Japan’s nuclear disaster continues to unfold this morning, and as workers at the Fukushima Daiichi continue to pump seawater into the crippled reactors there a new threat is emerging: the spent fuel rods in the cooling pools at each reactor. While reactors Nos. 1-3 remain unstable due to core heating, the cooling pool at reactor No. 4 was boiling as of late Tuesday (local time), and the temperatures were rising in the cooling pools at No. 5 and No. 6.