Monday, June 6, 2011

Tackling TEPCO's water problem

The utility has 15 million gallons of radioactive liquid created by efforts to cool its crippled nuclear reactors

aquariusTEPCO is a bearer of a water problem that sets daunting challenges ahead of its ambitious plan to decommission the Fukushima reactor site. In many ways, getting the millions of gallons of radioactive water out of buildings at the site is a path to getting the decommissioning program in place and working against a predictable schedule.

The basic rule in responding to emergencies involving environmental cleanup is that you can never do just one thing without something else coming up. In the case of TEPCO's three crippled reactors at Fukushima, pouring hundreds of tons a day of water on the leaking units to cool the hot nuclear fuel inside them has resulted in creating a huge volume of radioactive water.

Left uncontrolled the water is seeping into the ground and running off into the sea. Evaporation leaves a residual trail of radioactive debris which will be hard to clean up. The Bloomberg wire service reported June 2 that TEPCO may see radioactive water overflowing the storage it has on-site by early this week resulting in flows of the contamination into the sea. TEPCO has to get control of the water before it makes the situation at Fukushima even worse.

Estimates of the amount of radioactive water that has accumulated since March 11 range from 15-to-28 million gallons. The rate of accumulation of new radioactive water is estimated to be 300-to-500 tons per day or 72,000-to-120,000 gallons per day. In a month the amount of new radioactive water would be to 2.1 million to 3.6 million gallons.

For the purposes of analysis, this blog will use the 15 million gallon figure for the volume that exists so far and an accumulation rate of 300 tons per day.

Water is the biggest challenge at Fukushima

Fukushima workersIn fact, according to a Washington Post report for June 3, uncontrolled radioactive water is the biggest worry at Fukushima.

Lake Barrett, the project manager for cleanup of the heavily damaged Three Mile Island Reactor, told the newspaper continued use of outside water to cool the crippled Fukushima reactors is just making a bigger mess.

Fixing the reactor recirculation systems is taking time and workers are limited in how much time they can spend in certain areas of the plant because of the presence of the radioactive water.

TEPCO has two challenges.

  • First, it must collect the radioactive water from turbine buildings, basements, and other places, and put it in containers to hold it until a treatment system can start operation.
  • Second, the radioactivity must be removed from the water and then safely removed to another location for permanent disposal. Then the cleaned water can be made available to be used to cool the reactors.

In the balance of this blog post I will try to describe the issue ahead using numbers published in the news media. Some of these numbers may turn out to be overtaken by new estimates. TEPCO has been revising information about the scope of the contamination as it discovers new problems. The basic issues of meeting the two challenges noted above remain constant.

Get control of the water

tepco tank According to the Daily Yomiuri and the Associated Press, last week TEPCO started to transport tanks, each with a storage capacity of 120 tons and measuring 15.1 meters long and 3.7 meters in diameter, to Fukushima Prefecture from a tank manufacturer in Kanuma, Tochigi Prefecture. (photo left: TEPCO)

TEPCO plans to deliver a total of 370 tanks -- 170 units with a storage capacity of 120 tons and 200 units with a capacity of 100 tons. The operation is expected to continue until mid-August, according to the utility. So how much water will the tanks hold?

A gallon of water weighs 8.35 pounds. So how much water can TEPCO put in the tanks? It turns out TEPCO will be able to put about half of the 15 million gallons of uncontrolled radioactive water in the tanks. See Table below. If metric measurements are used, then the total is slightly higher.

TEPCO Storage of

Radioactive Water



Total
Nmbr of Capacity Tons
Tanks Tons Storage Gallons
100 100 10,000 2,400,000
170 120 20,400 4,890,000

270


30,400 7,290,000
NOTES


One gallon weighs 8.35 lbs


One ton (2,000 lbs) equals 240 gallons




Store once treat once

precipitateThe next step is to develop a treatment system to process the water to get the radioactivity out of it. It turns out Areva, the French state-owned nuclear giant, has such a process which it uses at the La Hague fuel reprocessing plant. Areva is building a similar facility at Fukushima which it expects to start up on June 15.

There are significant challenges facing the Areva project. In a spent fuel reprocessing plant, the nuclear chemistry numbers are well known, predictable, and controlled in small batches and in an established industrial environment. At Fukushima, no one is quite sure what is in the water, and the working conditions are anything but ideal.

Capture the flag

Areva plans to use a coprecipitation process to remove radioactivity like isotopes of cesium and iodine and other radionnuclides that are coming from the water leaking out of the reactor containment buildings. As the water flows down into turbine buildings and other underground vaults, it mixes with seawater used to cool the reactors in the first days of the crisis, mud, industrial debris, etc. This makes for a very different water chemistry than a spent fuel plant. The salt content of the seawater alone may require new thinking about how to make the process work.

Conprecipitation works by a method that could be called "capture the flag." You put in chemicals that will bind to the material you want to remove. At the same time you want the resulting mass to become insoluble which will then precipitate out either by gravity or centrifuge process. Then you can wrap the unwanted and now insoluble material in some other substance, pump it into leakproof containers, and then get rid of it that way.

Areva's plan is to treat contaminated water from reactor cooling systems by adding chemicals that bind to radioactive isotopes and then settle out. In its press statement last April, Areva did not specify the exact technical method it plans to use at Fukushima. There are numerous methods in the technical literature.

According to several media reports, the process it uses at La Hague uses a specific mix of chemicals which capture the radionuclides which are removed from the water as a highly radioactive chemical sludge.

The sludge is mixed with bitumen petroleum product similar to asphalt. That mixture is poured into 55 gal drums, which are sealed and then buried in a special landfill for radioactive materials.

Water quality or mixed nuts?

mixed nutsThe efficiency of coprecipitation of radioactive materials from water depends on what you want to remove and how much of it you can expect to remove given the input chemicals and the amount of radioactivity in the water at the start of the process.

What this means for Areva is that given the difficult mixed nuts quality of the water chemistry at Fukushima, it is unclear how much radioactivity will be removed by the plant. Some is better than none.

Nuclear expert Lake Barrett told the Washington Post that at Fukushima, "you have massive volumes and a very heterogeneous chemistry."

Areva spokesperson Patricia Marie told the newspaper, "Honestly, it is hard to say how it will work."

Video of Areva plant equipment at Fukushima

Some progress is better than none at all

For its part Areva says that is bringing the water treatment process to Japan based on a request from TEPCO. Developed by AREVA and used in the Marcoule and La Hague facilities, the conprecipitation process uses chemical reagents to separate and recover the radioactive elements. AREVA also called on the skills of Veolia Water a multi-national civil engineering firm specializing in water treatment technologies and building facilities that use them.

Areva said in its statement . . .

“The contaminated water must be treated rapidly as it is preventing Tepco from repairing the power plant’s power supply and cooling systems. The unit will sharply reduce the radioactivity levels of the treated water, which could be reused in the power plant’s cooling systems.”

The treatment unit, which will be provided by Veolia Water, can process 50,000 liters (13,200 gallons) of water per hour, Areva Chief Executive Officer Anne Lauvergeon said at an April briefing in Tokyo.

How much how soon?

With an estimated 15 million gallons of water needing treatment, the timeline for optimal performance will be about two months. However, water treatment processes, especially those with a mixed effluent of unknown concentrations of contaminants, don't work a the optimum level right out of the box.

It could take some time for Areva to adjust the system to do its job efficiently. Also, there are the likelihoods of equipment breakdowns and the difficulties that will be encountered if the plant leaks radioactive water itself.

Stocks and flows

TEPCO's on site storage of radioactive water once its its new canisters arrive will be about 7.3 million gallons which means the other 7.6 million gallons will be uncontrolled and the volume will grow with each new day of external cooling until the recirculation systems are restored to service if in fact that is even technically possible.

In terms of stocks and flows, Areva’s plant will have to be reliable for some period of time. TEPCO is reportedly pouring up to 100 tons a day of water (24,000 gallons) on each of the three unstable reactors. With start-up of the new facility scheduled for June 15, and a 50 day or more backlog of water already needing treatment, in another two months, there will be another three-to-six million gallons of water needing treatment. Rainwater and snow melt will add volume what's coming off the reactors.

Measures of real progress

BWR recirculation systemThe water treatment plant is a stop gap action. For real progress to be made, TEPCO must find a way to achieve recirculation of cooling water inside the reactors and contain the leaks. (Reactor recirulation diagram: Nuclear Street)

Removing as much of the initial volume through storage and treatment is a good first step because it will allow plant workers to enter areas that are closed to them now due to the high levels of radioactivity in the water.

Areva will have some steep technical challenges to meet to achieve these results. TEPCO and the Japanese government are betting the ranch that the French nuclear giant can do it.

The other choice is almost unfathomable, and that is to run a pipe from the Fukushima shoreline to the continental shelf and pump the radioactive water into the abyss of the Pacific trench.

This solution seems unlikely as local fishermen and environmental groups are reporting radioactive contamination of fish as much as 12 miles offshore from Fukushima. Adding more radioactive water to the sea for dilution at deep levels might make technical sense, as a last resort. It may not fly in the supercharged political environment that is threatening to claim the political leadership of Japanese PM Naoto Kan.

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2 comments:

Anonymous said...

I'm somewhat familiar with the water treatment steps required, and I don't really see all that much difficulty. You pump the water through particulate filters followed by resin-negative and resin-positive filter, repeatedly until the batch is clean, then you can put the cleaned up water back into the sea. The equipment required is very generic. Some batches will need one pass, others will need many. Not that tough really!

djysrv said...

The chemical process is not that hard. The issue is the volume of contaminated water and the mixture of contaminants that have to be processed in a short period of time. TEPCo appears to have under-invested in the needed infrastructure to do the job.