CA2024879C - Neutron-absorbing materials - Google Patents
Neutron-absorbing materialsInfo
- Publication number
- CA2024879C CA2024879C CA002024879A CA2024879A CA2024879C CA 2024879 C CA2024879 C CA 2024879C CA 002024879 A CA002024879 A CA 002024879A CA 2024879 A CA2024879 A CA 2024879A CA 2024879 C CA2024879 C CA 2024879C
- Authority
- CA
- Canada
- Prior art keywords
- neutron
- absorbing material
- boron
- polyethylene
- boron carbide
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
- 239000011358 absorbing material Substances 0.000 title claims abstract description 26
- 239000004698 Polyethylene Substances 0.000 claims abstract description 24
- -1 polyethylene Polymers 0.000 claims abstract description 24
- 229920000573 polyethylene Polymers 0.000 claims abstract description 22
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 claims abstract description 12
- 229910052796 boron Inorganic materials 0.000 claims abstract description 12
- 229910052580 B4C Inorganic materials 0.000 claims description 50
- INAHAJYZKVIDIZ-UHFFFAOYSA-N boron carbide Chemical compound B12B3B4C32B41 INAHAJYZKVIDIZ-UHFFFAOYSA-N 0.000 claims description 23
- 239000002245 particle Substances 0.000 claims description 9
- 238000005245 sintering Methods 0.000 claims description 7
- 238000000034 method Methods 0.000 claims description 6
- 150000001639 boron compounds Chemical class 0.000 claims description 5
- 238000001816 cooling Methods 0.000 claims description 5
- 239000000203 mixture Substances 0.000 claims description 5
- 229910052799 carbon Inorganic materials 0.000 claims description 3
- 239000003381 stabilizer Substances 0.000 claims description 3
- 238000000196 viscometry Methods 0.000 claims description 3
- 238000004519 manufacturing process Methods 0.000 claims 1
- 238000002156 mixing Methods 0.000 claims 1
- 239000000463 material Substances 0.000 description 20
- 230000009102 absorption Effects 0.000 description 13
- 238000010521 absorption reaction Methods 0.000 description 13
- 229920006448 PE-UHMW Polymers 0.000 description 10
- 229910052739 hydrogen Inorganic materials 0.000 description 5
- 239000001257 hydrogen Substances 0.000 description 5
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 4
- 229920003023 plastic Polymers 0.000 description 4
- 239000004033 plastic Substances 0.000 description 4
- 239000003054 catalyst Substances 0.000 description 3
- 238000002360 preparation method Methods 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- 229920000785 ultra high molecular weight polyethylene Polymers 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- LYCAIKOWRPUZTN-UHFFFAOYSA-N Ethylene glycol Chemical compound OCCO LYCAIKOWRPUZTN-UHFFFAOYSA-N 0.000 description 2
- 239000004952 Polyamide Substances 0.000 description 2
- 239000004699 Ultra-high molecular weight polyethylene Substances 0.000 description 2
- 125000002947 alkylene group Chemical group 0.000 description 2
- 230000002238 attenuated effect Effects 0.000 description 2
- 229910052793 cadmium Inorganic materials 0.000 description 2
- BDOSMKKIYDKNTQ-UHFFFAOYSA-N cadmium atom Chemical compound [Cd] BDOSMKKIYDKNTQ-UHFFFAOYSA-N 0.000 description 2
- 150000001875 compounds Chemical class 0.000 description 2
- 239000000470 constituent Substances 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 239000000945 filler Substances 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 229920002647 polyamide Polymers 0.000 description 2
- 229920000728 polyester Polymers 0.000 description 2
- 230000005855 radiation Effects 0.000 description 2
- 238000012216 screening Methods 0.000 description 2
- 229910052582 BN Inorganic materials 0.000 description 1
- PZNSFCLAULLKQX-UHFFFAOYSA-N Boron nitride Chemical compound N#B PZNSFCLAULLKQX-UHFFFAOYSA-N 0.000 description 1
- GHKOFFNLGXMVNJ-UHFFFAOYSA-N Didodecyl thiobispropanoate Chemical compound CCCCCCCCCCCCOC(=O)CCSCCC(=O)OCCCCCCCCCCCC GHKOFFNLGXMVNJ-UHFFFAOYSA-N 0.000 description 1
- 239000003508 Dilauryl thiodipropionate Substances 0.000 description 1
- 239000002656 Distearyl thiodipropionate Substances 0.000 description 1
- 239000004743 Polypropylene Substances 0.000 description 1
- 239000006096 absorbing agent Substances 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 239000003963 antioxidant agent Substances 0.000 description 1
- 125000004429 atom Chemical group 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- KGBXLFKZBHKPEV-UHFFFAOYSA-N boric acid Chemical compound OB(O)O KGBXLFKZBHKPEV-UHFFFAOYSA-N 0.000 description 1
- 238000004364 calculation method Methods 0.000 description 1
- 239000013065 commercial product Substances 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 235000019304 dilauryl thiodipropionate Nutrition 0.000 description 1
- PWWSSIYVTQUJQQ-UHFFFAOYSA-N distearyl thiodipropionate Chemical compound CCCCCCCCCCCCCCCCCCOC(=O)CCSCCC(=O)OCCCCCCCCCCCCCCCCCC PWWSSIYVTQUJQQ-UHFFFAOYSA-N 0.000 description 1
- 235000019305 distearyl thiodipropionate Nutrition 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- WGCNASOHLSPBMP-UHFFFAOYSA-N hydroxyacetaldehyde Natural products OCC=O WGCNASOHLSPBMP-UHFFFAOYSA-N 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 230000035515 penetration Effects 0.000 description 1
- 230000000704 physical effect Effects 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 238000003825 pressing Methods 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- QQONPFPTGQHPMA-UHFFFAOYSA-N propylene Natural products CC=C QQONPFPTGQHPMA-UHFFFAOYSA-N 0.000 description 1
- 125000004805 propylene group Chemical group [H]C([H])([H])C([H])([*:1])C([H])([H])[*:2] 0.000 description 1
- 230000002285 radioactive effect Effects 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 239000007858 starting material Substances 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 229920001169 thermoplastic Polymers 0.000 description 1
- 229920001187 thermosetting polymer Polymers 0.000 description 1
- 239000004416 thermosoftening plastic Substances 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
Classifications
-
- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
- G21F1/00—Shielding characterised by the composition of the materials
- G21F1/02—Selection of uniform shielding materials
- G21F1/10—Organic substances; Dispersions in organic carriers
- G21F1/103—Dispersions in organic carriers
-
- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21C—NUCLEAR REACTORS
- G21C5/00—Moderator or core structure; Selection of materials for use as moderator
- G21C5/12—Moderator or core structure; Selection of materials for use as moderator characterised by composition, e.g. the moderator containing additional substances which ensure improved heat resistance of the moderator
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
- C08K3/00—Use of inorganic substances as compounding ingredients
- C08K3/38—Boron-containing compounds
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E30/00—Energy generation of nuclear origin
- Y02E30/30—Nuclear fission reactors
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Dispersion Chemistry (AREA)
- General Engineering & Computer Science (AREA)
- High Energy & Nuclear Physics (AREA)
- Medicinal Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Health & Medical Sciences (AREA)
- Polymers & Plastics (AREA)
- Organic Chemistry (AREA)
- Plasma & Fusion (AREA)
- Compositions Of Macromolecular Compounds (AREA)
- Solid-Sorbent Or Filter-Aiding Compositions (AREA)
- Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)
- Ceramic Products (AREA)
Abstract
The invention relates to boron-containing polyethylene having a mean molecular mass of at least 2.5 x 106 g/mol as a neutron-absorbing material.
Description
Neutron-absorbinq materials The invention relates to a neutron-absorbing material. It is composed of ultrahigh molecular weight polyethylene in which a boron compound, preferably boron carbide B4C, is embedded.
In contrast to alpha- and beta-particles, neutrons do not have a charge and therefore cannot lose energy by ener-gization on passing through matter. Consequently, their penetration power is extremely high. Neutrons are subject exclusively to the action of the nuclear forces and are scattered on atomic nuclei. According to the collision laws, the energy releases to the body undergoing a collision are the greater in such scattering processes, the more similar the mass thereof is to the mass of the colliding body. Therefore, a bundle of neutron beams, which penetrates lead plates of several meters thickness without significant attenuation, is very greatly attenu-ated when passing through hydrogen-contAining substances of a few cm thickness. On average, the energy is reduced to l/e on collision with a proton, whereas the energy release to atomic nuclei of higher mass is less, due to inelastic collision. It is known from the literature that on average 18 collisions are necessary in hydrogen and on average 114 collisions are necessary in carbon in order to brake a neutron down to thermal energy. ,hese thermal, i.e. slow neutrons can then be completely absorbed by elements of high cross-section, such as cadmium or boron.
In neutron absorption, binding energy is released in the form of secondary gamma-radiation. It depends on the absorber material and can be of considerable magnitude.
Thus, the gamma-radiation energy is 6 MeV in the absorp-tion of neutrons by cadmium, 2.2 MeV in that by hydrogen and only 0.5 MeV in that by boron.
- As the materials which protect against neutron radiation, especially water and paraffins as well as plastics cont~in;ng significant quantities of hydrogen, such as polyethylene, polyesters and polyamides, are used.
Thus, according to the teaching of German Auslegeschrift 1,297,869, moldings of thermoplastic or thermosetting plastics, in which the carbon/hydrogen ratio or the residual atom/hydrogen ratio is in the range from 1 : 2.1 to 2 : 1 and the molecular weight of which is less than 200,000, are used for protection against gamma-radiation and neutron radiation. Such plastics can be from the classes of high- and low-pressure polyethylenes, poly-propylenes, alkylene/propylene or alkylene/butylenecopolymers, polyamides and polyesters.
In German Auslegeschrift 1,162,694, a neutron-absorbing material is described, in which granulated polyethylene is embedded in a hydrogen-cont~ining liquid which remains liquid or cures to give a plastic.
However, the known neutron-absorbing materials have properties which restrict their applicability. Thus, although plastics have a low density, their processibil-ity frequently causes difficulties. Moreover, their mechanical behavior does not always meet all requirements and their heat resistances are frequently unsatisfactory.
The invention is based on the object of providing a neutron-absorbing material which cannot only be processed by conventional methods but is also mechanically strong and resistant to thermal influences and has a low density.
This object is achieved by a neutron-absorbing material, in which boron is embedded in polyethylene. It is defined by a mean molecular mass, measured by viscometry, of the predominantly linear polyethylene of at least 2.5 x 106 g/mol.
ri ne~r polyethylenes having a mean molecular mass of at least 2.5 x 106 g/mol and up to 1 x 107 g/mol are also `~ ~ 3 ~ 202487 q described as ultrahigh molecular-weight polyethylenes (PE-UHMW). The molecular mass quantified above is under-stood to mean the values determined by viscometry. A
method for measuring them is described, for example, in CZ-Chemietechnik 4 (1974), 129 et seq.
The preparation of PE-UHMW is known. It can be carried out by various processes. A proven process, which is operated under low pressure with mixed catalysts of titanium(III) halides and aluminum-organic compounds, is described in German Auslegeschrift 2,361,508.
Ultrahigh molecular-weight polyethylene is distinguished by a number of advantageous physical properties. Its high wear resistance, its low coefficient of fraction against other materials, its excellent toughness behavior and its remarkable resistance to numerous chemicals should be singled out.
PE-UHMW having molecular masses of between 2.5 x Io6 g/mol and 8 x 106 g/mol, especially 3 x 106 g/mol and 6 x 106 g/mol has proven particularly suitable for the neutron-absorbing material according to the invention.
In order to ensure that no long-lived radioactive iso-topes are formed by the nuclear process taking place on neutron capture, the polyethylene must be substantially free of impurities. In particular, the compounds still present from the preparation, which were used as cata-lysts or constituents of catalysts, must not exceed a content of 200 ppm by weight, preferably 150 ppm by weight, relative to the polymer.
Furthermore, it is advisable to protect the PE-UHMW from effects of heat, light and oxidation. Examples of com-pounds, alone or in combination, which have proven suitable as stabilizers are as follows: 4,4'-thiobis-(3-methyl-6-tertiary-butyl-1-phenol), dilauryl thiodipropionate, distearyl thiodipropionate, tetrakis-[methylene-(3,5-ditertiary-butyl-4-hydroxy-hydro-cinn~m~to)]-methane, n-octadecyl-~-(4~-hydroxy-3,5~-ditertiary-butylphenyl)-propionate and glycol bis-[3~3-bis-(4'-hydroxy-3'-tertiary-butylphenyl)-butanoate~. They are in general added in quantities of from 0.1 to 0.2 %
by weight, relative to the total mixture. The addition of antioxidants is important for the reason that polyethyl-ene is oxidized in the presence of oxygen under the action of gamma-radiation. It is then transformed into low molecular-weight, waxy products, embrittles and loses its extensibility.
As a further constituent, the novel material contains boron in the form of boron compounds such as boric acid (H3BO3). Boron carbide B4C has proven particularly suit-able. Boron nitride is less suitable because of its thermal properties. Mixtures of different boron compounds can also be used, but a chemically homogeneous substance is preferred. Boron carbide is used in the commercially available purity. For use of the novel neutron-absorbing material in practice, it is essential that it is homo-geneous. It is therefore advisable to incorporate boron carbide, which is as finely dispersed as possible, into the polyethylene, i.e. boron carbide of a particle size which corresponds to the size of the polyethylene particles. It has proven advantageous to use boron carbide of a particle size of from 10 to 200 ~m and especially from 20 to 80 ~m. This has the result that no segregation of the components occurs during the process-ing of the material and no irregularities arise in itsstructure. Surprisingly, the outstanding mechanical properties of PE-UHMW are hardly impaired by the addition of boron carbide, and certain physical features, e.g. the attrition behavior, are even improved.
The boron carbide content in the novel material depends on the layer thickness in which it is used. It has been found that, in the case of thin thicknesses of material, i.e. at layer thicknesses of up to 5 mm, the screening properties are markedly improved with increasing B4C
content. At layer thicknesses above about 20 mm, an increase in the B4C concentration in the material to more than 1 %, relative to the material, hardly has any further effect on the absorption behavior. At a given degree of attenuation, the required layer thickness for absorption of thermal neutrons can therefore be deter-mined via the B4C content.
Allowing for the desired material properties, the prepar-ation and the processibility of the novel material, it is advisable to adjust the B4C concentration to values of from 5 to 50 % by weight, preferably from 10 to 40 % by weight and especially from 20 to 30 % by weight, each relative to polyethylene cont~ining boron carbide.
The neutron-absorbing material of the invention is prepared by homogeneously ~ixing the starting materials PE-UHMW, boron compound and, if desired, additives in a suitable mixer and subsequently sintering the mixture under pressure at temperatures of from 180 to 250C, especially from 200 to 230C. The sintering pressure is from 5 to 10 MPa, especially from 8 to 10 MPa. Cooling is also carried out under pressure, and 3 to 5 MPa, prefer-ably 4 to 5 MPa, have proven suitable. The sintering and cooling times depend on the thickness of the material and on the filler content. Thus, the sintering time is, for example, 5 hours for plates of 60 mm thickness, which are composed of 70 % by weight of polyethylene and 30 % by weight of B4C.
The novel material can be mechanically worked in a conventional manner, for example drilled, milled and sawn, and allowance must of course be made here for the properties of the boron carbide, in particular its hardness; it can be formed by pressing.
The invention is explained in more detail in the - 6 - 202487~
following example.
ExamPle For the irradiation tests, laboratory plates of PE-UHMW, having a molecular mass of about 3 x 106 g/mol ~R~(Hostalen GUR 412) with 1, 5, 10, 20 and 30 % by weight of boron carbide were prepared in different thicknesses of 1,5,20 and 60 mm under standard conditions (pressure on sintering 5 MPa, pressure on cooling 10 MPa, sintering and/or cooling time depending on the thickness of mater-ial and on the filler content). Unfilled PE-UHMW of the same molecular mass and in the same dimensions was used as a comparison.
-~3 The boron carbide used was the commercial product TETRABOR F 280 from Elektroschmelzwerk Kempten GmbH, ~, having a particle size of 22-59 ~m.
For preparing the laboratory plates, the particular components were homogeneously mixed in a laboratory mixer.
The samples were irradiated with thermal neutrons of an energy less than 1 eV.
The neutron absorption coefficients were calculated by the following equation:
~ tot X
o where:
X thickness of the sample ~tot total absorption coefficient which contains all the absorption components and scattering components in PE and boron.
* ~e.--~A~
Io counting rate of the neutron beam, measured before the use of every sample thickness, in order to eliminate changes in the reactor power.
I attenuated counting rate at layer thickness X of the sample.
The measured results show that, at low layer thicknesses, the screening of thermal neutrons by the novel material increases with increasing B4C content. At large layer thicknesses, B4C concentrations above about 1 % by weight (relative to the PE-UHMW filled with B4C) do not lead to any improvement in the absorption behavior. Thus, the attenuation is then independent of the boron carbide concentration.
~ - 8 - 2024879 Material Zero Counting ~tot thickness counting rate I (mm~l) (mm) rate I~ with S (particles/ sample second) (parti-cles/sec.) Hostalen GUR 1.08 5270 4172 0.2164 5.25 5302 1881 0.1974 20.1 5269 264.3 0.1489 60.2 5243 11.303 0.1020 + 1 % B4C 1.11 5263 4100 0.2250 5.24 5244 1490 0.2401 20.3 5309 73.15 0.2111 60.2 5249 1.155 0.1399 + S % B4C 1.06 5252 3654 0.3424 4.76 5237 792.7 0.3967 19.8 5229 5.497 0.3464 58.8 5284 1.021 0.1454 + 10 % B4C 1.16 5248 3029 0.4736 4.95 5276 357.1 0.5440 19.7 5236 1.573 0.4117 60.1 5298 0.919 0.1441 + 20 % B4C 1.00 5270 2484 0.7522 5.02 5257 83.44 0.8253 21.2 5225 0.986 0.4045 60.3 5247 0.862 0.1445 + 30 % B4C 1.03 5297 1904 0.9934 5.07 5202 28.87 1.0245 20.7 5231 0.966 0.4153 60.9 5298 0.856 0.1434 A comparison of the measured value shows that the absorp-tion coefficients at small thicknesses are considerablyhigher than those at thicknesses of > 20 mm. This be-havior can be explained by the fact that the thermal neutrons are almost completely absorbed in thinn~r layers and a small proportion of fast neutrons cont~ine~ in the bundle of neutron beams is braked in thicker layers of material to lower speeds and then absorbed. A substan-tially small absorption coefficient must be expected for these thermallized, originally fast neutrons.
In the table which follows, the particular material thickness, at which 95 % of the thermal neutrons are absorbed, is indicated in accordance with the equation for calculating the absorption coefficients. This cal-culation was based on the averages of the absorptioncoefficients for thin material thicknesse (s 5 mm) from the absorption measurements for each B4C content.
Layer thickness~tot (mm~l) (mm) at 95 absorption PE-UHMW 13.6 0.22 (Molecular mass:
about 4 x 106 mol/g) + 1 % B4C 12.0 0.25 + 5 % B4C 7-7 0 39 + 10 % B4C 5.7 0.53 + 20 % B4C 3.8 0.80 + 30 % B4C 3.0 1.02 It will be clearly seen that the layer thickness is considerably reduced with increasing B4C content.
In contrast to alpha- and beta-particles, neutrons do not have a charge and therefore cannot lose energy by ener-gization on passing through matter. Consequently, their penetration power is extremely high. Neutrons are subject exclusively to the action of the nuclear forces and are scattered on atomic nuclei. According to the collision laws, the energy releases to the body undergoing a collision are the greater in such scattering processes, the more similar the mass thereof is to the mass of the colliding body. Therefore, a bundle of neutron beams, which penetrates lead plates of several meters thickness without significant attenuation, is very greatly attenu-ated when passing through hydrogen-contAining substances of a few cm thickness. On average, the energy is reduced to l/e on collision with a proton, whereas the energy release to atomic nuclei of higher mass is less, due to inelastic collision. It is known from the literature that on average 18 collisions are necessary in hydrogen and on average 114 collisions are necessary in carbon in order to brake a neutron down to thermal energy. ,hese thermal, i.e. slow neutrons can then be completely absorbed by elements of high cross-section, such as cadmium or boron.
In neutron absorption, binding energy is released in the form of secondary gamma-radiation. It depends on the absorber material and can be of considerable magnitude.
Thus, the gamma-radiation energy is 6 MeV in the absorp-tion of neutrons by cadmium, 2.2 MeV in that by hydrogen and only 0.5 MeV in that by boron.
- As the materials which protect against neutron radiation, especially water and paraffins as well as plastics cont~in;ng significant quantities of hydrogen, such as polyethylene, polyesters and polyamides, are used.
Thus, according to the teaching of German Auslegeschrift 1,297,869, moldings of thermoplastic or thermosetting plastics, in which the carbon/hydrogen ratio or the residual atom/hydrogen ratio is in the range from 1 : 2.1 to 2 : 1 and the molecular weight of which is less than 200,000, are used for protection against gamma-radiation and neutron radiation. Such plastics can be from the classes of high- and low-pressure polyethylenes, poly-propylenes, alkylene/propylene or alkylene/butylenecopolymers, polyamides and polyesters.
In German Auslegeschrift 1,162,694, a neutron-absorbing material is described, in which granulated polyethylene is embedded in a hydrogen-cont~ining liquid which remains liquid or cures to give a plastic.
However, the known neutron-absorbing materials have properties which restrict their applicability. Thus, although plastics have a low density, their processibil-ity frequently causes difficulties. Moreover, their mechanical behavior does not always meet all requirements and their heat resistances are frequently unsatisfactory.
The invention is based on the object of providing a neutron-absorbing material which cannot only be processed by conventional methods but is also mechanically strong and resistant to thermal influences and has a low density.
This object is achieved by a neutron-absorbing material, in which boron is embedded in polyethylene. It is defined by a mean molecular mass, measured by viscometry, of the predominantly linear polyethylene of at least 2.5 x 106 g/mol.
ri ne~r polyethylenes having a mean molecular mass of at least 2.5 x 106 g/mol and up to 1 x 107 g/mol are also `~ ~ 3 ~ 202487 q described as ultrahigh molecular-weight polyethylenes (PE-UHMW). The molecular mass quantified above is under-stood to mean the values determined by viscometry. A
method for measuring them is described, for example, in CZ-Chemietechnik 4 (1974), 129 et seq.
The preparation of PE-UHMW is known. It can be carried out by various processes. A proven process, which is operated under low pressure with mixed catalysts of titanium(III) halides and aluminum-organic compounds, is described in German Auslegeschrift 2,361,508.
Ultrahigh molecular-weight polyethylene is distinguished by a number of advantageous physical properties. Its high wear resistance, its low coefficient of fraction against other materials, its excellent toughness behavior and its remarkable resistance to numerous chemicals should be singled out.
PE-UHMW having molecular masses of between 2.5 x Io6 g/mol and 8 x 106 g/mol, especially 3 x 106 g/mol and 6 x 106 g/mol has proven particularly suitable for the neutron-absorbing material according to the invention.
In order to ensure that no long-lived radioactive iso-topes are formed by the nuclear process taking place on neutron capture, the polyethylene must be substantially free of impurities. In particular, the compounds still present from the preparation, which were used as cata-lysts or constituents of catalysts, must not exceed a content of 200 ppm by weight, preferably 150 ppm by weight, relative to the polymer.
Furthermore, it is advisable to protect the PE-UHMW from effects of heat, light and oxidation. Examples of com-pounds, alone or in combination, which have proven suitable as stabilizers are as follows: 4,4'-thiobis-(3-methyl-6-tertiary-butyl-1-phenol), dilauryl thiodipropionate, distearyl thiodipropionate, tetrakis-[methylene-(3,5-ditertiary-butyl-4-hydroxy-hydro-cinn~m~to)]-methane, n-octadecyl-~-(4~-hydroxy-3,5~-ditertiary-butylphenyl)-propionate and glycol bis-[3~3-bis-(4'-hydroxy-3'-tertiary-butylphenyl)-butanoate~. They are in general added in quantities of from 0.1 to 0.2 %
by weight, relative to the total mixture. The addition of antioxidants is important for the reason that polyethyl-ene is oxidized in the presence of oxygen under the action of gamma-radiation. It is then transformed into low molecular-weight, waxy products, embrittles and loses its extensibility.
As a further constituent, the novel material contains boron in the form of boron compounds such as boric acid (H3BO3). Boron carbide B4C has proven particularly suit-able. Boron nitride is less suitable because of its thermal properties. Mixtures of different boron compounds can also be used, but a chemically homogeneous substance is preferred. Boron carbide is used in the commercially available purity. For use of the novel neutron-absorbing material in practice, it is essential that it is homo-geneous. It is therefore advisable to incorporate boron carbide, which is as finely dispersed as possible, into the polyethylene, i.e. boron carbide of a particle size which corresponds to the size of the polyethylene particles. It has proven advantageous to use boron carbide of a particle size of from 10 to 200 ~m and especially from 20 to 80 ~m. This has the result that no segregation of the components occurs during the process-ing of the material and no irregularities arise in itsstructure. Surprisingly, the outstanding mechanical properties of PE-UHMW are hardly impaired by the addition of boron carbide, and certain physical features, e.g. the attrition behavior, are even improved.
The boron carbide content in the novel material depends on the layer thickness in which it is used. It has been found that, in the case of thin thicknesses of material, i.e. at layer thicknesses of up to 5 mm, the screening properties are markedly improved with increasing B4C
content. At layer thicknesses above about 20 mm, an increase in the B4C concentration in the material to more than 1 %, relative to the material, hardly has any further effect on the absorption behavior. At a given degree of attenuation, the required layer thickness for absorption of thermal neutrons can therefore be deter-mined via the B4C content.
Allowing for the desired material properties, the prepar-ation and the processibility of the novel material, it is advisable to adjust the B4C concentration to values of from 5 to 50 % by weight, preferably from 10 to 40 % by weight and especially from 20 to 30 % by weight, each relative to polyethylene cont~ining boron carbide.
The neutron-absorbing material of the invention is prepared by homogeneously ~ixing the starting materials PE-UHMW, boron compound and, if desired, additives in a suitable mixer and subsequently sintering the mixture under pressure at temperatures of from 180 to 250C, especially from 200 to 230C. The sintering pressure is from 5 to 10 MPa, especially from 8 to 10 MPa. Cooling is also carried out under pressure, and 3 to 5 MPa, prefer-ably 4 to 5 MPa, have proven suitable. The sintering and cooling times depend on the thickness of the material and on the filler content. Thus, the sintering time is, for example, 5 hours for plates of 60 mm thickness, which are composed of 70 % by weight of polyethylene and 30 % by weight of B4C.
The novel material can be mechanically worked in a conventional manner, for example drilled, milled and sawn, and allowance must of course be made here for the properties of the boron carbide, in particular its hardness; it can be formed by pressing.
The invention is explained in more detail in the - 6 - 202487~
following example.
ExamPle For the irradiation tests, laboratory plates of PE-UHMW, having a molecular mass of about 3 x 106 g/mol ~R~(Hostalen GUR 412) with 1, 5, 10, 20 and 30 % by weight of boron carbide were prepared in different thicknesses of 1,5,20 and 60 mm under standard conditions (pressure on sintering 5 MPa, pressure on cooling 10 MPa, sintering and/or cooling time depending on the thickness of mater-ial and on the filler content). Unfilled PE-UHMW of the same molecular mass and in the same dimensions was used as a comparison.
-~3 The boron carbide used was the commercial product TETRABOR F 280 from Elektroschmelzwerk Kempten GmbH, ~, having a particle size of 22-59 ~m.
For preparing the laboratory plates, the particular components were homogeneously mixed in a laboratory mixer.
The samples were irradiated with thermal neutrons of an energy less than 1 eV.
The neutron absorption coefficients were calculated by the following equation:
~ tot X
o where:
X thickness of the sample ~tot total absorption coefficient which contains all the absorption components and scattering components in PE and boron.
* ~e.--~A~
Io counting rate of the neutron beam, measured before the use of every sample thickness, in order to eliminate changes in the reactor power.
I attenuated counting rate at layer thickness X of the sample.
The measured results show that, at low layer thicknesses, the screening of thermal neutrons by the novel material increases with increasing B4C content. At large layer thicknesses, B4C concentrations above about 1 % by weight (relative to the PE-UHMW filled with B4C) do not lead to any improvement in the absorption behavior. Thus, the attenuation is then independent of the boron carbide concentration.
~ - 8 - 2024879 Material Zero Counting ~tot thickness counting rate I (mm~l) (mm) rate I~ with S (particles/ sample second) (parti-cles/sec.) Hostalen GUR 1.08 5270 4172 0.2164 5.25 5302 1881 0.1974 20.1 5269 264.3 0.1489 60.2 5243 11.303 0.1020 + 1 % B4C 1.11 5263 4100 0.2250 5.24 5244 1490 0.2401 20.3 5309 73.15 0.2111 60.2 5249 1.155 0.1399 + S % B4C 1.06 5252 3654 0.3424 4.76 5237 792.7 0.3967 19.8 5229 5.497 0.3464 58.8 5284 1.021 0.1454 + 10 % B4C 1.16 5248 3029 0.4736 4.95 5276 357.1 0.5440 19.7 5236 1.573 0.4117 60.1 5298 0.919 0.1441 + 20 % B4C 1.00 5270 2484 0.7522 5.02 5257 83.44 0.8253 21.2 5225 0.986 0.4045 60.3 5247 0.862 0.1445 + 30 % B4C 1.03 5297 1904 0.9934 5.07 5202 28.87 1.0245 20.7 5231 0.966 0.4153 60.9 5298 0.856 0.1434 A comparison of the measured value shows that the absorp-tion coefficients at small thicknesses are considerablyhigher than those at thicknesses of > 20 mm. This be-havior can be explained by the fact that the thermal neutrons are almost completely absorbed in thinn~r layers and a small proportion of fast neutrons cont~ine~ in the bundle of neutron beams is braked in thicker layers of material to lower speeds and then absorbed. A substan-tially small absorption coefficient must be expected for these thermallized, originally fast neutrons.
In the table which follows, the particular material thickness, at which 95 % of the thermal neutrons are absorbed, is indicated in accordance with the equation for calculating the absorption coefficients. This cal-culation was based on the averages of the absorptioncoefficients for thin material thicknesse (s 5 mm) from the absorption measurements for each B4C content.
Layer thickness~tot (mm~l) (mm) at 95 absorption PE-UHMW 13.6 0.22 (Molecular mass:
about 4 x 106 mol/g) + 1 % B4C 12.0 0.25 + 5 % B4C 7-7 0 39 + 10 % B4C 5.7 0.53 + 20 % B4C 3.8 0.80 + 30 % B4C 3.0 1.02 It will be clearly seen that the layer thickness is considerably reduced with increasing B4C content.
Claims (14)
1. A neutron-absorbing material comprising boron embedded in predominantly linear polyethylene, wherein the mean molecular mass, measured by viscometry, of the predominantly linear polyethylene is at least 2.5 x 106 g/mol.
2. A neutron-absorbing material as claimed in claim 1, wherein the molecular mass of the polyethylene is 2.5 x 106 to 8 x 106 g/mol.
3. A neutron-absorbing material as claimed in claim 1, wherein the molecular mass of the polyethylene is 3 x 106 to 6 x 106 g/mol.
4. A neutron-absorbing material as claimed in claim 1 wherein the polyethylene contains boron in the form of boron carbide B4C.
5. A neutron-absorbing material as claimed in claim 2 wherein the polyethylene contains boron in the form of boron carbide B4C.
6. A neutron-absorbing material as claimed in claim 4 wherein the boron carbide has a particle size of from 10 to 200 µm.
7. A neutron-absorbing material as claimed in claim 5 wherein the boron carbide has a particle size of from 20 to 80 µm.
8. A neutron-absorbing material as claimed in any one of claims 1 to 3, wherein the polyethylene contains boron in the form of boron carbide B4C and the concentration of boron carbide is 5 to 50% by weight relative to the neutron-absorbing material.
9. A neutron-absorbing material as claimed in any one of claims 1 to 3, wherein the polyethylene contains boron in the form of boron carbide B4C and the concentration of boron carbide is 10 to 40% by weight relative to the neutron-absorbing material.
10. A neutron-absorbing material as claimed in any one of claims 1 to 3, wherein the polyethylene contains boron in the form of boron carbide B4C and the concentration of boron carbide is 20 to 30% by weight relative to the neutron-absorbing material.
11. A neutron-absorbing material as claimed in claim 8 which contains a stabilizer.
12. A neutron-absorbing material as claimed in claim 10 which contains a stabilizer.
13. A process for preparing a neutron-absorbing material as claimed in any one of claims 1 to 7, which comprises mixing polyethylene and boron compound, sintering the mixture under pressure at a temperature of from 180 to 250°C, at a pressure of from 5 to 10 MPa, and cooling the sintered product under a pressure of from 3 to 5 MPa.
14. A process according to claim 13 wherein the mixture is sintered at a temperature from 200 to 230°C at a pressure of 8 to 10 MPa and the sintered product is cooled at a pressure of from 4 to 5 MPa.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DEP3930887.1 | 1989-09-15 | ||
| DE3930887A DE3930887A1 (en) | 1989-09-15 | 1989-09-15 | NEUTRON ABSORBENT MATERIAL |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2024879A1 CA2024879A1 (en) | 1991-03-16 |
| CA2024879C true CA2024879C (en) | 1995-02-14 |
Family
ID=6389532
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002024879A Expired - Fee Related CA2024879C (en) | 1989-09-15 | 1990-09-07 | Neutron-absorbing materials |
Country Status (7)
| Country | Link |
|---|---|
| EP (1) | EP0417598B1 (en) |
| JP (1) | JPH03107797A (en) |
| KR (1) | KR930011108B1 (en) |
| AT (1) | ATE120300T1 (en) |
| CA (1) | CA2024879C (en) |
| DE (2) | DE3930887A1 (en) |
| ES (1) | ES2071719T3 (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE4131274C2 (en) * | 1991-09-20 | 1995-05-11 | Braas Gmbh | Neutron absorbing plastic sheet |
| KR100395886B1 (en) * | 1999-10-29 | 2003-08-27 | 한국수력원자력 주식회사 | Ultra-Low Level Shielder of External Radiation for a Gamma-ray Spectroscopy System |
| JP5609048B2 (en) * | 2009-09-09 | 2014-10-22 | 独立行政法人日本原子力研究開発機構 | Neutron beam absorbing sheet |
| JP2013205359A (en) * | 2012-03-29 | 2013-10-07 | Toshiba Corp | Gelatinous neutron absorber and molten core recovery method |
| CN110767340B (en) * | 2019-10-10 | 2023-03-03 | 中广核研究院有限公司 | Novel composite shielding material of tungsten boron crosslinked polyethylene with high tungsten content |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1337399A (en) * | 1962-07-30 | 1963-09-13 | Fonderie De Gentilly | Protective material against ionizing radiation |
| DE1297869B (en) * | 1963-02-08 | 1969-06-19 | Huels Chemische Werke Ag | Manufacture of molded bodies for protection against gamma and neutron radiation |
| US4123392A (en) * | 1972-04-13 | 1978-10-31 | Chemtree Corporation | Non-combustible nuclear radiation shields with high hydrogen content |
| DE3225788A1 (en) * | 1982-05-07 | 1984-01-12 | Selim Dipl.-Ing. 8000 München Mourad | Building material and process for the manufacture thereof |
| JPS58201099A (en) * | 1982-05-20 | 1983-11-22 | 電気化学工業株式会社 | Structure for neutron shield |
| JPS60194394A (en) * | 1984-03-15 | 1985-10-02 | 三井化学株式会社 | Shielding material for neutron |
-
1989
- 1989-09-15 DE DE3930887A patent/DE3930887A1/en not_active Withdrawn
-
1990
- 1990-08-24 KR KR1019900013149A patent/KR930011108B1/en not_active Expired - Fee Related
- 1990-09-04 ES ES90116919T patent/ES2071719T3/en not_active Expired - Lifetime
- 1990-09-04 EP EP90116919A patent/EP0417598B1/en not_active Expired - Lifetime
- 1990-09-04 DE DE59008749T patent/DE59008749D1/en not_active Expired - Fee Related
- 1990-09-04 AT AT90116919T patent/ATE120300T1/en not_active IP Right Cessation
- 1990-09-07 CA CA002024879A patent/CA2024879C/en not_active Expired - Fee Related
- 1990-09-07 JP JP2235948A patent/JPH03107797A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0417598A1 (en) | 1991-03-20 |
| DE59008749D1 (en) | 1995-04-27 |
| ATE120300T1 (en) | 1995-04-15 |
| JPH03107797A (en) | 1991-05-08 |
| KR910007003A (en) | 1991-04-30 |
| ES2071719T3 (en) | 1995-07-01 |
| EP0417598B1 (en) | 1995-03-22 |
| CA2024879A1 (en) | 1991-03-16 |
| DE3930887A1 (en) | 1991-03-28 |
| KR930011108B1 (en) | 1993-11-24 |
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