Just a few nits to pick until they bleed and scab over. Followed up with bombastic pendantics of more information than a normal person would ever want.
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Originally Posted by Raptor1
Certain types of U-235 Reactors (Not sure if the majority or not) DO have water come into direct contact with U-235, because the water blocks out the radiation just fine, however, U-238 reactors must use Heavy water, which must be separated from the core because it would make a lot of mass otherwise...
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It is important to know the differences between the coolant and the moderator in a nuclear reactor.
The moderator, which in contact with the fuel rods (never in direct contact with the Uranium) serves to slow down the neutrons. (skipping incomprehensible physics) Slower neutrons have an increased chance of causing fission (a good thing). This moderator, which can be a solid (graphite) or a liquid (many formulas) gets very hot and needs to be cooled.
The coolant (which can be a liquid or a gas) is designed to wick away the heat from the critical assembly to include the moderator. Depending on the design of the reactor, the coolant may be radioactive or not (many reactors have staged cooling systems to isolate the radiation). One would not want to drink the primary coolant. Not only is it hot, but tastes like crap and will probably kill you).
Can the moderator and the coolant be the same thing? Yes and no. Depends on the type of reactor.
In a Light Water Reactor, the low enriched 235U reaction is moderated by regular water (H2O). This water also wicks heat away from the critical assembly so it can also be considered a coolant. However in LWRs there needs to be a secondary cooling system (which usually uses water but does not have to) to remove the heat from the moderator. So you would have the water doing double duty of both moderating the reaction and acting as the primary coolant.
One thing to keep in mind is that in a power generating nuclear reactor this heat is the primary product. It needs to be saved and moved to where it can mechanically drive a turbine. Turbines (and the technicians responsible for the running of turbines) get bitchy when radioactive steam is introduced into their systems. One of several reasons for radiological isolation of the coolant systems.
“... DO have water come into direct contact with U-235...”
Strictly speaking the moderator never comes in direct contact with the 235U. The 235U is formed into what is called fuel rods (constructions of Uranium fuel pellets stacked in a specific way inside a metal case called “Cladding”. The purpose of the cladding is to maintain the structural geometry of the fuel stack as well as protecting the Uranium from contact with water. Uranium fuel pellets are like Gremlins. You don’t want to get them wet. They get real bitchy and stop talking to the neutrons.
“... U-238 reactors must use Heavy water”
238U is non fissile. 235U is fissile. Fissile is one of them fancy words meaning capable of maintaining a chain reaction or criticality. Whether a 235U reactor uses light water (H2O) or heavy water (D2O) depends on the enrichment of the 235U. Natural 235U (0.7%) needs heavy water, low enriched 235U (3-5%) needs light water.
So 238U is useless? Not at all
238U is required for the production of Plutonium.
When a 238U atom and a neutron like each other very very much........
238U has a well earned reputation of being greedy when it comes to Neutrons. 235U plays nice with neutrons and when hit with one nicely splits up into chunks, gives off a little heat, and gives back to society less than 3 neutrons on average. These less than three neutrons spread out and contact other atoms of 235U and smack in to them (sorta like a nuclear mosh pit). This is called a Chain Reaction because it is like a chain letter -- if a neutron fails to smack another atom of 235U, it is cursed with bad luck. While the other neutrons who do smack in to an atom of 235U keeps the chain going and is blessed with good luck.
On the other hand the greedy 238U atom is so self centered that if a neutron smacks into it, the 238U absorbs the neutron, gives nothing back (well almost nothing). Feeling pretty proud of it self, the now fat 238U transmogrifies into several other elements finally deciding on 239Pu (Plutonium). This is how you “grow” Pu. You overfeed a 238U atom with neutrons. The problem is that Pu keeps the greed of 238U and keeps absorbing neutrons and growing fatter (240Pu) and fatter (241Pu) and fatter.......
How much you feed your Pu buddy depends on what you want to do with it.
If you want to make Plutonium explosive devices, you want thin trim 239Pu.
If you want to fuel a Plutonium power reactor, then the fatter Pu’s work better.