U.S. regulators have completed new requirements for hardened vents at boiling water reactors in response to the Fukushima Daiichi accidents, while in Japan the plant reported another small tank leak and surveys found reduced doses in the surrounding countryside.

Recent developments related to the Tokyo Electric Power Co. reactors blacked out following Japan's 2011 earthquake and tsunami include:

NRC to Release Hardened BWR Vent Order

The Nuclear Regulatory Commission has finalized details of new requirements on the installation of hardened vents for early containment designs of General Electric reactors. According to an NRC release, the new order mandates wetwell venting upgrades at 31 reactors with Mark I or Mark II containment starting in June of 2014. It also requires plants to analyze their current drywell venting, potentially requiring upgrades to those systems starting in 2017. Once the NRC posts the order online, it can be found in full under accession number ML13130A067.

Leaking tank at Fukushima. Source: TEPCOSmall Leak Found in Above-Ground Tank

A Fukushima worker on patrol found contaminated water leaking from a seam in an above-ground tank Wednesday afternoon. The water leaked at a rate of one drop every three or four seconds, according to a TEPCO release. In four hours, it's estimated the 500-metric-ton tank leaked less than 1 liter, which TEPCO said was contained with an absorbent. A TEPCO official told the Asahi Shimbun that the tank, erected in May, is one of 63 installed at the plant. Workers are in the process of moving water into above-ground tanks from underground tanks found to be leaking earlier.

Doses Down

An aerial survey of evacuated areas near the plant showed airborne radiation levels have decreased an average of 40 percent. Nuclear Regulation Authority data released Wednesday showed that areas designated as uninhabitable for at least five years had seen average doses fall from 14.5 microsieverts per hour to 8.5 microsieverts per hour between November of 2011 and March of this year. Similar airborne dose reductions were reported in other categories of evacuated land around the plant, the Asahi reported, which the NRA attributed to natural decay and contaminants being washed into the sea.