• Question: How do you choose which fuel and coolant to use?

    Asked by anon-201342 to Sophia, Sarah, Meirin, George, Emily on 13 Mar 2019.
    • Photo: Emily Lewis

      Emily Lewis answered on 13 Mar 2019:


      A good question!
      For reactor fuel we want a ‘fissile’ material, something that splits apart and releases energy when we fire neutrons at it. Uranium 235 is good for this. Still OK but good are ‘fertile’ materials, which we can convert into fissile ones with enough neutrons, I think about this as damp wood we need to dry out before we can set it on fire. An example is uranium 238, a slightly larger uranium nucleus.

      Then we want to decide what form we want the fuel in, traditional reactors have long tubes of solid uranium pellets. The design I contribute to wants to dissolve the uranium into a liquid salt! this poses some problems as the salt would be corrosive… but also it would be easy to drain the fuel out in an emergency so it could be a bit safer

      Coolant wise we can choose from water, helium gas, liquid metal or the liquid salt again. It would depend on the type of reactor we are running, how hot it is and how much energy the neutrons from the reaction have, also whether we wanted our coolant to act as a neutron moderator in addition to cooling.
      So overall there are lots of options to choose from and lots of arguments to be made for all of the choices!

    • Photo: George Fulton

      George Fulton answered on 13 Mar 2019:


      Great question.

      The coolant depends on the thing you are cooling. Some things can be water cooled, if they are not near electronic equipment but other things require a special electrically insulating coolant. This mean that electrical equipment doesn’t get fried if there is a leak! Some things need to be cooled to very low temperatures, for these we use cryogenic (or very cold) coolants. In fusion reactors, we use liquid hydrogen and liquid helium to get to these very low temperatures and most of our diagnositic equipment will be liquid nitrogen cooled. (it’s a lot cheaper)

      In nuclear fusion, we try to recreate the reaction that happens in the sun. It turns out that the suns reaction is quite inefficient and takes a long time to happen. But the sun can get away with it because it is so big. We still fuse types of hydrogen like the sun to make helium, but we use slightly different types of hydrogen forms. We call these isotopes: and we use deuterium and tritium. These are used as fuels as they are the easiest the fuse together 🙂

    • Photo: Sarah O'Sullivan

      Sarah O'Sullivan answered on 14 Mar 2019:


      Something called reactor neutronics comes into play here. You might want to use fuel X with coolant Y but for a fission reactor, you’re reliant on controlling the number of neutrons that are released from previous fission events. you want just enough so the fission reaction keeps going but not so much that it goes out of control. How neutrons are absorbed in your reactor will depend on the materials used in the design, including the coolant and if this coolant should also be a moderator (to slow the neutrons down to the desired energy needed for more fissions). There’s lots of things that need to be considered in the reactor design too, like your optimal operating temperatures and how the coolant might affect your fuel and its cladding.

Comments