Kirk Sorensen wrote:
macpacheco wrote:
Then how is it that any reactor can attain criticality on natural uranium ?
They use moderators such as heavy water or graphite to slow their neutrons to thermal equilibrium with their surroundings, while avoiding much absorption in the moderator. In this way they are able to achieve criticality with the low fissile content of natural uranium.
Kirk, I understood 100% of what you said, in the level you explained.
But when I think about microscopic and macroscopic cross sections, it doesn't add up.
I watched through all of your videos, specially the whole protospace video at least 3x.
How about a thought experiment.
If we had a 1Km radius vat of natural uranium+salt coolant.
Neutron losses would be essentially irrelevant even for fast neutrons due to the ultra long distances neutrons would have to travel before they're lost.
Nearly every neutron would either find a fissile or fertile to hit.
If U235 fission produces 2.4 neutrons on average, there are enough neutrons to promote U238 to Pu239, fission more U235 and still some left.
Maybe the problem is too many neutrons would breed Pu239 and not enough U235 fission would happen, due to cross section differences between fast and thermal fission. So the reactor never starts up. But unless the proportion of breed/U235 fission is the same, then a thermal natural enrichment reactor would be impossible too. Get my point ?
That possibility was one of the reasons I suggested perhaps giving that design an ADS neutron source. Those are not fast neutrons.
Perhaps the energy budget for an ADS neutron source to produce enough Pu239 so fast fission can be sustained would be astronomical (say with a 1000 tons of natural uranium reactor).
Apologies for the persistence. And thanks.