Cosmic rays to pinpoint Fukushima cores

11 August 2014

A system using muon detectors should be able to determine the exact locations of the molten cores of the damaged Fukushima Daiichi reactors. It is to be supplied by Decision Sciences International Corporation (DSIC) through a contract awarded by Toshiba.

Fukushima muon detector 460 (LANL)
By comparing results from two detectors - one high in the building and the other low down - DSIC will be able to determine the path muons take through the reactor buildings and identify the exact location and condition of the fuel within them (Image: LANL)

Middleburg, Virginia-based DSIC will design, manufacture and deliver two muon detectors that will fit into the power plant building. The detectors will be part of Toshiba's overall Fukushima Complex project to determine the location and condition of the nuclear fuel inside the plant. The value of the contract has not been disclosed.

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. DSIC has already applied the technology in its Multi-Mode Passive Detection System, used at ports for scanning containers for radioactive materials.

The 3-D image produced by the detectors should give a clear picture of the condition and location of the fuel in the cores of the three damaged reactor at Fukushima Daiichi. This will assist Toshiba in developing a safe and effective remediation plan.

Different computer models have been used by Tepco and the Japan Atomic Energy Agency to analyse the positions of the Fukushima Daiichi cores, coming to the same conclusion for unit 1. Both models predict that all 77 tonnes of the fuel from unit 1 melted and passed from the reactor vessel to the drywell area immediately below. At units 2 and 3 the models differed but together indicated that 30-40% of their 107-tonne cores remain in the vessels, the rest in the drywell.

DSIC president and CEO Stanton Sloane said, "We are delighted to extend the application of our solution to assist in the recovery of the Fukushima power plant as well as support and secure a safe working environment for personnel."

Technology development

DSIC's muon detection system is based on technology originally invented at the Los Alamos National Laboratory (LANL).

Los Alamos chief scientist and leader of its muon tomography team Christopher Morris recently noted, "Los Alamos researchers began working on an improved method for muon radiography within weeks of the 2011 earthquake and tsunami that damaged the Fukushima reactor complex. Within 18 months we had refined our technique and published a paper showing that the Los Alamos method was superior to traditional muon radiography techniques for remotely locating and identifying nuclear materials, and that it could be employed for field use."

LANL chief technology officer Duncan McBranch said that its technology "will allow plant operators to establish the condition of reactor-core material without the need to actually get inside." He added, "Invasive techniques such as video endoscopy or introduction of robots run the risk of releasing radiation. Furthermore, those techniques at best offer a partial view of material location. Muon tomography will enable plant operators to see the location of the nuclear material inside, determine its condition, and provide critical insight that can inform the design of a safer and faster cleanup."

Researched and written
by World Nuclear News