The operator of the disaster-hit Fukushima No. 1 nuclear plant on Thursday began experiments using cosmic rays in an effort to look at the condition of the damaged reactor cores, hoping it will help determine how to remove the melted fuel they hold.

During the process of decommissioning the plant, which is expected to take up to 40 years, the eventual removal of fuel debris from the three reactors that suffered meltdowns in the 2011 earthquake-tsunami disaster is the hardest part. But the exact condition of the melted fuel inside the reactors is unknown, as high radiation levels have prevented anyone from obtaining data.

So Tokyo Electric Power Co. is now turning to technology utilizing muons, a type of cosmic ray that constantly showers down on the Earth’s surface. While muons pass through most objects, they are blocked by heavy substances such as uranium, which is used in nuclear fuel. This means muons can be used to create an image of the condition of the fuel inside the reactors similar to an X-ray, the operator said.

Tepco has installed devices — developed by Japan’s High Energy Accelerator Research Organization — around the reactor 1 building and began collecting data on Thursday. The same technology has been used to identify the condition of magma inside a volcano, the utility said.

Tepco plans to collect data for about a month and announce its results in March. But there are limitations to the muon technology and the operator aims to grasp the rough condition of the melted fuel as the first step.

The utility believes that most of the fuel in reactor 1 melted through the bottom of the pressure vessel, but details remain unknown nearly four years after the outbreak of the nuclear crisis.