Comprehensive nuclear materials 2 01 the actinides elements properties and characteristics

Comprehensive nuclear materials 2 01 the actinides elements properties and characteristics Comprehensive nuclear materials 2 01 the actinides elements properties and characteristics Comprehensive nuclear materials 2 01 the actinides elements properties and characteristics Comprehensive nuclear materials 2 01 the actinides elements properties and characteristics Comprehensive nuclear materials 2 01 the actinides elements properties and characteristics

R J M Konings, O Benesˇ, and J.-C Griveau

European Commission, Joint Research Centre, Institute for Transuranium Elements, Karlsruhe, Germany

ß 2012 Elsevier Ltd All rights reserved.

Abbreviations

dhcp Double hexagonal close-packed

fcc Face-centered cubic

IUPAC International Union of Pure and

Applied Chemistry

OECD/NEA Organisation for Economic

Cooperation and Development/

Nuclear Energy Agency

2.01.1 Introduction

The actinides are the 15 elements with atomic numbers

89–103 in the periodic system The International Union

of Pure and Applied Chemistry (IUPAC) has

recom-mended that these elements are named actinoids

(meaning ‘like actinium’), but this has never found

gen-eral acceptance In these elements, the 5f electron

sub-shell is progressively filled, leading to the generalized

[Rn 7s25fn] configuration Unlike the lanthanides, in which the 4f electrons lie in the interior of the xenon core region and thus hardly contribute to the chemical bonds (called ‘localized’), the 5f electrons show a much more diverse character, particularly in the metallic state.1 The 5f electrons in the elements thorium to neptunium are placed in the valence shell (often called

‘itinerant’ or ‘delocalized’) and show substantial covalent bonding, whereas the 5f electrons in the elements amer-icium to lawrencium are localized Plutonium and americium have a transition position, showing both localized and delocalized behavior depending on tem-perature, pressure, and magnetic field.2

The actinides are radioactive elements, their iso-topes having strongly variable half-lives Owing to the short half-life, compared with the age of the earth, majority of the actinides have decayed and cannot be found in nature Only the long-lived isotopes 232Th,

235

U, and238U are of primordial origin, and possibly

244

Pu Also,231Pa is found in very low concentrations

in natural minerals (e.g., pitchblende ores), but it is a

1

product of the235U (4nþ 3) decay chain.3

Most other actinides are man-made elements They were

synthe-sized by nuclear reactions using reactors and

accelera-tors in the period 1940 (Np) to 1961 (Lr) The metals

from Th to Cm are available in gram quantities that

have allowed experimental determination of (some of)

their physicochemical properties; Bk and Cf metals

have been prepared in milligram quantities and Es in

microgram quantities and therefore only limited

inves-tigations have been possible The metals Fm and

beyond have not been prepared in pure form

The main technological relevance of the actinides

is their use as fuel for nuclear fission reactors,

partic-ularly the nuclides233U,235U, and239Pu, which

fis-sion with thermal neutrons.235U and239Pu occur in

the so-called U/Pu fuel cycle.235U is present in 0.7%

in natural uranium;239Pu is formed when uranium is

irradiated in a reactor as a result of neutron capture

by238U.233U is formed by neutron capture of232Th

in the Th/U fuel cycle The vast majority of nuclear

power reactors use oxide fuel, but carbide and nitride

as well metallic alloys fuels have been studied since

the early days of reactor development.4

In this chapter, we discuss the physicochemical properties of the actinide metals, with emphasis on the elements Th to Cm for which experimental data

on bulk samples generally exist The trends and sys-tematics in the properties of the actinide series will

be emphasized and compared with those of the 4f series These physicochemical data are essential for understanding and describing the properties of mul-tielement alloys (seeChapter2.05, Phase Diagrams

of Actinide Alloys) and actinide containing com-pounds (Chapter2.02, Thermodynamic and Ther-mophysical Properties of the Actinide Oxides)

2.01.2 Crystallographic Properties 2.01.2.1 Crystal Structure

The stable crystallographic modifications of the acti-nides at atmospheric pressure are listed inTable 1 Compared to the lanthanide series in which the hex-agonal close-packed (hcp) and the face-centered cubic (fcc) structures dominate, the actinide metals show a remarkable variation in the structural

Table 1 The crystal structure of the actinide metals

Structure Space group a (pm) b (pm) c (pm) Angle(s) V m (cm 3 mol1) r (g cm 3 )

g Cubic

a P42/mnm, P4 2 /nm or P4n2.

Source: Edelstein, N M.; Fuger, J.; Katz, J J.; Morss, L R In The Chemistry of the Actinide and Transactinide Elements; Morss, L R.,

2 The Actinides Elements: Properties and Characteristics

properties at room temperature, as shown inFigure 1.

Particularly, the elements Pa–Pu have unusual low

symmetry (distorted) crystal structures a-Pa is

body-centered tetragonal, and a-U and a-Np are

orthorhombic but with slightly different space

groups a-Pu has a monoclinic crystal structure

with 16 atoms in the unit cell at room temperature

Plutonium is unique in the periodic table of the

elements with six allotropes at atmospheric pressure

and one more at elevated pressure

This complexity of the structural properties of

the actinides is also evident from Figure 2, which

shows the variation of the molar volume of the

a-phases of the actinides at room temperature and

atmospheric pressure, indicating that the actinides Pa

to Pu follow the trend in the (itinerant) d-transition

metals, whereas the actinides Am to Bk follow that

of the (localized) 4f metals It is generally accepted that this complex behavior is due to the active role

of the f-electron in the metallic bond and the changes in temperature and pressure by which the f-electron bonding character is affected Experimen-tal observations and electronic structure calculations have indeed shown that the bonding in the transition metals is dominated by d-electron contributions, that

in the lanthanides there is a lack of f-electron contri-bution, and that the actinides fall in between.5 2.01.2.2 Effects of Pressure

Pressure is expected to drive the atoms in the crystal lattice closer to each other, forcing the electrons to

(a)

(d)

Figure 1 The crystal structures of the actinides at room temperatures: (a) a-Th, (b) a-Pa, (c) a-U, (d) a-Np, (e) a-Pu, (f) a-Am.

Ac 0

1 0

2 0

3 0

4 0

Vm

3 mol

1 )

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Figure 2 The molar volume of the actinide elements () compared with that of the lanthanides ( ○) and the 4d transition metals ( □).

participate in the binding (delocalization),6 which

particularly affects the heavy actinides with

loca-lized f-electron behavior at ambient pressure

Recent studies using diamond anvil cells coupled

to synchrotron radiation have provided strong

evi-dence for that As discussed by Heathman et al.,7

americium shows a remarkable decrease in volume

with increasing pressure (at ambient temperature)

with three transitions up to 100 GPa (Figure 3) Its

structure changes from hcp (Am-I) through fcc

(Am-II) to orthorhombic (Am-III and Am-IV),

indi-cating the appearance of the itinerant character 5f

electrons This behavior is also observed in curium,

with a puzzling supplementary magnetically

stabi-lized Cm-III structure at 40–60 GPa.8 Uranium

shows a comparatively straightforward behavior and

the a-structure is stable up to 100 GPa, with a much

smaller volume decrease.6A similar behavior has been

found for protactinium, its a-form being stable up to

80 GPa This is clearly reflected in the isothermal bulk modulus (Table 2), which is around 100 GPa for the elements Pa to Np but around 30–40 GPa for Am and

Cm The Am-IV phase shows a large bulk modulus (more similar to that of uranium), as expected for a metal with appreciable 5f-electron character in its bonding This is also evident from the comparison of the actinide and lanthanide metals (Figure 4) Uncertainty still exists about the bulk modulus

of a-plutonium As discussed by Ledbetter et al.,12 the published B0 values at ambient range show a large variation, as do the theoretical calculations The most accurate results for the isothermal bulk modulus vary between 51(2) GPa13and 43(2) GPa.14 2.01.2.3 Effects of Temperature

Detailed studies show that the crystal lattice of most actinide metals expands with increasing temperature

1.00

0.95

Am I

Am II Am III

Am IV

Cm I

II

Cm III

Cm

V

0.85

0.80

2 %

11.7 %

α-U

Pa I

Pa II

0.75

0.70

0.65

0.60

0.55

0.50

0.45

Pressure (GPa)

Figure 3 The relative volumes as a function of pressure of several actinide metals.

Table 2 The isothermal bulk modulus (B 0 ) and its pressure derivative (B00) of the actinide elements at ambient temperature

4 The Actinides Elements: Properties and Characteristics

and evolves to a simple cubic arrangement close to

their melting temperature, similar to the lanthanide

elements (For numerical data on the thermal

expan-sion, seeSection 2.01.4.1) As the atoms move away

from each other, the electrons in the 5f metals tend

to favor a localized state As discussed by Vohra and

Holzapfel,15 this is particularly important for Np

and Pu, which are on the threshold of localization/

itinerancy The case for plutonium is much more

complex, as shown in Figure 5 The crystal lattice

of plutonium expands for the a-, b-, g-, and e-phases,

and the g- to d-transition has a positive expansion

The d- and d0-phases have negative thermal

expan-sion and the d- to d0- and d0- to e-transitions show a

negative volume change, as is the case upon melting

Dynamic mean field calculations show that the

monoclinic a-phase of Pu is metallic, whereas fcc d

is slightly on the localized side of the localization–

delocalization transition.16

Moreover, the stability of the crystalline state of the actinide metals varies significantly The melting temperature is high for thorium, similar to that of the transition metals in group IVB, and low for Np and

Pu (Figure 6)

When applying high temperature as well as high pressure to the actinides, phase changes can be sup-pressed, as is shown inFigure 7 For example, the triple point for the a–b–g equilibrium in uranium is found at about 1076 K and 31.5 kbar; above this pressure, ortho-rhombic a-U directly transforms in fcc g-U.17In plu-tonium, the g-, d-, and d0-phases disappear at relatively low pressure and are replaced by a new phase desig-nated z In contrast to the other actinides, plutonium shows a negative slope for the liquidus down to the b-z-liquid triple point (773 K, 27 kbar) reflecting the increase in density upon melting.17

2.01.2.4 Effects of Radiation The a-decay of the actinides taking place in the crystal lattice creates an alpha particle and a recoil atom The recoil atom produced has a range of about

12 nm and causes a dense collision cascade with typi-cally about 2300 displacements (Frenkel pairs) within

a short distance, around 7.5 nm in size The a-particle has a path of about 10 mm, with a cascade of about 265 displacements at the end of its range.18 Although recombination will take place, point defects and eventually extended defects (dislocations, dislocation loops) will survive in the crystal lattice, resulting in changes in the properties of the materials Computer simulations of the radiation effects in fcc plutonium have shown that the defect recombination stage is much longer than that in other metals and that the vacancies do not seem to form clusters.19In addition

to the radiation damage, helium ingrowth takes place

As discussed by Hecker and Martz,20the expan-sion of the lattice of a-Pu is significant due to

α

β

γ

δ δ⬘

ε Liquid

8

6

4

2

0

T (K)

Figure 5 The thermal expansion of Pu Made after

Schonfeld, F W.; Tate, R E Los Alamos National

Laboratory, Technical Report LA-13034-MS; 1996.

150

100

B0

50

0 La

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 4 The isothermal bulk modulus (B 0 ) of the actinide elements ( ○) compared with that of the lanthanides ().

self-irradiation, when held at cryogenic

tempera-tures, saturating at about 10 vol.% In contrast, the

(Ti-stabilized) b-phase shows a slight contraction

and the (Al-stabilized) d-phase a substantial

contrac-tion, the latter saturating at 15 vol.% Of course this

is also reflected in other properties such as electrical

resistivity.21,22 The radiation effects recover upon

annealing to room temperature, a few percent of

the damage remaining Gorbunov and Seleznev23

observed that a-Pu containing predominantly239Pu

retains its crystal structure after prolonged storage

at room temperature A sample of predominantly shorter lived 238Pu (t1/2¼ 87.74 years) contains both the a- and b-forms at immediate examination and additionally the g-, Z-, and e-phases after a similar storage period Chung et al.24 showed by X-ray diffraction and dilatometry measurements on

238

Pu-doped d-phase plutonium samples that the lattice expansion by self-irradiation appears to be the primary cause for dimensional changes during

La 2500 2000 1500

Tfus

1000

500

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 6 The melting point of the lanthanide () and actinide ( ○) metals The estimated values are indicated by .

0

400

600

800

1000

1200

P (kbar)

P (kbar)

α

β

γ

Uranium

300

400

500

600

700

800

900

1000

γ

β

δ

δ⬘

ε

ζ

Liquid

1100 1200 1300 1400 1500 1600

Americium

Liquid

γ β

500 600 700 800 900 1000 1100

γ

β α

Neptunium Liquid

Figure 7 The pressure–temperature phase diagrams for U, Pu, Np, and Am Reproduced from Lee, J A.; Waldron, M B Contemp Phys 1972, 13, 113–133.

6 The Actinides Elements: Properties and Characteristics

the initial 23 years of aging Following the initial

transient, the density change is primarily caused by

a constant helium ingrowth rate as a result of particle

decay The two effects were combined in an equation

for the expansion DL/L with an exponential

(radia-tion damage) and a linear (helium ingrowth) part:

DL=L ffi A½1  expðBt Þ þ Ct ½1

where A, B, and C are constants and t is time

The self-irradiation is one of the main causes that

complicates the study of the heavy actinide metals For

example, berkelium metal (t1/2¼ 314 days; 0.2%

249

Cf growth per day) shows signs of amorphization

(weak and diffuse X-ray spectra) at room temperature,

which improved after annealing and thermal cycling,

and the samples were found to contain two

crystallo-graphic structures at room temperature, double

hexag-onal close-packed (dhcp) and fcc, of which the former is

the stable form.25 An extreme case is Es; its crystal

structure has been resolved only by rapid electron

diffraction of thin film material due to the very short

half-life of the isotope used.26

2.01.3 Thermodynamic Properties

Many critical reviews of the thermodynamic

proper-ties of the actinide metals have been made since the

1960s The first milestone was the review by Oetting

and coworkers,27 which gave recommended values

for Th to Cm Ward et al.28treated the same elements

but also gave recommendations for Cf and Es

In addition, the room temperature thermodynamic

properties for the major actinides Th and U have

been reviewed by the CODATA team for key values

for Thermodynamics,29while Th, U, Np, Pu, and Am

have been reviewed by the OECD/NEA team.30–33

The most recent evaluation was made by Konings

and Benesˇ,34with emphasis on the high-temperature

properties There are no large differences between

these studies for the major actinides and it is thus

clear that the recommendations given in this chapter

rely heavily on these studies (Tables 3 and 4)

2.01.3.1 Heat Capacity and Entropy of

the Crystalline State

The low-temperature heat capacity has been

mea-sured for the actinides Th through Am, in most

cases showing anomalies The origin of these

anoma-lies has generally not been explained adequately35

but is likely related to ordering phenomena and

f-electron promotion The measurements for the major actinides Th, U, and Pu in the a-structure were made on gram-scale quantities, and the results should thus be of an acceptable accuracy

However, although the low-temperature heat capacity of plutonium was measured by a remarkably large number of authors,36–42 there is considerable scatter among the results above 100 K (seeFigure 8), probably due to self-heating and radiation damage But even the results for242Pu samples from the same batch,40,41 which are affected less due to its much longer half-life, differ considerably The differences

in the heat capacity have a pronounced effect on the standard entropy at T ¼ 298.15 K: 56.03 J K1mol1,39 56.32 J K1mol1,40 54.46 J K1mol1,41 and 57.1

J K1mol1.42 Especially, the results of Lashley

et al.42 indicate a very different shape of the heat capacity curve of a-Pu, rising much steeper up to

T ¼ 100 K and saturating at a lower value near room temperature Although the relaxation method used

in that study is less accurate ( 1.5% as claimed by the authors) than the traditional adiabatic technique used in the other studies, the difference is significant Lashley et al.42attributed this to the buildup of radia-tion damage at the lowest temperatures, which they tried to avoid by measuring upon cooling, and below

T ¼ 30 K by intermediate annealing at room temper-ature However, other authors also addressed this issue For example, Gordon et al.41performed a heat-ing run from room temperature to T ¼ 373 K before each low-temperature run Moreover, no substantial difference between the results for 239Pu and 242Pu was observed in that study

The electronic Sommerfeld heat capacity coeffi-cient (ge), a property proportional to the density

of states at the Fermi level, varies strongly in the actinide series (Table 5) It increases steadily up to

Pu but is very low for Am For d-Pu the electronic heat capacity coefficient geis even three times higher than that of a-Pu This corresponds well with the results of photoemission spectra48 that show a-Th has a small density of states at the Fermi level com-pared with that of a-U, a-Np, and a-Pu (Figure 9)

In a-Am, the valence band is well removed from the Fermi level The low-temperature heat capacity

of other modifications of plutonium has been measured recently Specifically, the d-structure sta-bilized by Am or Ce doping shows clearly enhanced values of the electronic heat capacity coefficient geat very low temperature.50,51

The standard entropies derived from the low-temperature heat capacity data are given inTable 3,

Table 3 Recommended entropy (J K1mol1) and the heat capacity (J K1mol1) of actinide elements in the solid and liquid phase

range (K)

Source: Konings, R J M.; Benesˇ, O J Phys Chem Ref Data 2010, 39, 043102.

and the variation along the actinide metal series is shown in Figure 10 The entropies of the elements

Th to Am are close to the lattice entropies of the corresponding lanthanides, showing the absence of magnetic contributions The entropies of the other actinide elements must be derived from estimations,

as experimental studies do not exist To this pur-pose Ward et al.28 suggested a general formula by correlating the entropy with metallic radius (r), atomic weight (M), and magnetic entropy (Sm):

Suð298:15K Þ ¼ Skð298:15K Þru

rkþ3

2R lnMu

Mkþ Sm ½2 where u refers to the unknown (lanthanide or actinide) element and k refers to the known element

Sm is taken equal to Sspin¼ (2J þ 1), where J is the total angular momentum quantum number The entropy of Cm thus obtained is significantly higher than that of the preceding elements, showing its magnetic character

The heat capacity of the actinide metals from room temperature up to the melting temperature has been reported for Th, U, and Pu with reasonable accuracy and for Np for the a-phase only The values for the other metals are based on estima-tions For example, Konings52 estimated the heat capacity of americium metal from the harmonic, dilatation, electronic, and magnetic contributions,

Cp¼ Charþ Cdilþ Celeþ Cmag, whereas the heat capac-ity of g-americium was obtained from the trends in the 4f and 5f series The high-temperature heat capac-ity data for the actinide metals was analyzed in detail

by Konings and Benesˇ,34who gave recommendations for the elements Ac to Fm The results for the elements

Th to Cm are summarized inTable 3

Figure 11 shows the variation of the sum of the transition entropies from the crystalline room temperature phase to the liquid phase for the lantha-nide and actilantha-nide series This value is about constant

in the lanthanide series but shows large variation in the actinide series, particularly for the elements U–Np–Pu The deviation from the baseline

Table 4 Recommended transition temperatures (K),

enthalpies (kJ mol1), and entropies (J K1mol1) of the

actinide metals

Transition T trs (K) D trs H D trs S

b !liq 2020 10 13.8 1.3 6.83

b !liq 1843 50 12.3 2.0 6.67

b !g 1049 2 4.62 0.50 4.40

g !liq 1407 2 8.47 1.00 6.02

g !liq 913 3 3.2 0.5 3.50

Pu a !b 399 1 3.706 0.030 9.29

b !g 488 1 0.478 0.020 0.98

g !d 596 2 0.713 0.050 1.20

d !d 0 741 4 0.065 0.020 0.09

d0!E 759 4 1.711 0.050 2.25

e !liq 913 2 2.766 0.1 3.03

Am a !b 1042 10 0.34 0.10 0.33

g !liq 1449 5 8.0 2.0 5.52

b !liq 1619 50 11.7 1.0 7.23

Source: Konings, R J M.; Benesˇ, O J Phys Chem Ref Data

2010, 39, 043102.

0

10

20

Cp

–1 )

30

40

100

100 25 30 35

200

T (K)

Figure 8 The low-temperature heat capacity of

plutonium; ◊, 37 ; , 38 ;, 39 ; r, 40 ; D, 41 ; , 42 ; ○, 43

Table 5 The electronic heat capacity coefficient (g e ) and Debye temperature (Y D ) of the actinide elements

a These values are for single crystal material, g ¼ 9.9 mJ K 2mol1and Y ¼ 184 K for polycrystalline material.

correlates well with the atomic volume of the metals that is also anomalous for these elements, indicating that the itinerant behavior of the 5f electrons and the resulting lowering of the room temperature crystal symmetry require additional entropy to reach a similar disordered liquid state

2.01.3.2 Heat Capacity of the Liquid State The heat capacity of the actinide elements in the liquid state is relatively poorly known Experimental data exist for Th, U, and Pu, and only the values for Th and U are known with an acceptable accu-racy They were measured by drop calorimetric techniques in a reasonable wide temperature range Semi-empirical models for liquid uranium suggest

a large electronic contribution to the heat capacity

of this element.53The data for Pu, also obtained by calorimetry, are scattered and measured in a limited temperature range and the heat capacity value for the liquid of this element is thus uncertain

Figure 12 also shows the estimated values for Am and Cm, based on assumptions considering the elec-tron configurations.52,54

a-Th a-U a-Np a-Pu a-Am a-Cm

0

Energy below Ef (eV)

Figure 9 Valence-band photoemission spectra of the

actinide metals Modified from Moore, K T.; van der Laan, G.

Rev Mod Phys 2009, 81, 235–298 by adding the results for

a-Cm by Gouder et al 49 Note that the spectrum for a-Th is

scaled up compared to the other spectra so that it is easily

visualized In reality, it is much lower in intensity due to a

small f density of states at the Fermi level.

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 40

60 80 100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 10 The standard entropies of lanthanide () and actinide ( ○) metals at T ¼ 298.15 K; estimated values are indicated by ( ).

Ac

La 25 20 15 10 5 0

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Figure 11 The sum of the transition entropies of the lanthanide () and actinide ( ○) metals The estimated values are indicated by .

10 The Actinides Elements: Properties and Characteristics