Patent Number: 055457975
Section: description

EXAMPLE 1 Preparation of Zr.sub.1-x Pu.sub.x SiO.sub.4 in the Laboratory Dissolve Pu in hydrochloric acid (HCl) and add nitric acid (HNO.sub.3. Evaporate off HCl at about 100.degree. C. Add more HNO.sub.3 and evaporate again (if necessary, repeat to dissolve Pu completely). Add HNO.sub.3 and dilute with water to form an aqueous Pu-nitrate solution. Mix stoichiometric quantities of the Pu-nitrate solution, zirconium nitrate [Zr(NO.sub.3).sub.4 .multidot.yH.sub.2 O] and tetraethylorthosilicate (TEOS) in ethyl alcohol and water to achieve the desired loading of Pu (x-value). Add gadolinium nitrate [Gd(NO.sub.3).sub.2 .multidot.yH.sub.2 O] solution in a small quantity, if a neutron poison is necessary for criticality control. Gd will partially substitute for Pu or Zr. At this stage, a .UPSILON.-radiation emitter, e.g., Co-60, can also be added in small quantities, if easy physical access to the final waste form is to be prevented. Heat this solution to 40.degree. to 50.degree. C. for several days to allow nucleation to occur. Add ammonium hydroxide (NH.sub.4 OH) to form a precipitate. Remove the precipitate and dry at about 90.degree. to 100.degree. C. Calcine the dried precipitate at about 800.degree. C. to remove residual water and to decompose nitrate. The powder can be processed into a final waste form by cold pressing and subsequent sintering at about 1800.degree. C. to produce a high density, impervious, and chemically durable solid that can be placed in a metal canister for transportation, storage, and final disposal in a geologic repository. Alternatively, a metal-sheathed high-density waste form can be obtained by uniaxially cold pre-pressing and subsequently hot pressing the powder in a metal bellows container at temperatures from 1150.degree. to 1350.degree. C. These bellows can then be placed in metal containers for transportation, storage, and disposal. EXAMPLE 2 Preparation of Zr.sub.1-x Pu.sub.x SiO.sub.4 at a Larger Scale For obvious reasons, this process has not yet been tested but is envisaged to be conducted as follows: Convert Pu metal to Pu-nitrate as in Example 1 and dry the nitrate at 90.degree. to 100.degree. C., or convert Pu metal to Pu-oxide by oxidation in air or in oxygen. The rate of oxidation can be controlled by the amount of oxygen or air in the reaction cell. Mix stoichiometric quantities of Pu-oxide with Zr-oxide (ZrO.sub.2) and silicon oxide (SiO.sub.2) powders to achieve desired waste loading. Add neutron poison as an oxide (Gd.sub.2 O.sub.3) powder, if desirable. If Pu is added as nitrate, calcine the mixture at 650.degree. C. to remove water and decompose nitrate. Intimate mixing of the powders, e.g. in a screw blender, is necessary to facilitate the solid state reaction and to keep the reaction temperatures and pressures as low as possible. If necessary, ZrO.sub.2 and SiO.sub.2 powders could be obtained by hydrolysis of mixtures of respective organic precursors (e.g., TEOS or TMOS for SiO.sub.2). Amorphous silica or other reactive products such as xerogels can also be used. After mixing, the powder can be processed as described above. The powder must be transferred into a bellows feeder from where it can be vibrated into the bellows. Prior to cold pressing a small quantity of zircon (ZrSiO.sub.4) doped with a .UPSILON.-emitter, or the .UPSILON.-emitter as such, could be added, if desirable. The .UPSILON.-doped zircon need not be distributed homogeneously and can be introduced into the feeder or into the bellows. Hence, only the steps of bellows filling and cold and hot pressing are conducted in a shielded environment. The exact conditions of the hot pressing step (temperature, pressure and time) of large scale processing of the waste form, starting with oxide mixtures, depend on the details of the process. Approximate values should be as follows: Temperatures 1150.degree. to 1350.degree. C., pressure 15-30 MPa, time 1 to 2 hours. It should be noted that the cold pressing and the heating are carried out in the same bellows. The bellows are first cold pressed to increase the thermal conductivity of the powder, whereupon heating is effected. This will decrease the reaction time at temperature and under pressure. From the foregoing, it can be seen that the inventive method offers a number of advantages over the heretofore known methods. For example, due to the relatively high waste loading that is possible as well as the smaller volume that is achieved, deep, permanent disposal of plutonium in geologic environments where a borosilicate waste form glass would not be stable is possible. Furthermore, due to the high durability of the zircon structure, disposal in an open system in which ground water is present is also possible. The reason that zircon is an improvement over glass for deep disposal is threefold. First of all, zircon is stable at higher temperatures, and deep disposal brings glass into a temperature range in which it is not stable due to the geothermal gradient. Secondly, a higher waste loading in zircon is possible, and this reduces the volume of material that must be placed down a drill hole. Higher waste loading is possible pursuant to the present invention because zircon is durable at high temperatures and the low release rate due to chemical corrosion means that the probability of release, concentration, and ultimately criticality is minimized. Thirdly, methods of criticality control of PuO.sub.2 are well-known from mixed-oxide full (MOX) fabrication and can be applied to the fabrication of zircon. In summary, due to the extremely durable phase of the zircon structure the latter can be used as a plutonium host for the disposal of large quantities of plutonium. This long-term durability of the zircon structure has been confirmed from natural occurrences in diverse and extreme geologic environments over extremely long periods of time. The very low solubility of the zircon structure ensures that plutonium will not be concentrated by later cycles of geochemical alteration to values that might lead to criticality. Finally, the lower volume provided by the inventively produced waste product, and the greater durability, particularly at elevated temperatures, expands the range of possible geologic disposal sites. The present invention is, of course, in no way restricted to the specific disclosure of the specification and examples, but also encompasses any modifications within the scope of the appended claims.