TEPCO says "as a precaution". It looks ALPS Line B wasn't removing enough all-beta.
The multi-nuclide removal system ALPS is still on the extended "hot" test run using the waste water, treating less than 200 tonnes of water per day.
ALPS comes after the desalination (reverse osmosis) that comes after the cesium absorption by either SARRY or Kurion. These systems before ALPS haven't stopped. The water processed through SARRY/Kurion and reverse osmosis goes back into the reactors to cool the corium, and ALPS is to treat the waste water that is high in all-beta.
Even if ALPS is stopped, the water treatment and cooling the reactors continue.
TEPCO's email alert to the press (3/18/2014) says:
多核種除去設備(ALPS)では、汚染水処理設備にて処理した廃液を用いた試験(ホット試験)を行っていますが、本日(3月18日)、3系統(A系,B系,C系)あるうちの1系統(B系)について、午後0時4分にフィルタの酸洗浄のため停止しています。
We've been conducting the "hot" test of the multi-nuclide removal system ALPS using the waste water after it is processed in the contaminated water treatment systems. Today (3/18/2014), one of the three lines, Line B, has been stopped at 12:04PM to acid-clean the filters.
同日、B系の処理後の出口水の全ベータの分析結果(3月17日採取分)が10の7乗Bq/L程度であることを確認しました。
Also today, we noted that the nuclide analysis of the water treated by Line B (collected on March 17) showed all-beta to be [in the order of] 10^7 Bq/L [10,000,000 Bq/L].
多核種除去設備(ALPS)の入口水については、全ベータで10の8乗Bq/L程度であり、処理が不充分となっている可能性があることから、念のため、A系について同日午後1時38分、C系について午後1時39分に処理を中断しました。
The water before entering ALPS has about 10^8 Bq/L [100,000,000 Bq/L] of all-beta, so there is a possibility that the treatment by ALPS is not adequate. As a precaution, we stopped Line A at 1:38PM and Line C at 1:39PM.
Strontium-90, a beta nuclide, is mostly removed in the pre-treatment process of ALPS that uses iron coprecipitation and carbonate coprecipitation. TEPCO hasn't found what part of ALPS may be malfunctioning this time.
For my post on why ALPS may already be obsolete, click here.