A PRL paper published last week had an interesting proposal. Use muons generated in the upper atmosphere due to cosmic ray collisions to peek into the Fukushima reactors.
To radiograph inaccessible parts of Fukushima reactors, Borozdin et al. propose a similar approach based on muon detectors placed right outside the reactor building. The authors compared two imaging methods: attenuation radiography, which measures how muons are absorbed inside the reactor, and scattering radiography, which monitors how their path is deviated. They show that scattering radiography would deliver more reliable images of the nuclear core after only a few weeks of measurement, allowing the visualization of melted fuel as well as debris.That report has a link to the PRL paper that you can get under the Creative Commons license.