This is a well known notion. Changing the temperature at which a chemical reaction is occurring changes the rate of the reaction, whereas no such influence of temperature is noted in nuclear reactions. I have a feeling that this is somewhat of a half truth. True at temperatures normally attainable through available technologies, but perhaps not true under more extreme temperatures.
Take the case of fusion of hydrogen nuclei. Impossible at room temperature (unless you’re a liar!). Use an fission bomb as a fuse, though, and say bye, bye, atoll. My impression of this is that incredible temperatures (and pressures??) are produced as a result of the fission process and these conditions are required to initiate nuclear fusion.
So what gives? When can and when can’t temperature influence the rate of nuclear reactions? Not only processes like fusion, but also more mundane nuclear reactions like nuclear decay? And why are nuclear processes “resistant” to the influence of normal temperatures?
Temperature is a measure of the average kinetic energy of individual atoms/molecules.
Chemical reactions speed up with increased temperature because: the atoms/molecules are moving faster, therefore, they bump into each other more often and with more force.
Fission & nuclear decay don’t vary with temperature (that I know of) because they don’t rely on the energy of the atom as a whole, only on the imbalance between the weak & strong nuclear forces holding a nucleus together.
Fusion requires high temps & pressures (if you don’t have one of them magic cold fussion doo-hickies). The high temp strips off the electrons from the nuclei. The high pressure squeezes them together so they can fuse. I would assume that the reaction would increase with increasing temp/pressure, but IANANP (I am not a nuclear physicist).
the high temp also gives the nuclei enough K.E. to clide.
For fusion, the heat and pressure of the reaction feed it but in fission it is the release of high speed nutrons that feed the reaction, which is itself can be considered heat - but it’s much more indirect.
Someone correct me if I’m wrong here:
Fission has a critical mass that once exceeded the chain reaction can start on it’s own. Is this critical mass temp. dependant?
I would doubt it. Atomic bombs work at environmental temperatures. I guess it would interesting to know if this can happen at temperatures near absolute zero.
how low is too low for conventional explosives to explode?
jb
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*Originally posted by k2dave *
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In a sense it is. In a fission reactor (as opposed to an A-bomb)the rate of the reaction is dependand on temperature. Most modern commercial nuclear reactor are of a type called Presurized Water Reactors (PWRs). In PWR, the water that is used to cool the core, and to transfer the heat produced by the reaction to make steam, also serves to slow down, or “moderate” the neutrons produced by fisson. The neutrons slow when they collide with the water molecules.
Slower neutrons (called “thermal neutrons”) are more easily absorbed by the uranium, which goes on to fission again, sustaining the chain reaction. As the temperature of the core (and the water) increases, the water becomes less dense, thus for a given volume of water, there are fewer molecules for the neutrons to colide with. Therefore, there are fewer thermal neutrons and the reaction slows down. Obversly, colder water speeds up the reaction.
This is a Good Thing[sup]tm[/sup]. Even in the absense of operator intervention, in the event of an increase in temperature of the reactor (say due to loss of power to the coolant pumps) the reaction will slow itself down and prevent a disaster. The relation ship between the temperature of the moderator and the speed of the reaction is called the “temperature coefficient of reactivity,” and is symbolized as “alpha[sub]T[/sub]”. In a PWR, alpha[sub]T[/sub] is a negative value. In older-style reactors (like Chernobyl) graphite was used as the moderator instead of water and alpha[sub]T[/sub] is positive. The consequences of that are left as an exercise to the reader.
So, what does this have to do with A-bombs, which I think was the OP? Not much actually. In a fission bomb, the reaction happens so fast, and there are so many neutons around, that the slower thermal neutons are not needed to maintain the reaction. It is said to be “fast-critical”