Designing the PRISM Reactor

Ben -   Nuclear power has been firmly in the news this month as we mark the anniversary of the Fukushima power plant failure - the result of an earthquake and a tsunami that hit Japan on the 11th of March 2011.  The Fukushima plant was commissioned back in February 1971 and reactor technology has developed considerably in those 40 years with new designs being safer and more efficient.  Even now, new breeds of reactor are capable of closing the nuclear cycle by generating electricity at the same time as dealing with the problem of radioactive waste.  One of these is PRISM that stands for Power Reactor Innovative Small Module and that's been developed by GE Hitachi and we are joined by their Chief Engineer, Dr. Eric Lowen.  Eric, thank you ever so much for joining us.  I wonder if you could start off by explaining, what's the push to need this technology?

Eric -   I would say the push is that as humans grapple with different energy sources going forward, the PRISM technology represents a different fuel source that has vast potential to generate a lot of carbon free energy.

Ben -   So, what is the PRISM reactor itself and how does it differ from a traditional nuclear reactor?

Eric -   A PRISM reactor is not too different from a conventional reactor that it still does the basic physioprocess of breaking big atoms in half to extract energy.  The PRISM technology came from work that General Electric started in 1981 and then the US government funded an advanced liquid metal reactor programme. So at this point, the technologies really had three decade's worth of development that we feel is ready to be commercialised in different market segments.

Ben -   So, how does it actually work?  So we need a radioactive source, but it seems that the PRISM reactor can use a far wider range of sources than the reactors that we're used to seeing.  How does it take that reaction and turn it into useful energy?

Eric -   What's unique about the PRISM reactor is that when a large atom such as uranium 235 or plutonium breaks in half, or fissions, it gives off neutrons that are very high energy and in our current commercial reactors with water cooling, those neutrons slow down or they lose their energy very rapidly and that limits the amount of reactions you can do. 

So in the PRISM reactor, we use sodium as a coolant that keeps the neutron energy still high, or relatively fast, and that allows us to use different fuel to fuel this and that's where the potential of this energy comes from.

Ben -   So why would sodium allow things to remain faster than using water as a coolant?  What's the advantage?

Eric -   Well as we know, water is made up of 3 atoms - oxygen and then 2 hydrogen atoms - and hydrogen atoms have one proton in the centre and they're about the same mass as a neutron.  So, when we have a fission reaction that occurs with water cooling, that neutron comes and strikes the proton and the water slows down. It gives off its energy because they're about the same mass.  So it would be similar to playing billiards where the cue ball, the white ball, hits the 8 ball and transfers its energy.  With sodium, because it's a bigger atom, it has 24 protons and neutrons in the centre.  When that neutron hits it, it typically bounces off and keeps its energy.  

So an analogy that I like to use, if we look at the Earth's orbit, it takes us 365 days to get around the sun.  The fast neutrons in the PRISM reactor would go around the sun, if it followed that orbit, in 73 hours.  If we look at the slower neutrons in a water cooled reactor, it would take 8.3 years.  So it's a big difference in energy that we have and that's why we can use different fuels.

Ben -   So you're getting these sort of almost elastic collisions and that means you keep hold of a lot more energy.  Does that also mean that if there's a problem, it takes you a lot longer to get rid of the energy to cool everything down?

Eric -   Actually, it's kind of the reverse when the PRISM reactor uses metallic fuel, it's in metallic cladding and a metallic coolant called sodium, in a metal vessel called the reactor pressure vessel. That allows us to have air come on the outside, to remove the heat.  The unique thing with any nuclear power plant is that once we turn it off, it still generates heat from the radioactive decay and typically, that's about 7% when it initially gets turned off.  So the PRISM reactor has the ability to remove that heat very easily with air coming on the outside of the reactor vessel and it can do that not for hours or days, or weeks, but forever.  So that's the unique part of the design that we've come up with.

Ben -   We saw in Fukushima that there was a problem with the coolant and they were able to bring in seawater.  Now, the seawater obviously ended up contaminated.  It wasn't an ideal situation, lots of steam, and in fact, it was the very hot steam that led to some of the explosions that people early on thought were a meltdown.  Presumably, with the sodium, it will just continue to convect and continue to pass that heat out into the air and will get lots of hot air, but nothing explosive, nothing building up pressure.  So actually, this will be a lot safer.

Eric -   Yes.  So if we look at what happened to Fukushima, they were in an event which we call beyond a station blackout.  They lost all of their offsite alternating current, all their onsite alternating current, and all their batteries.  And so, they had to grapple with those operators to remove that heat, initially at 7% when they turn it off. 

So the way that PRISM is designed is that when it turns off, the way to remove the heat is we pipe in air right beside the reactor vessel.  We have a different design and that allows us to continuously remove that heat so we don't build it up.

Ben -   And what fuels can a PRISM reactor actually use?  What can you take advantage of?

Eric -   One of the fuels we can take advantage of is the plutonium that has been separated in the United Kingdom during your reprocessing and that has a great potential to produce a lot of electricity. 

With the work that I've done in the United States, we looked at using this technology to extract fuel out of our used nuclear fuel.  So that's another possibility. And then if you mix some of our fuel with thorium, you could extract energy that way.  So there's a lot of variants that you can use with PRISM as far as extracting energy from big atoms, is what we call it, to produce green electricity.

Ben -   And what then happens to the fuel in there?  Obviously, with traditional plants, you're left with quite radioactive, unpleasant stuff that we have a bit of a problem and a bit of an argument as to how we get rid of it.  What's left at the end of the life of a PRISM reactor?

Eric -   In the PRISM reactor that we have been talking to the US government about, we take the fuel that comes out of the reactor and we do separations using electricity.  Not acids but electricity, and when we do that separations, we take out what is called fission products. Those are the small atoms that when we break big atoms in half like caesium, krypton, rubidium - superman type materials. And we put the elements that normally occur in nature as a mineral, we put that into a very robust ceramic.  And then the other one that are normally as metals, we put that into a metal alloy. And those two waste products, this rock or ceramic and this piece of metal, are then, after about 300 years are less radioactive than the uranium ore.

Ben -   So clearly, it's a cleaner way of doing this as well and we're going to have fewer dangerous products at the end of it.  What sort of power can we get out of these PRISM reactors?  Are they again, equivalent to the reactors that we're currently seeing in the surface?

Eric -   They're a little bit smaller.  So what we have proposed for the United Kingdom, for the disposition of plutonium at the Sellafield site would be one PRISM power block and its total electricity output is about 600 megawatts electric.  And that 600-megawatt electric over its lifetime could disposition that plutonium that's currently stored at the Sellafield site.

Ben -   So not only are you going to generate 600 megawatts of electricity, but you're also going to get rid of or use up, or at least make use of a stockpile of plutonium that we currently have, just taking up space and being a problem?

Eric -   Yes, we would turn that into an energy asset.  We don't look at that plutonium as a waste and your country has done a very good job of safely storing that plutonium for this future use. The current policy as I understand in the UK is that they don't want to reprocess or recycle, like I described, so they just want to push it through a reactor system such PRISM, extract some energy and then place that in a geological repository. 

Now if we took the full vision of the full capabilities of this technology, that 100 tonnes of plutonium could generate between 200 and 500-gigawatts electricity.  So there's a great energy potential there, should it be chosen to be used.

Ben -   Well thank you very much.  That's was Dr. Eric Lowen.  He's a Chief Engineer at GE Hitachi.