TECHNICAL FIELD
[0001]
The present invention relates to a granulated
radioactive iodine adsorbent of zeolite X, and a method for
5 treating radioactive iodine contained in steam discharged from
a nuclear power facility.
BACKGROUND ART
[0002]
10 Nuclear power facilities, such as nuclear power plants
and the like, are conventionally equipped with a filter for
removing radioactive iodine. A flow of radioactive iodinecontaining
steam generated in a nuclear power facility is
passed through the filter so that radioactive iodine is
15 adsorbed and removed before being discharged from the nuclear
power facility. This process is very important, and therefore,
research and development efforts have been and are still being
undertaken in order to achieve improvements in the radioactive
iodine adsorbing effect of the filter. Among these
20 improvements is an adsorbent having high efficiency of
removing radioactive iodine even at high humidity (see, for
example, Patent Document 1). Patent Document 1 indicates that
the efficiency of removal of methyl iodide which is a
radioactive iodine compound is improved by an adsorbent
25 including alumina having a large number of pores having an
average pore diameter of 200–2000 Å, on which a metal or a
compound containing the metal is supported.
[0003]
Zeolite may be used as a support in a radioactive iodine
30 adsorbent (see, for example, Patent Document 2). Patent
Document 2 describes a radioactive iodine adsorbent in which
silver is supported on zeolite having a silica-to-alumina mole
ratio of 15 or more. Document 2 indicates that, in this
radioactive iodine adsorbent, only a small amount of silver
3
supported is required to achieve highly efficient removal of
radioactive iodine.
CITATION LIST
5 PATENT LITERATURE
[0004]
Patent Document 1: Japanese Unexamined Patent
Application Publication No. S54-4890
Patent Document2: Japanese Unexamined Patent Application
10 Publication No. S60-225638
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0005]
15 In both of the adsorbents described in Patent Documents
1 and 2, the crystal structure of zeolite, which has such a
pore size as to provide a molecular sieve effect, is utilized
to selectively adsorb radioactive iodine. The adsorbents
described in both of the documents are considered to have a
20 measure of success in adsorbing radioactive iodine. However,
there is still a demand for higher-performance radioactive
iodine adsorbents for reliably preventing leakage of
radioactive iodine into the outside.
If an extraordinary situation such as a nuclear reactor
25 accident or the like occurs in a nuclear power facility, a
large amount of radioactive materials including radioactive
iodine is released into a large area. Therefore, nuclear
reactor accidents must be prevented. To this end, a plan to
provide, to a nuclear reactor building, a filter vent for
30 reducing pressure in a nuclear reactor when an extraordinary
situation occurs in the nuclear reactor is under way. However,
the radioactive iodine adsorbents described in Patent
Documents 1 and 2 are not intended for addressing
extraordinary situations where filter venting or the like is
4
required. Therefore, further research and development efforts
are required to provide a radioactive iodine adsorbent that
can be used even when an extraordinary situation occurs, or a
process of using such a radioactive iodine adsorbent. Hydrogen
5 generated in a nuclear reactor is considered to be a factor in
nuclear reactor accidents. The reduction of such hydrogen is
not at all described in Patent Document 1 or 2.
[0006]
With the above problems in mind, the present invention
10 has been made. It is an object of the present invention to
provide a radioactive iodine adsorbent that can adsorb
radioactive iodine more effectively than in the conventional
art, and remove hydrogen that is a factor in nuclear reactor
accidents, and a method for treating radioactive iodine, which
15 can be used to address extraordinary situations where filter
venting or the like is required.
SOLUTION TO PROBLEM
[0007]
20 To achieve the above object, a radioactive iodine
adsorbent according to the present invention is characterized
in that it is a granulated radioactive iodine adsorbent of
zeolite X, wherein ion exchange sites of the zeolite X are
substituted with silver so that a size of minute pores of the
25 zeolite X is suited to a size of a hydrogen molecule, and the
radioactive iodine adsorbent has a silver content of 36 wt% or
more when dried, a particle size of 10  20 mesh, a hardness of
94% or more, and a water content of 12 wt% or less when dried
at 150C for 3 h and thereby reduced in weight.
30 [0008]
In the radioactive iodine adsorbent having this feature,
a product obtained by granulation of zeolite X is used as a
base material. There are a variety of zeolites, which have
different crystal structures. Zeolites have a characteristic
5
feature that each crystal structure has considerably uniform
pore diameters. Such a characteristic pore diameter allows
zeolite to be used for molecular sieve, selective adsorption
of molecules, and the like.
5 In the radioactive iodine adsorbent of the present
invention, zeolite X, which is a zeolite that has a relatively
large pore diameter, is used. Sodium at ion exchange sites of
zeolite X is substituted with silver. As a result, radioactive
iodine can be adsorbed in the form of silver iodide.
10 Therefore, even if an extraordinary situation such as a
nuclear reactor accident occurs, radioactive iodine can be
prevented from being released into the outside of a nuclear
reactor.
In addition, sodium of zeolite X is substituted with
15 silver so that a size of minute pores of zeolite X is suited
to a size of a hydrogen molecule, and the silver content when
dried is 36 wt% or more, the particle size is 10  20 mesh, the
hardness is 94% or more, and the water content when dried at
150C for 3 h and thereby reduced in weight is 12 wt% or less,
20 and therefore, the zeolite X after the substitution can
efficiently collect hydrogen molecules. As a result, even in a
situation where hydrogen is generated due to a nuclear reactor
accident or the like, the radioactive iodine adsorbent of the
present invention can be used to remove hydrogen so that a
25 nuclear reactor accident can be avoided.
[0009]
In the radioactive iodine adsorbent of the present
invention, 97% or more of the ion exchange sites of the
zeolite X are preferably substituted with silver.
30 [0010]
In the radioactive iodine adsorbent having this feature,
97% or more of the ion exchange sites, i.e., sodium of the
zeolite X are substituted with silver, and therefore, the
zeolite X can effectively adsorb radioactive iodine with high
6
efficiency. In addition, the efficiency of removal of hydrogen
is improved. Therefore, an extraordinary situation in a
nuclear reactor can be avoided.
[0011]
5 In the radioactive iodine adsorbent of the present
invention, the ion exchange sites of the zeolite X are
preferably not substituted with any material other than
silver.
[0012]
10 In the radioactive iodine adsorbent having this feature,
the ion exchange sites, i.e., sodium of the zeolite X are not
substituted with any material other than silver. Therefore,
the radioactive iodine adsorptivity is sustained over a long
period of time.
15 [0013]
To achieve the above object, a method for treating
radioactive iodine according to the present invention is
characterized in that it is a method for treating radioactive
iodine contained in steam discharged from a nuclear power
20 facility, including: a filling step of filling an airpermeable
container with the above radioactive iodine
adsorbent; and a flow passing step of passing a flow of the
steam discharged from the nuclear power facility, through the
container filled with the radioactive iodine adsorbent.
25 [0014]
In the method for treating radioactive iodine that has
this feature, effective radioactive iodine adsorption and
hydrogen removal with high efficiency can be achieved by
performing the above two steps.
30 The method for treating radioactive iodine may be
performed, for example, after a treatment by filter venting.
Filter venting refers to an operation of discharging highpressure
steam in a nuclear reactor from a nuclear reactor
building in order to control or prevent nuclear reactor
7
accidents or leakage and release of radioactive iodine
accompanying such accidents when an extraordinary situation
occurs in the nuclear reactor. By performing the method for
treating radioactive iodine according to the present invention
5 after filter venting, radioactive iodine and hydrogen
contained in high-pressure steam discharged by filter venting
can be adsorbed and thereby reliably removed. As a result, the
risk of release of radioactive iodine or a nuclear reactor
accident can be avoided or controlled.
10 [0015]
In the method for treating radioactive iodine according
to the present invention, the steam discharged from the
nuclear power facility preferably contains hydrogen molecules.
[0016]
15 In the method for treating radioactive iodine that has
this feature, the steam discharged from the nuclear power
facility contains hydrogen molecules. Therefore, if, in the
flow passing step, the radioactive iodine adsorbent of the
present invention is used, hydrogen molecule contained in the
20 steam can be removed. As a result, the risk of a nuclear
reactor accident can be controlled.
[0017]
In the method for treating radioactive iodine according
to the present invention, the steam discharged from the
25 nuclear power facility is preferably superheated steam having
a temperature of 100C or more.
[0018]
In the method for treating radioactive iodine that has
this feature, even if the steam is at considerably high
30 temperature as described above, then when the radioactive
iodine adsorbent of the present invention is used in the flow
passing step, hydrogen contained in the steam can be
efficiently adsorbed.
[0019]
8
In the method for treating radioactive iodine according
to the present invention, in the filling step, the filling
density of the radioactive iodine adsorbent is preferably
adjusted to 1.0 g/ml or more.
5 [0020]
In the method for treating radioactive iodine that has
this feature, the filling density of the radioactive iodine
adsorbent is adjusted to 1.0 g/ml or more, which allows for
effective adsorption and removal of radioactive iodine and
10 hydrogen with high efficiency.
[0021]
In the method for treating radioactive iodine according
to the present invention, in the flow passing step, a period
of time for which the steam is retained in the container
15 filled with the radioactive iodine adsorbent is preferably set
to 0.06 sec or more.
[0022]
In the method for treating radioactive iodine that has
this feature, even when the retaining time is as short as 0.06
20 sec or more, the radioactive iodine adsorbent can effectively
collect and remove radioactive iodine and hydrogen with high
efficiency.
[0023]
In the method for treating radioactive iodine according
25 to the present invention, in the flow passing step, the steam
preferably has a pressure of 399 kPa or more.
[0024]
In the method for treating radioactive iodine that has
this feature, even when the pressure of the steam is as high
30 as 399 kPa or more, the radioactive iodine adsorbent can
effectively adsorb and remove radioactive iodine and hydrogen
with high efficiency in the flow passing step.
[0025]
In the method for treating radioactive iodine according
9
to the present invention, in the flow passing step, the
container filled with the radioactive iodine adsorbent
preferably has a humidity of 95% or more.
[0026]
5 In the method for treating radioactive iodine that has
this feature, even when the humidity in the container filled
with the radioactive iodine adsorbent is as high as 95% or
more, the radioactive iodine adsorbent of the present
invention can be used. It is difficult in the conventional art
10 to adsorb radioactive iodine at high humidity. The method for
treating radioactive iodine that has this feature allows for
removal of hydrogen while achieving adsorption of radioactive
iodine at high humidity, resulting in high safety.
15 BRIEF DESCRIPTION OF DRAWINGS
[0027]
[FIG. 1] FIG. 1 is a diagram for describing zeolite X
used as a radioactive iodine adsorbent according to the
present invention.
20 [FIG. 2] FIG. 2 is a schematic diagram of a
configuration of nuclear reactor equipment.
[FIG. 3] FIG. 3 is a schematic diagram of a
configuration where a radioactive iodine adsorbent according
to a first embodiment is provided in a boiling water reactor.
25 [FIG. 4] FIG. 4 is a schematic diagram of a
configuration where a radioactive iodine adsorbent according
to a second embodiment is provided in a boiling water reactor.
[FIG. 5] FIG. 5 is a schematic diagram of a
configuration where a radioactive iodine adsorbent according
30 to a third embodiment is provided in a boiling water reactor.
[FIG. 6] FIG. 6 is a schematic diagram of a
configuration where a radioactive iodine adsorbent according
to a fourth embodiment is provided in a pressurized water
reactor.
10
[FIG. 7] FIG. 7 is a graph showing increases or changes
in the temperature of a radioactive iodine adsorbent.
DESCRIPTION OF EMBODIMENTS
5 [0028]
Embodiments of a radioactive iodine adsorbent according
to the present invention and embodiments of a method for
treating radioactive iodine according to the present invention
will now be described with reference to FIGS. 1–7. Note that
10 the present invention is not intended to be limited to
configurations described below.
[0029]

Firstly, zeolite X that is used in a radioactive iodine
15 adsorbent according to the present invention will be
described. FIG. 1 is a diagram for describing a zeolite
included in the radioactive iodine adsorbent of the present
invention. FIG. 1(a) is a schematic diagram of the crystal
structure of zeolites. FIG. 1(b) is a diagram for describing a
20 reaction in which sodium sites of zeolite 13X are substituted
with silver. FIG. 1(c) is a diagram for describing the
reduction of a pore diameter size as a result of substitution
of sodium sites of zeolite 13X with silver.
As shown in FIG. 1(a), zeolites, which are a type of
silicate, have (SiO4)4 and (AlO4)5 25 having a tetrahedron
structure as repeating units, which are three-dimensionally
linked together to form a crystal structure. The repeating
units are linked in different ways to form different crystal
structures. Each crystal structure formed has a specific
30 uniform pore diameter. This uniform pore diameter allows
zeolites to have properties such as molecular sieve,
adsorption, and ion exchange capability.
[0030]
The radioactive iodine adsorbent of the present
11
invention includes zeolite 13X, which is a type of zeolite X.
Zeolite 13X is widely industrially used. The composition of
zeolite 13X is Na86[(AlO2)86(SiO2)106]276H2O. As shown in FIG.
1(b), the radioactive iodine adsorbent of the present
5 invention is formulated by ion-exchanging sodium sites of
zeolite 13X with silver (the sodium sites are ion exchange
sites). The silver ion exchange ratio in the radioactive
iodine adsorbent is 97% or more, preferably 98% or more.
Moreover, it is preferable that the ion exchange sites in
10 zeolite X be not ion-exchanged with any material other than
silver. In other words, in the radioactive iodine adsorbent of
the present invention, substantially all sodium sites in
zeolite 13X are ion-exchanged with silver. Such a high ion
exchange ratio allows the radioactive iodine adsorbent of the
15 present invention to have considerably higher adsorptivity
than that of conventional radioactive iodine adsorbents.
Incidentally, when sodium sites in zeolite 13X are ionexchanged
with silver, the resultant zeolite 13X has a smaller
pore diameter size than that of the original zeolite 13X. The
20 present inventors have extensively studied to find that
zeolite 13X that is adapted to have a smaller pore diameter
size is effective in adsorbing hydrogen, and have conceived of
using such zeolite 13X as a radioactive iodine adsorbent.
Specifically, as shown in FIG. 1(c), the pore diameter (about
25 0.4 nm) of zeolite 13X that has sodium sites before being ionexchanged
with silver is too great to hold a hydrogen molecule
(molecule diameter: about 0.29 nm). Meanwhile, the present
inventors found that zeolite 13X, sodium sites of which have
been ion-exchanged with silver, has an optimum pore diameter
30 (about 0.29 nm) so that a hydrogen molecule fits into the
pore, and as a result, zeolite 13X having silver obtained by
ion exchange can effectively adsorb, with high efficiency, not
only radioactive iodine but also hydrogen molecules.
[0031]
12
The radioactive iodine adsorbent of the present
invention is preferably formulated so that, in addition to the
feature that the above ion exchange ratio is achieved, the
proportion of the silver component (silver content) in the
5 adsorbent when dried is 36 wt% or more, the particle size is
10  20 mesh (JIS K 1474-4-6), the hardness is 94% or more (JIS
K 1474-4-7), and the water content of the adsorbent when dried
at 150C for 3 h and thereby reduced in weight is 12 wt% or
less. As used herein, the term “10  20 mesh” with respect to
10 the size of a particle means that the particle can pass
through a 10-mesh sieve, but not through a 20-mesh sieve,
i.e., that the particle size is 10–20 mesh. If a radioactive
iodine adsorbent is formulated to have such properties, the
radioactive iodine adsorbent can more effectively exhibit the
15 above high hydrogen molecule adsorptivity. Radioactive iodine
adsorbents are exposed to a severe environment (high
temperature, high pressure, high humidity), and therefore, are
required to have a certain high particle strength. With this
in mind, the radioactive iodine adsorbent of the present
20 invention is preferably adapted to have a loss on attrition of
3% or less (ASTM D-4058). As a result, even if the radioactive
iodine adsorbent is placed under severe conditions such as
filter venting or the like, the radioactive iodine adsorbent
can maintain its particle shape, and therefore, continue to
25 exhibit high hydrogen molecule adsorptivity.
[0032]

Before describing a method for treating radioactive
iodine using the radioactive iodine adsorbent formulated as
30 described above, the structure of a typical nuclear power
plant will be described with reference to FIG. 2. FIG. 2 is a
schematic diagram of the configuration of nuclear reactor
equipment. FIG. 2(a) is a schematic diagram of the
configuration of a boiling water reactor (BWR) 100. FIG. 2(b)
13
is a schematic diagram of the configuration of a pressurized
water reactor (PWR) 200. In Japan, two types of nuclear
reactors, i.e., boiling water reactors (BWR) and pressurized
water reactors (PWR), are used as nuclear reactor facilities.
5 Nuclear reactor equipment mainly includes a nuclear reactor
building, a nuclear reactor containment building, a nuclear
reactor pressure vessel, a turbine, and a generator. As shown
in FIG. 2(a), the boiling water reactor 100 includes a nuclear
reactor building 10, a nuclear reactor containment building
10 11, a nuclear reactor pressure vessel 12, a turbine 13, and a
generator 14. In the boiling water reactor 100, water is
boiled in the nuclear reactor pressure vessel 12, steam thus
produced is sent to the turbine 13 as indicated by a solid
line arrow shown in FIG. 2, and the nuclear reactor water is
15 recirculated as indicated by a dashed line arrow. The steam
directly drives the turbine 13 to rotate, which causes the
generator 14 to produce electricity. Meanwhile, as shown in
FIG. 2(b), in the pressurized water reactor 200, a nuclear
reactor containment building 20 includes a nuclear reactor
20 pressure vessel 21, a pressurizer 22, and a steam generator
23. The pressurizer 22 is used to control water in the nuclear
reactor containment building 20 so that the water is always
maintained at high pressure and thereby prevented from boiling
even at high temperature. The steam generator 23 is used to
25 generate steam (indicated by a solid line arrow in FIG. 2(b))
from water separated from water flowing inside the nuclear
reactor (indicated by a dashed line arrow in FIG. 2(b)). This
steam drives a turbine 24 to rotate, which causes a generator
25 to produce electricity. In a first embodiment below, a
30 method for treating radioactive iodine using a radioactive
iodine adsorbent will be described in relation to the boiling
water reactor of FIG. 2(a).
[0033]
[First Embodiment]
14
(Filling Step)
FIG. 3 is a schematic diagram of a configuration where a
radioactive iodine treatment unit 1 that contains a
radioactive iodine adsorbent K according to a first embodiment
5 of the present invention is provided in the boiling water
reactor 100. In the first embodiment, a method for treating
radioactive iodine that is provided, assuming that an
extraordinary situation occurs due to an accident or the like
in a nuclear reactor, will be described. A filter vent 15 is
10 provided outside the nuclear reactor building 10 in case the
nuclear reactor containment building 11 is damaged due to an
accident occurring in the nuclear reactor. The filter vent 15
is a piece of equipment which is provided so that when, for
example, the nuclear reactor containment building 11 is
15 damaged due to an accident, steam is sent from the nuclear
reactor containment building 11 to the filter vent 15 through
a pipe 16 as indicated by a solid line arrow in FIG. 3 in
order to reduce the internal pressure, and the filter vent 15
collects radioactive iodine in the steam to reduce the amount
20 of radioactive iodine before discharging the steam into the
outside of the nuclear reactor building 10. As shown in FIG.
3, the radioactive iodine treatment unit 1, which includes a
container 2 for containing the radioactive iodine adsorbent K,
is disposed and coupled to the filter vent 15. As described
25 below, the container 2 is preferably formed of a heatresistant
and corrosion-resistant material because steam or
gas that has passed through the nuclear reactor containment
building 11 or the filter vent 15 flows therethrough. Such a
material for the container 2 is, for example, stainless steel,
30 and may be an aluminum alloy or the like. The container 2
needs to have air permeability so that steam or gas is allowed
to flow through the radioactive iodine adsorbent K. To this
end, the container 2 is provided with a plurality of minute
pores. The container 2 is filled with the radioactive iodine
15
adsorbent K, where the filling density is adjusted to 1.0 g/ml
or more, preferably 1.2 g/ml or more (filling step). Such a
filling density allows the radioactive iodine adsorbent K to
exhibit an optimum adsorption effect. It is desirable for
5 workers to work as easily and quickly as possible in a nuclear
reactor facility in order to pay a maximum level of attention
to the safety. In this regard, the radioactive iodine
treatment unit 1 has a simple configuration as described
above, and therefore, when the adsorption effect of the
10 radioactive iodine adsorbent K has become weak, the
radioactive iodine adsorbent K is only removed from the
container 2 and replaced with a new radioactive iodine
adsorbent K, i.e., only a simple work is required. Therefore,
a load on workers can be reduced, so that the safety of
15 workers can be ensured.
[0034]
(Flow Passing Step)
While, as described above, the filter vent 15 can be
used to reduce the amount of radioactive iodine, it is
20 necessary to substantially completely remove radioactive
iodine before discharging the steam or gas from the nuclear
reactor building 10 because radioactive iodine has serious
deleterious effects on human bodies and environments.
Therefore, the radioactive iodine adsorbent K of the present
25 invention is used to substantially completely remove
radioactive iodine. As shown in FIG. 3, the steam treated by
the filter vent 15 is sent to the radioactive iodine treatment
unit 1 through the pipe 16 as indicated by the solid line
arrow of FIG. 3. Thereafter, a flow of the steam passes
30 through the radioactive iodine adsorbent K with which the
container 2 of the radioactive iodine treatment unit 1 is
filled (flow passing step). As described above, the
radioactive iodine adsorbent K has a pore diameter suited to a
hydrogen molecule, and is contained in the container 2 having
16
air permeability, and therefore, effectively removes hydrogen
contained in the steam passing through the radioactive iodine
treatment unit 1. Thereafter, the steam after the adsorption
of radioactive iodine and the removal of hydrogen is
5 discharged from the nuclear reactor facility through a
discharge pipe. Here, the steam flowing in the radioactive
iodine treatment unit 1 is superheated steam having a
temperature of 100C or more and a pressure of 399 kPa or more.
In addition, the humidity in the container 2 is 95% or more.
10 Under such a severe condition, the radioactive iodine
adsorbent K can remove radioactive iodine and hydrogen.
Moreover, in the method for treating radioactive iodine
according to the present invention, a retaining time for which
the steam flowing in the radioactive iodine treatment unit 1
15 is retained in the container 2 is set to 0.06 sec or more. If
the nuclear reactor containment building 11 is damaged, it is
necessary to take measures against this as quickly as possible
in order to avoid or control leakage or release of radioactive
iodine or nuclear reactor accidents. To this end, the
20 treatment by the filter vent 15 and the treatment of
radioactive iodine need to be completed as quickly as
possible. Here, in the present invention, as described above,
the steam retaining time in the container 2 is considerably
short, and therefore, in emergency situations, the adsorption
25 of radioactive iodine and the removal of hydrogen can be
completed more quickly than when conventional radioactive
iodine treatment methods are used. Thus, the present invention
provides a method considerably effective in ensuring the
safety.
30 [0035]
[Second Embodiment]
In the first embodiment, the boiling water reactor 100
is provided with the radioactive iodine treatment unit 1 that
is not disposed directly adjacent to the nuclear reactor
17
containment building 11. In contrast to this, in the second
embodiment, as shown in FIG. 4, the radioactive iodine
treatment unit 1 is disposed between the filter vent 15 and
the nuclear reactor containment building 11. In this case, as
5 indicated by a solid line arrow in FIG. 4, steam discharged
from the nuclear reactor containment building 11 is sent to
the radioactive iodine treatment unit 1 through the pipe 16.
In other words, radioactive iodine and hydrogen are adsorbed
by the radioactive iodine treatment unit 1 before being
10 treated by the filter vent 15. The radioactive iodine
adsorbent K of the present invention can effectively adsorb
and remove radioactive iodine and hydrogen even when severe
steam flows in the container 2, such as superheated steam
having a temperature of 100C or more, or the like. Therefore,
15 steam discharged from the nuclear reactor containment building
11 can be sent directly to the radioactive iodine treatment
unit 1, which can effectively treat the steam. Thus,
radioactive iodine is adsorbed and hydrogen is removed by the
radioactive iodine treatment unit 1 before the steam is sent
20 to the filter vent 15, and therefore, a load on the following
filter vent 15 can be reduced, and the treatment can be
smoothly performed by the filter vent 15. The cost of building
the filter vent 15 is high, and the heavy use of the filter
vent 15 may accelerate deterioration. Therefore, if the method
25 for treating radioactive iodine according to the present
invention is performed in advance (upstream) of the filter
vent 15, the lifespan of the filter vent 15 can be extended so
that the filter vent 15 can continue to work over a long
period of time.
30 [0036]
[Third Embodiment]
In the first and second embodiments, it is assumed that
an emergency situation occurs due to an accident or the like
in a nuclear reactor facility (the boiling water reactor 100).
18
The radioactive iodine adsorbent K and the method for treating
radioactive iodine according to the present invention can be
used in not only emergency situations but also other
situations. In particular, in the boiling water reactor 100,
5 steam is sent directly from the nuclear reactor pressure
vessel 12 to the turbine 13 as described above, and therefore,
it is necessary to strictly manage the amounts of radioactive
iodine and hydrogen in order to ensure the safety. To this
end, as shown in FIG. 5, the radioactive iodine treatment unit
10 1 may be provided between the nuclear reactor pressure vessel
12 and the turbine 13 so that radioactive iodine can be
adsorbed and hydrogen can be removed by the radioactive iodine
adsorbent K before the steam being sent to the turbine 13. By
such an arrangement, the turbine 13 can be driven to rotate
15 using safe steam, and therefore, risks that are caused by
radioactive iodine and hydrogen can be avoided.
[0037]
[Fourth Embodiment]
The first to third embodiments are all directed to
20 boiling water reactors. The radioactive iodine adsorbent K and
the method for treating radioactive iodine according to the
present invention are also applicable to pressurized water
reactors (PWR). As shown in FIG. 2(b), in the pressurized
water reactor 200, water that contains radioactive materials
25 is not directly sent to the turbine 24. Thus, pressurized
water reactors are safer and easier to maintain than boiling
water reactors. However, nuclear reactors are a piece of
equipment that handles nuclear fuel, that is a considerably
harmful material, and therefore, strict risk management is
30 required. To this end, in pressurized water reactors, the
radioactive iodine adsorbent K can be used to address
emergency situations. When the radioactive iodine adsorbent K
is applied to the pressurized water reactor 200, the
radioactive iodine treatment unit 1 may be provided at some
19
point in the pathway of steam sent from the steam generator 23
to the turbine 24 as shown in FIG. 6, for example. As in
boiling water reactors, in order to take measures when a
nuclear reactor is damaged due to an accident or the like, the
5 radioactive iodine treatment unit 1 may be provided adjacent
only to a filter vent, or between a nuclear reactor
containment building and a filter vent (not shown).
Examples
[0038]
10 [Example 1]
In Example 1, a test for adsorption of radioactive
iodine using the method for treating radioactive iodine
according to the present invention was conducted.
[0039]
15 Initially, 97% of the sodium sites of zeolite 13X were
ion-exchanged with silver, followed by granulation, so that
the silver component was 36 wt%, the particle size was 10  20
mesh (JIS K 1474-4-6), and the water content when dried at
150C for 3 h was 12 wt%. An air-permeable container was
20 filled with the resultant zeolite 13X, where the filling
density was 1.0 g/ml. Thus, a radioactive iodine adsorbent was
formulated. The radioactive iodine adsorbent thus formulated
had a hardness of 94% (JIS K 1474-4-7). Next, various
radioactive iodine adsorbents having different thicknesses
25 were measured in terms of the steam retaining time in the
container and the methyl iodide adsorption effect, with
respect to steam, where the steam had a humidity of 95%, a
temperature of 130C, and a pressure of 399 kPa, and contained
1.75 mg/m3 of methyl iodide (CH3
131I), and had a linear speed of
30 20 cm/sec and 41 cm/sec. The result of the measurement is
shown in Table 1.
20
[0040]
[Table 1]
20 cm/sec 41 cm/sec
Thickness
(cm)
Retaining time
(sec)
Adsorption rate
(%)
Thickness
(cm)
Retaining time
(sec)
Adsorption rate
(%)
2.5 0.123 99.032 2.5 0.061 97.989
5.0 0.246 99.967 5.0 0.123 99.673
7.5 0.369 > 99.999 7.5 0.184 99.843
10.0 0.492 > 99.999 10.0 0.246 99.974
21
[0041]
As can be seen from the result of Table 1, even when the
linear speed was set to 41 cm/sec, the methyl iodide
adsorption rate was high. In particular, even when the
5 retaining time was as short as 0.061 sec, the methyl iodide
adsorption rate was 97.989%, which is a good result.
[0042]
[Example 2]
In Example 2, the radioactive iodine adsorbent
10 formulated in Example 1, that had a thickness of 5.0 cm, was
measured in terms of the methyl iodide adsorption effect at
various temperatures, with respect to steam, where the steam
had a pressure of 101 kPa and contained 17 mg/m3 of methyl
iodide (CH3I), and had a linear speed of 46 cm/sec. The result
15 of the measurement is shown in Table 2.
[0043]
[Table 2]
Temperature (C) 110 125 150
Adsorption rate (%) 99.90 99.95 99.95
[0044]
20 As can be seen from the result of Table 2, even when the
steam had a temperature of as high as 100C or more, the methyl
iodide adsorption rate was as high as 99% or more.
[0045]
[Example 3]
25 In Example 3, a filter (100 cm  83 cm) formed of the
radioactive iodine adsorbent formulated in Example 1, that had
a radioactive iodine adsorbent thickness of 26 mm and a mass
of 26 kg, was measured in terms of the methyl iodide
adsorption effect at various temperatures, with respect to
30 steam, where the steam had a pressure of 101 kPa and contained
0.608 mg/m3 of methyl iodide (CH3I), and had a linear speed of
20 cm/sec. The result of the measurement is shown in Table 3.
22
[0046]
[Table 3]
Temperature (C) 30 40 70 150
Adsorption rate (%) 99.77 99.85 99.91 99.98
[0047]
5 In Example 3, unlike Examples 1 and 2, the radioactive
iodine adsorbent was measured in terms of the adsorption rate,
where the radioactive iodine adsorbent had a size and a mass
similar to those in actual use. As can be seen from the result
of Table 3, in such a case, even when the temperature of the
10 steam increased to as high as 150C, the methyl iodide
adsorption rate was still high. Therefore, it was demonstrated
that the radioactive iodine adsorbent of the present invention
has practical utility.
[0048]
15 [Example 4]
In Example 4, the radioactive iodine adsorbent
formulated in Example 1 was measured in terms of the methyl
iodide adsorption effect for various radioactive iodine
adsorbent thicknesses, with respect to steam, where the steam
20 had a pressure of 103 kPa, a temperature of 66C, and a linear
speed of 20.3 cm/sec, and contained 1.75 mg/m3 of methyl iodide
(CH3
131I), and the humidity was 70%. The result of the
measurement is shown in Table 4.
[0049]
25 [Table 4]
Thickness (mm) Retaining time (sec) Adsorption rate (%)
50.8 0.250 > 99.999
76.2 0.375 > 99.999
101.6 0.500 > 99.999
23
[0050]
[Example 5]
In Example 5, the radioactive iodine adsorbent
formulated in Example 1 was measured in terms of the methyl
5 iodide adsorption effect related to the thickness and
temperature of the radioactive iodine adsorbent, with respect
to steam, where the steam had a pressure of 103 kPa and a
linear speed of 20.3 cm/sec, and contained 1.75 mg/m3 of methyl
iodide (CH3
131I), and the humidity was 95%. The result of the
10 measurement is shown in Table 5.
[0051]
[Table 5]
Thickness Adsorption rate (%)
(mm)
Retaining
time (sec) 30C 60C 90C
50.8 0.250 98.738 99.685 99.970
76.2 0.375 99.850 99.950 99.983
101.6 0.500 99.960 99.987 99.995
[0052]
15 As can be seen from the result of Table 4, when the
humidity was 70%, the methyl iodide adsorption rate was as
high as 99.999% or more in all the cases. Meanwhile, the
result of Table 5 shows that even when the humidity was as
high as 95%, the methyl iodide adsorption rate was still high.
20 Therefore, it is demonstrated that the radioactive iodine
adsorbent of the present invention has a high adsorption
effect even at high humidity.
[0053]
[Example 6]
25 In Example 6, the radioactive iodine adsorbent
formulated in Example 1 was measured in terms of the methyl
iodide adsorption effect at various temperatures in a dried
state, with respect to steam, where the steam had a pressure
of 101 kPa and a linear speed of 20 cm/sec, the radioactive
24
iodine adsorbent had a thickness of 5.0 cm, the retaining time
was 0.25 sec, and the steam contained 1.75 mg/m3 of methyl
iodide (CH3
131I). The result of the measurement is shown in
Table 6.
5 [0054]
[Table 6]
Temperature (C) 30 80 150
Adsorption rate (%) 99.9 99.9 99.9
[0055]
As can be seen from the result of Table 6, even when the
10 temperature was as high as 150C, the methyl iodide adsorption
rate was high.
[0056]
[Example 7]
In Example 7, the radioactive iodine adsorbent
15 formulated in Example 1 was measured in terms of the methyl
iodide adsorption effect at various humidity at a temperature
of 80C, with respect to steam, where the steam had a pressure
of 101 kPa and a linear speed of 20 cm/sec, the radioactive
iodine adsorbent had a thickness of 5.0 cm, the retaining time
20 was 0.25 sec, and the steam contained 1.75 mg/m3 of methyl
iodide (CH3
131I). The result of the measurement is shown in
Table 7.
[0057]
[Table 7]
Humidity (%) 0 70 90
Adsorption rate (%) 99.9 99.9 99.9
25
[0058]
As can be seen from the result of Table 7, even when the
humidity was as high as 90% at a temperature of 80C, the
methyl iodide adsorption rate was high.
25
[0059]
[Example 8]
In Example 8, the radioactive iodine adsorbent
formulated in Example 1 was measured in terms of the methyl
5 iodide adsorption effect related to the thickness and
temperature of the radioactive iodine adsorbent in a dried
state under atmospheric pressure, with respect to steam, where
the steam had a pressure of 104 kPa and a linear speed of 20
cm/sec, and contained 75 mg/m3 of iodine (131I). The result of
10 the measurement is shown in Table 8.
[0060]
[Table 8]
Thickness (cm) Retaining time (sec) Adsorption rate (%)
2.5 0.123 100
5.0 0.246 100
10.0 0.492 100
[0061]
15 As can be seen from the result of Table 8, the iodine
adsorption rate was 100% in a dried state under atmospheric
pressure in all the cases. It was demonstrated that the
radioactive iodine adsorbent has considerably high
performance.
20 [0062]
[Example 9]
In Example 9, the radioactive iodine adsorbent
formulated in Example 1 was measured in terms of the hydrogen
adsorption effect, where the radioactive iodine adsorbent had
25 a thickness of 5 cm. The result of the measurement is shown in
Table 9.
26
[0063]
[Table 9]
Flow rate of
humidified
air (ml/min)
Flow rate
of hydrogen
(ml/min)
Retaining
time
(sec)
Hydrogen
content before
passage of flow
(%)
Temperature
(C)
Increase in
temperature
(C)
Hydrogen content
after passage of
flow
(%)
54 - 1.2
2200 45 2.62 2.0 62 - 0.8
79 - 0.5
77 - 1.0
2200 68 2.60 3.0 117 2 < 0.5
137 15 < 0.5
75 1 > 1.5
6600 205 0.87 3.0 120 15 < 0.5
136 17 < 0.5
27
[0064]
As can be seen from the result of Table 9, when the
temperature is as high as 100C or more, the hydrogen content
after a flow of the steam was passed through the radioactive
5 iodine adsorbent was 0.5% or less, i.e., about 83% or more of
hydrogen were adsorbed. It was also observed that when the
temperature was 137C or 136C, the temperature increased by
15C and 17C, respectively. Even in this case, the hydrogen
content after a flow of the steam was passed through the
10 radioactive iodine adsorbent was 0.5% or less. This
demonstrates that the radioactive iodine adsorbent of the
present invention is a high-performance adsorbent that can
adsorb hydrogen in a stable manner even at high temperature.
[0065]
15 [Example 10]
In Example 10, the radioactive iodine adsorbent
formulated in Example 1 was measured in terms of an increase
in temperature, with respect to steam having a hydrogen
content of 3%, where a flow of the steam was passed through
20 the radioactive iodine adsorbent having a temperature of 136C,
and the hydrogen content after a flow of the steam was passed
through the radioactive iodine adsorbent was 0.5% or less. The
result of the measurement is indicated by a graph shown in
FIG. 7.
25 [0066]
As can be seen from the graph of FIG. 7, the temperature
of the radioactive iodine adsorbent did not significantly
increase, and the radioactive iodine adsorbent adsorbed
hydrogen in a stable manner.
30 [0067]
The results of Examples 1–8 demonstrate that the
radioactive iodine adsorbent of the present invention and the
method for treating radioactive iodine using the same have a
28
considerably excellent adsorption effect on steam having high
temperature, high pressure and high humidity, and in addition,
a quick and efficient adsorption effect. Moreover, the results
of Examples 9 and 10 demonstrate that the radioactive iodine
5 adsorbent of the present invention effectively adsorbs
hydrogen as well as radioactive iodine with high efficiency.
INDUSTRIAL APPLICABILITY
[0068]
10 The radioactive iodine adsorbent and the method for
treating radioactive iodine according to the present invention
are effective in avoiding or controlling the risks of leakage
and release of radioactive iodine, nuclear reactor accidents,
and the like in a nuclear reactor facility.
15
REFERENCE SIGNS LIST
[0069]
1 RADIOACTIVE IODINE TREATMENT UNIT
2 CONTAINER
20 10 NUCLEAR REACTOR BUILDING
11, 20 NUCLEAR REACTOR CONTAINMENT BUILDING
12, 21 NUCLEAR REACTOR PRESSURE VESSEL
100 BOILING WATER REACTOR
200 PRESSURIZED WATER REACTOR
25 K RADIOACTIVE IODINE ADSORBENT
29
We Claim:
1. A granulated radioactive iodine adsorbent of
zeolite X, wherein
5 ion exchange sites of the zeolite X are substituted with
silver so that a size of minute pores of the zeolite X is
suited to a size of a hydrogen molecule, and
the radioactive iodine adsorbent has a silver content of
36 wt% or more when dried, a particle size of 10  20 mesh, a
10 hardness of 94% or more, and a water content of 12 wt% or less
when dried at 150C for 3 h and thereby reduced in weight.
2. The radioactive iodine adsorbent of claim 1,
wherein
15 97% or more of the ion exchange sites of the zeolite X
are substituted with silver.
3. The radioactive iodine adsorbent of claim 1 or 2,
wherein
20 the ion exchange sites of the zeolite X are not
substituted with any material other than silver.
4. A method for treating radioactive iodine
contained in steam discharged from a nuclear power facility,
25 comprising:
a filling step of filling an air-permeable container
with the radioactive iodine adsorbent of any one of claims 1-
3; and
a flow passing step of passing a flow of the steam
30 discharged from the nuclear power facility, through the
container filled with the radioactive iodine adsorbent.
5. The method of claim 4, wherein
the steam discharged from the nuclear power facility
30
contains hydrogen molecules.
6. The method of claim 4 or 5, wherein
the steam discharged from the nuclear power facility is
5 superheated steam having a temperature of 100C or more.
7. The method of any one of claims 4–6, wherein
in the filling step, the filling density of the
radioactive iodine adsorbent is adjusted to 1.0 g/ml or more.
10
8. The method of any one of claims 4–7, wherein
in the flow passing step, a period of time for which the
steam is retained in the container filled with the radioactive
iodine adsorbent is set to 0.06 sec or more.
15
9. The method of any one of claims 4–8, wherein
in the flow passing step, the steam has a pressure of
399 kPa or more.
20 10. The method of any one of claims 4–9, wherein
in the flow passing step, the container filled with the
radioactive iodine adsorbent has a humidity of 95% or more.