Wednesday, October 15, 2008

A Primer on Nuclear Safety: 2.4 Defense in Depth

A Primer on Nuclear Safety:
2.4 Defense in Depth
A Brief Note on ESBWR Safety

How Safe is the ESBWR? Given a probabalistic approach to nuclear safety, we first note that the ESBWR designers GE has estimated that a likelihood of core meltdown every 29 million years. But a core meltdown is far from a release of a large amount of radiation from the reactor into the environment. Thus we must look at the likelihood of a failure of not only the presser vessel, but of the core catcher, a system designed to trap and contain molten material from the core and to prevent its movement out side the reactor containment system. The core catcher is a passive safety system that uses the force of gravity,to move molten core materials into a series of dead end underground passages. Finally, our large amount of radioactive material must escape the massive outer containment dome of the reactor. Let us assume that each of these containment structures works as intended. We know, for example, that the pressure vessel will contain a core meltdown, because the core of the Three Mile Island reactor was contained by its pressure vessel. Thus it is very unlikely that the pressure vessel of a ESBWR would fail if its core did melt down. Let us consider that the likelihood of that the average time to pressure vessel breach by a molten core is average time to core meltdown multiplied by a factor of 10. That would give us a figure of 290,000,000 years to pressure vessel breach.

Now the failure of the core catcher is far more unlikely than the breach of the pressure vessel because the core catcher relies on natural forces to capture and hold the molten core. Let us assume for the sake of argument that the likelihood of a core catcher failure is the average time to pressure vessel failure multiplied by a factor of 10. That would give us a period of time of about 2,900,000,000 years before core catcher failure.

Once the core catcher fails there are still the massive outer containment walls of the reactor to prevent a large scale release of radioactive materials. Again we will assume that this structure will increase the likelihood of containment by a factor of 10. This would give us a catastrophic release of radioisotopes into the environment once every 29 billion years. That figure happens to be over twice the age of the Universe, and several times the expected lifespan of the earth.

Given a probabalistic world, many natural catastrophic events are far more likely than containment failure including the eruption of the Yellowstone super-volcano and event which could kill millions of people, and which is likely to occur sometime within the next 160,000 years.

Of course if more ESBWRs are built, our probability of catastrophic containment failure will increase. With a set of 1000 ESBWRs, we end up a gain with the figure of once every 29,000,000 million years between containment failures. To put this figure into some perspective collisions between the Earth and astroids of at least 5 km in diameter occur once every 10 million years. The impact of such an astroid with the earth would cause enormous damage to human society. Such events are three times as likely to occur as the catastrophic failure of containment in a ESBWR in a thousand reactor system. The worst possible consequences of reactor core containment failure would be very small compared to the consequences of a once every 10 million year astroid impact event.

It is my contention then that the dangers posed to the population of the world by a massive 1000 reactor system of ESBWRs is insignificant when compared with far more likely natural disasters.  However, as safe as the ESBWR is, it is not the ultimately safe reactor.  I will turn next to an exploration of how to make reactors even safer than the ESBWR is.

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