Muons suggest location of fuel in unit 3 — World Nuclear News

” Some of the fuel in the damaged unit 3 of the Fukushima Daiichi plant has melted and dropped into the primary containment vessel, initial results from using a muon detection system indicate. Part of the fuel, however, is believed to remain in the reactor pressure vessel.

Muons are high-energy subatomic particles that are created when cosmic rays enter Earth’s upper atmosphere. These particles naturally and harmlessly strike the Earth’s surface at a rate of some 10,000 muons per square meter per minute. Muon tracking devices detect and track these particles as they pass through objects. Subtle changes in the trajectory of the muons as they penetrate materials and change in direction correlate with material density. Nuclear materials such as uranium and plutonium are very dense and are therefore relatively easy to identify. The muon detection system uses the so-called permeation method to measure the muon data.

Tokyo Electric Power Company (Tepco) installed a muon detection system on the first floor of unit 3’s turbine building. Measurements were taken between May and September this year.

Tepco said analysis of muon examinations of the fuel debris shows that most of the fuel has melted and dropped from its original position within the core.

Prior to the 2011 accident, some 160 tonnes of fuel rods and about 15 tonnes of control rods were located within the reactor core of unit 3. The upper and lower parts of the reactor vessel contains about 35 tonnes and 80 tonnes of structures, respectively.

The muon examination indicates that most of the debris – some 160 tonnes – had fallen to the bottom of the reactor pressure vessel and resolidified, with only about 30 tonnes remaining in the reactor core. Tepco said another 90 tonnes of debris remains in the upper part of the vessel.

The bulk of the fuel and structures in the core area dropped to the bottom of the reactor pressure vessel (RPV), Tepco believes. While part of the molten fuel is understood to have then fallen into the primary containment vessel (PCV), “there is a possibility that some fuel debris remains in the bottom of the RPV, though this is uncertain”, the company noted.

Similar muon measurements have already been conducted at units 1 and 2 at Fukushima Daiichi. Measurements taken at unit 1 between February and September 2015 indicated most of the fuel was no longer in the reactor’s core area. Measurements taken between March and July 2016 at unit 2 showed high-density materials, considered to be fuel debris, in the lower area of the RPV. Tepco said that more fuel debris may have fallen into the PCV in unit 3 than in unit 2.

Tepco said the results obtained from the muon measurements together with knowledge obtained from internal investigations of the primary containment vessels using remote-controlled robots will help it plan the future removal of fuel debris from the damaged units. ”

by World Nuclear News

source with illustration of Unit 1-3

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Radiation measured at deadly 9.7 sieverts in Fukushima reactor — The Japan Times

” Tokyo Electric Power Co. said Monday that radiation in the primary containment vessel of the No. 1 reactor of the Fukushima No. 1 power station gets as high as 9.7 sieverts per hour — enough to kill a human within an hour.

The radiation levels at six locations in the western section of the first floor of the PCV ranged from 7.0 to 9.7 sieverts per hour, the beleaguered utility said in disclosing data collected by a remote-controlled robot on Friday.

By contrast, the temperatures at the six locations monitored were cool, ranging from 17.8 to 20.2 degrees.

Tepco sent the robot into the primary containment vessel on Friday, expecting it to stay alive for 10 hours. But the robot failed within three hours after completing about two-thirds of its planned route. Tepco has given up on recovering the robot.

The survey involved eight Tepco employees and 36 other workers who were hired by contractors. The maximum radiation dose logged was 1.73 millisieverts.

Tepco official Teruaki Kobayashi said the survey found no major obstacles around an opening leading to the underground part of the vessel, which is good news for future surveys needed to extricate the molten nuclear fuel.

The No. 1 reactor is one of the three damaged by core meltdowns during the Fukushima nuclear crisis in March 2011. ”

source