Minimization of Radioactive Material Deposition in Water-Cooled Nuclear Reactors

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. 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 .sup.64 Zn 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 .sup.65 Zn, in an amount proportional to the concentration of .sup.64 Zn. In accordance with the present invention, the production of .sup.65 Zn is lessened if not eliminated entirely by using zinc in which the .sup.64 Zn is either reduced in proportion to the other isotopes or entirely absent.

Naturally occurring zinc has an approximate isotopic composition as follows:

The zinc used in accordance with the present invention has a composition in which the .sup.64 Zn 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 proportoin of .sup.64 Zn 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 .sup.64 Zn 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 electromagnetic separation, liquid thermal diffusion, and laser excitation. In the laser excitation process, zinc vapor is ionized by means of a tunable lazer specific to a wavelength which selectively excites .sup.64 Zn 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 .sup.64 Zn 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 about 1 to about 1,000 ppb (parts per billion by weight). preferably from about 3 to about 100 ppb, maintained in the reactor water during operation of the reactor will provide the best results.

The invention may be applied to any waterbearing 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 about 25% to about 95% by weight, preferably from about 40% to 80%. Slurries will generally contain from about 0.1% to about 20% by weight, preferably from about 1% to about 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 commonly assigned copending application Ser. No. 900,927, filed Aug. 27, 1986, which is incorporated herein by reference.

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.

The foregoing is offered primarily for purposes of illustration. It will be readily apparent to those skilled in the art that numerous modifications and variations of the features of construction and operation disclosed herein may be made without departing from the spirit and scope of the invention.