Method for preventing Co-60 contamination of cooling water circuits in nuclear reactor

A method for preventing or reducing the formation of Co-60 contaminated material on the surfaces of a cooling water circuit of a nuclear reactor. The method is carried out during shutdown of the reactor. First, Co-60 contaminated material formed on the surfaces of the cooling water circuit is chemically removed. Then at least one iron compound is added to the cooling water in an amount sufficient to scavenge cobalt from the cooling water. Thereafter, oxygen is injected into the cooling water to provide a dissolved oxygen concentration sufficient to form an oxide film on the surfaces of the cooling water circuit. The oxide film so formed is substantially free of Co-60 isotope. This treatment reduces the radiation hazard to nuclear reactor personnel.

This invention relates to the operation and safety of water-cooled nuclear 
reactors, and in particular to measures for reducing the hazards of 
possible exposure of operating and maintenance personnel to a source of 
radiation dispersed throughout the cooling water circulating system of 
such nuclear reactors. 
BACKGROUND OF THE INVENTION 
A major hazard in water-cooled nuclear reactors is the accumulation of 
radioactive substances in the structural portions of the reactor. The 
buildup of radioactive nuclides occurs on the inner surfaces of components 
which are in contact with the reactor water. This includes both the 
primary recirculation circuit and the reactor water cleanup system. 
During reactor shutdown, workers are exposed to radiation emanating from 
stainless steel internal walls and inner surfaces of piping. Radioactive 
materials retained in oxide films which have accumulated on wall and 
piping surfaces are a major source of radiation exposure. The 
radioactivity has been found to be predominantly due to the Co-60 isotope. 
As a result, a substantial effort has been made to identify the key 
parameters which affect Co-60 buildup and to determine and implement 
methods for limiting that buildup. 
Cobalt, derived from a number of different alloys commonly employed in 
components of the reactor's mechanisms or structures, is subject to 
induced radioactivity, especially the Co-60 isotope. This radioactive 
Co-60 isotope, or ions or compounds thereof, can be carried in the 
circulating cooling water flowing through the cooling water circuit, 
whereby the radioactive substances are spread and deposited throughout the 
cooling water circuit or primary loop system of the reactor plant. Such 
radioactive substances are prone to be taken up and incorporated into the 
normally occurring oxide films which form and progressively accumulate on 
the inner surfaces of the cooling water circuit. 
A number of proposals or potential solutions to this problem of dispersion 
of radioactive substances throughout the cooling water circuit or system 
and incorporation of radioactive substances into the inherently produced 
oxide films forming on the surfaces of cooling water circuit or system 
have been considered or made. One approach to controlling the potentially 
hazardous cobalt source of such radiation has been the application of zinc 
as disclosed in U.S. Pat. No. 4,756,874 to Ruiz et al. and U.S. Pat. No. 
4,759,900 to Peterson et al. 
SUMMARY OF THE INVENTION 
The present invention comprises a method for preventing or reducing the 
contamination of cooling water circuit surfaces by Co-60 isotope in a 
water-cooled nuclear reactor. The method encompasses the addition of iron 
compounds and the creation of attendant physical conditions which scavenge 
Co-60 isotope from the cooling water and also impede subsequent buildup of 
the Co-60 isotope on the surface of metal components or structures that 
form the primary cooling circuit system of a water-cooled nuclear reactor. 
In particular, the method for preventing or reducing the formation of 
material contaminated with Co-60 on the surfaces of a circuit carrying 
cooling water in a nuclear reactor comprises the step of adding at least 
one iron compound to the cooling water in an amount sufficient to maintain 
an iron concentration in the cooling water in a range from 50 to 200 ppb. 
The iron compound is added while the reactor is shut down. Prior to the 
addition of the iron compound, the cooling water is treated to attain a pH 
thereof in the range of about 7.5 to about 8.0 measured at a cooling water 
temperature of about 25.degree. C. Thereafter the cooling water is heated 
to a temperature of at least about 230.degree. C. and maintained at that 
temperature while the iron compound is present in the aforementioned 
concentration. The result of this treatment is that Co-60 is scavenged 
from the cooling water in the cooling water circuit. 
The method further comprises the step of maintaining a dissolved oxygen 
concentration in the cooling water in a range from 200 to 400 ppb. The 
oxygen is added for the purpose of accelerating the formation of oxide 
film on the cooling water circuit surfaces, which oxide films, because 
Co-60 has been scavenged from the system, will be substantially free of 
radioactive Co-60 isotope. Such oxide films act as a barrier to the 
formation of further oxide films 
The method of the invention further comprises the step of adding at least 
one iron compound to the cooling water in an amount sufficient to maintain 
an iron concentration in the cooling water in a range from 50 to 100 ppb 
during operation of the reactor after the shutdown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The method of the invention comprises a chemical process for preventing the 
formation of cobalt-contaminated oxide films on the surfaces of metal 
structures providing the cooling water circuits of water-cooled nuclear 
reactors, such as inner portions of pipes, conduits, vessels, tanks, 
chambers, etc. 
Cobalt derived from metal alloy materials utilized in nuclear reactor 
plants is known as a major source of radiation and, in turn, is a health 
hazard to operating and maintenance personnel working about the nuclear 
reactor structures. Cobalt, particularly the Co-60 isotope, is carried in 
the cooling water throughout the nuclear reactor coolant circuit or system 
and becomes entrained and/or embedded in the mass of oxides commonly 
forming and accreting over the exposed metal surfaces of vessels, 
conduits, etc. of the cooling water circuit system. Reducing the presence 
of cobalt by replacing cobalt-containing alloys with alloys free of cobalt 
to minimize its source is expensive and most often impractical. 
Chemical decontamination procedures for removing cobalt-contaminated oxide 
films from inside surfaces of the cooling water circuit have been proposed 
whereby the hazardous radiation fields are substantially reduced through 
oxide film removal by chemical means. However, due to extremely high 
corrosion rates, the decontaminated surfaces rapidly pick up Co-60 isotope 
from the circulating cooling water and retain it in the accreting body of 
oxides forming on exposed metal surfaces. Thus, radiation levels measured 
one cycle after decontamination are frequently as great as before 
decontamination. 
In accordance with the method of the present invention, a chemical 
technique is provided which controls and/or minimizes contamination in 
water-cooled nuclear fission reactor systems prior to any significant 
initial contamination or following decontamination. By minimizing 
recontamination, the method of this invention can be a more effective 
means of reducing radiation exposure of personnel in a boiling water 
nuclear reactor. 
The chemical measures of this invention entail a combination of conditions 
that reduce the soluble (ionic) Co-60 concentration in reactor cooling 
water and preoxidize the surfaces of the cooling water circuit with an 
oxide film which is substantially free of Co-60. The steps of the 
invention comprise adjusting the pH of the cooling water circulating 
within the cooling water circuit to a slightly basic condition of about 
7.5 to about 8 when measured at a water temperature of about 25.degree. 
C., and adding a solution of an iron compound, including, but not limited 
to, freshly prepared insoluble species such as Fe(OH).sub.3, Fe.sub.2 
O.sub.3 and Fe.sub.3 O.sub.4, or water-soluble compounds such as ferrous 
oxalate and ferric citrate, in amounts sufficient to maintain a soluble 
iron concentration in the cooling water in the range of 50 to 200 ppb. 
Under these conditions, preferably augmented by elevated water 
temperatures, the soluble (ionic) Co-60 in the reactor cooling water is 
effectively scavenged. Moreover, while the soluble Co-60 concentration in 
the cooling water is reduced, the surfaces of the cooling water circuit 
can be oxidized to form a substantially cobalt-free protective film prior 
to initial operation or following cobalt purging. 
Preferred conditions for the practice of this invention comprise adjusting 
the cooling water pH to about 7.5 to about 8 with adequate Fe(OH).sub.3 
addition to maintain the iron concentration at approximately 200 ppb with 
the cooling water at a temperature of at least about 230.degree. C. 
Generally optimum effects are obtained when these conditions of pH, iron 
concentration and temperature are maintained in the cooling water of the 
reactor coolant system over a period of at least about 500 hr. 
The elevated temperatures of the cooling water can be provided without 
nuclear fission-produced heat in accordance with a pre-startup treatment 
by any suitable means or source, such as heat generated by recirculation 
pumps which drive the cooling water through the reactor coolant system. 
In a typical reactor pre-startup treatment in accordance with the 
invention, suitable amounts of ferric hydroxide in a slightly basic water 
solution are injected into the reactor coolant for attaining the desired 
pH of about 8 and iron concentration of about 200 ppb, with the pH being 
determined at a water temperature of about 25.degree. C. The temperature 
of the cooling water is maintained at about 230.degree. C. or higher. 
Given these conditions, the soluble Co-60 in the cooling water can be 
reduced to less than about 1% of the total Co-60 concentration in the 
reactor water. 
To foster oxidation of the surfaces of the cooling water circuit upon 
purging of Co-60 from the cooling water, the dissolved oxygen content in 
the reactor cooling water is maintained at about 200 to about 400 ppb. The 
oxygen can be provided by introducing oxygenated water, such as control 
rod drive water, or by injecting oxygen. Preferably the operations of pH 
adjustment combined with iron solution addition for Co-60 purging of the 
cooling water system, and oxygen level control are carried out as long as 
is practical before startup of the nuclear reactor, for example, at least 
about 500 hr. 
Following starting up of the water-cooled nuclear fission reactor, the iron 
content of the cooling water may be depleted rapidly, whereby a high iron 
solution injection rate can be appropriate or required to maintain the 
iron content in the range of about 50 to about 100 ppb. The pH of the 
water should be maintained in the range of about 7.5 to about 8. Then the 
nuclear reactor is operated under the given conditions for approximately 
500 hr before the iron solution injection is terminated. At this stage the 
iron content of the cooling water should be maintained at about 5 ppb. 
This can be achieved by means of feedwater quality control.