Japanese engineers are trying to gain control of the Fukushima Daiichi nuclear power plant, 240 km (150 miles) north of Tokyo, which was destroyed by the huge March 11 earthquake and tsunami. Radiation continues to seep into the sea and the air, although at far lower levels than at the peak of the crisis  in mid-March.

Japan's nuclear safety watchdog estimated last month that the amount of radioactive substance emitted since March 11 was about one tenth of that released at Chernobyl in 1986.

Two of the six reactors at the plant, operated by Tokyo Electric Power Co (TEPCO), are considered stable but the other four are volatile. Following are some questions and answers about efforts to end the world's worst nuclear crisis since Chernobyl:

What Is Happening?
Workers are trying to fill the reactors with enough water to bring the nuclear fuel rods to a "cold shutdown," in which the water cooling them is below 100°C and the reactors are considered stable.

TEPCO has been pouring water into the reactor vessels containing the rods since the disaster happened. TEPCO is filling the containment vessels - an outer shell of steel and concrete that houses the reactor vessel - with water in a procedure called water entombment. In April it started by increasing the amount of water being poured into the No.1 reactor and is aiming to fill much of the containment vessel in about three weeks.

At the same time, it is working to restore the reactors' cooling systems. TEPCO is trying to install a separate system to theNo.1 reactor which will cool water in pipes circulating through the reactor with fans.

TEPCO opened the double doors at the reactor building on Sunday. The nuclear safety watchdog said the opening of the doors would not impact the environment. Japan said on Sunday it had notified the international community of the decision beforehand.

How Long Will The Crisis Last?
On April 17, TEPCO announced a timetable for its operations. Within the first three months it plans to cool the reactors and the spent fuel stored in some of them to a stable level and reduce the leakage of radiation.

TEPCO then hopes to bring the reactors to a cold shutdown in another three to six months.

But some experts said the process could take longer. TEPCO said constant aftershocks, power outages, high levels of radiation and the threat of hydrogen explosions were factors that could hamper its work.

Weather conditions, such as the approaching rainy season and typhoons and lightning during the summer, could also pose problems.