Introduction
Since the first operational nuclear power plants, uranium dioxide (UO2) has been used as the primary uranium-carrying compound in the fuel composition. However, following the 2011 Fukushima Daiichi nuclear disaster, the United States Department of Energy’s Accident Tolerant Fuel (ATF) campaign has funded research to find safer alternatives to UO2 in light of the disaster. Uranium mononitride (UN) is a major contender for the development of ATFs, research into it starting even prior to the ATF campaign due to its high 235U loading potential and thermal conductivity. UN’s higher thermal conductivity allows for lower operating temperatures and increased safety margins, both of which would increase safety in an accident scenario.
However, UN’s primary drawback, and the reason for a lack of UN usage in the past is that it has a rapid hydrolysis (shown in Equation 1) reaction with water, making it non-viable for usage in light water reactors (LWR).
Equation 1, UN Hydrolysis reaction
Jolkkonen et al. confirmed the reactivity of UN with superheated steam in a 2016 study. They prepared the UN into pellets using spark plasma sintering and subjected them to a superheated steam-argon gas. They found that there is no thermal threshold of rapid onset for the reaction, though the rate increases with the temperature and pressure of the steam. This relatively rapid hydrolysis in superheated steam presents a major barrier for the usage of UN in LWRs.
UN itself is still potentially a viable candidate as an ATF in future Gen IV reactors which use metallic and gaseous coolants, but one alternative viable in LWRs that is being explored is a composition with another Uranium compound that has higher resistance to hydrolysis. This composition can eliminate the hydrolysis issue and allow for the safe usage of UN in LWRs. This review will examine the current state of research into various UN fuel compositions, such as UN-U3Si2 and UN-U3Si5.
UN-U3Si2 Composition
Triuranium disilicide, uranium silicide, or U3Si2, has received significant attention in the ATF research campaign due to its high density and thermal conductivity similar to UN. However, U3Si2 has a higher resistance to hydrolysis than UN, allowing it to be used as a barrier from the water in a composition with UN. Early research into this composition was focused on a route for fabrication of a viable pellet design, while more recent research has been in direct testing and improvements on these designs in active scenarios.
Early in the ATF campaign, Ortega (2016) et al. demonstrated a viable method to construct fuel pellets with a UN-U3Si2 composition. Powdered UN and U3Si2 were combined at varying compositions, compressed in a hydraulic press, and sintered at around 1400C in a research furnace for varying times. Analysis of the methodology proposed in this study shows that an average of 30% increase in uranium density can be achieved from UO2. This study demonstrated an early strategy to create pellets with UN-U3Si2, and many later studies employed this methodology in their further examination of the composition.
Alternatively to standard sintering, White et al. (2017) tested liquid phase sintering to create UN-U3Si2 pellets. This involves sintering at higher temperature, closer to U3Si2’s melting point (1938K). The sintering temperatures were varied around 1938K to examine the efficacy of liquid phase sintering. The densities of resulting pellets were tested and compared to the theoretical densities of the compositions. They found that the closest they got to the theoretical densities was when the sintering temperature was slightly higher than the melting point of U3Si2, verifying the liquid sintering methodology. They also observed microcracks forming between the nitride and silicide during cooling, which though small is a detriment to potential reactor performance. White et al.’s study proved that liquid phase sintering is potentially a more viable alternative to solid sintering, though more investigation is warranted regarding the formation of microcracks in the structure.
Following the establishment of viable fabrication routes for UN-U3Si2 composite pellets, Lopes et al. (2016) sought to evaluate the reactivity of UN-U3Si2 pellets in steam to support the viability of this composition in limited hydrolysis. The study set out to compare the water reactivity of UN-U3Si2 pellets with that of pure UN pellets. Both pellets were subjected to steam at high temperatures, and they found that UN-U3Si2’s reaction with the water occurred in a different manner and, crucially, over five times slower than that of UN with water. The pure UN pellets underwent a process where the oxide phase formed on the grains exposed to the steam would precipitate, separating the grains from the pellet entirely. This then further exposes more grains, resulting in increased reaction rate and intergranular cracking. However in the UN-U3Si2 pellets, the U3Si2 phase acted as a stabilizing agent for the UN and reacted favorably with the steam. The UN-U3Si2 pellets only experienced intra-granular cracking, which required more energy to proliferate and resulted in less damage. This study overall demonstrated conclusively that UN-U3Si2 is a viable composition to reduce detrimental reactivity of UN-based fuel pellets with steam and water.
In 2020, the Idaho National Laboratory, a major DOE nuclear research lab, performed the ATF-1 experiment involving an irradiation experiment of several ATF candidates, including UN-U3Si2. The experiment involved the irradiation of the rodlets in a test reactor and the non-destructive and destructive analysis of the effects on the rodlets. The UN-U3Si2 in particular performed as expected on the given irradiation, but pellet-cladding mechanical interaction occurred during irradiation. This included permanent deformation of the cladding during irradiation, including hourglassing of the pellet due to increased local stresses. This crucial experiment overall highlights the necessity in future research to optimize the physical stability of the UN-U3Si2 pellets and their cladding.
A recent study conducted by Song et al. attempted to address this issue by utilizing a neural network to tailor the microstructure of UN-U3Si2 to increase thermal conductivity and compressive strength. The microstructure of composite materials such as UN-U3Si2 has a significant effect on physical properties such as compressive strength and thermal conductivity, but it is often difficult to optimize for these qualities. This study attempted to use novel AI techniques to evaluate and optimize for an increase in these physical properties. A CNN model was employed and trained to identify microstructure-proptery pairs using transfer learning, a method in which a large and diverse model is trained with specialized data to make up for a lack of data. They found that optimization for both thermal conductivity and compressive strength is an important tradeoff for the development of ATFs, since the features are often opposing microstructurally. They proposed a framework for the optimization of these features that can possibly be used in future studies to decrease deformation and cracking under irradiation.
UN-U3Si5 Composition
Another UN composite fuel that has stirred recent conversation is a composition with U3Si5. While U3Si5 has less uranium density compared to the extremely dense UN and U3Si2, it offers numerous advantages that make it desirable in the development of ATFs. For one, U3Si5 has a similar neutronic performance to UO2, which would reduce operational changes for its implementation in LWRs.
The ATF-1 experiment performed at Idaho National Laboratory (INL), discussed earlier, also tested several rodlets with UN-U3Si5 compositions. They conducted a low burnup irradiation experiment in the INL Advanced Test Reactor, before conducting extensive non-destructive and destructive analyses on the rods. The UN-U3Si5 rods performed as expected in irradiation and numerous analyses have been conducted on the results of this important first step.
A 2023 study conducted by Hanson et al. examined the results of the previously discussed ATF-1 experiment as it was conducted on UN-U3Si5. The analysis found that UN-U3Si5 performed similar to other successful ATF-1 tests on other materials. They found that the rodlets contained no significant neutronic irregularities and only had extremely minor cracking in the UN-U3Si5 phase. UN-U3Si2 and U3Si2 both underwent slightly more microstructural cracking under irradiation than U3Si5, though still far less than UO2, prompting more research into this fuel. This study highlights that though U3Si5 has lower uranium density than U3Si2, its thermal conductivity and high performance in the ATF-1 test indicate that more research should be conducted into this potentially viable fuel.
Conclusions
Both UN-U3Si3 and UN-U3Si5 have significant potential as potential ATF candidates, and recent research has highlighted this further. UN-U3Si2 has had substantially more research done, due to its very high thermal conductivity and uranium density, and several viable fabrication routes have been shown to produce potential fuel pellets. Ortega et al. (2016) and White et al. (2017) identified two viable routes in solid and liquid phase sintering, which have both been employed in many future studies. UN-U3Si5 has also employed similar fabrication methods as UN-U3Si2, and several variations of both compositions were tested in the ATF-1 experiment, which tested many ATF candidates under irradiation in a test reactor. UN-U3Si5 performed significantly better in the ATF-1 experiment, as was noted by Hanson et al. in a 2023 analysis of the results of the experiment. Both pellets experienced significant corrosion and cracking under irradiation, though UN-U3Si2’s was of greater severity. This motivated Song et al. (2023) to use novel AI microstructure optimization techniques to find methods to increase the compressive strength and thermal conductivity of UN-U3Si2. Both of these ATF fuel candidates have the potential to be viable candidates, however more research needs to be conducted. UN-U3Si2 must have more research conducted into the cause of deformation in the irradiation scenario and how to optimize its features such as compressive strength. UN-U3Si5 must have more research into fabrication routes and optimizing thermal conductivity and density, since its reduced deformation compared to UN-U3Si2 makes it a currently more viable candidate.
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