Fukushima Nuclear Accident – 17 March update

Fukushima Nuclear Accident – 17 March update Barry Brook | | URL: http://wp.me/piCIJ-14k

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.

Read more of this post