It's straightforward. Consider what would happen (for example) if all the fuel in a reactor is compressed into a more compact configuration.
In a thermal reactor, there's no problem, as there's now no moderator. There was massive rearrangement and compaction of melted fuel at the TMI accident, but criticality was not going to be a serious issue for the fundamental reasons I gave above.
In a fast reactor? It can only become more reactive. Anything else there was only absorbing neutrons, not helping, and the geometric change reduces neutron leakage.
Edward Teller somewhat famously warned about the issue in 1967, in a trade magazine named "Nuclear News":
“For the fast breeder to work in its steady state breeding condition, you probably need half a ton of plutonium. In order that it should work economically in a sufficiently big power producing unit, it probably needs more than one ton of plutonium. I do not like the hazard involved. I suggested that nuclear reactors are a blessing because they are clean. They are clean as long as they function as planned, but if they malfunction in a massive manner, which can happen in principle, they can release enough fission products to kill a tremendous number of people.
… But if you put together two tons of plutonium in a breeder, one tenth of one
percent of this material could become critical. I have listened to hundreds of analyses of what
course a nuclear accident could take. Although I believe it is possible to analyze the
immediate consequences of an accident, I do not believe it is possible to analyze and foresee
the secondary consequences. In an accident involving plutonium, a couple of tons of
plutonium can melt. I don’t think anyone can foresee where one or two or five percent of this
plutonium will find itself and how it will get mixed with other material. A small fraction of
the original charge can become a great hazard."
(Natrium is not a breeder but the same argument holds.)
That no fast reactors have yet exploded is of course no great argument. How many fast reactors have been built, particularly large ones? Not many. And we've already seen a commercial fast reactor suffer fuel melting (Fermi 1).
In a thermal reactor, there's no problem, as there's now no moderator. There was massive rearrangement and compaction of melted fuel at the TMI accident, but criticality was not going to be a serious issue for the fundamental reasons I gave above.
In a fast reactor? It can only become more reactive. Anything else there was only absorbing neutrons, not helping, and the geometric change reduces neutron leakage.
Edward Teller somewhat famously warned about the issue in 1967, in a trade magazine named "Nuclear News":
“For the fast breeder to work in its steady state breeding condition, you probably need half a ton of plutonium. In order that it should work economically in a sufficiently big power producing unit, it probably needs more than one ton of plutonium. I do not like the hazard involved. I suggested that nuclear reactors are a blessing because they are clean. They are clean as long as they function as planned, but if they malfunction in a massive manner, which can happen in principle, they can release enough fission products to kill a tremendous number of people.
… But if you put together two tons of plutonium in a breeder, one tenth of one percent of this material could become critical. I have listened to hundreds of analyses of what course a nuclear accident could take. Although I believe it is possible to analyze the immediate consequences of an accident, I do not believe it is possible to analyze and foresee the secondary consequences. In an accident involving plutonium, a couple of tons of plutonium can melt. I don’t think anyone can foresee where one or two or five percent of this plutonium will find itself and how it will get mixed with other material. A small fraction of the original charge can become a great hazard."
(Natrium is not a breeder but the same argument holds.)
That no fast reactors have yet exploded is of course no great argument. How many fast reactors have been built, particularly large ones? Not many. And we've already seen a commercial fast reactor suffer fuel melting (Fermi 1).