Materials that adopt the pyrochlore structure tend to generate significant interest from the scientific community as their diverse chemistry leads to a wide range of chemical and physical properties, including interesting magnetic21–23 and luminescent24 behaviour, in addition to ionic
and electrical conductivity.25–29 The wide variety of properties exhibited by
pyrochlores is due to the high level of structural flexibility afforded by this structure type, allowing the incorporation of many combinations of cations, and tolerance of different oxidation states, resulting in the known preparation of over 500 pyrochlores with different compositions.1 With the
highly tuneable nature of the pyrochlore structure allowing different properties to be selected by varying structure and composition, it is unsurprising that many are used for important technological and industrial applications, including in energy materials, catalysis, sensors, and thermal barrier coatings.30–38
components of potential wasteform for radioactive lanthanides and
actinides, including plutonium, uranium and thorium.39–44 Finding a way to
safely immobilise radioactive waste for a prolonged period of time is particularly important given the current social climate, where the need for safe and affordable energy will only increase with the rise in human population, and where there is a need to find a solution to address economic, environmental and security concerns. The UK was one of the first nations to use nuclear fission to produce electricity, meaning that since the 1950s radioactive waste has been accumulating, with the 15 reactors currently operational continuing to add to this dangerous stockpile. The UK government divides radioactive waste into four categories according to the degree of possible contamination: high level waste (HLW) such as spent nuclear fuel which, due to the high level of radioactivity, generates significant amounts of heat; intermediate level waste (ILW), which encompasses a wide array of materials including reactor components, filters and glove boxes; low level waste (LLW), denoting items that have low levels of radioactive contamination, including protective equipment and resins, with the majority of LLW resulting from dismantled nuclear reactors; and very low level waste (VLLW), which includes items from hospitals or universities that have only been contaminated with very low levels of radiation.45
The HLW, generated as a highly acidic liquid, is heated to dryness then mixed with glass in a furnace, producing a molten substance that is poured into corrosion resistant cylindrical stainless steel canisters and allowed to cool and solidify. This process, known as vitrification, reduces the volume of HLW by approximately one third. However, as of yet there is no strategy for the safe disposal of HLW, only a storage procedure.
In a recent report produced by the Department of Business, Energy and Industrial Strategy (BEIS) and the Nuclear Decommissioning Authority (NDA), it was concluded that based on existing stores as of 1st April 2016,
and based on future forecasts, by 2125 the amount of packed radioactive waste produced by the UK will be 4.77 million cubic metres, equivalent to
the volume of Wembley Stadium. Although only 0.03% (1,500 m3) of the total
uranium and plutonium of approximately 200,000 and 114 tonnes, respectively. As the highly radioactive isotopes 235U and 239Pu, used and
produced during the fission process have half-lives of 700 million and 24,100 years, respectively,40
it is worryingly clear that if it is not feasible to dispose of these radioactive waste sources, at the very least a long-term containment and storage strategy needs to be implemented.
The first class of materials employed to immobilise HLW materials, termed first-generation wasteforms were based on borosilicate or aluminophosphate glasses.40,46–48 Depending on the nature of the cation being immobilised, the
composition of the glass would vary to maximise durability and minimise phase separation. Though they are still being used to this day, glass-based wasteforms have several shortcomings, namely, low levels of waste loading and reactivity towards moisture that reduces durability. In addition, radiation damage from the immobilised cations can lead to deformation that can compromise structural integrity. Therefore, it seems that rather than the glasses themselves exhibiting high chemical or physical durability, it is the combination of this immobilisation matrix, surrounded by the additional
containment measures, i.e., the reinforced, corrosion resistant steel
containers, that together are designed to prevent any leaching of the radioactive cations and contamination of the local ecosystem. In order to ensure the safe long-term encapsulation of radioactive materials, it is clear that alternative wasteforms, with improved chemical and physical durability, must be found.
A potential second-generation wasteform for radioactive cations is synthetic
inorganic rock (SYNROC) systems, first developed by Ringwood et al. in the
late 1970s.49–51
This material was designed to mimic naturally occurring minerals that have successfully immobilised radioactive cations for millions of years. For example, natural zirconolite (CaZrTi2O7) can contain up to 20
wt% (weight present) ThO2 and 14 wt% UO2, 52
while natural pyrochlores have been seen to contain as much as 9 wt% ThO2 and 30 wt% UO2.53 The
crystalline nature of ceramics mean that the radioactive cations should in principle be distributed homogenously through the material, and occupy
distinct crystallographic sites, in contrast to the potential cation distribution in amorphous glass-based encapsulation systems. This bodes well for the chemical durability of ceramic-based waseforms which, in order to be effective at safely encapsulating radioactive species for centuries, must be thermodynamically stable and highly resistant to any variation in pH, temperature, moisture levels, as well as many other conditions. Compared to other wasteforms, titanium-containing ceramics have shown particularly good chemical durability,54,55
with the excellent resistance to leaching exhibited by SYNROC due predominantly to the presence of titanium oxide.46 Hollandite (BaAl
2Ti6O16), zirconolite (CaZrTi2O7) and perovskite
(CaTiO3) were the main mineral components of the first synthetic rock
material, SYNROC-C, which was developed to immobilise liquid HLW from used nuclear reactor fuel. Cations including Cs+, K+, Rb+ and Ba2+ can be
immobilised within the hollandite phase, whereas the zirconolite and perovskite minerals primarily immobilise radioactive species including Sr
and Pu, with SYNROC-C capable of storing up to 30 wt% HLW.52
The advantage of SYNROC over earlier wasteforms is its compositional flexibility, which allows it to be tuned for the particular HLW species needing to be immobilised. This tailoring has led to a series of SYNROC variations, including SYNROC-D, which contains nepheline ((Na,K)AlSiO4)
rather than hollandite, and SYNROC-F, a compositional variant containing
pyrochlore ((Ca,Gd,U,Pu,Hf)2Ti2O7), with zirconolite-rich materials
specifically developed for the immobilisation of excess Pu.56–58
The immobilisation of radioactive species will expose the wasteform to α- decay, β-decay and γ-irradiation processes, which can lead to the generation of excess heat, species transmutation and atomic displacement through collisions.59 The α-decay process involves the ejection of an α-particle (He
nucleus) from a radioactive species, causing the parent nucleus to transmutate, for example, converting 239
Pu to 235
U, with the daughter species known as an α-recoil ion. The energy from emitted α-particles is dispersed through ionisation processes, leading to heat being generated in the
wasteform. The α-particle also causes minor displacement of the atoms in its path. However, more localised and therefore more severe atomic
displacement is caused by the much heavier α-recoil ion, causing a cascade
of perturbation in atomic positions and thus significant amorphisation of the structure.60,61 The release of He during the α-decay process can cause the
pressure within the material to increase, meaning wasteforms must exhibit good mechanical strength, to minimise structural damage.62 In addition, as
the daughter species formed during α-decay may have different properties to
the parent atom, wasteforms designed to immobilise HLW must also possess the structural flexibility that enables them to cope with the structural change resulting from this radiation damage process.
β-decay involves an electron being ejected from a radioactive species, causing transmutation of the parent cation. As an electron is significantly lighter than an α-particle, this radiation process results in much less structural disruption compared to the 1500 atom displacements resulting from a single α-decay process.59
The emitted β-particle triggers ionisation
processes, leading to heat generation and structural damage. Indeed, β-decay
is responsible for the majority of wasteform heating during the first 500 years of HLW immobilisation.63
Radioactive species can also damage wasteforms through γ-irradiation,
where highly energetic γ-rays are emitted. As this process does not involve
the ejection of a particle, the γ-irradiation process has a negligible effect on the displacement of atoms in a wasteform.63 However, as γ-rays are highly
energetic, their release does contribute to wasteform heating.
The compositional range and resulting structural flexibility exhibited by pyrochlore ceramics has made them potentially promising radioactive wasteforms. The response to ion-beam irradiation for a series of rare-earth A2B2O7 pyrochlores, in which B = Ti, Sn and Zr has previously been
resistant to radiation damage compared to Ti-bearing pyrochlores. It was concluded that the relative cation size (rA/rB) was a significant factor in
determining how a particular material responds to irradiation. As Sn4+ and
Zr4+
are both larger than Ti4+
, the Sn- and Zr-bearing pyrochlores have lower rA/rB, meaning these structures are closer to the threshold where a defect
fluorite phase may form and thus are more able to account for radiation damage by adopting this alternative crystalline phase, in preference to amorphisation.64
A relationship between radiation resistance and rA/rB has
also been observed for Ga2ZrxTi2–xO7 pyrochlores, where systematically
increasing the Zr content results in a reduction in susceptibility to radiation- induced amorphisation.65 The difference in radiation tolerance for Er
2Ti2O7
and Er2Zr2O7,66 which adopt pyrochlore and defect fluorite structures,
respectively, further highlights how key the rA/rB ratio is in determining the
response of a ceramic material to radiation-induced defects. This study again showed that structures containing A and B cations with highly disparate sizes, i.e., Er2Ti2O7, are less resistant to radiation damage, leading to
amorphisation, whereas compounds for which rA/rB is smaller, such as
Er2Zr2O7, have a higher radiation tolerance.66 In a more recent study of a
Y2SnxTi2–xO7 pyrochlore solid solution, it was observed that susceptibility to
radiation damage decreased linearly with increasing Sn content, again
showing how materials with lower rA/rB are more capable of accommodating
radioactive species.67 A similar trend was seen during an investigation in
which a series of Ln2Sn2O7 pyrochlores (Ln = Y and La-Lu) were irradiated. It
was found that the response of these stannate pyrochlores to ion beam radiation varied greatly, with La-, Nd- and Gd- bearing compounds becoming amorphous following irradiation, whereas the Y and Er pyrochlore derivatives, which both have lower rA/rB, instead adopt the crystalline defect
fluorite structure.68
The seemingly linear relationship between the ionic radius ratio (rA/rB) and
the radiation resistance of a ceramic indicates that it should be possible to synthesise ceramic-based materials that have the correct balance of high chemical durability, as exhibited by Ti-containing materials, combined with the impressive radiation resistance of Zr- and Sn-containing materials, to
produce wasteforms capable of safely immobilising HLW species for thousands of years.