Process and apparatus for cleaning nuclear reactor cooling water

A process and an apparatus for cleaning nuclear reactor cooling water with cation exchange resin whose ion-exchanging groups have a bonding energy of not more than 300 KJ/mole are disclosed, whereby the radiation exposure of operators in an atomic power plant can be considerably reduced, and the waste ion exchange resin can be readily disposed.

BACKGROUND OF THE INVENTION 
This invention relates to a process and an apparatus for cleaning nuclear 
reactor cooling water in atomic power plants, and particularly to a 
process and an apparatus for cleaning nuclear reactor cooling water in a 
filtration-desalting system using powdery ion exchange resin capable of 
reducing the radiation exposure of operators working in an atomic power 
plant and of being readily susceptible to waste disposal. 
With recent increase in the number of atomic power plants in operation, 
reduction in radiation exposure of operators during the normal operating 
period and the periodic inspection period has been keenly desired, and 
thus it is necessary to efficiently remove radioactive materials contained 
in the nuclear reactor cooling water, such as fine particles having 
particle sizes of about 0.1-10 .mu.m, composed mainly of iron oxides 
called "cruds", or radioactive metal ions such as .sup.60 Co.sup.2+, 
.sup.59 Fe.sup.2+, etc. 
In a boiling water-type nuclear reactor, the cooling water recovered by a 
condenser after the driving of a turbine has been so far cleaned by a 
filteration desalter precoated with powdery ion exchange resin and an 
ordinary desalter using a mixed bed of granular cation exchange resin and 
anion exchange resin, and fed to the nuclear reactor. The powdery ion 
exchange resin used in the filtration desalter has been a powdery mixture 
of pulverized cation exchange resin and anion exchange resin. Usually, the 
filtration desalter and the desalter together are generally referred to as 
"apparatus for cleaning nuclear reactor cooling water". 
Reasons why the apparatus for cleaning nuclear reactor cooling water has a 
remarkable effect upon reduction in the radiation exposure will be 
described in detail below. 
The main cause for radiation exposure is deposition of radioactive cruds 
and radioactive .sup.60 Co.sup.2+ on the piping, and the piping dosage is 
increased thereby. Furthermore, a cause for forming radioactive cruds and 
.sup.60 Co.sup.2+ is radioactivation of non-radioactive iron or cobalt 
dissolved from the condenser or piping into cooling water through neutron 
irradiation in the nuclear reactor. Thus, the reduction in the radiation 
exposure can be made by removing iron and cobalt in both crud and ion 
states in cooling water, irrespective of the radioactive or 
non-radioactive nature. Thus, apparatuses for cleaning nuclear reactor 
cooling water, based on a combination of a filtration desalter and a 
desalter, have been used, where the filtration desalter provided on the 
upstream side removes cruds and metal ions as radioactive materials in the 
cooling water at the same time, whereas the desalter removes the remaining 
metal ions which have not been completely removed in the filtration 
desalter. 
In the foregoing prior art, the filtration desalter and the desalter use 
benzenesulfonic acid-based cation exchange resin and quaternary 
ammonium-based anion exchange resin as the powdery or granular ion 
exchange resins. Their molecular structures are shown below: 
##STR1## 
The reasons why the benzenesulfonic acid-based resin is selected as a 
cation exchange resin and the quaternary ammonium-based resin as an anion 
exchange resin among so many kinds of ion exchange resin are that they 
have a good heat resistance and a good radiation resistance, and moreover 
the benzenesulfonic acid-based resin is strongly acidic and the quaternary 
ammonium-based resin is strongly basic, so that they have a distinguished 
ability to remove neutral salts such as NaCl, etc., if present in the 
cooling water due to a leakage of sea water from the condenser. 
As described above, use of an apparatus for cleaning cooling water, which 
comprises a filtration desalter and a desalter, can considerably reduce 
the radiation exposure of operators working in an atomic power plant. 
However, still much more reduction in the radiation exposure has been 
nowadays desired. 
There has been proposed a process for using weakly acidic ion exchange 
resin, for example, weakly acidic cation exchange resin having carboxyl 
groups as ion-exchanging groups in an apparatus for cleaning nuclear 
reactor cooling water [Japanese Patent Application Kokai (Laid-open) No. 
58-76146]. However, as a result of extensive studies made by the present 
inventors, it has been found that a portion of the weakly acidic cation 
exchange resins combined with non-radioactive metal ions leaks into the 
cooling water and deposits onto fuel rods, as in the case of 
benzenesulfonic acid-type cation exchange resin, and the non-radioactive 
metal ions are radioactivated, and redissolved into the cooling water to 
make deposition onto pipings and increase the radiation dosage as will be 
described in detail later. 
To clasify the causes for the phenomena, the present inventors have made 
further studies and have found that, among the weakly acidic cation 
exchange resins, those whose ion-exchanging groups are directly bonded to 
the benzene rings have a relatively high bonding energy and show the 
similar phenomena to those of the sulfonic acid type resins. As a result 
of further studies, it has been found that only weakly acidic cation 
exchange resins whose ion-exchanging resins have a bonding energy of not 
more than 300 KJ/mole are effective for cleaning the nuclear reactor 
cooling water. The present invention is based on the said findings. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process and an apparatus 
for cleaning nuclear reactor cooling water with an ion exchange resin 
capable of reducing a radiation exposure of operators working in an atomic 
power plant and of being readily susceptible to waste disposal. 
According to the present invention, there is provided a process for 
purifying nuclear reactor cooling water which comprises contacting nuclear 
reactor cooling water with a cation exchange resin whose ion-exchanging 
groups bonded to the main chain of an aromatic ring-containing polymer 
have a bonding energy of not more than 300 KJ/mole, thereby trapping cruds 
or cations in the cooling water. 
The present inventors have found that the radiation exposure of operators 
can be much more reduced by improving the apparatus for cleaning nuclear 
reactor cooling water now in use. That is, powdery ion exchange resin is 
used in a filtration desalter, where the powdery ion exchange resin has an 
average particle size of about 30 .mu.m, but its particle size 
distribution is so broad that the involved smallest particle size is less 
than 5 .mu.m. When such powdery ion exchange resin (a mixture of cation 
exchange resin and anion exchange resin) is used in the filtration 
desalter, a portion of the resin powder having particle sizes of less than 
5 .mu.m leaks out of the filtration desalter and enters into the cooling 
water. The leaked powdery ion exchange is brought, as such, into the 
nuclear reactor to increase the radiation exposure of operators. The 
foregoing has been found by the present inventors. This will be described 
in detail below, referring to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 3 shows a case where cooling water 3 containing no leaked ion exchange 
resin, but only non-radioactive metal ions dissolved from pipings, etc. 
such as cobalt ions, etc. flows into a nuclear reactor. In the nuclear 
reactor, fuel rods 8 whose surfaces are covered by an oxide film are 
provided. When an electrically charged state of metal ions 7 flowing into 
the nuclear reactor and the surfaces of fuel rods 8 are taken into 
account, the metal ions are, needless to say, positively charged, and the 
surfaces of fuel rods 8 also have a positive surface potential, because 
the zeta potential of the oxide film is positive in water at pH 5 to 8. In 
such a state, the metal ions 7 and the surfaces of fuel rods 8 are 
positively charged, and thus the metal ions 7 are substantially not 
deposited on the surfaces of fuel rods 8, as shown in FIG. 3. 
FIG. 4 shows a case where the ion exchange resins (cation exchange resins 9 
and anion exchange resins 10) leaked from the filtration desalter enter 
into the cooling water. The cation exchange resins 9 are negatively 
charged, whereas the anion exchange resins 10 are positively charged. 
Since the surfaces of fuel rods 8 are positively charged, a portion of the 
negatively charged cation exchange resins 9 deposits onto the surfaces of 
fuel rods 8. Non-radioactive metal ions such as cobalt ions, etc. are 
ionically adsorbed on the deposited cation exchange resins 9. Usually, the 
cation exchange resins 9 are not saturated with the adsorbed metal ions, 
and thus will also ionically adsorb metal ions 7 suspended in the cooling 
water 3. Thus, it has been found that, once cation exchange resins leak 
into the cooling water, the amount of non-radioactive metal ions deposited 
on the surfaces of fuel rods 8 will be increased. That is, it has been 
found that, when cation exchange resins leak into the cooling water, the 
average residence time of the non-radioactive metal ions in the nuclear 
reactor will be prolonged. 
Cobalt 59 (.sup.59 Co) is typical of non-radioactive metal ions contained 
in the cooling water 3, but the .sup.59 Co will be partially converted to 
cobalt 60 (.sup.60 Co) when subjected to neutron irradiation in the 
nuclear reactor. It has been found that, when the cation exchange resins 9 
leak into the cooling water 3, .sup.59 Co, etc. will stay in the nuclear 
reactor for a prolonged residence time, and consequently the yield of 
radioactive metals such as .sup.60 Co, etc. will be increased. That is, 
the radiation exposure of operators will be increased. 
The yield of radioactive metals in the nuclear reactor will be increased by 
leakage of cation exchange resins from the filtration desalter, and a 
portion of radioactivated metals peels from the fuel rods 8 and again 
dissolves into the cooling water. As a result, the radioactive density of 
cooling water will be increased. Since a portion of the radioactive metal 
ions in the cooling water deposits on the piping, the dosage of piping 
will be increased. For the same reasons as explained, referring to FIGS. 3 
and 4, the amount of radioactive metal ions deposited on the piping will 
be increased by the presence of cation exchange resins. 
When the cation exchange resins leak into the cooling water, the yield of 
radioactive metals will be increased and the amount of radioactive metals 
deposited on the piping will be increased in this manner, resulting in 
increasing radiation exposure of operators. 
The anion exchange resins 10 are positively charged and thus hardly deposit 
on the surfaces of fuel rod or piping, because these surfaces are 
positively charged. 
The present inventors have obtained the foregoing finding through the 
following test, using a test apparatus shown in FIG. 5. A circumstance 
corresponding to the core water conditions for a boiling water-type 
nuclear reactor was made by heating pure water 11 containing about 1 ppb 
of .sup.60 Co to 280.degree. C. under 70 atmospheres by a heater 12, and 
passing the heated water through a piping 14 by a pump 13. The amount of 
.sup.60 Co deposited on the piping 14 was determined after the passage 
through the piping 14. When the pure water contained about 0.01 ppm of 
cation exchange resins, it was found that the amount of .sup.60 Co 
deposited on the piping was 1.5-2-fold increased, as compared with that 
when the pure water contained no cation exchange resins. Furthermore, when 
the pure water contained the anion exchange resins, no increase was found 
in the amount of .sup.60 Co deposited on the piping 14. 
It is seen from the foregoing results that it is effective to prevent 
leakage of cation exchange resins into the cooling water to reduce the 
radiation exposure of operators. 
The ion exchange resins are used in both filtration desalter and desalter, 
which constitute an apparatus for cleaning nuclear reactor cooling water, 
and a mixture of cation exchange resins and anion exchange resins is used 
in these two units. In the desalter, granular ion exchange resins having 
particle sizes as large as about 500 .mu.m are used, and leakage of the 
ion exchange resin into the cooling water hardly occurs, whereas in the 
filtration desalter the average particle size of ion exchange resin is as 
small as about 30 .mu.m with a broad particle size distribution of from a 
few .mu.m to about 100 .mu.m, and thus ion exchange resins having smaller 
particle sizes are liable to leak into the cooling water. Thus, it is 
necessary to provide a means for preventing the cation exchange resin 
leakage from the filtration desalter. Such a means would be use of only 
powdery ion exchange resins having particle sizes above a predetermined 
size, for example, above 5 .mu.m, and is indeed effective for the leakage 
prevention, but disadvantageous from the viewpoint of cost, because the 
powdery ion exchange resins are prepared by pulverizing granular ion 
exchange resins, and thus inevitably contain those having particle sizes 
as small as 1 .mu.m, and when only those having particle sizes above the 
predetermined size are to be used, it is necessary to single them out from 
the thus prepared powdery ion exchange resins by screening. Furthermore, 
such singling-out becomes much complicated and costly, because the 
particle size to be singled out is as small as a few .mu.m. 
The present inventors have studied the presence of cation exchange resins 
incapable of increasing the radiation exposure of operators even if the 
cation exchange resins leak into the cooling water and have found that it 
is effective to use cation exchange resins whose ion-exchanging groups are 
bonded to other elements than the carbon atoms constituting a benzene 
ring. 
When the cation exchange resins leak into the cooling water 3, they will 
deposit on the surfaces of fuel rods and the average residence time of the 
non-radioactive metal ions such as .sup.59 Co, etc, in the nuclear reactor 
will be prolonged thereby, and the yield of radioactive metals will be 
increased, as already described above. Heretofore, benzenesulfonic 
acid-based resins whose ion-exchanging groups (SO.sub.3.sup.-) are 
directly bonded to the benzene ring have been used as cation exchange 
resins, and it has been found that such resins have a high heat resistance 
and a high radiation resistance, and are hardly decomposed even used in 
the primary system of a nuclear reactor. 
The reasons why the cation exchange resins are liable to deposit on the 
fuel rods 8 are that the cation exchange resins 9 are negatively charged, 
and the reasons why the cation exchange resins are negatively charged are 
as follows: the polymer body (copolymer of styrene and divinylbenzene) is 
electrically neutral, but the ion-exchanging groups are negatively charged 
as their property, and thus the cation exchange resins are negatively 
charged on the whole. 
When the cation exchange resins enter into a nuclear reactor, they are 
exposed to a higher temperature and an intense radiation, and thus are 
more susceptible to decomposition. Decomposition of cation exchange resin 
starts at first at the positions of the ion-exchanging groups having the 
lowest chemical bonding energy, as given by the following chemical 
equation: 
##STR2## 
If it is presumed that a cation exchange resin is decomposed as shown by 
the foregoing equation, an electrically neutral polymer body and a 
negatively charged sulfite ion (SO.sub.3.sup.2-) are formed. The sulfite 
ion migrates into the cooling water 3 owing to its solubility, whereas the 
remaining polymer body becomes electrically neutral, and thus hardly 
deposits onto the surfaces of fuel rods 8. Even if the polymer body 
deposits thereon, it has no more ion-exchanging capacity, and thus the 
non-radioactive metal ions in the cooling water are not retained on the 
surfaces of fuel rod for a prolonged period. Furthermore, the cation 
exchange resin whose ion-exchanging groups have been decomposed is 
electrically neutral, and thus hardly deposits even on the piping. 
Consequently, deposition of radioactive metals such as .sup.60 Co, etc. is 
no more promoted thereby. 
In the foregoing, the case of liberating the ion-exchanging groups from the 
cation exchange resin by decomposition has been described. Heretofore, 
benzenesulfonic acid-based resins having a high heat resistance and a high 
radiation resistance have been used as cation exchange resins. That is, 
the ion-exchanging groups have been hard to liberate from such cation 
exchange resins by decomposition, and the said effect of reducing 
radiation exposure of operators has not been obtained. In other words, the 
effect of reducing the radiation exposure of operators can be obtained by 
using cation exchange resins readily susceptible to thermal decomposition. 
Cation exchange resins readily susceptible to thermal decomposition will be 
described below. 
Cation exchange resins can be classified into the following two large 
groups on the basis of species of elements to which the ion-exchanging 
groups are bonded: one is the group where the ion-exchanging groups are 
bonded to the carbon atoms constituting a benzene ring, as will be 
hereinafter referred to as "benzene ring type", and another is the group 
where the ion-exchanging groups are bonded to other elements than the 
carbon atoms constituting a benzene ring, as will be hereinafter referred 
to as "straight chain type". 
Table 1 shows examples of benzene ring type and straight chain type cation 
exchange resins. 
TABLE 1 
__________________________________________________________________________ 
Bonding energy 
Group Molecular structure (KJ/mol.) 
__________________________________________________________________________ 
Benzene ring type 
.circle.A 
##STR3## 339 
.circle.B 
##STR4## -- 
.circle.C 
##STR5## 447 
.circle.D 
##STR6## -- 
Straight chain type 
.circle.E 
##STR7## 289 
.circle.F 
##STR8## 260 
.circle.G 
##STR9## 293 
__________________________________________________________________________ 
Remark: 
##STR10## 
Seven kinds of cation exchange resins given in Table 1 were subjected to a 
test to find out thermally weak resins. The test was carried out in the 
following manner. Seven kinds of the cation exchange resins were dipped in 
hot water at 280.degree. C. under 70 atmospheres to show changes in ion 
exchange capacity to investigate how the ion-exchanging groups were 
liberated by decomposition with time. 
FIG. 6 shows the test results, where the axis of abscissa shows a dipping 
time in the hot water and the axis of ordinate logarithmically shows 
changes in ion exchange capacity. It is seen from the test results that 
cation exchange resins readily susceptible to thermal decomposition are 
.circle.E , .circle.F and .circle.G , that is, the straight chain type 
ion exchange resins given in Table 1. Ready susceptability to thermal 
decomposition of the straight chain type ion exchange resins seems due to 
the bonding energy of the ion-exchanging groups to the polymer body (see 
Table 1). That is, it seems that the ion-exchanging groups bonded to the 
carbon atoms in a benzene ring (benzene ring type) has such a large 
bonding energy that they are hard to liberate by thermal decomposition, 
whereas the ion-exchanging groups bonded to other elements than the carbon 
atoms in the benzene ring (straight chain type) has such a small bonding 
energy that they are easy to liberate by thermal decomposition. 
FIG. 7 shows the time t.sub.1/10 until the ion exchange capacity becomes 
1/10 for the respective ion exchange resins, obtained from changes in the 
ion exchange capacity shown in FIG. 6, where the axis of ordinate shows 
t.sub.1/10 and the axis of abscissa shows the bonding energy of the 
ion-exchanging group in the corresponding ion exchange resin. It is seen 
from FIG. 7 that, when the bonding energy is not more than 300 KJ/mol, 
t.sub.1/10 will be not more than one hour, and the ion exchange resins are 
readily susceptible to thermal decomposition. Particularly, straight chain 
type ion exchange resins are readily susceptible to thermal decomposition, 
because their bonding energy is usually not more than 300 KJ/mol, and even 
benzene ring type ion exchange resins can readily undergo thermal 
decomposition, so long as their bonding energy is not more than 300 
KJ/mol. 
It can be seen from the foregoing that it is effective for reducing the 
radiation exposure of operators to use straight chain type cation exchange 
resins or cation exchange resins whose ion-exchanging groups have a low 
bonding energy as cation exchange resins for the filtration desalter. 
Up to now, more than 100 kinds of cation exchange resins have been known, 
and most of the resins now in use belong to the benzene ring type, and the 
straight chain types are not so many. Particularly, there have been no 
examples of using a straight chain type cation exchange resin in an 
apparatus for cleaning nuclear reactor cooling water. 
The straight chain type cation exchange resins include the following types: 
(1) Oxybenzylsulfonic acid type: as shown by .circle.E in Table 1. 
(2) Acrylic carboxylic acid type: as shown by .circle.F in Table 1. 
(3) Methacrylic carboxylic acid type: molecular structure of this type is 
as follows: 
##STR11## 
(4) Aromatic carboxylic acid type: molecular structure of this type is as 
follows: 
##STR12## 
(5) So called chelate resin: as shown by .circle.G in Table 1, and those 
having the following molecular structures: 
##STR13## 
When cation exchange resin containing divinylbenzene as a 
molecule-constituting member is used in the present invention, the content 
of the divinylbenzene in the resin is 1 to 20% by weight, preferably 2 to 
16% by weight, on the basis of the resin. 
Which ion exchange resin is most preferable for a filtration desalter 4 
shown in FIG. 2 or a desalter 5 shown in FIG. 1 will be described in 
detail below. 
Impurities contained in the cooling water include fine granular materials 
such as cruds, etc., which will be hereinafter referred to as impurities 
a, cations produced by corrosion of materials such as Co.sup.2+, 
Fe.sup.2+, Mn.sup.2+, etc., which will be hereinafter referred to as 
impurities b, and anions such as carbonate ions, silicate ions, etc., 
which will be hereinafter referred to as impurities c, and furthermore 
include neutral salts such as NaCl, etc., when sea water leaks into the 
cooling water from the condenser 2 as shown in FIG. 1 (the neutral salt 
will be hereinafter referred to as impurities d). 
According to the prior art, a mixture of benzenesulfonic acid-based resin 
as a strongly acidic ion exchange resin and a quaternary ammonium-based 
ion exchange resin as a strongly basic ion exchange resin is used in the 
filtration desalter, as described earlier, and all of the said impurities 
a to d can be removed in the filtration desalter. That is, the desalter is 
provided in an auxiliary sense. 
In the present invention, on the other hand, a desalter 5 as shown in FIG. 
1 plays an important role, because the straight chain type cation exchange 
resin generally belongs to a weakly acidic ion exchange resin, and thus 
has a low ability of decomposing the impurities d (neutral salt) 
(NaCl.fwdarw.Na.sup.+ +Cl.sup.-) to adsorb these ions. Thus, when sea 
water leaks into the cooling water from the condenser 2 as shown in FIG. 
1, and when straight chain type ion exchange resin is used as cation 
exchange resin in the filtration desalter 4, the neutral salt cannot be 
completely removed in the filtration desalter. Thus, if there is a 
possibility of leakage of sea water from the condenser 2, it is preferable 
to use a mixture of strongly acidic granular ion exchange resin such as 
benzenesulfonic acid-based resin, etc., and a strongly basic granular ion 
exchange resin such as quaternary ammonium-based resin in the desalter 5. 
The neutral salts can be completely removed in the desalter 5 thereby. 
In the foregoing description, mention has been not substantially made of 
anion exchange resin among the powdery ion exchange resins to be used in 
the filtration desalter 4. Even if the anion exchange resin leaks in the 
cooling water 3, it will not increase the radiation exposure of operators, 
and thus the same quaternary ammonium-based resin as so far used, or any 
other resins can be used satisfactorily. Other anion exchange resins than 
the foregoing include primary to tertiary amine-based anion exchange 
resins, as given below: 
##STR14## 
Heretofore, quaternary ammonium-based resins have been used as anion 
exchange resin for use in the filtration desalter, because the quaternary 
ammonium-based resins are strongly basic resins, and thus have a high 
percent removal of the neutral salts in the filtration desalter when used 
in mixture with the strongly acidic benzenesulfonic acid-based resin. 
In the present invention, however, removal of the neutral salts (impurities 
d) is carried out in the desalter 5, and thus the anion exchange resin for 
use in the filtration desalter 4 is not limited to the quaternary 
ammonium-based resins. 
As already described above, the radiation exposure of operators can be 
considerably reduced by using cation exchange resins whose ion-exchanging 
groups are bonded to other elements than the carbon atoms constituting a 
benzene ring in the filtration desalter 4 using the powdery ion exchange 
resins. 
Furthermore, the following effects can be obtained with the said structure 
of the present invention. 
When the powdery ion exchange resin precoated in the filtration desalter 4 
is used for a prolonged period, for example, 10 to 50 days, the resin 
layer undergoes clogging as a result of trapping the cruds in the cooling 
water 3, and the pressure drop through the resin layer will be increased. 
When the pressure drop reaches a predetermined value, the resin layer is 
back-washed and the ion exchange resin is exchanged with fresh one. The 
used ion exchange resin is handled as a radioactive waste. About half of 
the radioactive wastes now discharged from the boiling water-type nuclear 
reactor is the used ion exchange resins discharged from the filtration 
desalter 4 (the used ion exchange resin will be hereinafter to as "waste 
resin"). 
As means for reducing the volume of waste resin, there has been developed a 
process for thermal decomposition or incineration, as disclosed in 
Japanese Patent Application Kokai (Laid-open) No. 59-107,300. The waste 
ion exchange resin according to the present invention can be readily 
treated by the said process for thermal decomposition or incineration. The 
reasons will be described below, referring to thermal decomposition 
treatment of benzenesulfonic acid-based resin ( .circle.A in Table 1) and 
acrylic carboxylic acid resin ( .circle.F in Table 1) in comparison. 
FIG. 8 is a diagram showing changes in weight by heating when the 
benzenesulfonic acid-based resin (curve .circle.A ) and the acrylic 
carboxylic acid resin (curve .circle.F ) were subjected to thermal 
decomposition treatment in a nitrogen gas atmosphere. As is apparent from 
FIG. 8, the benzenesulfonic acid-based resin produces about 50% by weight 
of residues, even if subjected to the thermal decomposition at 500.degree. 
C. or higher, owing to its high heat resistance, whereas more than 95% by 
weight of the acrylic carboxylic acid resin can be decomposed, producing 
only small amount of the residues, when subjected to the thermal 
decomposition at 450.degree. C. or higher, owing to its low heat 
resistance. That is, it is evident therefrom that, when the waste resin 
used in the present invention is subjected to a thermal decomposition 
treatment, the amount of the wastes can be considerably reduced. 
As a result of investigations of thermal decomposition characteristics of 
other cation exchange resins according to the present invention, it has 
been found that more than 95% by weight of methacrylic carboxylic acid 
resin, aromatic carboxylic resin and chelate resin are decomposed and even 
about 70% by weight of oxybenzylsulfonic acid is decomposed when subjected 
to thermal decomposition at 500.degree. C. in a nitrogen gas atmosphere. 
That is, any of the cation exchange resins for use in the present 
invention is more readily susceptible to thermal decomposition than the 
benzenesulfonic acid-based resin. 
The same results were obtained when they were subjected to an incineration 
treatment in an oxygen-containing atmosphere. That is, the conventional 
benzenesulfonic acid-based resin was hardly incinerated owing to its high 
heat resistance, and a portion of the residues deposited onto the furnace 
wall shortened the life of the furnace material, because the incineration 
was carried out at 800.degree. C. or higher, and thus a portion of the ion 
exchange resin was melted, and was much liable to deposit on the furnace 
wall. On the other hand, the cation exchange resins for use in the present 
invention could be readily incinerated. 
Furthermore, when the conventional benzenesulfonic acid-based resin is 
subjected to thermal decomposition or incineration, harmful gases, such as 
H.sub.2 S, SO.sub.x, etc. are evolved, because the resin contains sulfur 
atom. In the present invention, on the other hand, no such harmful gases 
as H.sub.2 S and SO.sub.x are evolved at all, even if the acrylic 
carboxylic acid resin, aromatic carboxylic resin, etc. according to the 
present invention are subjected to thermal decomposition or incineration 
treatment, because they contain no sulfur atom. Thus, the gas treatment 
system can be simplified, and also corrosion of materials by H.sub.2 S, 
etc. can be prevented. When the cation exchange resins according to the 
present invention are used in the filtration desalter 4, not only the 
radiation exposure of operators can be reduced, but also the radioactive 
waste disposal can be facilitated. 
The conventional benzenesulfonic acid-based resin is used as granular 
cation exchange resin in the desalter 5 also in the present invention, and 
thus no effect of facilitating the waste disposal can be obtained, but 
there is no serious problem at all, because the amount of the wastes 
discharged from the desalter 5 is not more than 1/10 of the amount of the 
wastes discharged from the filtration desalter 4. Furthermore, a trouble 
at the disposal of the benzenesulfonic acid-based resin discharged from 
the desalter 5 can be eased by treating the waste resin discharged from 
the desalter 5 together with the waste resin discharged from the 
filtration desalter 4 simultaneously, for example, by thermal 
decomposition or incineration. The concentration of SO.sub.x or H.sub.2 S 
evolved from the benzenesulfonic acid-based resin can be relatively 
lowered by the simultaneous treatment, and thus the problems such as 
corrosion of materials, etc. can be eased, as compared with the single 
treatment of the benzenesulfonic acid-based resin. 
In the foregoing, it has been described that the desirable powdery cation 
exchange resins for use in the filtration desalter 4 are those whose 
ion-exchanging groups are bonded to other elements than the carbon atoms 
constituting a benzene ring, but it is inevitable to use those whose 
ion-exchanging groups are partly bonded to the carbon atoms in the benzene 
ring directly on account of process conditions, etc. 
In summary, the preferable modes of the present invention are as follows: 
(1) Powdery cation exchange resins whose ion-exchanging groups are bonded 
to other elements than the carbon atoms constituting a benzene ring 
(straight chain type) are used in a filtration desalter in an apparatus 
for cleaning nuclear reactor cooling water. 
(2) A mixture of benzenesulfonic acid-based cation exchange resin and 
quaternary ammonium-based anion exchange resin is used as granular ion 
exchange resins in a desalter in an apparatus for cleaning nuclear reactor 
cooling water. 
PREFERRED EMBODIMENTS OF THE INVENTION 
The present invention will be described in detail below, referring to 
Examples and Drawings. 
EXAMPLE 1 
A first embodiment of the present invention is shown in FIGS. 1 and 2, 
where FIG. 1 shows a flow diagram of an apparatus for cleaning cooling 
water in a boiling water-type nuclear reactor, and FIG. 2 shows a 
partially cutaway view of a filtration desalter used in the apparatus for 
cleaning cooling water shown in FIG. 1. 
Cooling water 3 recovered by a condensor 2 after driving a turbine 1 is 
cleaned by a filtration desalter 4 and a desalter 5, and returned to a 
nuclear reactor 6 through a feedwater heater 16 by a feedwater pump 15. 
In the desalter 5 a mixture of the said granular benzenesulfonic acid-based 
resin and quaternary ammonium-based resin in a mixing ratio of 2:1 by 
weight is packed. In the filtration desalter 4, powdery ion exchange resin 
is precoated. That is, acrylic carboxylic acid resin ( .circle.F in Table 
1) having an average particle size of about 30 .mu.m as powdery cation 
exchange resin and quaternary ammonium-based resin having an average 
particle size of about 30 .mu.m as powdery anion exchange resin are 
charged in a ratio of 2:1 by weight of the former to the latter into a 
precoat tank 17 at first, and further about 0.05 to about 1% by weight of 
a water-soluble polymeric electrolyte such as polyacrylic acid, 
polymerized maleic acid, etc. is added therto. Then, the mixture is 
stirred by a stirrer 18. As a result, flocs composed of the mixture of 
cation exchange resins 9 and anion exchange resins 10 are formed. The 
flocs are supplied to the filtration desalter 4 through a valve 19 by a 
precoat pump 20. Nylon or stainless steel filtration elements 21 are 
provided in the filtration desalter 4, as shown in FIG. 2, and are 
precoated with the flocs 22. 
The boiling water-type nuclear reactor provided with the apparatus for 
cleaning cooling water, as described above, was operated for one year, and 
the surface dosages of various pipings were measured. It was found that 
the dosage of recycle piping 23 was highest and was 20 mR/h. When a 
conventional apparatus for cleaning cooling water, using benzenesulfonic 
acid-based resin and quaternary ammonium-based resin in the filtration 
desalter 4, was used, the surface dosage of recycle piping 23 was 30 mR/h. 
The nuclear reactor 6 was operated while changing the species of the 
powdery ion exchange resin in the filtration desalter 4, and the surface 
dosage of recycle piping 23 was measured. The results are shown in Table 
2. 
TABLE 2 
______________________________________ 
Surface 
Filtration desalter dosage of 
Cation exchange Anion exchange 
recycle 
resin resin piping (mR/h) 
______________________________________ 
Benzenesulfonic 
Quaternary 
Conven- acid-based ammonium-based 
30 
tional resin resin 
Acrylic Quaternary 
carboxylic ammonium-based 
20 
acid resin resin 
Oxybenzyl- Quaternary 
sulfonic acid 
ammonium-based 
25 
resin resin 
The Methacrylic Tertiary amine- 
Inven- carboxylic acid 
based resin 20 
tion resin 
Aromatic Secondary 
carboxylic amine-based 20 
acid resin resin 
Primary amine- 
Chelate resin 
based resin 25 
______________________________________ 
As is obvious from Table 2, the radiation exposure of operators according 
to the present invention can be reduced to 1/3-1/6 of that according to 
the prior art. For example, when the surface dosage of recycle piping is 
20 mR/h, the annual radiation exposure of operators amounts to about 60 to 
about 100 man.rem. 
In the present invention, it has been found that the life of powdery ion 
exchange resin for use in the filtration desalter 4 can be prolonged, as 
will be described below, referring to FIG. 9, where the dotted line curve 
A shows changes in differential pressure when cooling water 3 was cleaned 
by a precoat of a mixture of acrylic carboxylic acid resin and quaternary 
ammonium-based resin in a mixing ratio of 2:1 by weight of the former to 
the latter, and the full line curve B shows changes in differential 
pressure when a precoat of a mixture of the conventional powdery ion 
exchange resins (benzenesulfonic acid-based resin and quaternary 
amine-based resin) was used. It is obvious from FIG. 9 that the 
differential pressure curve of the present invention rises much later than 
that of the prior art, and the life can be about 1.5-fold prolonged. Thus, 
in the present invention, the filtration life of the powdery ion exchange 
resin can be prolonged, and not only the cost can be reduced, but also the 
resin can be used for a longer time, and consequently the amount of the 
waste resin can be decreased. That is, the amount of the radioactive 
wastes can be effectively reduced. 
In the foregoing description, a combination of acrylic carboxylic acid 
resin and quaternary ammonium-based resin has been exemplified as the 
powdery ion exchange resins for use in the filtration desalter 4. 
Equivalent effects can be also obtained from combinations of other kinds 
of ion exchange resins according to the present invention. That is, 5 
kinds of powdery cation exchange resins, such as oxybenzylsulfonic acid 
resin, acrylic carboxylic acid resin, methacrylic carboxylic resin, etc., 
and 4 kinds of powdery anion exchange resins such as quaternary 
ammonium-based resin, tertiary amine resin, etc. were selected, and their 
filtration lives were experimentally determined on the basis of 
combinations thereof. The results are shown in Table 3, where the 
filtration lives on the basis of various combinations according to the 
present invention are shown as relative values to the filtration life of 
the conventional powdery ion exchange resin as unity. 
TABLE 3 
______________________________________ 
Quaternary 
Tertiary Secondary 
Primary 
Filtration 
ammonium- amine-based 
amine-based 
amine-based 
Life based resin 
resin resin resin 
______________________________________ 
Oxybonzyl- 
sulfonic 1.2 1.2 1.1 1.1 
acid resin 
Acrylic 
carboxylic 
1.5 1.6 1.4 1.5 
acid resin 
Methacrylic 
carboxylic 
1.8 1.6 1.6 1.4 
acid resin 
Aromatic 
carboxylic 
1.4 1.2 1.3 1.3 
acid resin 
Chelate 1.4 1.3 1.2 1.4 
resin 
______________________________________ 
*Filtration life: Relative to the conventional resin as unity. 
As is obvious from Table 3, any of the combinations according to the 
present invention can make the filtration life 1.1 to 1.8-fold longer. 
In the present embodiment, not only the radiation exposure of operators can 
be reduced to 1/3-1/6 of that of the prior art, but also the life of ion 
exchange resin in the filtration desalter can be made about 1.5-fold 
longer, as described above, and thus cost and the amount of discharged 
radioactive waste can be reduced to about 1/3. 
Tests were carried out in the present apparatus on the condition that sea 
water leaked into the cooling water 3 from the condenser 2, but no trouble 
appeared. When sea water leaked in the cooling water from the condenser 2, 
neutral salts such as NaCl, etc. in the cooling water could be removed in 
the filtration desalter 4 in the conventional apparatus, whereas in the 
present invention the neutral salts could not be removed, because weakly 
acidic cation exchange resin was used in the filtration desalter 4. 
However, in this embodiment of the present invention, the strongly acidic 
benzenesulfonic acid-based resin and the strongly basic quaternary 
ammonium-based resin were used in the desalter 5 and thus the neutral 
salts that could not be removed in the filtration desalter 4 could be 
completely removed in the desalter 5. That is, there was no problem at 
all, even when sea water leakage took place. 
In this embodiment of the present invention, three components, i.e. cation 
exchange resin, anion exchange resin and polymeric electrolyte, were mixed 
in the precoat tank 17, but when both or any one of the cation exchange 
resin and the anion exchange resin, to which the polymeric electrolyte has 
been added in advance, for example, by surface treatment, etc., are used, 
only two components, i.e. the cation exchange resin and the anion exchange 
resin, are to be mixed in the precoat tank 17. 
EXAMPLE 2 
This embodiment had the same basic structure as in Example 1, but the 
mixing ratio of powdery cation exchange resin to the cation exchange resin 
to be used in the filtration desalter 4 was changed from that of Example 
1. 
In Example 1, it was shown that the life of the powdery ion exchange resins 
to be used in the filtration desalter 4 could be prolonged in the present 
invention. In this embodiment changes in the life of the powdery ion 
exchange resins were investigated by changing the mixing ratio of the 
powdery cation exchange resin to the powdery anion exchange resin. The 
same test procedure as shown in FIG. 9 was used, where the differential 
pressure changing curves were experimentally plotted to determine the life 
of the resins. The test results are shown in FIG. 10, where the axis of 
abscissa shows the resin proportion and the axis of ordinate shows the 
filtration life. The filtration life on the axis of ordinate shows a life 
relative to the life of the powdery ion exchange resins used in the 
filtration desalter in the conventional apparatus, i.e. a mixture of 67% 
by weight benzenesulfonic acid-based resin and 33% by weight of quaternary 
ammonium-based resin, as unity. In FIG. 10, the full line curve C shows 
powdery ion exchange resins based on combinations of acrylic carboxylic 
acid resin and quaternary ammonium-based resin, and the dotted line curve 
D shows powdery ion exchange resins based on combinations of methacrylic 
carboxylic acid resin and tertiary amine resin. 
As is obvious from FIG. 10, the filtration life could be prolonged by 
setting the ratio of the cation exchange resin to the anion exchange resin 
to 50:50- 90:10% by weight. The reasons why the filtration life depended 
on the mixing ratio of the cation exchange resin to the anion exchange 
resin seems as follows: there are much more cations such as Co.sup.2+, 
Fe.sup.2+, etc. as impurities in the cooling water 3 than anions, and if 
the cation exchange resin and the anion exchange resin are used in equal 
weights, the cation exchange resin will adsorb such a larger number of 
ions than the anion exchange resin. Once at least any one of the cation 
exchange resin and the anion exchange resin adsorbs much more ions, the 
filtration capacity seems to be lowered. In order to prolong the 
filtration life, the ratio of the cation exchange resin must be 50% or 
more. 
According to the prior art, neutral salts such as NaCl, etc. from sea water 
leakage is designed to be removed in the filtration desalter 4, where 
equal equivalent weights of cations (Na.sup.+) and anions (Cl.sup.-) must 
be removed. Thus, the ratio of the cation exchange resin cannot be made 
too large. However, in the present invention, the neutral salts are 
designed to be removed not in the filtration desalter 4, but in the 
desalter 5, and thus the ratio of the cation exchange resin in the 
filtration desalter can be made considerably larger without any problem. 
It is also seen from FIG. 10 that the filtration life will be shortened 
when the ratio of the cation exchange resin exceeds 90% by weight. This is 
because no suitable flocs (coagulates of cation exchange resin and anion 
exchange resin) for the filtration can be formed owing to too small an 
amount of the anion exchange resin. 
As described above, it is desirable that the ratio of the powdery cation 
exchange resin to the powdery anion exchange resin is 50:50 to 90:10% by 
weight, preferably 60:40 to 85:15% by weight, whereby the filtration life 
can be considerably prolonged. 
EXAMPLE 3 
This embodiment shows a case of applying the present invention to an 
apparatus for cleaning cooling water in a pressurized water-type nuclear 
reactor, whose flow diagram is shown in FIG. 11. 
Cooling water 3 heated in nuclear reactor 6 is recycled to the nuclear 
reactor 6 through a steam generator 24 by a primary cooling water pump 25, 
but a portion of the cooling water 3 is supplied to a filtration desalter 
4 through a heat exchanger 26. In the filtration desalter 4, a mixture of 
cation exchange resin and anion exchange resin having the following 
molecular structures is used in a ratio of the former to the latter of 3:1 
by weight as powdery ion exchange resins: 
##STR15## 
According to the prior art, the pressurized water-type nuclear reactor uses 
a mixed bed desalter using granular ion exchange resins in place of the 
filtration desalter 4, but the capacity of removing cruds in the cooling 
water can be considerably increased by using the filtration desalter of 
this Example, and also the radiation exposure of operators can be largely 
reduced. 
The present apparatus for cleaning cooling water can be applied also to a 
core water-cleaning system in a boiling water-type nuclear reactor. 
According to the prior art, a portion of cooling water in the nuclear 
reactor recycle system is cleaned in the filtration desalter in the 
boiling water-type nuclear reactor, and a mixture of benzenesulfonic 
acid-based resin and quaternary amine-based resin is used as powdery ion 
exchange resins for the filtration desalter. However, a portion of the ion 
exchange resins leaks in the cooling water even in the filtration 
desalter, and there is a consequent possibility to increase the radiation 
exposure of operators. When the ion exchange resins according to the 
present invention are used in the filtration desalter in the core 
water-cleaning system for cleaning a portion of the cooling water in the 
recycle system, an increase in the radiation exposure of operators can be 
minimized, even if the ion exchange resin leakage takes place. 
Furthermore, the life of the filtration desalter in the core 
water-cleaning system can be prolonged as in Example 2, and the waste 
disposal can be also facilitated thereby. 
EXAMPLE 4 
In Example 1, a combination of the filtration desalter 4 and the desalter 5 
arranged in series as an apparatus for cleaning nuclear reactor cooling 
water is exemplified, but the desalter 5 is not always required, and only 
the filtration desalter 4 can perform the required duty. That is, the 
filtration desalter 4 also uses ion exchange resins as in the desalter 5, 
and most of ions and cruds can be removed in the filtration desalter 4, 
because the desalter has only a backing function. When the ion exchange 
resins according to the present invention are used in the filtration 
desalter 4, the neutral salts are not completely removed therein, and thus 
the desalter 5 can be only omitted when there is a very low possibility 
for the sea water leakage from the condenser 2, or when 
corrosion-resistant materials, such as stainless steel, etc. are used in 
the condenser 2, or when there is no possibility at all for the sea water 
leakage, for example, when river water or well water is used in place of 
the sea water as the condenser cooling water. 
According to this embodiment of the present invention, the same effect as 
in Example 1 can be obtained, and the cost can be much more reduced by 
omitting the desalter 5. 
EXAMPLE 5 
When the ion exchange resins used in the apparatus for cleaning nuclear 
reactor cooling water run down, the waste ion exchange resins are treated 
as radioactive wastes. The radioactive wastes are now stored and preserved 
in atomic power plants, and the amount of such wastes is increasing year 
after year. 
This embodiment concerns a thermal decomposition treatment of waste ion 
exchange resins (waste resins) discharged from the filtration desalter 4 
shown in Example 1, and will be explained, referring to the flow diagram 
shown in FIG. 12. 
Waste resins 27 are in a slurry state, because they are discharged from the 
filtration desalter 4 by backwashing, and are stored in a waste resin tank 
28 for a while. The waste resins in the waste resin tank 28 are supplied 
in the slurry state at a concentration of about 10% by weight through a 
valve 29 at a constant flow rate to a thermal decomposition unit 31 by a 
slurry pump 30. The waste resins used in this embodiment are composed of 
60% by weight of acrylic carboxylic acid resin, 30% by weight of 
quaternary ammonium-based resin, and 10% by weight of impurities such as 
cruds, etc. The thermal decomposition unit 31 is a rotary kiln of 
continuous treatment type, operated at 500.degree. C. In the thermal 
decomposition unit 31 an inert gas atmosphere is kept by nitroge gas 
purging. The waste resins 27 supplied to the thermal decomposition unit 31 
are subjected to drying and thermal decomposition at the same time, and 
the thermal decomposition residues are stored in a powder hopper 35 for a 
while. A flue gas evolved at the thermal decomposition is composed mainly 
of steam and hydrocarbons, and led through a valve 33 to a flue 
gas-treating unit 34 and treated. After the thermal decomposition residues 
32 are stored in the powder hopper 35 for a while, they are mixed with 
cement 37 or plastics, or the like in a mixer 36 and then poured into a 
drum 39 through a valve 38, and solidified. 
The functions and effects of this embodiment will be described below: 
By thermal decomposition of the waste resins 27 in the thermal 
decomposition unit 31, the waste resins 27 can be reduced to 20% by weight 
and 10% by volume of the charged entire waste resins, whereas the waste 
resins 27 whose cation exchange resin is the conventional benzenesulfonic 
acid-based resin is reduced only to 40% by weight and 25% by volume of the 
charged entire waste resins by the thermal decomposition. In the case of 
the waste resins according to this embodiment the flue gas evolved by the 
thermal decomposition contains only steam and hydrocarbons, whereas in the 
case of the waste resins containing the benzenesulfonic acid-based resin 
SO.sub.x or H.sub.2 S is evolved by the thermal decomposition in addition 
to the steam and the hydrocarbons owing to the sulfur atoms contained 
therein, as is obvious from the said structure. SO.sub.x or H.sub.2 S is a 
harmful gas component, and its removal requires an alkali scrubber, etc., 
complicating the flue gas-treating unit 34. The present invention has no 
such problems. 
Thus, such effects as considerable reduction in the volume of waste resins 
and simplification of the flue gas-treating unit can be obtained by using 
the ion exchange resins according to the present invention and by thermal 
decomposition of the waste resins. 
In this embodiment, the waste resins discharged from the filtration 
desalter in the boiling water-type nuclear reactor have been exemplified, 
but the waste resins discharged from the filtration desalter in the 
pressurized water-type nuclear reactor shown in Example 3 can be likewise 
treated according to the present invention. 
EXAMPLE 6 
In Example 5, the waste resins 27 are treated by thermal decomposition, but 
the same effects as in Example 5 can be obtained by incineration treatment 
in place of the thermal decomposition. In this case an incineration 
furnace is used in place of the thermal decomposition unit 31, and the 
waste resins are incinerated at 600.degree. C. or higher in the air. So 
far as the ion exchange resins according to the present invention are 
used, a considerable reduction in the volume can be attained, and also the 
flue gas-treating unit can be simplified. 
In this embodiment, the following effects can be further obtained. In the 
case of incineration, waste resins are treated at 600.degree. C. or 
higher, usually at 800.degree. to 1,500.degree. C., and thus a portion of 
the waste resins is melted and the melted resin deposits on the furnace 
wall, shortening the life of the incineration furnace. The ion exchange 
resins according to the present invention have a low heat resistance, and 
thus can be readily gasified below 600.degree. C. That is, the present ion 
exchange resins are not substantially melted. Thus, the trouble of waste 
resin deposition on the furnace wall can be considerably reduced, and the 
life of the incineration furnace can be prolonged. 
EXAMPLE 7 
In the foregoing embodiments, the straight chain type cation exchange 
resins are used in the filtration desalter, but a filter of membrane 
structure capable of mechanically removing cruds, such as hollow fiber 
filter, etc. can be used as a filtration desalter. In this embodiment, 
there is no leakage of ion exchange resin into the cooling water or no 
function to increase the surface dosage of pipings as in the case of the 
powdery ion exchange resins. Furthermore, when polymers whose 
ion-exchanging groups are bonded to other elements than carbon atoms 
constituting the benzene rings are used as the filter of membrane 
structure having ion-exchanging groups, a high volume reduction effect can 
be obtained at the disposal of the resulting waste polymers. That is, the 
same functions and effects as in Examples 5 and 6 can be obtained owing to 
the use of straight chain type polymers as .circle.E , .circle.F and 
.circle.G in Table 1. Thus, in this embodiment of using polymers whose 
ion-exchanging groups are bonded to the straight chains, equivalent or 
superior effects to those of the foregoing Examples can be obtained. 
In Example 1, powdery straight chain type cation exchange resins having an 
average particle size of about 30 .mu.m are used, but the present 
invention is not limited to the said particle size. For example, fibrous 
straight chain type cation exchange resins, which are extended in one 
direction, can be used. Such fibrous straight chain type cation exchange 
resin is effective for reducing the resin leakage from the filtration 
desalter. Both or any one of cation exchange resin and anion exchange 
resin can be such a fibrous ion exchange resin. 
When the fibrous cation exchange resin is long enough not to pass through 
nylon or stainless steel filtration elements, the cation exchange resin 
can be used alone. That is, since the cruds to be removed are positively 
charged and also ion species such as Co, etc. are positively charged, they 
can be removed only by the cation exchange resin. The amount of waste 
resins can be largely reduced in this manner without using the anion 
exchange resins. 
Furthermore, the straight chain type cation exchange resins have a lower 
degree of dissociation (smaller .sub.P K.sub.A, where .sub.P K.sub.A is a 
logarithm of the reciprocal of the dissociation constant) than the 
sulfonic acid bonded to the benzene ring, and are of weakly acidic type. 
Thus, their ability to decompose the neutral salts is lowered, as already 
described, and it is hard to remove NaCl when sea water leaks into the 
cooling water. In other words, if the straight chain type ion exchange 
resins can have ion exchanging groups of higher degree of dissociation 
(.sub.P K.sub.A of less than 3), removal of NaCl can be made without 
providing a condensate desalter on the downstream side. That is, when a 
possibility of large sea water leakage is substantially eliminated by an 
increased reliability of apparatuses and machinery of an atomic power 
plant in the future, the condensate desalter 5 as shown in FIG. 1 can be 
omitted by using the straight chain type cation exchange resins having ion 
exchanging groups of high degree of dissociation in the filtration 
desalter. 
EXAMPLE 8 
Characteristics of filtration desalter 4 can be further improved by 
optimizing the particle size of the ion exchange resin according to the 
present invention, or by adding fibers thereto. 
Characteristics required for the filtration desalter 4 are the following 
two: 
The first is a longer filtration life. The longer the filtration time, the 
more the amount of cruds adsorbed in and removed by the precoat layer. 
Thus, clogging takes place in the precoat layer, increasing the filtration 
differential pressure (pressure drop). When the differential pressure 
reaches a predetermined value (usually 1.75 kg/cm.sup.2), the powdery ion 
exchange resins as a precoat material is back-washed, and discharged, and 
exchanged with a new precoat material. Thus, in order to reduce the cost 
and the amount of the discharged wastes, it is desirable that the 
differential pressure increases slowly, that is, the filtration life is 
longer. 
The second characteristic as required is a higher percent removal of cruds 
from the cooling water to be treated, and usually percent removal of 90% 
or higher is required. 
The present inventors have found, as a result of basic tests, that when the 
cation exchange resins according to the present invention are used, a 
longer filtration life can be obtained as desired than that of the 
conventional strongly acidic sulfonic acid-based resin, but sometime 
cracks are developed in the precoat layer during the filtration, lowering 
the percent removal of cruds to 60-90%, and continuous use of the 
filtration desalter in such a state becomes inappropriate. 
The cation exchange resins according to the present invention include 
carboxylic acid-based resin, chelate resin, etc., and at first the case of 
using carboxylic acid-based resin will be described below: 
Methacrylic carboxylic acid resin, acrylic carboxylic acid resin, etc. are 
known as the carboxylic acid-based resin. Characteristics of these resins 
used as the precoat material are substantially equal to one another, and 
thus will be hereinafter referred to all together as "carboxylic 
acid-based resin". The known carboxylic acid-based resin is the so called 
granular resin having particle sizes of about 500 .mu.m, but such granular 
resin is not suitable for the precoat and has an insufficient filtration 
effect owing to the larger particle size, and is also not practical owing 
to the low efficiency in ion exchange reaction. 
Thus, the carboxylic acid-based resin is pulverized to prepare powdery 
carboxylic acid-based resin having an average particle size of about 50 
.mu.m. The thus prepared resin is mixed with powdery anion exchange resin 
in a ratio of the former to the latter of 2:1 by weight, and a small 
amount of a polymeric coagulating agent such as polyacrylamide, etc. is 
added to the mixture to prepare a precoat material. Water containing cruds 
is treated with the thus obtained precoat material. The results are shown 
in FIG. 13, as compared with the results of using the conventional 
sulfonic acid-based resin as cation exchange resin. As is obvious from 
FIG. 13, the filtration differential pressure increases with filtration 
time, but the filtration differential pressure of carboxylic acid-based 
resin increases more slowly than that of sulfonic acid-based resin. The 
filtration life of the carboxylic acid-based resin is about 1.5 times as 
long as that of sulfonic acid-based resin, where the filtration time is 
defined as the filtration time or the trapped crud amount at the time when 
the filtration differential pressure reaches 1.75 kg/cm.sup.2, and thus a 
better result can be obtained in the present invention. 
However, the percent crud removal decreases with filtration time, and 
reaches about 75% in the case of carboxylic acid-based resin at the time 
when the filtration differnetial pressure reaches 1.75 kg/cm.sup.2, and 
the percent crud removal of 90% or higher as generally required cannot be 
attained. It has been found that the low percent crud removal is caused by 
development of cracks in the precoat layer with increasing filtration 
time. The detail will be described below, referring to FIG. 14. 
Generally, water 42 is filtered after a precoat layer 41 having a thickness 
of 2 to 20 mm has been formed on a filtration element 21, as shown in FIG. 
14(a). The precoat layer is shrunk with increasing filtration time, and 
thus cracks 43 are developed, as shown in FIG. 14(b), lowering the percent 
crud removal. Such phenomena are known also in the case of the 
conventional sulfonic acid-based resin, and known effective means for 
preventing development of cracks 43 is addition of fibers thereto. The 
results of sulfonic acid-based resin shown in FIG. 13 are based on the 
precoat layer containing fibers. It is known that in the case of the 
conventional sulfonic acid-based resin it is appropriate to add 30 to 60% 
by weight, preferably 50% by weight, of fibers thereto. 
The present inventors carried out filtration tests in the following manner. 
After mixing carboxylic acid-based resin with anion exchange resin, 50% by 
weight of acrylic fibers (size: about 10 .mu.m, length: about 100 .mu.m) 
was added to the mixture on the total basis to prepare a precoat material. 
The results are shown in FIG. 15, together with the results obtained 
without any addition of the fibers. It is obvious from FIG. 15 that the 
percent crud removal can be considerably improved by the addition of 
fibers, and crack development in the precoat layer can be prevented. 
However, even in the proportion of fibers of 50% by weight, which is deemed 
most appropriate in the case of the sulfonic acid-based resin, percent 
crud removal of more than 90% as generally required could not always be 
obtained, as in obvious from FIG. 15. The present inventors tried to 
clarify its causes and find out a step for solving this problem. 
To clarify the causes for crack development in the precoat layer in the 
case of carboxylic acid-based resin, the present inventors at first 
investigated causes for crack development in the case of the conventional 
sulfonic acid-based resins. As a result, it was found that the crack 
development was caused by shrinkage of precoat layer 41, and the cause for 
the shrinkage of precoat layer 41 was due to the synergistic effect of the 
following two factors. That is, the first factor is that, among ion 
exchange resins, cation exchange resin particles have negatively charged 
surfaces, whereas anion exchange resin particles have positively charged 
particles, and the precoat layer composed of a mixture of these two is a 
loose layer of so called flocs by electric repulsive forces. When cruds 
are adsorbed on the precoat layer, the surface electric charges are 
offset, and the electric repulsive forces are reduced, whereby the precoat 
layer is shrunk and turns into a dense layer. 
The second factor is that the conventional sulfonic acid-based resin is a 
shrinkable resins in which the resin particles shrink when they adsorb the 
cruds, and thus the precoat layer is shrunk thereby, causing crack 
development. In the case of the conventional sulfonic acid-based resin, 
cracks develop by reduction in the electric repulsive forces by the crud 
adsorption and shrinkage of the resin itself, and fibers are added thereto 
to prevent the crack developments. 
In the case of carboxylic acid-based resin, on the other hand, the 
mechanism of crack development was found to be quite different from the 
foregoing. That is, the said first factor (reduction in the electric 
repulsive force) was quite identical with the foregoing, but the second 
factor was quite different. That is, even if the carboxylic acid-based 
resin adsorbs cruds, the resin particles are not shrunk, but rather 
expand. That is, the carboxylic acid-based resin is an expandable resin. 
Thus, the second factor acts to lessen the shrinkage of the precoat layer, 
and it has been found that the carboxylic acid-based resin has less 
precoat layer shrinkage than the sulfonic acid-based resin. The present 
inventors presumed that, in contrast to the appropriate proportion of 
fibers of 30 to 60% by weight (dry basis) in the case of sulfonic 
acid-based resin, crack development in the precoat layer could be 
prevented by a much small proportion of fibers in the case of carboxylic 
acid-based resin. The present inventors conducted filtration tests by 
changing the proportion of fibers. The results are shown in FIG. 16, where 
the percent crud removal is shown as a function of proportion of fibers 
when the filtration differential pressure reaches 1.75 kg/cm.sup.2 in the 
precoat layer. It has been found that in the case of sulfonic acid-based 
resin, cracks develop in the precoat layer in a proportion of below 30% by 
weight (dry basis), and the percent crud removal is drastically lowered, 
whereas in the case of carboxylic acid-based resin no cracks develop even 
in a smaller proportion of fibers, as presumed, and the percent crud 
removal of 90% can be obtained even in the proportion of fibers of 10% by 
weight (dry basis). On the other hand, the percent crud removal is again 
lowered even in the region of a larger proportion of fibers, as shown in 
FIG. 16. The cause for such lowering is that the proportion of ion 
exchange resin having a high filtrability is decreased with increasing 
proportion of fibers. In the case of sulfonic acid-based resin, the 
percent crud removal becomes less than 90% in the proportion of fibers 
above 60% by weight (dry basis), whereas in the case of carboxylic 
acid-based resin the percent crud removal becomes less than 90% in the 
proportion of fibers above 40% by weight (dry basis). The reasons are that 
the sulfonic acid-based resin is a strongly acidic resin and thus has a 
high ability to remove cruds, and the desired ability can be maintained 
even if the proportion of fibers is rather increased, whereas the 
carboxylic acid-based resin is a weakly acidic resin, and has a rather low 
ability to remove cruds, and thus the proportion of fibers cannot be made 
too large. 
As described above, it has been found that the carboxylic acid-based resin 
and the sulfonic acid-based resin have different optimum proportions of 
fibers from each other owing to different physical properties. That is, in 
the case of carboxylic acid-based resin, percent crud removal of 90% or 
higher can be obtained, if the proportion of fibers is in a range of 10 to 
40% by weight (dry basis), and such a range is quite preferable when 
applied to a filtration desalter. Any of acrylic fibers, nylon fibers, 
plant fibers, carbon fibers, etc. can be used as fibers for use in the 
present invention, and the fiber size can be in a range of a few .mu.m to 
a few tens .mu.m and the length can be in a range of a few tens .mu.m to a 
few mm, as in the conventional size and length. 
The optimum proportion of fibers has been clarified in the foregoing, and 
the carboxylic acid-based resin has such an essential defect that the 
ability to remove cruds is rather low, as already mentioned above. That 
is, in the case of carboxylic acid-based resin, the percent crud removal 
reaches maximum 93% when the proportion of fibers is about 20% by weight 
(dry basis), whereas in the case of sulfonic acid-based resin maximum 97% 
crud removal can be obtained, as is obvious from FIG. 16. 
To investigate the reason fully, the present inventors investigated 
distribution of cruds adsorbed through the precoat layer. The results are 
shown in FIG. 17, where the axis of ordinate shows the concentration of 
adsorbed cruds, and the axis of abscissa shows the depth of precoat layer 
41 from the surface toward the filtration element 21. It is seen therefrom 
that, since the sulfonic acid-based resin is a highly acidic resin, it has 
a high ability to remove cruds, and thus most of cruds are adsorbed near 
the surface of the precoat layer 41, whereas the carboxylic acid-based 
resin is a weakly acidic resin, cruds are adsorbed throughout the precoat 
layer, and a portion of cruds reaches even the filtration element 21. That 
is, since the carboxylic acid-based resin is weakly acidic, cruds cannot 
be completely adsorbed in the precoat layer, and a portion of the cruds 
reaches the filtration element. That is, the percent crud removal is low. 
To give such carboxylic acid-based resin an equivalent ability to remove 
cruds to that of the conventional sulfonic acid-based resin, the present 
inventors presumed that reduction in particle size of the resin would be a 
preferable step. In the conventional sulfonic acid-based resin, powdery 
resins having particle sizes of 60 to 400 meshes, that is, an average 
particle size of 50 to 150 .mu.m have been used from the viewpoint of 
precoatability, etc. In the case of carboxylic acid-based resin having a 
rather low ability to remove cruds, the present inventors presumed that an 
effective ability to remove cruds could be increased by reducing the 
particle size, thereby increasing the reactive surface area, and 
consequently a higher percent crud removal than that of the sulfonic 
acid-based resin could be obtained. Thus, the present inventors conducted 
filtration tests by changing the particle sizes of carboxylic acid-based 
resins. The results are shown in FIG. 18, where the proportion of fibers 
was always kept at 20% by weight. 
The percent crud removal shown in FIG. 18 shows values obtained when the 
filtration differential pressure reaches 1.75 kg/cm.sup.2, and the 
filtration life is defined as follows. In the conventional sulfonic 
acid-based resin, powdery resin having particle sizes of 60 to 400 meshes, 
preferably 100 to 200 meshes, that is, an average particle size of about 
80 .mu.m, as disclosed in Japanese Patent Publication No. 47-44903, are 
used. Thus, the filtration life of sulfonic acid-based resin having an 
average particle size of 80 .mu.m is presumed to be unity, and the 
filtration life of carboxylic acid-based resin is shown as a relative 
value thereto. As is obvious from FIG. 18, the smaller the average 
particle size, the larger the reactive surface area and the higher the 
percent crud removal. To obtain percent crud removal of 90% or higher, it 
has been found that it is desirable that the average particle size is not 
more than 60 .mu.m. On the other hand, the larger the average particle 
size, the longer the filtration life. The reasons are that the smaller the 
average particle size is, the more often clogging takes place in the 
precoat layer at the crud adsorption, and consequently the increase in the 
filtration differential pressure is accelerated. Thus, to make the 
filtration life equal or superior to that of the conventional sulfonic 
acid-based resin, it is desirable that the average particle size of 
carboxylic acid-based resin is 30 .mu.m or larger. That is, to obtain 
percent crud removal of 90% or higher and make the filtration life equal 
or superior to that of the conventional sulfonic acid-based resin, it is 
desirable that the average particle size of carboxylic acid-based resin is 
30 to 60 .mu.m. 
The reasons why an average particle size of 50 to 150 .mu.m (60 to 400 
meshes) is desirable in the case of the conventional sulfonic acid-based 
resin, whereas 30 to 60 .mu.m is desirable in the case of carboxylic 
acid-based resin can be summarized as follows: that is, to obtain a high 
percent crud removal, the carboxylic acid-based resin must have a larger 
reactive surface area owing to the weak acidity (whereas the sulfonic 
acid-based resin is strongly acidic), and thus it is desirable that the 
average particle size is smaller than that of the conventional sulfonic 
acid-based resin. Sulfonic acid-based resin is a shrinkable resin in which 
the resin particles shrink when they absorb cruds, and thus the filtration 
life is drastically shortened when the average particle size is less than 
50 .mu.m, whereas the carboxylic acid-based resin is an expandable resin 
to the contrary, the filtration life equal to that of the conventional 
sulfonic acid-based resin can be obtained even if the average particle 
size is about 30 .mu.m. 
In FIG. 18, the cases with the proportion of fibers of 20% by weight are 
shown, but the present inventors have found that a high filtrability can 
be obtained in proportions of fibers ranging from 10 to 40% by weight 
shown in FIG. 16 by making the average particle size 30 to 60 .mu.m. This 
will be explained, referring to FIG. 19. When the proportion of fibers 
shown on the axis of abscissa is less than 10% by weight, cracks develop, 
and the percent crud removal becomes less than 90%, whereas above 40% by 
weight the proportion of ion exchange resin is lowered, and the percent 
crud removal is lowered. When the average particle size shown on the axis 
of ordinate is more than 60 .mu.m, the reactive surface area is 
insufficient, and the percent crud removal is low, whereas below 30 .mu.m 
clogging takes place in the precoat layer, and the filtration life becomes 
short. Since the carboxylic acid-based resin is expandable and weakly 
acidic, and is different in the physical properties from the conventional 
sulfonic acid-based resin, the optimum conditions for powdery ion exchange 
resins are quite different therebetween. 
In the foregoing, carboxylic acid-based resin has been exemplified, but the 
same results can be obtained from other ion exchange resins, so long as 
they are the resins according to the present invention. One example is a 
chelate resin shown as .circle.G in Table 1. Test results obtained by 
mixing the cation exchange resin shown as .circle.G in Table 1 with 
anion exchange resin in a mixing ratio of the former to the latter of 1:1 
by weight, and adding fibers thereto are shown in FIG. 20, where percent 
crud removal at the time when the filtration differential pressure reaches 
1.75 kg/cm.sup.2 is shown for chelate cation exchange resins having 
average particle sizes of 30, 60 and 100 .mu.m when the proportion of 
plant fibers having a size of about 5 .mu.m and a length of a few tens 
.mu.m are changed in a range from 0 to 60% by weight. In this case, higher 
percent crud removal can be obtained in proportions of fibers from 10 to 
40% by weight as in FIG. 16, and percent crud removal of 90% or higher can 
be obtained when the particle size of the cation exchange resin is from 30 
to 60 .mu.m, as is obvious from FIG. 20. 
When the powdery ion exchange resin is used as a precoat material, the 
weakly acidic cation exchange resin and anion exchange resin are used in a 
mixture in a predetermined ratio (usually the ratio of the cation exchange 
resin to the anion exchange resin is in a range of 4:1 to 1:2), and 
powdery quaternary, tertiary, secondary and primary amine-based resins, 
etc. can be used as the anion exchange resin, as in the prior art. 
According to the present invention, the following effects can be obtained: 
(1) Non-radioactive metals such as .sup.59 Co, etc. are not retained in a 
nuclear reactor for a long time, and thus formation of .sup.60 Co, etc. is 
suppressed and deposition of radioactive metals onto pipings can be also 
suppressed. That is, radiation exposure of operators in an atomic power 
station can be considerably reduced. 
(2) Even if the ions or cruds are trapped from nuclear reactor cooling 
water, an increase in the filtration differential pressure or crack 
development can be suppressed, and thus the life of a filtration desalter 
can be prolonged. That is, not only the cost, but also the amount of 
discharged wastes can be reduced. 
(3) The cation exchange resins for use in the present invention have 
ion-exchanging groups of low bonding energy, and thus can be thermally 
decomposed simply. That is, the waste disposal can be readily made, and a 
considerable reduction in the waste volume can be attained without 
generation of harmful gas components at the waste disposal and the waste 
disposal facility can be simplified.