Research on New Cladding and Fuel Materials for Safer Nuclear Reactors

Around the world nuclear energy is a vital source of energy. It is needed because it is a clean energy source and reduces the carbon emissions from fossil fuels. However many people believe that the risk of nuclear accidents does not outweigh the advantages of using nuclear energy. Michael Tonks, assistant professor of mechanical and nuclear engineering at Penn State and director of the Microstructure Science and Engineering Laboratory at Penn State is involved with three projects through the Department of Energy’s Nuclear Energy University Program (NEUP). These projects aim to discover new materials for nuclear fuel, to make the existing light water reactors (LWRs) safer.

Image shows a mesoscale simulation used to predict the impact of fission gas bubbles on uranium dioxide fracture strength. Similar simulations will be used to investigate microcracking in silicon carbide composite cladding. (credit: Michael Tonks)

The projects cover accident tolerant fuels, or fuels with improved tolerance to bear the loss of coolants in a nuclear accident for a much longer time, compared to conventional fuels, providing reactor operators a little more time to resolve issues before large scale damage occurs. Accident tolerant nuclear fuels have to be cost-effective and need to have similar or better performance compared to the existing fuels.

The issues with the Fukushima Daiichi nuclear reactor accident were actually direct issues with the choice of material for the fuel and cladding. And so the idea is that maybe we can change the fuel material or the cladding material, but keep everything else in the reactor the same.

Michael Tonks, Assistant Professor of Mechanical and Nuclear Engineering, Penn State

Metallic cladding surrounds a stack of fuel pellets, thus separating the fuel and the coolant that is present inside the reactor.

Changing the cladding and the fuel is a more near-term and cost-effective solution than replacing the current nuclear reactors with newly-designed reactors, and could also change the future of nuclear energy and improve safety aspects.

Uranium dioxide is the nuclear fuel used in all LWRs in the U.S., and a zirconium alloy is used for the cladding material in these LWRs. The attributes of these materials make them good choices to be employed in nuclear reactors, proven by their good performance. However, their performance is not very good in accident conditions.

Due to its very low thermal conductivity, uranium dioxide traps heat inside the fuel pellet. The low thermal conductivity is counterproductive to a nuclear reactor’s objective of generating heat, it can also cause overheating and melting the fuel pellets if the reactor loses the coolant.

The cladding material, zirconium alloy, is reactive with water, particularly with the steam generated when the coolant water is heated up during an accident. The steam oxidizes the alloy with the result that hydrogen, a highly combustible gas, is released.

The main aim of Tonk’s research is to discover how the structure of the material affects its behavior. For the three projects, he is exploring how the microstructures of new fuel materials and cladding materials are affected under radiation conditions.

It’s well understood that the microstructure has a direct impact on the properties of the material, but my research focuses on harsh environments, where, because of the environment, the microstructure doesn't stay static, but actually changes with time. It's not enough just to design a microstructure that's going to give you the behavior you want. You have to make sure that even as the microstructure evolves, it doesn't ever result in behavior that's going to cause your part or your reactor to fail.

Michael Tonks, Assistant Professor of Mechanical and Nuclear Engineering, Penn State

In order to understand the microstructures, Tonks employed computational models for creating simulations ranging from 1 to 10 µm, which is much smaller than a human hair strand. The simulations display the behavior of a material under various conditions.

Tonks and his group are working on three projects that search for safer reactor fuel alternatives by utilizing the simulations. For cladding, the team is exploring layering other materials over the existing zirconium alloy cladding. With material layers, the researchers hope to obtain the strengths of various metals and avoid the weaknesses. The layer can protect the metal cladding from reacting with steam, and prevent hydrogen from being produced. However, the layers may be more susceptible to damage caused by radiation. Tonks is employing modeling techniques to simulate reactor conditions, and studying the changes these materials undergo.

The research team is also exploring whether a new material, such as a silicon carbide composite, could be used as the cladding material. Silicon carbide has similar benefits to zirconium alloy, and has been employed in non-nuclear applications. It also has the advantage of not reacting with coolant water, so it would not degrade and prevent generation of hydrogen inside the reactor. However the composite material is not easy to fabricate, and can also undergo cracking. Tonks is employing fracture simulations under both non-accident (normal) and accident conditions to determine how radiation causes cracking. The simulations also help to discover whether microcracks allow fission products to escape.

To address the issue of the low thermal conductivity of uranium dioxide, the team is simulating several fuel additives to raise its thermal conductivity. It is also looking at the possible side effects caused by the addition of the additives under a harsh reactor environment.

Our role is developing the models for these systems. No one has ever done this before so there are no models. We are developing the models from scratch and then using them to help evaluate if these concepts are viable or not.

Michael Tonks, Assistant Professor of Mechanical and Nuclear Engineering, Penn State

In particular, the researchers are exploring the potentially damaging interactions that may take place between the new candidate materials and the radiation in accident and non-accident conditions.

We are hoping to be able to apply the tools that we have developed for understanding uranium dioxide and zirconium alloy, but now extend them to look at these new materials.

Michael Tonks, Assistant Professor of Mechanical and Nuclear Engineering, Penn State

A main tool that Tonks is employing for the projects is MARMOT, a mesoscale fuel performance tool being developed by the U.S. Nuclear Energy Advanced Modeling and Simulation Program. Tonks was the main developer for MARMOT when he was at the Idaho National Laboratory.

The work by Tonks and his team can help to evaluate accident tolerant fuels in a quicker manner in comparison to when only experimental data is utilized. Modeling offers data more easily and cheaper than when full nuclear tests are run. The simulations can guide the experimental work of the collaborators by pointing at the likely viable fuels so that the experimental work of the researchers can be prioritized.

The accident tolerant fuels work is funded through the U.S. Department of Energy Nuclear Energy University Program. The University of Wisconsin-Madison and Penn State are collaborating for the silicon carbide work. The multi-layer composite cladding work is a collaborative effort between Ohio State, MIT, Penn State, Aalto University in Finland, and VTT Technical Research Centre in Finland. The MIT, University of Wisconsin-Madison, Texas A&M, Penn State, Idaho National Laboratory, AREVA, and ANATECH, are involved in the work on fuel additives.

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