It was one of America's darkest hours -- a terrorist attack struck on American soil, killing 2,974 people. It was a tragedy felt worldwide, as 90 countries were represented among those who died.
The most devastating part of the attack in terms of damage was not the impact of the airplanes themselves, but the fires they caused. The fires burned in the middle stories of the buildings at temperatures of around 500C (932F). They weakened the steel supports, which eventually collapsed, taking the rest of the building with them, killing many in the top floors. Part of the mystery was how this occurred -- steel's melting point is much higher.
Dr Sergei Dudarev, principal scientist at the United Kingdom Atomic Energy Authority (UKAEA) explains, "Steels melt at about 1,150C (2,102F), but lose strength at much lower temperatures. [The steel] becomes very soft. It is not melting but the effect is the same."
This deadly mystery was not first observed in the 9/11 attacks. It was widely known among the scientific community before them that steel became pliable at lower temperatures. This was unfortunate as steel's normal strength would lend itself nicely to many ultra-hot applications, such as lining the wall of a fusion reactor. The phenomena went largely unexplained and unexplored until after 9/11 when the topic was brought into sharp focus.
Now, Dr. Dudarev and his fellow scientist have experienced a breakthrough developing new insight into how steel weakens at higher temperatures. What they discovered was that tiny irregularities in the steel's structure disrupt the internal magnetic fields. When the metal is cool it is hard, but when it is heated the irregularities damage the magnetic fields which hold the atoms rigid, allowing them to slide past each other.
The phenomenon is similar to that exploited since the days of ancient Rome to work iron. While not able to reach temperatures high enough to melt the iron, sometimes, blacksmiths discovered if they heated it to a relatively hot temperature, the metal became pliable, able to be shaped into weapons. While this was good for the Romans, it was not so good for modern applications which demand heat resistance, such as architecture and fusion power.
Architects have previously tried to protect buildings from this phenomenon by placing insulating panels around the support beams. However, as the 9/11 attacks showed, these panels can be easily ripped off by a disaster. This is precisely what is thought to have happened -- the collision with the airliners ripped off the insulation, exposing the steel to the fire. Ultimately this elasticity doomed the structure.
Trying to make something good come of such a negative event, Dr. Dudarev and the UKAEA first worked out the mechanism for this weakness. Now they are working on developing steel that lacks the irregularities and thus is able to operate at blistering temperatures. Such Iron Man-like steel would be a crucial step towards achieving clean cheap power for mankind through nuclear fusion. The group is working on the ITER reactor, the international community's largest experimental fusion reactor yet.
The 500 MW ITER reactor must be lined with ultra-robust materials. By mixing steel with other elements, Dr. Dudarev is confident he and his team can develop exotic steels to fit the bill. He is confident that the problem is not a difficult one, and just comes down to experimentation. He states, "We need to look at the magnetic properties of steel, [and] vary their chemical composition in a systematic way in order to get rid of this behavior."