Inside the destroyed Fukushima reactors lie hundreds of tons of melted fuel — a mixture of uranium, zirconium, iron, and fission products. This debris is called fuel debris. It must be extracted, transported, and securely packaged for decades. But how will this volatile mixture behave under heat, compression, or accidental impact? To answer that, we need precise numbers, not guesses.

One key component of the debris is the uranium-iron compound UFe₂. There is plenty of iron in the reactor: it makes up pipelines, structural components, and fasteners. At temperatures above 2500 °C, uranium melted together with iron into a tough intermetallic. Researchers in Japan created an almost pure sample of UFe₂, using arc melting and spark plasma sintering, and ran it through a series of tests. The result surprised them: at 306 K (about 33 °C), the alloy conducts heat like steel—10 W/(m·K)—and by 1073 K (800 °C), its thermal conductivity rises to 25 W/(m·K)—higher than many of its iron relatives.

Mechanical tests painted the opposite picture: UFe₂ is a pliable material. Its Young's modulus (tensile stiffness) is only 69 GPa — similar to aluminum alloys. For comparison, pure iron has about 210 GPa. Vickers hardness is 5.6 GPa, almost half that of Fe₂Zr and Fe₂B. In practice, this means that cutting tools will meet less resistance, and the material itself will be less prone to cracking under thermal shock. Both factors are critical for remote dismantling.

These numbers will underpin computer simulations of reactor dismantling. Engineers will be able to more accurately calculate cutting forces, container cooling rates, and the risk of spontaneous cracking. A small ingot tested in the lab shortens the path to safely retrieving hundreds of tons of real debris.