Tepco has found that airborne radiation above and around the damaged units 1 and 2 of the Fukushima Daiichi nuclear power plant are all well below the legal density limit for air breathed in by radiation workers (2×10-3 Bq/cm3 for Cs 134, 3×10-3 for Cs 137).

Results ranged from below a hundredth of these limits in the upper parts of unit 1, to up to a half of these limits near the lower part of the unit 2 blow-out panel, according to Tepco, although unit 2 roof area results have not yet been released.

Tepco’s plans to cool the damaged Fukushima Daiichi unit 3 using an additional method have been postponed. It has been injecting water into the unit 3 reactor building via the feedwater system, as it is at units 1 and 2. The feedwater is injected beneath the active core, the latent heat of steam production doing most of the cooling. But despite injecting double the amount water into unit 3 as each of the other two units, (7.1 m3/hr, as of 30 August) the temperature of the feedwater nozzle and the bottom of the primary containment pressure vessel remain above 100°C. Also, the World Nuclear Association’s WNN reports that Tepco also wishes to reduce the amount of highly radioactive water produced in cooling the reactors. So Tepco has announced plans to inject water into the reactor using the core spray system, which sprays the top of the fuel via the CS ring header. Site surveys that took place in late August confirmed that the CS injection valve (MO-14-12B) could be actuated electrically.

Although the system was installed in several stages from 22 August to 25 August, its commissioning has been delayed pending investigation of a leak using the same equipment. A slow leak (one drop/30 seconds) has been found in the unit 4 spent fuel pond alternative cooling system. Once commissioned, the amount of injection via core spray will be slowly increased, and that from feedwater injection will be slowly reduced over the period of about a week, until 3m3/hr is injected into the reactor by each method.

Tepco has concluded that most of the fuel in the spent fuel ponds is sound, based on the results of an August 19-20 spent fuel pool analysis, although it did not elaborate how it came to this conclusion. Tepco said that the results were similar to a previous analysis. The numbers also show the extent of salt contamination in units 2, 3 and 4, which have about 1500-2000 ppm of the chloride ion, presumably from emergency cooling water injected from the sea. The pH of the pools also varied quite widely.

•In other news, Tepco has found evidence consistent with its theory of the cause of the explosion at the unit 4 spent fuel pool. After the fuel rods were found to be covered by water, and therefore could not have produced combustible hydrogen, in May Tepco suggested an alternative cause for the explosion. It suggested that hydrogen flowed backwards into unit 4 from unit 3 via the exhaust stack and the standby gas treatment system. Radiation measurements around the SGTS filters show higher radiation doses at locations closer to unit 3.

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