Heavy rare earth elements (HREEs) comprise the latter half of the lanthanide series, typically from gadolinium (Gd) to lutetium (Lu). These elements are characterised by their filling 4f electron shells and a gradual decrease in ionic radius across the series, a phenomenon known as the lanthanide contraction. In nuclear engineering, HREEs are of particular interest due to their exceptionally high thermal neutron absorption cross-sections, which makes them valuable for reactor control, safety, and fuel management strategies.
All lanthanides, including HREEs, possess a typical electronic configuration of , where ranges from 8 for gadolinium to 14 for lutetium. The 4f electrons are largely shielded from chemical interactions, leading to similar chemical properties across the series. However, the increasing nuclear charge results in the lanthanide contraction, causing a steady decrease in ionic radii for the ions. This subtle change in size allows for the chemical separation of HREEs from light rare earth elements (LREEs) and from each other.
The most significant nuclear characteristic of many HREEs is their large thermal neutron absorption cross-sections. This is primarily due to specific nuclear isomers or stable isotopes within their isotopic composition. For example, gadolinium has isotopes such as and with absorption cross-sections of and barns respectively for thermal neutrons, making natural gadolinium one of the strongest neutron absorbers known. This property is crucial for maintaining criticality and ensuring the safe operation of nuclear reactors.
Several HREEs are specifically utilised or encountered within the nuclear industry:
HREEs, particularly gadolinium and dysprosium, are incorporated into control rods, which are designed to regulate the neutron flux and thus the reactor's power level. Their ability to rapidly absorb neutrons allows for effective control over the chain reaction.
Burnable poisons, such as gadolinia, are mixed directly into the nuclear fuel. They compensate for the initial excess reactivity of fresh fuel, ensuring that the reactor operates safely within its design parameters. As the fuel burns, the HREEs gradually transmute into less absorptive isotopes, allowing the reactivity to decrease at a more controlled rate throughout the fuel cycle. The reactivity change over time can be represented by the equation:
where is the effective multiplication factor as a function of time, influenced by the burnup of the neutron poison.
Due to their high density and neutron absorption capabilities, some HREEs can be considered for specialised neutron shielding applications. Furthermore, research continues into potential roles for HREEs in the immobilisation or transmutation of nuclear waste, leveraging their robust chemical forms and neutron interaction properties.
While HREEs are generally considered to have low chemical toxicity compared to other heavy metals, their handling requires standard industrial safety precautions. Of particular concern in nuclear applications are potential radioactive impurities, such as thorium and uranium, which can be found in naturally occurring rare earth ores. Strict purity specifications are therefore essential for HREEs intended for reactor use.