Comprehensive nuclear materials 2 11 neutron reflector materials (be, hydrides)

Comprehensive nuclear materials 2 11 neutron reflector materials (be, hydrides) Comprehensive nuclear materials 2 11 neutron reflector materials (be, hydrides) Comprehensive nuclear materials 2 11 neutron reflector materials (be, hydrides) Comprehensive nuclear materials 2 11 neutron reflector materials (be, hydrides) Comprehensive nuclear materials 2 11 neutron reflector materials (be, hydrides) Comprehensive nuclear materials 2 11 neutron reflector materials (be, hydrides)

2.11 Neutron Reflector Materials (Be, Hydrides) S Yamanaka and K Kurosaki Osaka University, Suita, Japan ß 2012 Elsevier Ltd All rights reserved 2.11.1 Required Properties 307 2.11.2 2.11.2.1 2.11.2.2 2.11.2.3 2.11.3 2.11.3.1 2.11.3.2 2.11.3.3 2.11.3.4 2.11.3.5 2.11.3.6 2.11.3.7 2.11.3.8 2.11.3.9 Beryllium Introduction Production and Processing Methods1 Basic Properties Fundamental Properties of Metal Hydrides Introduction Production of Zirconium Hydride20 Lattice Parameter20 Elastic Modulus and Hardness20,21 Electronic Structure22 Electrical Conductivity22 Heat Capacity20 Thermal Conductivity of Metal Hydrides23 Comparison of Thermal Conductivity of Zirconium Hydride with those of the Hydrides of Titanium and Yttrium Conclusion Summary 308 308 308 308 312 312 312 314 314 315 316 317 317 2.11.3.10 2.11.4 References Abbreviations DOS Density of state MO Molecular orbital Symbols B D E G H Hv Tm Ttr l lel Bulk modulus Diffusivity Young’s modulus Shear modulus Enthalpy Vickers hardness Melting temperature Transformation temperature Thermal conductivity Electronic contribution to thermal conductivity Lattice thermal conductivity llat r Electrical resistivity s Electrical conductivity DmH Enthalpy of fusion DtrsH Enthalpy of transition 318 320 321 321 2.11.1 Required Properties Two highly desirable properties of both neutron reflectors and moderators are efficient neutron slowing and low neutron absorption The first requires effective slowing of neutrons over short distances, thus reducing the required volume of the reflector or moderator in the reactor core Moreover, in a reactor core of a given shape and volume, this reduces the leakage of neutrons in the course of their slowing For reflectors in particular, the key requirements include a high reflectivity, a large macroscopic crosssection, and efficient neutron slowing The reflectivity of a material is inversely proportional to its diffusion ratio (D/L), which is the ratio of its diffusivity (D) to its diffusion length (L) This ratio is generally considered to decrease as scattering becomes large in comparison with absorption It is essential, moreover, to obtain high reflectivity without excessive thickness, and for this purpose, to use a material with a large macroscopic total cross-section In a thermal reactor, the performance of the reflector is enhanced if it does not simply reflect the neutrons but rather slows and then reflects them, 307 308 Neutron Reflector Materials (Be, Hydrides) and for this reason, the same material is often used as both reflector and moderator In general, materials whose nuclides have low mass number and neutron absorption may be used as moderators and reflectors The most commonly used materials are light water (H2O), heavy water (D2O), and graphite (C) In addition, hydrocarbons, zirconium hydride, and other such materials are often used as moderators Heavy water is particularly effective because of its very low absorption level Graphite is second to heavy water in its low absorption level, is lower in cost, and has the added advantage of suitability for use at high temperatures Beryllium is generally used as a reflector rather than as a moderator In addition to the aforementioned materials, there exist other candidates as neutron reflectors For example, Commissariat a` l’Energie Atomique (CEA) is studying zirconium silicide as the reflector for next generation reactors.1 Tungsten carbide has also been used as neutron reflectors (http://en.wikipedia.org/ wiki/Tungsten_carbide) For fusion reactors, various materials such as titanium carbide and boron carbide are considered as reflectors.2 This chapter outlines the basic properties of beryllium and zirconium hydride that are fundamental to their utilization as neutron reflectors and moderators in nuclear reactors 2.11.2 Beryllium 2.11.2.1 Introduction Apart from its use as a neutron reflector and moderator in nuclear reactors, beryllium is in strong demand for use in X-ray windows of medical and industrial equipment, acoustic speaker diaphragms, galvano mirrors for laser drilling, reflected electron guard plates in semiconductor production equipment, and various other applications It is also widely used in the electrical and electronic industry, particularly in beryllium– copper alloys for wrought metal production and for molds and other forging tools and dies In electronics, in particular, the need for beryllium has been growing rapidly in recent years with the trend toward lighter, thinner, and smaller electronic components In the following sections, we outline the methods of its production and processing and discuss its basic properties 10–14% beryllium oxide (BeO) At present, the two main industrial processes used to extract BeO from beryl are the fluoride method and the sulfuric acid method Both of these yield BeO of industrial-grade purity, which is used as a raw material for Be–Cu mother alloys, electronics manufacture, refractories, and other fields of application For use in nuclear reactors, BeO is further purified by recrystallization or precipitation Metallic beryllium (Be) is produced from BeO or Be(OH)2 by either of two industrial processes One involves the formation of BeF2 followed by its thermal reduction with Mg to produce Be pebbles, and the other involves the formation of BeCl2 followed by its electrolysis to produce Be flakes The resulting pebbles and flakes are high in Mg and Cl2 content, respectively, and these impurities are removed by vacuum melting The principal techniques of Be processing are molding by powder metallurgy, warm or hot working, and joining or welding In hot-press sintering, which has been widely developed for Be molding, the starting material is commonly À200 mesh Be powder, which is inserted between graphite dies and then pressure molded in vacuum at high temperatures (1323 K) The resulting moldings are commonly called ‘hot-press blocks,’ and can be obtained with high integrity and near theoretical density Other molding methods that may be employed include spark plasma sintering and cold-press sintering Cold working of Be at room temperature is extremely difficult because of its low elongation, and it is accordingly formed into plates, rods, or tubes by ‘warm working’ at 773–1173 K or ‘hot working’ at 1273–1373 K In either case, the Be must be covered with mild steel or some other material and the intervening air withdrawn before it is heated, as it readily oxidizes at high temperatures Various methods have been developed for Be joining and welding These include mechanical joining and resin bonding, electron-beam and diffusion welding, and brazing and soldering Because of its high oxygen affinity, however, any process in which the Be is heated must be performed under an appropriate inert gas or vacuum 2.11.2.3 2.11.2.2 Production and Processing Methods1 Among the 30 or so naturally occurring ores, the most economically important is beryl, which contains Basic Properties The crystal structure of Be is closed-packed hexagonal with a c/a ratio of 1.5671 and lattice parameters a ¼ 0.22866 nm and c ¼ 0.35833 nm.3 Table shows the basic properties of Be.4,5 It weighs only about Neutron Reflector Materials (Be, Hydrides) two-thirds as much as aluminum (Al), and both its melting point and its specific heat capacity are quite high for a light metal It is widely known for its high Young’s modulus and other elastic coefficients Its nucleus is small in neutron absorption cross-section and relatively large in scattering cross-section, both of which are advantageous for use as a moderator or reflector Its superior high-temperature dynamical Table 309 properties are also advantageous for use in nuclear reactors It emits neutrons under g-ray irradiation and can thus be used as a neutron source Its soft X-ray absorption is less than one-tenth that of Al, making it highly effective as a material for X-ray tube windows Figure shows the temperature dependence of the specific heat capacities of various Be samples.3 The following equations describing the specific heat capacity of Be are reported.3 CP ẳ 11:8 ỵ 9:12 103 T Basic properties of Be Crystal structure Density (near room temperature) (g cmÀ3) Melting point (K) Boiling point (K) Heat of fusion (kJ molÀ1) Heat of vaporization (kJ molÀ1) Heat capacity (302 K) (J KÀ1 molÀ1) Thermal conductivity (300 K) (W mÀ1 KÀ1) Thermal expansion coefficient (302 K) (KÀ1) Speed of sound (room temperature) (m sÀ1) Young’s modulus (GPa) Shear modulus (GPa) Bulk modulus (GPa) Poisson ratio Vickers hardness (GPa) Scattering cross-section (barn) Absorption cross-section (barn) Moderating ratio Diffusion ratio Hexagonal 1.85 1560 2742 7.895 297 16.443 200 11.3  10À6 12 870 287 132 130 0.032 1.67 0.009 0.0597 0.0273 Source: Genshiryoku Zairyou Handbook; The Nikkan Kogyo Shimbun: Tokyo, 1952; http://en.wikipedia.org/wiki/Beryllium; Rare Metals Handbook, 2nd ed.; Reinhold: New York, NY, 1961 ð J KÀ1 gÀ1 atom; from 600 to 1560 KÞ CP ẳ 25:4 ỵ 2:15 103 T J K1 gÀ1 atom; from 1560 to 2200 KÞ Temperature dependences of the thermal expansion coefficient and the electrical resistivity of Be3 are given in Figures and 3, respectively Figure shows the temperature dependence of the thermal conductivities of various Be samples.3,6 Be exhibits relatively high thermal conductivity values around 200 W mÀ1 KÀ1 at room temperature, and the values decrease with temperature The effect of high-dose neutron irradiation on the thermal conductibility of Be has been investigated.7,8 It is reported by Chakin et al.7 that neutron irradiation at 303 K to a neutron fluence of  1022 cmÀ2 (E > 0.1 MeV) leads to sharp decrease of thermal conductivity, in particular at 303 K, the thermal conductivity decreases by a factor Specific heat capacity, CP (J K-1 g-1) 0 200 400 600 800 Temperature, T (K) 1000 1200 Figure Temperature dependence of the specific heat capacity of various Be samples Different marks mean different samples Reproduced from Beeston, J M Nucl Eng Des 1970, 14, 445 310 Neutron Reflector Materials (Be, Hydrides) Thermal expansion coefficient (´10-6 K-1) 70 60 50 Volume 40 30 Linear, perpendicular to hexagonal axis 20 10 Linear, parallel to hexagonal axis 400 600 800 1000 Temperature, T (K) 1200 1400 Figure Temperature dependence of the thermal expansion coefficient of Be Reproduced from Beeston, J M Nucl Eng Des 1970, 14, 445 35 Electrical resistivity, r (μΩ cm) 30 25 20 15 10 200 400 600 800 Temperature, T (K) 1000 1200 Figure Temperature dependence of the electrical resistivity of Be Different marks mean different samples Reproduced from Beeston, J M Nucl Eng Des 1970, 14, 445 of five, but short-term high-temperature annealing (773 K for h) leads to partial recovery of the thermal conductivity In addition to the data listed in Table 1, the thermodynamic properties of Be have been reported recently,9 in which the temperatures of transformation Ttr and melting Tm, and the enthalpies of transformation DtrH and melting DmH are measured by difference thermal analysis and by anisothermal calorimetry It is reported by Kleykamp9 that the results for hcp–bcc transformation of Be are Ttr ẳ 1542 ặ K and DtrH ẳ 6.1 ặ 0.5 kJ mol1 and those for the melting process are Tm ẳ 1556 ặ K and DmH ẳ 7.2 ặ 0.5 kJ mol1 A fine, transparent BeO film of about 10À6 cm thickness forms on Be in air, and it therefore retains Neutron Reflector Materials (Be, Hydrides) 311 Thermal conductivity, l (W m-1 K-1) 250 200 150 100 50 400 600 800 1000 Temperature, T (K) 1200 1400 Figure Temperature dependence of the thermal conductivities of various Be samples Different marks mean different samples Adapted from Beeston, J M Nucl Eng Des 1970, 14, 445; Chirkin, V S Trans Atom Ener 1966, 20, 107 its metallic gloss when left standing This results in its passivation in dry oxygen at up to 923 K, but the oxidized film breaks down at temperatures above about 1023 K and it thus becomes subject to progressive oxidation.10 It reacts with nitrogen at 1173 K or higher, forming Be2N3, and with NH3 at lower temperatures.10 Be undergoes passivation in dry CO2 at up to 973 K, but only up to 873 K in moist CO2.11,12 Its resistance to corrosion by water varies with temperature, dissolved ion content, pH, and other factors; it is reportedly poor in water containing Cl (110 ppm), SO42 (515 ppm), Cu2ỵ (0.15 ppm), Fe2ỵ (1–10 ppm), or other such ions.10 Among the various compounds formed by Be, BeO and Be2C may be taken as typical The basic properties of BeO are shown in Table 2.4 Its melting point and thermal conductivity are both high,13 its heat shock resistance is excellent, its thermal neutron absorption cross-section is small, and its corrosion resistance to CO2 at high temperatures is also excellent Be2C is formed by reaction of Be or BeO with C Its basic properties are density, 2.44 g cmÀ3; specific heat capacity, 41.47 J KÀ1 molÀ1 (303–373 K); thermal expansion coefficient, 10.5  10À6 KÀ1 (298–873 K); and electric resistivity, 0.063 O m (303 K) It is reportedly unstable in moist air.10 Intrinsically, BeO is an excellent moderator and reflector material in nuclear reactors Various utilizations of BeO in reactors14 and behavior of BeO under neutron irradiation have been reported.15 Especially, Table Basic properties of BeO Crystal structure Density (near room temperature) (g cmÀ3) Melting point (K) Boiling point (K) Thermal conductivity (293 K) (W mÀ1KÀ1) Thermal expansion coefficient (293–373 K) (KÀ1) Electrical resistivity (1273 K) (O cm) Scattering cross-section (barn) Absorption cross-section (barn) Moderating ratio Diffusion ratio Hexagonal wurtzite 3.02 2780 4173 281 5.5  10À6 8.0  107 9.8 0.0092 0.0706 0.0273 Source: Genshiryoku Zairyou Handbook; The Nikkan Kogyo Shimbun: Tokyo, 1952; Gregg, S J.; et al J Nucl Mater 1961, 4, 46 the effect of neutron irradiation on the thermal conductivity of BeO has been widely studied.16,17 Figure shows the temperature dependence of the thermal conductivity of unirradiated and irradiated BeO.17 It is observed that irradiation of BeO with neutrons considerably reduces the thermal conductivity It has also been reported that the irradiation-induced change in thermal conductivity can be removed by thermal annealing, but complete recovery is not achieved until an annealing temperature of 1473 K is reached One further important property of Be that must be noted is its high toxicity The effect of Be dust, vapor, and soluble solutes varies among individuals, 312 Neutron Reflector Materials (Be, Hydrides) Thermal conductivity, l (W m-1 K-1) 300 250 Unirradiated 1.2 ´ 1020 nvt 1.5 ´ 1019 nvt 4.0 ´ 1020 nvt 200 150 100 50 260 280 300 320 Temperature, T (K) 340 360 Figure Temperature dependence of the thermal conductivity of unirradiated and irradiated BeO Reproduced from Pryor, A W.; et al J Nucl Mater 1964, 14, 208 but exposure may cause dermatitis and contact or absorption by mucous membrane or respiratory tract may result in chronic beryllium disease, or ‘berylliosis.’ Maximum permissible concentrations in air were established in 1948 and include an 8-h average concentration of mg mÀ3, a peak concentration of 25 mg mÀ3 in plants, and a peak concentration of 0.01 mg mÀ3 in plant vicinities.18 In relation to workplace health and safety, particular care is necessary in the control of fine powder generated during molding and mechanical processing Dust collectors must be installed at the points of generation, and dust-proof masks, dustproof goggles, and other protective gear must be worn during work In Japan, Be is subject to the Ordinance on Prevention of Hazards due to Specified Chemical Substances 2.11.3 Fundamental Properties of Metal Hydrides 2.11.3.1 Introduction Zirconium hydride is used as a material for neutron reflectors in fast reactors The evaluation of the thermal conductivity, elastic modulus, and other basic properties of zirconium hydride is extremely important for assessing the safety and cost-effectiveness of nuclear reactors Metal hydrides, of which zirconium hydride is a typical example, are also very interesting because they exhibit unique properties and shed light on some fundamental aspects of physics As part of work on metals such as zirconium, Yanamana et al have successfully created crack-free, bulk-scale metal hydrides, and systematically investigated their fundamental properties – particularly at high temperatures Here, we present an outline of the results on the fundamental properties of zirconium hydride Figure shows the zirconium–hydrogen binary phase diagram.19 2.11.3.2 Production of Zirconium Hydride20 We used polycrystalline (grain size: 20–50 mm) ingots of high-purity zirconium as the starting material for producing hydrides The main impurities present in the zirconium were O (0.25 wt%), H (0.0006 wt%), N (0.0024 wt%), C (0.003 wt%), Fe (0.006 wt%), and Cr (0.008 wt%) The hydride was generated with high-purity hydrogen gas (7 N) at a prescribed pressure, using an advanced ultra-high vacuum Sieverts instrument Details of the instrument configuration are given in Figure The procedure for synthesizing hydrides varies according to the type of metal This is due to the phase transition, from metal to hydride that is accompanied by a massive increase in volume due to hydrogenation, and to differences in the strength of the hydride Figure shows the external appearance of zirconium hydride substances produced by the author’s group Neutron Reflector Materials (Be, Hydrides) 0.2 0.4 Weight percent hydrogen 0.6 0.8 313 1.2 1.4 1.6 1.8 1000 900 863 ЊC 800 (b-Zr) Temperature (ЊC) 700 600 δ (a-Zr) 550 ЊC ~37.5 5.93 56.7 ε 500 400 300 200 100 10 Zr 20 30 40 50 Atomic percent hydrogen 60 70 80 Figure Binary phase diagram of the zirconium–hydrogen system d and e represent the face-centered cubic d-phase hydride and the face-centered tetragonal e-phase hydride, respectively Adapted from Zuzek, E.; Abriata, J P.; San-Martin, A.; Manchester, F D Bull Alloy Phase Diagrams 1990, 11(4), 385–395 13 11 14 12 : 15 10 11 Absolute capacitance manometer (25 ktorr) Absolute capacitance manometer (1 ktorr) Absolute capacitance manometer (10 torr) Calibrated vessel (~50 ml) Calibrated vessel (~500 ml) Reactor for high pressure (steel) Mantle heater (