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Following the NPP accident, several hundred tons of heat-generating corium and fuel debris have been cooled permanently by millions of m 3 of flowing. Knowledge on the interaction with water is crucial for any decommissioning planning.Starting from knowledge on the evolutions of the accident in the three reactor cores and associated fuel debris formations and some additional isotopic and physio-chemical information on debris fragments collected in Fukushima soils, we review the temporal evolution of the chemistry and leached radionuclide contents of the cooling water, comparing measured concentration ratios of the actinides and fission products in the water to reported results of laboratory leaching studies with either spent nuclear fuel or simulated fuel debris.As for spent fuel leaching, the fractions of inventories of 134,137 Cs in the cooling water are orders of magnitude larger than that of the actinides. After more than 10 years of fuel debris/ water contact, 137 Cs release rates have decreased by about a factor of 100. The total release of actinides from the fuel debris is orders of magnitude lower than that of 134,137 Cs or of 90 Sr. This high stability makes direct disposal of fuel debris in suitable containers after decommissioning a viable option.

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A total activity of ∼1019 Bq, including ∼1016 Bq of 137Cs, was released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in 2011, among which 137Cs (30.1 years half-life) will remain in the environment for decades either in the form of: (i) Cs bound to clays, or (ii) highly radioactive Cs-rich microparticles (CsMPs). CsMPs are nano- to microscale particles that were dispersed as far away as ∼230 km, thus the characterization of CsMPs has been extremely challenging. This chapter summarizes the technical challenges in the application of state-of-the-art analytical techniques including atomic-resolution electron microscopy, secondary ion mass spectrometry, and synchrotron-based micro X-ray analysis. CsMPs consist of a glassy silicate matrix and contain Cs (<0.55–30 wt%), Fe, Zn, as well as other trace elements. The 134Cs/137Cs activity ratios of individual CsMPs are ∼1, confirming their FDNPP origin. The nanoscale texture of CsMPs indicates that intrinsic Cs phase(s) and other fission fragment nanoparticles formed in the reactors during meltdown. Nanoscale fragments of fuel debris, incorporated into the CsMP matrix, reveal a variety of physico-chemical properties including euhedral, uraninite crystals. 235U/238U isotopic ratios within the CsMPs range from ∼0.019 to ∼0.030 reflect the variation in the burn-up of the nuclear fuel in different reactors. Trace Pu occurs associated with U(iv) oxide nanoparticles, which are incorporated into the CsMPs. Thus, CsMPs are not only an important medium with localized Cs; microparticulates also provide a mechanism for the transport of debris fragments of incorporated U and Pu into the environment in a respirable form.