Research and Development Related to the Accident at TEPCO’s Fukushima Daiichi NPS

1-7 Toward the Recovery of Uranium from Fuel Debrisᴪ Development of a Chlorination Method for Slightly Soluble Components ᴪ(a) Chlorination

Thermocouple

10000

Isothermal furnace: UCl4

Sealed glass tube
Steel container

6000
4000
2000
010Sample(about 100 mg)(b) Distillation of by-products

Intensity (a.u.)

8000

2030

40

5060

Diffraction angle 2θ(degree)
Temperature
gradient furnace

Highest temp. Low temp.

Fig.1-19 XRD profiles of the product obtained from areaction between (U0.5Zr0.5)O2 and MoCl5
Observed peaks were identified as tetragonal UCl4 obtained bya chlorination reaction.
ProductBy-product

Thermocouple

Thermocouple

SampleSealed glass tube
High temp.

Low temp.

Fig.1-18 Outline of a chlorination experiment with fueldebris from MoCl5
A mixture of (U0.5Zr0.5)O2 and MoCl5 is sealed in a glass tube invacuum. It is converted to chloride by homogeneous heating
at 573–773 K and by-products are separated by distillation at573 K under a temperature-gradient condition.

Fig.1-20 Appearance of the product after separation of byproducts
By chlorination of (U0.5Zr0.5)O2, green UCl4 was obtained asa product (left side in the figure), and MoOCl3 and ZrCl4 byproducts were separated by distillation (right side in the figure).

As a result of the accident at the TEPCOʼs Fukushima
Daiichi NPS, the treatment of fuel debris generated bysolidification of melted fuels has been studied. One possible
treatment option that has been proposed is the application of
pyrochemical reprocessing technologies of spent nuclear fuels
to treat fuel debris. According to the pyrochemical method,uranium and plutonium in spent fuels are separated fromﬁssion products and recovered to a cathode by electrolysis in amolten salt. A large issue with this method is the developmentof a pretreatment technique for converting fuel debris intochlorides.
Conventional chlorination techniques using chlorine gas over873 K require the use of corrosion-resistant structural materials.
In contrast, the chlorination technique using molybdenum
pentachloride (MoCl5) is expected to have advantages such asa lower reaction temperature, ease of handling the solid MoCl5.
By-products of this method can also be efficiently separated
from chloride products by distillation.
In this study, we propose a new chlorination method of fueldebris using MoCl5 and study the applicability of this method
through basic experiments.
Using uranium–zirconium oxide solid solution ((U0.5Zr0.5)O2)

powder and sintered pellets as simulated fuel debris,
chlorination experiments with MoCl5 were conducted and thereaction principle of uranium chlorination and separation of
by-products by distillation (Fig.1-18) were conﬁrmed.
Regarding the chlorination reaction, most uranium in thesample was successfully converted to uranium tetrachloride(UCl4) at 573 K (for the powder) or 773 K (for the sinteredparticles) based on the following equation (Fig.1-19):2(U0.5Zr0.5)O2 + 4MoCl5 → UCl4 + ZrCl4 + 4MoOCl3
By-products of this reaction include MoOCl3 and zirconium
tetrachloride (ZrCl4). These were successfully separated from
UCl4 by distillation at 573 K (Fig.1-20).
On the basis of these results, we proposed a new chlorination
method for fuel debris using MoCl 5 and confirmed its
applicability in principle.
These results were obtained in accordance with the projectfor development of the fuel debris characterization andtreatment technologies as funded by the Agency for Natural
Resources and Energy, Ministry of Economy, Trade andIndustry of Japan (METI).

Reference
Sato, T. et al., Chlorination of UO2 and (U, Zr)O2 Solid Solution using MoCl5, Journal of Nuclear Science and Technology, vol.52, issue 10, 2015, p.1253-1258.

JAEA R&D Review 2016-17

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