The Spallation Neutron Source is an accelerator-based neutron source in Oak Ridge, Tennessee , USA Source: Oak Ridge National Laboratory

Professor Robin Grimes, director of London’s Imperial College Rolls-Royce University Technology Centre in Nuclear Engineering, shares with deputy editor Tim Probert his views on plant lifetime extensions, Generation IV nuclear reactors and the prospects for a nuclear renaisssance.

PEi: Do you believe that there is a nuclear renaissance?

Professor Robin Grimes, Imperial College London
Grimes: There is certainly a global nuclear renaissance and it is taking place in two parts. Firstly in the existing nuclear countries, which are moving towards working out how they are going to rebuild their nuclear programmes, either by replacing reactors or extending reactor lifetimes.

Secondly is the emergence of new entrant countries. Your question is a good one in terms of new entrant nuclear countries because, while the existing countries have all necessary infrastructure set up – the government agencies, the independent regulatory network – apart from Iran, there isn’t a complete commitment to new nuclear build. They haven’t actually laid much concrete.

I think certain countries are going to build new reactors, but if you ask me that question in six months to a year, then I may give you a different answer.

Even in the UK, it is not certain. Hinkley Point C, which is set to be the site of UK’s first new nuclear plant since Sizewell B was opened in the 1990s, went through the planning process to build the country’s second pressurized water reactor (PWR), but it was never built. British Energy gave up.

PEi: The nuclear power sector has had a somewhat chequered history from an engineering point of view. Is the industry ready to succeed in a largely ‘liberalized’ energy sector?

Grimes: In the early years of the nuclear power industry, reactor designs were usually for a safety case of 20–30 years, and latterly 40 years. As we’ve learned more over the years and designs have changed, the design criteria can be lengthened. There are a number of reasons for this.

We have better materials, we have a better understanding of scheduling maintenance, and we can design structures with the replacement of parts in mind. Cars used to have the wheels bolted on and when the rubber wore out, you threw away the car. But now we change the wheels!

The designs of the reactors are different, but we’ve learned a lot from previous designs. From new designs, we have learned how to extend the lives of existing reactors. This is particularly important financially, because once you’ve paid for the reactor, additional years are very lucrative, as the debt has been paid off.

Many countries will be extending the lifetimes of their nuclear power plants as many will be facing a generation capacity shortfall in the coming years. Plant lifetime extensions are extremely important in countries like the UK, Germany and, particularly, Belgium.

In the UK, however, it is more problematic due to what I call ‘The British Issue’. Because the UK was first out of the blocks in designing commercial nuclear reactors – i.e. the Magnox reactors – we built each of those designs slightly differently, even down to the fuel cans. Of course, from an engineering point of view, this was not terribly efficient. When the UK moved on to the AGRs (advanced gas-cooled reactors), it still designed bespoke reactors, but at least the fuel was consistent.

The consequence of the AGR fleet is that the length of time taken to actually have the reactors go critical was very variable and the problems the reactors have had are very different. Some are more problematic than others, so some of the lifetime extensions of the reactors will be quite different to others.

Contrast that with France – which incidentally also went down the AGR route, as it’s a sensible design and we’ll see them again with the Generation IV reactors. France, which had time to take a look around, decided against the AGR and went with the US PWR design, and built a large, virtually identical fleet. Even though it was not the best design, by deciding not to tweak each design as it went along, France pursued a very pragmatic path.

In fact, after a few years France did swap to an upgraded PWR design – so essentially they have two fleets. Nevertheless, they have several reactors that are essentially identical. This makes the lifetime extensions much simpler.

PEi: Is there a need for new reactors in countries like Germany and the UK?

Grimes: Yes there is. Again, it is like cars. It’s possible to keep your old, favourite car running, but there comes a point where it’s so inefficient and takes up so much resource to keep it on the road, it’s more cost-effective to buy a new one.

There will come a point when the regulator no longer accepts life extension to a particular plant. France is aware of this and it is embarking on a new build programme.

PEi: How will they be paid for?

Grimes: Nuclear plants are very profitable as long as the electricity price is maintained. What did for British Energy was a low electricity price, which left them with a fleet of reactors that did not make much money.

I believe very strongly in a balanced mix of generation. The French situation of having 75 to 80 per cent of their electricity from nuclear makes it very difficult for them. It creates other problems, like load balance, which means they have to have reactors that load follow as a consequence. It’s better to have reactors producing baseload power.

The problem for the UK is balancing baseload power from nuclear with wind and other renewables like tidal power. I’d like to see the UK with an energy capacity of 35–40 per cent nuclear, 30 per cent from wind, some from hydropower and tidal power, with the rest from gas for peaking power. That would ensure security of energy supply at a cost not much higher than today, at far lower carbon emissions.

PEi: Do you think there needs to be an incentive to build nuclear power stations like the generous subsidy schemes available for renewables?

Grimes: I just don’t think that’s politically acceptable. The progress made by the nuclear industry over the past five years by convincing people that nuclear is a necessary option has been remarkable, but I don’t think an incentive is possible at this point in time.

Having said that, I would like to see a more level playing field for nuclear power. The subsidies available for wind farms, particularly offshore, are absolutely huge. I think there should be some recognition that nuclear is carbon free, but there should also be some recognition of the cost of nuclear waste. The CO2 cost is predicated on the idea that the emission of carbon dioxide will have an impact on the environment. With nuclear, the CO2 cost could be replaced with the cost of waste. I’ve seen numbers that suggest that the waste cost would be about half that of the carbon cost.

PEi: Will new nuclear reactors be mostly built where the national governments are of the more, shall we say, authoritarian variety?

Grimes: China is going to build an astounding number of new reactors and yet nuclear power will still only make up a relatively small proportion of its total generating capacity. And yes, it is a state issue. If they want something to happen, it will happen. Of course, it’s completely different in the West, as the power industry here needs to generate revenue, which is why its nuclear renaissance will take a different route.

I was lecturing recently on this very issue of what it takes to build a new reactor in both the East and the West as part of our new master’s degree in nuclear engineering, which started in October. This includes a 20-hour overview over the whole of nuclear engineering so that students have a context for all the major courses that they will go on to undertake sequentially.

PEi: Has the so-called nuclear renaissance led to a greater interest among students in studying nuclear engineering?

Grimes: Absolutely. It is astounding. At Imperial College, we have three nuclear degrees: mechanical engineering, chemical & nuclear engineering, and material & nuclear engineering. Undergraduates undertake the nuclear engineering components together in the third and fourth years, when they have completed enough mathematics and so forth. One particular tutor group has one chap from France, one from Singapore, one from the UAE and one from Indonesia, as well students from the UK.

These are the people who will be the future leaders in nuclear engineering. We have been teaching the undergraduate nuclear engineering courses for four years, but we have been offering modules in nuclear engineering since 1961.

PEi: Nuclear engineering has come a long way since 1961. How do you see the next generation of nuclear reactor designs developing?

Grimes: Fast breeder reactors will make a reappearance in the Generation IV reactors. Of course, we’ve had fast breeder reactors since day one. The EBR 1 in the United States, the very first to generate electricity, was a sodium-potassium eutectic breeder reactor. Unfortunately the fuel economy was greater than one.

The important part is the breeding ratio, i.e. the number of fissile nuclei created in the fuel compared to the number used up. The key question is for every uranium-235 isotope that undergoes fission, how many nuclei in the plutonium-239 do you produce in compensation? If you produce one for one, then the breeding ratio is one. I understand you have one child, so you and your wife have a breeding ratio of 0.5; we have three so we have a ratio of 1.5!

The thing that’s good about a fast breeder reactor is that because of the breeding ratio, you can utilize all of the uranium, not just uranium-235. You can also use uranium-238 to breed plutonium to make what is essentially an in-situ MOx fuel that then continues to burn.

It then goes through a recycle process to re-establish the plutonium or uranium/plutonium fuel to go back into a fast breeder reactor or into a thermal reactor and then reprocess.

There are all sorts of options, but the key point is that it uses up the plutonium, so you don’t end up with a large amount of civil plutonium. Furthermore, because the minor actinides can be separated out – i.e. the americium, curium and neptunium – you can also make ‘true fuels’ – i.e. transuranic fuels, which can also be burned in a fast reactor in a fast neutron spectrum. This brings the radioactivity of the waste compared to the uranium ore down to 300 years instead of the 100 000 years of that from the spent fuel of a thermal reactor.

France has them, Russia has had one for 40 years, and the UK had Dounreay. It’s a hell of a challenge, but it’s been done and it works. Unfortunately it’s very expensive and the technology has not been established sufficiently, and the price of electricity is not currently high enough to allow us to use fast breeder reactors.

PEi: And if that continues to remain the case?

Grimes: Well, if a fast reactor programme is not pursued, and if many countries continue their conventional nuclear programmes, then global uranium resources will be used up in around 100 years. Thoria has, therefore, the opportunity to become an interesting fuel for a number of different reasons.

Firstly, thoria doesn’t have a fissile isotope, so it has to be mixed with uranium-233, which is bred from thoria. This needs a chemical reaction, but reactors can be designed so that enough neutrons are being generated from one part, and then you have a blanket of thoria on the outside, with which you use the neutrons from the inside to create uranium-233 in the thoria fuel in-situ.

India went down this route due to the Non-Proliferation Treaty. They also have a fast reactor in IGCAR (Indira Gandhi Centre for Atomic Research) in Chennai, which has been used successfully for 25 years. This is used to create uranium-233 from thoria. It’s all to do with the neuton economy and how you get the neutron density in the right place at the right time to maintain not only criticality but also the stability of the reactor.

PEi: Do you think that accelerator-driven subcritical reactors could play a role?

Grimes: In accelerator-driven subcritical reactors [ADSRs), the critical flux of neutrons is created through a beam. A spallation source is bombarded with protons, and this bashes off neutrons, creating a beam of neutrons. The neutron beam hits then hits the target, i.e. the nuclear fuel, thus achieving criticality. Such a reactor would be incredibly inherently safe because you can effectively just turn it on or off with literally a flick of a switch.

And because of the type of spectrum created using this mechanism, you could use it to burn up minor actinides. Americium, curium and neptunium have very little practical use, so zapping them and transmuting them into other elements that are not long lived is useful. The spallation sources already exist, as we already use neutron pulses for scientific experiments.

The ISIS Pulsed Neutron in Oxford is a world leader in this field. Oak Ridge in the USA also has the light source. These are quite something to see; gallons of mercury hurtling around cooling things! But ADSRs are still really at the research laboratory stage.

The ISIS pulsed neutron source at the Rutherford Appleton Laboratory in Oxfordshire, UK Source: Science and Technology Facilities

PEi: As is nuclear fusion. Are we still 50 years hence from nuclear fusion criticality?

Grimes: People in conventional nuclear engineering always like to say that, but it’s a little unfair. Nuclear fusion has actually made tremendous progress; it’s just a hell of lot more difficult than people at first appreciated. It may be that it is still 40–50 years away, and if so that makes uranium-based fission reactors a bridging technology from a mostly carbon-based economy.

Fusion/fission hybrids are another possibility. Fission reactors would generate neutrons to be used in fusion reactors. In other words, some of the necessary particles which help to drive a fusion reactor would be generated through fission. US Energy Secretary Steven Chu said the hybrid fission/fusion reactor is a means to both generate electricity and break down nuclear waste.

Earlier this year in the UK, Lord Drayson also called for research into these reactors. China’s Institute of Plasma Physics has gone one step further and plans to build a prototype by 2020.

PEi: What are the barriers to developing these reactors?

Grimes: These reactors would of course be very expensive, but I am concerned about the gaps in the nuclear workforce. Thirty years down the line many nuclear workers won’t be there. People will be thinner on the ground by a factor of five.

The lack of people in the nuclear industry is a worry, particularly when there are so many presently non-nuclear countries that want to build reactors. This will drain skilled workers from the West and other established nuclear countries. The industry needs to bring people on quick.


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