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)
Trang 1S Yamanaka and K Kurosaki
Osaka University, Suita, Japan
ß 2012 Elsevier Ltd All rights reserved.
2.11.3.9 Comparison of Thermal Conductivity of Zirconium Hydride with those of the
Abbreviations
DOS Density of state
Symbols
conductivity
2.11.1 Required Properties Two highly desirable properties of both neutron reflectors and moderators are efficient neutron slow-ing 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 cross-section, 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
Trang 2and 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
mod-erators and reflectors The most commonly used
and graphite (C) In addition, hydrocarbons,
zirco-nium hydride, and other such materials are often
used as moderators Heavy water is particularly
effec-tive 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
suitabil-ity for use at high temperatures Beryllium is
gener-ally used as a reflector rather than as a moderator
In addition to the aforementioned materials, there
exist other candidates as neutron reflectors For
exam-ple, Commissariat a` l’Energie Atomique (CEA) is
studying zirconium silicide as the reflector for next
used as neutron reflectors (http://en.wikipedia.org/
wiki/Tungsten_carbide) For fusion reactors, various
materials such as titanium carbide and boron carbide
This chapter outlines the basic properties of
be-ryllium and zirconium hydride that are fundamental
to their utilization as neutron reflectors and
modera-tors in nuclear reacmodera-tors
2.11.2 Beryllium
Apart from its use as a neutron reflector and
modera-tor in nuclear reacmodera-tors, beryllium is in strong demand
for use in X-ray windows of medical and industrial
equipment, acoustic speaker diaphragms, galvano
mir-rors for laser drilling, reflected electron guard plates in
semiconductor production equipment, and various
other applications It is also widely used in the
electri-cal 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
pro-duction and processing and discuss its basic properties
Among the 30 or so naturally occurring ores, the most
economically important is beryl, which contains
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
ther-mal reduction with Mg to produce Be pebbles, and
its electrolysis to produce Be flakes The resulting
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
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 work-ing’ 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 join-ing and weldjoin-ing These include mechanical joinjoin-ing and resin bonding, electron-beam and diffusion weld-ing, 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
The crystal structure of Be is closed-packed
Trang 3two-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
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 1 shows the temperature dependence of
The following equations describing the specific heat
Temperature dependences of the thermal expansion
shows the temperature dependence of the thermal
relatively high thermal conductivity values around
decrease with temperature The effect of high-dose neutron irradiation on the thermal conductibility of
that neutron irradiation at 303 K to a neutron
decrease of thermal conductivity, in particular at
303 K, the thermal conductivity decreases by a factor
0 1 2 3 4
-1 g
-1 )
Temperature, T (K)
Figure 1 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.
Table 1 Basic properties of Be
Density (near room temperature) (g cm3) 1.85
Heat of fusion (kJ mol1) 7.895
Heat of vaporization (kJ mol1) 297
Heat capacity (302 K) (J K1mol1) 16.443
Thermal conductivity (300 K) (W m1K1) 200
Thermal expansion coefficient (302 K) (K1) 11.3 10 6
Speed of sound (room temperature) (m s1) 12 870
Vickers hardness (GPa) 1.67
Scattering cross-section (barn) 6
Absorption cross-section (barn) 0.009
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.
Trang 4of five, but short-term high-temperature annealing
(773 K for 3 h) leads to partial recovery of the thermal
conductivity
thermodynamic properties of Be have been reported
by difference thermal analysis and by anisothermal
results for hcp–bcc transformation of Be are
thickness forms on Be in air, and it therefore retains
0 5 10 15 20 25 30 35
Temperature, T (K)
Figure 3 Temperature dependence of the electrical resistivity of Be Different marks mean different samples.
Reproduced from Beeston, J M Nucl Eng Des 1970, 14, 445.
0 10 20 30 40 50 60 70
Linear, parallel to hexagonal axis Linear, perpendicular to hexagonal axis
Volume
-6 K
-1 )
Temperature, T (K)
Figure 2 Temperature dependence of the thermal expansion coefficient of Be Reproduced from Beeston, J M Nucl Eng Des 1970, 14, 445.
Trang 5its 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
Its resistance to corrosion by water varies with
tem-perature, dissolved ion content, pH, and other factors;
Among the various compounds formed by Be, BeO
resistance is excellent, its thermal neutron absorption
cross-section is small, and its corrosion resistance to
formed by reaction of Be or BeO with C Its basic
resistivity, 0.063 O m (303 K) It is reportedly unstable
Intrinsically, BeO is an excellent moderator and
reflector material in nuclear reactors Various
the effect of neutron irradiation on the thermal
shows the temperature dependence of the thermal
observed that irradiation of BeO with neutrons consid-erably 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,
0 50 100 150 200 250
-1 K
-1 )
Temperature, T (K)
Figure 4 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.
Table 2 Basic properties of BeO Crystal structure Hexagonal wurtzite Density (near room temperature)
(g cm3)
3.02
Thermal conductivity (293 K) (W m1K1)
281
Thermal expansion coefficient (293–373 K) (K1)
5.5 10 6
Electrical resistivity (1273 K) (O cm) 8.0 10 7
Scattering cross-section (barn) 9.8 Absorption cross-section (barn) 0.0092
Source: Genshiryoku Zairyou Handbook; The Nikkan Kogyo Shimbun: Tokyo, 1952; Gregg, S J.; et al J Nucl Mater 1961,
4, 46.
Trang 6but 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
and safety, particular care is necessary in the control
of fine powder generated during molding and
mechan-ical processing Dust collectors must be installed at
the points of generation, and proof masks,
dust-proof 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
Zirconium hydride is used as a material for neutron
reflectors in fast reactors The evaluation of the
ther-mal conductivity, elastic modulus, and other basic
properties of zirconium hydride is extremely
impor-tant 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
have successfully created crack-free, bulk-scale metal hydrides, and systematically investigated their funda-mental properties – particularly at high tempera-tures Here, we present an outline of the results on the fundamental properties of zirconium hydride
Figure 6 shows the zirconium–hydrogen binary
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 pres-sure, using an advanced ultra-high vacuum Sieverts instrument Details of the instrument configuration
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 accom-panied by a massive increase in volume due to hydro-genation, and to differences in the strength of the
of zirconium hydride substances produced by the author’s group
0 50 100 150 200 250
300
1.2 ´ 10 20 nvt 4.0 ´ 10 20 nvt
Temperature, T (K)
Unirradiated 1.5 ´ 10 19 nvt
-1 K
-1 )
Figure 5 Temperature dependence of the thermal conductivity of unirradiated and irradiated BeO Reproduced from Pryor, A W.; et al J Nucl Mater 1964, 14, 208.
Trang 70
100
200
300
400
500
550⬚C
~37.5 (a-Zr)
(b-Zr)
ε
δ 600
700
800
900
1000
Atomic percent hydrogen Weight percent hydrogen
Zr
Figure 6 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.
1 Absolute capacitance manometer (25 ktorr)
2 Absolute capacitance manometer (1 ktorr)
3 Absolute capacitance manometer (10 torr)
4 Calibrated vessel (~50 ml)
5 Calibrated vessel (~500 ml)
6 Reactor for high pressure (steel)
7 Mantle heater (<723 K)
9 Low temperature incubator (inner temperature: 298 K)
10 Turbo-molecular pump
11 Oil rotary vacuum pump
12 Ionization vacuum gauge
13 Liquid nitrogen trap
14 Compressed hydrogen gas cylinder
15 Reactor for high temperature (quartz glass)
16 Electric resistance furnace (<1273 K)
1
6
8
9
13
14
11
7
15 16
12
:
Figure 7 Schematic diagram of advanced Sieverts instrument.
Trang 82.11.3.3 Lattice Parameter20
Zirconium hydride or deuteride described here was
crys-tals with a fluorite structure The lattice parameters
at ambient temperature of zirconium hydride or
hydride increases slightly with increasing hydrogen
content, according to the following formula:
Figure 10 illustrates the hydrogen content depen-dence of the elastic modulus of zirconium hydride or deuteride, determined using an ultrasonic pulse echo method The elastic modulus of zirconium hydride is higher than that of the pure metal, and decreases slightly with increasing hydrogen content The hydrogen content dependence of the elastic modulus
of zirconium hydride is expressed by the following equations (E: Young’s modulus, G: Shear modulus,
Figure 11illustrates the hydrogen content depen-dence of the Vickers hardness of zirconium hydride and deuteride The graph clearly shows that the Vickers hardness of the hydride is higher than that
of pure zirconium, and that it decreases slightly with increasing hydrogen content Generalizing these results, we can conclude that increasing the hydrogen content has the effect of making zirconium hydride and deuteride plastically ‘softer.’ The relationship between the hardness and hydrogen content depen-dence for zirconium hydride is expressed by the following formula:
Figure 8 Bulk-scale zirconium hydride.
0.476 0.477 0.478 0.479
Hydrogen content, CH (H–Zr)
Yamanaka Kempter Ducastelle Beck Sidhu Moore Cantel
Figure 9 Hydrogen content dependence of the lattice parameter of zirconium hydride and deuteride.
Trang 9HVðGPaÞ ¼ 7:190 2:773 CHðH=ZrÞ
Figure 12shows the density of states (DOS) of
zirco-nium hydride, determined by DV-Xa molecular
orbital (MO) calculations Here, 0 eV corresponds
to the Fermi energy The new band resulting
from the hydrogen is generated immediately below the d-band of the hydride cluster, in the region
zirconium hydride With increasing hydrogen con-tent, there is a marked decrease in the bond order
order of Zr–H covalent bonds does not change This reduction in bond order is likely due to a decrease
in the electric charge of the matrix of Zr bonds
0 20 40 60 80 100 120 140 160
Hydrogen content, CH (H–Zr)
a-Zr d-ZrH 2-x
d-ZrD 2-x
E G B
Figure 10 Hydrogen content dependence of the elastic modulus of zirconium hydride and deuteride.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
HV
Hydrogen content, CH (H–Zr)
α-Zr δ-ZrH 2-x
δ-ZrD 2-x
Figure 11 Hydrogen content dependence of the Vickers hardness of zirconium hydride and deuteride.
Trang 10Since bond order can be thought to be related to the
spring constant of interatomic bonds, these results
can be understood to mean that the effective spring
constant of zirconium hydride, as a whole, decreases
with increasing hydrogen content This hypothesis
offers a good explanation for the hydrogen content
dependence of the various properties of zirconium
hydride
Figure 14 shows the temperature dependence of the electrical conductivity of zirconium hydride
In line with the behavior of most metals, electrical conductivity decreases with increasing temperature for zirconium hydride The hydride has a lower elec-trical conductivity than the pure metals
0 1 2 3 4
-1 )
Electron energy, E (eV)
α-Zr δ-ZrH 1.0
δ-ZrH 1.5
δ-ZrH 2.0
Figure 12 Hydrogen content dependence of the DOS of zirconium hydride.
2.0 1.5
1.0 0.0
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Hydrogen content, CH (H–Zr)
Zr–Zr bond Zr–H bond
Figure 13 Hydrogen content dependence of the bond order of zirconium hydride.