Interactions between Nuclear Fuel and Water at the Fukushima Daiichi Reactors

Abstract

Used nuclear fuel is a redox-sensitive semiconductor consisting of uranium dioxide containing a few percent of fission products and up to about one percent transuranium elements, mainly plutonium. The rapid increase in temperature in the cores of the Fukushima reactors was caused by the loss of coolant in the aftermath of the damage from the tsunami. Temperatures probably well above 2000 °C caused melting of not only the UO2 in the fuel but also the zircaloy cladding and steel, forming a quenched melt, termed corium. Substantial amounts of volatile fission products, such as Cs and I, were released during melting, but the less volatile fission products and the actinides (probably >99.9%) were incorporated into the corium as the melt cooled and was quenched. The corium still contains these radionuclides, which leads to a very large long-term radiotoxicity of the molten reactor core. The challenge for environmental scientists is to assess the long-term interactions between water and the mixture of corium and potentially still-existing unmelted fuel, particularly if the molten reactor core is left in place and covered with a sarcophagus for hundreds of years. Part of the answer to this question can be found in the knowledge that has been gained from research into the disposal of spent nuclear fuel in a geologic repository

Uranium transport in acidic brines under reducing conditions

“…Reducing conditions in uranium-bearing systems are not limited to IOCG deposits and some models explaining the formation of unconformity-type uranium deposits involve reduced basement rocks containing uranium oxides 25 . Moreover, our data suggest that seawater used to cool fuel assemblies after the Fukushima nuclear accident, which likely remained at temperatures near or above boiling for months following the event 8 , 9 would have the potential to dissolve and mobilize uranium at ppm concentrations. These and other natural and anthropogenic scenarios involving uranium transport under reducing conditions, for which our study represents a starting point, require further investigation.…”

“…This extends also to catastrophic accidents, such as that at the Fukushima power plant, for which it is believed that reducing conditions will inhibit uranium migration. A lack of thermodynamic data for uranium species at elevated temperatures is a key factor preventing accurate predictions of uranium behavior during such an event 8 , 9 . These and other conclusions about the behavior of uranium in the presence of aqueous fluids, whether it be for nuclear industry applications or models of uranium ore formation, routinely draw upon the dogma that “uranium is immobile in the reduced state”.…”

Caesium-rich micro-particles: A window into the meltdown events at the Fukushima Daiichi Nuclear Power Plant

“…Until now, the reactions that occurred in the FDNPP reactors have been only inferred based on indirect evidence3. It is believed that radioactive Cs was liberated from the irradiated fuel when the temperature of the fuel rose above 2,200 K8 after the cooling systems shut down in Units #1–3. Other radionuclides were released in amounts depending on their respective volatilities9 rather than in amounts based on their estimated presence in the nuclear fuel, which was primarily composed of UO 2 810.…”

“…It is believed that radioactive Cs was liberated from the irradiated fuel when the temperature of the fuel rose above 2,200 K8 after the cooling systems shut down in Units #1–3. Other radionuclides were released in amounts depending on their respective volatilities9 rather than in amounts based on their estimated presence in the nuclear fuel, which was primarily composed of UO 2 810. Thus, a large portion of the fission products (FPs), including radioactive Cs still remain in the damaged reactors and in contact with the cooling water11.…”

Ten years after the NPP accident at Fukushima : review on fuel debris behavior in contact with water

“…The change became smaller in mid- 2012. The leaching of radionuclides from damaged fuel and structural materials can be classified into two categories: rapid transport (IRF, instant release fraction) and slow transport [61] (see also discussion on experience on spent fuel behavior in chapter 5). A two-stage model consisting of dilution of the initial inventory and constant leaching from the source term was applied to reproduce the analytical data of the trends of Cs concentration as shown in Figure 9 [60].…”