Minimization of radioactive material deposition in water-cooled nuclear reactors

In water-cooled nuclear reactors where zinc is added to the water to remove or lessen the accumulation 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 .sup.64 Zn or removing this isotope entirely.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF THE INVENTION 
Naturally occurring zinc has an approximate isotopic composition as 
follows: 
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Isotope Concentration (%) 
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.sup.64 Zn 48.6 
.sup.66 Zn 27.9 
.sup.67 Zn 4.1 
.sup.68 Zn 18.8 
.sup.70 Zn 0.6 
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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.