Muons again

 February 12, 2015

TEPCO turns to cosmic rays to get peak inside Fukushima reactors

http://mainichi.jp/english/english/newsselect/news/20150212p2a00m0na017000c.html

Fukushima No. 1 nuclear plant operator Tokyo Electric Power Co. (TEPCO) is turning to cosmic rays to get a look inside the stricken power station's wrecked reactors, and help decide how to extract the melted nuclear fuel inside.

Since the March 2011 triple-meltdown nuclear disaster at the Fukushima No. 1 plant, due to high radiation levels it's been impossible to see inside the three reactors affected to ascertain the state of the fuel -- specifically how much is left in the pressure vessels and how much melted through to the containment vessel. As poking a hole in the reactor vessels would be far too dangerous, TEPCO is turning to a technique similar to X-ray exams at a doctor's office: muon detectors.

Muons are subatomic particles produced when cosmic rays strike Earth's atmosphere, and rain down constantly onto the planet's surface. Muons pass through most materials, including concrete and steel, unhindered, but they are blocked by very dense substances such as nuclear fuel. As such, the melted fuel in the Fukushima No. 1 reactors will block muons passing through the pressure vessels, and show up as shadows on a muon detector readout. The same technique is used to study magma in volcanoes.

By Feb. 10, TEPCO had set up two muon detectors next to the Fukushima plant's No. 1 reactor -- one each on the north and northwest side of the reactor building -- and was scheduled to begin taking readings on Feb. 12. TEPCO calculates that most of the fuel in the No. 1 reactor melted through the bottom of the pressure vessel and onto the floor of the containment vessel. The lower part of the containment vessel is below ground level, so the muon detectors won't be able to detect the fuel there. The pressure vessel, however, is above ground, meaning TEPCO can check how much fuel remains.

According to the Tsukuba, Chiba Prefecture-based High Energy Accelerator Research Organization (KEK), which developed the muon detectors, the devices can pin down the position of nuclear fuel 30-50 centimeters in size.

TEPCO and its partners plan to take about a month's worth of measurements, and finalize the results in late March.

"It will be very important simply to confirm the hypothesis that the fuel is no longer in the pressure vessel," said KEK professor emeritus Fumihiko Takasaki. A TEPCO spokesperson commented, "If we can ascertain the position of the nuclear fuel debris, we'll be able to decide to some extent how we should go about extracting it (from the reactors)."