Zubrin lies like he breathes, not bothering even calculate in Excel, not to mention take integrals!
But they continue to believe him — because it's Zubrin!
When they discussed his "engine on salt of Uranium" [NSWR — zaitcev] I wrote that the engine will not work in principle, because a reactor can only work in case of effective deceleration of neutrons, but already at the temperature of the moderator of 3000 degrees (like in KIWI), the cross-section of fission decreases 10 times, and the critical mass increases proportionally. But nobody paid attention — who am I, who is Zubrin!
The core has to be hot, and the moderator has to be cool, this is essential.
But they continued to fantasize, is it going to be 100,000 degrees in there, or only 10,000?
No matter how much I pointed out the principal contradiction — here is the cold sub-critical solution, and here is super-critical plasma, only in a meter or two away, and therefore neutrons from this plasma fly into the solution — which will inevitably capture them, decelerate, and react, and therefore the whole concept goes down the toilet.
But they disucss this salt engine over decades, without trying to check Zubrin's claims.
All "normal" nuclear reactors work only in the region between "first" (including the delayed neutrons) and "second" (with fast neutrons) criticalities. Only in this region, control of the reactor is possible. By the way, the difference in breeding ratios is only 1.000 and 1.007 for slow neutrons and 1.002 for the fast ones (in case of Plutonium, this much is the case even for slow neutrons).
And by the way, average delay for delayed neutrons is 0.1 seconds! The solution has to remain in the active zone for 100 milliseconds, in order to capture the delayed neutrons! Not even the solid phase RD-0410 reached that much.
Therefore, Zubrin's engine must be critical at the prompt neutrons. And because the moderator underperforms because it's hot, prompt neutrons become indistinguishable from fission neutrons, and therefore the density of plasma has to be the same as density of metal in order to achieve criticality — that is to say, almost 20 g/sm^3 for Uranium.
But this persuades nobody, because Zubrin is Zubrin, and who are you?
It all began as a discussion of Mars Direct among geeks, but escalated quickly.