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Small modular nuclear – is small beautiful?

By Dave Elliott

A new report ‘The role for nuclear within a low carbon energy system’ from the Energy Technologies Institute, claims that the UK could have 50GW of nuclear power plants by 2050, including Small Modular Reactors (SMRs). Although it says, due to basic economies of construction and operational scale, ‘large reactors are best suited for baseload electricity production’, it notes that, based on using existing sites, there is ‘an upper capacity limit in England and Wales to 2050 from site availability of around 35 GWe’, and it could be less (e.g. if CCS plants need some of the sites). However, there could be more room for small nuclear plants (under 300kW) on new sites, at least 21GW and in theory up to 63GW.

While the ETI notes ‘ the diseconomies of scale associated with smaller reactors’, it says ‘potential attributes include more extensive modularisation, shorter site construction times, and manufacture and assembly of the reactor system in a factory with associated quality and productivity benefits’. It adds that some designs of SMRs ‘seek competitive advantage by moving away from established LWR technology, including High Temperature Gas Reactors and Molten Salt Reactors’.

It also says that, although SMRs ‘may be less cost effective for baseload electricity production, SMRs could fulfil an additional role in a UK low carbon energy system by delivering combined heat and power (CHP) – a major contribution to the decarbonisation of energy use in buildings’ – assuming the necessary district heating infrastructure was available. Crucially, the ETI says ‘SMRs offer more flexibility with deployment locations that could deliver heat into cities via hot water pipelines up to 30km in length’.

So the report looks to a possible major nuclear expansion, and suggests that, with SMRs included, the ‘total nuclear contribution in the 2050 energy mix could be around 50 GWe’, although it warns that ‘SMRs contributing nuclear capacity above 40 GWe will require flexibility in power delivery to aid balancing of the grid’.

The problem ETI is battling with is baseload. Nuclear plants are usually run 24/7 to recoup their high construction costs and avoid varying output, since that introduces operational and safety problems. So they are well suited to meeting baseload requirements. But the continuous level of energy demand falls to around 20GW in the UK in summer at night so, even if all other plants have been shut down, if you have more nuclear plants than that, you will at times be generating excess power. Absent major storage or export options, it will be wasted. So large baseload plants aren’t very useful beyond a certain level. It may be possible to vary output to a degree, to meet the daily demand peaks and troughs – that’s what France does, with slow ramping up and down. But this can’t be done rapidly or repeatedly, e.g. in order to balance the frequent near-random variations in output from variable renewables like wind and solar.

However, some types of small nuclear plants can vary output more easily – the small reactors used in submarines are designed to do this. And in theory, if new small units are also designed so that the heat output can be used direct, rather than just being used to generate power, then that would add more flexibility, assuming there was a nearby heat load. ETI sees 30 km as a maximum distance for heat transmission without too much heat loss, so that defines a potential role for mini-nukes. Most large nuclear plants are located well away from centres of population for safety’s sake, and to get access to cooling water, and so could not supply heat to cities. But it’s claimed that mini-nukes will be safer, and so more acceptable near cities, so they could meet heat loads, and be flexible grid-balancing plants too. A big set of assumptions! The economics also depend crucially on whether heat grids are available. If not, then the cost of building heat networks has to be added to the cost of SMRs:

The ETI report looked at all this in the context of its 2050 scenarios, and concluded that 21GWe of SMR ‘was sufficient for most scenarios’, including it seems for the Clockwork scenario, which has 40GW of nuclear. It would presumably be more than sufficient for their Patchwork scenario that only has 16GW of nuclear, unless SMRs were used to balance the 75GW of wind it has!

Should SMRs be explored in the UK? The ETI says that the case for SMR v. large reactors is not that strong, but would be much improved by heat production if heat grids existed – and also presumably by offering balancing capacity. Maybe. But there are many other, arguably safer and cheaper, options for balancing and heat supply, most of which are available now, whereas it will take decades to develop SMRs. ETI say 2030 at the earliest.

The UK government’s position is fairly cautious. It still sees large conventional reactors as being the main urgent focus, but has backed more research on SMRs, with ‘at least £250 million over the next 5 years’ allocated in the autumn budget to ‘an ambitious nuclear research and development programme’. It has highlighted regulatory issues as being important. It says that, with potentially many plants being sited across the UK near to cities, it is important that any new reactor designs are subjected to rigorous safety assessment. Public confidence in SMRs would suffer if it were perceived that standards were being lowered to facilitate speedier design assessment’.

Globally, progress on SMRs seems to be limited, even though some developers tout them as likely to be cheap, assuming mass production:

It’s not immediately obvious, however, that they will be. So far the rule has been that larger plants are more cost-effective. But that depends on what they are used for. Westinghouse say that their 225MW mini-nuke offers ‘improved energy security and reduction in the overall lifecycle carbon footprint when used to provide power for liquid transportation fuel from resources of oil sands, oil shale and coal-to-liquid applications’. So they come into their own for use at remote oil shale/tar sand extraction sites. Just what we need! A technical fix for fossil fuel extraction…

Though to be fair, they can also be used elsewhere (an early idea was in developing countries) and Westinghouse says that they might be used to balance variable renewables, since ‘daily load follow can be performed from 100% to 20% power at a rate of 5% change per minute; in continuous load follow, the plant can perform load changes of ±10% power at a rate of 2% per minute’. That doesn’t make them that flexible, compared with some new gas turbine designs that can change output much faster, without significant loss of efficiency. For example, GE’s 510MW FlexEfficiency 50 is claimed to be able to ramp up at a rate of 51 MW per minute (i.e. 10% per minute) and still deliver 61% energy conversion efficiency, while Siemens and Alstrom have gas turbines that can evidently ramp even faster:

However, they still all use gas, and the possibility of using mini-nukes in CHP mode for low carbon heat as well as power production may offer some advantages in some locations, if heat grids exist and if the local population would accept the use of these plants near them. That is unclear. There will be safety and security issues, with fissile material and active wastes scattered around the country in small plants. All in all, given the many other options for local heating, it does seem like a desperate last gasp attempt to find a role for nuclear power. Though naturally the IAEA was all for it: But would you want one near you?


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