Polymer compositions and methods for shielding radioactivity

Abstract

A urethane based polymer composition is provided that exhibits superior shielding properties during and after exposure to high level radiation. The composite is formed by mixing a liquid isocyanate monomer, preferably 4,4′-diisocyanate monomer with a liquid phenolic resin, preferably phenol formaldehyde resin, and a phosphate ester flame retardant. An optional pyridine catalyst may be added to shorten the cure time. The resulting composition cures at room temperature and can be utilized in several manners, including spraying or pouring the composition prior to curing over radioactive material to prevent leakage of radiation. The uncured composite can be sprayed on the walls of a room or container to prevent leakage of radiation and can also be used to contain radiation prior to demolition. The uncured composite can also be molded into bricks or panels for use in construction. In a preferred embodiment, the polymer composition further incorporates radioactive waste, namely depleted uranium oxide, and can be used in conjunction with specially designed containers for storing radioactive material. The resulting polymer / waste composition cures at room temperature and does not deteriorate or suffer structural damage when exposed to higher levels of gamma radiation, nor do the mechanical or chemical properties undergo any detectable change. The composition is resistant to biodegradation and combustion, and does not creep or shrink during thermal cycling.

Description

[0001] This application claims the benefit of U.S. Provisional Application Ser. No. 60 / 206,853 filed May 24, 2000; U.S. Provisional Application Ser. No. 60 / 206,888 filed May 24, 2000; U.S. Provisional Application Ser. No. 60 / 210,393 filed Jun. 9, 2000; and is a division of U.S. patent application Ser. No. 09 / 775,359.BACKGROUND OF THE INVENTION [0002] The present invention relates generally to a composition and method for shielding radioactivity and, more particularly, to a modified urethane / phenolic resin polymer composition shielding material capable of encapsulating and stabilizing nuclear waste within a polymeric matrix, and methods for using the polymer composition. Nuclear waste generated by the nuclear energy industry must be immobilized for safety and environmental reasons. The polymer composition of the present invention effectively immobilizes nuclear waste in the polymeric matrices safely and in a cost-effective manner. Further, when depleted uranium is the waste incorpora...

AI Technical Summary

Benefits of technology

[0029] A catalyst may be utilized in applications where a short curing time is necessary. Phenylpropyl pyridine is presently used as a catalyst, reducing the composition cure time to about 20 minutes, depending on environmental conditions.

[0031] In one of the preferred embodiments, the waste that is encapsulated into the polymer matrix is depleted uranium. In a preferred composition, a mixture of about 6% of the liquid monomer defined above and about 94% depleted uranium in powder form is mixed until homogeneous. The mixture is then allowed to cure at room temperature. The cured polymer composition provides excellent radiation shielding characteristics, without damage to the polymer matrix or leaching of the radioactive material. When depleted uranium or other low level radioactive material is incorporated, this composition solves two problems. The composition deals with a problematic waste material and provides a useful product.

[0033] The shielding composition may be utilized in several manners. The monomers can be mixed and the resulting solution sprayed or poured, prior to curing, over the radioactive material, in order to contain the material and prevent leakage of radiation. In addition, the uncured composition can be sprayed on the walls of a room or container to prevent leakage of radiation, and can also be used to contain radiation prior to demolition. Further, the polymer composition can be molded to produce bricks or panels that may be utilized as part of the construction of a containment vessel or room.

[0034] In a preferred use, the polymer is utilized in a thermal desorption / polymer based immobilization method to safely deal with U-238 stored under contaminated mineral oil. In this process, the oil and other organic material is distilled off at reduced pressure in the presence of the inert gas helium. The distilled oil and vapors are combusted in a high efficiency burner. The combustion products are further oxidized by a series of catalytic oxidizers. The off-gases are scrubbed chemically, filtered and the helium is recycled. The uranium metal is cooled in the helium atmosphere and then immobilized with the radiation shielding polymer of the present invention. Fire hazards are eliminated as all the processes are conducted in a helium or flame retardant atmosphere at room temperature.

Problems solved by technology

Depleted uranium is a very low radiation source, but is very toxic and environmentally hazardous.

Further, transportation to other storage sites is not feasible for economic and safety reasons.

This mixture cannot be treated by conventional stabilization techniques.

Treatment of radioactive waste is often further complicated by the presence of metal ions in the waste.

However, conventional methods of containment of the chemically immobilized waste have been shown to be ineffective.

Conventional stabilization / solidification techniques using cement grout have been successfully used to contain certain heavy metal contaminants, typically lead and mercury, but do not perform successfully when the total organic content exceeds about 3%.

At high levels of organic compounds, which is common in the radioactive waste industry, any final product made with cement will remain in paste form, which is unacceptable under Emergency Response Disposal Facility (ERDF) criteria.

In addition, radioactive isotopes cause structural damage to cement based binders.

Vitrification of the cement based binders provides a glassy matrix which is not self healing, increasing leaching problems.

Solubility limitations lead to large volume increases making this method economically infeasible.

Further, vitrification provides only partial remediation of waste, typically about 8% of the clear liquids.

Solvent washing techniques also result in a very large volume of secondary waste streams, contaminated with radionuclides.

Further, when the uranium has been completely washed of oily organics, the metallic uranium, which is pyrophoric, will spontaneously ignite.

The final product therefore does not meet non-leachability and non-pyrophorocity ERDF criteria.

Unfortunately, the amount of bonded water tends to decrease with time, causing a reduction in shielding ability, and the improvement in performance does not justify the increased expense.

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