Nuclear Technology
There are presently 617 power reactors in 31 countries. It is a mature and reliable technology and 116 older reactors have already safely completed their working life. In general these were small machines employing old technology. The oldest Calder Hall at Seascale, Cumbria, England achieved criticality in January, 1956. But the bulk of them, like the failed older plant at Fukushima, commenced construction in the 1960's and early 1970's.
| Status | Net generating capacity (MWe) | Number of Reactors |
| Operating | 372,791 | 441 |
| Under construction | 52,615 | 56 |
| Shut down | 38,476 | 116 |
| Not operating | 2,568 | 4 |
| Grand Total | 466,450 | 617 |
Most of us today would be dubious about boarding a Mk1 De Havilland Comet. But that several of these crashed due to metal fatigue did not put an end to jet airliners. We can easily distinguish various aircraft types and are generally happy with the recent technology despite the occasional crash. We accept the very high levels of sophistication required to avoid more frequent disasters.
But few it seems are able to distinguish one nuclear reactor technology from another.
There are at least six broad technologies in current commercial use and each has variations depending on age and builder.
All fission reactors employ neutrons slowed-down by a moderator. When slowed these combine with the nucleus of a heavy element such as uranium or plutonium to destabilise it. The destabilised nucleus splits and releases further neutrons and heat. These new neutrons are in turn slowed to result in further combinations with more heavy nuclei; and hence establish a 'chain reaction'.
Most reactors operating in the world today employ at high pressure water as a moderator to slow the neutrons. The high pressure water has a high boiling temperature and can be used in a heat exchanger to make steam in a secondary loop to drive turbines to make electricity. High pressure reactors do not normally allow boiling to take place within the high pressure stage. As a result they have a built in safety factor in that if the water in the reactor boils, due to pressure collapse or excessive heat; for example if the secondary cooling fails, moderation reduces and the reactor turns itself down.
In addition, fission reactors are typically controlled by means of control rods containing neutron absorbing materials like: silver; indium; cadmium; boron; cobalt and hafnium; these are lowered into, or raised from, the core containing the fuel, to optimise reactor performance.
Cruas Nuclear Power Station in France
Comprising four pressurized water reactors of 900 MW each - totalling 3600 MW
Just three such plants would replace all the coal burning generation in NSW
Some older water based reactors, like Fukushima 1 to 4, were not of this pressurised design. They are designed to produce steam directly to run the turbines; boiling the water within the reactor.
When fully inserted the control rods stop the chain reaction. But as we have seen at Fukushima, it can be some time before all heat generation stops and without cooling or replacement, the water in the reactor is at risk of boiling away; resulting in a potential 'meltdown'. This did not happen to the two newer reactors (5&6) at Fukushima that were subjected to the same events.
Some other older designs like the reactor at Three Mile Island and in the old Eastern Block, like Chernobyl, use graphite as an additional moderator so that less enriched uranium could be used as fuel but these get rapidly hotter if the cooling water boils away and graphite has proven to be an additional fire/explosion risk if the reactor goes critical.
Yet other designs use a different fluid in the primary heat loop such as molten sodium metal. Some new designs, that are inherently meltdown immune, use pelletised fuel to heat gas that can be used to power a gas turbine.
For a more in-depth discussion of the Fukushima situation follow this link