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Muon Scattering Tomography: a Healing Method for the Fukushima Nuclear Scar?

On March 11th 2011, the Fukushima I Nuclear Power Plant on the northeastern coast of Japan suffered a catastrophic failure resulting from a chain of events set off by powerful tsunami waves following in the wake of the Tōhoku Earthquake – the largest recorded earthquake ever to hit Japan. Three of the six nuclear reactors underwent complete meltdown and caused the release of substantial amounts of radioactive materials, eventually ranking this major accident at level 7, the highest level of the International Nuclear and Radiological Event Scale (INES). The event, commonly known as the Fukushima Daiichi Nuclear Disaster, remains the largest nuclear accident after the 1986 Chernobyl Disaster. Although there were no short-term radiation exposure fatalities reported, more than 300,000 people were evacuated and the 2013 World Health Organization Report asserted the highly increased risks of thyroid cancer development for the individuals living in the most affected areas near the Power Plant.

But what exactly happened?

According to many analysts, the causes of the disaster were mostly man-made and its occurrence was foreseeable as the plant was incapable of withstanding the effects of this earthquake and the resulting tsunami waves that ensued. In fact, investigators brought forward the nuclear center’s failure to meet some of the most basic safety requirements, such as the assessment of the probability of damage, the preparation for the containment of collateral damage from such disaster and the development of evacuation plans.

The three independent investigations from the Tokyo Electric Power Company TEPCO, the Nuclear and Industrial Safety Agency NISA/NSC and the government body promoting the nuclear power industry METI all ascertained the man-made nature of the catastrophe, mostly claiming that it was rooted in “regulatory capture” –i.e. “the situation where regulators charged with promoting the public interest defer to the wishes and advance of the agenda of the industry or sector they ostensibly regulate” –resulting in a network of corruption, collusion and nepotism. They go on to state that the lobbying of the plant’s regulators by those with a vested interest in specific policy or regulatory outcomes influenced the regulators choices and actions, explaining why the risks of operating nuclear power reactors in Japan were systematically downplayed and mismanaged, compromising operational safety.

Additionally, the acting Prime Minister of Japan during the time of the tsunami and the Daiichi Disaster admitted the government also shares the blame with the regulatory agency for the extent of the disaster, as it omitted to heed warnings that the Nuclear Power Plant should never have been built so close to the ocean. He acknowledged the flaws in the handling of the crisis by the authorities, notably the poor communication and coordination between regulators, utility officials and government, and claimed: “the disaster laid bare a host of even bigger man-made vulnerabilities in Japan’s nuclear industry and regulation, from inadequate safety guidelines to crisis management, all of which need to be overhauled.” Amory Lovins, a physicist and environmentalist, also added that Japan’s “rigid bureaucratic structures, reluctance to send bad news upwards, need to save face, weak development of policy alternatives, eagerness to preserve nuclear power’s public acceptance, and politically fragile government […] also contributed to the way the accident unfolded.”

What is the situation today?

Regardless of the various measures that could have prevented some of the extensive consequences of the 2011 Disaster, there are still to this day many signs that radioactive material remains in the area surrounding the nuclear power plant, requiring the development of an effective action plant to accelerate the cleanup process. In fact, the pollution of the sea water along the coast of the nuclear plant persists due to the continuous arrival of radioactive material transported toward the sea by the surface water running over contaminated soil.

Continuous leaks from storage tanks of highly contaminated water into the ground require a necessary maintenance of the surveillance of marine life living in coastal waters off Fukushima. The water surrounding the Fukushima coast also remains one of the world’s strongest current, and as such, the contaminated waters were and are still transported far in the Pacific Ocean, causing a great dispersion of radioactive elements.

What are the risks?

Overall, while the radiation risks are presently quite below the level considered harmful to marine animals and human consumers, the caesium-137 isotopic concentration – one of the main substance released in the ocean during the disaster – in the water near the coast remains 10 to 1000 times greater than prior to the accident, necessitating governmental measures to restrict the distribution and consumption of contaminated or contamination-prone items. As mentioned above, the disaster was rated 7 on the International Nuclear Event Scale, defining it as an accident causing widespread contamination with serious health and environmental effects. Prior to this event, only the Chernobyl disaster has been recorded as a 7 on this scale.

Monitoring Reading of the intensity of the radioactive emissions in the regions surrounding the Fukushima Nuclear Plant a few weeks after the disaster.

What policy changes, if any, are being implemented?

Many debates over energy policies and strong anti-nuclear sentiments were sparked as a result of the Fukushima Daiichi Nuclear Disaster, as the event shattered the Japanese government’s claims on the safety of nuclear power and highly increased public awareness of the importance of regulated processes for energy use. In fact, many energy policy analysts have begun calling for a reduction of the nation’s reliance on nuclear power in Japan and the development of other energy resources, such as the afore mentioned Amory Lovins, who claims: “Japan is poor in fuels, but is the richest of all major industrial countries in renewable energy that can meet the long-term energy needs of an energy-efficient Japan, at lower cost and risk than current plans. Japanese industry can do it faster than anyone – if Japanese policymakers acknowledge and allow it.”

Japan does indeed have an incredible renewable energy base, including a total of 324 GW of achievable potential in the form of onshore and offshore wind turbines, geothermal power plants, additional hydroelectric capacity, solar energy and agricultural residue. Many claim the disaster could be the nation’s great opportunity to “get it right” on energy policy, as “Japan – having suffered a painful shock but possessing unique technical capacities and societal discipline – can be at the forefront of an authentic paradigm shift toward a truly sustainable, low-carbon and nuclear-free energy policy.”

As of September 2012, the majority of the Japanese population supported the elimination of nuclear power usage. Prime Minister Noda and his government announced plans to make the country nuclear-free by 2030, including the termination of the construction of new nuclear power plants as well as instituting a 40-year limit on existing nuclear plants. These policies, notably the elaboration of new safety standards that must be met for nuclear plant restarts, will require the investment of about $500 billion over 20 years.

But what about the cleanup process and the specific healing of the Fukushima Nuclear Scar?

Regardless of the ongoing nuclear crisis caused by the Fukushima Daiichi Disaster, the Power Plant cannot, to this day, be fully shut down until the assessment of the full extent and location of the damage to the reactors is complete. In fact, while a new phase of nuclear cleanup and decommissioning began in December of 2011, the dismantling of reactors without an approximate idea of just how much damage the cores of the reactors have endured nor the knowledge of the location of the nuclear remains such as melted fuel render the process a lot harder.

Many researchers have thus been focusing on this particular problem for the past few years, and one of the most promising innovative idea that has recently been brought forward is certainly the use of cosmic-ray radiography and muon scattering to gather greater information on the damaged cores of the nuclear reactors that melted during the disaster. This concept was first introduced by Edward Milner and his fellow scientists from the Los Alamos National Laboratory in New Mexico, USA. In fact, the American team of scientists published a paper in the Physical Review Letters that elaborated on a comparison study of two methods that could be used for cosmic-ray or muon radiography to collect images of the nuclear remains within the core of a nuclear reactors and where one of these reactors highly resembled the first reactor of the Fukushima Nuclear Plant that was destroyed on March 2011. Their findings were very conclusive: the scattering method for cosmic-ray radiography, a muon scattering method, is considerably more efficient than the traditional transmission method used to capture high-resolution image data of potentially damaged nuclear material. The lead author of the paper and member of the Los Alamos’ Subatomic Physics Group, Konstantin Borozdin, indeed explains: “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.”

What exactly are cosmic rays? Muons?

Cosmic rays can be defined as energetic particles originating from deep space. They usually stem from accelerated remnants of supernova explosions that hit the Earth’s atmosphere. As these cosmic particles penetrate our atmosphere, they completely rip their way through with atom smashing power and an incredible amount of energy, usually around 1020 electron volts corresponding to a much greater energy level than any human-built particle accelerator, such as the Large Hadron Collider, can achieve. The cosmic particles’ interactions in the earth’s atmosphere produce an impressive amount of high energy particles and anti-particles, initially positive and negative pions and kaons that subsequently decay into muons and muon neutrinos, including protons and neutrons as a result of nucleonic decay.

Once at ground level, the resulting flux of particles is majorly composed of those muons, of specific interest in this article, as well as electrons, with a ratio of about 75:25 percent, both having considerable energy (more than 4 GeV) and a speed near that of light (about 0.998 c). These muons, product of the interaction of cosmic rays and the Earth’s atmosphere, thus initially hold impressively high levels of energy as they reach the planet’s surface, and although they gradually lose it, they are capable of ionizing (positively or negatively charging) many atoms because they are completely depleted. Their ability to travel at nearly the speed of light, as well as their very little mass, highly inhibits muon-muon interactions, yet permits them to travel through substantial lengths of matter before their energy is exhausted––i.e. penetrating buildings, mountains, bodies, deep into the Earth’s surface, usually without even leaving a trace of their existence. They can achieve all of this within their measured lifetime of 2.2 microseconds, making them a fantastic demonstration of Einstein’s theory of relativity. Logically, considering their life span, muons travelling nearly at speed of light should not be able to travel more than 660 meters, according to recent calculations; they should not even have the time to reach the ground once they are produced from the decay of cosmic particles. However, if Einstein’s theory that time goes by slower for particles that travel nearly at speed of light is considered, then the muons lifetime could be increased by a factor of ten or more as it travels close to the speed of light, so they would have plenty of time to reach the ground.

These specific characteristics of cosmic rays and muons have given them a particularly high value in the view of many scientists interested in particular physics and, as such, have become the main focus of various researchers in the field for the past century.

One of these scientists, E.P. George, elaborated a process of muon tomography around the 1950s that is at the core of the current research done in the Los Alamos National Laboratory. Muon tomography is a technique that generates three-dimensional images of volumes through the use of cosmic rays and muon scattering processes. It is a similar process to X-ray imagery; however, muon scattering prevails on many very important points, especially since it reduces the risks of damage to the materials exposed to the process.

Additionally, the muon scattering angle increases with the element’s atomic number; thus, core materials such as ceasium-137 – which as mentioned previously is one of the main elements that was emitted during the meltdown of the reactors, as well as other core materials that compose a reactor – can be highly perceptible by the muon-based cosmic rays. It then has the ability to pinpoint the exact location of nuclear material residues within the power plant buildings.

This technique’s predominance over the traditional transmission method, usually using X-rays, was also demonstrated through a simulation of a nuclear reactor where parts of its core was removed and placed in random areas within the building. Both the muon scattering and the traditional methods were used to identify the locations of the displaced parts. Over six weeks, the observations showed that the muon scattering method permitted the production of high-resolution images that clearly showed the material that had been moved from the main core, as well as its exact location, while the traditional method only provided a blurry image of the core itself during the same time period. As such, the muon scattering method not only surpasses the X-ray based technique on its effectiveness in imagery processes, but also permits the collection of crucial data regarding the inside of the damaged reactor cores, without any human risks of exposure to highly radioactive material that remains in the surroundings of the reactor buildings.

Applications

In his paper, Konstantin Borozdin indeed explains: “muon images could be valuable in more effectively planning and executing faster remediation of the reactor complex.” It is now viewed as one of the most promising techniques to assess the extent of the damage around the nuclear cores and the location of the radioactive materials that remain in their surroundings, and it is believed that with only a few months of measurements through the muon scattering or radiography, a plan could be made regarding the dismantling of the reactors and the cleanup of the Fukushima Nuclear Power Plant.

Los Alamos National Laboratory Muon Radiography team members stand in front of the damaged Fukushima Daiichi reactor complex during a visit to evaluate whether Los Alamos' Scattering Method for cosmic-ray radiography could be used to image the location of nuclear materials within the reactor buildings.

In addition to its possible contribution to the acceleration of the cleanup process of the Japanese Nuclear Plant, muon radiography has also been recognized as a potential tool to serve for counter-terrorism and non-proliferation purposes, i.e. to detect the presence of nuclear material in cargos, ships or vehicles. In fact, after the 9/11 attacks, researchers pioneered a concept of noninvasive muon radiography portals that would be able to detect contraband nuclear materials, even the most heavily shielded, without breaking the device or container in which it is transported. This technique has been proven useful in detecting materials with a high atomic number (Z), and has recently started to serve for specific purposes like detecting nuclear cargo entering ports or crossing over borders. Muon radiography, an extension of muon tomography, as a means for nuclear waste imaging also compensates for many challenges and issues of nuclear waste characterization that have been faced by researchers in the past, such as historical waste – non-traceable waste stream such as tanks with liquids, fabrication facilities contaminated before decommissioning, interim waste storage sites – or the difficulty with measurement and characterization of some waste forms such as encapsulated alpha/beta emitters or heavily shielded wastes. Other problems also include the homogeneity of the waste that needs characterization, for example the sludge in tanks or the homogeneities in cemented waste, or even the condition of the waste and the waste package, i.e. if there are breaks of containment, corrosion or voids. Muon tomography thus remains an incredibly useful tool to assess the characterization of waste, radiation cooling, and condition of the waste container.

As such, although muons may pass through each and every one of us every day and not leave a single trace of their existence, these tiny particles might as well be at the core of one of the most fascinating and cutting-edge innovations in the field of nuclear waste imaging that has yet to be pioneered. Just imagine this: the scattering of muons, with their tiny size and mass, evaluated at about 1.88 10-28 kg, can permit the imaging of some of the most dense elements in the world, as well as help deal with some of the world's worst disasters.

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