Pu-loaded glasses and crystals: evolution due to self-irradiation - PowerPoint PPT Presentation

Pu-loaded glasses and crystals: evolution due to self-irradiation Shiryaev A.A. Frumkin Institute of Physical chemistry and electrochemistry RAS, Moscow With contributions by: B.E. Burakov, S.V. Stefanovsky, V.O. Yapaskurt and many other

Immobilisation of actinides • High-purity Pu (weapons-grade, i.e. mostly 239 Pu with low amounts of 238, 240 Pu) can be used in new generation of power plants in MOX (mixed oxide) fuel. • Lower quality Pu, “ scrap ” etc. is not suitable economically for MOX => must be safely immobilised (in US ~20 metric tons … + Russia, UK, France, China … ). Composition of waste for immobilisation heavily depends on initial fuel chemical and isotopic composition, cladding type, burn-up degree. No universal form (matrix) for all waste types can be made!! Waste separation for subsequent separate immobilisation is extremely important.

Amount of a radionuclide which does NOT require special permission to handle (Soviet Radiation safety rules from 1987) Isotope Activity (microCi/(microgram)) 238 Pu 230Th 0.1/(5) 232Th 100/(900 grams) 233U 1/(100) 235U 1/(50000) 238U 100/(300 grams) 2500 g 239 Pu 237Np 0.1/(140) 238Pu 0.1/(0.006) 239Pu 0.1/(0.03) 244Cm 0.1/(0.001)

Pu-doped glasses Two “ complementary ” mechanisms of An-doped waste forms degradation: 1) Thermomechanical stresses 2) Radiation damage (swelling, amorphisation/disordering … .)

Pu-doped single crystals Zircon doped with 2.4 wt% 238 Pu (T 1/2 =87.7 y), grown in July 2001. ~7x10 17 decays/gram. (Eu,Pu)PO 4 monazite doped with 4.9 wt% 238 Pu, grown in Dec. 2003. Now approx. ~2x10 19 decays/gram. ~1.1x10 18 At decays/gram dispersed particles has appeared; at ~5.2x10 18 decays/gram “ peeling ” has started. Images: B.E. Burakov

Pu decay He range ≤ n·10 µm U range ~ n·10 nm µm

Pu-loaded glasses (Lanthanide borosilicate – LaBS and borosilicate SON68-like)

Lanthanide-Borosilicate glass • Maximum PuO 2 concentration in conventional borosilicate glasses is ~3-4 wt.%. Lanthanide- Borosilicate (LaBS) glasses are potentially capable to dissolve up to ~10 wt.% PuO 2 . They are much more durable in water solutions than conventional borosilicate glass (Strachan et al., 1998). • Behaviour of Pu and of some other constituents in LaBS glasses is still poorly constrained.

Glass preparation • PuO 2 powder mixed with chemicals, heated to 1500  C at a rate of 10  C/min, kept for 30 min and quenched. • In contrast to Pu-free glasses of the same composition (Pu → Hf) it is very difficult to obtain homogeneous glass if high PuO 2 loads are used. In some runs the sample is clearly segregated into crystal-like and glassy parts. Range of target chemical compositions (wt%) Al 2 O 3 B 2 O 3 Gd 2 O 3 HfO 2 La 2 O 3 Nd 2 O 3 PuO 2 SiO 2 SrO 8-20 10-22 8-12 4-14 11-18 11-14 0-9.5 18-28 2.5

The LaBS glass seems to be homogeneous on mm-scale (RBS data), but is markedly heterogeneous on sub-mm scale if high PuO 2 loads (>5 mass%) are used. 8000 40000 7000 6000 5000 30000 Pu 4000 Intensity 3000 2000 20000 1000 0 760 780 800 820 840 860 880 900 920 940 10000 0 0 200 400 600 800 1000 Channels

Phase composition of the LaBS glass (9.5 wt% PuO 2 ) Identified phases (XRD+SEM/EDX) PuO 2 : crystallites with sizes of >50 nm. PuO 2 Solid solution of (Pu,Hf)O 2 with a fluorite structure (SEM/EDX/XRD) Britholite Britholite: (approx. REE 10 Si 6 O 24 (OH) 2 ) is a “ real ” powder. • Heterogeneously distributed britholite Young (1 year storage) • Non-stochiometric PuO 2 (?) Old (1.5 year storage) • The silicate network is depolimerised (mostly Q 2 units) Intensity, arb. units PuO 2 1000 2000 3000 10 20 30 40 2  , degrees Raman shift, cm -1

XAS results: Pu L III -edge • Pu is mainly tetravalent(XANES and XPS) First shell shows similarity to PuO 2+x • 1 year storage (PuO 2.2 ?) 1.5 years storage • The main fraction of Pu is in the vitreous FT Magnitude phase. • With increasing storage time the splitting of the first sphere becomes more pronounced. In the fresh glass it comprises two subspheres, whereas for the 2 y.o. glass – three (similar to Conradson et al., JACS, 126, 13443, 2004 ). 0 1 2 3 4 5 6 R, A Sample Atom Distance , А Occupation 2 y.o. glass 1.87-1.92 0.15-0.47 О 2.09-2.12 ~1.2 О 2.20-2.27 4 ± 1 О Pu 3.74 2 ± 0.5 Fresh glass 2.13 1.3 О FT peak of the first coordination shell is asymmetric – superposition О 2.25-2.28 5 of contributions from various Pu 3.66-3.69 2.5 ± 0.5 phases.

Evolution of Pu environment with glass storage time: Wavelet approach 1 year 4 4 0 0 Pu 0.1687 0.02925 0.3375 0.05850 0.5062 0.08775 0.6750 0.1170 3 3 0.8437 0.1463 1.013 • Sharpening of the maxima in R- 0.1755 1.181 0.2048 R, A 1.350 R, A 0.2340 space indicates better separation of REE or Hf(?) 2 2 contributions from glassy and O B crystalline phases. A 1 1 -5 0 5 10 15 20 • PuO 2 and britholite grains are -5 0 5 10 15 20 k, A-1 k, A-1 precipitated, the grain size and crystallographic perfection 1.5 years increases. 4 4 0 0 0.04213 0.2750 0.08425 0.5500 0.1264 0.8250 0.1685 1.100 3 3 0.2106 1.375 0.2528 1.650 0.2949 1.925 R, A R, A 0.3370 2.200 2 2 C D 1 1 -5 0 5 10 15 20 -5 0 5 10 15 20 k, A-1 k, A-1 k 2 -weighting k 3 -weighting

XAS results II: Hf L III -edge • The nearest oxygen is at 1.8 Å; small amount of a heavy element (possibly Pu) is present around 2.5 Å; at 3.6 Å minor oxygen coexists with a heavy element (most likely REE). Hafnium L3 • The peaks due to the second coordination sphere are weak => Hf is mostly in the vitreous phase with coordination number close to 6. • Some (minor) fraction enters the 0 1 2 3 4 5 6 phase with approximate formula R, A GdHfO 1.75 . No evolution of Hf • environment with time.

Hf in Pu-free glass Raman scattering 4.0 0 0.06675 0.1335 3.5 0.2003 Frit 1 high Hf, low Al and Si 0.2670 0.3337 Frit 3 3.0 0.4005 0.4672 Frit 4 0.5340 2.5 R, A Intensity, arb. units Frit 5 low Hf, high Al and Si 2.0 1.5 1.0 -5 0 5 10 15 20 k, A -1 4.0 0 0.07000 200 400 600 800 1000 1200 1400 1600 1800 2000 0.1400 3.5 0.2100 0.2800 Raman shift, cm -1 0.3500 3.0 0.4200 0.4900 No spectral manifestations of HfO 2 as a separate phase. 0.5600 2.5 R, A Hf is mostly in vitreous environment, but weak 2.0 second coordination sphere is also present. 1.5 1.0 Strong dependence on glass composition. -5 0 5 10 15 20 k. A -1

The “ heavy spots ” Glass piece after 2 years of storage. Many bright dots; look like some dust …

The “ heavy spots ” : a closer look Silicates Precipitates of (Pu, Hf)O 2 solid solution and of REE-Al phase!! Dendritic morphology consistent with CaF 2 -structural type dendrites Exsolution (rapid?) of excess PuO 2 ?

LaBS glass with 9.5 wt% PuO 2 : SEM of partly crystallized sample • Precipitation of britholite (?) REE 10 Si 6 O 24 (OH) 2 . The glass retains Al, Hf (supports XAS). • REE, Sr and Si partition to the precipitate. • Pu concentrations are roughly similar for the glass and the precipitate.

The Pu-rich LaBS sample is a glass-crystalline ceramics. Britholite is resistant to radiation. But … . What happens if hot water contacts the glass?

Surface alteration: Pu-free glass Formation of altered layer, which may crack and detach from underlying bulk Rather uniform process on the whole glass surface

Alteration of Pu-rich glass in water

SON68-like glass with 238 Pu 56 days at 90 ° C in distilled water

SON68 with 238 Pu Changes in the photoluminescence spectra of Eu admixture indicate more ordered environment of Eu in the alteration layer in comparison with the bulk glass. Possibly this is related to higher Al/Si ratio.

Just a scientific curiosity? Real life examples

Cracking of the Chernobyl lava Infra-red spectroscopy shows presence of OH-groups in (Zr, U)O 2 and in chernobylite (not in UO 2 !) Moisture-induced spontaneous transition of tetragonal zirconia to monoclinic phase is accompanied by considerable volume increase: possible mechanism of the lava cracking.

UO 2+x inclusions in Chernobyl lava Variable morphology : • Dendrites (quenched supersaturated solution?) • Rounded ( “ molten ” ) pieces Cracks!! Undissolved fuel pellet? UO 2 precipitated from the melt always contains Zr admixture

Aerosols Aerosols (2011-2013 гг)

Spontaneously detached glass particles • Sizes of the glass chips reach 150x200 μ m 2 . • UO 2+x inclusions are between ~1-2 and 5-7 μ m.

Secondary mineralisation Secondary minerals are easily lost α -tra racks: Particle size: 200x100 μ m! 10 min 2h Wt % O 39 UO 2+x 022 1950 1800 Na Na 26 1650 S 1500 S 15 1350 O 1200 Counts U 10 1050 U Na, U 900 Zr U U 750 Si U Cl 600 Ca sulphates (?) Al K S 450 Zr K Mg Cl Ca 300 150 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 keV

Mineral-like ceramics