Patent Number: 
Section: description

The present invention is a process applied to graphite materials previously used as the moderator in the core of a thermal nuclear reactor and which are no longer required for this purpose. It also applies to any other graphite materials (fuel element sleeves, braces etc.) irradiated in the neutron flux of a nuclear reactor core. In a preferred embodiment, the present invention provides a process including the following steps: (i) reacting the radioactive graphite with superheated steam or gases containing water vapor to form hydrogen and carbon monoxide; (ii) reacting the hydrogen and carbon monoxide from step (i) to form water and carbon dioxide; (iii) reacting the carbon dioxide from step (ii) with a magnesium or calcium oxide to form magnesium or calcium carbonate; and (iv) processing of radioactive contaminants. In step (i) of the process of the present invention, the reaction of superheated steam or gases containing water vapor with graphite is carried out at a temperature within the range of 250xc2x0 to 900xc2x0 C., preferably between 600xc2x0 to 700xc2x0 C., to form hydrogen and carbon monoxide. This type of process is generally referred to in the art as xe2x80x9csteam reformingxe2x80x9d. The reaction in step (i) may be carried out with the addition of oxygen to the steam or gases containing water vapor to provide exothermic reaction energy for the process. The addition of oxygen also enables the temperature of the steam reforming reaction to be controlled. The gases from step (i) are then further oxidized in step (ii) with oxygen to form carbon dioxide and water. During the process, the gases are retained in an enclosed vessel under an inert atmosphere. In the preferred embodiment, the carbon dioxide and water are disposed of in a third step including the chemical processing of the carbon dioxide to create a suitable solid waste form for disposal. Preferably, the carbon dioxide is reacted with magnesium or calcium oxides to produce insoluble magnesium or calcium carbonate salts. Alternatively, the carbon dioxide and water which are produced in the process may be disposed of subsequently by a number of procedures including the following: (1) controlled discharge of the carbon dioxide to the atmosphere, after further processing necessary to minimize its radioactive content; (2) compression and liquification of the carbon dioxide for temporary storage, processing transportation or disposal; (3) condensation of steam to provide water for treatment, disposal or release. Finally, in the fourth step of the process of the present invention, the remaining radioactive contaminants contained in the radioactive graphite are processed. The process of the present invention may be carried out either within the containment a decommissioned nuclear reactor or may be applied (in externally provided equipment) off site. Radioactive secondary waste from the process of the invention or from further processing of the carbon dioxide produced in the process prior to discharge, can be dealt with in any conventional manner appropriate to normal procedures of the nuclear plant concerned. In-situ processing of graphite requires that the graphite in the nuclear reactor containment be subjected to conditions suitable for the gasification of graphite. The in-situ reactions can be performed by various methods, as discussed below. In a first method, carbon dioxide, nitrogen or other inert gas maybe recirculated through the reactor containment using normal in-plant equipment with the addition of small, controlled amounts of steam and/or oxygen, when required. A side stream is continuously extracted from the loop for the removal of carbon monoxide, hydrogen, and carbon dioxide. To mitigate the potential of a hydrogen explosion a catalytic hydrogen converter is preferably inserted in the treatment loop to convert any hydrogen to water. This option requires the injection of a small amount of oxygen into the catalytic converter. For the addition of trace amounts of oxygen, the reactor circuit is maintained above 250xc2x0 C. for oxidation reactions to proceed in sufficient time to allow the use of less than 5% oxygen concentration in the recirculating gases. The use of restricted oxygen levels is recommended to eliminate potential explosion reactions. For the addition of steam, the reactor circuit is maintained above 350xc2x0 C. for the reformation reactions to proceed at reasonable rates. In a second method, gases are injected into and removed from the nuclear reactor containment without the use of other in-plant equipment. This method involves the isolation of the graphite moderator from the balance of plant systems. An external gas recirculation loop can be utilized to inject gases into the nuclear reactor containment and provide removal of gaseous reaction products. Selected areas of the reactor containment can be maintained at high temperature by the injection of superheated gases at 400xc2x0 to 900xc2x0 C., or by the generation of the needed heat inside the reactor containment. Heat generation inside the reactor containment can be achieved by the insertion of electrical or combustion tube heaters placed in one or more of the fuel channels. The in-situ reaction utilizing this method allows the preferential removal of the graphite in selected areas of the nuclear reactor in order to remove graphite in a planned sequence. This feature is an extremely valuable safety mechanism, because it allows the graphite to be removed in a structurally secure manner, avoiding the possibility of collapse of a weakened moderator structure during the later stages of removal. The feasibility of local removal of graphite by this method is further aided by the decreased thermal conductivity of graphite in end-of-life moderators, which occurs as a result of neutron irradiation. It is estimated that over 75% of the graphite could be removed this way. The injected gases may consist of an inert gas and steam, together with oxygen, as required. Final removal of the last traces of graphite could, for example, be achieved by reverting to the previous method discussed above. When the process of the present invention is carried out on pieces or particles of the graphite which have been removed from the reactor core, the process may be carried out as a continuous, semi-continuous or batch process. The process may be carried out using a stationary bed formed from the graphite particles or pieces or, alternatively in a fluidized bed reactor. Preferably, the bed will be fluidized using the steam reactant as the fluidization aid, but it will be appreciated that the reactant bed may be fluidized using an inert gas, such as nitrogen or carbon dioxide, with the appropriate injection of steam and/or oxygen, to enable the reaction to proceed. Inert bed material can be utilized in the fluid bed to stabilize the temperature where steam and/or oxygen is injected into the vessel. The steam reforming reaction proceeds according to the equation: C+H2Oxe2x86x92CO+H2  In the second stage of the process of the present invention the carbon monoxide and the hydrogen are oxidized to carbon dioxide and water. This is generally carried out using oxygen gas as the oxidizing agent. The oxidation reaction may be carried out in the same vessel as, or a different vessel from, that in which the steam reforming reaction is carried out. For example, when the steam reforming reaction is carried out in a fluidized bed reactor, the oxygen may be introduced into the upper portion of the fluidized bed reactor, so that both steps of the process are carried out in a single reactor vessel. The advantage of the process of the present invention, as compared to the combustion of radioactive graphite, is that it can be carried out under appropriately controlled containment conditions. The loss of hazardous or radioactive materials in the off-gas is therefore reduced or even eliminated. Another significant benefit is the low volume of off-gas that simplifies handling including the possibility of achieving substantially zero gaseous emissions. Further, the process enables the Wigner energy stored in the radioactive graphite to be released in a controlled manner. The present invention will be further described with reference to FIG. 1 of the accompanying drawings which is an overview flow diagram of one means of carrying out the process of the present invention. Referring to the drawing, radioactive graphite is remotely removed from a nuclear reactor core by means of water jet or mechanical cutters. Graphite pieces and water are introduced into a size reduction wet grinder 1 where the graphite is reduced to  less than 1.0 cm size. The size-reduced graphite is then mixed with water in vessel 2 and the slurry is fed directly into the fluidized bed reformer 4 by means of a slurry injector pump 3, without any other pre-treatment or handling required. Alternatively, graphite can be size reduced to  less than 12.0 cm, preferably less than 4.0 cm for direct injection into the reformer 4 by means of a mechanical screw conveyor 5. The fluidized bed reformer 4 serves to evaporate all water from the graphite slurry and other liquid waste feeds and to pyrolyze any organic components through destructive distillation (pyrolysis). Energy needed to evaporate the feed water and drive the endothermic reformation process is provided by operating the fluid bed in an autothermal steam reforming mode. The off-gas from the reformer 4, which leaves the reformer along line 7, contains fine particulates, including most radionuclides and non-volatile inorganic materials, such as silica and calcium and gaseous components such as steam, carbon dioxide and gaseous radionuclides, particularly tritium, carbon-14 and iodine. The solid residue is elutriated from the reformer 4 by the fluidizing steam and gases. The particulates in the off-gas from the reformer are removed from the off-gas stream by a high temperature filter or wet scrubber 8. If only graphite is to be processed by the process, a high temperature particulate filter is all that is needed to clean all non-volatile radionuclides from the off-gas. If other streams are being processed, the wet scrubber is utilized as shown in FIG. 1. Table 1 provides a list of typical radionuclides found in moderator graphite and how the radionuclides found in moderator graphite and how the radionuclides are partitioned in the process of the present invention Utilization of the wet scrubber 8 cleans the off-gas by removing particles elutriated from the reformer 4 and neutralizes any potential acid gases. The scrubber solution is concentrated by the hot off-gas from the reformer 4 to 1% to 20% by weight solids. The pH in the scrubber solution is controlled between 5.0 and 7.0 to minimize carbon dioxide absorption and to ensure removal of acid gases. The salt solution can be directed along line 9 for treatment by conventional means, such as direct discharge (if radioactivity levels permit), discharge after selective removal of radioactive species, or encapsulation to form solid waste. Insoluble constituents in the scrubber solution can be removed by filtration if a discharge route is chosen. The warm, water-saturated off-gas stream leaves the scrubber 8 along line 10 and can be further processed to remove essentially all the water vapor by means of a refrigerated condenser 11. The condensed water leaving the condenser 11 along line 12 will include essentially all the tritium from the graphite. The condenser water, with trace levels of tritium, can be handled by one or more of the following methods. It can be recycled to provide for water cutting duty, or to supply superheated steam to the reformer. Alternatively it can be discharged as water vapor or liquid water, or used to mix with cement for solidification of other radioactive waste. Some of the iodine in the off-gas will also tend to be carried with the water leaving the condenser 11 along line 13. The cool, dry off-gas consists almost exclusively of carbon-dioxide and small amounts of oxygen and nitrogen. If allowed by regulation, the carbon-dioxide rich off-gas can be HEPA filtered in vessel 14, monitored at 15, and then discharged to the facility stack at 16. If required by regulation, carbon dioxide can be removed from the off-gas by a refrigerated CO2 condenser 18. The concentrated carbon dioxide can be transferred along line 20 for conversion into a solid carbonate salt. The remaining non-condensable gases can then be circulated along line 19 to the HEPA filter 14, monitored and then discharged to the facility stack 16. The final small off-gas flow represents less than 5% of the off-gas flow from the outlet of the reformer. The concentrated carbon dioxide stream 20 issuing from the condenser is next converted to a solid, inert carbonate compound. Preferably, the carbon dioxide is reacted with calcium or magnesium oxides or metals in reaction vessel 21 to produce insoluble magnesium or calcium carbonate salts. Alternatively, carbon dioxide may be reacted with MAGNOX fuel element debris waste is described in several publications (e.g., xe2x80x9cCEGB dissolves Magnox fuel element debris at Dungenessxe2x80x9d by FH Passant, CP Haigh and ASD Willis Nuclear Engineering International, Feb. 1988 pp 44-51). Once the carbonate salt is formed, in can then be conveniently used to fill void spaces in existing radioactive waste disposal containers. It will be apparent to those skilled in the art that many substitutions and modifications can be made to the foregoing preferred embodiments without departing from the spirit and scope of the present invention, defined by the appended claims.