INHIBITION OF RADIOACTIVE COBALT DEPOSITION IN WATER-COOLED NUCLEAR REACTORS

Abstract: Deposition of radioactive cobalt on the inte­ rior surfaces of a water-cooled nuclear reactor is in­ hibited or substantially prevented by the continuous injection of zinc oxide to the reactor water. The zinc oxide may be prepared in the form of a paste, a slurry, or a preformed aqueous solution.

Description: This invention relates to the operation and safety of water-cooled nuclear reactors, and in particular to methods to prevent the deposition of radioactive cobalt in the water-bearing vessels of such reactors. A major hazard in water-cooled nuclear reactors is the accumulation of radioactive substances in the structural portions of the reactor. During reactor shutdown, for instance, workers are exposed to internal walls and tubing surfaces, and radioactive materials retained in oxide films which have accumulated on these surfaces are a major source of radiation exposure. The introduction of certain metallic ions, including zinc, has been used to remove or lessen such deposition. The form and manner in which the zinc has been added, however, has had certain disadvantages. For instances, zinc has been added as a salt with anions which have been detrimental to the reactor, and the use of the active metallic ions before startup or during cleaning shutdowns has been of limited effectiveness in situations where long-term operation has occurred. It has now been discovered that effective inhibition of radioactive cobalt deposition may be achieved by the continuous addition of zinc oxide to the water entering a water-bearing vessel of the reactor throughout the operation of the reactor. This provides the advantage of steady control of the radioactive cobalt accumulation, as well as efficient use of the zinc oxide, and total avoidance of the presence of anions harmful to the internal reactor surfaces. The zinc oxide may be added in a variety of forms, examples of which are disclosed in detail below. Figure 1 is a flow diagram of one embodiment of the invention, designed to inject zinc oxide into the system in the form of a paste. Figure 2 is a flow diagram of the second embodiment of the invention, designed to add zinc oxide addition as a solution. Figure 3 is a flow diagram of the third embodiment of the invention, designed to add zinc oxide as a slurry. In accordance with the present invention, zinc oxide is added to the water in water-bearing vessels which tend to accumulate matter deposited in films along their surfaces. The zinc oxide addition is done on a continuous basis throughout the operation of the reactor, providing a constant replenishment of zinc ion. A very small amount of zinc oxide will be sufficient to achieve inhibition of the radioactive cobalt deposition. The actual amount is not critical, however, and may vary over a wide range. For most applications, a concentration of from 1 to 1,000 ppb (parts per billion by weight), preferably from 10 to 100 ppb, maintained in the reactor water during operation of the reactor will provide the best results. The invention may be applied to any water-bearing vessel in a nuclear reactor in which deposition of radioactive cobalt may occur. Such vessels may include tubes, shelves, feed and recirculation piping, and transfer and storage vessels in general. Recirculation piping is of particular concern, since it is a major source of exposure to plant workers during maintenance shutdowns. The zinc oxide may be added through feedlines to such vessels, or where appropriate, to recirculation lines branching off of such vessels. The zinc oxide may be added in any form which permits it to be dissolved in the reactor water. Examples include slurries, pastes, and preformed solutions. When pastes or slurries are used, the zinc oxide is preferably in the form of a finely divided powder, fumed zinc oxide being most preferred. The zinc oxide contents in these pastes and slurries are not critical, since the concentration in the reactor vessels where the zinc oxide is needed may be controlled by the rate of addition of the paste or slurry to the incoming water. In most cases, pastes will have zinc oxide contents ranging from 25% to 95% by weight, preferably from 40% to 80%. Slurries will generally contain from 0.1% to 20% by weight, preferably from 1% to 5%. The figures illustrate various ways in which the zinc oxide may be added. In Figure 1, the zinc oxide is added to the water-bearing vessel 10 by first combining the zinc oxide with water feed to the vessel through a feed conduit 11. Regulation of the flow rate through the feed conduit is achieved by a pump 12. The arrangement shown in Figure 1 is designed for the injection of a zinc oxide paste which is stored in a cylinder 13 fitted with an internal piston (not shown). For a nuclear reactor of typical size, it is contemplated that a typical cylinder size may be 3″ (7.6 cm) in diameter and 20″ (50.8 cm) in length. The shaft 14 of the piston is moved slowly forward by a motor-driven linear actuator 15. This may, for example, consist of an electric motor operating through a speed reducer and linear actuator moving at a rate of 0.001 inch (0.0025 cm) per minute. As the piston moves to the left, the paste is forced out of the cylinder 13 through tubing 16. A stop valve 17 permits an operator to remove a depleted cylinder 13 without shutting down the feed line 11 or affecting the operation of the pump 12. A full cylinder may then be installed. Rather than injecting the paste directly into the feed conduit 11, the paste is injected into a recirculation loop 18 which bridges a high pressure point 19 in the feed conduit (at the discharge end of the pump) with a low pressure point 20 (at the inlet or feed side of the pump 12). Circulation through this recirculation loop thus occurs in the clockwise direction in accordance with the arrangement shown in the drawing. An arrangement of static mixers 21 is positioned downstream of the injection point 22 where the paste from the cylinder 13 enters the recirculation loop 18. In Figure 2, the zinc oxide is added in the form of a solution formed by passing water through a bed of sintered zinc oxide pellets. Here, the flow arrangement is similar to that shown in Figure 1--using a recirculation loop 30 branching off of a conduit 31 leading to the water-bearing vessel (not shown) sought to be protected from cobalt deposition. The conduit 31 may itself be a recirculation line or a feedwater line, and the flow rate through it is again controlled by a pump 32. The recirculation loop 30 brackets the pump 32. Water drawn into the recirculation loop enters a solid-retaining region 33 of the loop. This region contains solid zinc oxide with which water is in contact as it passes through. The region may be a trap, an expanded portion, or merely a segment of the piping closed off by screening or other construction designed to permit the passage of water while retaining solids. Alternatively, the solid-retaining region may be a segment having walls lined with solid zinc oxide. It is preferred, however, that the zinc oxide be in particulate form, providing a high degree of surface area for contact between the water and the solid surface. A fixed or fluidized bed of zinc oxide particles may be used. The zinc oxide is preferably sintered, in pellets or a convenient shape. As the water passes through the pellet bed, it dissolves a portion of the zinc oxide, up to the solubility limit. A sufficient amount of zinc oxide may be charged to this retaining receptacle to last the entire life of the reactor. Figure 3 demonstrates the addition of zinc oxide in the form of a slurry, using an alternative configuration. The slurry is supplied directly from a reservoir 40 in which the slurry 41 is maintained in substantially homogeneous form by a stirrer 42. A pump 43 delivers the slurry at a fixed preselected rate to a flow conduit 44 which supplies water to the water-bearing vessels sought to be protected. As before, the relative flow rates of water through the flow conduit 44 and slurry through the pump 43 will determined the final concentration of dissolved zinc in the water-bearing vessel. The present invention is applicable to water-cooled nuclear reactors in general, including light water reactors and heavy water reactors. The invention finds particular utility in pressurized water reactors and boiling water reactors.

Claims: A method for inhibiting the deposition of radioactive cobalt in a water-bearing vessel of a water-cooled nuclear reactor, characterized in that it comprises: adding zinc oxide to water entering said water-bearing vessel, continuously during operation of said water-cooled nuclear reactor. A method in accordance with Claim 1 characterized in that the amount of zinc oxide added is selected to achieve a concentration of 1 to 1,000 ppb zinc oxide in said water-bearing vessel. A method in accordance with Claim 1 characterized in that the amount of zinc oxide added is selected to achieve a concentration of 10 to 100 ppb zinc oxide in said water-bearing vessel. A method in accordance with Claim 1 characterized in that said zinc oxide is added in the form of a member selected from the group consisting of a water-based slurry, a water-based paste and a water-based solution. A method in accordance with Claim 1 comprising adding said zinc oxide to a conduit selected from the group consisting of a recirculation water line and a feedwater line discharging into said water-bearing vessel. A method in accordance with Claim 5 comprising adding said zinc oxide to a recirculation loop branching off of said conduit. A method in accordance with Claim 5 in which flow through said conduit is maintained by a pump in said conduit, and said method comprises adding said zinc oxide to a recirculation loop drawing water from the discharge side of said pump and returning said water to the feed side of said pump. A method in accordance with Claim 1 characterized in that said zinc oxide is added in the form of a paste comprised of zinc oxide and water. A method in accordance with Claim 8 in which the amount of Zinc oxide in said paste is from 25% to 95% by weight. A method in accordance with Claim 8 in which the amount of zinc oxide in said paste is from 40% to 80% by weight. A method in accordance with Claim 8 comprising adding said paste to a conduit supplying water to said water-bearing vessel at a flow rate maintained by a pump in said conduit, by mixing said paste with water passing through a recirculation loop drawing water from the discharge side of said pump and returning said water to the feed side of said pump. A method in accordance with Claim 11 in which said mixing of said paste with said water passing through said recirculation loop is achieved by continuously injecting said paste into said recirculation loop at an injection point therein from a motor-driven hydraulic cylinder, and passing said paste through a static mixer in said recirculation loop downstream of said injection point. A method in accordance with Claim 1 wherein water is drawn from a conduit supplying water to said water-bearing vessel and said drawn water is passed through a recirculation, loop discharging back into said conduit, continuously during the operation of said water-cooled nuclear reactor; characterized in that it is added in the form of a paste comprised of zinc oxide at 40% to 80% by weight in water injected into said recirculation loop at an injection point at a rate selected to achieve a zinc oxide concentration of 10 to 100 ppb in said conduit. A method in accordance with Claim 1 characterized in that said zinc oxide is added in the form of an agueous solution of zinc oxide. A method in accordance with Claim 14 characterized in that said aqueous solution is formed by passing a stream of water drawn from said water entering said water-bearing vessel through a receptacle containing solid zinc oxide retained therein. A method in accordance with Claim 15 characterized in that said solid zinc oxide is a bed of zinc oxide particles. A method in accordance with Claim 16 characterized in that said zinc oxide particles are sintered zinc oxide. A method in accordance with Claim 14 characterized in that said aqueous solution is formed in a recirculation loop branching off of a conduit supplying said water to said water-bearing vessel, said recirculation loop containing a bed of sintered zinc oxide particles, and said aqueous solution is formed by passing a stream of water drawn from said conduit through said recirculation loop. A method in accordance with Claim 18 characterized in that the flow rate through said conduit is controlled by a pump, and said recirculation loop is positioned to draw water from a point at the discharge side of said pump and to discharge water at the feed side of said pump. A method in accordance with Claim 1 characterized in that said zinc oxide is added in the form of a slurry of zinc oxide in water. A method in accordance with Claim 20 characterized in that the zinc oxide content of said slurry is from 0.1% to 20% by weight. A method in accordance with Claim 20 characterized in that the zinc oxide content of said slurry is from 1% to 5% by weight. A method in accordance with claim 20 characterized in that said zinc oxide in said slurry is fumed zinc oxide. A method in accordance with Claim 20 characterized in that the rate of addition of said slurry to said water is selected to produce a zinc oxide content in the water of said water-bearing vessel of 1 to 1,000 ppb. A method in accordance with Claim 20 characterized in that the rate of addition of said slurry to said water is selected to produce a zinc oxide content in the water of said water-bearing vessel of 10 to 100 ppb.

Inventors: PETERSEN, GEORGE ERVIN, ROBINSON, RANDALL NORMAN, RUIZ, CARL PHILLIP
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1987
Main classification: G21C19
IPC codes: G21C19, G21C17