Name: laser-heating and radiance spectrometry for the study of nuclear materials in conditions simulating a nuclear power plant accident
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Description: Watch this Scientific Journal Video about Laser-heating and Radiance Spectrometry for the Study of Nuclear Materials in Conditions Simulating a Nuclear Power Plant Accident at JoVE.com

The authors are indebted to the European Commission for funding the present research under its institutional research programs. In addition, part of the presented research was financed through the EC 6th Framework Program under the F-BRIDGE project and 7th FP under the SAFEST and GENTLE projects.

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State of Knowledge. , 1835-1838 (2015).</li><li>Manara, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+ZrC-C+eutectic+structure+and+melting+behaviour:+A+high-temperature+radiance+spectroscopy+study.">The ZrC-C eutectic structure and melting behaviour: A high-temperature radiance spectroscopy study.</a> J. Eur. Ceram. Soc. 33, 1349-1361 (2013).</li><li>Manara, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+melting+behaviour+of+calcium+monoxide+under+different+atmospheres:+A+laser+heating+study.">On the melting behaviour of calcium monoxide under different atmospheres: A laser heating study.</a> J. Eur. Ceram. Soc. 34, 1623-1636 (2014).</li><li>Welland, M. J., Thompson, W. T., Lewis, B. J., Manara, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Computer+simulations+of+non-congruent+melting+of+hyperstoichiometric+uranium+dioxide.">Computer simulations of non-congruent melting of hyperstoichiometric uranium dioxide.</a> J. Nucl. Mater. 385, 358-363 (2009).</li><li>Olander, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nuclear+Fuels-+Present+and+future.">Nuclear Fuels- Present and future.</a> J. Nucl. Mater. 389, 1-22 (2009).</li><li>De Bruycker, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+melting+behaviour+of+plutonium+dioxide:+A+laser-heating+study.">The melting behaviour of plutonium dioxide: A laser-heating study.</a> J. Nucl. Mater. 416, 166-172 (2011).</li><li>Manara, D., Boboridis, K., Morel, S., De Bruycker, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+UC2-x+-+Carbon+eutectic:+A+laser+heating+study.">The UC2-x - Carbon eutectic: A laser heating study.</a> J. Nucl. Mater. 466, 393-401 (2015).</li><li>Barrachin, M., Chevalier, P. Y., Cheynet, B., Fischer, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+modelling+of+the+U-O-Zr+phase+diagram+in+the+hyper-stoichiometric+region+and+consequences+for+the+fuel+rod+liquefaction+in+oxidising+conditions.">New modelling of the U-O-Zr phase diagram in the hyper-stoichiometric region and consequences for the fuel rod liquefaction in oxidising conditions.</a> J. Nucl. Mater. 375, 397-409 (2008).</li><li>Gu&#233;neau, C., Chartier, A., Van Brutzel, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermodynamic+and+thermophysical+properties+of+the+actinide+oxides.">Thermodynamic and thermophysical properties of the actinide oxides.</a> Comp Nucl Mater. 2, 21-59 (2012).</li><li>Preston-Thomas, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+International+Temperature+Scale+of+1991+(ITS-90).">The International Temperature Scale of 1991 (ITS-90).</a> Metrologia. 27, 3-10 (1990).</li><li>Preston-Thomas, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Erratum:+The+International+Temperature+Scale+of+1990+(ITS-90).">Erratum: The International Temperature Scale of 1990 (ITS-90).</a> Metrologia. 27, 107 (1990).</li><li>Bedford, R. 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The authors have nothing to disclose.

The laser-heating radiation spectroscopy technique presented here is recognized as an innovative and effective method for the investigation of very high-temperature and melting behavior of refractory materials15, 16. Thanks to its remote and almost container-less nature, it is particularly suited for the experimental study of radioactive nuclear materials and the simulation of core meltdown accidents in NPPs, as shown by the example results presented here.
While evaluating experimental data obtained with the current method, one should no doubt be careful about the correct assignment of experimental points to phase transitions. In fact, at very high temperatures, material kinetics can be extremely fast, and several difficult-to-control phenomena may occur, such as non-congruent vaporization, segregation, compound dissociation, etc. As the comparison with more traditional heating methods (like induction furnaces) demonstrates, the possible occurrence of such phenomena justifies the use of a fast heating and cooling technique like the current one. On the other hand, doubts may arise about the effective stabilization of thermodynamic equilibrium conditions under the current heating conditions. As explained in the procedure section, such conditions cannot be guaranteed during the fast laser-heating part of the thermal cycles. However, thermodynamic equilibrium conditions are certainly produced on the cooling stage. This statement was validated with the help of computer codes simulating the current experiments and based on near-equilibrium mass and heat diffusion in the presence of local phase transitions11. Nonetheless, thermodynamic equilibrium conditions should always be cross-checked experimentally, typically by measuring well-assessed phase transition temperatures in compounds that can be taken as references. This was realized in the present work with the melting/solidification points of W, Mo (recommended as secondary reference temperatures in the International Temperature scale of 199017,18,19), UO2, and the ZrC-C eutectic9. Measuring such reference points is also necessary in order to assess the accuracy and uncertainty of the present approach.
Given the extreme conditions and phenomena produced in the laser-heating experiments, a precise uncertainty analysis is paramount for the usability of the data produced. For successful measurement campaigns, the cumulative uncertainty affecting the current phase transition temperature data should amount to &#177;1% of the absolute temperature, with a 2-standard-deviation coverage factor (95% confidence). Such uncertainty bands can be larger for complicated materials, where, for example, non-congruent vaporization may change the actual sample composition in an uncontrollable way during the experiments. Such uncertainty should take into account the errors due to the calibration procedure, the NSE determination, the sample stability (i.e., the repeatability, over successive laser shots, of experimental phase transition temperatures), etc. An example of uncertainty analysis for the melting/freezing point of PuO2 is reported in Table 1. The various uncertainty contributions can be considered as independent and combined according to the error propagation law3.
<img alt="Table 1" src="/files/ftp_upload/54807/54807table1.jpg" /> 
Table 1: Example of uncertainty analysis for the melting/freezing point of PuO2 (Reference13). 
The meaning and value of c2 is reported in the introduction section with the comments on Equation 1. &#916;&#949;&#955; stands here for two standard deviations around the average experimental value obtained for&#160;&#949;&#955; by fitting experimental radiance spectra within the grey body assumption. &#948;Tc and &#948;Td represent two standard deviations around the average standard lamp extrapolated temperature curve and the average experimental solidification temperature value, respectively.
Some improvements can be made on the present experimental approach. In particular, connecting the pressure vessel with a mass spectrometer through a complex pipe system will allow the detection, at least qualitatively, of the species present in the vapor plume released by the hot material. Moreover, the implementation of a thermo-camera is foreseen for the two-dimensional study of the temperature distribution over the hot sample surface in order to detect possible inhomogeneities and segregation effects. Finally, improvements in the safety system built around the current equipment are foreseen. Actually, the current Plexiglas glove box used here is suited for the study of highly-radioactive materials, such as uranium and transuranium elements, thanks to the fact that it effectively blocks &#945; radiation. However, this shield is not sufficiently safe for the investigation of strong &#947; emitters, like the nuclides contained in real irradiated nuclear fuel. A new facility including a lead-walled cell is foreseen for the study of spent nuclear fuel coming from real NPPs.

Although nuclear fission is broadly presented as a promising large-scale, practically inexhaustible energy source, its full public acceptance is still stalled by some safety, security, and safeguard risks. The experimental approach presented in this work aims at answering some fundamental materials science questions relating to one of these risks, the occurrence of severe accidents (SAs) leading to core meltdown in a nuclear power plant (NPP). This can result in a possible release of highly-radioactive material in the environment, with severe consequences, both for people&#39;s health and the country&#39;s economy. Major SAs of this type have occurred three times in NPPs, at Three Mile Island (USA, 1979), Chernobyl (former USSR, 1986), and Fukushima (Japan, 2011). Hence, NPP SAs are the focus of considerable research in a few facilities worldwide, encompassing many challenging phenomena and complicated by very high temperatures (often exceeding 3,000 K) and the presence of radioactive materials.
In this scenario, a recent directive by the European Council1 requires EU countries to give the highest priority to nuclear safety at all stages of the lifecycle of a nuclear power plant. This includes carrying out safety assessments before the construction of new nuclear power plants and also ensuring significant safety enhancements for old reactors.
In this context, a controlled-atmosphere, laser-heating and fast radiance spectro-pyrometry facility2,3,4 has been implemented at the European Commission&#39;s Joint Research Centre&#39;s Institute for Transuranium Elements for the laboratory simulation, on a small scale, of NPP core meltdown. Due to the limited sample size (typically on a cm- and 0.1-g-scale) and the high efficiency and remote nature of laser heating, this approach permits fast and effective high-temperature measurements on real nuclear materials, including plutonium and minor actinide-containing fission fuel samples. In this respect, and in its capability to produce a large amount of data concerning materials under extreme conditions, the current experimental method is recognized worldwide as being unique. In fact, other complementary investigation techniques based on induction heating have been shown to suffer from the rapid high-temperature interactions between the sample material and containment5. In addition, if such techniques allow and mostly need larger amounts of material for analysis, they are less suited than the present method for the investigation of real nuclear materials, due to the high radioactivity and limited availability of the samples.
In the current experiments (schematized in Figure 1), a sample, mounted in a controlled-atmosphere autoclave contained in an &#945;-shielded glove box, is heated by a 4.5-kW Nd:YAG CW laser.
<img alt="Figure 1" src="/files/ftp_upload/54807/54807fig1.jpg" /> 
Figure 1: Laser-heating and radiance spectro-pyrometry experimental set-up. 
The sample is fixed with graphite (or tungsten or molybdenum) screws in a gas-tight vessel under a controlled atmosphere. The picture reported in the bottom-left corner shows, as an example, a PuO2 disk fixed with graphite screws. If the sample is radioactive, the vessel should be mounted inside an alpha-tight glove box. The sample is heated by a 4.5-kW Nd:YAG laser at 1,064 nm. A fast two-channel pyrometer is used for recording the sample temperature and the reflected signal from a lower-power Ar+ laser. A slower multi-channel spectro-pyromenter is employed for in situ analysis of optical properties of the hot sample. <a href="http://cloudflare.jove.com/files/ftp_upload/54807/54807fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Radiation pyrometers measure the sample radiance Lex. This is the electromagnetic radiation power density per unit surface, wavelength, and solid angle emitted by the sample at a given temperature6. It is linked to the sample surface temperature T through a modified Planck function:

<img alt="Equation 1" src="/files/ftp_upload/54807/54807equation1.jpg" />
where L&#955; is the radiative power, &#949;&#955; is the spectral emissivity, c1 = 2&#183;h&#183;c02 is the first radiation constant, c2 = h&#183;c0/kB = 14,388 &#181;m&#183;K is the second radiation constant, c0 is the speed of light in vacuum, h is Planck&#39;s constant, and kB Boltzmann&#39;s constant. The spectral emissivity takes into account the fact that a real body will radiate, at a given wavelength and temperature, only a fraction equal to of the power emitted by an ideal blackbody at the same temperature. Therefore, takes values between 0 and 1, with 1 corresponding to the ideal blackbody case for which Planck&#39;s law was derived. Since pyrometers in the present work were always set up near normal with respect to the sample surface, the angle dependence of &#949;&#955; was not considered, and &#34;emissivity&#34; will always refer to normal spectral emissivity (NSE). The NSE must be determined in order to convert, through equation 1 and a pyrometer calibration procedure, Lex into absolute temperature T.
The specimen temperature is detected using a fast pyrometer calibrated against standard lamps up to 2,500 K at &#955; = 655 nm and. An additional 256-channel radiance spectro-pyrometer operating between 515 nm and 980 nm was employed for the study of the NSE (&#949;&#955;) of the sample. Determination of the NSE is possible by completing a non-linear fit of the thermal emission spectrum with Equation 12, 3, T and&#160;&#949;&#955; being the only two free parameters. This approach has been demonstrated to be acceptably accurate in refractory materials7 like those usually present in a NPP, for which the NSE can be assumed to be wavelength-independent (grey body hypothesis) on a broad spectral range. Once the temperature of the laser-heated sample is correctly measured as a function of time, thermal analysis can be performed on the resulting temperature-time curve (thermogram). Inflections or thermal arrests in the thermograms give information related to phase transitions (solidus, liquidus, and isothermal phase transformations). Moreover, besides being necessary for NSE determination, direct spectral analysis of the radiance Lex emitted by the hot sample also permits an in situ study of some optical properties of the studied surface. This constitutes another supporting tool for the identification of high-temperature phenomena, such as phase transitions, chemical reactions between condensed materials and the gas phase, or segregation effects. An additional technique called reflected light signal (RLS) analysis2, 3 is used to confirm phase transitions. It is conducted by using the second channel of the pyrometer tuned to a low-power (1 W) Ar+ laser (&#955; = 488 nm). This channel detects the laser beam originating from the Ar+ cavity and reflected by the sample surface. A constant RLS signal indicates a solid surface, while random oscillations appear after melting due to surface tension-induced vibrations on the liquid sample surface.
In general, water-cooled reactors using solid fuel elements, currently the most common type of NPP, possess four successive barriers to ensure the containment of radioactivity8. The first barrier is that the fuel pellet itself, thanks to its crystalline structure and micro-macroscopic porosity, can hold the solid fission products and part of the volatile ones. In general, the entire fuel element is placed in a metallic (Zircaloy or steel) cladding that works as the second protection stage. In case of failure of the cladding, the third barrier is the whole NPP inner vessel, in general confined by a steel wall that is a few cm thick (primary system). Finally, the containment building (m-thick concrete) is the last safety barrier before release into the environment.
In case of failure of the water cooling system, a NPP SA can take place, leading to core overheating and meltdown. Overheating is initially due to fission heat. However, in the absence of cooling, overheating can also continue long after the termination of nuclear chain reactions, due to the residual decay heat of fission products and other highly-radioactive species contained in the nuclear core debris. In general, core melt will start from the central part of the fuel element, unless lower-melting compounds (possibly eutectics) are formed at the interface between the fuel and cladding. The first objective of the present research consists of establishing whether such lower-melting compounds can be formed in real fuel-cladding systems, and, in this case, what the resulting melting temperature depression would be. In order to answer this question, the melting behavior of pure and mixed fuel compounds should first be soundly assessed, which therefore constitutes an even more important goal of the current approach. If fuel and cladding melt together, the liquid mass will rapidly fall to the bottom of the primary vessel and start reacting with the steel wall and with the remaining water and steam, if any. At this stage, steel can also be melted together with the fuel/cladding hot mixture. The resulting lava-like liquid is called &#34;corium&#34;. This hot, highly-radioactive mixture can diffuse outside the primary containment if the steel wall is melted through and end up reacting even with the concrete constituting the most external barrier. The elevated heat and the high reactivity of the species present in the corium can lead to water dissociation and the production of hydrogen. This might result in an additional risk of steam and hydrogen explosions (cf. the SAs in Three Mile Island and Fukushima), heavy oxidation, or (less likely) hydration of the corium mass and the NPP structural materials. The current experimental method permits the separation and experimental analysis of several of the many complex physicochemical mechanisms related to the described sequence of events. Besides the mentioned pure component melting analysis and fuel-cladding interaction, several high-temperature interaction mechanisms can be investigated in simplified systems, such as between Pu-containing fuel and steel, between fuel and concrete, etc. Corium formation can potentially be studied in the presence of different atmospheres (inert gas, air, traces of hydrogen or steam), producing important reference data for a comprehensive understanding of SAs.
The present approach, particularly suited for the laboratory investigation of high-melting materials, has also been employed for the successful analysis of other, more innovative types of nuclear fuels (based, for example, on uranium carbides or nitrides) and other refractory compounds, such as zirconium9, tantalum and hafnium carbides, metallic superalloys, calcium oxide10, etc.

<table border="0" cellpadding="0" cellspacing="0" width="1498">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="142">
			<td height="142" style="height:142px;width:299px;">Two-channel fast pyrometer</td>
			<td style="width:281px;">Assembled privately</td>
			<td style="width:421px;"></td>
			<td style="width:497px;">Fast pyrometer. Photodiode detectors at 650 nm and 488 nm, assembled with focussing objective and fast logarithmic amplifier.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:299px;">Laser TRUMPF HLD4506, TRUMPF,&#160;</td>
			<td style="width:281px;">TRUMPF Schramberg, Germany</td>
			<td style="width:421px;">HLD4506</td>
			<td>Heating agent</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:299px;">CDI spectrometer</td>
			<td style="width:281px;">CDI</td>
			<td style="width:421px;">Optical Spectrograph card, 256 channels</td>
			<td>Multi-wavelength spectro-pyrometer array&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:299px;">Ar+ laser</td>
			<td style="width:281px;">Ion Laser Technology</td>
			<td style="width:421px;">5500A-00</td>
			<td>0.75 W RLS laser</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:299px;">Oscilloscope NICOLET</td>
			<td style="width:281px;">NICOLET, Madison, Wi, USA</td>
			<td style="width:421px;">Pro 44C Transient Digitizer</td>
			<td>AD converter, data acquisition system</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:299px;">SETNAG Oxygen analyser</td>
			<td style="width:281px;">SETNAG, Marseille, France</td>
			<td style="width:421px;">JC24V-M</td>
			<td>ZrO2 electrochemical cell for oxygen analysis in the autoclave</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:299px;">Blackbody source</td>
			<td style="width:281px;">POLYTECH CI Waldbronn, Germany</td>
			<td style="width:421px;">Customized</td>
			<td>Black body source for spectro-pyrometer calibration</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:299px;">Standard calibration lamps</td>
			<td style="width:281px;">POLARON, Watford, UK</td>
			<td style="width:421px;">P.224c and P213c</td>
			<td>Lamps for pyrometer and spectro-pyrometer calibration</td>
		</tr>
	</tbody>
</table>

laser-heating and radiance spectrometry for the study of nuclear materials in conditions simulating a nuclear power plant accident

Major and severe accidents have occurred three times in nuclear power plants (NPPs), at Three Mile Island (USA, 1979), Chernobyl (former USSR, 1986) and Fukushima (Japan, 2011). Research on the causes, dynamics, and consequences of these mishaps has been performed in a few laboratories worldwide in the last three decades. Common goals of such research activities are: the prevention of these kinds of accidents, both in existing and potential new nuclear power plants; the minimization of their eventual consequences; and ultimately, a full understanding of the real risks connected with NPPs. At the European Commission Joint Research Centre&#39;s Institute for Transuranium Elements, a laser-heating and fast radiance spectro-pyrometry facility is used for the laboratory simulation, on a small scale, of NPP core meltdown, the most common type of severe accident (SA) that can occur in a nuclear reactor as a consequence of a failure of the cooling system. This simulation tool permits fast and effective high-temperature measurements on real nuclear materials, such as plutonium and minor actinide-containing fission fuel samples. In this respect, and in its capability to produce large amount of data concerning materials under extreme conditions, the current experimental approach is certainly unique. For current and future concepts of NPP, example results are presented on the melting behavior of some different types of nuclear fuels: uranium-plutonium oxides, carbides, and nitrides. Results on the high-temperature interaction of oxide fuels with containment materials are also briefly shown.

Figure 3 displays real temperature thermograms measured on uranium dioxide with various oxidation levels (UO2+x with 0 &#60; x &#60; 0.21)2. Uranium dioxide is the essential component of the most common fuel in current NPPs. Its oxidation to various oxygen hyper-stoichiometry levels can occur in normal and off-normal reactor conditions12. With the current method, it was shown that UO2 oxidation can result in a dramatic decrease of its melting/solidification point by up to 700 K. In this case, experiments had to be carried out under a rather high inert gas pressure (He at 10 MPa) in order to suppress the highly non-congruent vaporization at high temperatures.
<img alt="Figure 3" src="/files/ftp_upload/54807/54807fig3.jpg" /> 
Figure 3: Thermograms measured on laser-heated stoichiometric and hyperstoichiometric uranium dioxide samples (after 2). 
An example double-pulse laser profile is shown in the graph. Thermograms are recorded for several UO2+x compositions. Solidification arrests occur at significantly-different temperatures and with different features, depending on the sample composition, revealing the evolution of the melting/freezing temperature and solidification dynamics in the U-O system. <a href="http://cloudflare.jove.com/files/ftp_upload/54807/54807fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Figure 4 shows pyrometer (straight line) and spectro-pyrometer thermograms recorded on a plutonium dioxide sample laser heated under an oxidizing atmosphere (compressed air at 0.3 MPa). Also, PuO2 is an essential nuclear fuel component. In the same figure, two radiance spectra measured by the spectro-pyrometer at different temperatures are also displayed in the insets, together with curves fitting experimental data and the corresponding T and&#160;&#949;&#955; values. Thanks to the present study, the PuO2 melting/freezing temperature was reassessed to be 3,017 K &#177; 28 K, over 300 K higher than previously indicated by more traditional heating methods. Those methods yielded results certainly affected by extensive high-temperature interactions between the sample and containment, an issue that has been largely solved with the present remote heating approach.
<img alt="Figure 4" src="/files/ftp_upload/54807/54807fig4.jpg" /> 
Figure 4: Thermograms measured on a plutonium dioxide sample laser heated beyond the melting point. 
Main graph: the black solid line and the full black circles represent the thermograms recorded on a PuO2 sample under an oxidizing atmosphere by the fast pyrometer and the multi-wavelength spectro-pyrometer, respectively. The white circles represent the spectral emittance values obtained by fitting experimental radiance data with Planck&#39;s radiance law12. The two insets show example spectra recorded (black circles) and fitted (red solid lines) in liquid and freezing PuO2, respectively, within the grey body assumption. In these plots, the radiance L&#955; is normalized to the first radiation constant c1 for the sake of simplicity. The main thermogram was obtained using an average constant emittance of 0.83. <a href="http://cloudflare.jove.com/files/ftp_upload/54807/54807fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Figure 5 shows a series of laser-heating pulses performed on a mixture of UO2 and ZrO2 under different atmospheres. This test is representative of conditions that may be produced in case of an accidental temperature excursion in a NPP. The melting/solidification point occurs at a well-repeatable temperature over successive shots if experiments are carried out in argon. On the other hand, the melting/solidification temperature decrease over the laser shots if laser-heating cycles are performed in compressed air. This shows that, in the latter case, the sample gets increasingly oxidized during the laser-heating treatments. Also, in the case of mixed UO2- ZrO2 oxides, a melting point depression occurs in oxygen hyper-stoichiometry conditions.
<img alt="Figure 5" src="/files/ftp_upload/54807/54807fig5.jpg" /> 
Figure 5: Thermograms measured mixed UO2-ZrO2 samples in pressurized argon and air. 
The melting/solidification point occurs at a well-repeatable temperature over successive shots if experiments are carried out in argon (black thermograms). On the other hand, the melting/solidification temperature decrease over the laser shots if laser-heating cycles are performed in compressed air (green thermograms). <a href="http://cloudflare.jove.com/files/ftp_upload/54807/54807fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
A further example concerns another kind of material, uranium dicarbide. This is envisaged as a possible material for an alternative concept of nuclear fuel, potentially working at higher temperatures and considerably reducing the risk of a meltdown accident. A new composition containing a large excess of carbon (nominally UC2.8) was investigated for the first time with the current approach14. In this case, the UC2-C eutectic temperature, established to be at 27,37 K &#177; 20 K, was used as a radiance reference together with the cubic-tetragonal (&#945;&#8594;&#946;) solid-state transition, fixed at 2,050 K &#177; 20 K. The NSE of the carbon-richer compound was measured to increase up to 0.7 at 650 nm, whereas the value&#160;&#949;&#955; = 0.53 was established for pure uranium dicarbide at the limit of the eutectic region. This increase was analyzed in light of the demixing of excess carbon and used for the determination of the liquidus temperature (3,220 &#177; 50 K for UC2.8). Due to fast solid-state diffusion, also fostered by the cubic-tetragonal transition, no obvious signs of a lamellar eutectic structure could be observed after quenching to room temperature. The eutectic surface C/UC2-x composition could be qualitatively, but consistently, followed during the cooling process with the help of the recorded radiance spectra, as shown in Figures 6 a and b. Interestingly, the current NSE analysis showed that, whereas in the liquid phase the external liquid surface was almost entirely constituted of uranium dicarbide, it got rapidly enriched in demixed carbon upon freezing. Demixed carbon seemed to quickly migrate towards the inner bulk during further cooling. At the &#945;&#8594;&#946; transition, uranium dicarbide again covers almost the entire external surface. All of these details on the very high-temperature material behaviour are essential for the analysis of this type of compound in case of uncontrolled temperature increase in the reactor core. They were deduced only on the basis of radiance spectroscopy analysis, whereas they would be hardly accessible to any other experimental investigation technique.
<img alt="Figure 6" src="/files/ftp_upload/54807/54807fig6.jpg" /> 
Figure 6: Thermogram and radiance spectra measured on a UC2.8 sample in pressurized argon14. 
a) The cooling stage of a thermogram recorded on a UC2.8 sample. Full circles identify the time points at which the radiance spectra were recorded by the spectro-pyrometer. b) Four examples of radiance spectra recorded at different temperatures. One of them was recorded in liquid UC2.8, while the other three correspond to the thermal arrests visible in Figure 5a. <a href="http://cloudflare.jove.com/files/ftp_upload/54807/54807fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Laser-heating and Radiance Spectrometry for the Study of Nuclear Materials in Conditions Simulating a Nuclear Power Plant Accident at JoVE.com

Laser-heating and Radiance Spectrometry for the Study of Nuclear Materials in Conditions Simulating a Nuclear Power Plant Accident

We present experiments in which real nuclear fuel, cladding, and containment materials are laser heated to temperatures beyond 3,000 K while their behavior is studied by radiance spectroscopy and thermal analysis. These experiments simulate, on a laboratory scale, the formation of a lava-phase following a nuclear reactor core meltdown.

Major and severe accidents have occurred three times in nuclear power plants (NPPs), at Three Mile Island (USA, 1979), Chernobyl (former USSR, 1986) and ...

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