Can reactor fuel debris be safely removed from Fukushima Daiichi?

Can reactor fuel debris be safely removed from Fukushima Daiichi?, Science Daily, :January 25, 2022Source:University of Helsinki

Summary:Decommissioning and clean-up are ongoing at the Fukushima Daiichi Nuclear Power Plant (FDNPP); however, many difficult problems remain unaddressed. Chief amongst these problems is the retrieval and management of fuel debris.

Decommissioning and clean-up are ongoing at the Fukushima Daiichi Nuclear Power Plant (FDNPP); however, many difficult problems remain unaddressed. Chief amongst these problems is the retrieval and management of fuel debris. Fuel debris is the name given to the solidified mixture of melted nuclear fuel and other materials that now lie at the base of each of the damaged reactors (reactor Units 1 — 3). This material is highly radioactive and it has potential to generate enough neutrons to trigger successive nuclear fission reactions (uranium-235 breaks into two elements after capturing neutrons, emitting enormous amounts of energy, radiation, and more neutrons). Successive fission reactions would present a serious safety and material management risk.

One of the materials in nuclear reactors that can lower the number of neutrons interacting with uranium-235 is boron carbide (B4C). This was used as the control rod material in the FDNPP reactors, and it may now remain within the fuel debris. If so, it may limit fission events within the fuel debris.

Can the fuel debris be safely removed?

On March 11th 2011, the control rods were inserted into the FDNPP reactors to stop the fission reactions immediately after the earthquake, but the later tsunami destroyed the reactor cooling systems. Fuel temperatures soon became high enough (>2000 °C) to cause reactor meltdowns. Currently, the fuel debris material from each reactor is cooled and stable; however, careful assessment of these materials, including not only their inventories of radioactive elements but as well their boron content, a neutron absorber, is needed to ascertain if successive fission reactions and associated neutron flux could occur in the fuel debris during its removal. Many important questions remain: was boron from the control rods lost at high temperature during the meltdown? If so, does enough boron remain in the fuel debris to limit successive fission reactions within this material? These questions must be answered to support safe decommissioning.

Study shows direct evidence of volatilization of control rods during the accident.

Despite the importance of this topic, the state and stability of the FDNPP control rod material has remained unknown until now. However, work just published in the Journal of Hazardous Materials now provides vital evidence that indicates that most of the control rod boron remains in at least two of the damaged FDNPP reactors (Units 2 and/or 3).

The study was an international effort involving scientists from Japan, Finland, France, and the USA………………….. https://www.sciencedaily.com/releases/2022/01/220125093041.htm