Fukushima’s Ice-Wall – A Fridge Too Far

The Tōhoku earthquake and tsunami on March 11, 2011 caused significant damage to the TEPCO Fukushima Daiichi nuclear power generation site. The damage inflicted to the plant’s cooling system, caused a ‘Loss of Coolant Accident’ resulting in nuclear meltdowns and releases of radioactive materials from several of its reactors. It was the largest nuclear disaster since the Chernobyl disaster of 1986 and only the second disaster (along with Chernobyl) to measure Level 7 on the International Nuclear Event Scale.

The reactor buildings were severely damaged to their foundations, and having been built on ‘made ground’ above a highly active and porous aquifer up to 50 metres deep, ground water began to penetrate the damaged reactor building’s basement at a significant rate. Initially this proved an aid to the immediate situation, with the cooling system out of action an emergency system was set up utilising site waters to cool the damaged reactors, with 400 tons of water being continuously poured into the damaged reactor buildings every day to cool them. On the downside, this cooling water became contaminated by the exposed molten fuel. Added to that, approximately 400 tons per day of groundwater flowing into the basements of the damaged buildings also became contaminated due to cracks in the reactor containment vessels. Approximately 800 tons of contaminated water was required to be pumped up every day from the damaged buildings and treated to minimise its harmful contaminant content. Even after treatment, these stored waters contained significant amounts of caesium-134, caesium-137, strontium-90 and tritium. The water that was not reused for cooling was stored in holding tanks. Needles to say the contaminated water is accumulating as such a rate that some discharges to the sea will become inevitable. 

The technical problems posed for the authorities are immense.  High level contamination around the damages reactors, massive structural damage, derelict buildings and radioactive debris spread over an extensive area.  And an apparently unstoppable flow of ground water flooding buildings wherein the corium stumps of 3 melted-down reactors still lay. And as if to make matters worse, the water levels in the basement behaved tidally, indicating that the contaminated waters had a seriously large conduit or ‘preferential pathway’ to the open sea.  With all of these issues, even with the Chernobyl experience, the Fukushima clean-up project is a massive, unique and highly challenging situation, that may take as long as 50 years or more to fully address.

In years following the disaster the Japanese authorities, struggling to meet the daunting challenges, came under increasing internal and external pressure to seek external assistance in the clean-up and remediation of the Fukushima plant. In response, in mid-2013, Japan’s International Research Institute for Nuclear Decommissioning Authority (IRID) made a worldwide call for technologies to address their radio-chemical contaminated water, and other technologies to assist to remediate the site.  In a global ‘brainstorm’ they drew in a significant amount of good ideas and prospective valuable new technologies.

There are several key stages to ‘brainstorming‘.  Setting the context and defining the problems faced at Fukushima are largely self evident.  The plant needs to be made safe and decommissioned, and the wider environment beyond the plant needs to be remediated and restored, at least as far as is possible.  In generating ideas, there needs to be a flow of ideas that are uncompromised by ‘mindset’.  To this end those involved in the process are generally selected from both within and without the problem owning group and from as wide a range of expertise as possible.  So as not to prohibit radical ideas or ideas that would be outside the technical culture of the problem owning group, it is quite normal to reserve any critical review at this stage, until everything is on the table.  Thereafter, the filtering of ideas commences. The most promising are shortlisted and then follows a more detail examination of the pros and cons of each, where their merits and de-merits weighed.  On selection of the best idea, any specific problems are addressed and if acceptable, the front runner goes forward to be implemented as an operational project.

Consider the operation requirements of decommissioning the Fukushima reactor buildings.  There needs to a be robust containment wall put in place, to (a) control the immediate ingress and discharge of water (b) prevent spread of contamination during decommissioning and (c) a coffer must be installed to contain for what will remain a site of significant radiation risk for hundreds of years to come.  The ‘brainstorming process seems to have fallen short at Fukushima. The concept of the ice-wall was mooted long before IRID’s call for technology, and advanced as the optimum solution before any wide-ranging brainstorming took place.  Moreover, it would appear that the overseeing authorities had become ‘mindset’ on this solution, announcing the decision to construct the Ice-wall in September 2013 despite IRID still seeking and collecting worldwide technology submissions.  Installation of facilities to create the ice-wall commenced in June 2014 and was completed on February 9, 2016 at an estimated to cost some ¥34.5 billion ($339 million).  Activation was on March 31 this year, with commencement of the freezing of the seaward side wall.  Freezing of the land-side wall commenced on June 6 and has as yet to achieve and control over the water ingress to the ice-walled coffer. Yet despite this commitment to the ‘ice-wall’ as a solution to the problem, serious questions arise as to whether this technology is capable of meeting the short term needs, let alone the  medium or long-term containment needs.

Ice-wall technology has been used in Japan on hundreds of occasions in civil engineering projects to stem flooding and avoid collapse issues in tunnelling. The purported principal benefit of using a frozen barrier compared with a physical barrier is that it avoids the challenges of building a wall around such underground obstacles as pipes, which it can freeze plug, and if complete, create a seamless barrier. Once in place, frozen walls take a long time to melt and therefore if the site were hit by another earthquake or tsunami the wall might stay intact for a couple of months, allowing time for its refrigeration plant to be repaired and power restored.

As for the cons, relative to what is required at Fukushima, ice-wall technology has only ever been used on a short term basis, and never for a semi-permanent installation. None have run for the decades that Fukushima’s wall would need to be in place. The Fukushima wall at 1,500 metres in length, 30 metres in depth and at circa 70,000 cubic metres in volume would be nearly double the size of the largest prior ice-wall ever constructed.  Curiously it was designed only as a partial barrier in that it doesn’t reach to 50 meters to the impermeable rock strata below the aquifer and thus it has no containment floor beneath the site.  Such a wall has never been constructed on such a highly active aquifer and it is quite a different matter to freezer moving water.  As an added complication, due to the proximity of the sea to the site and the existence of preferential pathways to the sea, the groundwater would have a high mineral content and be highly saline, containing salts of sodium, potassium and critically calcium. Owing to this mixed salinity, freezing to below 0oC would not be nearly enough to freeze the soil-water column solid and stop the water flow.  The ground soil-water column would have to be taken to below -21oC and possibly to -41oC.  TEPCO are utilising a CaCl2/card-ice eutectic coolant, which has a minimum freezing temperature of -41oC ‘at the pump’ and closer to -25oC in the cooling pipes.  It would be hard pressed to get the ground temperature to -21oC due to heat ingress, and even at its coldest it won’t freeze a calcium rich saline system solid.  As for the heat ingress into the system, we mustn’t forget that we are trying to enclose 3 very warm meltdown corium stumps, effectively comprising a ‘hot-spring’ at the centre of the ice-wall structure.   Over and above that heat, the Fukushima site is located next to the Pacific and has seasonally warm southerly currents bathing the site’s shore front during the summer months bringing yet more heat into the system.  Even with a heat exchanger rated at 12.6 Mega Watt, (that’s about enough energy to run a small town), it’s a big ask, and I fear that given the geotechnical circumstances the desired ice-wall project outcomes are beyond the capacity of this technology.

A complication of ice-wall technology is that it causes ground heave.  The ice causes the ground to swell, creating a sheer between the unfrozen ground and the ice swollen frozen ground.  So, further damage to the foundations of the stricken buildings and localised subsidence is likely.  A greater problem might ensue when the wall is thawed.  The chewing of the ground by the ground heave process would likely destroy subsoil texture and leave the ground more permeable to water than before.

Given the pros and cons of an ice-wall I ask the question; why didn’t TEPCO opt for a jet grouted cement/mortar double wall that could have totally enclosed the site, as this was the method of choice for controlling groundwater migration at Chernobyl?  It would be possible to jet grout below the buildings and flexible ‘soft wall’ mortars could be used rather than Ordinary Portland Cement (OPC) to guard against fracture by future earthquakes.

At present TEPCO contend that the ice wall project is going to plan.  However, Japan’s Nuclear regulatory authority (NRA) aren’t yet convinced, pointing out that the ice wall has yet to impact the collection of water in waterfront wells.  Test wells within and without the ice wall indicated water levels tracking each other over time, showing the internal and external groundwater systems still interconnected.  Moreover, the much vaunted advantage of the ice wall in being able to seal around a plug pipes appears not to be the case a Fukushima, where underground pipes and conduits remain warm and are probably acting as the preferential pathways for water ingress and egress.  NRA committee member Toyoshi Fuketa recently stated, “This is not a wall in a true sense. Perhaps it’s more akin to a bamboo screen, with groundwater trickling through the gaps”. It would now seem that in response to criticism and to control the water flow TEPCO are now resorting to a hybrid approach by trying to cement closed the holes in the wall.  The problem with cement is, it doesn’t set well below 0oC, but other related sealant options are available.

Thus far, it would appear that after 5 years with the bill racing toward $500 million, all TEPCO’s Ice-wall project has achieved is a very expensive steaming ‘slushy’ and no control over water ingress into the site.  Indeed there is little control on water egress from the site other than by continual pumping from the reactor building basement to tanks to maintain the basement water levels below groundwater, and in doing so hope migration of contamination into the sea is prevented.  Maybe it’s time to ‘call it a day’, purge the mindset and re-brainstorm the problem.

https://www.linkedin.com/pulse/fukushimas-ice-wall-fridge-too-far-peter-j-hurley

July 12, 2016 Posted by dunrenard | Fukushima 2016 | Fukushima Daiichi, Ice wall | Leave a comment