Patent Number: 054250722
Section: description

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a portion of a steel object 10 is shown having a surface 12 with an internal layer 13 in which radionuclides 14 are embedded. A laser source 16 is shown directed at the surface 12 to apply a local area 18 of intense heat to the surface 12. The laser source 16 as shown in FIG. 2 is arranged to pass in a raster manner, as shown by the arrows, across the surface 12 to pass the local area 18 of intense heat across the surface 12. In operation, the local area 18 of intense heat applied by the laser source 16 is arranged to cause local melting at the surface 12 without vaporization thereof, the molten surface 12 subsequently solidifying and fixing the radionuclides 14 therein as the laser source 16 passes across the surface 12. In an alternative application of the invention shown in FIG. 3 to a concrete object 50 having a surface 52 contaminated with radionuclides (not shown), a layer 54 of a sealant is applied to the surface 52 and is melted by a local area 55 of intense heat applied by a laser source 56 so as to fix the radionuclides to the surface 52. Suitable sealants include: an inorganic paste such as water glass, metal powder, ceramic powder, glass powder, pozzolana and chamotte, or a mixture thereof, and may be applied by conventional techniques such as spraying. The application of pozzolana and chamotte to a concrete surface causes a reaction with free lime at elevated temperatures. This generates a ceramic bond of the coating to the concrete surface, and leaves a glassy substantially poreless coating after application of the intense heat. More than one such layer 54 may be applied. The invention may be performed by alternative heat sources such as: flame, plasma ion, ultrasonic energy, microwaves, and induction heating, for example to melt the layer 54. Suitable laser sources include: a CO.sub.2 laser, a Nd-YAG laser, an excimer laser , or a semi-conductor laser. A neodymium-yttrium aluminium garnet (Nd-YAG) laser source is preferred since the radiation therefrom may be transmitted through a fibre optic cable. Such a cable is readily movable to facilitate movement of the transmitted local area of intense heat from the laser source across the surface. If desired the use of an appropriate sealant layer 54 nay be applied to non-concrete surfaces, eg steel. For most applications of the invention, a local area of intense heat of at least 150 W/cm.sup.2 is preferred. It will be understood that instead of or as well as moving the laser source or the fibre optic cable in the afore-described applications of the invention, the object having the contaminated surface may be moved to pass the local area of intense heat across the surface. Referring to FIG. 4, a portion of concrete object 60 is shown having a surface 62 contaminated with radionuclides (not shown). A first layer 64 of cementitious material is applied to the surface 62, and is set on the surface 62 with the assistance of heat from a laser source 66 arranged to be traversed across the first layer 64, it is soaked with water for about one minute from a water source 68 to reverse the dehydration of lime in the first layer 64, and allowed to reset for more than twenty four hours. A second layer 70 of cementitious material similar to the first layer 64 is applied to the first layer 64, and heat from the laser source 66 is then traversed across the second layer 70 in `x-y` raster manner to set the second layer 70 and produce a vitreous surface 72. The cementitious material for the first layer 64 preferably comprises a mixture in optimum proportions of: Chamotte--70% PA1 Pozzolana--10% PA1 industrial water glass--20% PA1 Pozzolana--40% PA1 Pozzolan--35% PA1 Chamotte--20% PA1 industrial water glass--5% PA1 water PA1 Chamotte PA1 sand/granite PA1 Pozzolana (small amounts) PA1 industrial water glass PA1 water and the second layer 70 preferably comprises a mixture in optimum proportions of: Such a cementitious material should provide sufficient silicate content for the formation of glass in the second layer 70 after heating by the laser source 66, although if desired the first layer 64 and the second layer 70 may have compositions that differ from each other. It is an advantage if the direction of traverse of the laser source 66 on the second layer 70 is perpendicular to the direction of traverse of the laser source 66 on the first layer 64, since this should lead to a smoother surface with improved impact resistance of the second layer 70. Some advantage might be gained in impact resistance of the second layer 70 by adding small amounts of granite powder, or metal powders such as stainless steel to the cementitious mixture. Small amounts of zinc powder in the mixture should also improve the smoothness of the layers 64, 70. For some applications, a thickness of each layer 64, 70 of between 0.5 mm and 0.8 mm should be satisfactory. Suitable lasers include a 2 kW Electrox CO.sub.2 laser, and a 400 W Lumonics Nd-YAG laser. The Nd-YAG laser can be transmitted through optical fibres. A laser beam of spot size between 4 to 8 mm diameter may be used. If desired the surface to be heated by the laser source 66 may be protected by an inert shroud gas such as nitrogen or Argon. Referring now to FIG. 5, a portion of a concrete object 80 is shown having a surface 82 contaminated with radionuclides (not shown). A thick layer 84 (eg &lt;5 mm) of cementitious material is applied to the surface 82, and heat from a laser source 86 then applied to the layer 84 to form a vitreous coating (1 mm) at the surface 88 of the layer 84. The layer 84 preferably comprises a mixture of: Use of a relatively high percentage of Pozzolana/Pozzolan at the top of the layer 84 assists in the formation of the vitreous coating at the surface 88. A laser source 86 similar to the laser source 66 may be used. The thickness of the layer 84 inhibits heat from the laser source 86 reaching the surface 82 at a temperature high enough (500.degree. C.) to cause substantial dehydration of free lime in the layer 84 at the surface 82. Before the layer 84 is applied to the surface 82, an initial heat treatment may be applied to the surface 82 by the laser source 86.