Sampling boat launched; major reports published

22 November 2011

TEPCO was due to launch a remote-controlled boat to collect water radioactivity samples near the damaged Fukushima Daiichi nuclear reactor site in late November.

Fukushima Daiichi sampling boat
Remote-controlled sampling boat

The boat contains a water sampler, underwater dose rate gauge and current meter. It will be used to sample water up to 10km offshore, within the 20km exclusion zone. A planned route between Fukushima Daiichi and Daiini would include five investigation points northeast, east and southeast of Fukushima Daiichi. It generally would navigate within stationary measurement points anchored at the shoreline and about 15km offshore. The boat, which has a 100km cruising radius, was a joint development of the University of Tokyo, Mitsui Engineering & Shipbuilding Co.

Also in mid-November, several major reports have been published about the incident.

First, the US-based Institute of Nuclear Power Operations has compiled a standardised event timeline for the progression of the incident, from the time the earthquake struck on 11 March, to four days later, drawing on information from TEPCO, the Japanese government, the IAEA, and a few other Japanese organisations. The report aims to only list facts and events, without providing analysis or interpretation. The report can be downloaded from a link on the right-hand side of the page.

Second, the Japanese nuclear training authority, Japan Nuclear Technology Institute, has compiled a thorough review and analysis of the event, from the point of view of the Japanese nuclear industry (utilities and manufacturers). It also recommends improvements to nuclear power plants to mitigate the risks demonstrated by the incident at Fukushima. The summary and report can be downloaded from links on the right-hand side of the page.

The report concludes that there were three turning points in the events at Fukushima: inability to supply AC power to the station, inability to remove heat from the reactor, and leakage of hydrogen into the building and hydrogen explosions. It further breaks down the first problem-station blackout-into four subsidiary problems: inability to supply external power, inability to supply power from emergency diesel generators, inability to interchange power sources, and the impossibility of early power recovery. The document analyses each of these issues, lists the reasons why they could not be accomplished (for example, inundation of electric panels by tsunami) and proposes particular resulting issues to be examined.

It concludes: "The essence of the problem is that preparations assuming a contingency brought about by tsunami strike were ineffectual." It goes on to organise issues and potential actions in five categories: preparation for earthquakes and tsunami (natural hazards), preparation of power sources, measures to counter heat sink loss, hydrogen countermeasures and preparations for emergency situations. Examples include enhancing seals on penetrations and containment vessel gaskets and installation of bulkheads around seawater pumps. Recommendations are listed below by topic.

In other news, TEPCO reports that the unit 1, 2 and 3 RPV bottom and primary containment vessel (PCV) internal temperatures have remained below 100°C for about six weeks. Both unit 1 temperatures are about 40°C; unit and unit 3 temperatures are about 75°C. TEPCO said that any fuel that leaked out of the reactor and into the PCV should be sufficiently cool not to generate steam (and thus reduce the amount of fission product release).

As of mid-November, the water treatment system had processed 162,000 tons of water. It now consists of multiple decontamination and desalination lines. As a result, the amount of accumulated water in the unit 1 and unit 2 turbine building has fallen from 3.8m to about 3.1m above Ohama Port levels.

TEPCO has set up a regular radioisotope sampling programme over units 1, 2 and 3. It says that the current release of radioactive materials from the three PCVs is about 60 million Bq/h, which translates into a radiation dose of 0.1 mSv/y at the site boundary. At unit 1, a permanent exhaust line has been installed under the apex of the roof of the new covering; this air is sampled pre-and post-filtration, and then vented through the site stack. At unit 2, a sensor monitors exhaust from the blow-out panel. At unit 3, a crane trails a sensor over the as-yet uncovered reactor building.

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