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Problems in nuclear fusion, radiation risks – some active wastes, intermittency

Fusion- some new issues June 19, Renewables are doing very well these days, with costs falling, but some say that we will also need other non-fossil options to respond to climate change. Nuclear fission is one, but it is having problems- it’s proving to be expensive and, some say, risky. Some are hopeful that new technology will improve its lot, but for others the big hope is that, at some point in the future, nuclear fusion will be available and will avoid the problems that fission faces.

It is usually claimed that fusion will be cleaner and safer, with no fission products to store and no risks of core melt downs.  Moreover, since it uses hydrogen isotopes (deuterium and tritium), which are relatively easily obtained (deuterium from sea water, tritium from lithium), fusion can provide energy more or less indefinitely, into the far future. It may not be a renewed resource, but it is large. An exciting high tech solution – that could, some say, be available soon!

However, the reality is a bit more complex, with there being issues at each stage of the fuel-to-energy process, and a lot more work to do. In terms of fuel, it takes energy to extract deuterium from water, and lithium reserves, although relatively large, may be increasingly depleted given the growing demand for Lithium Ion batteries for electric vehicles. In terms of fusion plant operation, there will be radiation exposure risks and the potential for accidental release of active materials – tritium has a 12.3 year half-life, and tritiated water can be a major health hazard.  Depending on the fusion system used, there will also still be some active wastes to deal with- the components and containment structures will be activated by the high radiation fluxes and have to be regularly stripped out. They will be less long-lived than fission wastes, but they are still an issue.

More generally there is the issue of plant operation in power terms. It is early days yet, since we only have experience with small prototype test projects, like JET at Culham, and no detailed plans for full scale power stations. However, it seems likely that the plants will not be run continually, but in pulses.  When eventually finished, and fully commissioned (maybe by 2030?) the 500 MW rated €15bn ITER project being built in the south of France is expected to generate power in up to 10 minute bursts, and for at the most 1 hour. The proposed larger DEMO follow up (in the 2040s?) willevidently also only run in bursts, but of 2-4 hours.

One implication of this intermittent generation is that commercial scale fusion reactors, when and if they emerge, may be used not to generate base-load continuous power, but for producing hydrogen in batch-production mode. That can be used as a storable fuel for heating or be converted into various synfuels for vehicle use. It may thus be that fusion will focus on these more lucrative markets rather than trying to compete in the very tight electricity market.

There are other approaches to fusion which might offer other power options. The USA’s laser-fired ‘ignition’ system has its fans. Certainly some see the ‘inertial confinement’ approach, with tiny fuel pellets being compressed, using multiple focused laser beams, to reach fusion conditions, as winning over Tokomak magnetic constriction plasma systems like ITER. We shall see, with Google even entering the field, offering advanced electronics. Germany, Japan, South Korea and China are also in the game, as is Russia, which is where the original Tokomak design came from.  TheUK national hopes rest with the MAST spherical Tokomak at Culham and derivatives like the ST40.

Few of these technologies seem likely to be running at full scale before the 2030s or even 2040’s, but some do claim that they can be ready earlier.  In 2014, Lockheed surprised everyone by claiming that for their ‘compact fusion’ programe they were aiming for a ‘prototype in 5 years, defence products in 10, clean power for the world in 20 years’.  We may see, but for the moment it all seems rather speculative and long term.  Some of the rivals may get there faster, but, even assuming everything goes to plan, a commercial-scale ITER follow up is not now seen as likely to be available to feed power to the grid until after 2050!

Breakthroughs in smaller-scale laser fusion or some such are possible, and some reports seem to suggest imminent success (or at least a sustained positive output by 2024), but for the moment, there are the practicalities of the large scale Tokomak approach being developed by ITER to face. Some of the issues are quite worrying. The high radiation fluxes will present some operational safety issues. Indeed, a recent paper in Nature has warned that not enough attention had so far been given to safety.

It compared the current 500MW rated ITER project with the hypothetical DEMO commercial-scale follow-up project, maybe running in the 2040/50s. In ITER, it said, the risk of radiation exposure comes from fusion neutrons emitted from the plasma, γ-radiation emitted by neutron-activated components, X-rays emitted by some heating and current drive generators, and the β-radiation emitted from tritium. DEMO, would have a similar range of radiation – the main difference being the size of the inventories of typical radioactive products. It would presumably be the workforce who were most at risk, but there could also be public exposure issues, especially if there was a major loss of containment

The Nature article says that it’s been calculated that the radioactivity due to materials activation in a future fusion reactor may be three orders of magnitude more than that in a typical fission reactor with the same electrical power output, while the total radioactivity is comparable. It adds ‘from this point of view, fusion reactors may be potentially unsafe if low-activation materials are not deployed. Note that this finding may also be applicable to the more recent fusion reactor concepts with even low-activation materials adopted. This means that radiation exposure control for fusion reactor design and operation is of critical concern […] Thus, several radiation protection provisions, such as confinement barriers, radiation shielding and access control, must be applied in order to meet the maximum public dose limits required by the regulatory body and at the same time to keep individual occupational doses for workers as low as reasonably achievable.’ 

It also says ‘a fusion demonstration reactor is generally expected to have an order of magnitude more decay heat power than ITER, comparable to that of a fission reactor with the same electrical output power’ And finally, ‘in DEMO, radio-active waste activity after 100 years, assuming that low/reduced-activation materials are used for the first wall & structure material, could be around 20–50 times more than for ITER. The larger tritium inventory is also significant for tritiated waste management. In fact, this large amount of radioactive waste and especially tritiated waste will result in a large burden for waste disposal sites in the country where DEMO is located’.

There do seem to be some serious issues, and the ITER project has attracted its fair share of criticism.    Breakthroughs are always possible, but artificial fusion may not be the way ahead after all! We may have to rely on the (free) fusion reactor we already have- the sun. Maybe a safer option. And a faster one- we have working renewables now: we don’t need to wait for fusion topossibly start dealing with climate change decades hence.

June 4, 2019 - Posted by | 2 WORLD, technology

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