The crisis isn’t over at Japan’s crippled Daiichi nuclear plant, but the situation is still improving.

US Energy Secretary Steven Chu told Fox News on Sunday that “with each passing hour, each passing day, things are more under control”. Japanese deputy-chief cabinet secretary Tetsuro Fukuyama echoed this in a press conference yesterday, claiming: “We are getting closer to bringing the situation under control.”

And early this morning (Australian time), the International Atomic Energy Agency added its voice to the chorus, saying: “There have been some positive developments in the last 24 hours, but the overall situation at the Fukushima Daiichi nuclear power plant remains very serious.”

Best news is that power has been restored to reactors 5 and 6, whose cooling systems are now working properly. As the IAEA put it shortly after midnight AEST:

“Workers today have successfully placed reactor unit 5 and 6 into cold shutdown. This means that the reactors are in a safe mode, with cooling systems stable and under control, and with low temperature and pressure within the reactor.”

Temperatures in the spent-fuel pools at units 5 and 6 — which were rising dangerously — are also down, from 70 degrees on Saturday morning to around 40 degrees at midday yesterday — so we can take these two off our list of concerns.

The next piece of good news is that radiation levels at the plant are still falling. The latest measurements 500 metres from reactor 3 — where they have been highest — show levels below 3 millisieverts per hour, which is half what you would get from a typical CT scan. The levels peaked at 400 millisieverts per hour between unit 3 and 4 last Tuesday (much closer to the reactor) after an explosion and fire.

But the crisis isn’t over. Yesterday, the plant’s operator TEPCO decided it might need to let off pressure building up inside reactor 3 and release another large burst of radiation. Luckily, the pressure fell back again and it wasn’t necessary, but no one seems to know why that latest mini crisis at unit 3 occurred (or abated).

The damage to reactors 3, 2 and 1 has now been upgraded to 5 on the NES scale of 1 to 7 (Three Mile Island was 3; Chernobyl 7). TEPCO says half the fuel in these units is still exposed, damaged and liable to heat up, despite continuous efforts to cool the reactors with seawater. Moreover, the containment vessels in units 2 and 3 are cracked or damaged.

The only way to fix this is to get the cooling systems working again, and that depends on restarting the pumps, but no one yet knows if the pumps can be made to work. Meanwhile, power is finally back on at unit 2 and is forecast to be working at 3 and 4 within a couple of days.

The other key concern involves the pools at reactors 3 and 4. As the IAEA puts it:

“Spent fuel removed from a nuclear reactor is highly radioactive and generates intense heat. This fuel needs to be actively cooled for one to three years in pools that cool the fuel, shield the radioactivity, and keep the fuel in the proper position to avoid fission reactions. If the cooling is lost, the water can boil and fuel rods can be exposed to the air, possibly leading to severe damage and a large release of radiation.”

Over the weekend, the Tokyo Fire Department wheeled in its Super Pump truck and Hyper-Rescue unit to spray 3000 litres (or three tonnes) of water per minute into the damaged unit 3 building. As to why this combination — which has a 22-metre arm for the spray — wasn’t brought in earlier is a mystery.

Water spraying also started on Sunday into unit 4, where almost three times as many spent fuel rods (around 1500) are stored. The key question here is whether the pool (which sits on top of the reactor and near the top of the damaged building) will actually retain water sprayed or pumped in.

Put simply, no one yet knows that either. But if the pool is damaged it’s going to be difficult — or perhaps impossible — to repair it. And on the IAEA’s timetable, that could mean another 33 months of fire hoses.

Clearly, in that scenario, TEPCO would have to copy the Russians and bury the Daiichi plant under masses of concrete and sand. Indeed, Japanese authorities have already admitted this may ultimately be necessary.

Meanwhile, amid the euphoria of finding two more survivors alive, the grisly death toll from the earthquake and resulting tsunami continues to rise — 8277 are now confirmed dead, and 12,722 remain missing. Some 400,000 people are homeless.