Fukushima and Reactor Safety – Unknown Unknowns

No technology including nuclear power can be made absolutely safe.

One of the best and well-known historical illustrations of this is the design of the Titanic. The plan incorporated a system of sixteen water- tight compartments that allowed the ship to float if two of the first four were flooded. Thus, the ship should survive any conceivable collision scenario. The media of the time and perhaps the designers talked of an “unsinkable” ship but who could imagine an accident in which the vessel would scrape along an iceberg slitting open six of the forward compartments?

The root of the problem is the “unknown unknowns”. This bit of pentagon-speak nicely sums up the idea that there are accident scenarios that one can’t imagine in advance. I also like the terminology “Black Swans” which I take to mean the same thing and will use that way.

Who could imagine that the operators at Three Mile Island would manually turn off the reactor’s Emergency Core Cooling System? Who could imagine that the Chernobyl operators would drive a reactor with no containment into a parameter region of unstable operation? Who could imagine that the design of the Fukushima power reactors would not withstand the largest possible earthquake and that the emergency power system was vulnerable to inundation?  Unfortunately, the answer is that no one imagined these possibilities and so the last two of these accidents became catastrophes.

It’s not just the big ones. Many other unanticipated nuclear accidents have occurred over the sixty years of nuclear power that avoided disasters with public consequences through the prompt operation of safety systems, skilled operator intervention and plain good luck.

Preliminary accounts of the Fukushima accident s show the importance of two well-known issues of nuclear safety. The first was inadequate redundancy in the fuel cooling systems because the reactor station design was deficient in protecting against earthquakes and tsunamis.   Who could imagine that in seismically active Japan, a country with 120 active volcanoes?  The second was the failure to vent in time the hydrogen generated by fuel exposure although the operators had many hours to do so. The hydrogen mixed with the oxygen from ambient air caused explosions that wrecked buildings and much of the reactor equipment in them further compounding the difficulty of restoring the necessary fuel cooling.  Ironically, the same “hydrogen bubble” was a big concern at Three Mile Island some thirty years before and hydrogen explosions no doubt occurred at Chernobyl.  Who could imagine that the operators would be so indecisive as to bungle hydrogen venting?

It’s easy to second guess and criticize others in hindsight. However, my intention here is not to assign blame for Fukushima but to show that nuclear accidents are fundamentally unavoidable. As history now shows, every once in a while there will be a big nuclear accident when a Black Swan comes to roost. Even the most sanguine unsinkable ship fans can no longer deny that.

When nuclear authorities in Canada and elsewhere say “our reactors are safe” what they really mean is that they have gone through all of their event trees, probabilistic calculations and voluminous safety documents of numerous accident scenarios and found no fault with them. In effect they are saying:  “we have solutions for every accident scenario we have been able to imagine.”  They are sincere in their exhaustive analysis and certainly want no accidents at occur. However, they can make no plausible claim for a process for identifying unknown unknowns. I wish they were more honest by adding these qualifiers to their declarations of safety.

To me this leads to two important conclusions. First, the nuclear industry should recognize publically that serious nuclear accidents will occur from time to time in spite of their best efforts to prevent them. Therefore, nuclear accidents are a price that society has to pay for increasing population growth, high energy consumption and perhaps low-carbon energy as argued in my first posting on Fukushima. Secondly, the public must acknowledge that its expectation of perfect nuclear safety is unrealistic and like any other industrial technology accidents will continue to be a feature of nuclear energy in the future.  I can detect some of that feeling of public acceptance already emerging.

Lake Ontario isn’t Walden Pond and never will be.

ORIGINALLY POSTED June 5, 2011

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