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Saturday, 12 March 2011

Meltdown Shock Horror – Some Facts

Following on from the severe earthquake that has affected much of northern Japan, and generated a tsunami which has reached the west coast of the USA, much attention has been focused on the nuclear power station at Fukushima, where there has been an explosion and talk of one reactor melting down.

So there have been comparisons with other well known nuclear power station incidents, and this inevitably means Three Mile Island Reactor 2 (TMI2), near Harrisburg, Pa., and most notoriously Chernobyl, in the Ukraine near Kiev. Let’s start with the latter, and lay one scare story to rest immediately: the Russian designed RBMK reactor used at Chernobyl used water as a coolant, and graphite as a moderator. It did not feature a full containment structure.

The Fukushima reactors – there are six at the station – are Hitachi licensed Boiling Water Reactor (BWR) technology: this uses water as both coolant and moderator. There is no graphite in the core, so any “flaming graphite core” rumours can be disregarded.

However, the BWR use of water as a moderator is shared with the Pressurized Water Reactor (PWR) technology used in the UK’s last completed nuclear plant, Sizewell B. And TMI2 is a PWR. The main difference between the two reactor types is that the PWR containment also houses steam generators which supply the electricity generating turbine(s), and the primary coolant circuit therefore stays within that containment. The BWR primary circuit drives the turbine(s) directly.

But, as noted, both BWR and PWR technology features a full containment building around the reactor. At Sizewell B, this takes on the appearance of a giant dome. This made the difference during the incident at TMI2, where there was a leak of highly radioactive liquid from the reactor pressure vessel.

The leak at TMI2 led to a subsequent build up of hydrogen within the pressure vessel, and there was a (clearly audible) detonation, but the vessel held it. The containment building then ensured that the radioactive liquid went no further.

Thus TMI2 was ultimately an economic disaster, rather than a humanitarian one as at Chernobyl. TMI2 went from being a $1 billion asset to being worthless in short order, and with an open ended clean-up bill attached.

At Fukushima, if there has been any catastrophic melt-down, the containment is most likely to hold it, with a result similar to TMI2. As to whether nuclear power stations should be built in earthquake zones, well, that’s another story.

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