Events at the quake- and tsunami-stricken Fukushima Daiichi nuclear powerplant in Japan went well at the weekend, with two reactors there successfully brought into cold shutdown under off-site power, power lines hooked up to other cores being cooled using seawater and some progress in refilling spent-fuel storage pools. Initial food sampling from the region around indicates that no significant quantities of hazardous radioisotopes have escaped from the plant.
World Nuclear News reports that reactors 5 and 6, which were hooked up to a new power line from off site on Friday, were then able to restart their cooling systems and bring their cores to "cold shutdown", where the core coolant is at less than 100°C. Spent-fuel cooling ponds at these units are also being chilled effectively and brought to normal status.
Elsewhere at the plant, operators reported that offsite power was now also available at reactors 1 and 2, bringing various instruments and readings back online. As of Sunday, plant owner TEPCO hoped that main cooling would also be brought back at these units as it survived the quake in functional condition and was only lost when backup diesel generators were destroyed in the following tsunami. TEPCO expects to restore power at units 3 and 4 within days.
The focus of workers at the site has remained the spent-fuel pool at No 3, where waters levels were feared to have fallen dangerously low last week - raising the prospect of damage to the rods from their own internal heat. The elite Tokyo fire-brigade "Hyper Rescue" unit arrived at the site and commenced operations over the weekend, deploying a truck with a 22-metre arm able to spray 3 tonnes of water per minute - "in combination with Super Pump Truck", reports WNN.
Hyper Rescue reportedly sprayed the No 3 fuel pond with this equipment for more than 13 hours continuously yesterday (it can be operated remotely) delivering more than two thousand tonnes of water before shifting to No 4. The spent-fuel pool there had caused the Japanese authorities less concern than the one at No 3 as helicopter overflights last week had appeared to show it still held some water, though US officials chose to differ and described it as the priority problem at the plant. WNN reports that the general temperature observed at the two buildings' rooftops using thermal imagers is now below 100°C, and that radiation nearby continues to drop steadily. Latest readings there indicate levels of 2.75 millisievert/hr, indicating that workers need to carefully manage their time spent close to the two units: they can sustain a total annual dose of 250 millisievert under emergency rules before being withdrawn from the operation.
There was briefly some concern on Sunday at rising pressure levels inside the core of Reactor 3, and it appeared that heavy venting of radioactive steam would be required - potentially from the reactor containment rather than the suppression chamber, raising the prospect of heightened radiation levels similar to those seen last Tuesday when it is thought that the suppression chamber of Reactor 2 was damaged. However in the event no such venting was required and operations on site were able to continue.
Meanwhile sampling of food from farms in Fukushima province revealed that so far, in line with expectations, no dangerous radioisotopes have been released from the plant in significant quantities. The primary health threat is iodine-131, which can be dangerous to children and younger adults even in very small amounts as it concentrates in their thyroid glands. This can cause a slightly elevated risk of thyroid cancer later in life, enough that over a large population the effects will be apparent - though no individual need worry greatly as there will still be only one chance in several thousand of ever developing thyroid problems even for those very severely affected.
Another positive factor is that radio-iodine has a half-life of just eight days and thus it disappears within months. According to the Japanese government, the levels detected in food samples thus far would have to be consumed for a lifetime to do harm: if someone drank milk containing radio-iodine at the levels seen in the affected samples for a year, the effect on the thyroid would be the same as a single CT scan. (That would be impossible in this case as all the radio-iodine from Fukushima will have decayed away within weeks.) Nonetheless the Japanese government has advised that evacuees below the age of 40 from the 20km zone around the plant should take iodine pills (or syrup for children) as a precaution: the presence of non-radioactive iodine in the body prevents iodine-131 being taken up by the thyroid.