The “ultimate” nuclear energy source may well be that from Fusion reactors. I shall assume anyone reading this is at least vaguely familiar with the process of fusion power, so I won’t dwell on it too long. If you’re not familiar, follow these links to wikipedia here , here and here
9.1 Benefits of Fusion power
The key benefits of fusion are that the nuclear waste products from such reactions produce a higher yield of energy per kg of fuel and substantially less nuclear waste, which generally has a relatively short half life. The primary fusion fuels, Deuterium, Tritium (which can be obtained from Lithium subjected to neutron bombardment, easily done in a Fusion reactor core) or hydrogen are all relatively abundant substances, at least compared to the world’s limited stockpiles of Uranium or Thorium. In theory nuclear safety should be vastly simplified and the proliferation risks are fairly low (there maybe a slight problem with some countries diverting Tritium or Deuterium into thermo-nuclear bombs, but without the fission primary it will be useless).
9.2 The Deuterium – Tritium path to Fusion, present progress
The preferred D-T reaction works out like this: D + T →He (3.5 MeV) + 1n (14.1 MeV). Assuming that the Tritium initially came from Lithium, then this yields an energy output of 7.5m kWhth per kg of lithium (Ongena and Van Oost, 2006). By comparison a PWR gets 8,000 kWhe from 30 g’s of enriched Uranium fuel, or about 0.8m kWhth per kg of Uranium, i.e. Fusion power yields over 9 times the energy output per kg of fuel.
There has been considerable progress in fusion research over the last few decades using the Inertial confinement method. JET (Joint European Torus) , using D-T reactions, produced a net positive energy output for some seconds, a major step forward. The current plan now is to take this forward using the ITER magnetic confinement fusion experiment, which is sceduled to come online in 2020. ITER will be capable of sustaining Fusion energy pulses for periods of up to 1,000 seconds, a significant technical step forward it should be said, but still some way from a system capable of running 24/7, and of course doing so commercially. The purpose of ITER is to essentially prove the concept as well as acting as a research tool, which will allow several of the outstanding technical barriers towards a commercial fusion reactor to be overcome. These experiments are expected to continue running until 2035-2040. http://en.wikipedia.org/wiki/Iter#Objectives
Indeed ITER is in fact two projects rolled into one. ITER in France will work on the reactor, while IFMIF in Japan will focus on the issue of materials research. One of the problems that results from the D-T fusion method is the high neutron flux generated. You will recall the diagram above, showing the D-T reaction. You will note, not only that neutron shooting out but the very high kinetic energy attached to it. The result is that any D-T reactor will have a neutron flux at least 100 times that of a similar sized LWR (at around 1×1018 n/m2-s) or about 14-25 times more than a Fast neutron reactor generates (4 – 7 x1018 n/m2-s) . The goal of IFMIF is to develop some new material capable of withstanding these high neutron fluxes, as well as the high radiant heat loads and yet still give a reasonable service life. The smart money is currently on either Tungsten or Molybdenum (both expensive, brittle and difficult to form) or in all likelihood an alloy of either (or both) and possibly Graphite (fire risk).
Of course the critics already have a name for this new material, Unobtainium! As was discussed in the materials section, the sort of material needed to construct a working Fusion reactor, at least one with a decent service life and that’s cheap and easy to put together, simply may not exist. We could, I suspect compromise, use a graphite blanket around the core to reduce neutron fluxes with the rest of the reactor built out of a mix of Tungsten alloys, and where possible, ceramics. However, such a reactor would have a number of draw backs. Some exposed parts that can’t be shielded (it is integral to the design that they be exposed to the core) and would need to be regularly replaced, eating into capacity factors and making for an expensive operation. Alternatively we could simply build our Fusion plants with a short operating life, say 20 years rather than the current 50 years standard at present for the nuclear industry. Unfortunately, this would obviously depreciate the economics of Fusion power. And as it is, building a complex machine out of the materials I’ve suggested isn’t going to be cheap.
Worse still, all of that material, particularly the graphite, will now be mildly radioactive and need to be put into ILW storage, with some parts likely requiring HLW storage. So such a plan would mean our Fusion power program producing some quantities of radioactive nuclear wastes, a fraction of what we currently generate yes, but certainly not zero.
Another cause for concern is the lengthy timetable towards fusion power. ITER is scheduled to finish operations in 2038 (or more likely the 2040’s at the current pace). As noted, ITER’s goal is merely to prove the concept, not provide a 24/7 operating reactor, never mind a commercially viable one generating electricity. The ITER groups own figures has the design and construction of the follow on DEMO reactor taking place from around 2035-2045. This will be, as the name implies, a demonstration Fusion Reactor unit. While it will run for extended periods it certainly won’t be running on anything like a commercial basis, that will require waiting for PROTO (the first commercial prototype reactor) coming about 10-20 years later. Assuming no hold-ups, either technical or political, to stop or slow down this process (worth noting that the ITER program was held up for 5 years while everyone argued over where ITER was to be built, and indeed its already behind schedule), this puts the first generation of commercial Fusion Reactors as being built around the 2050’s at the earliest and more than likely the 2060’s or even 2070’s if we’re realistic. Indeed its likely even the timetable I’ve outlined above might slip as ITER is now not expected to acheive fully operational status until 2026.
ITER (next set as the PC language above refers to it as) and the preceding phases of fusion development
But we cannot wave a magic wand and the world will start sprouting fusion reactors like daisies. Assuming a well supported government campaign of building (which will be dependent on the costs being reasonable, if they are higher the build rate will be slower) then we should hopefully be able to match the maximum ever Fission reactor build rate of 30 GW/yr (or 235 Billion kWh/yr) after a 15-20 year lead time (i.e. time to train everybody, tool factories, built new ones, sort out planning issues, etc.) Unfortunately if you do you’re sums this would have Fusion power just about succeeding in replacing our existing nuclear capacity of around 5% of global energy use sometime around the 2100’s, quite some time away, with a lot of potential showstoppers in between.
When presented with the above, Fusion power supporters usually react with indignation and accuse one of “manipulating the figures”. They seem to believe that, cometh the hour, they’ll be given an unlimited budget and unlimited resources to complete roll out of nuclear power at an unrealistically fast schedule – many think the first commercial reactors will be built by the 2050’s. While that is still possible, assuming no hold ups (and as discussed there’s already been a few!), one or two fusion reactors (or even one each in every nation that’s a member of ITER) aren’t going to make a huge difference to the global energy picture, we’d need hundreds or thousands of GWe scale units to achieve that, and it would take a considerable time to build those (decades), especially if they are made out of exotic materials where there is only a very limited manufacturing capacity.
And of course, if the economics of nuclear energy don’t work out as well as hoped (remember these are the same people who told us Fission reactors would be “too cheap to meter”) then they’ll be getting very little government or private industry support, possibly none! And of course there is the possibility that ITER and IFMIF might fail. The whole point of ITER is to prove once and for all if the Tokamak magnetic confinement approach is actually viable….or not. If it’s no, then it’s back to the drawing board for the scientists and we have an even longer wait.
Another problem is the fact that Fusion energy using the D-T approach is limited by the global stockpiles of Lithium. The (pro-nuclear) author Dr D. Mc Kay gives a figure for 10 kWh/person/day from current known stocks of Lithium (he also gives an unrealistically high figure from unconventional sources from sea water which would not be economic to extract). By comparison he gives similar figures of 0.55 kWh/p/day for Uranium and 4 kWh/p/day for Thorium (again he also gives a range of highly unrealistic figures for unconventional resources of both that essentially ignores economics and practical factors, see here). Mc Kay also gives a UK energy consumption rate of energy of around 125 kWh/p/day, thought there are some who say he got his sums wrong, implying a figure of 52 kWh/p/day would be more accurate. This would imply that D-T fusion can only supply 8% (if we use Mc Kay’s figures) or possibly 20% of global energy (using his critic’s figures). Where does the other 92-80% of our energy come from?
But are there any other options to the D-T fusion in a Tokamak reactor? The answer is yes. Are they viable? Hard to say! I’ll attempt a brief summary.
9.6 Alternatives Reactor designs to the Tokamak
Firstly, to tackle the question of reactor design, there is the idea of using lasers to drive the fusion process. Such experiments are taking place with the American NIF as we speak. I would note that the primary goal of the NIF is weapons research, with Fusion energy research merely piggybacked onto it (the critics say, purely for the purposes of PR than any real intent to generate Fusion power). Also, there is the matter of that big laser. Have a look at the official website (here) and tell me how commercially viable it would be to build a massive laser like that next to every commercial nuclear power station!
Another possibility is the Fusor proposed by the scientists Farnsworth and Hirsch or the similar Polywell proposed by Bussard. These have all shown promising results, the Polywell in particular. However, the project has been starved of funding, and the recent death of Bussard, hasn’t helped matters. Also, the scientists involved have had difficulty proving the concept. Currently any Fusor/polywell devices have consumed considerably more energy to create Fusion pulses that they subsequently generated (i.e a net energy sink). The supporters claim that if scaled up sufficiently, this would turn into a net positive, and indeed that such devices would be much more efficient than any Tokamak reactor.
Whether they are correct or not is difficult to say. Much like the LFTR, the Fusor has something of a cult following online and this makes any meaningful assessment of its potential difficult to access. Suffice to say the smart money is on them being wrong and ITER and the Tokamak design being right. Of course, if ITER fails in its objectives, expect a sudden change of course towards the Fusor sometime in the 2030’s!
A nice picture of a Fusor experiment glowing away nicely, more on this here
9.7 Alternative fuel cycles
There are a range of different fuel cycles to the D-T method currently proposed. Firstly, there is the option of D-D fusion. This produces substantially less high energy neutrons (only 18% of the energy output v’s 81% for D-T, at least according to wikipedia!). The global stockpiles of Deuterium are much larger, practically limitless indeed if it could be successfully extracted from seawater. Either way, fuel supply issues will not really be an issue with the D-D reaction.
However, the downside with the D-D reaction is that the energy confinement must be much better (30 times according to wikipedia) and the energy density will be much less (68 times less according to wikipedia). This means D-D fusion is much harder to do at present and would require much larger reactors for a similar power output as a D-T reaction.
The D-D fusion process, the resulting Tritium particles will typically then fuse with Deuterium (producing a free neutron), as will the He-3 (producing a free proton as shown below)
Another option is so-called Aneutronic Fusion, which is essentially any fusion process that carries no more than 1% of its generated energy away in the form of neutrons. A lower neutron flux solves a lot of the material issues mentioned earlier. Better still, there is the option of using the charged particles generated by many of these fusion processes produce (those would be the p and e things in the charts below you see flying around) to generate electricity directly, without needing to muck around with turbogenerators. This last option is very useful. You’ll note I’ve largely glossed over the whole issue above of turbo-generator plant for Fusion power stations, largely because the answer entirely depends on what the state-of-the art will be when Fusion power becomes available (i.e in 40-100 years time!). With present technology it would be “challenging”, so obviously if these Aneutronic processes could save us the need to build a big expensive turbine set, then that would be a huge advantage.
However, Aneutronic fusion comes with a number of significant drawbacks. The temperatures required by the reactor are much higher (10 times higher) and the power densities may well work out as much lower. In short, its harder to do.
There is also an issue with fuel supplies. Boron (one of the favoured fuels) is not nearly as abundant as Deuterium or even Lithium. Another favoured fuel choice is He-3. This is an extremely rare substance. There may be substantial reserves of it on the moon (deposited by the solar wind), but the density of such fuel is extremely low, well below what would currently be consider in anyway practical or economic to extract with presently available mining technology…..on Earth! Obviously mining the moon for it with currently available mining, and space launch technology is simply a non-starter, thought in the distant future that might of course change……but this isn’t the distant future!
A rough but unfortunately accurate summary of He-3 fusion proposals, see more here
In summary, Fusion power does offer some intriguing possibilities. However, it is also clear that we cannot set any particular timetable for the arrival of commercial fusion power, indeed we need to contemplate the possibility that it simply never arrives! This is important as the way some nuclear advocates talk you’d swear it was already a given! The truth is we’re making progress yes, but it’s a case of slow and steady progress, and we’re still a long way still from achieving the necessary breakthroughs. Let’s not start counting our chickens before they’ve hatched!
To build any nation’s energy strategy on the blind assumption that fusion power will arrive within a certain time window, and at a reasonable cost, makes about as much sense as selling your house and all your worldly goods because some Preacher told you the rapture was coming on the 21st of May 2011.
Even if and when we do develop fusion power, it’s inevitable that the build rate of reactors will initially be slow and the capital costs will be high (as is always the case for any new technology). While these conditions may both improve over time, it certainly won’t take place overnight and ultimately there may well be a limit to how much energy we can ever hope to get from Fusion power. We will still need something else to plug this future energy gap.