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Why not nuclear and renewables?

By Dave Elliott

Nuclear plants do not generate carbon dioxide, so why can’t we have nuclear AND renewables, supporting each other, as a response to climate change? In evidence to the Energy and Climate Change Select Committee in July Amber Rudd MP, DECC Secretary of State, suggested that despite its high cost nuclear baseload ‘enables us to support more renewables’ and was needed since, ‘as we all know, until we get storage right, renewables are unreliable’. Can nuclear really support renewables, and is it really low carbon?

The first point to make is that although nuclear plants themselves do not generate CO2, producing the fuel they use does. The mining and fabrication of nuclear fuel is an energy-intense, and hence (at present) carbon-intense, activity and, as demand for this fuel rises, the energy (and carbon) debt will rise since lower grade uranium ores will have to be used, undermining the carbon saving benefits of using nuclear plants.

In theory, nuclear energy or even (perversely) renewables, could be used to power nuclear fuel production so as to avoid this problem but there would still be diminishing returns – there are finite reserves of uranium. Overall, if we attempted to expand the use of nuclear dramatically to deal with climate change, we would exhaust the reserves rapidly unless new more fuel-efficient nuclear plants were developed e.g. fast breeders, and even that would not extend the life of the uranium resource indefinitely.

Nor would it deal with the other problems of nuclear power – safety, security, weapons proliferation and terrorist attack risks, rising costs, inflexible operation and active waste disposal. Indeed it could make them worse. There may be some technical options for limiting some of these problems (e.g. the development of smaller plants, plants using thorium and perhaps recycling some nuclear wastes) but, although there are (strong!) disagreements, some say nuclear fission may not be a significant energy supply option for the future.

Even so, it might be argued that nuclear plants can still prove useful in the interim, before the fuel scarcity problem kicks in, for example to backup variable renewables, as Rudd suggested. For good or ill, in fact it does not seem so. Nuclear plants can’t vary their output rapidly or regularly without safety problems. It takes time for the activated xenon gas that is produced when reaction levels are changed to dissipate – it can interfere with proper/safe reactor performance. In any case nuclear plants need to be run 24/7/365 to recoup their large capital cost. So nuclear plants can just about deal with some of the daily energy demand cycles (demand peaks in the evening, low demand at night) but not with the fast irregular variations likely with wind etc. on the grid – they can’t be used to back up the short-term variable output from renewables. It is conceivable that they could be used to cover the occasional longer periods when wind etc. is at minimum. This seems to be what is offered as one option in a new report from the Energy Research Partnership: http://erpuk.org/wp-content/uploads/2015/08/ERP-Flex-Man-Full-Report.pdf   However, that would mean running the plants at lower levels at other times, ready to ramp up slowly to meet the lull periods, which would undermine their economics.

Moreover, if there is a large nuclear contribution and also a large renewables contribution, there can be head-to-head operational conflicts when energy demand is low e.g. at night in summer, when in the UK demand is around 20 GW. The UK is aiming for 16 GW of nuclear by around 2030 and more later (there is talk of 75 GW by 2050) and maybe 30 GW of renewables by around 2020, perhaps more later. Assuming you can’t export all the excess, or store it all, which do you turn off when demand is low?  The nuclear operators do not want nuclear output to be “curtailed”. Neither do the renewable plant operators – they would lose money. It would be a waste either way.

Basically the two technologies are incompatible at large scale on the grid. What you need is one or the other: large, essentially inflexible, nuclear plants with large (very expensive) energy stores to take excess output at low energy demand times, coupled possibly with exporting any excess (as France does) OR a renewables-based system, with a flexible smart grid that balances the variations, using back-up plants (small cheap-to-run gas-fired plants initially, but biomass-fired increasingly), some energy storage (but not much – it is expensive) and demand-side management to reduce/delay peak demand until later. Surplus power at times of low demand can be exported (as with nuclear) and balanced with power imported from overseas if available – the time difference in demand and local variations in wind availability, e.g. across the EU, would help. Having a large inflexible nuclear base-load component on the grid, in such a system, just gets in the way, though a small nuclear component might just about be accommodated. Basically, in the new system, unless you have a vast energy storage capacity (which would be very expensive), having large base-load plants is a PROBLEM not a solution. The old system, with base-load plus top-up, was OK with large inflexible plant, although wasteful (with huge thermal conversion losses), but if we are to use variable renewables on a large scale we need a more flexible system. See the helpful “no wind, no sun” grid balancing chart in this presentation: http://projects.exeter.ac.uk/igov/presentation-renewables-how-far-can-we-go/

There are some other angles: the surplus power from renewables can be converted into hydrogen gas by electrolysis of water and stored, ready for use in a gas turbine plant to make power when demand is high. Or for use as a vehicle fuel. Germany is already doing this via several wind-to-gas/power-to-gas plants, some of them converting the hydrogen to methane gas, using CO2 captured from the air or from power station exhausts, to feed into the national gas main. It has been argued that if you do happen to have a large, already built, nuclear component (as in France) you could do the same with the excess power from that at night, but that seems to be just a way to sustain the over large nuclear fleet for a bit longer! It would not be economic to build large numbers of new nuclear plants to do this, even if their fuel supply could be guaranteed and low carbon long term. On that last point, interestingly, a new study suggests that using thorium could lead to higher net carbon emissions: www.sciencedirect.com/science/article/pii/S0196890415003957

It is conceivable that nuclear fusion may be viable in the longer term (possibly post 2050). Some say that, rather than being used for base-load, fusion might be used for hydrogen production, in which case it might offer a way to balance variable renewables. However that is very speculative, and fusion is still some way off. Certainly, even if all goes well with the current research work, fusion won’t be available in time to deal with the urgent problem of climate change, or to help renewables to do that in the near term.

In terms of the main focus for energy supply, both now and long term, it seems that we really do need to make a choice between nuclear and renewables.

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2 comments

  1. robertok06

    “The mining and fabrication of nuclear fuel is an energy-intense, and hence (at present) carbon-intense, activity and, as demand for this fuel rises, the energy (and carbon) debt will rise since lower grade uranium ores will have to be used, undermining the carbon saving benefits of using nuclear plants.”

    Totally false… as a matter of fact the nuclear fuel cycle has become LESS carbon-intensive, thanks mainly to the increased use of instead of diffusion process.
    On a recent issue of Nature a paper has been published about the extraction of uranium from sea water…

  2. robertok06

    “On that last point, interestingly, a new study suggests that using thorium could lead to higher net carbon emissions: http://www.sciencedirect.com/science/article/pii/S0196890415003957

    ???

    Do you read the papers you link???

    Last sentence of the abstract:

    “Overall, for these four
    nuclear energy technologies, the range of CO2(eq) emissions per kWh generated (6.60–13.2
    gCO2(eq)/kWh) appears to be low in comparison to the majority of electricity-generating technologies.”

    13.2 gCO2/kWhe… how much lower can one go with emissions? What’s the emission rate of PV in baseload mode, i.e. guaranteeing electricity 24h/24?
    C’mon!

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