Tokyo Electric Power Company (Tepco) on 13 May temporarily halted the injection of water into the damaged reactor at Fukushima Daiichi unit 2 to obtain data on how the temperature inside the reactor could rise in the event of an emergency. Tepco aims to use the information to update its planned response.

Tepco has been pumping water into the reactors at units 1-3 for more than eight years in order to cool the melted fuel debris. Unit 2 usually receives around three tons of coolant each hour. The temperature at the bottom of the reactor pressure vessel stood at about 24.5 Celsius at the start of the seven-hour test, and Tepco expected the reading to rise by up to 4 C. Although the condition of the reactors remains relatively stable as a result of the continuous cooling, a massive amount of contaminated water has accumulated at the plant as a result.

The Mainichi Shimbun reported on 13 May that the Japanese government is considering keeping contaminated water at the  plant in storage tanks for the long term. Five options to deal with the contaminated water are being considered: releasing it into the sea; piping it into a deep stratum of the Earth's crust; releasing it into the atmosphere as steam; encasing it in cement and burying it; and using electrolysis to hydrogenate tritium in the water before releasing it into the air.

However, strontium-90 was discovered in treated water in huge concentrations just before August 2018 public hearings on the contaminated water problem, and public concerns about releasing the water into the environment have caused the government to reconsider. A Ministry of Economy, Trade and Industry expert committee on the contaminated water issue, scheduled for June will now add long-term tank storage to the other five options.

There is already over 1 million tonnes of contaminated water stored at the Fukushima site, and current plans will see this increase almost 40% by 2020. At the current rate of increase, all the 10-metre-tall tanks will be full in four to five years.