Comprehensive nuclear materials 2 03 thermodynamic and thermophysical properties of the actinide nitrides

Comprehensive nuclear materials 2 03 thermodynamic and thermophysical properties of the actinide nitrides Comprehensive nuclear materials 2 03 thermodynamic and thermophysical properties of the actinide nitrides Comprehensive nuclear materials 2 03 thermodynamic and thermophysical properties of the actinide nitrides Comprehensive nuclear materials 2 03 thermodynamic and thermophysical properties of the actinide nitrides

the Actinide Nitrides

M Uno

University of Fukui, Fukui, Japan

T Nishi and M Takano

Japan Atomic Energy Agency, Tokai-mura, Ibaraki, Japan

ß 2012 Elsevier Ltd All rights reserved.

HD (diamond point) Hardness

LTE Linear thermal expansion

Uranium nitride UN not only has the same isotropic

a higher melting point, higher metal atom density, and

thus has advantages as a nuclear fuel compared to

proper-ties have been known for a long time However, UNhas some disadvantages as a nuclear fuel because of its

61

Plutonium nitride and thorium nitride have been

also well studied, mainly with regard to their

suitabil-ity as nuclear fuels Other actinide nitrides with

higher atomic number are also important as potential

nuclear fuels but the data on these fuels are

insuffi-cient because they are difficult to obtain and handle

In this section, the physicochemical properties of

the actinide nitrides, mainly uranium nitrides and

plutonium nitride, are discussed First of all, phase

stability and crystal structures of the nitrides are

described Then, their thermal, thermodynamic, and

mechanical properties which are relevant to their

suitability as nuclear fuels, are discussed

Character-istics of their preparation and irradiation as nuclear

Structure

References for specific data are given separately for

each section below, but readers are also referred to a

the viewpoint of suitability as nuclear fuels, the

vari-ous properties of not only the nitrides but also the

other compounds The binary phase diagram shown

inFigure 12is taken from data published in 1960,3–7

and is still valid The phase stability of U–N systems

an NaCl-type cubic structure, and the latter has an

nitrogen pressure (126 atm); and its lattice parameterdecreases with an increase in nitrogen content The

Although the phase diagram, where nitrogen

have a wide range of nonstoichiometry, at lowernitrogen pressure; UN decomposes such that UN

Figure 312,13; here theb-U2N3in the previous graph

nitrogen pressure of 2 atm is reported to be nearly

decomposes to U and nitrogen at nitrogen pressurebelow 2.5 atm As the decomposition of UN mustinfluence the properties of the fuel pellets, and the

forma-tion of UN through carbothermic reducforma-tion, the

important from the viewpoint of their use as nuclear

12 10 8 6

4 2

PN2>> 10 5 Pa

Figure 1 U–N phase diagram at nitrogen pressure larger

than 105 Pa Data from Levinskii, Yu V Atom Energ 1974,

37(1), 216–219; Sov Atom Energ (Engl Transl.) 1974,

37(1), 929–932.

(c)

U N

Figure 2 Crystal structures of (a) UN, (b) a-U 2 N 3 , and (c) b-U N

fuel The reported decomposition curves are shown in

Figure 4.3It is seen from these graphs, for example,

1620 K at nitrogen pressure of 1 atm More detailed

decomposition behavior of UN as well as other

only one structure for the mononitride, PuN: an

NaCl-type face-centered cubic (fcc) structure with

a ¼ 4.904 A˚ PuN is a line compound with little

non-stoichiometry, and is reported not to congruently

there is a study on the safety assessment of fuels on

the basis of vaporization behavior in which the

melt-ing temperature of Pu–N is given as 2993 K under a

Though thorium is a fertile material, recent research

on thorium and its compounds as nuclear fuel is

the former is an NaCl-type cubic structure with

UN 2

10 5 Pa

UN2+

N 2

a-U + UN b-U + UN

Figure 3 U–N phase diagram at nitrogen pressure smaller

than 2 atm Data from Storms, E K Special Report to the

Phase Equilibria Program; American Ceramic Society:

Westerville, OH, 1989; Muromura, T.; Tagawa, H J Nucl.

1 2

5 6

P Gross, C Hayman and H Clayton, ‘‘Heats of Formation

of Uranium Silicides and Nitrides’’; In Thermodynamics of Nuclear Materials- Proceedings of Symposium on Thermodynamics of Nuclear Materials, Vienna, May 1962, International Atomic Energy Agency.

2 atm Hypo- and hyper-ThN appear above 1350C.

important to consider the temperature and nitrogenand oxygen pressures during the preparation of ThN

Curium NitridesThese nitrides are also usually prepared by car-

difficult to prepare bulk samples due to their highradioactivity, there have been no systematic studies

on their phase stability However, it has been lished that there is only mono nitride in these systemsfrom the fact that no nitrogen absorption occurredupon cooling in nitrogen atmospheres during car-bothermic reduction These mono nitrides have anNaCl-type face-centered cubic structure, and their

structure of these three nitrides, as well as uraniumand plutonium nitrides, is advantageous as nuclear

g b 500

-483 ⬚C -483 ⬚C -320⬚C -215 ⬚C -125⬚C

N solubility in Th (Gerds and Mallett) 115

ThN + fccTh

ThN + bccTh

Th3N4+ ThN

1800 ⬚ ± 25 ⬚

1605 ⬚ ± 20 ⬚

Figure 6 Phase diagram of Th–N system Reproduced

from Benz, R.; Hoffman, C G.; Rupert, G N J Am Chem.

Soc 1967, 89, 191–197, with permission from Elsevier.

fuels, especially as accelerator-driven system (ADS)

targets of nitride solid solutions that contain a large

amount of minor actinides (MAs) Experimental

research on their vaporization behavior has revealed

that the congruent melting temperature of NpN was

stability and other properties of pure CmN Some data

on Cm and U or Pu solid solutions have been reported,

and these will be discussed in the next section

Mixtures

As (U,Pu)N were some of the most promising

candidates for the first breeder reactors, they are the

best studied nitride solid solution fuels UN and PuN

form a continuous solid solution, and the lattice

param-eter increases with an increase in the plutonium

con-tent, and is accompanied by a large deviation from

nonideality of the solution A diagram of the calculated

suggests that there is a relatively narrow range of

pos-sible (U,Pu)N compositions, as is the case with U–N

and Pu–N binary systems It is suggested that the

As uranium monocarbide and plutonium

mono-carbide, as well as other actinide carbides, have an

NaCl-type fcc structure, actinide nitrides and

acti-nide carbides form solid solutions Some research

performed on actinide nitride carbides, for example,

suit-ability of these carbonitride fuels and the impurities

in nitride fuels after carbothermic reduction Phase

stability graphs of U and/or Pu–N–C, both with andwithout oxygen, also have been constructed in order

behav-ior of (U and/or Pu)–N–C fuels also has been

scope of this chapter

As MAs are usually burnt with uranium and tonium for transmutation, and as Am originally exists

plu-in Pu, (MA,U)N or (MA,Pu)N have also been wellstudied As mentioned above, the vaporization behav-

vaporization of Pu and Am was observed The latticeparameters of (U,Np)N and (Np,Pu)N increase withincrease in Np and Pu content, and with a small

Although scarcely any data for pure CmN has been

Inert matrix fuels, where MA as well as uraniumand plutonium are embedded in a matrix, are alsobeing considered for use in ADS for transmutation.Recent research in MAs has focused on using variousnitride solid solutions and nitride mixtures as inertmatrix fuels For ADS targets, matrices have beendesigned and selected so as to avoid the formation

of hot spots and to increase the thermal stability,especially in the case of Americium nitride Con-sidering their chemical stability and thermal conduc-tivity, ZrN, YN, TiN, and AlN were chosen as

Table 1 Lattice parameter of some actinide nitrides

Source: Minato, K.; et al J Nucl Mater 2003, 320, 18–24, with

permission from Elsevier.

UN−NpN

Vegard’s law

0.8 1 0.6

Composition

0.4 0.2 0

0.4890

0.4895 0.4900 0.4905

UN UN NpN

PuN NpN PuN

UN−PuN NpN−PuN

Figure 7 Lattice parameter of some actinide nitride solid solution Reproduced from Minato, K.; et al J Nucl Mater 2003, 320, 18–24.

fcc structure with a ¼ 4.580 A˚ and has nearly the same

thermal conductivity as UN, has a high melting point,

good chemical stability in air, and a tolerable

dissolu-tion rate in nitric acid Recently, abundant data

have been made available for ZrN-based inert matrix

fuels It is planned that (Pu,Zr)N, with about 20–25%

The lattice parameter of (Pu,Zr)N decreases with an

increase in the Zr content, and is between that of

It has also been estimated, using a model, that (Pu,Zr)N with 20–40 mol% PuN, does not melt till up

to 2773 K; this is based on experimental

reported that two solid solutions are obtained when

Am content of the two phases have been estimated,from the lattice parameter, to be 14.5 and 43.1 mol%

A thermodynamic modeling of a uranium-free inert

PuO2

PuN

CmN CmO2

CmO2

0.6 0.8 1

PuO2

CmN

Figure 9 Lattice parameter of (Pu,Cm)N and (Pu,Cm)O 2

Reproduced from Minato, K.; et al J Nucl Mater 2003,

320, 18–24, with permission from Elsevier.

matrix fuel, for example, (Am0.20Np0.04Pu0.26Zr0.60),

In contrast to ZrN, TiN does not dissolve MA

nitrides even though TiN also has an NaCl-type fcc

structure This is explained by the differences in

A mixture of PuN and TiN was obtained by several

heat treatments above 1673 K, and the product, in

which one phase was formed, did not contain the

may be hypostoichiometric although (Pu,Zr)N is

hyperstoichiometric

In this section, the melting points and decomposition

temperatures of actinide mononitrides are discussed

in conjunction with the nitrogen pressures because

this behavior depends on the nitrogen partial

pres-sure of the system The vapor prespres-sure of a metal gas

over the solid nitride is discussed in the next section

as ‘vaporization behavior.’

The liquid mononitride MN (liq.) can be observed

when congruent melting occurs under a pressurized

nitrogen atmosphere; otherwise the solid

mononi-tride MN (s) decomposes into nitrogen gas and liquid

metal that is saturated with nitrogen, according to the

following reaction,

Olson and Mulford have determined the

by the optical observation of the nitride granules

when they were heated under controlled nitrogen

the nitrogen pressure p (atm) in logarithmic scale and

The solid curves show the following equations:

atm) The congruent melting for PuN was notachieved in the nitrogen pressure range up to 24.5 atm.The presence of an oxide phase, as an impurity,seems to lower the melting point and decompositiontemperature In the case of ThN mentioned above,the melting point and decomposition temperature

of a specimen containing 0.6 wt% oxygen fell by

130 K from those of the oxygen-free specimens( 0.04 wt% oxygen) A similar experiment conducted

that has the same crystal structure, has revealed adecrease in the melting point by 200–300 K when theoxygen content increased from 0.15 to 0.5–1.0 wt%.Some data sets on the equilibrium nitrogen pres-

Congruent melting

ThN

3.2 3

U(C,N), as measured by the Knudsen-cell and

mass-spectroscopic technique at lower temperatures, are

curve represents the correlation for UN developed

given as:

with a decrease in x, together with a lowering in the

is approximately one-fifth of that of UN When

con-sidering a nitride or carbide as nuclear fuel for fast

reactors, it should be noted that the decomposition

pressure of nitrogen can be lowered and that the

reactivity of carbide with moisture can be moderated

by employing the carbonitride instead of the nitride

or carbide

No experimental data on the melting behavior of

transplutonium nitrides such as AmN and CmN have

relationship between the decomposition temperatureand the instantaneous coefficients of linear thermalexpansion (CTE) and used it to predict the decom-

CTE at 293 and 1273 K plotted against reciprocaldecomposition temperature under 1 atm of nitrogenfor some transition metal nitrides (TiN, ZrN, HfN)and actinide nitrides (UN, NpN, PuN) The data used

references Except for the large CTE value for PuN at

293 K, a reasonable linear relationship is shown by theagreement of the broken lines From the CTE valuesfor AmN, determined by the high-temperature X-raydiffraction technique, the decomposition temperature

of AmN under 1 atm of nitrogen was roughly dicted to be 2700 K, which is much lower than that

pre-of PuN

In this section, the vapor pressure of a metal gas over

a solid actinide nitride is summarized

Timofeeva Brundiers TPRC Smirnov Houska

Kruger Benedict Aldred Takano

2.6 4 6 8 10 12 14

Figure 13 Coefficients of linear thermal expansion at 293 (open symbols) and 1273 K (closed symbols) for some transition metal nitrides and actinide nitrides plotted against reciprocal decomposition temperature under 1 atm of nitrogen For references see Table 2

8

3 4

1

2 9

5: x = 1 6: x = 0.79 7: x = 0.5 8: x = 0.36 9: x = 0.2

Figure 12 Decomposition pressures of U(C,N) as a

function of reciprocal temperature below 2400 K Solid

lines by Ikeda et al.44and broken lines by Prins et al.45

Dotted line for UN reviewed by Hayes et al.46

The major vapor species observed over UN are

U(g) UN(g) can also be detected in addition to U(g)

mag-nitude lower than that of U(g); therefore, the

contri-bution of UN(g) can be ignored in practice Some

fitting the data from eight experimental investigations

According to that paper, the reported data on

UN(s) vary somewhat The U(g) pressure obtained by

from the equation developed by Hayes et al., but is

It is well known that the evaporation of UN is

accompanied by the precipitation of a liquid phase,

The reported vapor pressure of U(g) over UN(s) is

suggested that the dissolution of nitrogen and/or

impurity metal from crucibles into the liquid phase

could affect the observed partial pressure Some

scat-tering of the previously reported data on U(g) over

UN(s) may be also caused by a reaction of the liquid

phase in UN with the crucible material From this

viewpoint, it appears that the partial pressure of U(g)

over UN(s) should be a little higher than that posed by Hayes et al

pro-The vapor species over PuN are nitrogen

PuN(g) is not detected because PuN is more unstable

Table 2 Summary of melting point, decomposition temperature and linear thermal expansion coefficient (CTE) for some transition metal nitrides and actinide nitrides

Nitride Congruent

melting point (m.p.) (K)

Decomposition temperature a (K)

References CTE (106K1) References Method or

: U(g), Hayes et al.46

: U(g), Suzuki et al.61

: U(g), Alexander et al.62

Figure 14 Partial pressure of N 2 (g) and U(g) over UN (s) as a function of temperature Adapted from Hayes, S L.; Thomas, J K.; Peddicord, K L J Nucl Mater 1990, 171, 300–318; Suzuki, Y.; Maeda, A.; Arai, Y.; Ohmichi, T J Nucl Mater 1992, 188, 239–243;

Alexander, C A.; Ogden, J S.; Pardue, W H J Nucl Mater 1969, 31, 13–24.

Pardue et al.,66and Campbell and Leary.67These dataseem to agree with each other.

The vapor pressure of Pu(g) over PuN(s), as a

values reported in the different studies almost

range of 1400–2400 K, which suggests that PuN

Pu(g) over PuN(s), at temperatures lower than

1600 K, is a little higher than the values extrapolatedfrom the high-temperature data, and that it approachesthat over Pu metal with further decrease in tempera-ture There is some possibility that a liquid phaseforms at the surface of the sample during the coolingstages of the mass-spectrometric measurements,because PuN has a nonstoichiometric compositionrange at elevated temperatures, while it is a linecompound at low temperatures

The vapor pressure of U(g) and Pu(g) over UN(s),

Table 3gives the vapor pressures of U(g) and Pu(g)

logarithmic temperature coefficients It is noteworthythat the vapor pressure of Pu(g) over mixed nitridewas observed to increase with an increase in the PuNcontent

of Np(g) over NpN(s) in the temperature range of1690–2030 K by using the Knudsen-cell effusion mass

function of temperature The partial pressure of Np(g) can be expressed using the following equation:

The vapor pressures of Np(g) over NpN(s) obtained

by Nakajima et al are similar to those of Np(g) over

10 4/T (K–1 ) 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Figure 15 Partial pressure of Pu(g) over PuN(s) as a

function of temperature Data from Suzuki, Y.; Maeda, A.;

Arai, Y.; Ohmichi, T J Nucl Mater 1992, 188, 239–243;

Kent, R A.; Leary, J A High Temp Sci 1966, 1, 176–183;

Sheth, A.; Leibowitz, L ANL-AFP-2, Argonne National

Laboratory; Chemical Engineering Division: Argonne,

: (U0.80Pu0.20)N : (U0.65Pu0.35)N : (U0.40Pu0.60)N : (U0.20Pu0.80)N : PuN

Figure 16 Partial pressure of U(g) and Pu(g) over UN(s),

PuN(s) and mixed nitride as function of temperature.

Reproduced from Suzuki, Y.; Maeda, A.; Arai, Y.; Ohmichi,

T J Nucl Mater 1992, 188, 239–243.

Table 3 Partial pressure of U and Pu over UN, PuN, and (U,Pu)N

Compound Vapor species Vapor pressure log p (Pa) Temperature range (K)

liquid Np metal found by Ackermann and Rauh71;

decomposition mechanism is considered to be

of Am(g) over AmN by using values of the Gibbs

The evaporation of AmN obeys the following

vapor pressure of Am over AmN, expressed as a

function of temperature, is

log p AmðgÞðPaÞ ¼ 12:913  20197=T

The calculated vapor pressures of Am over AmN are

The vapor pressure of Am over AmN is higher than

those of other actinide vapor species over their

respective nitrides

Data on the heat capacities of actinide nitrides are

very limited due to the experimental difficulties

In this section, the heat capacities of uranium nitride

UN, plutonium nitride PuN, neptunium nitride NpN,and americium nitride AmN are summarized

heat capacity of UN based on a comparison of ninedata sets; these seem to agree with each other at lowtemperatures but their data are limited, and to someextent scattered, at elevated temperatures The pre-viously reported values for the heat capacity of UNexhibit an almost linear increase with temperature,

trend at temperatures over 1500 K This behavior isanalogous to that of the actinide carbides, as pointed

the results of Conway and Flagella Thus, the heatcapacity data reported by Hayes et al can be consid-ered reliable The heat capacity of UN, expressed byHayes et al., is as follows:

Information on the heat capacity of PuN is veryscarce and is limited to the low temperatures More-over, the two data sets on PuN given by Alexander

Pu(g) over PuN Np(g) over NpN

Figure 17 Temperature dependence of partial pressure of

Np(g) over Np(1), Np(g) over NpN(s) and Am(g) over AmN(s)

together with those of U(g) and Pu(g) over UN(s) and PuN(s)

as a function of temperature Adapted from Suzuki, Y.;

Maeda, A.; Arai, Y.; Ohmichi, T J Nucl Mater 1992, 188,

239–243; Nakajima, K.; Arai, Y.; Suzuki, Y J Nucl Mater.

1997, 247, 33–36; Ackermann, R J.; Rauh, E G J Chem.

Thermodyn 1975, 7, 211–218; Takano, M.; Itoh, A.;

Akabori, M.; Minato, K.; Numata, M In Proceedings of

GLOBAL 2003, Study on the Stability of AmN and (Am,Zr)N,

New Orleans, LA, Nov 16–20, 2003; p 2285, CD-ROM.

500 50

60 70

1000 1500 2000

Figure 18 Heat capacities of PuN Data from Alexander,

C A.; Clark, R B.; Kruger, O L.; Robins, J L Plutonium and Other Actinides 1975; North-Holland: Amsterdam, 1976; pp 277; Oetting, F L J Chem Thermodyn 1978, 10, 941–948.

et al.65 and Oetting75are not consistent Matsui and

of PuN and have argued that the Oetting correlation

is more reliable Therefore, the heat capacity data, as

reported by Oetting, is given here The heat capacity

function of PuN given by Oetting is

The heat capacities of UN, PuN, and (U,Pu)N are

solutions can be estimated from those of its raw

materials with the same structure on the basis of the

additive law, it can be expected that the values for

the (U,Pu)N solid solutions are an intermediate

between those of UN and PuN However, the heat

capacities of (U,Pu)N, as reported by Alexander

UN, by Hayes et al., and PuN, by Oetting In addition,

the temperature dependencies of the heat capacities

of PuN and its solid solutions are almost linear,

although it has been suggested that they can shift

toward larger values at elevated temperatures, as

does UN It is considered that these discrepancies

are probably due to the lack of experimental data

Thus, it is necessary to obtain the accurate heat

capacity of PuN and (U,Pu)N

Recently, the heat capacities of NpN and AmN

of NpN and AmN were prepared by the carbothermicreduction of their respective oxides The enthalpyincrements were measured using a twin-type dropcalorimeter in a glove box The heat capacities weredetermined by derivatives of the enthalpy increments.The measured heat capacity of NpN is expressed by

distinct differences, the heat capacity of AmN wasslightly lower than those of UN, NpN, and PuN.The heat capacities of (Np,Am)N and (Pu,Am)Nsolid solutions were also obtained The heat capa-cities decreased slightly with an increase in Amcontent This tendency was attributed to the heatcapacity of AmN being slightly smaller than those

Some data on the Gibbs free energy of formation foractinide nitrides exist In this section, the Gibbs freeenergy of formation of uranium nitride, UN, pluto-nium nitride, PuN, uranium and plutonium mixednitride, (U,Pu)N, neptunium nitride, NpN, andamericium nitride, AmN are summarized

Figure 19 Heat capacities of UN, PuN, and (U,Pu)N Data

from Hayes, S L.; Thomas, J K.; Peddicord, K L J Nucl.

Mater 1990, 171, 300–318; Oetting, F L J Chem.

Thermodyn 1978, 10, 941–948; Alexander, C A.;

Ogden, J S.; Pardue, W M Thermophysical properties of

(UPu)N In Plutonium 1970 and Other Actinides PT.1; 1970,

17, 95–103; Kandan, R.; Babu, R.; Nagarajan, K.; Vasudeva

Rao, P R Thermochim Acta 2007, 460, 41–43.

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