Nuclear Waste Management

It is now generally recognized (apart from a few recidivists and those with deep investment in the fossil fuel industries) that we have to find alternatives to fossil fuels as energy sources. There is great enthusiasm for solar and wind power, despite the obvious shortcomings for total replacement that are generally overlooked, but one of the obvious replacements, nuclear power, is shunned. There are two reasons: the danger of reactor explosions, such as at Chernobyl and Fukushima, both of which were caused by stupidity, which, unfortunately, is never in short supply, and the hazard of nuclear waste. Whether we can get around stupidity is debatable, but we should be able to design so that the effects are minimal. In this post I want to think about nuclear waste, and I am going to mainly consider the current standard reactor process. However, I argue the main problem is social: people are so against nuclear power it is difficult to get the required programs implemented.

The usual waste is extremely dangerous, and comprises two subsets: fissioned products, which tend to have shorter half-lives, e.g. strontium ninety has a half-life of 29.1 years, and actinides, which have longer half-lives, the plutonium 239 has a half-life of 24,000 years. Because of the latter, very long storage is required, and the usual thought is it has to be stored for a minimum of 100,000 years. My personal view is that is far too short if plutonium is present. If we are going to bury this, we need geological structures that will remain unchanged for that length of time. The amount of waste is fortunately not excessive; a plant that produces 1 GW (and earns more than $1 million/day) apparently produces 20 t/day. The good news is that there are plenty of rock formations that have not changed in 100 million years, so the problem now is to put the waste down in a form that will stay where you put it.

The waste comes out in small ceramic pellets that have been heated at several hundred degrees Centigrade for a number of years. They are tough and can be dumped right then. The usual next step is to encase the radioactive nuclides in molten glass, which is then put into a stainless steel container, which is then put into a copper container. Provided there is no oxygen or sulphur-containing gas, copper simply does not react with anything found naturally in rock. The container then goes into a repository at a depth of several hundred meters in a hard rock, and that is further surrounded by bentonite clay, which is water-tight. 

That is the ideal. Unfortunately, so far there appear to be no such operational depositories, so the waste is buried in somewhat less desirable ways. There is a further problem in that a clay cannot be water-tight forever, and water-tightness may be a problem. A recent report (https://www.nature.com/articles/s41563-019-0579-x) has indicated a possible flaw that may make it necessary to revisit the current storage. When the glass or ceramic is placed in the steel cannister, as it cools a thin gap is created. If water gets into this gap, corrosion might occur, and if it does, the water gets progressively more acid, and that acid might start leaching waste. This is possible because when the waste is vitrified, it is distributed through the glass and some is at the glass surface. In my opinion there are various ways around this. The most obvious is to have only glass on the surface following vitrification, and that may mean two processes, the gap can be enclosed with silicone, and perhaps the steel could be enclosed in molten basalt. I do not know the answers, but I am reasonably convinced there is an answer.

We can do more. Reprocessing recovers plutonium and unburnt uranium, and it is possible to recover more fuel than you started with. This is because the initial fuel is uranium 235, but it would be surrounded by much more uranium 238; this is what absorbs a neutron and converts to plutonium 239. There will also be other useful radioactive isotopes there, such as americium for fire alarms, cobalt sixty for medical use, but of course there will remain a lot of rather noxious material. Most of the rest can be transmuted into more harmless material by irradiating them. Simple, so why don’t we do it? The basic reason is that most of the reactors currently used are of the light-water kind, and these cannot easily be so used. 

Different reactor designs can help the problem. Most current ones use a moderator to slow the neutrons. The advantages of this are stated to be that the required enrichment of uranium is much lower, and the plants are cheaper. The alternative of using fast neutrons (and no moderator) produce much less transuranic waste because the actinides are fissionable with fast neutrons. Paradoxically, Iran’s higher enrichment program could be used in fast neutron reactors and it would be much harder for them to produce bombs, but this seems not to be considered by the anti-Iran brigade. Molten salt reactors are claimed to produce less than one thousandth of the actinides. The actinides are the longest living waste, and they tend to be highly poisonous as well. So why are moderated reactors the predominant reactor? Possibly because they yield far more plutonium, and that is needed for bombs by the nuclear powers. It is alleged they are also cheaper. However, burning off these nuclides economically and safely is some distance away. It would involve a lot of money to set things up, and it would be preferable to develop much better robotic technology because you do not want to expose workers to the radiation while doing the processing. There is a further problem. If you have a large number of countries with nuclear power plants, using current technology, you have a large number of countries producing plutonium. The prospect of rogue countries developing bombs to blast out their neighbours is a deep problem, but there are ways around that. Unfortunately, that involves a somewhat radical change in the way some countries play politics.