In this section we describe several applications of neutrons to investigate the crys­tallography and phase composition of uranium, particularly in fuels. Metallic uranium has an orthorhombic crystal structure, leading to anisotropic single-crystal properties such as a negative thermal-expansion along the crystallographic b axis. This leads to substantial integrity problems when the material is heated. Therefore, the vast majority of nuclear fuels in power reactors consist of cubic uranium-oxide whereas in research reactors metallic uranium-molybdenum alloys are also used, with the molybdenum stabilizing the cubic gamma-structure. During operation, i. e. during heating and irradiation, atoms rearrange and phase transformations may occur, with the phases in a spent fuel having different material properties to those of a fresh fuel. For example, thermal gradients of several hundred degrees exist between the centre and outside of a fuel rod, leading to a spatial distribution of crystallographic phases over a distance on the order of a centimeter. The identification of the new phases, determination of their formation conditions and kinetics, as well as establishing their properties, are of paramount importance for new and existing fuel types. Similar considerations apply to structural materials, e. g. cladding or pressure-tubing materials in accident scenarios, and actinide-bearing minerals for mining and waste deposition [45].

In fuels, the elements of interest are high Z-number (uranium and other actinides) and low Z-number (oxygen, nitrogen, carbon). Neutron diffraction is better at deter­mining these structures while X-ray diffraction is biased towards the heavy atoms.

There are three structure models proposed for cubic UC2 [46-48] a non — quenchable phase existing between * 1,823 and *2,104 °C [49]. The three crystal structures differ in the arrangement of the carbon atoms. Simulated X-ray diffraction patterns are similar due to the bias towards the uranium lattice. Simulated neutron diffraction patterns can discriminate between the three different structures (Fig. 4.12) due to the sensitivity to the carbon atoms. Experimental neutron-dif­fraction data matches well with the structure proposed by Bowman [48].

This example illustrates the great advantages neutron diffraction offers over X-ray diffraction for crystal-structure investigations of nuclear materials and in particular nuclear fuels.

The smaller low Z-number elements are typically the mobile species and their rearrangement as a function of temperature leads to phase transitions, and neutron diffraction may be sensitive to these while X-ray diffraction will not. As many phases are non-quenchable, in situ techniques offer great advantages. Classical methods to study phase transitions, such as dilatometry or calorimetry, do not identify the

newly-formed phases and are also sensitive to changes in chemical composition, e. g. rearrangement of the oxygen atoms, without formation of a new, distinct phase. Not surprisingly, neutrons have played a vital role over the past decades in elucidating the properties of nuclear fuels.