Ron
3rd January 2008 - 02:00 PM
QUOTE (Engineer_Dave+Jan 3 2008, 01:28 PM)
Hi
Happy New Year!
I have recently been learning about nuclear reactors, since my interest was raised after watching a documentary about Chernobyl,
My questions are:
1. How does a nuclear reactor get started? What is the stimulus for the chain reaction?
2. Can a reactor be 'stopped' once started. i know the reaction can be slowed by introducing boron rods (or other materials) into the reactor, but can it be stopped altogether?
....and while we're on the subject....
3. Why is it that only radioactive materials are fissile?
Hi Dave,
This is just from reading"The Making of the Atomic Bomb" and other similar books/articles.
1) Chernobyl was not a run away chain reaction, it was a meltdown. A nuclear reactor is just a glorified steam engine. What happened in Russia was alot of bad events at once which caused the reaction to not stay cool. The radioactive material got so hot it melted through it's containment and into the air, ground, ...
2) The first controlled chain reaction was Enrico Fermi's Univ. of Chicago experiment where he introduced rods that would absorb some of the neutrons that would otherwise cause too many uranium splits.
3) U-238 is reactor-grade uranium, U-235 is weapons grade (it fissiles more easily.
4) the stimulus for the chain reaction is called a critical mass. Once you have that much uranium together, the neutrons (which spontaneously split from the atom, will cause a chain reaction (Little Boy was actually a gun that shot a pellet of u-235 into another mass of U-235 that together reached critical mass and Boom)
5)A nuclear weapon chain reaction cannot be stopped because it happens too fast (1 neutron makes 2, which makes 4, ...) In a reactor, because U238 is less fissile, the rods can actually stop the reaction by sucking up all the free neutrons.
6) Radioactive materials are so heavy that they cannot remain stable if another mass is introduced. Radio active is just a deterioration of the atom, which will continue to break down until it reaches Iron. The end of the radioactive chain.
This is how I understand these things, I hope some one corrects any errors I may have made.
Peace,
Ron
Engineer_Dave
3rd January 2008 - 02:21 PM
Hi Ron,
Thanks for the info. Sorry , I may have confused the issue by mentioning Chernobyl. It was just this that sparked an interest in things nuclear. My questions are pertaining to a normally functioning reactor.
Answer number 4 is what I was looking for. Basically it occurs spontaneously if there's sufficient material around. That makes sense to me and also explains the concept of 'critical mass'. I'm guessing that the physical arrangement may also be a factor.
Answer number 6 does it for me too! I take it then that by definition, radioactive materials ARE fissile, hence the decay.
Answer number 5. I see. So presumably once the neutron-absorbers are removed, the controlled chain reaction spontaneously starts again.
Thanks Ron. I had asked these questions to my friend who is a chemistry teacher, and he could only satisfactorily answer 2 of them (and even then he wasn't sure)!
Best Wishes
Dave
Ron
5th January 2008 - 07:23 PM
Hi Dave,
NP. Just as an aside, I found the Manhattan Project (and what grew from it) to be completely amazing. I mentioned how little boy worked (shooting an uranium projectile into a mass of uranium to make it reach critical mass), that's why Little Boy was long and thin (there was basically a cannon inside of it!). When the Gov't realized how hard it was to enrich enough Uranium for 1 bomb, they began working with Plutonium for Fat Man. Fat Man used shaped charges around a core of Plutonium (It looked like a soccer ball). The only test of a nuclear weapon before Hiroshima was Fat Man type of plutonium weapon (I'm sure you've seen the video).
Those early bombs were fission bombs (induced chain reaction of neutrons to bang into heavy elements to release more neutrons...) but If you've heard of the Hydrogen bomb, that is a fusion weapon. The problem with fusion (even though the energies released are 100's to 1000's of times greater than fission)is when you fuse 2 of the smallest atoms (actually an isotope of hydrogen) the outward energy fights against any inward pressure to fuse. What the Gov't does now for some fusion bombs is they wrap a conventional shaped charge around a plutonium fission bomb, around a hydrogen core. Kaboom.
Thanks for letting me ramble, I really find it amazing!
Peace,
Ron
Enthalpy
18th January 2008 - 04:54 PM
Hi Dave, Ron and everybody!
I just can't resist putting my very one comments, even if they add little to the existing replies...
1) U235 undergoes spontaneous fission (an unusual form of radioactivity), emitting neutrons. This small flux of neutrons is enough to start a reactor or a bomb.
1b) Less easy with plutonium, depending on if it contains only Pu239 (military grade Pu) or also Pu240 (produced together with Pu239 in uranium reactors unless one takes the appropriate steps).
1c) North Korean bomb was probably made of non-military-grade Pu, hence "only" 5kt. It would destroy a city over 1km radius instead of 2km at Hiroshima: Big deal!
1d) Producing military-grade Pu is rather easy, as is efficient use of non-military grade ("booster" weapons). Thus this distinction has been irrelevant for over 30 years. Hence any used fuel rods should be protected accordingly.
1e) History shows a country can use 20000 people for 5 years to get nukes. So used uranium rods or recycled Mox rods should be protected against governments sending 20000 troops at least.
2) Nuclear reactors have a passive stability. In pressurized water reactors, a faster reaction heats the liquid water which then expands (the liquid dilates by a factor of 4 when reaching the critical point). As less liquid water fills the reactors, it becomes less efficient at slowing neutrons, and this makes the overall reaction less efficient.
2b) Control rods alone would be too insecure, because technology is unreliable. Physics is better.
2c) With U235 and slow neutrons, a few % (2% ?) of the neutrons are emitted several seconds later. So as long as the chain reaction efficiency is under 102%, the cooling water and your rods have a few seconds to react. But in an accident, if this efficiency gets over 102% (or 100% for the immediate neutrons alone), you loose this help.
2d) Other reactors, especially fast neutrons reactors (brewers), don't have this delayed neutrons. Some don't use a neutron moderator nor have water that expands. Such reactors have less natural feedback and are intrinsically more dangerous.
3) Only U235 is fissile, over naturally occurring elements. Reactors use concentrations from natural 0.7% (Candu technology) to a few % U235 (enriched uranium). Bombs use nearly 100% U235, I guess reactors could as well. Small reactors, for instance in submarines, use more enriched U.
3a) Other nuclides are fissile, like Pu239 (is produced from U238) or U233 (would be produced from Th232) and more exotic (Americium, Californium etc).
3b) Fission is a form of great instability. Usual radioactivity is a more common instability. Same explanations with other words.