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Christopher Morris and his fellow group of Los Alamos scientists will not forget their trip to Japan anytime soon.
The team was in Japan to lend credence to their research that muon imaging may offer the best hope of assessing damage to the reactor cores and locating the melted fuel.
Muon imaging, which utilizes naturally occurring muons created in the atmosphere by cosmic rays to image dense objects, should solve the problem of determining the spatial distribution of the reactor fuel in the short term, the LANL team said.
“Muons are scattered more strongly by high-Z materials such as uranium fuel in Fukushima’s reactor,” explained LANL researcher Haruo Miyadera. “By measuring the scattering angle, and understanding the physics of Coulomb multiple scattering, one can assess the locations and amount of the melted fuel.”
While they were excited about their research, Morris said they were equally stunned by what they saw on a trip to Fukushima last summer.
“We left from Tokyo, spent the night in the hotel and went up the coast the following day,” Morris said. “As we drove up the coast, I noticed there were a lot of concrete paths. Then I realized this is where the houses were before the tsunami.
“The other thing we saw were just piles of cars that were piled up from the tsunami.”
What struck Morris when the team arrived in Fukushima was the number of people working at the project.
“They had a major catastrophe and it was amazing to see so many people working up there,” Morris said. “They built a building outside of Reactor 1 and most of them were working on cleanup.”
Morris said the team was not nervous about being at the site.
“To a man we were pretty excited,” Morris said. “We are all radiation workers. We knew what we were going to encounter and we knew the risk and we were prepared for it. We were not frightened. We were excited to see it.”
Shortly after the earthquake, tsunami, and core meltdowns at the reactors in Fukushima, Japan in March, 2011, several groups in both the United States and Japan realized that cosmic ray radiography might be able to provide information about the damaged cores. Two methods of radiography using cosmic rays have been described in the past, attenuation 1-3 and scattering.
Since deploying either of these methods to study the damaged cores of the Fukushima reactors involves a major human investment because of the high radiation fields surrounding the reactors, it quickly became apparent the one that worked the best was the scattering technique, which was researched by the Los Alamos group.
The team did a lot of their work at the Los Alamos Neutron Center at LANL.
“Within weeks of the disastrous 2011 tsunami, Los Alamos’ Muon Radiography Team began investigating use of Los Alamos’ muon scattering method to determine whether it could be used to image the location of nuclear materials within the damaged reactors,” said Konstantin Borozdin of Los Alamos’ Subatomic Physics Group.
“As people may recall from previous nuclear reactor accidents, being able to effectively locate damaged portions of a reactor core is a key to effective, efficient cleanup. Our paper shows that Los Alamos’ scattering method is a superior method for gaining high-quality images of core materials.”
Muon radiography (also called cosmic-ray radiography) uses secondary particles generated when cosmic rays collide with upper regions of Earth’s atmosphere to create images of the objects that the particles, called muons, penetrate. The process is analogous to an X-ray image, except muons are produced naturally and do not damage the materials they contact.
Massive numbers of muons shower the earth every second. Los Alamos researchers found that by placing a pair of muon detectors in front of and behind an object, and measuring the degree of scatter the muons underwent as they interacted with the materials they penetrated, the scientists could gather detailed images.
The method works particularly well with highly interfering materials (so-called “high Z” materials) such as uranium.
Because the muon scattering angle increases with atomic number, core materials within a reactor show up more clearly than the surrounding containment building, plumbing and other objects. Consequently, the muon scattering method shows tremendous promise for pinpointing the exact location of materials within the Fukushima reactor buildings.
Using a computer model, the research team simulated a nuclear reactor with percentages of its core removed and placed elsewhere within the reactor building. They then compared the Los Alamos scattering method to the traditional transmission method. The simulation showed that passive observation of the simulated core over six weeks using the scattering method provided high-resolution images that clearly showed material was missing from the main core, as well as the location of the missing material elsewhere in the containment building. In comparison, the transmission method was barely able to provide a blurry image of the core itself during the same six-week period.
“We now have a concept by which the Japanese can gather crucial data about what is going on inside their damaged reactor cores with minimal human exposure to the high radiation fields that exist in proximity to the reactor buildings,” Borozdin said. “Muon images could be valuable in more effectively planning and executing faster remediation of the reactor complex.”
Other Los Alamos National Laboratory co-authors of the paper include Steven Greene, Edward “Cas” Milner, Haruo Miyadera, Christopher Morris and John Perry; and (former Los Alamos post-doctoral researcher) Zarija Lukic of Lawrence Berkeley National Laboratory.
Cas Milner is credited by the team as the author of the original concept of applying muon imaging to Fukushima.