The term Passive Safety is being heard quite a lot these days. What is it?

It means using the laws of physics and chemistry to your advantage, so that they naturally move towards a safe situation without human intervention, and without the need for motive force to drive safety systems.

Let us take a moment to briefly consider the, now notorious, Fukushima incident. The largest earthquake on record struck in that region of Japan. The ground shook, and the Fukushima reactors switched themselves off, just as they were designed to do. So far …….no problem. But after a rapid switch-off nuclear reactors stay rather hot for a few days. They need to be cooled for some time.

But the earthquake had occurred under the sea, some distance from the coast. It consequently produced the largest tsunami on record. The tsunami took just under an hour to reach the Fukushima nuclear complex. When it did arrive, it jumped over the inadequate tidal defence barriers.

That is when the trouble started.

The tsunami washed away incoming power lines which supplied power to cooling pumps. These pumps supplied cooling water to the still hot reactors.

The pumps stopped working. Still no problem, because there was a backup system, diesel pumps. But …..oops……the tsunami washed away the outdoor diesel storage tanks. So it did not take long for the diesel pumps to stop…….then they had to rely on batteries.

All earlier nuclear reactors rely on such safety systems, ones that need electricity or diesel to drive pumps, and so on. But modern thought has advanced to introduce Passive Safety which does not rely on electrical pumps or diesel, it relies on natural physics, like gravity and thermal currents.

So Passive Safety does not rely on a pump being driven to supply cooling water, it will rather use a large tank of water placed high up, from which the water naturally falls under gravity. There are many other passive systems which have been developed for Generation IV technology, which all contribute to keep the nuclear reactor safe during any emergency situation.

From the inception of the ideas that led to the evolution of the HTMR-100 all conceivable Passive Safety systems were designed into the reactor. As a result, we have arrived at the current design which is such that the HTMR-100 is truly walk-away-safe.

It is impossible for the HTMR-100 TRISO fuel to melt down, as happened at Fukushima. If the worst of the worst were to happen in an HTMR-100, the best thing that the staff could do is to plain and simply walk away from the reactor, and leave it to cool itself off.