So your line of argument goes something like this: "->Do you concede that there are disaster scenarios that you didn't think of? ->Yes, I'm not God to know everything, I concede that. ->Well, QED, a fast reactor is unlicenseable." Case closed, right?

But the problem with this argument is that it can be applied to anything. In particular, if you really follow this argument, we should shut down all the current operating reactors, because, you know, there might be disaster scenarios that we didn't consider.

I hear you, I hear you. This argument only applies specifically to fast reactors, because this is what Dr. Teller said in 1967. But why did Teller say that?

In 1961 a nuclear reactor that the US Army commissioned with the stated intent to be virtually idiot proof, went boom. It became prompt critical. What happened was one of the most (if not the most) life-beats-fiction events in the history of nuclear reactors. It appears one of the operators decided to commit suicide by pulling a control rod as fast as he could [1]. Of course, since the guy died in the following half a second, we'll never know his true motives.

Another thing is that Teller was profoundly anti-communist. In 1957 another mind-bending accident happened in the Soviet Union, [2]. It had to do with plutonium, it went something like this: the Soviet Union was producing lots of plutonium to build bombs. Plutonium is extracted by chemical separation from spent uranium. After the separation, various steps are taken to increase the concentration, and eventually to purify the plutonium metal, but some chemical tailings that contained plutonium in low concentration remained. They were dumped in large underground tank. The solution contained ammonium nitrate, but it was mostly water. Water is a good moderator, so there's a good chance the solution would from time to time undergo prompt criticality excursions. Nothing major, no explosion happened. Yet. But because of the periodic heating, the water evaporated, and in time the ammonium nitrate became more concentrated, and it became a huge amount of (chemical) explosive. At some point this ammonium nitrate decided to go boom, like it happened in Beirut in 2020. The explosion was totally non-nuclear, but it put in the atmosphere a huge dirt cloud full of plutonium and other radioactive nasty stuff. The Soviet Union kept this a secret for many, many decades. But some people in the West learned some tidbits about this, and Teller, obsessed as he was with the Soviet Union, might have been one of the guys in the know. Plus, Teller (the father of the hydrogen bomb) had access to the highest level of top secret information in the US in regards to nuclear things. If anyone in the US knew anything about the Kyshtym, then Teller knew that too.

But why did Teller say that about fast reactors and not thermal reactors? Because that was the context. He was asked to testify if the US should build a sodium-cooled fast reactor. And he said it's a bad idea. If you know anything about Teller, is he said everything with a supreme confidence. Everything was very black and white for him, and he always knew with utmost certainty what was black and what was white. He thought for example that we should use hydrogen bombs to dig canals. Or that we should put anti-ballistic-missile thingies orbit that would zap the incoming missiles with a beam of X-rays produced by a nuclear bomb explosion [3]. This is not a joke.

Ok. Let's leave Dr. Teller aside for now. Are sodium-cooled reactors too risky to build. Well, patently not, considering that Russia is operating 2 right now and plans to build more. But let's say you are not happy with this. Will there be a point where you would concede that the experience the world has with building and operating such reactors is sufficient for the NRC to start granting such licenses? If not, what makes fast reactors any different that pressurized water reactors?

And by the way, the Natrium reactor is not designed to burn plutonium. It is not designed to be a breeder reactor. And the issue about prompt neutrons is actually less of a problem for such a reactor: U-238 has the highest ratio of delayed neutrons to prompt neutrons of any fissionable isotope and for fast reactors a larger proportion of fission events involve U-238 compared to thermal reactors [4].

[1] https://en.wikipedia.org/wiki/SL-1

[2] https://en.wikipedia.org/wiki/Kyshtym_disaster

[3] https://en.wikipedia.org/wiki/Project_Excalibur

[4] https://www.nuclear-power.com/nuclear-power/fission/prompt-n...