Modelling the future: how to build safer nuclear reactors

Research institutes across the world have started to explore new materials for nuclear fuel, which could reduce the amount of accidents that can occur in current light water reactors (LWRs). With safety the number one concern, new fuels and cladding materials could be on the horizon, so what’s the issue with the older materials and what are some of the alternatives being explored?

Nuclear power plant

Since the Fukushima nuclear disaster in 2011, there has been an industry-wide push to figure out how to make nuclear power generation safer. A long list of institutions has started collaborative research into developing new nuclear fuels and using materials that will reduce the risk of reactor accidents.

Nuclear power is an important energy source in the US, where it accounts for nearly 20% of the nation's total electricity generation. It is also gaining significant interest in hotspots around the world - notably China, Russia, the US and South Korea, among others - as a cleaner alternative to fossil fuels.

Hazards of traditional nuclear materials

Assistant professor of mechanical and nuclear engineering at Penn State University Professor Michael Tonks, who is involved with three nuclear research projects through the Department of Energy's Nuclear Energy University Program (NEUP), says there are a lot of design plans for future reactors that have improved safety, but the cost of constructing these new reactors is prohibitively high.

"We have this large fleet of existing reactors, so it's important that we can take those existing reactors and make them safer without the huge cost of building a new next-generation reactor," he says. "This is really being pushed throughout the world."

There are programmes in many countries, including France and the UK, to develop new fuel and cladding materials that will be incorporated into current reactors.

Tonks explains that the basic reactor won't be changed, but rather a few of the components inside, such as the zirconium alloy cladding and the uranium dioxide fuel.

"It's important to take existing reactors and make them safer without the huge cost of building a new next-gen reactor."

These materials have been reliable but they also come with certain risks. For example, uranium dioxide is ceramic and has very low thermal conductivity, meaning that it doesn't conduct heat very well. This can lead to heat being trapped inside fuel pellets and an added risk of the pellets overheating and melting if a reactor loses coolant.

Even if a coolant such as water stops flowing due to a power outage, as happened at Fukushima, the reactor will continue to generate heat. If the power station is turned off to stop the fission reaction, there will still be enough decay in the fuel to keep on producing heat and the temperature will rise and rise, and the cooling water will boil and produce steam.

Zirconium alloys are used for the cladding, which is the metal that surrounds the stack of fuel pallets and separates the fuel from the coolant inside the reactor. The main issue with using zirconium alloy is that it's highly reactive with water, particularly steam. If the coolant water heats enough to boil, then the steam starts to form a corrosive layer on the outside from an oxidation reaction, using the oxygen from the water and releasing the hydrogen.

"There are two very bad things that come from that," explains Tonks. "One is that some of that hydrogen gets into the cladding and makes it much more brittle, and so it's much more likely to break, especially in these very high-temperature conditions."

The other problem was what happened at Fukushima. At high temperatures, hydrogen escapes and congregates at the top of the reactor and, as hydrogen is so flammable, it exploded and blew the top off the Fukushima reactor.

"We want to make sure that we don't have this very bad oxidation reaction, so we need a cladding that doesn't corrode the way zirconium does," says Tonks. "And then come up with a fuel that has a lower thermal conductivity."

The method: material modelling

Tonks's role in the research is to try and understand material behaviour on a very small scale - which would require a very powerful microscope to see - and then identify how its structure impacts upon things such as the corrosion behaviour or how well it conducts heat.

"We use various modelling approaches to understand the impact of the small structure on the properties, and larger-scale models use those small-scale simulations or models as reference," he says. "The larger scale models would be something that an engineer could use at his desktop to actually design these new reactor types or these new fuel types, and make sure that everything was going to be safe and optimise the design."

Once they have used the modelling techniques, the team can narrow down the fuel and cladding alternatives for experiments.
"Experiments are very expensive, and they take a long time," says Tonks. "So we can help lower the number of required experiments and make more targeted experiments that are higher value by trying things out on the computer first."

When modelling certain materials, there is still a lot about the properties that the researchers don't understand; however, they have been using the system for a long time, so there is a huge amount of experimental data available about how materials behave.

"That's a big challenge: how to handle these new materials that we don't understand very much about, especially how they behave under the radiation in a reactor," says Tonks. "There are a lot of approximations and estimations, and then we need to compare against data to make sure that our approximations are okay."

Nuclear materials: some possible alternatives

So far, the team has looked at a few promising materials. Tonks says two of them are lower risk but also possibly lower return.

"One of these is to take uranium dioxide and try to mix something into it that will increase the thermal conductivity," he says. "There are a lot of potential additives that we're looking at, and I think some of those have some good potential for very near-term returns because they're a lot like what we know."

"The idea is to switch from uranium dioxide to silicon carbide."

They are exploring two concepts for the cladding and have high hopes for the one, although it may present problems, which Tonks says is higher risk and higher return. The idea is to switch to silicon carbide.

"It's very strong, it has a lot of the same benefits that zirconium does, and it doesn't corrode very much, but it can also be somewhat brittle," he says. "That one has a huge potential to be a good idea, but it also has a few things that are game stoppers if we're not careful."

Fixing nuclear energy's bad reputation

Nuclear power has a bad reputation among many people because of the accidents that have occurred during its history. However, Tonks believes new this research could give people more faith in nuclear, as long as there's a good way of demonstrating and convincing the public that it's becoming safer.

"I think that overall people are very receptive to trying to improve things, as long as they can trust that it really is being improved," he says. "But we also have to get to the point where we can prove it, and we're not there yet."

Tonks is confident that once they can prove things are working better, then public trust will follow.

"I think we can really help convince the public that things are [improving]; they don't have to have the same concerns that they may have had in the past," he says.

In the next few years, the first round of radiation tests will be complete and the team will be able to start to analyse that data. Modelling is just one part of the programme, which involves contributors from Ohio State, MIT, Penn State, as well as Finnish institutions Aalto University and VTT Technical Research Centre working on the cladding research. The work with fuel additives will be carried out in a collaboration between MIT, University of Wisconsin-Madison, Texas A&M, Penn State, Idaho National Laboratory, AREVA, and ANATECH.

"This is a large programme with a lot of really brilliant people who are working on it across the world," says Tonks. "I think because there are so many really good people working on this, there's a really high potential for some very good benefits to come out of this."