I have a question!

[Day Tripper]

I'm trying to do my homework, but can't fully concentrate. One of my questions that I'm totally stuck on is:

Why can a reactor not explode like a fission bomb?

Please, I'd be eternally grateful if anyone can explain this to me. It's like, kind of in my head, but not 100%. :(

[Puppet Strings]

Nuclear reactors are controlled and monitored, whereas in a bomb the chain reaction can happen very quickly and cause an explosion.

Something to do with the control over it anyway, and the fact that the reactor is much more controlled.

[Day Tripper]

I just don't get this nuclear stuff. I'm not going for nuclear stuff, I'm going to school for pharmacy stuff. :(

[Day Tripper]

Nuclear reactors use fission. I think...

[Puppet Strings]

Yeah, nuclear reactors use fission. They have control rods as one of the main control techniques, which they can move in and out of the reactor to control the reaction.

Nuclear fusion reactors, well apparently there are a few of them around, although they are very much experiemental and none can produce electricity as of yet.

This is enforcing a bit of physics revision upon me, haha.

[Day Tripper]

So what would you write down as the answer? Because of the control methods used in nuclear reactors, a meltdown occurs instead of a nuclear explosion?

[Ratatouille strychnine]

There isn't a critical mass of fissile material in the fuel rods unlike in a nuclear weapon.

The fuel is arranged in thin cylinders and this provides a high surface area to volume ratio and so the neutrons released by the fission (there are on average just over 2 released from each fission event) only go on to cause one other fission event. That is, it is designed so that on average, only one neutron released from a fission event goes on to cause another fission event and the others escape the fuel rod due to it's high surface are in relation to it's volume.

In a nuclear weapon however, the fissile material is arranged as a sphere. This provides a very low surface area to volume to ratio and so more than one neutron from each fission event goes on to cause another fission event. It is this that causes the chain reaction and the nuclear explosion.

As well as this, the fissile material used in fission reactors (usually U235 unless they are fast reactors that use PU239) is only approximately 2-5% U235. The rest is U238 which is not fissile. Therefore, the arrangement of the rods and the fuel used make sure it is at a subcritical mass.

The fissile material used in nuclear weapons is heavily enriched and about 85% U235 (or PU239 if it's a plutonium bomb) and so there is much more fissile material arranged in a sphere. This causes it to have a supercritical mass and thus explode with a very large bang.

[what_the?]

*feels stupid*
/pointless post

[Day Tripper]

Thanks you guys. And, Rob, so much detail. :)

*Looks down*

Oooooh, a penny!

Dicky!

*stares at the penny*

thats some fine lookin' coinage you got there Dicky...

Hope you're homework went okay Shan