Patent Number: 055457975
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method of immobilizing plutonium by atomic scale fixation in a crystalline ceramic in order to provide a durable, disposable waste form or product. With the end of the cold war, it is now necessary to be able to dispose of large quantities of highly pure plutonium, especially plutonium recovered from nuclear weapons. Such plutonium is here referred to as weapons plutonium, indicating that it is not mixed with other nuclides. Heretofore known or proposed methods for the disposal of plutonium are not suitable for disposal of these large quantities of plutonium. In addition, plutonium is highly fissile, and it is necessary to develop extremely durable waste forms which reduce the possibility of mobilization and concentration of plutonium in quantities that can lead to criticality. The known borosilicate glass waste form is not very durable, attains saturation damage due to exposure to radiation in an unacceptably short period of time, and is readily altered, both by physical degradation and chemical alteration under conditions at which waste forms should be stable. It is therefore an object of the present invention to provide a method that provides for the long-term disposal of plutonium in a waste form for which long-term durability can be confirmed and that overcomes the drawbacks of the heretofore known methods, with the inventive method providing a waste product or form that not only protects the environment but also ensures that the plutonium is not readily recoverable for use in weapons. This object, and other objects and advantages of the present invention, will appear more clearly from the following specification and examples. SUMMARY OF THE INVENTION The object of the present invention is realized by a method of atomic scale fixation and immobilization of plutonium to provide a durable, disposable waste product and includes the steps of: providing plutonium in the form of PuO.sub.2 or Pu(NO.sub.3).sub.4, providing ZrO.sub.2 and SiO.sub.2, mixing these three compounds together to form a mixture, cold pressing the mixture to form pellets, blocks, or any desired shape, and heating the pellets, blocks, or other shaped forms under pressure to form the durable, disposable waste product. As used in this application, the term immobilization indicates that the plutonium will not be able to migrate, and the term fixation is used to indicate that the plutonium is fixed at the atomic scale within the zircon structure. The method of the present invention makes it possible to immobilize large amounts of plutonium in a single crystalline phase. In particular, the inventive method involves the chemical reaction, for example hot pressing or sol gel techniques, of SiO.sub.2, ZrO.sub.2 and desirable quantities of PuO.sub.2 or Pu(NO.sub.3).sub.4 (e.g. 10 mole % or more) to form a single phase of zircon doped with plutonium, in other words (Zr, Pu) SiO.sub.4. It should be possible to incorporate this amount of plutonium in the zircon structure. However, if, for example, any of the PuO.sub.2 fails to react and fails to become incorporated into the atomic structure of zircon, then the PuO.sub.2 particles would be "encapsulated" in a matrix of zircon. This is still a highly stable and durable configuration for the waste form. Effective disposal of plutonium requires incorporation into a solid matrix that is suitable for transportation, is resistant to radiation damage and is inert in most near surface environments. The zircon structure produced by the method of the present invention satisfies these requirements. It also avoids criticality. Since the half-life of Pu-239 is 24,000 years, and it is desirable to isolate materials for at least 10 half-lives, this amounts to 240,000 years. This is well within the range for which data are available on the geochemical behavior of natural zircons. In particular, studies have been done on natural zircons which may be up to billions of years old. Thus, zircon is an extremely durable phase. In particular, its properties are known because zircon occurs naturally. For example, zircon is often found as a heavy mineral in stream sediments, and even after transport over great distances shows limited chemical alteration or physical degradation. The minor alteration that zircon undergoes over long periods of time and under extreme conditions makes it a far more desirable structure than the heretofore proposed glasses, which may more readily alter and degrade in relatively shorter periods of time. A 10 mole % substitution of plutonium for zirconium in the zircon structure has little effect on its chemical or physical properties. The study of the radiation effects of plutonium-doped zircon (8 mole %) and natural zircons (up to 4,000 ppm uranium) have shown that there is little difference in the radiation damage results (see The Radiation-Induced Crystalline-To-Amorphous Transition In Zircon, Weber, Ewing and Wang, Journal of Materials Research, Volume 9, Number 3, March 1994). Also, in distinct contrast to borosilicate glass, at temperatures below 80.degree. C. and at a nearly neutral pH, in other words conditions that are pertinent to nuclear waste disposal, zircon is extremely insoluble, so that leaching does not lead to the release, migration or concentration of plutonium. It should also be noted that the recovery of plutonium from the inventively produced zircon waste product is very difficult since the waste product is a highly refractory substance. With respect to criticality, concern thereof can be mitigated by adjusting the waste loading of plutonium in the zircon structure, and also by incorporating neutron absorbing nuclides, such as gadolinium, into the zircon structure. Natural zircons contain small quantities of gadolinium, which is an effective neutron poison. U.S. Pat. 3,959,172, Brownell et al, discloses the immobilization of radionuclides by a gel process or by a hydrothermal slurrying process. Unfortunately, such processes are not suitable for the large-scale provision of a durable, disposable waste product, and this reference certainly does not indicate how to make a (Zr, Pu)SiO.sub.4 single phase waste form. The following are examples showing the processing of Zr.sub.1-x Pu.sub.x SiO.sub.4 as a waste form for weapons plutonium, with the following Example 1 being intended merely to prove synthesis in the laboratory and not being pursuant to the present inventive method, whereas the actual production of a zircon structure waste product made by the inventive method for fixating or immobilizing plutonium is discussed in Example 2. The main goal of all processing techniques, in the laboratory or at large scale, is to achieve an intimate mixture of the reacting constituents in order to obtain maximum waste form performance (high chemical durability). The Pu concentration "x" can range from 0&lt;x&lt;1. The waste form can be produced in glove boxes. Handling techniques of large amounts of PuO.sub.2 powder are well established and used to produce UO.sub.2 -PuO.sub.2 (MOX) fuel for nuclear power reactors. .UPSILON.-radiation emitters can be incorporated in the waste form to limit accessibility. In this case, part of the processing equipment must be shielded.