5:15 PM - 6:45 PM
[MAG33-P02] Search for uranium-containing particles as fallout of the Hiroshima atomic bomb
Keywords:Atomic bomb fallout, Uranium-containing particle
Fallout of the Hiroshima atomic bomb included radioactive isotopes produced by the fission reaction of 235U, the structure materials of the bomb itself containing uranium, and neutron-activated materials of ground structures, etc. It is assumed that about 64 kg of enriched uranium, of which approximately 50 kg was fissile uranium 235U, was equipped with the bomb. However, it is estimated that the amount of 235U that contributed to the explosion was only about 0.8 kg, which means that approximately 49 kg of 235U was released into the environment. Therefore, we are focusing on the atomic bomb fallout containing uranium and are attempting to search for them around Hiroshima.
1. Method
The fireball generated immediately after the explosion was in a high temperature and high pressure state, and the uranium as well as the iron, tungsten those were structual material of the atomic bomb, and fission products, were in a plasma state. As the temperature decreased, the atoms might condense and changed to solid particles through liquid state, or attached to surface of natural aerosol particles in the atmosphere. In the accident at TEPCO's Fukushima Daiichi Nuclear Power Plant, insoluble spherical microparticles containing high concentrations of radioactive cesium have been discovered in the environment. This microparticle is called CsMP (Cesium-rich microparticle), and it has also been reported that CsMP containing uranium exists. Therefore, insoluble particles similar to CsMP containing high concentrations of uranium may have been generated by the atomic bomb.
In order to investigate the uranium-containing particle in soil samples collected around Hiroshima, following procedure using a solid-state track detector has been developed. 1) Separate high specific gravity particles from soil particles using SPT (sodium polytungstate) heavy liquid and collect them on a filter. 2) The filter that collects high-density particles is attached to surface of a solid-state track detector (BARYOTRAK), and neutron irradiation is performed at the KUR research reactor. 3) Etch the track detector by immersing it in a 7 M sodium hydroxide solution heated to 70°C for 30 minutes. 4) Observe the tracks of fission products generated on the surface of the track detector, and identify the position of the uranium particles on the filter based on the location where the tracks are concentrated. 5) Observe the specified position on the filter using SEM/EDX equipment and identify the uranium particles.
2. Results
Multiple clusters of tracks caused by fission reactions were observed on the detector. As a result of identifying the particles that generated these clusters using SEM/EDX, it was found that there were particles with a uranium content ranging from a few to ten percent. The uranium concentration of these particles is clearly higher than that of averaged natural soil. On the other hand, since the elements other than uranium were assumed to be naturally derived, such as rare earth elements, it is concluded that these particles originated not from the atomic bomb but from a part of uranium-containing natural rocks.
In this study, no uranium-containing particles derived from the atomic bomb could be found in the soil samples. However, we were able to demonstrate that a method using heavy liquid separation and a solid-state track detector is effective for identifying and analyzing particles containing several percent of uranium in soil. In the future, we will conduct analyzes using this method at other locations in Hiroshima to clarify the presence or absence of uranium-containing particles as atomic bomb fallout, and consider the physical and chemical processes from the formation of the fireball to the generation of the fallout.
1. Method
The fireball generated immediately after the explosion was in a high temperature and high pressure state, and the uranium as well as the iron, tungsten those were structual material of the atomic bomb, and fission products, were in a plasma state. As the temperature decreased, the atoms might condense and changed to solid particles through liquid state, or attached to surface of natural aerosol particles in the atmosphere. In the accident at TEPCO's Fukushima Daiichi Nuclear Power Plant, insoluble spherical microparticles containing high concentrations of radioactive cesium have been discovered in the environment. This microparticle is called CsMP (Cesium-rich microparticle), and it has also been reported that CsMP containing uranium exists. Therefore, insoluble particles similar to CsMP containing high concentrations of uranium may have been generated by the atomic bomb.
In order to investigate the uranium-containing particle in soil samples collected around Hiroshima, following procedure using a solid-state track detector has been developed. 1) Separate high specific gravity particles from soil particles using SPT (sodium polytungstate) heavy liquid and collect them on a filter. 2) The filter that collects high-density particles is attached to surface of a solid-state track detector (BARYOTRAK), and neutron irradiation is performed at the KUR research reactor. 3) Etch the track detector by immersing it in a 7 M sodium hydroxide solution heated to 70°C for 30 minutes. 4) Observe the tracks of fission products generated on the surface of the track detector, and identify the position of the uranium particles on the filter based on the location where the tracks are concentrated. 5) Observe the specified position on the filter using SEM/EDX equipment and identify the uranium particles.
2. Results
Multiple clusters of tracks caused by fission reactions were observed on the detector. As a result of identifying the particles that generated these clusters using SEM/EDX, it was found that there were particles with a uranium content ranging from a few to ten percent. The uranium concentration of these particles is clearly higher than that of averaged natural soil. On the other hand, since the elements other than uranium were assumed to be naturally derived, such as rare earth elements, it is concluded that these particles originated not from the atomic bomb but from a part of uranium-containing natural rocks.
In this study, no uranium-containing particles derived from the atomic bomb could be found in the soil samples. However, we were able to demonstrate that a method using heavy liquid separation and a solid-state track detector is effective for identifying and analyzing particles containing several percent of uranium in soil. In the future, we will conduct analyzes using this method at other locations in Hiroshima to clarify the presence or absence of uranium-containing particles as atomic bomb fallout, and consider the physical and chemical processes from the formation of the fireball to the generation of the fallout.