Most fuel in No.2 did not melt, says TEPCO

July 29, 2016

Fukushima No. 2 reactor ‘X-ray’ shows most fuel was contained

http://www.asahi.com/ajw/articles/AJ201607290050.html

THE ASAHI SHIMBUN

Most of the nuclear fuel inside the No. 2 reactor at the Fukushima No. 1 nuclear power plant did not melt through the pressure vessel as previously believed, research using muon tomography has revealed.

The assessment was made based on a study that utilized muons, an elementary particle that travels from outer space, to capture the interiors of the crippled reactor building like an X-ray.

Tokyo Electric Power Co., the ill-fated facility’s operator, made the announcement on July 28.

According to the utility, the technique captured a black shadow estimated to account for 180 to 210 tons of substances in the lower part of the reactor’s containment vessel.

There was about 160 tons of nuclear fuel inside the No. 2 reactor, which was operating when it suffered a meltdown in the accident that resulted from the 2011 Great East Japan Earthquake and tsunami.

As the nuclear fuel, control rods and other materials that would have melted down totaled about 210 tons in weight, TEPCO deemed that “a large portion of the nuclear fuel is presumed to be contained within the pressure vessel.”

The latest finding negates past studies that have suggested that most of the nuclear fuel inside the reactor had melted through the vessel.

“We hope to effectively come up with a method to extract the melted fuel,” said Naohiro Masuda, president of TEPCO’s Fukushima Daiichi Decontamination & Decommissioning Engineering Co.

Muons are known to travel through materials such as concrete, but are unable to penetrate highly dense substances such as uranium.

The International Research Institute for Nuclear Decommissioning, an organization made up of the utility and nuclear power plant manufacturers, had been using the particles’ properties to peer inside the No. 2 reactor since March.

July 29, 2016

Bulk of melted fuel at bottom of Fukushima No. 2 reactor vessel

TOKYO (Kyodo) -- Most of the melted nuclear fuel inside the No. 2 reactor at the disaster-hit Fukushima Daiichi power plant is likely located at the bottom of its pressure vessel, Tokyo Electric Power Company Holdings Inc. said Thursday.

According to a study that used a cosmic ray imaging system, an estimated 130 tons of the so-called fuel debris remains at the bottom of the vessel, the first time the location and amount of the melted fuel have been estimated.

The finding is important as the data could help the operator to narrow down methods to remove the fuel debris, the most challenging task in decommissioning the plant's Nos. 1 to 3 reactors that experienced meltdowns in the nuclear crisis that began in March 2011.

The study was carried out by a team involving Tokyo Electric and the High Energy Accelerator Research Organization in Ibaraki Prefecture.

As high radiation levels are continuing to hamper direct access to the reactors, researchers have tracked muon elementary particles, which are produced as cosmic rays collide with atmospheric particles and change course when coming into contact with nuclear fuel.

The No. 2 reactor was in operation when the nuclear crisis was triggered by a powerful earthquake and subsequent tsunami that devastated Japan's northeast.

About 160 tons of fuel assemblies are estimated to have been present inside the reactor vessel prior to the crisis. Most of the fuel is believed to have fallen to the bottom of the pressure vessel and mixed with nearby structures to form debris.

In the nuclear crisis, massive amounts of radioactive substances were released into the environment, with the Nos. 1, 3 and 4 reactor buildings damaged by hydrogen explosions.

The No. 4 reactor was offline for periodic maintenance work and all of its fuel was stored in the spent fuel pool, avoiding a meltdown.