Last year, 4/12/2011, 2.2E+8 Bq/m3 of Iodine 131 was measured from the water of SFP4 to suggest the spent fuel had recriticality after 311.
Cesium 134/137 were also measured, but Tepco states it’s about 1/100 of the amount in reactor1~3, they are likely to be blown from them.
However, Tepco did not show the comparison of the amount of Iodine 131 measured in SFP4 and reactor1~3.
Iori Mochizuki