The reason? The deadly combo of seawater and very high radioactive materials in it.
According to TEPCO's handout for the press on June 22, Kurion's system managed to reduce the radioactive materials as follows:
Iodine-131
Before the treatment: 6900 becquerels/cubic centimeter
After the treatment: 990 becquerels/cubic centimeter
Decon Factor: 6.97
Hoped-for DF: not known
Cesium-134
Before the treatment: 2,000,000 becquerels/cubic centimeter
After the treatment: 43,000 becquerels/cubic centimeter
Decontamination Factor: 46.5
Hoped-for DF: 1,000
Cesium-137
Before the treatment: 2,200,000 becquerels/cubic centimeter
After the treatment: 48,000 becquerels/cubic centimeter
Decon Factor: 45.8
Hoped-for DF: 1,000
So, Kurion's system achieved less than 1/20 (or 4.6%) of the performance hoped for.
The result for AREVA's system are listed as "ND" (not detected), but it was assumed by TEPCO that AREVA's system's Decon Factor was less than 400, where 1,000 was hoped for. (Information from Yomiuri Shinbun)
By using both Kurion's and AREVA's systems, TEPCO had hoped that the radioactive materials in the water could be reduced to 1/1,000,000.
If Kurion's system reduced the radioactive materials to about 1/46 and AREVA's system 1/400, the combined performance was only 1/18,400, short of the minimum performance needed for the smooth operation of the desalination system (by Hitachi).
Desalination is essential if TEPCO wants to use the treated water as cooling water for the reactors.
TEPCO's Matsumoto said in the press conference that he had hoped that the water treatment system would start running full-scale in a couple of days, but now he wasn't sure when. (Information from Asahi Shinbun)
The Tohoku region that include Fukushima Prefecture entered the rainy season on June 21.