BraveNewClimate

The crisis at the Fukushima Daiichi nuclear power station is approaching a weeks’ duration. The on-site situation remains extremely serious, with glimmers of hope being shrouded by a shadow of deep uncertainty.

If you’ve not been following the situation on BraveNewClimate, and want to recap, please read these recent updates:

Japan Nuclear Situation – 14 March updates

Further technical information on Fukushima reactors

Fukushima Nuclear Accident – 15 March summary of situation

Fukushima Nuclear Accident – 16 March update

These are assumed knowledge for understanding the rest of this post. The preparation of the material below was aided greatly by the private advice of my acquaintances in the nuclear engineering field.

As predicted yesterday, attention over the last 24 hours has focused on the critical situation with the ponds used for temporary storage of spent nuclear fuel at the individual reactor units, before it is moved to a centralised facility on site. Although this old fuel has lost much of its original radioactivity, the decline is exponential (see this figure) which means that thermal energy must continue to be dissipated for months.

This figure shows the location of the spent fuel ponds:

The problem, as is explained in this updated fact sheet by the NEI, is that as these ponds heat, their deep covering of water (which acts as a radiation shield and a cooling mechanism), starts to evaporate. If they reach boiling point, because of lack of operational maintenance systems, the evaporation rate will accelerate. If exposed, the there is a potential for these old fuel rods and their zirconium cladding to melt, and radiation levels will rise considerably. The heat generated in spent fuel depends on a number of parameters, including: (1) level of build-up of fission products (burn-up) and (2) length of time after having been taken out of the reactor.

The spent fuel pool temperature has been rising gradually since last Friday due to the loss of cooling pump (presumably no power source). As we know from previous updates, the side of the Reactor 4 building has been lost (it’s the left-most of the 4 buildings in the following image):

The Unit 4 reactor was already shut off for periodic maintenance when the earthquake struck. IF the fire was caused by hydrogen,  its only plausible source would be spent fuel degrading in steam. Under this scenario, initial inventory was probably reduced by sloshing during the earthquake, and heat generation and resulting evaporation/boiling would thereafter be more than double that in other pools due to it containing freshly off loaded fuel. Temperature indications in the absence of water would be that of the mixture of steam and air in the location of the thermowell.

Nothing can be confirmed at this stage. As has been the case throughout this crisis, information is hard to come by and must be pieced together.

Are the spent fuel in the pools in Units 3 and 4 are now uncovered? TEPCO claims that NRC Chief Jaczko was wrong in claiming this, that the spent fuel pools in both Units 3 and 4 need some refilling but are NOT dry. (The Japanese authorities are apparently saying they’ve seen water still in the Unit 4 pool.) The big concern here is that unlike the releases from damaged fuel in the reactor cores of Units 1, 2, and 3, which were largely filtered by scrubbing in the containment suppression pools (wetwell torus), releases of volatile fission products (e.g., cesium and iodine) from these spent fuel pools have direct pathways to the environment, if they remain dry for an extended period.

Efforts to deliver water to these pools have proven to be very difficult, and fuel damage may be occurring.  If they are exposed, then the use of the evaporation of salt water as a heat sink over periods of more than a few days is not viable because the quantities of salt deposited as the water evaporates becomes large in volume and plugs the flow paths through the fuel, degrading heat removal. Everything that is cooled becomes a heat sink to condense anything volatilised. Unfortunately, a fresh water supply seems difficult to come by.

One option is to bring fresh water by helicopter, but the amounts needed imply a large number of flights and gamma radiation levels are high above the pools making overflights hazardous. NHK has reported a number of  successful water dumps using helicopters today. If radiation levels on the ground increase further, personnel access will become more challenging. Additional spent fuel is stored in pools in Units 5 and 6 and in a large centralized storage pool. A key issue is how to continue to make up water to these pools in the longer term, particularly if site access becomes more difficult.

(more…)

Filed under: Hot News, Nuclear | 334 Comments »

I am currently compiling an update for the Fukishima Nuclear Accident as of March 17. The situation is developing very quickly and commenters on this thread (March 16 updates) are doing an excellent job at posting regular updates.

Meanwhile, here is a guest post from Luke Weston.

———————–

Looks like we need an update post on the situation with the Fukushima nuclear power reactors. The situation continues to change and develop, but good, sensible, detailed information is still hard to find.

In the wake of the Fukushima incident, it has really helped me to understand what happened in 1979 at Three Mile Island; with nonsense all throughout the media, and FUD, and panic spreading, with good information almost impossible to find, and with the over-abundance of bad information leading to hysteria.

But this is the first time it has happened to the Facebook and Twitter generation; I’ve yet to determine whether that’s a good thing or a bad thing. We need to keep working hard to make sure it’s a net benefit for the good information, not the dodgy information.

Design Basis Godzilla

Why wasn’t the earthquake design basis set high enough?, some people ask. What if the next earthquake is magnitude 10? Magnitude 12? Magnitude 20? But where does it stop? Where do you set the design basis? What if the reactor is attacked by Godzilla?

No matter where you set the design basis, you will always exceed it one day, eventually. And when you do, the anti-nuclearists will complain that the design basis is not set high enough.

There is always some really extreme, really catastrophic situation that you can imagine, but what is its probability in any given year?

It’s all about Probabilistic Risk Assessment.

You know what the average probability per year of a magnitude 8 earthquake in the area is, what the probability of a magnitude 9 earthquake is, what the probability of a large tsunami is, etc. I’m not sure what the probability of Godzilla attack is. Need to ask an expert. Somebody get Matthew Broderick in here; he’s the guy to ask.

And you decide what the acceptable probability of a severe core damage (“meltdown”) incident (which won’t hurt anybody but will probably write off the reactor, like TMI) for the nuclear reactor is in any given year – let’s suppose it is decided that one such failure per 50,000 reactor-years of operation is acceptable. Then, with that in mind, you design the degree of seismic hardening and safety engineering for the nuclear power plant so that you hit that target.

It is the safest way to do it which is actually realistic. You can’t say that it’s absolutely 100% resistant to any hypothetical scenario of destruction that you can imagine, because there is always something that you can imagine that is more destructive.

Seawater injection into the primary containment of Unit 1 – not into the reactor vessel itself

On 12 March, TEPCO announced that they planned to cool the Unit 1 reactor with seawater, adding boric acid to the water as a nuclear poison, to prevent any possibility of unintended criticality. The injection of fresh water and seawater into the primary containment vessel through a fire-extinguishing system line commenced on the 13th of March.

This is not an injection of seawater into any part of the nuclear reactor or the Nuclear Steam Supply System itself. It is an injection of seawater into the containment structure surrounding the reactor pressure vessel.

These reports confirm my earlier prediction that they were not talking about actually putting seawater into the nuclear steam supply system, despite the lack of any previous clear, sensible announcements in the press to this effect.

(more…)

Filed under: Hot News, Nuclear | 195 Comments »