Comment by dtgriscom | Hacker News Reader

Comment by dtgriscom | Hacker News Reader

OK, HN physicists: how did this work?

My first guess was that the beam of Cf252-emitted neutrons, when it hits the U235, triggers new neutrons moving in the same direction, rather than in random directions. This would ensure that any tertiary neutrons would join the crowd and help the amplification while not just heating the system up.

Or, maybe that's the point? It's a not-quite-critical collection of U235 that is pushed even closer to criticality by the Cf252, multiplying the Cf232's neutron flux by "up to 30 times". But, if the U235 neutrons trigger the same emissions as the Cf252 neutrons, then wouldn't that require a razor's edge of criticality?

https://en.wikipedia.org/wiki/Californium_neutron_flux_multi...

It works just like a nuclear reactor. Neutron hit U atoms, those split and emit more neutrons in a cascade. The configuration is set up to not be self sustaining. It’s a matter of geometry, enrichment level and such. Therefore it needs a source of neutrons. Cf-252 is such source. It fires neutrons constantly but has a relatively short half life (you have to replenish it yearly or so).

> then wouldn't that require a razor's edge of criticality?

Yup. From the device description in its decommissioning plan:

  The CFX was a sub-critical assembly of uranium-2-35 surrounding a Cf-252 source. The function of the U-235 fuel was to multiply the neutrons coming from the Cf-252 source, which fissions spontaneously. The CFX was designed never to exceed a Keff of 0.99. The CFX assembly yielded sufficient neutron fluxes for applications such as neutron activation analysis.

Keff is the fission neutron multiplication ratio; 1 is criticality.

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