The F word

12 May 2022

Is nuclear fission all that it’s cracked down to be?

The world is certainly headed toward an electrified vehicle fleet. The exact characteristics of such a fleet will evolve as it grows but regardless, it’s going to need electricity – lots of electricity. Carbon free electricity from renewable sources is said to be the answer.

Maybe.

Base-load is another term frequently mentioned in the discussion of our energy future. Base-load generally means big power stations. The government currently favors gas-fired plants for this role because they have an improved carbon footprint compared with coal. In the short to medium term, as the EV fleet grows, base-load generation of some kind will remain necessary for mass charging EVs at night when everyone plugs in and there’s no sunshine or wind, not to mention general industrial needs.

It’s clear that coal fired power stations won’t continue as an option, but gas also has its environmental opponents. Regardless, the government remains committed to gas and its recently released National Gas Infrastructure Plan considers the next 20 years. However, 20 years is a long time in politics and green issues are increasingly strong in public opinion. Further, while our medium term gas supply seems reliable it’s certainly not absolutely guaranteed into the longer-term future.

In consideration of the foregoing, is there anything that could replace fossil fuels altogether for a base-load generation? Renewables coupled with extensive battery storage may be able to meet the challenge. Even so, it could be some time before renewable infrastructure is ubiquitous and interconnected to a degree suitable for replacing fossil-fueled base-load capacity. Still, extensive renewables do remain a good idea, and for more than environmental reasons. More on that later.

In many countries, big, regionally located nuclear power plants are a common option for driving base-load generating capacity. Regardless of its bad publicity, nuclear power generation simply works. It’s true that nuclear has some genuine disadvantages but the process is safer than many of its opponents suggest. Education is needed. The following is a primer for the subject.

As virtually everyone would know atoms consist of neutrons, protons and electrons. Splitting atoms apart by having neutrons collide with uranium nuclei is the basis of fission. This is our only feasible means of harnessing nuclear energy for power generation. There’s also much talk about fusion but we’ve been waiting a long time for that magical process to become sustainable and useful in anything other than a hydrogen bomb. All indications are we’ll be waiting much longer again. The joke among those considering the process is that sustainable fusion is about 20 years away - and it always will be. So, fission it is.

Australia mines uranium ore and converts it to yellowcake (a mixture of uranium oxides) for export. About 99 per cent of natural uranium is uranium-238, which is not very radioactive (long half-life) and can’t be used as fuel in a conventional nuclear reactor. Also present is uranium-235, which is much more radioactive (short half-life) and well suited to the basic fission process. Enrichment involves separating uranium-235 from uranium-238.

It’s worth noting a long half-life is often cited as prima facie evidence of a nuclear hazard, but this isn’t entirely correct. The longer the half-life, the lower the radiation. Yellowcake has about the same level of radiation as the natural ore did in the ground, although it is more concentrated by volume. It’s fairly safe to handle with gloves but inhaling it is disastrous, so proper protective gear is mandatory. 

Uranium straight out of the ground is converted to a specific form of uranium oxide (U3O8). This is the main constituent of yellowcake. In turn, this is converted to uranium hexafluoride, which can be made gaseous under certain conditions. This gas is pumped into the centre of a tall, narrow, cylindrical centrifuge. The extra mass of the uranium-238 causes it to move to the wall of the rapidly spinning cylinder while the lighter uranium-235 remains closer to the centre. Each isotope thus separated is drawn off.

The amount of concentration achieved in each single centrifuge is minuscule so the gas passing out of each one is fed into another, then another, then another and so on in a cascade until the desired enrichment is achieved. This is the gas centrifuge method. A number of other processes can be utilised for gas separation but this is the most common modern method.

Importantly, all methods of separating uranium-238 and uranium-235 are time consuming, expensive and require considerable plant infrastructure. Also, the physical engineering requirements of a nuclear bomb are complex. The process is simply not available to a terrorist concocting a bomb in a garage.

Fears over fission reactors became, and remain, focused on the 1986 Chernobyl disaster in the Ukraine, controlled at the time by the Soviet Union. Chernobyl took over from Three Mile Island (1979) as shorthand for inevitable nuclear disaster and in 2011 Fukushima was added to the list. Much of what’s said about these accidents is wrong or at least misleading.

Although there were explosions at Chernobyl and Fukushima these were not nuclear explosions and nowhere near the magnitude of a nuclear explosion. In fact, a nuclear reactor core can never generate a nuclear explosion. The reason is that uranium fuel used in a nuclear reactor core is only enriched to perhaps four per cent. The material in a nuclear bomb is enriched to about 85 per cent.

It’s highly unlikely another Chernobyl-like event could happen. For a start, the Chernobyl reactor was different to current reactors. The complete lack of a thick concrete and steel containment vessel as seen on all modern reactors was a major design oversight. The Soviets realised this defect and built one – after the meltdown and the escape of much radioactive material!

The operational procedures, design characteristics and installation decisions that led to the disasters at Three Mile Island, Chernobyl and Fukushima occurred for a range of different reasons in each case. There are simply too many factors with too many details to recount in these pages, even in summary. However, the litany of failures in these disasters all led to the same result for each reactor – cooling system failure and meltdown of the core (partial at TMI). Also, the resultant heat boiled the residual cooling water into steam and then caused the steam to dissociate into hydrogen and oxygen. In each case, this volatile mix exploded (but no containment breach at TMI). As we said, these were not nuclear explosions.

There are many different types of nuclear reactors and the subject is rich in acronyms. PWRs (Pressurised Water Reactors) are the most common type (63 per cent). In these, the water that passes through the radioactive core doesn’t boil into steam. Rather, it’s pressurised so that it doesn’t boil at all, which allows it to reach temperatures much higher than 100 degrees Celsius. This high temperature water is pumped into a heat exchanger where it’s used to boil unpressurised water into steam and it’s this that drives the turbines. PWRs are less efficient than other common types because of the heat exchanger but they’re safer.

BWRs (Boiling Water Reactors) are the next most common type (18 per cent). In these, the water that passes through the core is boiled into steam and passes directly into the turbines. The lack of a heat exchanger (as in a PWR) is what makes them more thermally efficient. Of course, this allows water from the core to leave the core.

Both PWR and BWR units are classified as LWRs (Light Water Reactors) because they use ordinary water. HWRs (Heavy Water Reactors) use heavy water, which is water in which the hydrogen atoms have a proton and a neutron instead of just a proton, as with ordinary hydrogen. Chemically, it’s the same as ordinary water but it’s about 10 per cent heavier.

HWRs can use un-enriched or low-enriched uranium and save the cost of enrichment, but heavy water itself is much more expensive than ordinary water. PHWRs (Pressurised Heavy Water Reactors) are similar to PWRs in that water from the core is isolated from the turbines. PHWRs comprise 11 per cent of the world’s reactors.

The UK uses mainly GCRs (Gas Cooled Reactors). In these, liquid coolant can’t boil away because the carbon dioxide used as a coolant is already a gas. GCRs can also use unenriched uranium and run at higher temperatures to increase thermal efficiency. The type now in use in the UK is the AGR (Advanced Gas-cooled Reactor). These are, however, scheduled to be replaced with PWRs. The reactors mentioned above are the main types in service today.

The question, though, is what’s to stop further disasters similar to the big three mentioned above? One of the most important changes is the adoption of ‘passive safety’ principles. These are safety procedures that occur without human intervention. For example, more modern reactors are designed so the cooling water also controls, or moderates the speed of the neutrons (see illustration and caption). If all the water drains away the neutrons begin moving too quickly to interact with uranium nuclei, and the reaction stops.

Hardware has also changed. In older reactors the indication a valve or other component was open or closed came from the switch and the power supplied to it. In newer systems the state of the components comes from within the components themselves. Sensing and digital technology have also improved enormously. There have been many advances. In summary, it’s probably fair to say the past has taught us to build better and safer modern reactors. However, the most exiting thing about nuclear energy is the future and the new types of reactors in development.

Molten salt reactors use molten salt as a coolant rather than water. A number of groups are working on different designs. In some, the coolant also carries the reactor fuel, which means the core can be continuously refueled. Because molten salt can reach very high temperatures before boiling, the thermal efficiency of such reactors is better than PWRs. Also, the molten salt isn’t pressurised, so there’s less physical demand on the containment system. What’s more, because there’s no water present there’s no steam and no chance of evolved hydrogen causing an explosion.

If the molten salt coolant carrying fuel becomes too hot it expands and becomes less dense. The reduced density results in greater separation of the fuel elements and fission slows, which reduces temperature. Also, some designs have a ‘freeze plug’ set in the bottom of the core vessel. If temperatures rise too far the plug melts and all the uranium containing liquified salt runs out into holding tanks full of neutron absorbers that kill the fission process. These features are further examples of the passive safety principle.

Molten salt reactors can be designed to use a range of fissionable fuels including thorium, which is three or four times as plentiful as uranium. Again, Australia has plenty of it. Further, the waste products produced from thorium decay faster than those from the uranium fuel cycle so they don’t have to be stored for as long.

Molten salt reactors are just one example of a class known as small modular reactors. The name says it all. These would be manufactured at scale in factories and transported sites on trucks, ready to be activated. SMRs don’t produce as much power as large scale reactors but they’re modular; as many as needed can be linked together on one site or in a more widely distributed and interconnected network of locations. The principle can be further extended by utilising even smaller reactors known as micro-reactors, which can be very localised perhaps running a single large factory.

Regardless of how they’re powered, a modest number of big back-bone power stations constitute a vulnerability to disruption by hostile external interests. An unrealistic concern? Remember that Australia is rich in minerals essential for building the future. No one knows what international tensions might develop. A widely distributed network of micro-reactors coupled with battery backed solar on as many sky-facing surfaces as possible could be the best solution to the potential disruption of our electricity supply from external entities.

There are many aspects of the subject we haven’t covered here, for example, nuclear waste. That’s a whole subject on its own. This is just a brief overview of an extremely complex subject. To be clear, though, we’re not saying Australia should adopt nuclear energy. Maybe nuclear energy could drive our driving future, maybe not. We’re simply saying that the subject and all its complexities, both positive and negative, should be discussed openly and rationally. Then again, with future technical advances, flying pigs might even become a viable means of transportation.

Words: Paul Tuzson.

To be featured in Australasian Automotive June 2022