Hybrid Nuclear Reactors

18 July 2010

Interview with

Dr Bill Stacey, Georgia Institute of Technology

Chris -  Radioactive waste produced by the nuclear industry is a very big problem.  About 12,000 tonnes of it get produced around the world every single year and at the moment, it needs to be stored somewhere.  But what if there was a way to take this waste and actually use it as a fuel instead? That might sound too good to be true, but scientists think that it might be possible by building a nuclear fusion reactor that's nestling inside a fission reactor. This hybrid reactor will then be able to produce large numbers of extra neutrons that can be used to burn off the waste inside the reactor, and therefore make much more efficient use of the nuclear fuel. Bill Stacey, is from the Georgia Institute of Technology.  He's with us to explain how this could work.  Hello Bill...

Bill -   Hi.

Chris -   What's the general idea about this?  How does it work?  What's the concept?

Bill -   The basic idea is the actinides that Ian was just mentioning are all fissionable in certain types of reactors: instead of burying them, just put them back in reactors and burn them, which means to fission them. That's much easier said than done because it turns into competition for neutrons because as the fission products build up and the fission products are also competing for neutrons, it's hard to keep the reactor running.  So, there's a need for a few extra neutrons.  The idea of the fusion/fission hybrid is to have a fusion neutron source that provides these extra neutrons for the fission reactor that's burning up the actinides and spent nuclear fuel.

Chris -   Basically, what you've got is in the core of the reactor and at the moment with a fission reactor, where those fuel rods are with the pellets of uranium in them, the waste products that build up as the reaction goes on prevent the reaction from happening very efficiently.  In the end, you end up having to take the rods out even though only a tiny fraction of the fuel has been burned.  So if we could find a way of getting more bang for the buck by burning off the waste products with extra neutrons, then we'd save a whole lot of money and save a whole lot of waste as well.

Bill -   That's right.  The waste products that we're talking about burning off are primarily the actinides that are left in the fuel so they are unburned fuel for all practical purposes.  The waste products that are the fission products also are competing for the neutrons.  The problem is to get a few more neutrons and that's where the fusion idea comes in because the fusion reactor, that would be in the centre, would be producing extra neutrons, and you can dial up a number of extra neutrons that you need. In this way, you can keep the fission reactor running a lot longer and enable it to burn off the actinides.  Basically, the efficiency of doing this is that you can probably reduce by at least a factor of 10 the amount of material that would have to be stored in long term geological repositories. That means you would reduce by a factor of 10 the number of repositories that you had to build and that's substantial.

Chris -   Talking practically though Bill, we can't even get a fusion reactor to work at the moment sustainably, how practical is it to try and get one that we could then have in the core of a nuclear power station that's a fission reactor. You're going to put one kind of reactor inside another reactor.  It's huge, isn't it? A model of a cross section of ITER

Bill -   Yeah.  This is an idea that's been around for a long time because of the arguments that I just made, but the thing that's different now is that it is feasible to talk about building a fusion reactor.  The ITER project which is being built in Cadarache, in the South of France right now will have a fusion tokamak reactor that if we just took that technology and that physics, that would be sufficient for the neutron source for a fusion/fission hybrid. That device is being built now.  It will start operating in about 10 years and 10 to 20 years from now we'll demonstrate the physics and the technology that's needed for the neutron source.  So I think we would say that to be able to deploy something like this, starting in about 2030 or 2040 is a feasible thing.

Chris -   What about the safety aspect? Because you'd have a very, very hot and very, very powerful fusion reaction going on in a core of a super critical fission reactor.  Is there not quite a big element of danger there?

Bill -   Well, not really because if anything happens to the fusion reactor - the [core of the] fusion reactor is a gas like the sun, and basically, the problem is to keep it from going out. If anything happened to it, you would just end up with a spoonful of liquid hydrogen in the bottom of the vessel. There are of course interaction problems that have to be taken into account in the technology to do this, but it's not as if you've got an uncontrolled sun in the middle of a fission reactor.

Chris -   Just to look at the numbers to finish this off, apart from the waste issue, and you mentioned the 10-fold reduction in that at least, how much better would this be than what we can achieve at the moment with standard fission?

Bill -   Well, that's the question that really needs to be evaluated. The benefit of going with fission/fusion, as opposed to going with just a standard fission reactor to do this job, is that you can leave the fuel and burn it for longer. That means there would be fewer re-processing steps and so fewer re-processing facilities. That means more efficiency in burning up the fuel, so fewer repositories. The other thing is that that since each of these fission/fusion hybrids can use reactors that are completely fuelled with spent fuel you wouldn't need so many of them as you would if you had just conventional reactors which could only be partially fuelled with spent fuel. In fact, we've done some calculations and we estimate that a nuclear fleet that were sort of 75% light water reactors (LWRs) and 25% fusion/fission hybrid reactors would do this job very well.  Whereas if we were talking about a nuclear fleet made up LWRs and just plain fission burner reactors, it might be more like 75% the burner reactor and 25% LWRs.  So it's a matter of efficiency.

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