L.R. Blackburn1, S.A. Walling1, L.J. Gardner1, S.K. Sun1, R.C. Crawford1, C. L. Corkhill1, E. R. Maddrell2, M.C. Stennett1, N.C. Hyatt1

1Immobilisation Science Laboratory, Department of Materials Science and Engineering, The University of Sheffield, S13JD, UK2National Nuclear Laboratory, Sellafield, Cumbria, CA201PJ, UK

Heading

* Author email: lrblackburn2@sheffield.ac.uk

Hot Isostatic Pressing and Alteration of Ceramics Materials for

Immobilisation of Stockpiled UK Plutonium

The United Kingdom holds a substantial inventory of PuO2 (~140 teHM) as a result of PUREX

(Plutonium-Uranium-Reduction-Extraction) reprocessing of spent Magnox and AGR fuels. The

UK Nuclear Decommissioning Authority has identified potential options for reuse or disposal of

surplus Pu via the ‘credible options’ framework. Whilst fabrication of Mixed Oxide Fuels (MOX)

is preferable, immobilisation within a chemically durable ceramic matrix is deemed a suitable

pathway to disposition for Pu material that does not meet the standard for MOX fabrication [1].

Novel thermal treatment technologies such as Hot Isostatic Pressing (HIP) are candidates for

stockpile disposition.Zirconolite, general structure CaZrxTi3-xO7,

crystallises in the zirconolite-2M polytype for 0.80 ≤

x ≤ 1.35, consisting of alternating modules of

CaO8/ZrO7 and corner sharing HTB modules (Fig.

1). Polymorphic transitions are observed with

incorporation of lanthanide and actinides within the

structure. Polytypes are characterised by variations

in module stacking w.r.t. (001) plane. Pu can be

accepted within Ca2+ and Zr4+ sites via Ca1-

xPuxZrTi2-2xMxO7 and CaZr1-xPuxTi2O7 (where M3+ =

Al/Fe/Cr).Fig. 1) General structure of zirconolite-2M [2]

Background and Motives

Fig. 2) Schematic illustration of HIP unit [3]

Hot Isostatic Pressing is the preferred thermal

treatment for processing of UK Pu wastes & residues.

Ceramic and glass forming precursors are subject to

simultaneous heat and pressure applied through Ar gas

medium (Fig. 2). High density ceramic wasteforms can

be achieved in a once-through batch process. There

are many advantages to HIP over conventional thermal

treatment techniques: high density; volume reduction;

insensitivity to physicochemical nature of wastes,

minimal off-gas/secondary effluents; high Pu

accountability. The aim of this work is to characterise

the microstructure and chemical durability of

CaZr0.8Ce0.2Ti2O7 processed by HIP for immobilisation

of Pu.

Experimental Procedure and Characterisation

Pre-processing parameters for HIPed zirconolite wasteforms were optimised

based on our previous work [4]. Conventional solid state precursors CaTiO3,

CeO2, TiO2, ZrO2 were batched according to nominal stoichiometry

CaZr0.8Ce0.2Ti2O7. The sample was calcined ex situ and packed into a

stainless steel (316) canister (Fig. 3). The can was then evacuated under

300 ˚C to remove volatiles, and hermetically sealed.

Ceramic microstructure was characterised by helium pycnometry, powder X-

ray diffraction (XRD) and scanning electron microscopy (SEM-EDS).

Material from the 75 – 150 µm size fraction was subject to a modified PCT-B

style dissolution test (SA/V 600 m-1). Material was exposed to static leaching

in 1M HNO3, 1M NaOH, 1M HCl, 1M H2SO4 and 8M HNO3 at 90 ˚C for 7

days. Concentrations of Ce (Pu surrogate) in the leachate was measured by

ICP-OES analysis.Fig. 3) HIP canister

Results and Discussion

Fig. 4) Powder X-ray diffraction data for HIP sample. Major phase indexed as zirconolite-2M (space group C2/c)

• Reflections attributed to zirconolite-2M indexed by (hkl) values (Fig. 4). Theoretical

reflections from neutron diffraction of undoped CaZrTi2O7 given in red [5].

• Coexistence of zirconolite-2M and zirconolite-4M polytypes observed. Weak supercell

reflections at 2θ = 7.8˚ and 31.1˚ attributed to zirconolite-4M (unit cell doubling along c-axis)

• Considerable perovskite (nominally CaTiO3) observed: diagnostic peak at 2θ = 33˚.

• High yield of secondary perovskite may be detrimental to overall wasteform durability.

• Minor ZrO2 and CeO2 detected, suggesting incomplete immobilisation of Ce within the

ceramic framework.

SEM analysis of HIPed microstructure (ρ = 4.470 ± 0.004 g/cm3)

is shown in Fig. 5. Phases observed in agreement with XRD

measurements. Undigested ceria relic is highlighted. Minor free

ZrO2 and secondary zirconolite-4M phase is inferred from EDS

mapping. Morphological features for particulates within the 75 –

150 μm size fraction prior to dissolution are displayed in Fig. 6.Fig. 5) BSE micrograph of HIP microstructure and corresponding elemental maps

Fig. 6) Morphology of typical granule (75 – 150 μm) prior to static leaching

[1] – Hyatt, N. C. (2017) ‘Plutonium management policy in the United Kingdom: The need for a dual track strategy’, Energy Policy. Elsevier, 101, pp. 303–309.

[2] – Gilbert, M. R. et al. (2010) ‘Synthesis and characterisation of Pu-doped zirconolites - (Ca1-xPux)Zr(Ti2-2xFe2x)O7’, IOP Conference Series: Materials Science and Engineering, 9(012007).

[3] – Steven W. Shaw, Bruce Begg, Sam Moricca, Ken Bateman, “ Design features to facilitate hot isostatic Pressing in remote shielded facilities” conference proceeding 2009, Prague, Czech

Republic.

[4] – Thornber, S. M. et al. (2017) ‘The effect of pre-treatment parameters on the quality of glass-ceramic wasteforms for plutonium immobilisation, consolidated by hot isostatic pressing’, Journal of

Nuclear Materials. Elsevier B.V, 485, pp. 253–261.

[5] – Whittle, K. R. et al. (2008) ‘Neutron and Resonant X-ray Diffraction Studies of Zirconolite-2M’, in Mater. Res. Soc. Symp. Proc.

[6] – K.L. Smith, G.R. Lumpkin, M.G. Blackford, R.A. Day, K.P. Hart 1992: The durability of Synroc. J. Mater. Res. 190: 287-294.

[7] – Begg, B. D. and Vance, E. R. (1997) ‘The Incorporation of Cerium in Zirconolite’, in Mat. Res. Soc. Symp. Proc., pp. 333–340.

[8] – Zhang, Z. et al. (2005) ‘Aqueous dissolution of perovskite (CaTiO3): Effects of surface damage and [Ca2+] in the leachant’, Journal of Materials Research, 20(9), pp. 2462–2473.

Fig. 7) ICP-OES analysis of leachate prior to CaZr0.8Ce0.2Ti2O7 dissolution for 7 days at 90 ˚C

ICP-OES analysis indicates low retention of Ce from

the ceramic wasteform (Fig. 7), suggesting chemical

low durability. Extensive leaching was observed

particularly in 8M HNO3 and 1M H2SO4. Insignificant

leaching was observed in 1M NaOH, suggesting

increased wasteform durability in high pH

environments.

Zirconolite typically demonstrates chemically

excellent durability with other ceramic phases e.g.

perovskite, hollandite, rutile [6]. It was considered

that co-partitioning of Ce within CaTiO3 is

responsible for enhanced leaching rates.

Conclusions:

• Hot Isostatic Pressing is capable of processing high density ceramic wasteforms for Pu immobilisation

• Further optimisation of pre-processing parameters is necessary to improve microstructural homogeneity

• HIP environment promotes Ce4+→Ce3+ reduction, and preferential uptake in secondary perovskite phase

• Perovskite has lower aqueous durability with respect to zirconolite, leading to incongruent dissolution of Ce

from ceramic wasteform

• Promising implications for Pu immobilisation

• Further work aims for active validation (U/Th surrogates) and assessment of Pu retrievability from HIPed

wasteforms

Fig. 8) Powder XRD analysis of specimens subsequent to leaching

Fig. 9) Morphology measurements of particulates leached in 1M H2SO4 demonstrating incongruent dissolution and some apparent surface precipitation

BSE

Ti-Kα

• Powder XRD measurements were taken post

dissolution to determine whether preferential phase

dissociation was occurring (Fig. 8).

• The quantity of perovskite relative to zirconolite

decreases systematically with observed cerium

leaching. No variation in zirconolite intensity is seen.

• Perovskite can readily accept Ce in solid solution [7].

• Ce3+ more readily accommodated without charge

balancing species. Ce4+ → Ce3+ mechanism proposed

due to reducing environment of HIP canister.

• Surface morphology shows preferential phase

dissolution and surface precipitation (Fig. 9).

• EDS measurements suggest precipite may be

anatase (TiO2). Zhang et al., observed extensive

surface anatase when investigating aqueous

durability of CaTiO3 in H2O at 90 ˚C [8].

Implications for Pu: HIP may cause multiphase partitioning of Pu,

however propensity to reduce is greater for Ce. HIP of

CaZr0.8Pu0.2Ti2O7 could be expected to produce a minor secondary

(Ca/Pu)TiO3 phase, with lower demonstrated resistance to

proliferation.

References