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Two binary phases in the system Hf–O have been synthesized at pressures between 12 and 34 GPa and at temperatures up to 3000 K by reacting Hf with HfO2 using a laser-heated diamond anvil cell. In situ X-ray diffraction in conjunction with density functional theory calculations has been employed to characterize a previously unreported tetragonal Hf8O7 phase. This phase has a structure which is based on an fcc Hf packing with oxygen atoms occupying octahedral interstitial positions. Its predicted bulk modulus is 223(1) GPa. The second phase has a composition close to Hf6O, where oxygen atoms occupy octahedral interstitial sites in an hcp Hf packing. Its experimentally determined bulk modulus is 128(30) GPa. The phase diagram of Hf metal was further constrained at high pressures and temperatures, where we show that α-Hf transforms to β-Hf around 2160(150) K and 18.2 GPa and β-Hf remains stable up to at least 2800 K at this pressure.

The safe, reliable, and economic operation of the nation’s nuclear power reactor fleet has always been a top priority for the nuclear industry. Continual improvement of technology, including advanced materials and nuclear fuels, remains central to the industry’s success. Enhancing the accident tolerance of light water reactors (LWRs) became a topic of serious discussion following the 2011 Great East Japan Earthquake, resulting tsunami, and subsequent damage to the Fukushima Daiichi nuclear power plant complex. The overall goal for the development of accident-tolerant fuel (ATF) for LWRs is to identify alternative fuel system technologies to further enhance the safety, competitiveness, and economics of commercial nuclear power. Designed for use in the current fleet of commercial LWRs or in reactor concepts with design certifications (GEN-III+), fuels with enhanced accident tolerance would endure loss of active cooling in the reactor core for a considerably longer period of time than the current fue...

Transport of fission products (FPs) inside fuel pellets is an important mechanism that affects microstructure evolution as well as fuel performance. To study this phenomenon for low fuel burnups, w...

When a crystalline material is made with radioactive isotopes, the structure of that material will change as the radioisotope decays. Using density functional theory, we explore the potential structures formed from this decay, a process we term radioparagenesis. Using three systems as examples – CsCl, SrO, and Lu2O3 – we describe how in each case a here-to-fore unobserved crystalline phase of BaCl, ZrO, and Hf2O3 can be formed, resulting in novel crystalline materials. We examine how the formation of these phases depends on the parent structure and the pathways available to the system upon the decay of the radioisotope. We discuss the implications of this phenomenon for the formation of new materials.

Atomistic simulation techniques have been used to predict the preferential segregation of Y3+ ions to the (100), (101) and (110) surfaces of tetragonal zirconia (t-ZrO2). It is found that segregation energetics vary greatly between surfaces. In particular, dopant ions segregate to the top of the (101) surface. Conversely, although they also segregate towards the (100) and (110) surfaces, Y3+ becomes trapped just beneath these surfaces. For all of these surfaces, segregation effects are negligible below 12A. The surface orientation dependence will result in significant variations in the concentration of yttrium at different surfaces. As a consequence, properties that are a function of defect concentration and distribution will be surface dependent. Predictive understanding of such segregation effects will provide the possibility of better engineered devices for a variety of thermal and electrochemical applications.