Helicopter water dumps not having much effect
According to WNN, the helicopter drops "did not appear accurate enough to be effective" and radiation around the Nos 3 and 4 buildings fell only slightly, indicating that "the effect at present seems marginal at best".
An operation is now underway to move fire trucks equipped with high-pressure pumps to the buildings and cool the pools with water pumped from ground level. Police water cannon may also be employed. This effort is being delayed by the need to bulldoze a path for the vehicles through heavy debris and mud deposited by the tsunami and subsequent explosions at the site.
Radiation levels at Nos 3 and 4 have been recorded as running at between 3 and 4 millisieverts/hour. Japanese regulators have now raised the permitted dose that site workers can sustain from the incident before being withdrawn from 100 millsieverts to 250: this would indicate that some personnel who have been involved since the beginning could only work shifts near the two buildings for another 40 hours before being forced to retire from the struggle.
There is a risk of radiation sickness after sustaining a dose of 1,000 millisieverts, climbing to 50 per cent at 2,000. Most sufferers with lower doses recover but at 4,000 millisievert the death rate has climbed to 50 per cent. There is no indication that anyone at Fukushima will be permitted to get even close to 1,000 however.
The primary health concern for the workers there is an increased possibility of developing cancer in the long term, though the fact that protective measures (suits, masks, breathing apparatus, heavily shielded and filtered control rooms etc) prevent radioactive materials actually entering their bodies removes much of this issue. Cancer being a very normal way to die in all wealthy countries, and only small numbers of people being involved, future investigations in decades to come will probably have difficulty assigning any deaths directly to service at Fukushima.
Radiation levels away from the reactor buildings themselves appear to be dropping as of the early hours this morning UK time. Japan's nuclear regulators reported as of the early hours this morning (UK time) that readings at the plant gates had dropped from 0.7 millisievert/hour to 0.3 over the previous 12 hours: workers would be able to endure such levels for months if required, as these hourly doses are equivalent to those sustained by everyone on Earth from background radiation every few weeks.
The situation with respect to the actual reactor cores at Fukushima no longer appears to be causing huge concern. The three damaged reactors (1, 2 and 3) are all being cooled using seawater pumped into their pressure vessels using the site's firefighting systems. Periodically steam is vented off from the pressure vessels via the suppression tanks: it would normally be held here to let short-lived radioactives generated in the pressure vessels decay before venting onward to atmosphere, but it appears that the suppression tank at No 2 (and possibly at No 3) has suffered a breach and is leaking steam directly.