METHOD FOR NUCLEAR POWER PLANT DECONTAMINATION

PROCEDE DE DECONTAMINATION D'UNE CENTRALE NUCLEAIRE

English Abstract

Method effecting decontamination of metal components carrying an oxide layer
in a nuclear powered water cooled reactor wherein a low concentration of zinc
ions is introduced into the water to cause loosening and removal of the oxide
layer from the metal surface.

French Abstract

L'invention porte sur un procédé de décontamination efficace de composants métalliques supportant une couche d'oxyde dans un réacteur nucléaire à refroidissement par eau. Une faible concentration d'ions de zinc est introduite dans l'eau afin de décoller et retirer la couche d'oxyde de la surface métallique.

Claims

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WE CLAIM:
1. A method for effecting decontamination of metal
components carrying an oxide layer in a nuclear powered water
cooled reactor comprising introducing a low concentration of zinc
ions into the water to cause loosening of the oxide layer from the
metal surface.
2. A method according to claim 1, wherein said zinc ions
are introduced in a concentration of at least 30 ppb.
3. A method according to claim 2, wherein said zinc ion
concentration is 1-500 ppb.
4. A method according to claim 1, wherein said water is at
a temperature of at least about 450°F.
5. A method according to claim 4, wherein said temperature
is about 455 to 550°F.
6. A method according to claim 1, wherein said metal
surface carrying said oxide layer is exposed to said zinc ions for at
least 100 hours.
7. A method according to claim 6, wherein said exposure
time is about 2000 hours.

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8. A method according to claim 1, where the zinc ion is
substantially devoid of 64Zn.
9. A method according to claim 1 wherein the zinc ion is
added by dissolving zinc oxide in water.
10. A method according to claim 1 wherein the zinc ion is
added as a water-based paste, a water-based slurry or a water-
based solution.
11. A method of passivation of a metal surface comprising
immersing the metal surface initially free of oxide surface layer in
water at elevated temperature in the presence of zinc at a low
concentration for a time of at least 100 hours.
12. A method according to claim 11, wherein said water is
at a temperature of at least about 450°F.
13. A method according to claim 11, wherein said zinc is at
a concentration of 1-300 ppb.
14. A method for effecting decontamination of metal
components carrying an oxide layer comprising exposing the
component to an aqueous solution of zinc ions to cause loosening
of the oxide layer from the metal surface.

Description

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N~EETHOD FOR NUCLEAR POWER PLANT
DECQNTAMINATION
The present application relates to a method for
decontamination of nuclear power plant systems. More
particularly, the invention relates to the use of small amounts of
zinc to change the properties of the oxide film present on internal
walls and tubing surfaces of nuclear reactors to facilitate removal
of radioactive substances such as ~°Co therefrom.
BA~;KGROUND OF THE INVENTION
A major problem in water-cooled nuclear reactors is the
accumulation of radioactive substances in the structural portions
of the reactor system. For example, during reactor shut-down,
workers are exposed to radiation emanating from internal walls
and tubing surfaces, and radioactive materials retained in oxide
films which have accumulated on these surfaces are a ma,~or
source of radiation exposure.
The build-up of radioactive cobalt (~°Co) in recirculation
piping of nuclear power reactors, including boiling water reactors,
is a major source of radiation exposure, especially during reactor
shutdown. Efforts have been made during recent years to identify
parameters which affect the rate and magnitude of ~°Co buildup,
with a view to developing methods for limiting the buildup. It has
been shown in prior work that the msjority of ~°Co buildup in

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recirculation piping occurs by incorporation into the oxide film
during formation of the oxide film on stainless steel surfaces.
U.S. Patent No. 4,950,449 describes the use of zinc ions to
remove or lessen deposition of radioactive substances and reduce
intergranular stress corrosion cracking in water-cooled nuclear
reactors. The zinc may be added in the form of zinc oxide paste,
slurry or aqueous solution.
U.S. Patent No. 4,756,874 describes the use of zinc having a
Iower content of the ~Zn isotope in order to reduce accumulation
of radioactive cobalt without increasing the presence of the 65Zn
activation product of ~Zn. The zinc in this form may be added to
the reactor water in the form of a zinc salt or zinc oxide.
U.S. Patent No. 4,759,900 relates to the inhibition of
deposition of radioactive cobalt by continuous injection of zinc
oxide into the reactor water. The zinc oxide may be prepared in
the form of a paste, slurry or aqueous solution.
A need exists for reducing the buildup of radioactive
materials in nuclear power systems. The present invention seeks
to satisfy that need.

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SUMMLARY OF THE INVENTION
It has been discovered, according to the present invention,
that it is possible to introduce changes in the structure of an oxide
film present on a metal surface by exposure of the oxide bearing
surface to zinc ions at a low concentration, typically 1-3000 part
per billion (ppb), more usually 10-100 ppb, over a period of at
least 100 hours, typically in the region of 300 hours or more, and
at a temperature of at least 450, for example 45~ to 550°F. Such
exposure results in the oxide film changing from a tightly adherent
l0 structure to a loosely adherent structure that can be removed by
the circulating water of the system, thereby facilitating removal of
at least a portion of the film, including radioactive materials
contained therein. In this way, through changing the structure of
the oxide film, the invention provides a way of effecting
decontamination of the reactor to reduce the buildup of radioactive
materials, including ~°Co buildup, on the inner wall surfaces and
recirculation piping of nuclear power boiling water reactors.
It is also possible according to the present invention to
incorporate zinc into a passivating oxide film wherein stainless
steel, initially free of oxide film, is subjected to high temperature,
typically greater than 300°F, high purity water (i.e. water having a
quality consistent with that typically found in operating nuclear
reactors) used under conditions where the zinc concentration is no
higher than 500 ppb for an exposure time greater than or equal to

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about 100 hours. A passivating film covers the surface of the base
metal (e.g. stainless steel) and inhibits subsequent corrosion/oxide
formation Under these conditions, an oxide film is produced
which is substantially thinner, i.e. usually less than 0.5 microns,
more usually less than 0.2 microns, than non-oxide-bearing oxide
films formed under similar conditions in the absence of zinc.
Typically, film thicknesses in absence of zinc range from about 1
to 3 microns.
With the thinner films present on a stainless steel surface, it
is observed the at Co~° buildup is less than when the oxide film is
formed in the absence of zinc. When zinc is present, 6°Co levels
are typically less than l OpCi/cm2, while without zinc, the ~°Co
levels are typically between about l OpCi/cm2 and SOOpCi/cm2.
Moreover, removal of the loosened oxide layer can be effected by
the circulating water in the pipe.
In accordance with one aspect of the present invention,
there is provided a method for effecting decontamination of metal
components carrying an oxide layer comprising exposing the
component to an aqueous solution of zinc ions to cause loosening
of the oxide layer from the metal surface.
In accordance with another aspect there is provided a
method for effecting decontaminat;en of metal components
carrying an oxide layer in a nuclrsr Fowered water cooled reactor
comprising introducing a low concentration of zinc ions into the

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reactor water to cause loosening of the oxide layer from the metal
surface.
In accordance with yet another aspect, there is provided a
method of passivation of a metal surface comprising immersing
5 the metal surface initially free of oxide surface layer in water at
elevated temperature, typically in the region of 230 to 300°C in
the presence of zinc at a low concentration for example 1-300 ppb
for a time of at east 300 hours. It has been found according to the
invention that incorporation of low amounts of zinc in the
passivated film limits the buildup of radioactive materials therein,
including 6°Co.
The zinc may be added in the form of a salt, for example
zinc chromate or zinc oxide. It is also possible to use zinc which
has been treated to eliminate or reduce its ~Zn content, such as
described in U.S. patent 4,756,874 (herein incorporated by
reference). The zinc oxide may be added in any form which
permit it to be dissolved in the reactor water, such as by way of a
slurry, paste or preformed solution. A paste will typically have a
concentration of zinc oxide of about 25% to about 95% by weight,
whereas a slurry will have a zinc oxide content in the region of
about 0.1% to 20% by weight. Examples of ways in which the
zinc oxide is introduced are set forth in U.S. patent 4,756,874,
referred to above.

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An advantage associated with the method of the present
invention is that it may be applied directly to existing boiling
water reactor recirculation piping and other components without
prior oxide film removal. It may also be applied to oxide-free
metal components before their introduction into the reactor. The
inventors have found that the properties of the oxide film are
changed when subjected to the conditions of the present method.
At least part of the film is changed from a tightly adherent
structure to a loosely adherent structure, with simultaneous release
. of constituents. Thus, according to the invention, the application
of the process not only inhibits further Co~° buildup but also
effects a decontamination, including a decrease in Co6° levels.
E~,'.AMPLE
The following example illustrates the present invention.
Small stainless steel autoclaves were used to contain test
specimens fvr the formation of the oxide films in high temperature
water. The autoclaves used in these tests were previously
employed for corrosion specimen testing under similar conditions
without zinc present in the water. Under these conditions, after
long exposure (many thousands o f hours) the inner surfaces had
attained a heavy black oxide coating which was tightly adherent
and could not be removed by w i p i n ~.

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During a shut-down period for specimen removal, it was
noted that the inner autoclave surfaces which had been treated
according to the method of the present invention appeared lighter
in color. Wiping of the surfaces resulted in removal of some of
the dark outer surface oxide, leaving a dark bronze-colored oxide
inner film. The autoclaves have been operated with high
temperature water containing zinc for approximately 2,000 hours.
During the initial exposure, with the zinc containing water, a zinc
depletion was also noted. The zinc concentration in the water
l0 from the autoclaves was significantly lower than the inlet
concentration, indicating absorption on the stainless steel surfaces.
Zinc can thus be incorporated into an oxide film even in the
presence of an already existing oxide film. This results in release
of material from the previously existing film and also gives rise to
reduced Co~° buildup on the metal surfaces.
While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent
arrangements included within the spirit and scope of the appended
claims.