The safe disposal of tritiated water is a growing concern. Tritiated water is a form of radioactive water where the protium atoms have been replaced by tritium. Not only radioactive but highly corrosive, it needs to be treated – usually by flushing with seawater, to make it safe to discharge into the ocean and ensure background levels of tritium are not exceeded beyond 10Bq/L. Storing tons of radiation-laced water is something that can only be used as a solution for so long. After a few years, concern mounts as to how to find a plan to deal with this toxic waste. And never more so is this the case than in Japan.
Its now nearly nine years since the earthquake and 15-metre-high tsunami that caused the radiation leak at Fukushima nuclear power plant in the southernmost prefecture of Tōhoku region. How has the area coped with the resulting contamination since then, and what could be done going forward to help rehabilitate the region? According to the Japan Times, dramatic progress has been made to recover from the meltdown crisis of 2011. Although many areas are now designated as green (light protective wear only required) where contamination levels have been reduced, its still the case that local belief remains sceptical and the government has labelled some areas as ‘difficult to return to’ with radiation levels still considerably high. Tepco, who is in charge of decommissioning, need to keep water pouring over the contaminated fuel; highly contaminated coolant water is currently filtered and purified into tritium and water mix – a radioactive form of hydrogen that is difficult to separate from the water itself. But the 850 steel tanks currently holding this polluted water (tritiated water) are nearly at capacity. While the authorities stance is that tritiated water is relatively harmless to human health, and is routinely dumped at sea from other nuclear power plants as a by-product, many residents in the province, including fishermen, fear disposing of the water from Fukushima in this way could cause sea pollution. Additional objections from China, Korea and Taiwan have made this option politically difficult for Tepco. And so the question remains as to how to dispose of the increasing volume of radioactive water before capacity reaches the maximum, which Tepco estimate will be early in 2021. One possible solution could be packing industrial columns with Dixon rings. Dixon rings, developed in 1946 by Dr Olaf George Dixon while working for ICI, are a form of random packing used in chemical processing. They are currently used where manufacturers require high performance. Consisting of a stainless-steel mesh formed into a ring with a central divider, Dixon rings are intended to be packed randomly into a packed column. Wikipedia describes them as providing a large surface area and low-pressure drop while maintaining a high mass transfer rate, making them useful for distillations and many other applications. Although widespread across Asia and India, they are not yet widely used in Europe, despite their many benefits. The main advantages of random column packaging is that it offers:
• An increase in hydrodynamic capacity compared to packing of similar size and material
• Increasing the mixing when compared to trays at the same efficiency.
Recent research by Prof Kolaczkowski, Emeritus Professor in Dept Chemical Engineering at the University of Bath has used Dixon rings in distillation as a method of separating isotopes in aqueous streams (e.g. deuterium & tritium). And Croft Filters, one of the very few European suppliers of Dixon rings, have developed a revolutionary manufacturing method here in the UK that can be used in both counter-current absorption (scrubbing columns) including scrubbing of CO2 from air, as well as hard distillation separations for dangerous contamination removal – such as tritium from water. Dixon rings can also be used for distillation applications and are used explicitly for ‘difficult’ separations where the boiling points of the constituent parts are very close such as in tritium/water separation at 101.4°C/100.0°C. This makes them an ideal solution for water purification from areas such as nuclear power plants, where it is essential to recover clean water before disposal at sea takes place. Without doubt, in specific applications, Dixon rings performance exceeds that of any other conventional random column packings. This performance advantage makes Dixon rings the ideal choice of packing for some critical applications, such as those currently experienced at the Fukushima plant.