Climate change

December 15, 2009

IFR FaD 2 – fuel use

With the Chinese announcing a projected 10-fold increase in their country’s uranium demand by 2030, some observers are worrying that we face a uranium supply crisis. In the short term, there may indeed be bottlenecks, if mining expansion fails to keep pace with escalating demand. (Frankly, I find this unlikely — price will dictate resource investment decisions over this 20 year time frame.) But what about the broader, long-term question that arises from this supply problem? How much uranium is out there, and accessible, and if the world was run entirely on IFRs (or thorium-based LFTRs), how long would we be able to do this before the ‘energy metal’ fuel supply ran out?

This is an interesting and important issue, but it’s also a little complex. So I’m going to need to devote a couple of posts to answer it properly. (Keeping in mind that I want each IFR FaD post to be concise and have a single main message). In this post, I consider how much fuel an IFR would use.

Coal, natural gas and oil, which are the feedstock used to run fossil-fuel-powered thermal generators, embody a convenient and concentrated store of ancient sunlight. But as was discovered in the 1940s, we can also unleash the vast energy contained within the atom. Indeed, splitting (‘fissioning’) the nucleus of a heavy atom like uranium, releases over a million times more energy than chemically adding oxygen atoms to carbon (which is what combustion really does). So, compared to fossil fuels, all forms of nuclear power are incredibly efficient in terms of power density.

It takes 160 — 220 tonnes of natural uranium to fuel a modern 1 gigawatt (GW) nuclear power plant for an entire year (the total energy produced is called a gigawatt year, or GWyr). One GWe of power (recall that the ‘e’ stands for electrical power rather than ‘t’ for thermal power, or heat) is a huge amount. It’s enough to run 65 million desk lamps (assuming they used 15 W compact fluorescent globes), or more practically, to satisfy today’s electricity demand of a typical Australian or US city of about half a million people. For comparison, to deliver a GWyr of energy using a coal-fired power station, about 3 — 7 million tonnes of coal must be burned (the amount can vary depending on the grade of coal).

Most of the nuclear power stations in use today are called ‘thermal reactors’, or ‘light water reactors’ (LWR). They use ordinary (‘light’) water as a coolant, which takes heat away from the reactor core. The water also acts as a ‘moderator’, slowing down subatomic particles, called neutrons, which shoot out of the atom’s nucleus when a chain reaction is underway. These neutrons are responsible for causing unstable, heavy atomic nuclei to split apart and release energy. Other reactor designs use heavy water (enriched in ‘heavy hydrogen’: deuterium) or graphite (a form of carbon found in pencils) to moderate the neutrons, but the effect is similar.

These nuclear power plants need, as fuel, a form (isotope) of uranium that has 143 neutrons in its nucleus, called 235-U (or ‘uranium 235’). This is also called ‘fissile’ uranium because it will readily split apart (or ‘fission’ — the term was borrowed from cellular mitosis) when it absorbs a neutron. Yet natural uranium contains only ~0.7% 235-U; almost all of the other >99% is composed of an isotope that has 3 additional neutrons, called 238-U (or ‘uranium 238’). This much more abundant isotope is called ‘fertile’ uranium. As a result, today’s LWRs are able to extract less than 1% of the atomic energy content of uranium. The rest is discarded, either as used fuel (‘nuclear waste’) or as ‘depleted uranium’. The latter, composed mostly of 238-U, is the tails left over after the fuel has been ‘enriched’ to raise the concentration of 235-U to 3 – 5%. The end product of enrichment is~27 tonnes of uranium which is suitable to be fabricated into fuel rods, and 130 — 200 tonnes of depleted uranium, which is not currently used in the nuclear fuel cycle.

Compare this to the IFR — a ‘fast spectrum reactor’. These reactors are able to not only fission 235-U like LWRs, but also readily ‘breed’ other fissionable isotopes (like 239-Pu, an isotope of plutonium) from fertile 238-U (or 232-Th for LFTRs). This is due to their fast neutrons and greater neutron production at high energies (more on this in later IFR FaD posts). With repeated recycling (more on this later, too), this allows them to unlock virtually all of the energy in nuclear fuel. The amazing upshot is that instead of using ~180 tonnes of natural uranium to produce a GWyr of electricity, IFRs (and LFTRs) require only 1 tonne* of fertile energy metals. (Hang on, if it’s so much more efficient, why aren’t we doing this right now? Long question, answer will come, you guessed it, in future IFR FaD posts).

Okay, so this is a nice, simple ‘rule of thumb’ to remember. A 1 GWe (1000 megawatt) IFR must be supplied with ~1 tonne of natural or depleted uranium per year (it doesn’t matter which). It is roughly 150 to 200 times more efficient with its fuel use than current thermal reactors.

For perspective, to supply all of Australia’s current baseload electricity demand (~30 GWe) with IFRs, we would need to supply them with 30 tonnes of uranium a year (1 tonne of uranium would fit in a milk crate). In 2008, our mines produced 8,430 tonnes. If we instead used lignite coal (which we mostly do), we’d need to mine ~150 million tonnes of coal. This would fit into about 5,000 huge coal container ships. Spot the difference in energy density?

The above fuel figures apply to IFRs that are up and running. But what do you need to get them started? The short answer is ‘lots of fissile material’ (uranium 235, plutonium, or other transuranics) — which is a limitation. The long answer will be given in a future IFR FaD post — after I’ve taken a couple more posts to properly deal with the long-term uranium supply question with IFRs.

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