnant, non-circulating fluoride molten salt fuel. The expansion of the fluid fuel would overcome the effect from Xe, so the reactor would be stable.

whatisnuclear.com•31 days ago

Further analysis showed that stagnant salt would have a radial temperature gradient of a few hundred degrees: the peak temp would be close to the boiling point of the fuel! So they switched it over to the circulating fluid fuel system that we now know as the world's first molten salt reactor.

whatisnuclear.com•31 days ago

So it was molten salt + sodium cooled, both of which transferred heat to helium, which then went to water. Talk about advanced! It operated for 4 days and leaked a lot of fission products, but overall we learned a ton and progressed our ability to run interesting reactors.

whatisnuclear.com•31 days ago

I had to think about the xenon-related positive temperature coef of reactivity thing for a sec. After plotting these thermal neutron energy distributions vs. average temperature and the cross section, you can clearly see the issue.

whatisnuclear.com•31 days ago

At high power density there is A LOT of xenon during operation, and at high temperature, the hot tail of the thermal neutrons starts falling off this cliff-edge cross section.

whatisnuclear.com•31 days ago

If temperature increases in that condition, more neutrons move to the right, significantly reducing the number of them that get absorbed in Xe, and therefore increasing overall reactivity. Crazy!

philtonium.bsky.social•31 days ago

What are the rough dimensions of the reactor?

Impressed with the creativity of the engineers. Wonder how long it would take the NRC to license a reactor like this.

whatisnuclear.com•31 days ago

Here it is after its 4 days of operation on a truck on the way to the dump. some more info at https://en.wikipedia.org/wiki/Aircraft_Reactor_Experiment

ilovescience.bsky.social•30 days ago

This is interesting. Do you happen to have a reference for this?

whatisnuclear.com•30 days ago

Yes , everything I said is in the first few paragraphs of https://doi.org/10.13182%2FNSE57-A35495

ilovescience.bsky.social•30 days ago

Oh, thank you, thank you, thank you

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