JP2013160501A - Soil decontamination method - Google Patents

Abstract

The present invention provides a soil decontamination method capable of removing the surface layer of soil with a constant thickness.
In a soil decontamination method for decontaminating soil 2 containing a radioactive substance 1, a lawn growing process for growing turf 10 on soil 2, and topsoil 4 with turf 10 attached to root 14 of turf 10; And a lawn removing process that peels off integrally. In the lawn removing process, the peeled lawn 10 is wound in a roll shape with the root 14 facing inward. It is preferable to further include a classification step of sieving coarse soil particles (surface soil 4b) from the surface soil 4 attached to the roots 14 after the lawn removing step.
[Selection] Figure 2

Description

  The present invention relates to a soil decontamination method for decontaminating soil containing a radioactive substance.

  As a method for decontaminating radioactive substances that have come and fallen on the soil, there is a method for removing the surface layer of the soil at a predetermined depth. According to a survey conducted by the Ministry of Agriculture, Forestry and Fisheries in Iidate Village, Fukushima Prefecture, in uncultivated farmland, 95% of the fallen radioactive cesium (radioactive material) exists in the soil from the surface to a depth of 2.5 cm. It became clear that. Therefore, in viscous soil such as farmland, decontamination of radioactive material is almost completed by removing the surface layer of 5 cm of soil.

  In order to remove the soil, the topsoil is shredded shallowly using an agricultural machine such as vertical harrow, and then attached to a tractor and scraped off (conventional technology 1), or solidified with a solidifying agent applied to the soil, and then the surface layer. There are methods such as scraping off soil (conventional technology 2), and scraping and accumulating soil surface using a construction machine for leveling such as a grader (conventional technology 3).

  Although many decontamination methods for various facilities such as piping of nuclear power plants have been invented for a long time and there are many prior art documents, there is no prior art document on a method for decontaminating soil.

Ministry of Agriculture, Forestry and Fisheries Technical Conference, “Radioactive material removal technology (decontamination technology) on farmland soil”, [online], September 14, 2011, Ministry of Agriculture, Forestry and Fisheries, [Search November 1, 1999], Internet <URL: http: //www.s.affrc.go.jp/docs/press/110914.htm>

  In the prior art 1 and the prior art 3 described above, the shaved soil spills down, so that even if the surface soil is shaved, radioactive materials may remain on the exposed soil surface. Moreover, when the surface of the soil is not flat and has irregularities, there is a problem that it is difficult to scrape the soil with a certain thickness. On the other hand, when the soil is hardened with a solidifying agent as in the prior art 2, there is a problem that subsequent classification treatment becomes difficult and volume of the contaminated soil cannot be reduced.

  From such a viewpoint, an object of the present invention is to provide a soil decontamination method capable of removing the surface layer of soil contaminated with radioactive substances at a constant thickness.

  The invention according to claim 1 for solving such a problem, in a soil decontamination method for decontaminating soil containing radioactive substances, a lawn growing process for growing grass on the soil, and the lawn, A soil decontamination method comprising: a lawn removal step of stripping together with topsoil attached to the grass roots.

  According to such a method, even if there is unevenness in the soil, the unevenness of the soil is flattened in the lawn growing process, and the soil for the depth of the root is removed by peeling the turf together with the topsoil attached to the roots. Can be removed at a certain depth.

  The invention according to claim 2 is characterized in that, in the turf removing step, the peeled turf is wound up in a roll shape with the root facing inward. According to such a method, since the topsoil of the soil containing a radioactive substance comes to be wound inside, it is difficult for the topsoil to spill out.

  The invention according to claim 3 further includes a classification step of sieving coarse soil particles from the top soil adhered to the roots after the turf removing step. Of the topsoil that has been stripped together with the grass, silt and clay are attached to the grass roots, but radioactive substances such as radioactive cesium are easily adsorbed by the silt and clay, and coarse particles such as sand and gravel. Since the soil has a property that it is difficult to be adsorbed, it is possible to reduce the volume of contaminated soil by sifting away coarse particles from the top soil that has been stripped together with the turf.

  According to the soil decontamination method according to claim 1, it is possible to remove the surface layer of the soil containing the radioactive substance with a constant thickness. Moreover, according to the invention which concerns on Claim 2, it becomes possible to convey, without spilling topsoil from the stripped turf. Furthermore, according to the invention which concerns on Claim 3, the volume reduction of the contaminated soil which must be discarded can be aimed at.

(A)-(c) is sectional drawing which showed each process of the soil decontamination method concerning embodiment of this invention. (A), (b) is sectional drawing which showed each process of the soil decontamination method concerning embodiment of this invention. It is the figure which showed the radiation dose of the turf surface of each division of a measurement surface. It is the figure which showed the radiation dose of the soil after turf removal of each division of a measurement surface. It is the figure which showed the weight and radiation dose of the grass which peeled off each division of a measurement surface. It is the figure which showed the weight and radiation dose of the soil which fell off from the grass which peeled off each division of a measurement surface. It is the figure which showed the weight and radiation dose of the grass root of each division of a measurement surface, the soil attached to it. It is the graph which showed the weight ratio and radiation dose ratio of a grass part, the root, the soil attached to the root, and the soil which fell off.

  Hereinafter, a soil decontamination method according to an embodiment of the present invention will be described with reference to the accompanying drawings. As shown in FIG. 1 (a), in non-plowed farmland, school playgrounds, etc., 95% of the total radioactive material 1 exists in the range from the surface of the soil 2 to a depth of 2.5 cm. ing. It is considered that the radioactive substance 1 flying from a nuclear power plant or the like first accumulates on the surface of the soil 2 and then penetrates into the soil 2 together with water such as rainwater. The radioactive substance 1 is adsorbed on the topsoil 4 containing fine soil particles such as clay and silt in the surface layer 3 of the soil 2, but the topsoil 4 may dry out and scatter as dust, thus preventing early scattering. It is necessary to take measures and decontaminate.

  The soil decontamination method according to the present embodiment includes a lawn growing process, a lawn removing process, and a classification process.

(Lawn growing process)
The lawn growing process is a process of growing the lawn 10 on the soil 2 containing the radioactive substance 1. As shown in FIG. 1 (b-1), the lawn 10 grows by seeding seeds 11 and stalks 12 on the surface of the soil 2. The seed 11 and the cheek stalk are mixed with a paste material such as fiber together with nutrients and sprayed on the ground. When growing the turf 10, fertilizer and water are efficiently given and managed so that it grows early. The surface layer 3 may be crushed shallowly (4-5 cm) with vertical halo, and then the seeds 11 and the cheek stems 12 of the turf 10 may be sown. This is preferable because the survival of the turf 10 is promoted. In addition, in this embodiment, the surface layer 3 shows the range peeled off with the turf 10.

  About one month has passed after the start of the growth of the turf 10, and when the sprouting and roots 14 are attached, trimming is performed to spread the turf 10 sideways. At that time, the grass 10 is trimmed while rolling. In this way, unevenness of the surface of the soil 2 can be corrected. In addition, by trimming the stubble, weeds, and the like remaining in the soil 2 with the turf 10, it is possible to easily perform the subsequent stripping operation. When the grass 10 spreads sideways by cutting, the mushing stock and weed stock remaining in the soil 2 are connected, and the surface layer 3 can be easily peeled off at a constant thickness compared to when there is no grass 10. Will be able to. The root 14 of the turf 10 is said to enter a maximum depth of about 20 cm in half a year after sowing in viscous soil, but about 80% of the entire root system is distributed from the ground surface to 10 cm, and the soil particles are It attaches to the surface of the root 14 to form a rhizosphere, and efficiently absorbs nutrients and moisture.

  Therefore, the grass 10 grows until the roots 14 extend to a depth of 5 to 10 cm in the soil 2 as shown in FIG. When the seed 11 or the cheek stalk 12 is sown in the soil 2, it takes a period of 2 to 3 months for the root 14 to grow to a depth of 5 to 10 cm in the soil 2. In FIGS. 1 and 2, the root 14 is schematically shown. However, since the turf 10 grows over 10,000 stocks per square meter, the root 14 spreads in the left and right directions and involves the topsoil 4. It spreads densely.

  In addition, the installation method of the turf 10 is not limited to sowing the seed 11 or the cheek stalk 12 as described above. As shown in (b-2) of FIG. 1, the cut grass 13 cut into a mat shape may be laid on the soil 2. Depending on the weather and soil conditions of the place where the contaminated soil is to be decontaminated, a method of installing turf (whether to use a sowing lawn of seed 11 or stalk 12 or to use upholstered lawn of cut lawn 13) is selected. If the cut grass 13 is used, the grass 10 can be grown in a short time. As the method of laying the cut grass 13, solid tension (arranged without gaps), joint tension (with a gap between the cut grasses to provide a joint), checkered tension (stretched in a checkered pattern), or the like is employed. The laying method of the cut grass 13 is appropriately selected according to the area of the soil 2 to be decontaminated and the supply amount of the grass 10. Considering the growth time, the solid covering covers all the surface of the soil 2, which is preferable because the time for the lawn 10 to spread can be shortened. On the other hand, considering the cost, the checkered covering reduces the amount of the cut grass 13. It is preferable because it is possible.

(Turf removal process)
The turf removing step is a step of peeling the turf 10 together with the root system composed of the roots 14 and the topsoil 4 as shown in FIG. The topsoil 4 includes topsoil 4a for fine grains such as silt and clay, and topsoil (soil particles) 4b for coarse grains such as sand and gravel (see FIG. 2B). Silt and clay are attached to the surface of the root 14. In order to peel off the turf 10, first, a sod cutter (not shown) that cuts the turf seedling (sod) to a certain depth is cut so as to surround the periphery of the part to be peeled off. The sod cutter can arbitrarily set the thickness of the cut grass 10 within a range of 20 to 80 mm, for example. The cut-out thickness is set according to the distribution state of the radioactive substance 1 in the soil 2. In the present embodiment, the turf 10 is cut and cut to a depth of 5 cm. Thereafter, as shown in FIG. 2A, the turf 10 is pulled up and peeled off from the soil 2. At this time, the root 14 of the turf 10 holds the coarse topsoil 4b, the fine topsoil 4a adheres to the root 14, and the root 14 of the turf 10 forms a root system together with the topsoil 4. Therefore, the turf 10 and the topsoil 4 can be accurately cut out at a depth of 5 cm.

  The turf 10 is cut long and stripped with a width that allows winding. The peeled turf 10 is wound up in a roll shape with the root 14 (or a root system including the root 14) facing inward. When the turf 10 is wound up in such a shape, the outer side of the roll becomes a foliage portion of the turf 10, so that the topsoil 4 attached to the root 14 is difficult to spill and the radioactive material returns to the soil 2. Can be suppressed. Moreover, since the turf 10 can be handled in the state wound up in roll shape, conveyance can be performed easily.

(Classification process)
As shown in FIG. 2B, the classification step is a step of separating the topsoil 4 a of fine particles containing a large amount of radioactive material 1 and the topsoil 4 b of coarse particles containing little radioactive material 1. Specifically, the coarse soil particles (topsoil 4b) not attached to the roots 14 are removed from the topsoil 4 attached to the peeled turf 10 by sieving. The classification process is performed in an indoor intermediate storage facility where the radioactive substance 1 is not likely to be scattered. When the turf 10 wound up in a roll shape is spread and applied to a vibrating sieve (not shown), the topsoil 4a of fine particles such as silt and clay adhering to the root 14 is vibrated. The topsoil 4b of coarse particles such as sand and gravel is screened off without falling. Since the radioactive substance 1 (especially radioactive cesium) is mainly adsorbed by silt, it remains on the turf 10 side. On the other hand, the coarse soil particles (topsoil 4b) are less adsorbed with the radioactive substance 1 and can separate the topsoil 4a and the turf 10 containing a large amount of the radioactive substance 1 and the topsoil 4b containing little. The topsoil 4a and turf 10 containing a large amount of radioactive substance 1 are reduced in volume, for example, incinerated in an intermediate storage facility according to the content (contamination amount) of radioactive substance 1, and discarded. The topsoil 4b that does not contain much radioactive material 1 can be reused as a backfill material. In addition, since the topsoil 4b is a coarse-grained soil particle, the radioactive substance 1 can be washed away with water containing Prussian blue, which is known to adsorb water and radioactive cesium. Thereby, the radiation material 1 of the topsoil 4b can be further reduced, and the reuse variation of the topsoil 4b can be expanded.

  Next, how much radiation dose is decontaminated when decontamination is performed using the turf 10, the turf 10 containing the radioactive substance 1 in the turf where the radioactive substance actually falls, The result of measuring and examining the radiation dose of the soil after stripping 10 will be described. The survey was performed by actually peeling off the lawn and measuring the radiation dose of the remaining soil 2 and the topsoil 4 attached to the lawn 10. The measurement sample is selected from two squares (collecting points) A and B of 900 mm square at any place on the lawn, and each of the squares A and B is divided into small squares of 300 mm square each. Each radiation dose was measured.

(1) Radiation dose before turf removal As shown in FIG. 3, the radiation dose on the turf surface before turf 10 removal is 1.08 to 1.65 in the sections 1 to 9 of the collection point A. In the range of (μSv / h), the average is 1.32 (μSv / h), and in the sections 10 to 18 of the collection point B, the average is 1.30 in the range of 1.30 to 1.50 (μSv / h). 37 (μSv / h).

(2) Radiation dose after turf removal As shown in FIG. 4, the radiation dose on the ground surface after turf 10 removal is 0.12 to 0.22 (in the sections 1 to 9 of the sampling point A). In the range of μSv / h), the average becomes 0.16 (μSv / h), and in the sections 10 to 18 of the collection point B, the average is 0.14 in the range of 0.11 to 0.20 (μSv / h). (ΜSv / h). The reduction rate of the radiation dose by peeling off the turf 10 was 87.9% at the sampling point A and 89.8% at the sampling point B.

  The stripping thickness of the turf 10 is 2 to 3 cm in the sections 3, 6, and 9 at the sampling point A, and 5 cm in the other sections, and the sections 11, 14, and 17 are 2 in the sampling point B. ~ 3 cm, other compartments were 5 cm. Even if the stripping thickness is 5 cm or 2 to 3 cm, there is no significant difference in the radiation dose on the surface of the soil 2, and even if the stripping thickness is 2 to 3 cm, the radiation dose is higher than that of the 5 cm section. There was a part with a large reduction rate. Therefore, when the radioactive substance 1 falls on the turf 10, it is considered that a stripping thickness of 2 to 3 cm is sufficient. On the other hand, when the radioactive substance 1 falls on the exposed soil 2 and the turf 10 is grown thereon, it is preferable to increase the stripping thickness to 5 cm.

(3) Weight and radiation dose of peeled turf The weight and radiation dose of peeled turf 10 were measured by packing each grass 10 and topsoil 4 in a polyethylene bag. As shown in FIG. 5, in the sections 1 to 9 of the collection point A, the radiation dose is 1.15 (μSv / h) on average in the range of 1.00 to 1.30 (μSv / h), and the weight In the range of 2.46 to 6.25 (kg), the average is 4.78 (kg). In the sections 10 to 18 of the collection site B, the radiation dose is 1.18 to 1.50 (μSv / h) in the range of 1.29 (μSv / h) on average, and the weight is 2.52 to 7 The average is 5.52 (kg) in the range of .55 (kg). In addition, about the weight, since the peeling thickness of the turf 10 has a division of 2-3 cm and a division of 5 cm, the dispersion | variation has generate | occur | produced.

(4) Weight and radiation amount of soil removed by sieving The lawn 10 peeled off with a 5 cm stripping thickness was placed on a mesh having a screen size of 5 mm, and the topsoil 4b was screened off. And the topsoil 4b which sifted down was packed in the bag made from polyethylene, and the weight and the radiation dose were measured. As shown in FIG. 6, the weight and radiation dose of the topsoil 4b that has been sifted from the turf 10 are 0.34 to 0 in the sections 1, 2, 4, 5, 7, and 8 of the sampling location A. The average is 0.52 (μSv / h) in the range of .87 (μSv / h), and the weight is 1.99 (kg) on average in the range of 0.77 to 2.80 (kg). . In the sections 10, 12, 13, 15, 16, and 18 of the sampling location B, the radiation dose is 0.37 (μSv / h) on average in the range of 0.32 to 0.45 (μSv / h), The weight is 2.85 (kg) on average in the range of 2.45 to 3.45 (kg).

  From the above results, when the average of the sampling points A and B is obtained, the radiation dose is 1.35 (μSv / h) for the grass 10 before peeling, and 0.15 (μSv / h) for the soil 2 after peeling. μSv / h), 1.39 (μSv / h) is obtained by sieving soil from the peeled turf, and 0.45 (μSv / h) is obtained by sieving the topsoil 4b. The weight is 3.82 (kg) when the soil is removed from the peeled grass, and 2.42 (kg) when the top soil 4b is removed. From this, it was found that the radiation dose can be reduced by 89% by stripping the turf 10, and the weight of soil that must be discarded can be reduced by 39% by sieving the topsoil 4b.

  The relationship between the radiation dose and the weight is 61% of the weight of the turf 10 and the topsoil 4 as a whole, as shown in Fig. 8, with the soil removed from the removed turf (turf and root + rooted soil). However, 76% of radioactive material 1 is contained. The topsoil 4b that has been sifted out is 39% of the weight of the peeled turf 10 and the topsoil 4 as a whole, but contains 24% of the radioactive material 1. From the above, the concentration of the radioactive substance 1 increases as it is above the turf 10 and the soil 2, and the topsoil 4 b that has been sifted off is the soil from the turf that has been removed (the turf and roots). It was found that the radiation dose per unit weight was smaller than that of (+ rooted soil).

  In addition, 55% of the radioactive material 1 of the peeled turf 10 is accumulated in the turf part (soil is removed from the peeled turf, and the root and rooted soil are removed), and the remaining 45% is attached to the root 14. It was found that they were accumulated in the topsoil 4a and the topsoil 4b that had been sifted down. About 45% of the radioactive material 1 in the topsoil 4 is adsorbed by the topsoil 4a such as silt or clay adhering to the root 14 of the turf 10 or the like. However, the base of the turf land investigated this time is sandy soil, and it is considered that radioactive material 1 has migrated deeper into the ground than agricultural land, so a lot of radioactive material 1 adheres to the root 14 in agricultural land. Presumed to be contained in the soil. Therefore, in the farmland composed of clay and silt, it is expected that many radioactive substances 1 are stopped on the surface of the soil 2, so that the root 14 of the turf 10 adsorbs the radioactive substance 1. The effect of entwining 4 is considered high.

  On the other hand, by classifying the peeled turf 10, the weight of the turf 10 and the topsoil 4 that must be discarded can be reduced by about 40%. Moreover, even if the topsoil 4b that has been sifted down is backfilled in the soil 2, the decontamination effect is reduced by 67% with respect to the initial radiation dose (1.35 (μSv / h)) on the turf surface. Is also secured. In this way, it is possible to reduce the amount of soil that must be discarded while ensuring a certain decontamination effect.

  In this embodiment in which the turf 10 is grown after the radioactive substance 1 falls, it is estimated that the radiation dose in the turf portion 15 is reduced and the radiation dose in the soil 2 is increased as compared with the above-described investigation result. . In addition, considering that the base of the turf is sandy soil in this survey, it is considered that the proportion of the topsoil 4a in the silt and clay fine grains is larger in the agricultural land than in the ground of this survey. Therefore, even if more radioactive material 1 is contained in the soil 2 than the above-mentioned investigation results, a large amount of radioactive material 1 is adsorbed on the topsoil 4a. Guessed. Therefore, it is considered that this topsoil 4b can be reused as backfill soil.

  As described above, according to the soil decontamination method, the grass 10 grows on the soil 2 and is peeled off together with the topsoil 4 attached to the root 14, so that even if the soil 2 is uneven, The surface layer 3 of the soil 2 corresponding to the depth of the root 14 can be removed at a constant depth. As a result, the decontamination effect can be ensured evenly over the entire surface of the soil 2. In this embodiment where the turf 10 grows after the radioactive substance 1 falls, it is estimated from the above investigation results that the radioactive substance 1 in the soil 2 is increased, but by setting the cutting thickness to 5 cm, A sufficient decontamination effect can be secured.

  Moreover, by peeling off the topsoil 4 together with the turf 10, it is possible to suppress the topsoil 4 containing the radioactive substance 1 from scattering or flowing out, and to suppress secondary contamination at the decontamination site or during transportation. In particular, since the turf 10 is wound in a roll shape with the root 14 facing inward, the topsoil 4 containing the radioactive substance 1 is wound inward, and the turf 10 is less likely to spill onto the soil 2 after the turf 10 is peeled off. Become. Thereby, a reduction in decontamination effect and secondary contamination can be suppressed.

  Further, by removing the coarse soil particles (topsoil 4b) not sticking to the root 14 of the peeled turf 10, it can be easily separated from the topsoil 4a sticking to the root 14, and the radioactive substance 1 Is classified into the topsoil 4a containing silt or clay that is easily adsorbed, and the topsoil 4b that is difficult to absorb the radioactive substance 1. Thereby, the weight of the topsoil 4a to be disposed of can be reduced, and the volume of contaminated soil can be reduced. Furthermore, the topsoil 4b, on which the radioactive substance 1 is difficult to be adsorbed, has a low radiation dose (it is a radiation dose that can be used as backfill soil) and can be reused. In addition, since the topsoil 4b is a coarse particle | grain part, it can further reduce the radiation dose to contain by the simple operation | work of washing away with water.

  As mentioned above, although the form for implementing this invention was demonstrated, this invention is not limited to the said embodiment, In the range which does not deviate from the meaning of this invention, it can change suitably. For example, in the embodiment, the classification process is performed, but the classification process is not necessarily performed. In this case, the turf 10 and the topsoil 4 peeled off from the soil 2 are incinerated in an intermediate storage facility as they are, and the volume is reduced and discarded.

DESCRIPTION OF SYMBOLS 1 Radioactive material 2 Soil 3 Surface layer 4 Top soil 4b Top soil (soil particle of coarse particle)
10 turf 14 roots

Claims (3)

In the soil decontamination method for decontaminating soil containing radioactive substances,
A lawn growing process for growing grass in the soil;
A turf removal step of stripping the turf together with the topsoil attached to the turf root; and a soil decontamination method. The soil decontamination method according to claim 1, wherein, in the turf removal step, the peeled turf is wound up in a roll shape with the roots facing inward. The soil decontamination method according to claim 1 or 2, further comprising a classification step of sieving coarse soil particles from the top soil attached to the roots after the turf removal step.

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