Eric R. Vance

[NZP], a new radiophase for ceramic nuclear waste forms

The NZP structure is a candidate for immobilization of certain types of nuclear waste. It will incorporate 137Cs, 90Sr, and a range of other nuclides. The leach resistance of CsZr2(PO4)3 appears to be comparable with that of other phases under consideration for radio-Cs immobilization. This phase can be formed by sintering at ∼850°C; it is reasonably refractory, and it is compatible with monazite, a favored immobilizing agent for waste actinides.

Crystal data for rare earth orthophosphates of the monazite structure-type

Abstract Unit cell constants for REPO4 (RE = La to Gd) have been re-determined. Nine-fold coordinated ionic radii for Pu3+, Am3+, and Cm3+ in the monazite structure-type have been calculated.

CsAlSi5O12: a possible host for 137Cs immobilization

CsAlSi5O12 exhibits more acid resistance than pollucite (CsAlSi2O6). At pH values of 1.02 and 1.40, the extraction of Cs from CsAlSi5O12 at 25° C was approximately proportional to the square root of leach time. The Cs extraction at 25° C varied as [H+]0.36 over the pH range of 1 to 6. Also, the Cs extraction in various brines at 300° C and 30 MPa was comparable with that for pollucite. CsAlSi5O12 can be crystallized at about 1000° C from calcines if a small amount of CaO is present, but, in the absence of such sintering aids, crystallization temperatures of about 1400° C are necessary. Compatibility data were also obtained with respect to several other phases with which CsAlSi5O12 might be expected to co-exist in tailored ceramics designed for high-level defence waste.

The crystal chemistry of cerium in the monazite structure-type phase of tailored-ceramic nuclear waste forms

Monazite prepared at 1200°C in air was observed to incorporate cerium as 3+ and not 4+ even when charge compensation mechanisms for Ce4+ inclusion were present. The consequences of this phenomenon for the use of monazite structure-type solid solutions for transuranic nuclide isolation in radioactive waste management are discussed. The use of cerium as a substitute for the actinides is also considered.

Irradiated and metamict materials: relevance to radioactive waste science

The relation of the structures of metamict mineral phases and non-crystalline solids is discussed; the question of radiation effects in solidified nuclear waste forms is considered in this regard. In ceramics, if the actinides can be concentrated in radiation-resistant phases such as uraninite or monazite, the overall radiation effects on the ceramics should be quite small.

X-ray studies of iodine sorption in some silver zeolites

After saturation sorption, in a static system, of molecular iodine onto some silver zeolites at 130° C, powder X-ray diffraction analysis revealed sharp reflections due to crystalline α-Agl. However, this α-Agl contained only a fraction of the sorbed iodine and it seems likely that this α-Agl involves “unlocalized” ions such that a continuous three-dimensional network of α-Agl forms in the zeolite channels. Water in the zeolite channels is probably the underlying cause: heating to 200 to 300° C in air destroyed the sharp α-Agl reflections and no such reflections were observed if the sorption was carried out with the zeolites dehydrated beforehand. α-Agl formation was enhanced when the zeolite framework was broken down by heating in air at ≳ 800° C, heating in water or brine at 200° C, or γ-irradiation.

A Single Phase ([Nzp]) Ceramic Radioactive Waste Form

It has been shown that between 10 and 20% of a simulated PW–4b radwaste composition can be incorporated into a single nhase with the NZP (= ‘MaZr 2 P 3 o 12 ’) structure. By changing the P/Na and Zr/Na molar ratios (i.e., adjusting the crystal chemical model of where each ion is located in the structure) it has been possible to outline a very ‘forgiving’ compositional regime both at the 10% and the 20% waste loading level within which one obtains one ([NZP]) or two ([NZP] and [monazite]) phases. We have also succeeded in substituting Tio 2 for Zro 2 in making a TiO 2 -rich [NTP] waste form analogous to the [NZ]] materials. Thus we have succeeded in creating monophasic and diphasic ceramic waste forms which can be sintered below 1000° C. Only preliminary leach data have been obtained at 25° and 300°C, and they place this material with good ceramic forms.

Subsolidus phase relations in the systems CeO2RE2O3 (RE2O3 = C-type rare earth sesquioxide)

Abstract The systems CeO 2  RE 2 O 3 ( RE 2 O 3 = C-type rare earth sesquioxide) were studied to: (1) investigate the claims of several workers for the existence of a complete solid solution series between CeO 2 and RE 2 O 3 and (2) to characterize the weak C-type X-ray diffraction peaks reported by others from samples in the single-phase fluorite solid solution region. It is shown that a complete solid solution series does not exist, and an explanation for the observations of others reporting such is tendered. It is also shown that the observation of C-type reflections in the supposed single-phase fluorite field can be attributed to the partial reduction of Ce 4+ to Ce 3+ at the firing temperature, resulting in the movement of the bulk composition into a two-phase field of the CeO 2  RE 2 O 3 Ce 2 O 3 phase diagram, rather than the formation of a domain structure due to ordering.