Understanding the nuclear long game

With the UK likely to turn to nuclear power more in the future, the importance of how best to dispose of nuclear waste will become more pressing. We speak to Dr Claire Corkhill, about her work using the Diamond Light Source aiming to try and understand this nasty waste product.

Nuclear waste – is it all the same or are there various different forms?

There are different categories of radioactive waste: Low Level Waste (LLW) is usually found in very large volumes but is not very radioactive. An example of this would be gloves. LLW arises not only from the nuclear industry but also from universities and hospitals. This is deposited at the Low Level Repository in Cumbria, which is essentially a landfill site.

There are several different types of Higher Activity Waste (HAW). There is 300,000m³ of waste encapsulated in cement in the UK. The uranium and plutonium are separated from the fission products and minor actinides and they’re taken off in one direction and there’s current controversy as to whether they can be used as fuel or not. This mixture is about 95% uranium oxide. The remaining 4-5% is made up of fission products and minor actinides, which are very radioactive. The fission products and the minor actinides are placed inside a boiling liquid nitric acid solution, added into glass and encapsulated inside of a glass structure. There is an additional 150,000m3 of glass in the UK, spent fuel and miscellaneous waste.

What are the usual isotopes of nuclear waste that would be stored?

Fission products, the materials that go into the glass – technetium, neptunium, caesium, zirconium and strontium.

Are there particular elements collected in larger amounts than others?

I’m not sure but there are some from a waste point of view that would be better to sort out. For example caesium and strontium have a half-life of 30 years, so if you could separate them out and leave them to decay separately they’d be more or less safe after 300 years. Some of the other radioactive elements such as technetium have a half-life of 200,000 years so you can’t sit around and wait for it to decay – you need to put it somewhere.

You observe how nuclear waste behaves over time, why is there a need to understand this?

What I’m doing at Diamond is a series of experiments to try and understand the long term performance of materials used in nuclear waste immobilisation, so different cement materials, as part of our nuclear waste is composed of cement materials and chopped metal pieces inside the cement and in the geologic disposal facility. If you think about it, it’s like a series of Russian dolls, the idea being you have an engineer’s design where you have the nuclear waste in a container, and the container is surrounded by cement, acting as a backfill. The cement is the host geology, so I’m interested in the cement backfill part as well as the cement that’s within the waste itself. Because we need to dispose of the waste in an isolated place and wait for it to decay, looking forward at least 100,000 years, we want to have an idea of how safe this engineered facility is going to be over those time scales.

...So, the structural changes to the cement?

It’s more to do with chemical changes to the cement and that’s what Diamond and the Long Duration facility allows us to do – observe how the cement changes as a function of time over two years. Those changes occur when you add water to cement and it turns into a solid, hard material. During that process you form new materials and new minerals so you dissolve the cement minerals and reprecipitate new ones. That reaction keeps going as long as water is still inside your cement. We know this takes many thousands of years and what we’ve been doing at Diamond is to try and identify the phases that happen during that time period and it’s these phases that will absorb any radioactive elements that might be released from the waste. We’re trying to predict which phases are going to be present to stop the radionuclides from being released.

What have you found out so far?

Using this facility has been able to give us an unprecedented level of detail with the reaction we’re trying to follow; you can’t do this type of experiment in a laboratory as you can’t get the right level of detail. In the lab you’re changing the samples; you’re bringing the materials in and out of the cupboard, at Diamond we can follow very carefully in situ what happens. This has allowed us to create a thermodynamic model which can predict, with some level of certainty, how those cement phases will change up to two thousand years in the future. That’s really important as two thousand years is when you would expect groundwater to begin interacting with the cement.

Would you say the type of research you’re doing at the moment is new compared to say 20/30 years ago?

Trevor Rayment: Things in the nuclear industry are very slow, and they’re slow because of the difficulty in carrying out experiments and because if you get it wrong, you don’t even want to think about it. So everyone is very very safety conscious and even more so now because they are even more transparent about their processes, so if anything does go wrong in the UK they confess up about it very quickly. It just means things happen slowly but that doesn’t mean nothing is happen it just means it takes a long time for applications to bear fruit and Claire has talked about the work they’re doing on cement. There’s other work being carried out at Diamond about dealing with this waste that is stored in Sellafield. There is a really hideous problem as I understand it with chippings (swarf).

The work done at Diamond over the last few years has given them a hope of finding a quicker and cheaper process of storing the waste material in one of their rather hideous storage compartments, it’s slow but the use of the synchrotron is throwing light on how to do things.

What other experiments are going on at the moment to do with nuclear waste at Diamond?

TR: There are a good number of UK universities who use our facilities including Manchester, Sheffield, Imperial College, Bristol and Oxford. There are people doing very basic research in Birmingham.

If I wanted to highlight the themes and trends, at Manchester, they’re interested in the way that if these active materials do get into the environment, how they will travel through rocks and minerals.

There is a group in Bristol working on a radically simplified approach to packaging and disposal of intermediate level waste. The proposal is to replace a 22 step process with a three-step process which stores the waste ‘raw’ with concrete grout inside a shielded container. It also involves researchers from Leeds and London South Bank University and Sellafield.

Then again there’s a whole group of people who look at materials that are needed for the next generation of nuclear reactors, as the next generation of nuclear reactors will require new materials, new steel that is more resistant. These include researchers from Oxford and Manchester who use Diamond.

CC: We are also doing some of this at Sheffield too and are making MAX phases, which are highly radiation and heat resistant, so good for reactors. Some of the cement systems being worked on, in particular magnesium phosphate cements, are extremely promising for use in new reactor buildings as they also can stand high levels of radiation.

Trevor Rayment is Physical Science Director at Diamond Light Source.

Dr claire Corkhill is a Vice-Chancellor's research fellow from the University of Sheffield.