JPS6197600A - Back-filling material for radioactive waste - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to backfilling materials used in the geological burial disposal of radioactive waste.

[Traditional forceps]

As a method for disposing of high-level radioactive waste discharged from reprocessing plants, etc., and spent nuclear fuel discharged from nuclear reactors, etc., a geological disposal method, in which it is buried deep underground in bedrock, has been proposed. In this disposal method, a horizontal tunnel is created deep underground in rocks, etc., and a sealed stainless steel container (canister) filled with high-level radioactive waste is placed in a disposal pit inside the tunnel, and a disposal bit and This is to backfill the tunnel with backfilling material.

Therefore, the above-mentioned backfilling material used in this disposal method is required to have the following performance.

■ Radioactivity shielding High-level radioactive waste has quite strong radioactivity even when it is filled into a canister, so even after the canister is buried in the disposal bit, it must be sufficiently must be shielded to ensure a low quantity rate,
The shielding thickness of the backfill material is 2 m, and the density is 1. :l/d, the dose rate directly above the disposal pit is 5.4 x 10
mrem/hr, which is sufficiently low. Therefore,
We adopted @hidden as a similar shielding property, which is 1.3g/y
It is necessary that the value is greater than or equal to f.

＠ Water-stopping property In order to prevent corrosion of the canister due to groundwater seeping from the bedrock, the backfill material must have high water-stopping properties. For this reason, at least a good rock hydraulic conductivity (10cWL
/5ec), so 10
It is essential to have a hydraulic conductivity of cm/sec or less.

θ Adsorption Even if all of the radionuclides in the canister are released, if the backfilling material has sufficient adsorption capacity, all of them will be adsorbed by the backfilling material and leakage to the outside can be prevented.

Therefore, the backfilling material must have adsorption properties and is desired to have a large adsorption capacity (capacity). Specifically, we evaluated the adsorption properties of cation exchange valleys (IL(age)) of backfilling materials.
The larger the cation exchange capacity per unit volume, the more preferable it is.

[Phase] Chemical buffering properties, chemical stability In order to prevent corrosion of the canister, the backfilling material must be a material that has chemical buffering properties, is not oxidized to Cm, and is chemically stable. be done.

■ Thermal diffusivity level radioactive waste is 2k immediately after filling into the canister.
W, 0.5kL after 30 years l 0.1kW after 00 years
5. The required quality is the thermal diffusivity of typical granite, which requires a thermal diffusivity of at least the same as that of the surrounding rock. 5
The condition is that the temperature is Km/m -hr·°C or higher.

6. Mechanical properties The backfill material must withstand external forces such as earth pressure and thermal stress, as well as the heavy electric current of the canister, and is required to have mechanical strength above a certain level.

As an indicator thereof, it is required that the unconfined compressive strength is 2kliF/d or more.

[Purpose of the invention]

The object of the present invention is to provide a sealing material that satisfies the required performance as described above.

[Structure of the invention]

The radioactive leaf material of the present invention is composed of 5-95 mm% of smectite clay, 5-100% by weight of water, and 5-95 mm of graphite powder and/or chromite sand.
% is men.

Smectite clay (hereinafter referred to as smectite clay) is a clay that has high water-stopping properties and adsorption properties, typified by bentonite. This smectite clay is used after its water content is adjusted to 5 to 100% by weight.

The moisture content herein refers to the value obtained by dividing the weight of water by the weight of smectite clay multiplied by 100. Water content is 5
If it is less than 100% by weight, the water content is too low and molding after mixing with graphite powder or the like is impossible, so graphite powder and/or chromite sand is added to the 100% by weight smectite clay. As the graphite powder, it is possible to use a powder produced as cutting waste when processing a 1 m pole of industrially produced graphite using coal coke or petroleum pitch as a raw material, or natural graphite powder. This graphite powder has high thermal conductivity, and by mixing an appropriate amount with smectite clay, it can impart good thermal conductivity to the backfilling material.

In addition, chromite sand is chromite (FeO・0r01
) is a natural aggregate with E stone, and it is a material that contains CrO3, has high density, high thermal conductivity, and is chemically stable. Therefore, by blending it with smectite clay,
It can provide high density and high thermal conductivity without significantly impairing water-stopping properties and adsorption properties, and increases mechanical strength.

The blending ratio of smectite clay and graphite powder or chromite sand is 5 to 95% by weight of smectite clay. If the amount of smectite clay is less than 5% by weight, the water stopping and adsorption properties of the resulting backfilling material will be low, making it impossible to satisfy the above target value, and if the smectite clay is less than 95% by weight.
If kqlj is exceeded, the thermal conductivity is insufficient and the target value cannot be satisfied, and a large pressing force is required to satisfy the mechanical strength, which is disadvantageous.

Further, graphite powder and chromite sand may be added to the smectite clay individually, or may be mixed in any proportion and then added to the smectite clay.

Such a mixture is sufficiently kneaded by mixing machines such as mixers and mixers used in foundries and agricultural chemical factories.
It is considered as the backfilling material of the present invention. The backfill material thus obtained is a slightly moist clay-like clay mixed with sand, and can be compacted to solidify it.

This backfilling material will be used to backfill the buried canister within the disposal bit in the tunnel, but it will require pressurization after backfilling.

That is, it is necessary to put a sufficient amount of backfill material above the canister placed in the disposal pit and sufficiently compact it using a compaction machine such as a press machine. The pressing force during this compaction is required to be at least 30 to 9/d or more, preferably about 100 to 1000 kg/d. This compaction results in high density and unconfined compressive strength.

A canister filled with radioactive waste is
It will be stored in a closed state buried in backfilling material in a disposal bit.

In addition, there is a method in which the backfilling material of the present invention is pressure-formed in advance using gold m or the like into a molded body having a shape that can accommodate a canister, and the canister is housed in this molded body and stored in a disposal pit. [Function] Since one of the components of this type of backfilling material is smectite clay, which has high water-stopping and adsorption properties,
It has a low hydraulic conductivity of 0 crIL/Sec or less and has high adsorption properties, effectively preventing water from entering and completely preventing leakage of radionuclides from the canister. Adsorb.

In addition, since the other component is graphite powder or chromite sand, which has high density and high thermal conductivity, 1.3. S
Obtains a density higher than F/d, has immersed radiation shielding properties, and is 2.5 K! It exhibits a thermal conductivity of Ll/m-hr-'C or higher and has excellent heat dissipation properties.

Furthermore, graphite and chromite sand are used as aggregates and are high-strength materials, and when compacted, a uniaxial compressive strength of 2 Yu/cit or more can be obtained.

Furthermore, smectite clay, graphite, and chromite sand are all chemically stable, and this backfill material is chemically oil-free to the canis ζ- and does not cause corrosion or the like.

〔Example〕

Table 1 shows formulation examples of the backfilling material of the present invention and its characteristic values.

From Table 1, it can be seen that this backfilling material satisfies all required qualities.

Next, we investigated the relationship between the moisture content of bentonite (smectite clay), the density of the backfill material, and the unconfined compressive strength. Backfilling materials were produced by varying the moisture content of samples containing 20% by weight of bentonite and 80% by weight of graphite powder.
This was compacted at a pressure of 100 and 9/d, and the density and unconfined compressive strength of the solidified product were determined. The results are shown in the graph of FIG.

In addition, the relationship between compaction pressure, density, and unconfined compressive strength was investigated. 20% by weight bentonite with a water content of 39.4%
A sample containing 80% by weight of graphite powder was compacted to obtain a solidified product, and its density and unconfined compressive strength were determined. The results are shown in the graph of FIG.

〔Effect of the invention〕

The backfilling material for radioactive waste of this invention is made by blending specific amounts of high-density, highly thermally conductive graphite powder and/or chromite sand with water-stopping and adsorbent smectite clay. effect can be obtained.

1. Since the density can be made sufficiently high (1,311/crA or more), radiation from high-level radioactive waste can be sufficiently shielded.

It has a high water-stopping property of 2°, exhibiting a water permeability coefficient lower than that of good bedrock, and can be expected to be effective in preventing deer from flooding for about 100 years.

3. Even if the stainless steel container (canister) is flooded, it is surrounded by chemically stable materials, and corrosion of the container can be suppressed by adsorbing ions in underground water.

4. Since it has poor adsorption properties, it is effective in adsorbing and retaining radionuclides that should leak from the canister, thereby preventing external contamination.

5. It has high thermal conductivity, high heat capacity, and good high-temperature stability, so the heat released from buried radioactive waste can be quickly diffused to bedrock, etc., and heat does not get trapped inside.

6. Set appropriate mixing ratio, moisture content, and compaction pressure.

If this is done, the material will be sufficiently strong, and there will be no inconvenience such as the gearnister moving even during long-term storage.

[Brief explanation of the drawing]

The drawings all show the characteristics of the backfilling material of the present invention; Fig. 1 is a graph showing the relationship between water content, density, and uniaxial compression zero strength, and Fig. 2 is a graph showing the relationship between compaction pressure, density, and uniaxial compression zero strength. It is a graph showing the relationship with compressive strength.

Claims (1)

[Claims] A backfilling material for radioactive waste prepared by kneading 5 to 95% by weight of smectite clay, 5 to 100% by weight of water, and 5 to 95% by weight of graphite powder and/or chromite sand.