Constant hydrogen generation from high-level radioactive liquid waste measured

A research group including Researcher Tomohiro Toigawa and Leader Yuta Kumagai of the Research Group for Nuclear Chemistry, Nuclear Science and Engineering Center, Japan Atomic Energy Agency (JAEA), in collaboration with Japan Nuclear Fuel Limited (Executive President and CEO Naohiro Masuda), has confirmed that hydrogen generated from high-level radioactive liquid waste produced at spent nuclear fuel reprocessing facilities does not increase due to temperature rises. Concurrently, they compiled hydrogen generation behavior data covering conditions up to boiling. This achievement is expected to contribute to strengthening safety measures at reprocessing facilities. The results were published in the Journal of Nuclear Science and Technology on May 18.

High-level radioactive liquid waste generated at spent nuclear fuel reprocessing facilities contains various kinds of radionuclides. Water within the liquid waste is constantly decomposed by these radionuclides, generating hydrogen, while simultaneously absorbing radiation energy, which causes temperature rises.

Therefore, in storage areas, management is maintained to keep the hydrogen concentration below the flammability limit (4% by volume) by providing ventilation and cooling the liquid waste.

It is known that high-level radioactive liquid waste contains numerous metallic elements and fission products. Also, within the liquid waste, these metallic elements not only cause hydrogen to generate but also trigger reactions that consume hydrogen. It was also previously known that hydrogen generation increases as the radiation dose increases. However, in the event of power outages or equipment failures, there is a possibility that a temperature rise and hydrogen generation could occur simultaneously.

Therefore, with the objective of establishing more robust safety measures, the research group aimed to understand the impact of temperature changes on hydrogen generation from liquid waste. Because high-level radioactive liquid waste is a complex solution whose behavior is difficult to understand through theoretical calculations alone, actual liquid waste was utilized.

Using high-level radioactive liquid waste stored at the JAEA's Nuclear Fuel Cycle Safety Engineering Research Facility (NUCEF), they measured the amount of hydrogen generated under temperature conditions varied stepwise from room temperature (30℃) to the boiling point. Concurrently, they measured the hydrogen generation volume (the amount transferred from the liquid waste phase to the gas phase) under static conditions (without stirring) and agitated conditions (with stirring). The experiments were conducted by installing the apparatus inside a shielded concrete cell and performing remote operations using manipulators.

As a result, it was revealed that under the room-temperature static condition, the amount of hydrogen generated was kept lower than under the room-temperature agitated condition. It was found that under static conditions, a portion of the hydrogen is consumed within the solution, whereas adding agitation causes the hydrogen in the solution to transfer, thereby increasing the measured hydrogen generation volume.

The decomposition of hydrogen molecules by elements such as palladium is believed to be involved in this hydrogen consumption. Furthermore, when the liquid waste was heated step-by-step to the boiling point to measure the hydrogen generated under varying temperature conditions, it was also found that the impact of the temperature rise on the hydrogen generation volume was low.

Although the hydrogen generation volume increased upon heating compared to the room-temperature static condition, it was clarified that this increase occurred because bubbles generated by heating and boiling produced the same effect as agitation, making it easier for hydrogen to transfer into the gas phase.

Toigawa stated: "While this study focused on high-level radioactive liquid waste, the reprocessing process involves various separation steps, such as solvent extraction, and the chemical conditions differ for each process. Moving forward, we intend to proceed with acquiring foundational data on hydrogen generation behavior for various process solutions, aiming to deepen the understanding of hydrogen behavior to contribute to safety measures."

Journal Information
Publication: Journal of Nuclear Science and Technology
Title: Temperature effect on radiolytically generated hydrogen yield from a plutonium nitric acid aqueous solution
DOI: 10.1080/00223131.2025.2541067