For quite sometime the “holy grail” of the nuclear industry has been the “fast reactor”. Fast Reactor’s offer to intriguing possibilities, firstly the ability to “create” more nuclear fuel by breeding plutonium (or U-232 from Thorium). I would question the practical benefits of this ability, given that it relies on reprocessing, and if there’s one thing the French, British and Japanese nuclear programmes have proven its that reprocessing is a very efficient way of burning through lots of public money and generating lots of nuclear waste.
However, it is the other possibility of fast reactors that is perhaps more relevant. The potential ability of fast neutron reactors to “burn” off the more severe and nasty nuclear waste products (often referred to as Actinides). While you will still have nuclear waste coming out of the reactor, the half life of most of the products within it will be considerably shorter, and hence your geological storage facility does not need to be built to the same exacting standards.
7.1 Nuclear waste, the 100,000 year old problem
The Onkalo Waste facility and storage vessel (from the BBC)
For example, the only such facility being built right now, Onkalo in Finland, has to be built to contain said waste for 100,000 years. Nothing built in human history has lasted a fraction of that time. So long indeed is the potential lifetime of the Onkalo that the Finn’s are having to contemplate how to communicate the dangers of the nuclear waste with a future civilisation who may neither understand English or Finnish (indeed talk to most Scandinavians today and they’ll tell you they don’t understand Finnish either!) nor what the international radiation symbol stands for (bare in mind, that if anyone as early as 100 years ago saw this symbol they won’t have a clue what it meant), nor even know what radiation is. See these two links here and here.
A Fast neutron reactor would allow us to process waste to the point where, while it would still need storage of some kind, the facility might only need to work for 10,000’s of years or less, much more credible.
The field of thorns, a proposal is to place this, made out of concrete on top of the repository to leave the impression of something dangerous in the area.
7.2 Fast reactors, where nuclear dreams go to die
However, in many cases the nuclear industry’s pursuit of this “Holy Grail” has been about as successful and farcical as other fictional attempts to capture the Holy Grail of legend. Indeed at times the industry’s been left looking a bit like the Monty Python Black knight who doesn’t know when to give up.
The problem for Fast reactors, as I’ve detailed in a prior post, is that the try to do several things at once which are very technically challenging. They generally have both a high operating temperature and a high neutron flux, a very bad combination (if you read my summary of material issues). Worse still, because they also need a cooling fluid with a high thermal heat capacity but a low level of neutron absorption, most Fast reactor proposals to date have utilised liquid metal coolants. Designing a reactor capable of resisting these high temperatures, corrosive attack (from the coolant) and high neutron flux has proven difficult. Most have come in substantially over budget, Monju the world’s most recent Fast reactor project costs $ 5.9 Billion (about $21,000 per installed kW! 3 times the cost of PV at the time!), and most fast reactors have often been off more than they’ve been on, the French Superphenix for example managed a lifetime load factor of just 7.79% (much worse than any wind farm!).
7.3 The Gas Cooled Fast reactor
Schematic of a Gas Cooled Fast reactor working on an open cycle (from wikipedia)
The basic idea behind the Gas-cooled Fast Reactor is to essentially solve one of the problems listed previously, by using an inert gas such as Helium as our coolant, rather than a liquid metal. Helium has an even lower neutron cross section than Sodium. The GcFR combines ideas from the BWR, HTGR and the previous fast neutron reactor projects. In many respects it’s similar to the HTGR, except its fuel has a higher fissile content and lacks a moderator (i.e no graphite matrix). Thus the idea is to marry the high safety factors of the Gas reactor with the breeding/transmutation properties of the fast reactors. The preferred operating temperature range of the GcFR is around 850 °C , allowing them to utilise the Brayton cycle (as shown), indeed they just about fall within the window of being able to use the sulphur-iodine process to make hydrogen.
However, the energy production capability of a GcFR is, as it were a bonus. It’s the potential to reduce the many hundreds of thousands of tons of High level nuclear waste in the world, that is of particular interest.
7.4 Technical challenges of the GcFR design
Unfortunately, the GcFR still faces a number of technical challenges. While the coolant switch does solve a number of the problems associated with using liquid metals, it certainly doesn’t solve all the problems notably those associated with the high neutron flux. Our material choices are still somewhat constrained. Given the problems encountered with the UK AGR’s with neutron fluxes (which were modest in comparison to those of any fast reactor) I would rule out concrete as a material choice and the operating temperature and neutron flux in question is probably outside the limits of what we can hope any stainless steel to withstand. This limits us to build it out of Refractory’s (metals or ceramics) or Nickel alloys. Remembering that it’s not just the pressure vessel we need to worry about, all those moving parts inside that need to be also constructed out of said materials too. To cope with the neutron flux said parts, regardless of the material choice, will have to be essentially over-designed to ensure a long service life (else like Fast reactors of the past we’ll be turning the things off regularly to replace components that have failed).
Also, by using Helium, which again has a much lower thermal heat capacity and density to liquid metals (or water), a GcFR would likely be rather large. This is made worse by the fact that a Helium cooled core will have a relatively small thermal inertia (i.e potentially vulnerable to meltdown) necessitating a large reservoir of helium being kept within the core at all times (should you be wondering why there’s so much empty space in the sketch above). This naturally increases our reactor size (for a given power output) yet further upwards. And inevitably, a large reactor made out of expensive superalloys is likely to yield an expensive reactor, much more expensive than any other so far considered.
This issue of superalloy use again applies not just to the reactor but to the turbogenerators as well. As mentioned previously this will make such hardware potentially expensive. An open cycle running turbine (as shown above) is a possibility, though I suspect once those High-vis jacket wearing FMEA form-filling Killjoys have their say, we’ll need to split the cycle and insert a heat exchanger between the core and Brayton cycle turbines. Either that or they’ll insist on containing the entire turbogenerator set within the containment dome, so it would obviously be easier in this situation to just split the cycle in two. This would reduce thermal efficiency by about 10-15%, hardly crippling, but certainly inconvenient. While a GcFR eliminates the graphite core, which I spent sometime in the last chapter fretting over, it would also contain a lot of fairly “nasty” stuff (i.e spent fuel, plutonium from former weapons stockpiles, my used socks, the usual!) and as it lack the HTGR’s near immunity to meltdown, a containment dome would still be necessary. How large and expensive this dome would need to be is difficult to say at present as we have yet to construct any working GcFR’s with which to judge the margins of safety. Indeed that is in part the problem, there is a substantial R&D gap with this reactor design, so anyone planning on them being built in large numbers tomorrow, as unfortunately the US energy secretary Steve Chu seems to believe, is likely to be disappointed.
Another issue is the capacity factor. Given that these reactors would likely be used to either breed fuel or “burn” off Actinides, that is going to require regular shutdowns to swap over fuel assemblies. As these reactors lack the ability to fuel while running, this would mean a relatively low capacity factor. Probably not nearly as bad as with past Fast Reactor’s (8-10% worse than wind farms!), but certainly not very high (my guess? 40-60% against 90% with existing reactors). So obviously, this would make such reactors a not terribly reliable source of energy, given that they could well be frequently turned off at incontinent moments. So another source of energy, likely a thermal power station, hydroelectric plant or another nuclear reactor would need to back them up. But like I said, we’d be best looking on any energy these reactors actually generate as something of a bonus, rather than a means to an end. The primary role of them would be fuel processing.
7.5 Fast neutron transmutation of nuclear waste
Of course, if such a reactor could be proved to work and work well, then there would still be some merit towards building them, I just won’t hold out for private industry to do anything!
Given that nuclear waste disposal is a task that the industry has essentially delegated (rightly or wrongly) to governments, as indeed it has also delegated the task of securing future Uranium fuel reserves, any Fast Reactor program would have to be government sponsored, especially given the considerable amounts of R&D necessary to get this concept to work.
This is likely to prove unpopular with taxpayers who’ve been sold the mantra of “oh, we don’t want subsidies” for far too long. Granted, as I mentioned before no nuclear program is ever going to be subsidy free (indeed I’d question whether any energy source, even fossil fuels is truly subsidy free). But there’s a bit of a difference between governments slipping the nuclear industry a fiver from time to time (as is currently the case) while quietly picking up the tab for security, insurance and decommissioning – as opposed to blatantly signing an unending line of blank cheques as would be the case with a GcFR.
Given that governments are on the spot for paying for deep geological storage of waste anyway, if a cost benefit analysis could come out showing that GcFR’s and a much less exacting geological storage option are cheaper than the current proposals (of course part of the problem here is that most countries don’t have any disposal plans to do a CBA against!), then there may be merit in the GcFR’s.
Unfortunately, I doubt this is the case. Firstly, we are likely to still need to build some form of geological storage in the near term. As Fukushima showed, it is not sensible or wise to simply allow this nuclear waste to continue piling up in reactor cooling ponds or interim storage pens. Storing waste permanently in these manner is not a long-term solution as such facilities are vulnerable to a host of possible mishaps from terrorist attacks, flooding (many are close to water) a meltdown of the adjoining reactor, or indeed a fire started by a carelessly discarded cigarette.
Even if a major (and highly expensive) policy of building GcFR’s was undertaken tomorrow it would take decades to build enough of them to handle our current inventory of nuclear waste, and further decades, or maybe as long as a good century or two for them to actually process all of this waste. The USA for example has a current inventory of 75,000 tons or so of high level nuclear waste. Given the (likely) smaller capacity factor of these GcFR’s, the fact that the core will still need to be part filled with conventional uranium fuel (or MOX or Plutonium), we could probably only process, say 7-10 tons per year in one (I’m assuming a size of 500 MWe)…so it would take 50 of them 214 – 150 years to process America’s current waste inventory. And that’s just the commercial waste, the Military’s inventory would require another few reactors to handle that. Of course as we are still using nuclear power, and likely (if the nuclear industry has its way) to continue using it and of course these GcFR will be generating some nuclear waste themselves, inevitably we won’t be tackling the current nuclear waste problem terribly quickly and we’ll inevitably still have some amount of nuclear waste that needs to go into deep geological storage at some point in the future. Expensive as deep storage is, I doubt its going to any more expensive that a plan such as I’ve outlined, especially given the likely high costs of those 50 GcFR’s.
The one possibility that does remain is to build one or two of these reactors (i.e. a small handful of them) to “burn” off the world’s outstanding stockpiles of plutonium. But this would be purely a non-proliferation exercise with little to do with mass power generation.
7.6 GcFR’s and proliferation
Of course one major issue with GcFR’s is that they can be used not just to dispose of plutonium, but also to make more of it! So a program of installing them worldwide would include a fairly substantial proliferation risk as well. Thought I suspect, as with the CANDU’s, that entirely depends on who you’re selling them too! Certainly, Proliferation is a major political worry, but that’s the point it’s a political issue not a technical issue, and not something that can be easily accessed here. But certainly in a future world scenario where we are trying to prevent a nuclear arms race between certain states, GcFR’s would the last sort of reactors we’d want to see being built in large numbers worldwide.
So in summary the Gas cooled Fast Reactor has some merit… possibly! As one has never been built its impossible to gauge the possible positives and drawbacks at this time with out resorting to that most tried and trusted of engineering techniques W.E.G (Wild Educated Guess!). But looking at our criteria, the balance of probability is that they would be much more expensive to build, slower to build and have a much lower capacity factor than existing LWR’s. The GcFR would however be able to utilise the Thorium cycle, both in a closed cycle mode or open cycle.
They could be used to reduce our nuclear waste inventory, but again at exactly what pace, and what cost is an open ended question. The balance of probability is too slowly and at too high a cost. There is indeed a risk, if the Fusion power people are to be believed (more on that later) that any GcFR program would be obsolete before they were successfully brought to wide spread usage. While they can generate energy, the goal of said energy production would be to make the job of the energy minister who has to sign all the cheques every month that little bit less painful, rather than a means to an end. So the GcFR isn’t really an option as far as mass power generation is concerned.