4 ms·

> We're actually getting safer reactors that can eat spent fuel, Absolutely not my field, but can't CANDU reactors use spent fuel too? They seem pretty cool, i

> We're actually getting safer reactors that can eat spent fuel, Absolutely not my field, but can't CANDU reactors use spent fuel too? They seem pretty cool, it's not clear to me why they use them only in Canada!

acidburnNSA 2y ago
They can extract a little bit more energy out of spent fuel using a process like DUPIC [1], but they don't extract the vast energy remaining in the U-238, and they also do not transmute it into shorter-term waste like fast-neutron reactors do, so the total value is pretty low. There are lots of CANDUs outside of Canada though! [2] [1] https://dspace.mit.edu/bitstream/handle/1721.1/45495/42255431-MIT.pdf https://dspace.mit.edu/bitstream/handle/1721.1/45495/4225543... (thesis) [2] https://en.wikipedia.org/wiki/CANDU_reactor#Active_CANDU_reactors https://en.wikipedia.org/wiki/CANDU_reactor#Active_CANDU_rea...
- Retric 2y ago
  U-238 has almost zero economic value, and it’s definitely negative when part of spent fuel. It’s literally the waste product of uranium enrichment and mostly used in ammunition. CANDU avoids the need for enrichment which means throwing a lot less U-238 out at the beginning. However it’s all the other bits like maintenance keeping nuclear expensive right now, reactor grade enriched uranium is still cheap and plentiful (~0.9c/kWh in fuel rods) which is why CANDU reactors don’t take off in the first place.
  - acidburnNSA 2y ago
    The whole magic of breeder reactors is that they can turn U238 into Plutonium 239 for energy
    - Retric 2y ago
      All reactors do that. Breeder reactors operating on spent fuel can only utilize the U238 that wasn’t discarded in the initial enrichment process. That’s what happened to the vast majority of U238 in natural uranium used to make fuel. Roughly speaking to make 100kg of 5% enriched uranium fuel you need to throw away 1,000kg of U238 + ~3kg of U235 as 1,003 kg of depleted uranium.
      - acidburnNSA 2y ago
        Breeder reactors are defined as ones that turn more u238 into pu than the u235 they consume. The breeding ratio is greater than one in that case. So the net fissile material increases during operation.
        Retric 2y ago
        Which again sounds nice, but simply isn’t useful right now. Understanding why really comes down to following each step of the progression from natural uranium ore > natural uranium > etc. But basically U238 is really cheap, and nothing else in the process is.
credit_guy 2y ago
Candu reactors are cool, but there are some reasons not a lot of people use them outside Canada. The main one is that a certain reactivity feedback loop is positive. Meaning as things go bad, they tend to get worse. There are numerous feedback loops in a reactor, and the others are negative, so overall the reactors are safe. But inherently they are less safe than pressurized water reactors. Details here [1]. Another problem is the amount of waste. Because CANDU reactors use unenriched uranium, they extract less energy from the same amount of fuel. Put it differently, they need more fuel to get a certain amount of energy. About 10 times as much. After the fuel is used, it becomes waste, and you need to handle it somehow. Yet another problem is that deuterium is not as good a moderator as hydrogen. It does not absorb neutrons, and because of that it results in much better neutron economy, but it takes a larger volume of heavy water to achieve the same amount of moderation. So a Candu reactor tends to be bigger. It is also much more complex, because the heavy water and the regular water move in different circuits. All in all, a Candu reactor is a bit more difficult to maintain. For example, it needs to be overhauled after 30 years in order to get a life extension of another 30 years. A PWR can go for 40 years before an overhaul. That's not a huge difference, but it's something. Of course, the other thing is that you need the initial investment in the heavy water. Last time I read about it, that heavy water added about $1 BN to the construction cost. The idea was that over the life of the reactor, one would recoup that in the cheaper cost of the fuel, which does not need enrichment. Still, $1 BN is a big number. As for the spent fuel. You are right, Candu reactors can use the spent fuel from PWRs. But that's just in theory. I don't think in practice they do that. Reactors are complex enough. Dealing with the uncertainty of using a different fuel that the one they've been tested with is not something operators like to do without a very good reason. So in practice Candu reactors don't use spent fuel or thorium, or even MOX fuel (a mix of uranium and plutonium). [1] https://en.wikipedia.org/wiki/Void_coefficient https://en.wikipedia.org/wiki/Void_coefficient

acidburnNSA 2y ago

They can extract a little bit more energy out of spent fuel using a process like DUPIC [1], but they don't extract the vast energy remaining in the U-238, and they also do not transmute it into shorter-term waste like fast-neutron reactors do, so the total value is pretty low. There are lots of CANDUs outside of Canada though! [2] [1] https://dspace.mit.edu/bitstream/handle/1721.1/45495/42255431-MIT.pdf https://dspace.mit.edu/bitstream/handle/1721.1/45495/4225543... (thesis) [2] https://en.wikipedia.org/wiki/CANDU_reactor#Active_CANDU_reactors https://en.wikipedia.org/wiki/CANDU_reactor#Active_CANDU_rea...

Retric 2y ago
U-238 has almost zero economic value, and it’s definitely negative when part of spent fuel. It’s literally the waste product of uranium enrichment and mostly used in ammunition. CANDU avoids the need for enrichment which means throwing a lot less U-238 out at the beginning. However it’s all the other bits like maintenance keeping nuclear expensive right now, reactor grade enriched uranium is still cheap and plentiful (~0.9c/kWh in fuel rods) which is why CANDU reactors don’t take off in the first place.
- acidburnNSA 2y ago
  The whole magic of breeder reactors is that they can turn U238 into Plutonium 239 for energy
  - Retric 2y ago
    All reactors do that. Breeder reactors operating on spent fuel can only utilize the U238 that wasn’t discarded in the initial enrichment process. That’s what happened to the vast majority of U238 in natural uranium used to make fuel. Roughly speaking to make 100kg of 5% enriched uranium fuel you need to throw away 1,000kg of U238 + ~3kg of U235 as 1,003 kg of depleted uranium.
    - acidburnNSA 2y ago
      Breeder reactors are defined as ones that turn more u238 into pu than the u235 they consume. The breeding ratio is greater than one in that case. So the net fissile material increases during operation.
      - Retric 2y ago
        Which again sounds nice, but simply isn’t useful right now. Understanding why really comes down to following each step of the progression from natural uranium ore > natural uranium > etc. But basically U238 is really cheap, and nothing else in the process is.

credit_guy 2y ago

Candu reactors are cool, but there are some reasons not a lot of people use them outside Canada. The main one is that a certain reactivity feedback loop is positive. Meaning as things go bad, they tend to get worse. There are numerous feedback loops in a reactor, and the others are negative, so overall the reactors are safe. But inherently they are less safe than pressurized water reactors. Details here [1]. Another problem is the amount of waste. Because CANDU reactors use unenriched uranium, they extract less energy from the same amount of fuel. Put it differently, they need more fuel to get a certain amount of energy. About 10 times as much. After the fuel is used, it becomes waste, and you need to handle it somehow. Yet another problem is that deuterium is not as good a moderator as hydrogen. It does not absorb neutrons, and because of that it results in much better neutron economy, but it takes a larger volume of heavy water to achieve the same amount of moderation. So a Candu reactor tends to be bigger. It is also much more complex, because the heavy water and the regular water move in different circuits. All in all, a Candu reactor is a bit more difficult to maintain. For example, it needs to be overhauled after 30 years in order to get a life extension of another 30 years. A PWR can go for 40 years before an overhaul. That's not a huge difference, but it's something. Of course, the other thing is that you need the initial investment in the heavy water. Last time I read about it, that heavy water added about $1 BN to the construction cost. The idea was that over the life of the reactor, one would recoup that in the cheaper cost of the fuel, which does not need enrichment. Still, $1 BN is a big number. As for the spent fuel. You are right, Candu reactors can use the spent fuel from PWRs. But that's just in theory. I don't think in practice they do that. Reactors are complex enough. Dealing with the uncertainty of using a different fuel that the one they've been tested with is not something operators like to do without a very good reason. So in practice Candu reactors don't use spent fuel or thorium, or even MOX fuel (a mix of uranium and plutonium). [1] https://en.wikipedia.org/wiki/Void_coefficient https://en.wikipedia.org/wiki/Void_coefficient