Level the ground, pour in concrete slab, pile up a lot of sandbags. Decidedly conventional tech. And this has to last for no one knows how long. According to TEPCO, as of December 20, the total number of used cesium towers (Kurion and Toshiba) was 316, with the storage capacity of 393 , and the number of used cesium towers increased by 4 from the previous week. Even at this slow rate, the storage would have been full in 19 weeks or about 5 months. Therefore this "kick the can" construction of more storage space.
One area for Kurion (544 towers capacity), another for Toshiba's SARRY (200 towers). From the looks of it, TEPCO will move the existing used towers from the existing temporary storage space to these new temporary storage facility. That means Kurion's storage will be already more than half full (currently 288 towers), and Toshiba's storage will be more than 10% full (currently 28 towers) already.
Why do they need two separate areas for Kurion and Toshiba in the same facility? Because the size of the cesium towers from these companies is too different, as you can see in the TEPCO's diagram below. Kurion's tower is about 2.4 meter tall (probably built to the spec in inches and feet), and Toshiba's tower is about 3.8 meter tall (probably built in centimeters and meters). It seems Toshiba's SARRY towers do not need concrete lids on top, so they are doubling the height of the rack for SARRY towers and using concrete panels to create secure containment for Kurion towers.
Ah the infinitely long road to "recovery" at Fukushima I Nuclear Power Plant.
From TEPCO's press release on December 21, you see this diagram of the facility:
30-centimeter thick concrete slab on top of some sort of substrate. That doesn't inspire much sense of security, considering the recent leak from the evaporative condensation apparatus (desalination), whose concrete perimeter cracked because the concrete wasn't cured long enough.
Here's the facility up-close (click for a bigger photo):