MINIMIZATION OF RADIOACTIVE MATERIAL DEPOSITION IN WATER-COOLED NUCLEAR REACTORS

Abstract: In water-cooled nuclear reactors where zinc is added to the water to remove or lessen the accumu­ lation of radioactive cobalt, radioactivity arising from the zinc itself as a result of neutron capture is lessened or eliminated entirely by modifying the isotopic composition of the zinc prior to its injection into the system. The modification of the isotopic composition consists of lowering the proportion of ⁶⁴Zn or removing this isotope entirely.

Description: This invention relates to the operation and safety of water-cooled nuclear reactors, and in particular to methods for minimizing the dangers of exposure of workers to radioactive emissions during reactor shutdown. A major hazard in water-cooled nuclear reactors is the accumulation of radioactive substances in the structural portions of the reactor. During reactor shutdown, workers are exposed to stainless steel internal walls and piping 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. (See, for example US-A-4 756 874). Zinc however is itself a source of radioactivity in these reactors, and this radioactivity limits the effectiveness of the use of zinc. The present invention provides for using zinc which has a lower content of the ⁶⁴Zn isotope than naturally occurring zinc. This isotope is the isotope in greatest abundance in naturally occurring zinc, comprising approximately 50% thereof, and has a tendency to undergo neutron capture inside a nuclear reactor to produce ⁶⁵Zn, in an amount proportional to the concentration of ⁶⁴Zn. In accordance with the present invention, the production of ⁶⁵Zn is lessened if not eliminated entirely by using zinc in which the ⁶⁴Zn is either reduced in proportion to the other isotopes or entirely absent. Naturally occurring zinc has an approximate isotopic composition as follows: Isotope Concentration (%) ⁶⁴Zn 48.6 ⁶⁶Zn 27.9 ⁶⁷Zn 4.1 ⁶⁸Zn 18.8 ⁷⁰Zn 0.6 The zinc used in accordance with the present invention has a composition in which the ⁶⁴Zn is present in a substantially lower proportion than that indicated above, the term "substantially lower" referring to any amount which results in a significant lessening of the amount of radiation that arises from the zinc itself due to its exposure to neutron irradiation inside the reactor. In more specific terms, it is preferred that the proportion of ⁶⁴Zn be lowered to less than about 10%, particularly less than about 1%. It is most preferred that the zinc be substantially devoid of the isotope. Treatment of the zinc to reduce the ⁶⁴Zn content or to remove the latter entirely may be done according to conventional techniques for isotope separation of metals. Application of these techniques to zinc is within the routine skill of those skilled in the art. One such separation process is the gaseous-diffusion process. According to this process, zinc is first highly purified and converted to the vapor state, generally by reaction to form volatile compounds such as fluorinated zinc alkyls. The vaporized compound is then pumped through a series of diffusion aggregates arrayed in cells in a cascade pattern. The various isotopes diffuse through the cells at slightly different rates, permitting separation. High degrees of separation may be achieved by the use of multiple stages. Another example is centrifugal isotope separation, again using zinc in the vapor state. Dimethyl fluorinated zinc is one example of a volatile zinc compound which renders zinc susceptible to this kind of separation. Other methods of separation include electro-magnetic separation, liquid thermal diffusion, and laser excitation. In the laser excitation process, zinc vapor is ionized by means of a tunable laser specific to a wavelength which selectively excites ⁶⁴Zn atoms to form positive ions, which are then collected on a negative electrode. The remaining vapor is accordingly comprised of zinc depleted of this isotope. Still further methods will be known to those skilled in the art. Once the zinc has been treated to reduce or eliminate its ⁶⁴Zn content, it is added to the reactor water in any form which will result in zinc ion in solution. The zinc may thus be added in the form of a salt such as, for example, zinc chromate, or as zinc oxide. With zinc oxide, no extraneous anions are added. The use of zinc oxide is preferred. The major component of radioactive deposition on the walls of water-bearing vessels in nuclear reactors is radioactive cobalt. While inhibition of the radioactive cobalt deposition may be achieved with very small amounts of zinc, the actual amount used is not critical and may vary over a wide range. For most applications, a concentration from 1 to 1,000 ppb (parts per billion by weight), preferably from 3 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 radioactive depositions tend to occur. Such vessels may include tubes, shells, 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%. A convenient way of adding the zinc oxide as an aqueous solution is to pass a stream of the water entering the vessel over solid zinc oxide in a receptacle located either in the feed line or in a recirculation loop. A bed of zinc oxide pellets or particles, preferably sintered, will provide effective results. Examples of ways in which the zinc oxide may be added are described in EP-A-0 257 465 filed August 13, 1987. 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 boiling water reactors.

Claims: A method for inhibiting the deposition of radioactive material in a water-bearing vessel of a water-cooled nuclear reactor which comprises adding zinc ion to water entering said water-bearing vessel, characterized in that said zinc ion contains a substantially lower proportion of the ⁶⁴Zn isotope than naturally occurring zinc. The method of claim 1 wherein said radioactive material is radioactive cobalt. A method in accordance with Claim 1 or 2 in which the proportion of ⁶⁴Zn in said zinc ion is less than about 10%. A method in accordance with Claim 1 or 2 in which the proportion of ⁶⁴Zn in said zinc ion is less than about 1%. A method in accordance with Claim 1 or 2 in which said zinc ion is substantially devoid of ⁶⁴Zn. A method in accordance with Claim 1 or 2 in which said zinc ion is added by dissolving zinc oxide in said water. A method in accordance with Claim 6 in which the amount of zinc oxide dissolved is selected to achieve a concentration of 1 to 1,000 ppb zinc in said water-bearing vessel. A method in accordance with Claim 6 in which the amount of zinc oxide dissolved is selected to achieve a concentration of 3 to 100 ppb zinc in said water-bearing vessel. A method in accordance with Claim 6 in which said zinc oxide is dissolved from a member selected from the group consisting of a water-based slurry, a water-based paste and a water-based solution. The method of claim 1 characterized in that the addition of zinc ion is made by: adding a water-based zinc oxide paste to water entering said water-bearing vessel. A method in accordance with Claim 10 in which the amount of zinc oxide in said paste is from 25% to 95% by weight. A method in accordance with Claim 10 in which the amount of zinc oxide in said paste is from 40% to 80% by weight. A method in accordance with claim 2 characterized in that the addition of zinc ion is made by adding an aqueous solution of zinc oxide to water entering said water-bearing vessel. A method in accordance with Claim 13 in which 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 14 in which said solid zinc oxide is a bed of zinc oxide particles. A method in accordance with Claim 15 in which said zinc oxide particles are sintered zinc oxide. A method according to claim 2 characterized in that the addition of zinc ion is made by adding a slurry of zinc oxide in water to water entering said water-bearing vessel. A method in accordance with Claim 17 in which the zinc oxide content of said slurry is from 0.1% to 20% by weight. A method in accordance with Claim 17 in which the zinc oxide content of said slurry is from 1% to 5% by weight. A method in accordance with Claim 17 in which said zinc oxide in said slurry is fumed zinc oxide. A method in accordance with Claim 17 in which the rate of addition of said slurry to said water is selected to produce a zinc content in the water of said water-bearing vessel of 1 to 1,000 ppb. A method in accordance with Claim 17 in which the rate of addition of said slurry to said water is selected to produce a zinc content in the water of said water-bearing vessel of 3 to 100 ppb.

Inventors: BLAIES, DAVID MICHAEL, RUIZ, CARL PHILLIP
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1987
Main classification: G21F9
IPC codes: G21D3, G21F9, G21C19, G21D1