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

The overall goal of this procedure is to simulate the early stages of a core meltdown accident in a nuclear power plant on a laboratory scale, enabling the study of the melting behavior involved in the formation of corium. This experimental approach helps answer key questions in nuclear power plant accident research. For example, one can determine the temperatures that, depending on the atmosphere, can lead to failure of a core's thermomechanical stability.

The main advantage of this technique is that it is performed remotely, allowing the use of real radioactive materials in the experiment. Though this method can provide insight into the melting behavior of reactor core materials, it can also be applied to other refractory ceramics, high-temperature superalloys, and aerospace coatings. This is a unique method for studying materials at extremely high temperatures for short times.

It is therefore very useful in order to establish thermodynamic equilibrium reference states for the observed material behavior. First, calibrate the fast two-channel pyrometer and the spectro-pyrometer using reference standard lamps. Determine the calibration constants.

In a shielded glove box with optical quality windows, mount the sample in a holder with high-temperature zirconia glue or graphite, molybdenum, or tungsten screws. Place the sample in holder horizontally in a controlled atmosphere autoclave reactor with optical-quality windows. Secure the autoclave on an optical board in the glove box.

Mount a laser-absorbing graphite screen behind the autoclave. To ensure temperature homogeneity, select focusing unit lenses to produce a laser spot at least 10 times larger than the pyrometer sighting spot. Then, couple the focusing unit to a high-power laser fiber-optic system, being careful not to kink the fiber-optic lines.

Turn on the low-power red pilot laser, and align the beam so the spot is centered on the sample in the autoclave. Then, turn on the argon ion laser. Align the laser so the blue spot is in the center of the red pilot laser spot on the sample.

The two-channel pyrometer and spectro-pyrometer are mounted on the optical table in line with the sample. Check that the objectives are correctly focused on the sample. Then, shine a fiber-coupled light source into the eyepiece of the two-channel pyrometer, and verify that a sharply defined sighting spot is projected onto the sample surface.

Adjust the pyrometer so the sighting spot is centered within the red and blue laser spots. Align the spectro-pyrometer using the same technique. Then, check for parasite reflections of the red pilot laser from the sample surface, the glove box, and the autoclave windows.

Place graphite screens wherever parasite reflections occur. Next, evacuate the autoclave, and refill it with the reaction atmosphere five times. Then, fill the autoclave with the reaction atmosphere to the desired pressure.

Wait for the oxygen potential to stabilize before proceeding with the experiment. Connect both pyrometers and the high-power laser potentiometer to an oscilloscope acting as an analog/digital converter. Connect the oscilloscope to a computer.

In the oscilloscope software, set the parameters in the data acquisition triggers. Fill in the pyrometer and spectro-pyrometer calibration constants. In the high-power laser software, create a new heating program.

If the sample melting point is higher than 2, 500 kelvin, start the program with a preheating stage to a suited power level. Next, define cycles of three to four rapid laser shots to heat the sample well above its melting temperature. The sample must stay above room temperature during a cycle.

Create additional cycles with varying laser pulse intensity and duration. Then, from an appropriately shielded control room, test the laser heating program and the data acquisition triggers by shooting onto a water-cooled graphite absorber. Ensure the trigger system works properly and that the data are acquired.

Once the system is ready, remove the graphite shield by changing the laser path from absorber to sample. Deactivate the red pilot laser, and activate the high-power laser. Start the heating program.

After the preheating stage in the heating/cooling cycle, pause the program and check the sample appearance. Verify that the experimental thermograms indicate successful melting and solidifying of the sample. If the sample is intact, perform several more heating/cooling cycles, checking the sample after each cycle.

Continue this process until the sample is melted or broken. When the experiment has finished, turn off the high-power laser, vent the autoclave, and allow it to stabilize at atmospheric pressure. Transfer the sample and sample fragments from the autoclave to a shielded container for later characterization.

Use ethanol and laboratory wipes to clean the autoclave interior and windows. The melting and solidification behavior of uranium dioxide was evaluated in atmospheres with increasing oxygen contents. Real temperature thermograms showed that increased uranium dioxide oxidation levels resulted in melting solidification point decreases of up to 700 kelvin.

The melting/freezing temperature of plutonium dioxide was reassessed and found to be over 300 kelvin higher than previously reported. This was attributed to the remote heating method avoiding the high-temperature interactions, between the sample and its container, that would affect conventional heating processes. Mixed uranium dioxide-zirconium dioxide samples were studied in both argon and compressed air.

In the argon atmosphere, the sample melting and solidification temperatures stayed approximately the same in successive heating and cooling cycles. In compressed air, the melting and solidification temperatures decreased over successive cycles, suggesting sample oxidation. Normal spectral emissivity analysis of carbon-rich uranium dicarbide indicated that the surface was rapidly enriched by demixed carbon during cooling, but the demixed carbon had almost completely migrated away from the sample surface at the alpha-beta phase transition temperature.

Once the optimal experimental conditions are established for a given type of material, several thermal cycles can be performed on different samples. This is a large amount of experimental data points, useful for statistical analysis. While attempting this procedure, it's important to optimize the experimental parameters, like the atmosphere inside the vessel and the laser power by a trial and error method for each investigated material.

Other techniques can be performed following this procedure, like thermography, to measure the actual temperature distribution at the sample's edges, or like ultraviolet spectrometry, to determine the spectral emissivity behavior at high temperatures. Don't forget that working with high-power lasers involves risks that need to be evaluated by trained professionals. Precautions like wearing safety goggles and using appropriate laser shielding must always be taken while performing this procedure.

Although other approaches exist for the study of core information, only this one permits an investigation of materials containing plutonium and other transuranium elements. After watching this video, you should have some understanding of how one can study and simulate on a laboratory scale, but on real nuclear materials, the formation of corium in a nuclear power plant core meltdown accident. Be aware that the materials to leading this research work are highly radioactive.

These kind of activities should always be carried out in compliance with local radioprotection laws and under the supervision of authorized officers.