Status May 21, 2012

Status May 21, 2012

Work has just begun on the cover for the Unit 4 building.  To date only the Unit 1 cover has been erected.  The Unit 3 and 4 covers were awaiting completion of debris removal before proceeding.

Fukushima has multiple potential disasters which can lead to releasing radiation if another large earthquake/tsunami should hit in the next few years.  The Unit 4 building sustained substantial structural damage and the fuel pool has the largest inventory of fuel assemblies.  The pool received external cooling from the fire hoses during the first months after the earthquake.  One of the first actions after gaining access to the area was to shore up the fuel pool with 30 steel supports.  They continue to filter the water in the spent fuel pool to remove the salt water introduced to the system.

The challenge of remediating all the damage at Fukushima requires untested solutions.  There are 3 reactors with damaged fuel which has leaked out of the reactor vessel into the drywell. In 2 cases the drywell appears to be damaged, so the highly radioactive molten mass cannot be shielded via flooding.  The Unit 3 building is still open to the elements and debris needs to be removed.  Also, there is fuel in each of the 3 associated spent fuel pools.  Plans are being formulated on how the damaged reactor vessels can be accessed in order to remove the damaged fuel.  They will also need to gain access to the area below the reactor vessel to remove the fuel which melted through the vessel.  This is a procedure which has never been performed or planned.

They are storing a huge volume of highly radioactive water containing cesium and strontium in temporary tanks on site.  The water is being cleaned and recirculated for cooling, but the pumping and piping systems were quickly assembled and are prone to break downs and leaks. If a leak occurs the risk of runoff into the ocean is high.  Since the buildings are not sealed the inventory of water continues to increase when it rains.  The basements of several buildings are flooded with contaminated water.    

Removing the fuel from the Unit 4 pool requires substantial infrastructure repairs.  A 100 ton crane is needed to lift the fuel storage casks used for removing the fuel, but they still need to clear the debris which sits on top of the fuel.  The debris includes the crane which would normally be used to place the fuel into the storage casks.   The support systems for evacuating water from the cask after loading are probably damaged because wiring and motors have been water logged.    

The plan and work to cleanup Fukushima will take decades to complete.  The risk of a large earthquake and tsunami incurring substantial damage and releasing radioactivity into the environment will be there all the time.

Status April 2, 2012

A full year has passed since the tsunami wiped out the Fukushima plant.  Water continues to be treated and circulated into the reactor and reactor containment vessel to maintain the damaged fuel in a stable state.  The water pumping, storage, and recirculation system continues to have breakdowns and leaks.  On March 26, a leak from a storage tank was found.  Before the leak was stopped an estimated 80 liters of water containing strontium 90 had entered the ocean.  To prevent the spread of the contaminated sediment in the ocean around the plant, the seabed is being covered with a layer of cement.

The plans for removing the damaged fuel from the Units 1, 2, and 3 are being formulated.  Recently, an endoscope was inserted into the Unit 2 reactor containment vessel to determine the water level in the vessel.  The water level was found to be 0.60 meters, substantially lower than the 3 meters they expected.  The lower water level indicated damage to the containment vessel is greater than anticipated which further complicates the task of removing the highly radioactive damaged fuel because raising the water level is integral to the plans for removing the debris.

The structure which encases the Unit 1 reactor building has been completed.  Debris within the damaged building has not been removed.  Removal of debris from the Units 3 and 4 reactor buildings is progressing with completion expected at the end of 2012 and mid 2012, respectively.  The method for removing the fuel in the Unit 4 spent fuel pool is being determined.  There is an urgency to remove the fuel because the building sustained structural damage so there is a risk of the further damage to the pool in the event of a large earthquake.  Usually the spent fuel pool crane is used to remove fuel, but the crane was damaged and is contributing to the debris in the building.

Some towns around the Fukushima had been evacuated are now safe for habitation.  Most people have left the area and many are concerned about the residual contamination and are staying away.  

Status as of 6/29/2011

After a couple of attempts to begin recirculating the processed water, it appears all the piping leaks have been fixed.  They are now injecting a mix of processed water and fresh water into the Unit 1, 2, and 3 reactors for cooling. 

Construction of the structure around the Unit 1 reactor building has begun.  A construction crane is now in place for lifting a steel framework which is being preassembled offsite.  Panels of one millimeter-thick polyester fiber panes will be attached to the frame.  The structure will be about 162 feet high and is expected to be completed by late September.

Status as of 6/27/2011

Water from the water treatment plant was injected into the reactors for one and a half hours before the discovery of leaking pipes halted the recycle.  The original schedule had anticipated the recycling would be working by mid June.  An estimated 110,000 tons of water has collected in the basements and tunnels in the plant and 1850 tons has been processed.

Status as of 6/20/2011

The water treatment plant was started on Friday, but was stopped after 5 hours because a component reached its limit for radioactive contamination much faster than expected.  The contaminated water is to undergo three processes, the first separates the oil and water, the second reduces the radiation by absorbing the cesium, and the third removes the salt and other impurities.  The processed water will be stored and eventually used to cool the reactors.  The oil in the water is from the machinery which is in the basements of the flooded buildings.  The salt is from the seawater which was injected into the reactor for several days following the earthquake until a supply of fresh water could be obtained.  The cesium and other radioactive materials are from the damaged fuel and appeared to be at concentrations higher than TEPCO anticipated.  While it is expected that the water treatment plant will concentrate radioactive materials, they need to monitor the radiation level in order to avoid the problem of the ‘filter’ becoming too radioactive to be handled and disposed of properly.

The Unit 2 reactor building doors were opened to reduce the humidity in the building.  Efforts to cool the spent fuel pool apparently did not remedy the situation previously reported May 23.  The humidity was at 99%.  They want to reduce it to below 70% to create better working conditions.  

Question: What is the difference between a nuclear melt down and a melt through? What are the implications?

There is a bit of a difference in that a melt through is where the fuel melts through the lower core plate and through the lower head of the pressure vessel into the primary containment.  It makes clean up a bit more challenging.  There may be some additional releases to the environment.  However, there is nothing necessarily catastrophic about this problem.  It's a bit like the difference in a turbine explosion of having all the blades contained, or a few actually being ejected past the engine containment.  Either way, your day is going to suck.   This is not the nuclear equivalent of a disk explosion.

There is actually some benefit to having some melt through because the fuel is now spread out and you have a larger surface to volume ratio allowing more cooling potential of radio nuclide decay heat.   The fuel that falls to the bottom of containment is also under water and in contact with a huge heat sink.  Part of the trick to cooling the fuel is lots of wetted surface area.

See the diagram below.  The reactor pressure vessel (brown) is inside the primary containment (yellow).  Leaking out of the reactor vessel does not mean being released to the environment.  Assuming the primary containment is still substantially or completely intact, melting through the reactor vessel into the drywell should not cause much additional release to the environment.

The primary containment is protected from steam overpressure by venting into the Torus which is partially filled with cold water.  The steam released from the RV into containment during an accident is vented underwater in the torus and condenses reducing its volume and controlling pressure in the primary containment.

Status as of June 16, 2011

The water processing facility is undergoing final testing and could start operations by tomorrow.  After processing, the radioactivity in the water should be reduced between one-1000^th and one-10,000^th.  It will then be transferred to makeshift tanks at the plant and be used for cooling the fuel.  TEPCO estimates more than 110,000 tons of radioactive water has accumulated and will need to be processed.

TEPCO is also working on installing a system to clean the water in the intake bay.  It is pumping the water through zeolite to capture the radioactive cesium which has escaped into the bay.

Assembly of the framing which will support a polyester fabric for Unit 1 reactor building has begun.  The frame is 120 ft x 120 ft x 150 feet high.  After it is complete it will be lifted and placed over the damaged building then the fabric will be put into place.