KR19980078181A - Silicone rubber-based neutron shielding composition - Google Patents

Abstract

The present invention relates to a neutron shielding composition prepared by curing a mixture containing polypropylene, aluminum hydroxide and boron carbide in a silicone rubber as a base material. The composition of the present invention maintains fluidity for a predetermined period of time, And is excellent in thermal and mechanical properties, radiation resistance and neutron shielding ability while maintaining the characteristics of a silicone rubber excellent in flame resistance.

Description

Silicone rubber-based neutron shielding composition

The present invention relates to a neutron shielding composition comprising a silicone rubber and various additives in appropriate proportions, and more particularly to a silicone rubber as a base material, which is prepared by curing a mixture containing polypropylene, aluminum hydroxide and boron carbide To a neutron shielding material composition.

The neutrons generated from the transportation and storage of radioactive materials, nuclear reactors, liquid metals, etc. have high energy and permeability, and can generate secondary gamma rays by (n, γ) Materials that can safely shield neutrons are desired because they can damage structural members or equipment in nuclear facilities. Currently, water, concrete, polymer materials, cermet, boron alloys, etc. are used as neutron shielding materials depending on the place and conditions of use.

When water or a mixture of water and ethylene glycol is used as a neutron shielding material in a spent fuel transport vessel, an extra space is required in the transport container in consideration of the volume expansion due to the temperature rise. This extra space is complicated due to the effect of neutron shielding The thickness and weight of the transport container are increased and the payload of the transport container is reduced.

In a container having a complicated structure such as a spent fuel transportation container, a neutron shielding material having a fluidity for a certain period of time and having excellent processability is required. Silicone rubber is one of the most thermostable and flame resistant materials among hydrocyclic polymers, And since it is a liquid phase before curing, there is an advantage that a mold having a complicated shape can be obtained by utilizing its fluidity.

On the other hand, a silicon rubber-based neutron shielding material used in a conventional reactor is sometimes doped with a boron compound capable of absorbing thermal neutrals. Since the silicon rubber originally has a low hydrogen content, a high content of boron compounds leads to a low concentration of hydrogen atoms, There is a problem that the shielding ability is reduced. To solve this problem, Japanese Patent No. 86173198 discloses a neutron shielding material in which polypropylene, titanium hydride or the like is added so as to reduce the content of boron compounds and decrease the density of hydrogen atoms. Japanese Patent No. 86290400 discloses a silicone rubber-based neutron shielding composition to which a gadolinium compound having a higher thermal neutron absorption capability than a boron compound is added.

The inventors of the present invention have been studying to meet the properties required for neutron shielding materials used in nuclear fuel transport containers and the like, and have found that a liquid silicone rubber having the best heat resistance and flame retardancy among the hydrotropic polymer materials is used as a base material, The present invention has been accomplished by developing a silicone rubber-based neutron shielding composition having excellent radiation resistance and neutron shielding ability while taking advantage of the characteristics of the neutron shielding material.

It is an object of the present invention to provide a neutron shielding material composition comprising an additive capable of improving the neutron shielding performance by using a silicone rubber as a base material.

1 is a flow chart of a manufacturing process and a characteristic test analysis of a neutron shielding material,

2 is a graph showing the effect of radiation on the tensile strength of the neutron shielding material,

- - -: Shielding material -1, - - -: Shielding material -2, - Δ -: Shielding material -3

3 is a graph showing the effect of radiation on the compressive strength of the neutron shielding material,

- - -: Shielding material -1, - - -: Shielding material -2, - Δ -: Shielding material -3

FIG. 4 is a graph showing the effect of radiation on the hardness of the neutron shielding material. FIG.

- - -: Shielding material -1, - - -: Shielding material -2, - Δ -: Shielding material -3

To achieve the above object, the present invention provides a neutron liner material composition containing a liquid silicone rubber as a base material and polypropylene, aluminum hydroxide and boron carbide in powder form as additives.

Hereinafter, the present invention will be described in detail.

The liquid silicone polymer for room temperature curing, which is excellent in heat resistance and flame retardancy, is a polydimethylsiloxane, which is used as a base material of a neutron shielding material in the practice of the present invention. These are polydimethylsiloxane, base, part A and hardener (part B) Component type liquid silicone rubber which is divided into two components and is uniformly mixed at a weight ratio (or volume ratio) of 1: 1 between the main component and the curing agent immediately before use, do.

The additives used in the neutron shielding composition of the present invention include polypropylene, aluminum hydroxide and boron carbide. In order to add a large amount of the additive, it is necessary to lower the viscosity of the liquid silicone rubber. It is effective to limit the content of silica to 38% by weight.

Polypropylene is used for the purpose of increasing the density of the hydrogen atoms in the silicone rubber, and preferably has a particle size of 75 to 150 μm.

Boron carbide (B ₄ C) is the most commonly used compound in neutron shielding materials among boron compounds and is added because of its high shielding effect against low-rate neutrons or thermal neutrons. Since boron carbide has a large amount of boron in particular, it has a wide thermal neutron capture area and has many ideal characteristics as a neutron absorber, such as not generating secondary radiation of high level or secondary long-term by-product when irradiated with neutrons. To 100 m.

Aluminum hydroxide is a crystalline water-based compound containing about 35% water, which imparts flame retardancy to neutron shielding agents and promotes self-extinguishing properties without generating toxic gases. Aluminum hydroxide also works with silicon rubber to slow the high-speed neutrons to thermal neutrons, and dry aluminum hydroxide of 8 to 75 μm is effective.

The neutron shielding composition of the present invention comprises 35 to 70 wt% of silicone rubber, 2 to 20 wt% of polypropylene, 15 to 65 wt% of aluminum hydroxide, and 2 to 5 wt% of boron carbide.

The silicone rubber-based neutron shielding material of the present invention is made of Rheomix having a sigma type rotor to uniformly mix liquid silicone rubber as a base material and additives such as polypropylene, aluminum hydroxide, boron carbide in powder form, use. As a condition of mixing, the internal temperature of the mixer is room temperature, the internal pressure is atmospheric pressure, the rotating speed of the rotor is 60 to 100 rpm, and the mixing time is 20 to 25 minutes.

As shown in the flow chart of FIG. 1 , when mixing, each sample was prepared at a predetermined weight ratio, the liquid silicone rubber was mixed with the hardener in the same weight ratio (or volume ratio), and the powder samples were put in a mixer for a predetermined time Mix. As described above, since a small amount of bubbles exist at the interface between the liquid phase and the powder when a powdery additive, which is an inorganic substance, is added to the liquid silicone rubber, it is allowed to stand in a vacuum desiccator at a degree of vacuum of about 3 mbar for 25 to 30 minutes The bubbles inside the mixture are completely removed.

Hereinafter, the present invention will be described in detail with reference to the following examples. It is to be understood, however, that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

Example 1

49% by weight of silicone rubber (43% by weight of silica), 49% by weight of aluminum hydroxide and 2% by weight of boron carbide were put in a mixer and mixed for a certain period of time. Rheomix with a sigma rotor was used to uniformly mix the liquid silicone rubber and the powdery additives. The internal temperature of the mixer was room temperature, the internal pressure was atmospheric pressure, the rotating speed of the rotor was 60 to 100 rpm, and the mixing time was 20 to 25 minutes.

As described above, since a small amount of bubbles exist at the interface between the liquid phase and the powder when a powdery additive, which is an inorganic substance, is added to the liquid silicone rubber, it is allowed to stand in a vacuum desiccator at a degree of vacuum of about 3 mbar for 25 to 30 minutes And the bubbles in the mixture were completely removed to prepare the neutron shielding material of the present invention.

Thermal properties and mechanical properties of the silicone rubber-based neutron shielding material (shielding material-1) produced are shown in Tables 1 and 2 below. As a control, the performance of a neutron shielding material RX-237 (Boro-Silicone, Reactor Experiments Inc., USA) used in foreign countries was described.

The number density of hydrogen atoms (atoms / cm ³ ) Pyrolysis temperature (℃) Thermal conductivity (W / m · K) Thermal expansion coefficient (° C ^-1 ) Flame resistance ^* The shielding material-1 of the present invention 4.14 × 10 ²² 233 1.316 1.522 × 10 ^-4 ATB (sec) 5 AEB (mm) 5 RX-237 (control group) 4.49 × 10 ²² 205 2.430 1.70 x 10 ^-4 ATB (sec) 5 AEB (mm) 5 * Average Time of Burning (ATB): average burning time AEB (Average Extent of Burning)

As shown in Table 1, the hydrogen atom density and the thermal properties of the neutron shielding material of the present invention are found to be equal to that of RX-237. In particular, according to Korean Industrial Standard KS M3015, a general test method for thermosetting plastics, a material having a burning length of 25 mm or less is classified as incombustible, so that the flame resistance of the shielding material of the present invention is excellent.

Tensile strength (kg / mm ² ) Compressive strength (kg / mm ² ) Hardness (Shore A) Density (g / cm ³⁾ The shielding material-1 of the present invention 0.24 0.77 81 1.737 RX-237 0.04 0.32 66 1.59

As can be seen from Table 2, the mechanical properties of the shielding material of the present invention are superior to those of RX-237.

Example 2

(Silica content, 38 wt%) and 39 wt% of silicon rubber (silica content, 43 wt%), aluminum hydroxide 18 wt%, polypropylene 10 wt% and boron carbide 2 wt% , 59% by weight of aluminum hydroxide and 2% by weight of boron carbide (Shielding material-3) was used to prepare a neutron shielding material according to the method of Example 1 above. The radiation-resistant properties of the silicone rubber-based neutron shielding material produced are shown in FIGS . 2 , 3 and 4 , and the neutron shielding ability is shown in Tables 3 and 4 below.

The irradiation dose to investigate the radiation resistance characteristics of the neutron shielding material was performed by gamma irradiation with a high-level radiation source (Co-60, 86,500 Ci). The irradiation dose used in the test was 0.05 MGy, 0.5 MGy and 1.0 MGy, These were obtained by irradiating for 10 hours, 100 hours and 200 hours, respectively, at an average irradiation dose of 5.0 KGy / hr. The tensile test on the irradiated specimens was carried out with a Zwick Model 1446 tensile tester, the compression test was carried out by a universal material tester of MTS 25 ton capacity and the hardness test was carried out using a Rex Durometer Model 1000 In-spring hardness tester.

The irradiated dose affects the tensile strength of the shielding material, which is shown in Fig . As shown in FIG. 2 , when the dose of radiation was increased to 1.0 MGy, the tensile strength of the shielding material-2 tended to gradually increase slightly. The shielding material-1 showed a tendency to increase tensile strength The tensile strength of the shielding material-3 tended to decrease. Thus, it is judged that the polymer materials constituting the shielding material undergo decomposition reaction or cross-linking reaction by irradiation, depending on the components, properties and additives of the mixed polymer material. Therefore, it is necessary to control the silica content in the silicone rubber to maintain the tensile strength of the shielding material above a certain level.

The effect of radiation dose on the compressive strength and hardness of the polymer shielding materials is shown in FIGS . 3 and 4 , respectively. As can be seen from FIGS . 3 and 4 , the increasing tendency of the compressive strength and hardness of the shielding material-1 and the shielding material-2 was larger than that of the shielding material-3 as the radiation dose increased to 1.0 MGy. It is considered that the cross-linking between the free radicals generated by the side chain breaking of the silicon polymer as the basic material of the shielding material by radiation is superior to the shielding material-3 by the shielding material-1 and the shielding material-2, so that the compression strength and hardness are further increased . Particularly, in the case of the shielding material-2 added with polypropylene, as shown in FIG. 3 , the compressive strength was more increased than that of the shielding material-1 as the irradiation dose increased. This is because the cross-linking between the polypropylene and the silicone polymer is also performed.

As a result, it can be seen that the shielding material of the present invention is excellent in radiation resistance.

In order to investigate the shielding ability of the neutron shielding material, the shielding ability test was carried out by a voluntary fission neutron source, Californium-252, and the neutron source emission rate was 1.01 × 10 ⁹ n / s, and between the neutron source and the measuring instrument The distance between the neutron shielding material and the instrument is 15 cm with a distance of 50 cm and a specimen size of 350 mm (width, W) × 350 mm (height, H) × 25 mm (thickness, T). Neutron dose rates were measured with a 9 "remmeter with a polyethylene moderator. The shielding ability test results of the neutron shielding materials are shown in Table 3 below.

Shielding material -1 Shielding material -2 Shielding material -3 Neutron energy, E (MeV) 0.524 0.524 0.524 Shield material thickness, T (cm) 2.5 2.5 2.5 Distance between neutron source and instrument (cm) 50 50 50 Distance between neutron source and shield (cm) 35 35 35 Damping (D _i ) / Damping (D _o ) 0.741 0.774 0.732 Macroscopic removal cross section, Σ _R (cm ^-1 ) 0.120 0.102 0.125

As a result, it can be seen that the macroscopic removal sectional area ( _R ) of the shielding material of the present invention is superior to that of alkane hydrocarbon (removal cross section 0.11 cm ^-1 ) and water (removal cross section 0.10 cm ^-1 ).

Table 4 shows the results of the shielding analysis when this shielding material (thickness: 15 cm) is used as a transport container capable of loading seven bundles of spent nuclear fuel.

The design basis fuel used in the calculation of the source for the shielding analysis is a PWR 17 × 17 array aggregate corresponding to the high flammability nuclear fuel, having an initial concentration of 4.2 w / o and a specific power of 40.0 MW / MTU, And 50,000 MWD / MTU in the spent fuel reservoir of the nuclear power plant after the 1.5 year cooling period. The shielding analysis was carried out under the normal transport conditions of the transport container, and the transport container was specially loaded with spent fuel and the internal field tradition was air-cooled without water. The maximum radiation dose rate in the radial direction of the transport container was defined as one- The calculation code, ANISN code, was used to calculate the neutron dose rate at 2 m from the lateral and lateral surfaces of the transport vessel. Under the normal transport conditions of transport containers, the maximum allowable radiation dose is determined by the Ministry of Science and Technology, Article 85-8, Article 12, the IAEA Safety Series No. 6 para. 465, and the United States 10 CFR 71.47 In the case of dedicated loading, it is specified to be 2 mSv / hr at the surface of the vehicle and transport container and 0.1 mSv / hr at 2 m from the surface of the container.

Radiation dose rate (mSv / hr) Transport container surface 2 m from transport vessel surface Shielding material -1 0.358 0.070 Shielding material -2 0.425 0.083 Shielding material -3 0.330 0.065 Maximum radiation dose rate allowance 2.0 0.10

As a result, it can be seen that the shielding ability of the neutron shielding material of the present invention is excellent.

As described above, the composition of the present invention can be effectively used as a neutron shielding material by maintaining the characteristics of silicone rubber excellent in heat resistance and flame retardancy, and having excellent radiation resistance and neutron shielding ability.

Claims (5)

Wherein the additive type liquid silicone polymer is divided into two components, a base and a hardener, and various additives. The neutron shielding composition of claim 1, wherein the silicone polymer is polydimethylsiloxane. The neutron shielding composition according to claim 1, wherein the additive is polypropylene, aluminum hydroxide and boron carbide. The neutron shielding composition according to claim 3, wherein the polypropylene has a particle size of 75 to 150 μm, boron carbide of 5 to 100 μm, and aluminum hydroxide of 8 to 75 μm. 4. The absorbent article according to any one of claims 1 to 3, characterized in that it contains 35-70 wt% of silicone rubber, 2-20 wt% of polypropylene, 15-65 wt% of aluminum hydroxide and 2-5 wt% of boron carbide . ≪ / RTI >