Dust flux, Vostok ice core

Dust flux, Vostok ice core
Two dimensional phase space reconstruction of dust flux from the Vostok core over the period 186-4 ka using the time derivative method. Dust flux on the x-axis, rate of change is on the y-axis. From Gipp (2001).

Monday, June 27, 2011

Nuclear power and geologic hazards 3: flooding

The two main considerations for impacts of geologic hazards are predictability and prevention. How predictable is the event, and what can you do to prevent damage to critical facilities. Most of the thinking about geological hazards (especially with respect to nuclear power plants) tends to centre around earthquakes, because they are difficult to predict, and because any prevention has to be designed into them, as there is normally insufficient warning to take any meaningful measures prior to an earthquake.

Two other kinds of hazard would be in this same category--volcanic eruptions and mass movements (landslides and sinkholes). However these are usually mitigated by avoidance. How many nuclear plants have you seen built on a volcano or on the side of a mountain? Sinkholes occur in particular geologic terrains, and (hopefully) power plants are not built on such ground.

Other natural hazards (hurricanes, flooding, forest fires) are not considered as serious because they are somewhat predictable and it is possible to mitigate damage immediately prior to the event.

Floods in particular are avoidable. I admit to a blind spot in thinking about the hazards of flooding affecting nuclear reactors because I assumed most were built in places where flooding doesn't happen (the tsunami affecting Fukushima being an unexpected event).

So why is there a reactor facing flood damage in Nebraska?

The buildings are designed to withstand floods up to a certain height. The forecast for the current flood on the Missouri River falls nearly 2 m short of this height, and so in principle, the reactor should be fine. On the other hand, after the events at Fukushima, people should be forgiven for regarding official pronouncements of reactor safety with a degree of skepticism.

The official report:

The 2,000-foot berm collapsed about 1:25 a.m. Sunday due to “onsite activities,” OPPD officials said. The Aqua Dam provided supplemental flood protection and was not required under NRC regulations. 

“We put up the aqua-berm as additional protection,” said OPPD spokesman Mike Jones. “(The plant) is in the same situation it would have been in if the berm had not been added. We're still within NRC regulations.”

The NRC says its inspectors were at the plant when the berm failed and have confirmed that the flooding has had no impact on the reactor shutdown cooling or the spent fuel pool cooling. 

The NRC said there is a separate, earthen berm to protect the electrical switchyard and a concrete barrier surrounding electrical transformers. 

Hmm, earthen berms. Let's see how they have held up historically against floods.

Here's a good-looking one. Mississippi River 1890. This is probably much like the one that just collapsed in Nebraska (minus all the black folks).

Here's another nice sturdy earthen wall (from an earlier edition of Keller's textbook). Wow, okay, maybe that's a bad example.

Here we go, this gorgeous berm built by (and photographed by) the US Army Corps of Engineers is just what we need to protect that power plant. Berms like it have made New Orleans completely invulnerable to flooding . . . err, ahem.

The trouble with earthen berms is that they tend to be reasonably strong against single events. But if flood waters pile up around them, the chance of failure through undermining increases dramatically. It is worse if the dam is subject to repeated floods. Was the berm and the ground beneath it checked for signs of water seepage after the last flood? Were any repairs necessary?

Individual owners of properties protected by berms are urged to inspect them every month. How likely is it that this has actually been done (rather than simply having had some paperwork filled out by a contractor)?

Earthen berms in Iowa have already suffered similar damage according to this news story.

What constitutes safety in design? Normally you look at building to withstand the events with recurrence intervals of 100 to 1000 years (the 100-year flood or 1000-year flood). The problems that many reactors in the US have is that the size of such events has recently increased (or rather the probability of occurrence of large-magnitude flooding event has increased markedly) due to urbanization and landscape changes (particularly the numbers of levees in upstream settings), and the power plants themselves have been called upon to serve much longer than their projected lifespans.

When you build a power plant that is intended to last 35 years, there is only about a 1-in-30 chance of a thousand year flood during its projected life. But when you now double that lifespan, and at the same time increase the size of the projected thousand-year flood, the likelihood that the plant will experience a flood beyond its design capabilities rises considerably.

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