Process for Working Up Uranium-Thorium Wastes

The invention concerns a process which makes possible without expensive uranium-thorium separation the return without loss of uranium-thorium wastes in the form of a stable uranium-thorium solution in the production-casting process. In this process no byproducts are formed which could cause an additional waste problem.

To prepare high temperature reactor-fuel elements there are needed (U,Th)O.sub.2 particles having defined uranium-thorium ratios and specific particle properties. These particles are formed by the kernel casting process under special conditions and are solidified by drying, sintering and coating, e.g., see German Auslegeschrift No. 1,542,346 and related Hackstein U.S. Pat. No. 3,535,264. The entire disclosure of the Hackstein U.S. patent is hereby incorporated by reference and relied upon.

The kernel casting solution consists of a uranyl nitrate-thorium nitrate solution and a polyvinyl alcohol solution which are mixed together in specific ratios shortly before the casting process. Viscosity, pH and ammonium nitrate content substantially influence the formation of kernels and particle shape and therefore must be held within defined limits.

Numerous processes are known for the preparation of the valuable (U,Th)O.sub.2 waste ocurring in the various production steps in the form of, e.g., powders, kernels or particles, practically all of which have as the object the separation of uranium and thorium and therewith the separate return of uranium and thorium.

Thereby there were used exclusively extractive processes which brought about a separation of uranium and thorium from different nitric acid solutions by means of tributyl phosphate (Swiss Pat. No. 442,257) or other extracting agents (AAEC-Report TM310). In other processes thorium was separated through oxalate precipitation and the uranium recovered by purification precipitation or extraction.

However, these processes have various disadvantages. First extraction processes are very expensive and require a large expenditure of time and apparatus. Furthermore, in the working up byproducts occur and therefore, a recovering of uranium and thorium without loss in the casting process is not possible.

A direct recovery of the uranium-thorium solution obtained by dissolving (U,Th)O.sub.2 scrap in nitric acid-hydrofluoric acid was not previously accomplished since in the neutralization of the 6-8N nitric acid solution with ammonia uranium and thorium are precipitated. However, this neutralization is necessary in order to be able to again employ the solution in the production process.

Therefore it was the problem of the present invention to develop a process for working up and the return without loss of uranium-thorium wastes into the kernel casting process which does not require an extractive separation step and in which there is no precipitation of the metal hydroxide in the neutralization of a nitric acid uranium-thorium solution.

This problem was solved by evaporating a solution of uranium-thorium oxide waste dissolved in a nitric acid-hydrofluoric acid mixture up to the appearance of nitrous gases, the residue diluted in the hot condition with water and this solution brought to a pH between 2.5 and 3.5 with ammonia at a temperature below 40.degree. C.

The solution recovered by the process of the invention can be adjusted to any heavy metal (i.e., uranium-thorium) concentration up to about 300 grams/l at a pH of 3 to 3.5, is stable for over one month and is miscible with PVA (polyvinyl alcohol). This solution can be adjusted through further addition of uranium or thorium to the desired uranium-thorium ratio for the casting solution.

The nitric acid-hydrofluoric acid solution can contain for example HNO.sub.3 and HF in a molar ratio of from 100 to 1 to 400 to 1. The exact concentration of the nitric acid is not critical so long as it is strong enough that the mixture has a pH of not over 1.

According to the invention, all wastes can be returned without loss during the production process. This is likewise true for wastes which have a uranium-thorium ratio deviating from the current production.

The (U,Th)O.sub.2 -- waste is, in a given case after suitable pretreatment for removal of carbon and coating, dissolved with double the volume amount of HNO.sub.3 (65%) and 0.06N hydrofluoric acid (based on the HNO.sub.3) in a flask equipped with a reflux condensor. The time up to complete solution depends on the degree of fineness and the U-Th ratio of the scrap. The solution is then freed of excess HNO.sub.3 /HF by distilling twice. Thereby air or another gas is introduced for stirring up and delaying boiling in the heavy syruplike solution. Distillation was continued to the appearance of NO.sub.2 vapors. The thickened residue must still be diluted in the hot state portionwise with water. Generally, it is diluted with water to a heavy metal concentration of 500 to 650 g/l. Since the pH is around 1 it is still not possible to form a U-Th sol in this condition. In the subsequent neutralization to pH 2.5 to 3.5 with ammonia the solution must be unconditionally held below 40.degree. C. in order to prevent a sol formation at a pH above 2, which sol is made known by a dark red color.

The thus recovered, mostly yellow U/Th solution containing about 250 g/l of heavy metal can be added in any amount to the casting solution and yields after customary casting and further treatment particles which correspond to the known production quality, as well as after coating.

The process can comprise, consist essentially of or consist of the steps set forth employing the stated materials.

The process of the invention will be explained further in the following examples. Unless otherwise indicated, all parts and percentages are by weight.

From assorted uncoated nuclear waste there were dissolved 3 kg of uranium-thorium (corresponding to 3.410 kg of (U,Th)O.sub.2) in 6.8 liters of HNO.sub.3 (65%) and 17 ml of HF (40% ) with boiling using a reflux condenser. Depending on the order of fineness the time for dissolving was 30 to 70 hours.

By replacing the reflux condenser with a distillation bridge having a condenser connected thereto as well as a definite introduction of gases or air there were next carefully distilled off 4.4 liters of nitric acid. The syrupy residue was diluted with 3 liters of water and neutralized cold by the addition of about 1.5 liters of NH.sub.4 OH (25%) to a pH of 2.5 to 2.8. By filling up to a final volume of 12 liters at 20.degree. C. there was formed a solution containing about 250 g/l of heavy metal.

The typical analysis of such a solution for example yields the following concentrations and impurities:

This solution which is maintainable for weeks was mixed with two different casting formulations with final concentrations of 120 g/l. The nuclei cast and sintered therefrom corresponded to the customary quality requirements in regard to chemical and physical properties.

Thus, the following analytical values were found for example for sintered kernels with an average atomic ratio of U to Th of 1:10 produced with an addition of 10 to 20% scrap.

About 4 kg of nuclear waste coated with pyrolytic carbon were applied to flat sheets and annealed at 800.degree. C. with introduction of moist air. Then 3 kg U/Th corresponding to 3.410 kg (U,Th)O.sub.2 were weighed out and as described in Example 1 dissolved with boiling in 6.8 liters of HNO.sub.3 (65%) and 17 ml of HF (40%) and further treated up to obtaining of the solution neutralized to a pH of 2.5.

The average atomic weight ratio of U/Th = 1.8 in the scrap was maintained in the solution and yielded at about 260 g/l of heavy metal the following concentrations per kg of solution (density of the solution at 20.degree. C. = about 1.4):

This solution was used as a 20% addition to a plant-casting solution formulation. In the casting of these mixtures and customary further working there were formed (U,Th)O.sub.2 particles with the same properties as the particles recovered from the original casting solution. The analysis of the particles produced showed the following values: