Science Interviews


Tue, 27th Mar 2012

Designing the PRISM Reactor

Dr. Eric Lowen, GE Hitachi

Listen Now    Download as mp3 from the show Nuclear: About to Explode?

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 Fukushima I Nuclear Power Plantevent 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.


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I don't know?

Japan is stopping their nuclear technology it seems. Not that I see where they will get their electrical energy from?
The ocean maybe, it's an island after all. But when it comes to ceramics as some 'new' technology for storing nuclear waste in? That's no 'new technology' as far as I know? And those that tried found that the heat and radioactive decay created thin cracks in the material if I remember right. And then you have different humidities and temperatures working on those cracks. The thing is, as long as you can't guarantee one of two things.

Either that you have found a way to reuse the material until it is at the same level as the 'natural radioactive background'.
Or that you can guarantee that this 'clay ceramic glass' etc will contain the radioactivity for at least some thousands of years, and that is me solely, trusting in that we will solve the problems giving us such a time to do it in.

The other step is what I've always said. Build those waste facilities in the middle of our towns, constantly measuring the radioactivity. That way we will know if it doesn't work as predicted :) And by God, when it goes wrong we will at least react. Promises is only as good as the facts. yor_on, Wed, 28th Mar 2012

the problem it seems with nuclear technology is its danger when something unexpected happens those 1000 year natural events or just human error, ok you can say that the odds are against anything happening , but history has shown that if something can go wrong it will go wrong given time and the risk of disaster to so many lives isn't worth the risk. take a look at chernobyl
25 years and still the same.
i understand that was a different type of reactor and safety has improved , but you have to sit back and think what if.. and if that risk is to big then its not worth that risk.
steelrat1, Mon, 2nd Apr 2012

Excellent article.
I would wonder if the Sodium reactors would bring their own risks though.  Pure sodium is extremely reactive in certain situations.  In particular, it can explosively react with water.  Is it possible to choose a less reactive coolant?  Liquid Sodium Chloride?  Lead? CliffordK, Fri, 6th Apr 2012

Apart from the 200,000 odd people killed by the military at the end of WWII with nuclear bombs I doubt if an average of more than 5 people a year have died in nuclear mishaps since the start of the nuclear age , a number that have died on numerous occasions in petrol tanker accidents.
Yet this strange fear of nuclear power persist despite the large number of people that die from various other forms of power generation and the attendant pollution.    syhprum, Fri, 6th Apr 2012

The estimates of the effects of the Chernobyl disaster are all over the board.

Perhaps around 200 died within a year of so of the disaster due to the explosion and acute radiation exposure.

The number of people globally affected by the disaster are likely in the thousands. 
Likewise the Fukushima disaster has a small number of people that had immediate lethal doses of radiation, but there is debate about global spikes in infant mortality following the disaster, although perhaps no causative link has been established yet.

I do agree that much of this, however, is just panic and fear, but there do seem to be some real effects of increased birth defects, or increased rates of thyroid cancer in some areas. CliffordK, Fri, 6th Apr 2012

If a plane crashes or a petrol tanker or a malaysian ferry goes downkilling 300 or so people it is forgotten in a few days but the 7 mile island partial melt down that killed no one ruined the American nuclear industry and has gone down in history as a major disaster.
In 1952 there was an air pollution incident in London caused by coal burning power stations and domestic fires, it resulted in an increase of 4000 deaths compared to a comarable period. syhprum, Sat, 7th Apr 2012

  but the coal burning power stations are cleaner now and don't leave the area contaminated for 40 years after neither did they cause Thyroid Cancer, Leukemia, increase in nervous system disorders, diabetes, Birth defects,miscarriages, premature births, and stillbirth increases , neither was the farmland remained contaminated from the decaying components of plutonium.
After the Chernobyl accident, almost 400,000 people were forced to
leave their homes for their own safety – homes and villages that had been part of
their families for generations. Over 2,000 towns and villages were bulldozed to the ground, and was a global disaster .. this was just one nuclear power station mishap..there have been 22 incidents so far..  so you really think that is a good risk to take to get our power? steelrat1, Mon, 23rd Apr 2012

Yep, and you have studies done by Russian scientists presenting quite different figures than what you see in Western media.

Chernobyl. yor_on, Tue, 24th Apr 2012

This UN assessment summary supports a lower estimate of fatalities that can be blamed on the Chernobyl reactor disaster: northcoast, Fri, 11th May 2012

Sure they did :)
As did IAEA (International Atomic Energy Agency) and WHO (World Health Organization)

both interrelated through a agreement.

"In the early days of nuclear power, WHO issued forthright statements on radiation risks such as its 1956 warning: "Genetic heritage is the most precious property for human beings. It determines the lives of our progeny, health and harmonious development of future generations. As experts, we affirm that the health of future generations is threatened by increasing development of the atomic industry and sources of radiation … We also believe that new mutations that occur in humans are harmful to them and their offspring."

After 1959, WHO made no more statements on health and radioactivity. What happened? On 28 May 1959, at the 12th World Health Assembly, WHO drew up an agreement with the International Atomic Energy Agency (IAEA); clause 12.40 of this agreement says: "Whenever either organisation proposes to initiate a programme or activity on a subject in which the other organisation has or may have a substantial interest, the first party shall consult the other with a view to adjusting the matter by mutual agreement." In other words, the WHO grants the right of prior approval over any research it might undertake or report on to the IAEA – a group that many people, including journalists, think is a neutral watchdog, but which is, in fact, an advocate for the nuclear power industry."

And when it comes to IAEA (International Atomic Energy Agency) its own charter says.

"he Agency shall seek to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world. It shall ensure, so far as it is able, that assistance provided by it or at its request or under its supervision or control is not used in such a way as to further any military purpose" yor_on, Fri, 25th May 2012

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