Critical Mass question

[pierhogunn]

while I was driving this weekend I came up with a questino

what exactly is "Critical Mass" as it relates to nuclear fission of Uranium or Plutonium?

is it a set amount

is it a mass per volume of material sort of thing?

is it something that by my questions above that I don't understand?

[LCHIEN]

When you have a small amount of uranium, there's a normal slow fission rate (big atom splits) for U238, upon which each fission releases radiation (like some neutron particles) and some heat. If the radiation strikes more U238 it can cause additional fission beyond the normal decay rate.

When you have a lot of U238, more than a critical mass, you can start a chain reaction in which each fission causes more than one additional fission reaction at which time it avalanches very quickly and causes a nuclear explosion rather than just sitting there and running hot. E.g. one fission hits two more atoms and causes 2 fissions which in turn causes 4, 8, 16, 32, 64 etc exponentially in rapid succession causing a runaway fission reaction.

A fission bomb (e.g. hiroshima or Nagasaki-type) is made by holding two or more subcritical masses a safe distance apart and when you want to detonate it, hurling the subcritical masses together at supersonic speeds with dynamite (otherwise they would melt before combining).

The critical mass of a given nuclear material is well known to those skilled in the craft. for U238 I seem to recall its in the range of a few pounds or kilos of the stuff.

There, I've probably got this site banned for describing how to build a nuclear bomb.

Anyway for reactors, they have tubes of fissile material they keep separated. They have neutron absorbing boron or carbon barriers between them. The bring the rods together (by removing the boron material) to increase the fision rate and heat produced, which is then used to create steam and turn turbines.

IN reactors, the chief problem is moderating the uranium neutron release rate to control the reaction rate. It won't explode immediately since critical masses are kept apart but the main danger is failure of the cooling or neutron moderators which will cause a meltdown of the containment building and pooling of the molten uranium below the reactor vessel and mass contamination of the surrounding countryside.

[Tom Slick]

uuh ooh loring, homeland security is going to shut us down now!!!
I'll speak in code.
breaking it down in woodworking terms:

you decide to build a dresser, 6 bdft of 4/4 qswo isn't going to make a dresser but it is enough to make sawdust, a couple parts, and woodworking noises in the shop. you need to gather enough wood to be over the amount needed to finish the project for the project to be complete. The exact amount to finish would be your critical mass.

critical mass depends on the element and reaction being used.

[pierhogunn]

okay, that answers that question,

I tell ya, driving alone can be a dangerous thing...

[jackellis]

Since we're all about to be arrested anyway,

IN reactors, the chief problem is moderating the uranium neutron release rate to control the reaction rate. It won't explode immediately since critical masses are kept apart but the main danger is failure of the cooling or neutron moderators which will cause a meltdown of the containment building and pooling of the molten uranium below the reactor vessel and mass contamination of the surrounding countryside.

Apparently some of the new reactor designs, or perhaps they're proposed new designs, won't go critical if there's a loss of coolant. Instead, as the material heats up the reactions slows down and eventually stops. The idea is to ensure that the reactors are resistant to operator error (very careful choice of words. There's no such thing as idiot-proof.

[JimD]

I work in the nuclear industry but not the bomb industry - just making steam with nuclear fission instead of fossil fuel.

What Loring said is mostly right. It is not U238 that fissions, however, it is U235. Natural uranium is about .7% U235 and the rest of U238 which is inert under most circumstances. Boron is one absorber but it is only used in control rods, the parts of the reactor that shut things down, in some reactors. We typically use silver-indium-cadmium and hafnium is another good control rod absorber. PWRs use boron in the coolant but BWRs do not (PWR is Pressurized water reactor, BWR is Boiling water reactor). U235 naturally decays but the amount of heat given off is very tiny.

Critical mass is a function of which fissile materials. Electricity is usually made with low enriched uranium, meaning it has less than 5% U235. Low enriched uranium cannot be made into a bomb (at least I know no way to do it). It can go critical, however, since it has to in the reactor. During manufacturing, we have to use amounts, spacing and/or absorbers to make sure that the material does not go critical. We also avoid adding a moderator. Water is the moderator in light water reactors. Heavy water is used in Candu reactors (Canadian) and they go critical with natural uranium.

So it depends on the moderator you use (or if there is one) and the enrichment of the fissle material. When you get a self sustaining nuclear reaction, you get a lot of heat because you are destroying a very small amount of mass. I think a bomb is a form of a run-away fission reaction but I don't know much about bombs.

Big weapons are fusion reactions, however, not fission. Fission triggers are used to create the right circumstances to get hydrogen to form helium. That gives off a lot more power.

Jim

[cwithboat]

OK. You take a softball size of lower density material. Pack a little plastique around it inject some D2 and stand back (about 40 miles). Mass matters. So does density. And for you hands on people, so does color code.

[LCHIEN]

do i cut the blue wire or the red?

[crokett]

do i cut the blue wire or the red?

Neither. You cut the yellow one.