Cosmic Ray Radiography of the Damaged Cores of the Fukushima Reactors

The passage of muons through matter is dominated by the Coulomb interaction with electrons and nuclei. The interaction with the electrons leads to continuous energy loss and stopping of the muons. The interaction with nuclei leads to angle “diffusion.” Two muon-imaging methods that use flux attenuation and multiple Coulomb scattering of cosmic-ray muons are being studied as tools for diagnosing the damaged cores of the Fukushima reactors. Here, we compare these two methods. We conclude that the scattering method can provide detailed information about the core. Attenuation has low contrast and little sensitivity to the core.

Figure 1

Cutaway view of a boiling water reactor and a schematic of the detector placement for the Monte Carlo calculation. In the case of attenuation radiography, only trajectory information from the lower detector was used. The location of the 1 m diameter void in the core and its placement in the bottom of the pressure vessel are indicated by arrows.

Figure 2

Cosmic-ray muon energy spectrum at sea level. Solid symbols are from Jokisch et al. [8], and the open symbols are from Tsuji et al. [9]. Muons which penetrate the reactor lose 5–6 GeV.

Figure 3

Reactor reconstructions at different exposure times. In scattering radiography, the reactor core can be detected after about 10 hours of exposure. After four days, a 1 m diameter ($1\%$) void can be detected when compared to an intact core. After 6 weeks, the void is clear and the missing material can be observed. With the attenuation method, the core can be observed when compared to an empty scene in four days. The void is undetectable even after 6 weeks of exposure.