Characterization of neutrons emitted by an expected small amount of fuel debris in a trial retrieval from Fukushima Daiichi Nuclear Power Station

Retrieving objects with a small amount of fuel debris, such as a few grams, will begin soon at the Fukushima Daiichi Nuclear Power Station (1F) at the start of decommissioning. Objects retrieved from the primary containment vessel are not necessarily fuel debris; fuel debris is an object from which neutrons are emitted because it contains nuclear-fuel material. However, the characteristics of the neutrons emitted by fuel debris are unknown. Fuel debris was categorized into five types according to the elapsed time from the accident, burnup, and fuel type (UO2 or mixed oxide). The number and energy spectra of (alpha, n) and spontaneous fission neutrons emitted from 1 g of each fuel debris type were estimated using the SOURCES 4C code to obtain the neutron characteristics. The results showed that the average neutron energy is approximately 2.1 MeV, regardless of the type of fuel debris. However, the intensities of neutrons emitted from the fuel debris in 1F Units 2 and 3 varied by four orders of magnitude according to the fuel debris type.