CHRIS: So we have a nuclear reactor; it has
something fissionable, something capable of doing this in it, in this case: uranium. Neutrons
from that uranium hit the nuclei of other uranium atoms; they destabilise the nucleus;
it falls apart and in the process releases some of this energy and want more neutrons
so it can then do this again. PADDY: That’s the idea of the chain reaction.
CHRIS: So why doesn’t the power station meltdown or explode?
PADDY: There’s a very precise amount of neutrons that are produced in each nuclear
fission. And what you need for a sustainable chain reaction is exactly one of those neutrons
that’s released to produce fission in the next generation. In order to do that you basically
control the amount of neutrons that are in the reactor and that’s done by materials
called “control rods.” These are special elements, special lumps of material. They’re
made of things like boron, or hafnium, or cadmium, and they’re material that basically
drops into the reactor core and, for want of a better word, gobbles up neutrons. Takes
the neutrons away from causing fission on uranium and the amount of control rods you
put in will determine how many neutrons are still available to go on and cause fission.
If you put in too much control rod, you basically don’t have any nuclear fission; that happens,
they steal all the neutrons. If you take all of the control rods out, then
you would have an increased amount of fission. Most reactors wouldn’t be able to make a
bomb just because of the nature of the uranium fuel that’s in there.