Patent Application
20180190404
The present invention relates to a granulated radioactive iodine adsorbent of zeolite X, and a method for treating radioactive iodine contained in steam discharged from a nuclear power facility.
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 iodine-containing steam generated in a nuclear power facility is passed through the filter so that radioactive iodine is 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 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 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.
Zeolite may be used as a support in a radioactive iodine 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 supported is required to achieve highly efficient removal of radioactive iodine.
Patent Document 1: Japanese Unexamined Patent Application Publication No. S54-4890
Patent Document 2: Japanese Unexamined Patent Application Publication No. S60-225638
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 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 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 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 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 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.
With the above problems in mind, the present invention 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 can be used to address extraordinary situations where filter venting or the like is required.
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 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.
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 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.
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. 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 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, 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 nuclear reactor accident can be avoided.
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.
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 efficiency. In addition, the efficiency of removal of hydrogen is improved. Therefore, an extraordinary situation in a nuclear reactor can be avoided.
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.
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.
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 facility, including: a filling step of filling an air-permeable 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.
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.
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 high-pressure steam in a nuclear reactor from a nuclear reactor building in order to control or prevent nuclear reactor 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 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.
In the method for treating radioactive iodine according to the present invention, the steam discharged from the nuclear power facility preferably contains hydrogen molecules.
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 steam can be removed. As a result, the risk of a nuclear reactor accident can be controlled.
In the method for treating radioactive iodine according to the present invention, the steam discharged from the nuclear power facility is preferably superheated steam having a temperature of 100° C. or more.
In the method for treating radioactive iodine that has this feature, even if the steam is at considerably high 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.
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.
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 hydrogen with high efficiency.
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 filled with the radioactive iodine adsorbent is preferably set to 0.06 sec or more.
In the method for treating radioactive iodine that has this feature, even when the retaining time is as short as 0.06 sec or more, the radioactive iodine adsorbent can effectively collect and remove radioactive iodine and hydrogen with high efficiency.
In the method for treating radioactive iodine according to the present invention, in the flow passing step, the steam preferably has a pressure of 399 kPa or more.
In the method for treating radioactive iodine that has this feature, even when the pressure of the steam is as high 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.
In the method for treating radioactive iodine according 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.
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 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.
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
<Radioactive Iodine Adsorbent>
Firstly, zeolite X that is used in a radioactive iodine adsorbent according to the present invention will be described.
As shown in
The radioactive iodine adsorbent of the present 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
Incidentally, when sodium sites in zeolite 13X are ion-exchanged with silver, the resultant zeolite 13X has a smaller pore diameter size than that of the original zeolite 13X. The 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
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 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 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 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 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 exhibit high hydrogen molecule adsorptivity.
<Method for Treating Radioactive Iodine>
Before describing a method for treating radioactive iodine using the radioactive iodine adsorbent formulated as described above, the structure of a typical nuclear power plant will be described with reference to
(Filling Step)
(Flow Passing Step)
While, as described above, the filter vent 15 can be used to reduce the amount of radioactive iodine, it is 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 invention is used to substantially completely remove radioactive iodine. As shown in
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 containment building 11. In contrast to this, in the second embodiment, as shown in
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). 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, 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
The first to third embodiments are all directed to 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
In Example 1, a test for adsorption of radioactive iodine using the method for treating radioactive iodine according to the present invention was conducted.
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 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 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 (CH3131I), and had a linear speed of 20 cm/sec and 41 cm/sec. The result of the measurement is shown in Table 1.
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 retaining time was as short as 0.061 sec, the methyl iodide adsorption rate was 97.989%, which is a good result.
In Example 2, the radioactive iodine adsorbent 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 of the measurement is shown in Table 2.
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.
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 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.
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 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.
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 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 (CH3131I), and the humidity was 70%. The result of the measurement is shown in Table 4.
In Example 5, the radioactive iodine adsorbent formulated in Example 1 was measured in terms of the methyl 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 (CH3131I), and the humidity was 95%. The result of the measurement is shown in Table 5.
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. Therefore, it is demonstrated that the radioactive iodine adsorbent of the present invention has a high adsorption effect even at high humidity.
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 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 (CH3131I). The result of the measurement is shown in Table 6.
As can be seen from the result of Table 6, even when the temperature was as high as 150° C., the methyl iodide adsorption rate was high.
In Example 7, the radioactive iodine adsorbent 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 was 0.25 sec, and the steam contained 1.75 mg/m3 of methyl iodide (CH3131I). The result of the measurement is shown in Table 7.
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.
In Example 8, the radioactive iodine adsorbent formulated in Example 1 was measured in terms of the methyl 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 the measurement is shown in Table 8.
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.
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 a thickness of 5 cm. The result of the measurement is shown in Table 9.
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 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 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.
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 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
As can be seen from the graph of
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 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 adsorbent of the present invention effectively adsorbs hydrogen as well as radioactive iodine with high efficiency.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-219867 | Oct 2013 | JP | national |
The present application is related to and claims priority under 35 U.S.C. § 120 to commonly owned U.S. application Ser. No. 15/029,642, filed 15 Apr. 2016, now U.S. Pat. No. ______, which in turn claims priority under 35 U.S.C. 371 to PCT/JP2014/072011, filed 22 Aug. 2014, which in turn claims priority to Japanese Application No. 2013-219867, filed 23 Oct. 2013, the entirety of each of which is hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15029642 | Apr 2016 | US |
| Child | 15860900 | US |
