Defect evolution in burnable absorber candidate material: Uranium diboride, UB2

Abstract

The stability, diffusivity and clustering behaviour of defects in uranium diboride (UB 2) was investigated in light of the potential application as a burnable absorber in nuclear fuel. UB 2 was found to accommodate limited deviations from stoichiometry, which should be a consideration when manufacturing and operating the material. Self-diffusivity of both U and B was found to be sluggish (10 −14 cm 2 /s for B and 10 −19 cm 2 /s for U at 2000 K) and highly anisotropic, with migration along the basal planes being orders of magnitude faster than c-axis migration. The anisotropy of defect migration (both interstitials and vacancies) is predicted to hinder recombination of defects produced by collision cascades, thus limiting the radiation tolerance of the material. Boron and uranium vacancies exhibit a drive to cluster. Boron vacancies in particular, which are mobile on basal planes, are predicted to cluster into strongly bound di-vacancy, which in turn are less mobile. These are then predicted to grow into larger two-dimensional vacancy clusters on the B plane, leading to anisotropic swelling. We provide an analytical expression to predict the stability of these clusters based on purely geometrical considerations. Finally, the accommodation of Li, He and Xe onto vacancy clusters was considered. Li appears to stabilise the structure upon U depletion, while the retention of He and Xe appears to rise with increasing B depletion, through the formation of vacancy clusters.

A high density composite fuel with integrated burnable absorber: U3Si2-UB2

“…Projector augmented-wave potentials [15] were used with the GGA-PBE exchange correlation [16] supplied with the software. In accordance with the results from Burr et al [7], no Hubbard parameter was applied to the U f-electrons.…”

“…UB 2 enriched in 11 B has been the source of burgeoning interest as a fuel material, as a result of its physical and neutronic properties. Recent modelling and experiment suggests that UB 2 may be a significant ATF candidate in its own right [6,7,8] with a uranium density of 11.6 g/cm 3 [9].…”

Influence of boron isotope ratio on the thermal conductivity of uranium diboride (UB2) and zirconium diboride (ZrB2)

“…For example, DFT calculations predict a metallic structure for UO 2 until the additional localization term is included and thereby creates a gap in the electronic band structure [34]. Authors who have previously performed DFT calculations on UB 2 have justified using either a value of 2.0 eV for the Hubbard parameter (based on lattice parameters) [35], or excluded the parameter due to an incorrect prediction for the enthalpy of formation [15]. However, it has been shown experimentally that the 5 f electrons of UB 2 are itinerant due to the small U-U distance, which creates a large ff overlap [36].…”

“…It is a common structure shared with many other diborides including ZrB 2 , which is also used as a burnable absorber in some nuclear fuel designs [14]. Previous work has shown that UB 2 does not accommodate any significant deviations in stoichiometry and can therefore be considered a line compound in a similar manner to U 3 Si 2 [15] and ZrB 2 [16].…”

Steam performance of UB2/U3Si2 composite fuel pellets, compared to U3Si2 reference behaviour

“…However, the commercial production of enriched boron-10 for use in control rods necessarily produces material enriched in boron-11 as a by-product, which has a very small neutron cross section. There is currently renewed interest in UB 2 fuel concepts, most notably though atomic scale modelling to predict physical properties [4,6].…”