Safety above all

Despite the valiant move from Germany, I seriously doubt other countries such as the UK, France, China, India, Sweden and the US will give up their nuclear power or stop building new plants any time soon. Therefore, safety of this technology must remain a paramount point in any future development in this area.

Currently, the Light Water Reactors (LWR) enclose their fuel in a metallic material (zirconium alloy) called cladding. One of the consequences of using this alloy is that during off-normal conditions the temperature of the reactor can go above 1200°C. Temperature at wich it starts to react with water vapour resulting in further release of heat (exothermic reaction) and hydrogen. This release of hydrogen was the origin of the explosions observed in Fukushima. As the temperature keeps increasing the uranium based fuel melts and produce the feared core meltdown.

New designs have addressed this problem in different ways. The one described in this post will be of the High Temperature Reactor (HTR). Instead of having large amounts of material encapsulated in long metallic tubes as happens in current LWR, the original creators of the HTR decided to put in practice the very well-known phrase “divide and conquer”. The fuel in this reactor is made of tiny spherical uranium particles, 0.5 mm in diameter, coated with 4 layers of ceramics (pyrolytic carbon and silicon carbide). The final size of this single unit fuel…. 1 mm in diameter. This type of fuel is known as TRISO (tristructural isotropic) coated fuel particle and it was originally created in the late 1960s in the UK during the Dragon project (not in Germany despite some erroneous believes). Hundreds of thousands of this TRISO particles are then mixed with graphite to form fuel compacts in spherical (pebble bed reactor) or cylindrical shape (prismatic core). The size of a fuel pebble is slightly bigger than a baseball ball. This type of fuel has several advantages over current designs. First, the coatings covering the uranium kernel keep all fission products inside this fuel, effectively creating a miniature fission product containment vessel. Imagine, all that steel and concrete replaced by layers of only a fraction of a millimeter. Second, is the stability of the material. Under off-normal conditions the temperature of the reactor can reach a maximum of 1600°C. This temperature, although very high for metallic materials, is considerably lower than the temperature necessary to melt/evaporate carbon or silicon carbide (>2100°C), posing no threat to the safety of the fuel. Other features are included in the reactors that in theory makes it physically impossible to have a core meltdown as happened in Fukushima. Many could say that this idea looks good on paper but we could not know in practice. Well, in 2004 the Chinese, who have a prototype reactor called HTR-10, decided to carry out a test that would have been considered unthinkable in current reactors. They decided to remove all cooling system (like happened in Fukushima) and study the safety of the reactor. The result, no damage to the fuel or the reactor giving strong, realistic evidence of the safety of this technology.

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