Removal of thorium and uranium from surfaces by attapulgus clay suspensions

Abstract

In the operations connected to the nuclear fuel cycle, equipments and surfaces can undergo radiological contamination for accidental causes or consequently to their use; among radionuclides, Uranium and Thorium are often present and in a large amount. Protection of the operators requires the removal of radioactive substances from the materials involved in the working especially in the case of alpha emitters, whose effects on man are very dangerous. Severe limits to the contamination levels are imposed in the various countries: generally, alpha activity on the working surfaces must be below 1.85 Bq cm −2 Radioactivity elimination current techniques involve the use of physical and chemical methods. The physical or abrasive methods can changed in some degree treated materials and are not suited for a lot of instruments; chemical methods produce large quantities of radioactive wastes which often show some difficulties in handling. Both decontamination techniques can introduce some irradiation and contamination hazards for the operators. In order to reduce these disadvantages, there is arising a certain interest in the use of solid decontaminating agents as easily applicable pastes or gels. At C.A.M.E.N. we have used a suspension of a particular clay. The basic constituent of these decontaminating media is attapulgite, a widely used material utilized to remove resisting dirt present on artworks; its aqueous suspensions show high chemical inertness and thixotropic and not-newtonian properties. The present work describes some decontamination experiments of materials frequently used in the actinides technology, previously contaminated with uranyl nitrate and thorium nitrate. This decontamination is carried out by attapulgus clay aqueous suspensions. Samples, cured by B.S. 4247 specification, of the following materials, stainless steel AISI 304, mild steel UNI Aq 42, aluminum alloy UNI P-AlMg 3.5, lead, polymethylmetacrylate, polyvinylchloride, polytetraflouroethylene, polypropylene, have been contaminated with 0.2 ml of 2% solutions of uranyl nitrate and thorium nitrate (pH = 3.0) respectively. The suspension, obtained on mixing attapulgitic clay and water in predetermined proportions, was laid by a spatula with a 10–12 mm thickness, corresponding to 1.0–1.2 g cm −2. The samples were kept for 48 hours at room temperature; the suspension dries up and severs spontaneously. A light halo eventually remained on the samples and can be removed by wiping delicately surfaces with a lightly wet flock of cotton. The samples have been measured by an alpha scintillator, with ZnS(Ag) detector, after contamination and after each decontamination process; the efficiency of the instrument was determined with uranyl nitrate and thorium nitrate standard Results obtained in experiments are reported as decontamination factor D f(logarithm), where D f: Df = activity before contamination activity after decontamination In Tables I-II values of logarithm of D f found for uranyl nitrate and thorium nitrate for repeated applications of attapulgus clay aqueous suspensions are reported. From the data it appears that: - For most of the examined materials a high removal has been obtained: more than 95% of the initial activity, except for mild steel, where removal t001 TABLE I. Values of log D F for Thorium Nitrate After Decontamination by Apolications of Attapulgus Clay Aqueous Suspensions. Materials Number of Applications Mild steel 0.62 0.77 0.80 0.80 Stainless steel 2.22 2.51 2.53 2.53 Lead 1.38 1.73 1.80 1.80 Aluminium alloy 1.80 1.95 1.97 1.98 Polytetraflouroethylene 1.57 2.22 2.24 2.25 Polyviychloride 2.00 2.82 2.82 2.83 Polyproylene 1.70 2.39 2.41 2.41 Polymethylmetacrylate 2.12 2.89 2.90 2.90 t002 TABLE II. Values of log D F Found for Uranyl Nitrate after Decontamination by Applications of Attapulgus Clay Aqueous Suspensions. Materials Number of Applications Mild steel 0.58 0.72 0.75 0.76 Stainless steel 1.63 1.75 1.76 1.78 Lead 1.01 1.26 1.31 1.32 Aluminium alloy 1.33 1.66 1.72 1.73 Polytetraflouroethylene 1.41 1.54 1.63 1.64 Polyviychloride 2.20 2.41 2.43 2.43 Polypropylene 1.98 2.30 2.32 2.32 Polymethylmetacrylate 2.32 2.49 2.50 2.50 is about 78% for Uranium and 80% for Thorium respectively. - Decontaminated surfaces do not show visible damages except for the mild steel. - Total removed activity increases with the number of applications, to reach a level at which no further improvement is obtainable. - For investigated materials, whose log D F are lower than 1.40 after first attapulgus clay application, it is possible to foresee decontamination factors obtainable in the successive treatment by the following relation which we suggest: log (D F) n = decontamination factor obtained after the first application; (D F) 1 = decontamination factor obtained after the nth application; n = number of applications. - For all the investigated materials, except the mild steel, the residual activity found after four applications is below 1.85 Bq cm −2. The decontaminating method, in our experimental conditions and with the above mentioned exception, allows to reuse uranium and thorium nitrate contaminated materials.