The complexities of developing Accident Tolerant Fuel

Above: The basic assembly design of uranium oxide fuel pellets clad with zirconium alloy tubing has remained the fuel of choice for the vast majority of commercial nuclear power plants

There is a growing interest worldwide in the development and deployment of accident tolerant fuels (ATFs). Accident tolerant fuels are included in a wider generic group of Advanced Technology Fuels. ATFs are seen to offer both safety and economic benefits for nuclear power reactors with many designs under consideration. They can be defined as fuels:

• That have potential to enhance the safety in case of a severe accident in a reactor core for a longer time than the current UO2-zirconium alloy fuel system
• That can maintain or improve fuel performance during normal operation and operational transients
• That are compatible with all aspects of the nuclear fuel cycle (transport, storage, and possible use within a closed fuel cycle)

ATFs mostly involve new cladding and fuel pellet designs that increase the performance and severe accident response times of nuclear fuel. They take advantage of new materials that reduce hydrogen build-up, improve fission production retention, and are structurally more resistant to radiation, corrosion, and higher temperatures. In addition, they are expected to last longer than current fuel designs and may extend the time between refuellings from 1.5 to 2 years for pressurised water reactors. This would reduce the amount of fuel assemblies needed, leading to less waste production and a reduction in fuel costs.

Those technologies that are mature enough to start fuel qualification and achieve near-term deployment – within the coming decade in the existing fleet of light water reactors (LWRs) and in the LWR-based small modular reactor (SMR) designs – are termed “evolutionary accident tolerant fuels (eATFs)”. There are numerous designs and concepts that could be termed as eATFs, based on features such as enhanced cladding, modified uranium oxide fuel (including dopants), and increased enrichment.

A great deal of research and development has been undertaken to support the introduction of eATFs into commercial nuclear power reactors, with the first eATFs entering service in 2021 as part of a number of test programmes. These tests support the economic and safety cases for reactor core operations and provide preliminary information to anticipate potential impacts on the back end of the fuel cycle when compared with traditional power reactor fuels. This includes during storage, transportation, reprocessing and recycling, and disposal.

The origins of ATF

Nuclear fuel has been subject to continuous development over the past 40 years and has reached a stage where it can be safely and reliably irradiated up to 65 GWd/tU in commercial nuclear reactors. During this time, there have been many improvements to the original designs and materials, but the basic design of uranium oxide (UO2) fuel pellets clad with zirconium (Zr) alloy tubing has remained the fuel of choice for the vast majority of commercial nuclear power plants. However, severe accidents, such as those at the Three Mile Island plant in 1979 in the US and at Japan’s Fukushima Daiichi NPP in 2011 showed that, under extreme conditions, nuclear fuel will fail. Under certain circumstances, high temperature reactions between zirconium alloys and water will lead to the generation of hydrogen, with the potential for explosions to occur.

During the earthquake and tsunami that seriously damaged the Fukushima Daiichi plant in 2011, the reactors shut down immediately but fission products in the fuel continued to release decay heat. Zirconium alloys in the cladding of the fuel assemblies oxidise rapidly at temperatures a few hundred degrees higher than normal operating temperatures, leading to disintegration of the fuel rods and rapid corrosion of the cladding. This process released hydrogen causing explosions that further damaged the plant.

This disaster prompted researchers to begin developing accident-tolerant fuel solutions which would allow more time before active cooling is required in a loss of coolant accident (LOCA) or, even better, would be able to indefinitely withstand the sustained high temperatures created by decay heat and insufficient cooling, thus preventing or delaying the release of radionuclides during an accident.

Research, focused mainly on the design of fuel pellets and cladding, as well as interactions between the two, almost immediately got under way in key nuclear countries such as the USA, Russia, France the UK, South Korea, Japan and China. This research has been supported by international bodies such as the International Atomic Energy Agency (IAEA) and the OECD’s Nuclear Energy Agency (NEA).

ATF and the IAEA

The IAEA has been supporting international co-operation among member states in fuel development since the 1980s, long before the Fukushima accident. This included efforts to enhance the capacities of computer codes used for predicting fuel behaviour, within the framework of Coordinated Research Projects (CRPs). However, the Fukushima accident demonstrated the need for adequate analysis of all aspects of fuel performance to prevent a failure and to predict fuel behaviour in accidents, as well as the need to test and model the behaviours of ATFs.

The IAEA works through technical meetings, consultancy meetings, conferences, and CRP activity. Two CRPs were organised in the wake of Fukushima to focus on the modelling and testing of nuclear fuels and ATFs in design basis and severe accidents.

The first CRP on “FUel Modelling in Accident Conditions (FUMAC)” (2014-2018) sought to better understand fuel behaviour in accident conditions by identifying best practices in the application of relevant physical models and computer codes, used by different member states, and the enhancement of their predictive capacities. Well checked results of accident simulation experiments and their analyses with advanced fuel performance codes were carried out in the CRP and the results published in an IAEA technical document in 2019.

The second CRP on “Analysis of Options and Experimental Examination of Fuels with Increased Accident Tolerance (ACTOF)” (2015-2019) dealt with the acquisition of data through experiments on advanced technology fuel types and cladding materials. It also looked at development of modelling capacity to predict the behaviour of the components and the integral performance of ATF designs under normal and transient conditions and sought to demonstrate improvements under severe accident conditions. These results were published in an IAEA technical document in 2020.

Based on recommendation of an IAEA Technical Meeting (TM) on Modelling of Fuel Behaviour in Design Basis Accidents and Design Extension Conditions in May 2019, a new CRP on Testing and Simulations of ATF was launched in 2021 and is ongoing. It includes a set of experiments with some participants providing samples to several laboratories. The experiments then provide data on a new set of ATFs. The results will be available in 2024.

This CRP aims to address factors affecting the design, fabrication and in-pile behaviour of currently operating and innovative nuclear fuels and materials for power reactors, and to increase technology readiness. Objectives include:

• To perform experimental tests including single rod and bundle tests on ATFs’ performance under normal, Design Basis (DB) and Design Extension (DE) conditions
• To benchmark fuel codes against new test data either obtained during the CRP or from existing data relevant to advanced fuel and cladding concepts from member states’ experimental programmes
• To develop LOCA evaluation methodology for ATF performance with a view to NPP applications
• To look at part of the methodology for using ATF fuel in NPPs and to develop an open source library on the subject of ATF

ATF development progress

Dr Nicolas Waeckel a former employee of Electricite´ de France (EDF) who worked for many years on ATF development told NEI: “We started to work on ATF together with US DOE right after Fukushima, participating in expert groups in IAEA and OECD.” He added: “I was in charge of developing a new safety standard to qualify this new type of fuel. When you develop a new type of fuel you cannot apply current standards. You have to develop new criteria, a new way of validating the material... and maybe even changing the way of thinking about safety for a type of fuel which can be very different from current fuel.”

Since 2012, the US Department of Energy (DOE) has supported the development of ATF concepts through its Enhanced Accident Tolerant Fuel (EATF) programme. DOE is currently funding research by France’s Framatome as well as GE Hitachi and Westinghouse. Russia is also currently testing its TVS-2M ATF at unit 2 of the Rostov nuclear plant.

Waeckel pointed out that the DOE had received funding from Congress to develop safer fuel “so all the fuel manufacturers and the researchers worked quite hard on development”. As a result, even before Fukushima, many laboratories and manufacturers “had on their shelves in the closet some exotic fuel under development, but on standby,” he said. “However, there was no demand, no real need to put this fuel in a reactor because the current fuel was behaving quite well.”

He added: “People who developed the nuclear industry 15 years ago were pretty good, and they learned a lot from their mistakes and some bad experiences so that now we have achieved very good optimisation.” That, argues Waeckel, is why the fuel concept did not change. “And now, all of a sudden, because of Fukushima, just because DOE had the great idea of saving the world, we are considering change. I think we have to be a bit more humble.”

He said the driver to develop ATF used to be enhanced performance and behaviour in accidents, “but we realised that this is not really achievable in the short term”. So now the focus is on other things – cladding, fuel cycle optimisation – a longer cycle and higher burn up. “This may be of some interest to the operators... improving plant economics – that is the motor,” said Waeckel.

Among the multiple variants of ATF proposed by the various fuel suppliers (see Figure 1), there are two main categories of fuel, with respect to their technology readiness level, which relates to the time and effort needed to develop, qualify, and license the concepts. These are:

• Short-term evolutionary concepts, for example coated Zr-based claddings; stainless steel cladding (eg FeCrAl); high density fuel (eg U3Si2), and doped fuel pellets (eg Cr doped)
• Longer-term revolutionary concepts, such as refractory claddings (eg lined Mo), silicon carbide (SiC) claddings; microcells fuel; micro-encapsulated fuel pellet concepts; uranium nitride fuel

Key requirements for advanced fuels relate to in-reactor fuel performance, cladding performance, and compatibility with all system constraints.

Waeckel stressed that developing a new type of fuel is a long-term objective in terms of its qualification. “Putting new fuel in a reactor is not like putting a new app on your telephone. If the app doesn’t work you just dump it and the phone is still safe. But if you put the wrong fuel in a reactor it could be a nightmare for everyone. So, we have to be very prudent, very careful, think about everything that could happen. No operator in the world wants to experience bad surprises.”

However, this caution is not necessarily a negative thing. “For me, all this is very positive because it is motivating all the labs, all the researchers, and especially the young generation,” he said. “They see a vision for the future in developing this new type of fuel.” Some projects are very exotic and totally different from standard fuel while others are very similar - just fuel with a novel coating. Some of these evolutionary fuels are already under irradiation as part of the testing and qualification process.

However, Waeckel cautioned the ultimate progress for these evolutionary fuels is limited. “What we are talking about is grace time – we are going to gain a couple of minutes. It is not a game changer. It is a small change and offers some benefit. Is it real accident tolerant fuel? Not really, but it is an improvement,” he said.

Concluding, Waeckel said: “My main message is that development is always positive for the nuclear community because we are making progress in all areas – in research, in investigation techniques, in measurement techniques, in modelling, and all these improvements are beneficial for the nuclear industry as a whole,” he said, observing: “So even if the product itself is not going to be a magic answer or a magic solution, it is a platform for the young generation to think about safety, modelling, advanced modelling, advanced investigation.”

Author: Judith Perera, Contributing Editor, Nuclear Engineering International