My understand is that fission can be encouraged by slowing neutrons down (thermal neutrons) by surrounding the fuel elements with a "moderator" like graphite. The first controlled chain reaction was observed in a "pile" of graphite blocks interspersed with the uranium fuel rods. There were also "control rods" used to regulate the number of fission events by absorbing neutrons. Elements like boron or xenon gas come to mind as neutron absorbers. Rods all the way in to quench the reaction completely, partly withdrawn to permit fission events, all the way out gives one a "Chernobyl".
Regards,
GtG
PS Old timers still refer to nuclear reactors as "piles", old memories die hard.
Yes, you use dense matter as speed bumps then you use other mediums to soak them up once they are slowed down.
Graphite is the speed bump, Boron are the absorbers.
It takes many feet of graphite to slow them down or a similar element.
If you go to some of the links, these guys have nothing that would be able to take of this.
Neutrons can go right through lead, the guys running this experiment seem obvious to this fact.
The Great Fermi became a lover of all things American, after exile from his native Italy. This included American cutoms, and its language (loosely based on English ;).
He searched for a colloquial English word for his giant stack of carbon bricks and uranium slugs. He came up with the word "pile."
PS Old timers still refer to nuclear reactors as "piles", old memories die hard.