[WileyPost]

I'm not sure, but if the water absorbs an atoms neutrons, it destabilizes the atomic weight of the atom losing the neutron and that then creates the reaction with another atom. That's why as it steams out, the reaction settles don.

[WileyPost]

DOH! Burned by spellcheck!

[Robert A Cook PE]

Nah. It's even simpler than that.

Think of a single neutron, atomic weight 1, moving very fast as it leaves a fission reaction. The other two atomic nuclei that come from splitting the U235 are also moving (heat energy) and they also crash into adjacent atoms and cause THEM to heat up.

But we're gonna ignore those two and look at only the fast neutron for the time being.

If the neutron "hits" nothing, it escapes the core and eventually hits shielding or steel vessel walls and slows down, is absorbed, and is lost. A fraction do this.

If the neutron hits a nearby U238 nuclei it has a finite chance of being absorbed by the U238, turning it into U239 -> Np239 -> etc. and eventually into Pu239. That Pu239 can react later as new fuel. (Presto! Instant permanent fuel supply that Carter destroyed.)

If the neutron hits a material with a large cross section for absorption (such as Cadmium or boron or hafnium) then it is almost certain to be absorbed and lost to the reactor. These types of material are inserted into the core as control rods to stop the reaction, or removed to increase the reaction rate.

If the neutron its a heavy atom, but is NOT absorbed, then it merely bounces off (like a billiard ball hitting the edge of the table) and doesn't loose much energy. It stays in the reator, and continues to hit things. Or is lost when it eventually bounces out of the reactor. So low-absorbing but strong and corrosion resistance material is wrapped around the U235/U238/Pu239 to contain the material, prevent erosion and corrosion, and to contain the fission products.

If the neutron hits a "lightweight" atom, then the collisions energy is more evenly shared and BOTH the neutron and the lightweight target bounce away. The neutron bounces away with MUCH LESS kinetic energy (less heat energy), and the target atom is bounced away with much more kinetic energy (it become hotter.)

So, hydrogen-contianing material like water, or light metals like carbon, Al, polyethylene, plastic, and other light metals and liquids literally "slow down" the neutron better than heavier material.

These light-weight but non-absorbing material are called moderators in honor of our admin moderators who can't absorb anything either, but who control our reactions by bouncing ideas off of each other. 8<)

A perfect moderator also absorbs heat and transfer it out of the reactor. Again, like our admin moderator.

Remove the lightweight material, and the reaction slows down and eventually stops because no neutrons are slowed down by collisions..

One way of removing the light material is to boil off the water.