I hear alot about how North Korea is using nuclear plants to process plutonium but what reaction that goes on in a nuclear plant creates fissionable material (I thought it was just radioactive decay)? What makes a fuel rod become ‘spent’ and how is it processed into fissionable material? Is this done to create P-239 or U-235?
Nuclear material doesn’t come ready-to-use. The fissionable isotopes (i.e., a particular form of the atom) of Plutonium and Uranium must be purified and collected. This creates enriched Uranium or useful Plutonium. Different specific nuclear systems, weapons included, are designed to use different materials.
Once created, the readied material is put inside the reactor and the thing is turned on. I won’t go into the details, but this reduces the usable material. Eventually, you remove the nuclear material. This material is “spent”. Ideally, you then reprocess it and keep using the remaining active nuclear isotopes. The remainder goes into a holding facility
While in the reactor, U-238 absorbs a neutron, undergoes a beta decay, and turns into Pu-239. This plutonium is intimately mixed with the uranium that produced it. The spent (as in used up) fuel mixture then has to be chemically processed to separate out the plutonium.
As far as the U-235 goes, it is mined as natural uranium. Natural uranium has less than 1% U-235, with the rest U-238. The U-235 percentage needs to be increased to at least 3 - 5% to be able to sustain a chain reaction. This is very difficult to do because the two isotopes are identical chemically and so can’t be enriched chemically.
The most common way to do it is to turn the uranium into a gas and separate some of the heavier U-238 out by running the gas through a bunch of centrifuges, thus increasing the percentage of U-235 in the remaining uranium. This takes a lot of equipment, power, and time. The enrichment needed for uranium to make a bomb needs to be in the 90%+ range.
Also there are different reactor types. and from my understanding, A breeder reactor is very efficient since it’s waste can be used as fuel for the reactor, but it produced weapon grade materials. So we encourage a different type of reactors one that produced less energy and more waste.
For a nuclear reactor to work, it must have as fuel a fissionable material. While technically any atom except protium (H-1) is capable of being fissioned, what is meant by “fissionable” in this context is that its atoms self-produce neutrons capable of fissioning other atoms within it to cause a chain reaction at a critical (self-sustaining) level.
Only one isotope occurs naturally in more than trace quantities and is fissionable: U-235. It constitutes one part in 114 of natural uranium, the remainder being mostly U-238.
However, at least two other nuclides (“isotopes”) which occur in nature in reasonable quantity can be converted to fissionable isotopes: U-238 itself, which can be made into fissionable Pu-239, and Thorium 232 (100% of natural thorium), which can be made into U-233.
To run a reactor, in addition to the fuel and in addition to shielding and such, two other things are needed: a neutron absorber, to prevent the reaction from going supercritical and causing a meltdown, and a moderator, which reflects fast neutrons and slows them down to a speed where they can react with the fissionable fuel. If the reactor core is surrounded by U-238 or thorium, the slow neutrons will fuse with the U or Th nuclei, producing U-239 and Th-233, both of which beta decay twice, past Np-239 and Pa-233 to the desired Pu-239 and U-233.
This describes a breeder reactor, which can produce more fuel than it consumes. However, for security reasons explained by others above, the U.S. has chosen not to build breeder reactors and instead use U-235 reactors alone.
Nit: converting U to Pu needs fast neutrons, so there isn’t a moderator. They are thus called “fast” breeder reactors, and typically use liquid metal as a coolant.
Liquid metal? Isn’t metal as a liquid hot, except for mercury? How does it cool?
Metals have different melting points. The metals usually used are sodium, potassium, or a mixture of the two. These melt at low temperatures, around the boiling point of water. This is still much cooler than the fuel, so it carries heat away.
The metal has a high boiling point, so the coolant system can run at low pressure, and it has excellent heat transfer capability, which are good. It also needs heaters to keep it liquid when the reactor is shut down, and reacts violently if it comes in contact with water, which are bad.
Is there a secondary cooling loop of water to cool the primary metal loop?
The primary loop has radioactive sodium, the secondary loop contains non-radioactive sodium, and the tertiary loop is water.