Thermal conductivity and emissivity measurements of uranium carbides

Nuclear Instruments and Methods in Physics Research B 360 (2015) 46–53

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Thermal conductivity and emissivity measurements of uranium carbides S. Corradetti a,⇑, M. Manzolaro a, A. Andrighetto a, P. Zanonato b, S. Tusseau-Nenez c a

INFN Laboratori Nazionali di Legnaro, Viale dell’Università 2, 35020 Legnaro (PD), Italy Università di Padova, Dipartimento di Scienze Chimiche, via Marzolo 1, 35131 Padova, Italy c Institut de Physique Nucleaire (UMR8608), CNRS/IN2P3 – Université Paris Sud, 91406 Orsay Cedex, France b

a r t i c l e

i n f o

Article history: Received 5 May 2015 Received in revised form 6 July 2015 Accepted 27 July 2015 Available online 14 August 2015 Keywords: Uranium carbide Thermal conductivity Emissivity Radioactive ion beams

a b s t r a c t Thermal conductivity and emissivity measurements on different types of uranium carbide are presented, in the context of the ActiLab Work Package in ENSAR, a project within the 7th Framework Program of the European Commission. Two specific techniques were used to carry out the measurements, both taking place in a laboratory dedicated to the research and development of materials for the SPES (Selective Production of Exotic Species) target. In the case of thermal conductivity, estimation of the dependence of this property on temperature was obtained using the inverse parameter estimation method, taking as a reference temperature and emissivity measurements. Emissivity at different temperatures was obtained for several types of uranium carbide using a dual frequency infrared pyrometer. Differences between the analyzed materials are discussed according to their compositional and microstructural properties. The obtainment of this type of information can help to carefully design materials to be capable of working under extreme conditions in next-generation ISOL (Isotope Separation On-Line) facilities for the generation of radioactive ion beams. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The use of radioactive ion beams (RIBs) [1] in nuclear physics has been established as a fundamental method to both expand the current knowledge on nuclei very far from stability and provide a powerful tool for applications in several fields. Facilities which produce RIBs, or that are now being developed to do so in the near future, are traditionally divided into two main categories, depending on the way radioactive isotopes are generated and how the beam is formed and transported [2]: the in-flight method and the ISOL (Isotope Separation On-Line) technique. In the first case [3], radioactive isotopes are created by the interaction (fragmentation or fission) of a primary beam of heavy ions with a thin target. The reaction fragments are ejected in the forward direction with respect to that of the incident beam and subsequently separated with a fragment separator and sent to the experimental areas. In the ISOL technique [4], spallation or fission reactions are obtained by bombarding targets made of heavy elements (mainly uranium), with intense beams of light particles (typically protons). The produced neutral radioisotopes diffuse out of the target and effuse towards an ion source, where they are ionized. After passing through different stages of separation

⇑ Corresponding author. Tel.: +39 049 8068332. E-mail address: [email protected] (S. Corradetti). http://dx.doi.org/10.1016/j.nimb.2015.07.128 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

and manipulation, the formed beam is post-accelerated to the experimental halls. The core of an ISOL facility is represented by its target-ion source complex. In particular, the choice of the material constituting the target is vital to ensure excellent performances in terms of quantity and regularity of the isotopic yields over the duration of beam delivery. As stated above, during the operation of an ISOL facility the target material is involved in different processes:  Generation of particles through nuclear reactions between the primary beam and the target nuclei.  Diffusion of the produced isotopes inside the grains constituting the material.  Effusion of isotopes from the grain surface inside the material porosity towards the target surface, and subsequent effusion towards the ion source. Besides thermodynamical processes, chemical reactions of the produced isotopes with the surrounding materials (target and enclosures) are important factors which can affect their release. In order to increase the rates of diffusion and effusion, the target is kept at high temperature (2000 °C) in high vacuum (106 mbar or less) during operation [5]. This aspect, combined with the extreme thermomechanical stresses which are generated during the primary beam irradiation, makes the correct choice of the target material composition and properties even more fundamental.

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Due to its suitable nuclear (high cross section for the reaction between protons and 238U) and thermomechanical (high critical temperature) properties, and the relative facility to synthesize it, uranium carbide is by far the most used material as a target for RIB production in ISOL facilities [5]. In most cases, targets are not made by fully dense uranium carbide, but instead contain a variable amount of graphite and residual open interconnected porosity, conditions which have been found to favor the isotopic release [6]. This material is commonly referred to as UCX. The ActiLab Work Package in ENSAR, a project within the 7th Framework Program of the European Commission, brought together different European laboratories (CERN, INFN-LNL, GANIL, IPNO and PSI) with the aim of studying and testing the synthesis and characterization of innovative materials based on uranium carbide [4]. In this paper, thermal conductivity and emissivity measurements performed on uranium carbides of different composition and microstructural properties, synthesized in the framework of this Work Package, are reported. The obtainment of experimental data relative to these two properties is considered a fundamental step towards the development of future high power ISOL facilities, such as EURISOL [7], in which the target materials will be used in even more extreme environments with respect to the current ones. In literature, very few data are available relatively to the thermal conductivity of uranium carbides. The most notable results were obtained by De Coninck et al. [8–10], who reported thermal conductivity values for UC, UC2 and U2C3 by means of thermal diffusivity measurements using the laser-flash technique, and at the same time performed emissivity measurements using infrared pyrometers. More recently, thermal conductivity measurements on porous UC2 were performed by Greene and co-authors [11] in the framework of the research on targets for ISOL facilities. Other data about the thermal conductivity of UC and UC2 is available in [12–15]. A summary of the available data concerning thermal conductivity and emissivity of UC and UC2 is reported in Table 1. 2. Thermal conductivity estimation using inverse analysis The method here described for the estimation of thermal conductivity of uranium carbide is based on the one already reported

by Manzolaro et al. [16], successfully applied for graphite, silicon carbide and lanthanum carbide SPES [17] target prototypes at INFN-LNL. In order to make use of the method in the case of uranium carbide, a new setup was developed at Padova University in a dedicated actinide chemistry laboratory, in which the research and development of materials for the SPES target is carried out. Details about the experimental device, specifically developed for the measurements here reported, are given in section 3. The experimental technique is based on direct measurements of temperature and emissivity on a sample, under steady-state conditions, which are then converted to thermal conductivity data making use of the inverse analysis method [18]. From the experimental point of view, the method is based on the creation of a temperature gradient on the top surface of a thin disc, heated by thermal radiation thanks to a hot graphite crucible placed at a certain distance from it, directly facing its bottom surface. A setup of this type is schematized in Fig. 1 [16]. Since the heating of the crucible is induced by Joule effect and the graphite resistivity depends on temperature, the thermal and the electrical problems controlling the system are coupled [16]. Moreover, since the measurements are conducted in high vacuum, the obtained thermal interaction between the crucible and the disc is a result of conduction and radiation only. This type of problem is characterized by the diffusion of energy within the solid region V and the radiative heat transfer between the surfaces forming the enclosure Senc. The conductive problem can be expressed in general terms as [19]:

      @ @T @ @T @ @T @T þ þ þ q_ ¼ qc k k k @x @x @y @y @z @z @t

ð1Þ

where T(x,y,z) is the temperature field in the solid region V, t is the time, q is the density of the material constituting V, c is the specific heat, k is the thermal conductivity and q_ is the volumetric heat source. The radiative part of the problem can be represented as [20]: N  X dji i¼1

ei

 F ji

  1  ei

ei

 qenc;i ¼

N X ðdji  F ji Þ  r  T 4i

ð2Þ

i¼1

where dji is the Kronecker delta, ei is the hemispherical total emissivity of surface i, Fji is the radiation view factor, qenc,i is the net rate of radiative energy loss per unit area (flux) of surface i, r is

Table 1 Thermal conductivity and emissivity data reported in literature for UC and UC2. (a) Spectral emissivity at 0.65 lm, (b) spectral emissivity at 2.3 lm, (c) data relative to a porous sample containing UC2 and graphite, all the other values reported in this table are relative to highly dense materials.

Thermal conductivity (W/m °C)

UC

UC2

23  20 (650  1150 °C) 20  26 (1150  2250 °C) [8]

13  18 (600  1740 °C) 19  20 (1800  2060 °C) [9]

23  21 (50  450 °C) 21  80 (450  2300 °C) [12]

5  8 (1500  1880 °C) [11] (c)

21  17 (100  400 °C) [14] 22  24 (75  2130 °C) [15]

11  20 (0  1500 °C) [13] 12  21 (100  2300 °C) [12]

7  6 (200  400 °C) [14] Emissivity

0.49  0.48 (1100  2250 °C) [8] (a)

0.50  0.53 (1040  1740 °C) 0.55 (1800  2060 °C) [9] (a) 0.44  0.45 (1040  1740 °C) 0.50 (1800  2060 °C) [9] (b)

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Fig. 2. CAD view of the thermal conductivity estimation setup, showing the positions of the temperature measurement points. In both cases, a spot of 4 mm diameter was considered in the numerical analysis, corresponding to the experimental one (pyrometer spot).

where NHC is the number of current steps used to power the heater and so to generate temperature gradients in the sample top surface, TC_COMP_i and TP_COMP_i are the numerically computed temperatures at the center and at the periphery of the sample disc, respectively, TC_MEAS_i and TP_MEAS_i are the correspondent measured values. f is the vector of the unknown coefficients: Fig. 1. (a) CAD view of the thermal conductivity estimation setup, (b) sample heated by thermal radiation.

the Stefan–Boltzmann constant and Ti is the absolute temperature of surface i. A problem of this type can be solved iteratively [21], applying the radiation problem outputs (qenc) as boundary conditions for the conduction one, and vice versa (Ti) [16]. From the electrical point of view, the heat power dissipation per unit volume of material can be obtained by the scalar product [16]:

q_ ¼ rV  j

ð3Þ

where j(x,y,z) is the field of current density and rV is the gradient of the field of the electric potential V(x,y,z). In order to solve the problem above described, some specific properties (thermal conductivity, emissivity and electrical resistivity) of the materials constituting the tested sample, the crucible and the surroundings must be known. By making use of suppliers datasheets and by direct measurements (in the case of emissivity), the unknown quantities can only be limited to the thermal conductivity of the sample material. The parameterization of this property can be expressed in a linear way with respect to temperature, in the range of temperatures in which the here reported measurements were performed [16]:

k ¼ C0 þ C1 T

N HC X

½T C

COMP i ðfÞ

 TC

MEAS i ðfÞ

2

i¼1

þ ½T P

ð6Þ

The minimization of J with respect to f, is performed numerically making use of the First Order Optimization Method, an accurate optimization tool implemented in the ANSYSÒ [22] software. The solution of the direct problem is obtained by means of a finite element numerical model in ANSYSÒ, using a coupled thermal-electric element for the discretization of the physical domain, as reported in [16,23,24]. The confidence interval of the so obtained thermal conductivity estimation is based on the standard deviation of C0 and C1, according to [18,25]:

rf m

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u8" #1 9 u< 2 = @ JðfÞ u for m ¼ 1; 2 ¼ rt : @f p @f q ;

ð7Þ

mm

where r is the standard deviation of the temperature measurement. Considering a normal distribution for the measurement errors, confidence intervals at the 95% confidence level for the estimated parameters fm can be obtained as:

f m  1:96rf m < f m < f m þ 1:96rf m

for m ¼ 1; 2

ð8Þ

where f m is the exact value of the parameter.

ð4Þ

In the above reported expression, C0 and C1 are unknown parameters to be obtained, and T is the average temperature between center and periphery. Fig. 2 shows the approximate positions of the center and periphery measurement points. The inverse (optimization) problem is based on the minimization of the differences between the experimentally determined temperatures and those obtained numerically, in two different positions of the heated sample. The objective of the optimization is the minimization of the residual function [16]:

JðfÞ ¼

f ¼ fC 0 ; C 1 g

COMP i ðfÞ

 TP

2 MEAS i ðfÞ

ð5Þ

3. Experimental 3.1. Uranium carbide samples preparation The samples tested in this work were prepared in different laboratories, in the ActiLab framework. Table 2 shows the main geometrical and compositional properties of the samples. One of the samples (SPES MM) was produced at Padova University, in the setup described in [5]. Its final composition was obtained by reacting UO2 (Cerac) with graphite (Sigma–Aldrich, particle size < 45 lm) at 1700 °C in high vacuum (106 mbar), according to the reaction:

UO2 þ 6C ! UC2 þ 2C þ 2CO

ð9Þ

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Table 2 Properties of the tested samples. The wt.% percentages of UC, UC2 and C were calculated with Rietveld refinement on XRD data. In the case of SPES MM, XRD analysis was not available. Sample

Production site

Main phase

wt.% UC

wt.% UC2

wt.% C

Diameter (mm)

Thickness (mm)

Density (g/cm3)

SPES MM Gatchina ParrNe 894 ParrNe BP OXA COMP30

UNIPD PNPI IPNO IPNO IPNO IPNO

UC2 UC UC2 UC2 UC UC2

– 86.9 9.6 5.3 70.5 8.6

– 13.1 81.6 86.1 29.5 91.0

– 0.0 8.8 8.6 0.0 0.4

28.9 13.2 13.0 12.6 7.4 8.3

1.4 1.0 1.9 1.5 1.9 2.5

3.9 12.4 3.1 4.4 8.7 4.5

Prior to mixing it with graphite, UO2 powder was finely size reduced to below 20 lm by using a vibratory micro mill (Pulverisette 0, Fritsch) equipped with agate jar and grinding ball. According to [5], a minor amount of UC was with any probability obtained in this sample at the end of the thermal treatment. The sample referred to as Gatchina, made available to the ActiLab collaboration by PNPI (Gatchina, Russia), was composed mainly of UC, treated to obtain a high density material, as reported in [26]. The remaining pellets were synthesized at IPNO, some of them already reported in a previous off-line isotopes release study [6], by heat treating uranium and carbon sources up to 1800 °C in vacuum (105 mbar). Two samples (ParrNe 894 and ParrNe BP) were obtained by reacting UO2 (Areva) with graphite (Cerac, particle size < 44 lm), according to the same scheme of Reaction (9), leading to the formation of UC2 and C with an approximate 1:2 ratio, and a minor amount of UC. The only difference between them was relative to the UO2-graphite mixing methodology: in one case (ParrNe 894) a mixer mill (Retsch PM 200) was used, in the other (ParrNe BP) the powders were mixed and grinded making use of a planetary ball mill (Retsch PM 100). Uranium oxalate dihydrate (synthesized at IPNO) was used as a uranium source for the synthesis of the OXA and COMP30 samples, according to the reactions:

UðC2 O4 Þ2 ; 2H2 O þ 3C ! UC þ 2CO2 þ 4CO þ 2H2 O

ð10Þ

The difference between the two samples was the carbon source of Reaction (10): in the OXA case only graphite was mixed with oxalate, whereas to obtain COMP30 a 3:1 (molar) mixture of graphite and carbon fibers (TORAYCA T300, 7 lm diameter and 10–150 lm length) was used [6]. In both cases, the reagents were mixed in an alcoholic solution that was homogenized ultrasonically. Even if designed to obtain mainly UC, the COMP30 composition was demonstrated to give rise mainly to UC2 after thermal treatment, due to the presence of the carbon fibers [6]. 3.2. Setup for the measurements The core of the experimental apparatus, contained inside a water-cooled high vacuum chamber, is constituted by a graphite crucible, heated by Joule effect by means of intense electrical currents (up to 370 A) passing through it. This heating system was used for both the thermal conductivity estimations and for the emissivity direct measurements. What follows is a description of the experimental setup in the two cases. For thermal conductivity estimations, the sample to be tested is in the form of a thin disc resembling the typical SPES target geometry, but with 30 mm diameter instead of 40 mm. It is suspended above the crucible, at a distance of about 3.5 mm, by means of three tungsten bars mounted on a graphite structure (Fig. 3). Differently of what was reported for the setup previously developed at INFN-LNL [16], in this case the graphite structure is not mounted on a thin stainless steel plate, but is directly fixed to the furnace main base. The choice of materials and the specific geometrical configuration were accurately studied in order to

Fig. 3. Disc suspended over the crucible in the newly developed setup.

minimize the thermal expansion of the components at high temperature, thus maintaining the crucible-disc distance throughout all the measurements. The heating of the crucible, which presents a round top area, results in a homogeneous temperature distribution on it. This reflects in the creation of a temperature gradient on the suspended disc facing it, measurable by making use of a dual-frequency infrared pyrometer (IRCON ModlineÒ 5R) [27] placed on the top of the vacuum chamber. A boro-silicate glass window, almost completely transparent to infrared radiation, is placed on top of the chamber, allowing the measurements of both temperature and emissivity – necessary for the conductivity estimation – by the pyrometer. The data relative to the two different analyzed positions (center and periphery) of the samples top surface are collected in separate heating cycles, adopting the same ramps for the crucible heating currents. To carry out the emissivity direct measurements on the smaller size samples, the graphite-tungsten supporting system described before can be removed, allowing the direct placement of the samples on top of the graphite crucible. This allows reaching higher temperatures with respect to the thermal conductivity measurements case. The emissivity measurements were performed according to the method reported in [27]. 4. Results and discussion 4.1. Thermal conductivity As in the case of the previously developed setup [16], a calibration of the system was performed, by measuring the thermal conductivity of a graphite sample and comparing it to the supplier datasheet. The material chosen for the test sample was again angstrofine graphite (POCO EDM-AF5). Fig. 4 reports the temperature measurements (experimental) on a 30 mm diameter disc with a thickness of 1.3 mm, in center and periphery, compared to the value obtained during the ANSYSÒ simulation

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Fig. 4. Comparison between experimental and numerical temperatures obtained on the system calibration with a graphite disc. The error on each temperature measurement T corresponds to 0.5% of T plus 2 °C.

(numerical), imposing as a thermal conductivity value the one indicated in the supplier datasheet and for emissivity the one measured experimentally. The calculated numerical temperatures correspond to the mean values in the 4 mm diameter spots shown in Fig. 2. The method here reported, with the current setup geometry, is applicable for samples with diameters between 28 and 40 mm and thicknesses below 2 mm. Temperature measurements were carried out by increasing the heating current with steps of 5 A, from 160 A to 240 A, keeping it constant for about 15 min at each step, in order to measure temperature in a steady state. As in [16], the correspondence between the two sets of data is less evident in the case of the periphery temperatures with respect to those obtained in the center, due to uncertainties in the actual pyrometer spot positioning during the experimental measurements. In Fig. 5, the values of thermal conductivity for different temperatures are shown, with error bands calculated as in Section 2, compared to those reported in [16]. With the set of data here obtained, the calibration of the system was considered successful, so that the method was subsequently applied to the determination of the thermal conductivity of UCX, carrying out measurements on a SPES MM type sample. In Figs. 6 and 7, the comparison of the numerical and experimental temperatures and the thermal

Fig. 5. Thermal conductivity of a POCO EDM-AF5 sample. Comparison with the data obtained in [16] is reported.

Fig. 6. Comparison between experimental and numerical temperatures obtained on the SPES MM UCX disc. The error on each temperature measurement T corresponds to 0.5% of T plus 2 °C.

Fig. 7. Thermal conductivity of the SPES MM UCX sample.

conductivity trend with respect to temperature are reported, respectively. As in the case of the graphite sample, the heating current was increased with steps of 5 A, but in this case from 200 A to 290 A. Each reported experimental temperature is the average value of five repeated measurements. Fig. 8 reports the trends of the residual function J and the parameters C0 and C1 for different iterations of the calculation model, until convergence (C0 = 3.503 * 101 W/m °C, C1 = 1.848 * 102 W/m °C2). In Fig. 9 an image of the ANSYS model displaying the simulated temperature gradients is shown. The values here reported represent among the first experimental estimations of the thermal conductivity of materials specifically designed as ISOL targets, containing uranium dicarbide, graphite, a minor amount of UC and pores, even if the power output of the experimental system did not allow reaching temperatures comparable to those that will be obtained on-line. The comparison of these trend with the data shown in Table 1 is not immediate since in this case the material is not composed by fully dense UC2, as in most reported cases in literature. Data reported for a material similar to the one here tested [11] were obtained at higher temperatures than those here reported, showing an increasing trend of conductivity with temperature, so opposite to the one here

S. Corradetti et al. / Nuclear Instruments and Methods in Physics Research B 360 (2015) 46–53

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Fig. 8. J, C0, and C1 versus the number of numerical iterations for the SPES MM UCX sample. Initial guess: C0 = 40 W/m °C and C1 = 0.018 W/m °C2.

Fig. 10. Emissivity of the tested samples. Fig. 9. ANSYSÒ model displaying the simulated temperature gradient in the UCX disc at a current of 290 A. The temperature analysis spots are reported.

obtained. Moreover, according to most of the data reported in literature, highly dense and pure UC2 should exhibit increasing conductivity with temperature, but the presence of residual graphite, with its decreasing k with T, could have resulted in a different trend in the case of the material here reported. The fact that for the higher considered temperatures the numerical and experimental data tend to diverge, especially in the case of the periphery measurements (Fig. 6), suggests that for UCX a linear decreasing correlation between conductivity and temperature may not be maintained at high temperature, differently of what was found for graphite and silicon carbide [16]. 4.2. Emissivity measurements Fig. 10 reports the thermal emissivity trends with respect to temperature for the investigated materials. The results relative to the SPES MM sample are reported as a comparison, even if they were not obtained in the setup specifically designed for the emissivity evaluation, but during the temperature measurements carried out for the conductivity estimation described in Section 4.1. The accuracy and precision of the emissivity evaluation strongly

depends on the correct positioning of the pyrometer spot, especially in terms of focusing. For this reason, the measurements were repeated three times for each sample, resulting in every case in standard deviations below 5%. The accuracy of the emissivity measurements in this setup was evaluated testing a POCO EDM-AF5 sample and comparing its emissivity trend in the range 1000 °C  1600 °C with the one reported in [27], which was found to be consistent with data found in literature. Also in this case the differences were found to be way below 5%. The obtained results are very different from one another, in terms of both emissivity values and dependence on temperature. In general, with reference to Table 2, materials containing UC2 as a main phase were found to have higher and more stable values of emissivity, whereas in UC-based ones (Gatchina and OXA) a drop of emissivity at temperatures of about 1200 °C  1300 °C was observed. This behavior was accompanied by the sticking of the tested pellet on the heating crucible top surface, indicating the reaction between UC and C. As indeed reported in the U–C phase diagram by Manara et al. [28], above 1120 °C UC can exist in both hypostoichiometric and hyperstoichiometric forms, indicated as UC1±X. Moreover, at low oxygen pressures (104 mbar) above 900 °C UC can react with residual O creating layers of UC2 and UO2 on its surface, with liberation of CO and CO2 [29,30]. All

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COMP30 samples. In the case of SPES MM, no SEM investigations of the surface were made, but its microstructure can be related to the one reported in [5], where the same type of production process was used, with the difference that UO2 was not milled prior to mixing it with graphite. In that case, a regular distribution of UC2 grains and residual graphite was obtained. For the SPES MM sample it is probable that, having homogenized the size of the UO2 powder used as a reagent, a similar or higher microstructure regularity was obtained, leading to a uniform graphite and pores distribution on the top surface of the sample, which could have had a positive effect on emissivity with respect to the ParrNe composition. 5. Conclusions

Fig. 11. Pressure inside the vacuum chamber during the emissivity measurement of three different samples.

the aforementioned processes could have resulted in the instability of the emissivity value, leading to a final value of about 0.4 for both samples although their emissivity trend over the considered temperatures is different. Fig. 11 shows the comparison between the residual pressure inside the vacuum chamber in the case of Gatchina, ParrNe BP and ParrNe 894, in which the gas evolution at low temperatures and the general instability of the UC-based material is evident. Differences of emissivity among UC2-based samples could be attributed to either compositional (e.g. actual amount of residual graphite, presence of carbon fibers) or microstructural (type and amount of total/open porosity) aspects. In particular, the SPES MM sample seems to possess the highest emissivity among all the investigated materials, and the values here presented are in accordance with those reported in [5] for samples prepared with the same technique. In that case, an increasing trend of emissivity with respect to temperature was observed, but the measurements were carried out during the thermal treatment to produce and sinter the UC2 + 2C mixture, whereas in the case here reported the measurement campaign was conducted on a previously prepared and already heated sample. The fact that this material has a higher emissivity with respect to other UC2-based ones (ParrNe BP, ParrNe 894 and COMP30) could be attributed to differences in the UC2:C ratio (lower in SPES MM, higher in ParrNe), which could have been obtained during the production stage and resulted in a deviation from the designed 1:2 ratio. The presence of free carbon was found to have the effect of increasing the emissivity of UCX [5] with respect to the one of pure, fully dense and polished UC2 samples reported in [9] and already shown in Table 1. In the case of COMP30, XRD measurements [6] showed that it contained only a small amount of free carbon, if compared to ParrNe, resulting in a lower emissivity value throughout the entire investigated temperature field. This aspect is confirmed in Table 2, where the UC/UC2/C concentrations as calculated by Rietveld refinement on XRD data are reported. As already mentioned above the microstructure, in terms of porosity amount, type and distribution could have had an effect on the emissivity measured on the top surface of the samples. As reported in [6], important morphological differences were found between COMP30 and ParrNe internal and superficial structures. While for the former an irregular distribution of pores and residual carbon fibers was reported, in the latter, despite a more homogeneous pore size and dispersion in the sample, irregular grain sizes were found. This aspect was found to result in a small pore interconnectivity, which reduced the isotope release with respect to the

Thermal conductivity estimations were obtained applying the inverse parameter method to experimental data obtained on a uranium carbide (UCX) sample with geometry resembling that of the SPES target, and more in general of an ISOL target. The composition of the sample consisted of uranium dicarbide, graphite and a minor amount of uranium monocarbide, thus making it difficult to compare the results here reported with the one reported in literature for pure, fully dense UC2. The preliminary data obtained on a graphite test sample was consistent with the one obtained during the model validation in [16], consisting on a linear decreasing of conductivity with temperature in the 500  1200 °C range. In the case of UCX a deviation from linearity was observed at the higher tested temperatures (up to 1300 °C), suggesting the use of different models of k–T relationship for future studies, in which the setup will be upgraded to be able to obtain experimental data at higher temperatures, more similar to the one which are used in the on-line operation of targets for radioactive ion beam production. Emissivity of different types of uranium carbides, either composed mainly of UC or UC2, in the range of 1000  1600 °C was measured by means of a dual frequency infrared pyrometer. The results were related to the different composition and microstructure of the materials, showing that UC-based materials tend to have a more irregular emissivity evolution leading to smaller values with respect to those found for UC2-based ones at high temperature. Acknowledgments The authors would like to thank all the people directly involved in the ActiLab collaboration: T. Stora, A. Gottberg, H. FranbergDelahaye, J. Grinyer, C. Lau, I. Gunther-Leopold, M. Martin. References [1] Y. Blumenfeld, T. Nilsson, P. Van Duppen, Phys. Scr. T152 (2013) 014023. [2] T. Nilsson, Nucl. Instr. Meth. Phys. Res. B 317 (2013) 194–200. [3] B. Harss, R.C. Pardo, K.E. Rehm, F. Borasi, J.P. Greene, R.V.F. Janssens, C.L. Jiang, J. Nolen, M. Paul, J.P. Schiffer, R.E. Segel, J. Specht, T.F. Wang, P. Wilt, B. Zabransky, Rev. Sci. Instr. 71 (2000) 380–387. [4] T. Stora, Nucl. Instr. Meth. Phys. Res. B 317 (2013) 402–410. [5] L. Biasetto, P. Zanonato, S. Carturan, P. Di Bernardo, P. Colombo, A. Andrighetto, G. Prete, J. Nucl. Mater. 404 (2010) 68–76. [6] B. Hy, N. Barré-Boscher, A. Özgümüs, B. Roussière, S. Tusseau-Nenez, C. Lau, M. Cheikh Mhamed, M. Raynaud, A. Said, K. Kolos, E. Cottereau, S. Essabaa, O. Tougait, M. Pasturel, Nucl. Instr. Meth. Phys. Res. B 288 (2012) 34–41. [7] Y. Blumenfeld, P. Butler, J. Cornell, G. Fortuna, M. Lindroos, Int. J. Mod. Phys. E 18 (2009) 1960–1964. [8] R. De Coninck, W. van Lierde, A. Gijs, J. Nucl. Mater. 57 (1975) 69–76. [9] R. De Coninck, R. De Batist, A. Gijs, High Temp. High Press. 8 (1976) 167–176. [10] R. De Coninck, W. van Lierde, A. Gijs, J. Nucl. Mater. 46 (1973) 213–216. [11] J.P. Greene, T. Burtseva, J. Neubauer, J.A. Nolen, A.C.C. Villari, I.C. Gomes, Nucl. Instr. Meth. Phys. Res. B 241 (2005). [12] L. Grande, B. Villamere, L. Allison, S. Mikhael, A. Rodriguez-Prado, I. Pioro, J. Eng. Gas Turbines Power 133 (2011) 022901. [13] Thermophysical Properties of Materials for Nuclear Engineering: A Tutorial and Collection of Data, IAEA, Vienna, Austria, 2008.

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