Japanese utility TEPCO is preparing an application to the Japanese regulator to flood the Fukushima Daiichi unit 1 reactor vessel, and is planning a ventilation system to improve working conditions during planned stabilisation work.

Reactor-by-reactor, system-by-system summary from JAIF as of 3 May

Reactor-by-reactor, system-by-system summary from JAIF as of 3 May; yellow indicates abnormal/unstable; red means damaged/nonfunctional

Also, it announced that based on a 29 April survey by a remote-control robot, no significant water has leaked from the unit 1 primary containment vessel.

TEPCO announced a site stabilisation plan two weeks ago that included a plan to flood each of the reactors with water to a level higher than the top of the fuel assemblies. The water level in unit 1 is currently 1700 mm below the top of the fuel assemblies.

Last week, TEPCO announced a plan to double the rate of water injection from 6m3/hr to 14m3/hr for a period to test the potential effect of flooding. The test lasted from 27 to 29 April.

Now, TEPCO reports that the Japanese regulator has requested that it apply for permission to carry out the flooding. It said it would submit a report including an analysis of the impact of increased pressure on the primary containment vessel, reactor building, drywell and suppression chamber, how flooding would affect the earthquake resistance of the reactor, and the impact on current leaks at unit 1, and estimate of probable future leaks.

TEPCO continues to inject nitrogen gas to reduce the risk of hydrogen gas, building up because of a reaction of air and the zirconium fuel cladding, from exploding.

Now, to stabilise the reactor, it is also planning to install and calibrate a new reactor water level gauge and new cooling water circulation system.

In preparation for this work, it is performing surveys and collecting dose information and the density of radioactive substances with a remote-control robot.

TEPCO is also planning to set up an air filtration system to circulate air inside the building and filter out radioactive systems. Two 270m3/hr systems will blow air through temporary ducts into the turbine building, providing the turbine building with positive pressure relative to the reactor building. Reactor building air, channelled by temporary ducts, will be filtered by four 270m3/hr filtering units. Once the system has been set up, it will run for some time before workers enter the building and open the unit 1 reactor airlock.