Reply to thread

Fukushima Nuclear Accident – 17 March update

                   Posted on 17 March 2011 by Barry Brook               

                                        The  crisis at the Fukushima Daiichi nuclear power station is approaching a  weeks’ duration. The on-site situation remains extremely serious, with  glimmers of hope being shrouded by a shadow of deep uncertainty.

 If you’ve not been following the situation on BraveNewClimate, and want to recap, please read these recent updates:

 Japan Nuclear Situation – 14 March updates

 Further technical information on Fukushima reactors

 Fukushima Nuclear Accident – 15 March summary of situation

 Fukushima Nuclear Accident – 16 March update

 These are assumed knowledge for understanding the rest of this post.  The preparation of the material below was aided greatly by the private  advice of my acquaintances in the nuclear engineering field.

 As predicted yesterday, attention over the last 24 hours has focused  on the critical situation with the ponds used for temporary storage of  spent nuclear fuel at the individual reactor units, before it is moved  to a centralised facility on site. Although this old fuel has lost much  of its original radioactivity, the decline is exponential (see this  figure) which means that thermal energy must continue to  be dissipated for months.

 This figure shows the location of the spent fuel ponds:

[ATTACH=full]79466[/ATTACH]The problem, as is explained in this updated fact sheet by the NEI,  is that as these ponds heat, their deep covering of water (which acts  as a radiation shield and a cooling mechanism), starts to evaporate. If  they reach boiling point, because of lack of operational maintenance  systems, the evaporation rate will accelerate. If exposed, the there is a  potential for these old fuel rods and their zirconium cladding to melt,  and radiation levels will rise considerably. The heat generated in  spent fuel depends on a number of parameters, including: (1) level of  build-up of fission products (burn-up) and (2) length of time after  having been taken out of the reactor.

The spent fuel pool temperature has been rising gradually since last  Friday due to the loss of cooling pump (presumably no power source). As  we know from previous updates, the side of the Reactor 4 building has  been lost (it’s the left-most of the 4 buildings in the following  image):

[ATTACH=full]79467[/ATTACH]

The Unit 4 reactor was already shut off for periodic maintenance when  the earthquake struck. IF the fire was caused by hydrogen,  its only  plausible source would be spent fuel degrading in steam. Under this  scenario, initial inventory was probably reduced by sloshing during the  earthquake, and heat generation and resulting evaporation/boiling would  thereafter be more than double that in other pools due to it containing  freshly off loaded fuel. Temperature indications in the absence of water  would be that of the mixture of steam and air in the location of the  thermowell.

 Nothing can be confirmed at this stage. As has been the case  throughout this crisis, information is hard to come by and must be  pieced together.

 Are the spent fuel in the pools in Units 3 and 4 are now uncovered? TEPCO claims that NRC Chief Jaczko was wrong  in claiming this, that the spent fuel pools in both Units 3 and 4 need  some refilling but are NOT dry. (The Japanese authorities are apparently  saying they’ve seen water still in the Unit 4 pool.) The big concern  here is that unlike the releases from damaged fuel in the reactor cores  of Units 1, 2, and 3, which were largely filtered by scrubbing in the  containment suppression pools (wetwell torus), releases of volatile  fission products (e.g., cesium and iodine) from these spent fuel pools  have direct pathways to the environment, if they remain dry for an  extended period.

 [ATTACH=full]79468[/ATTACH]Efforts to deliver water to these pools have proven to be very difficult, and fuel damage may be  occurring.  If they are exposed, then the use of the evaporation of  salt water as a heat sink over periods of more than a few days is not  viable because the quantities of salt deposited as the water evaporates  becomes large in volume and plugs the flow paths through the fuel,  degrading heat removal. Everything that is cooled becomes a heat sink to  condense anything volatilised. Unfortunately, a fresh water supply  seems difficult to come by.

 One option is to bring fresh water by helicopter, but the amounts  needed imply a large number of flights and gamma radiation levels are  high above the pools making overflights hazardous.  NHK has reported a number of  successful water dumps using helicopters  today. If radiation levels on the ground increase further, personnel  access will become more challenging.  Additional spent fuel is stored in  pools in Units 5 and 6 and in a large centralized storage pool.  A key  issue is how to continue to make up water to these pools in the longer  term, particularly if site access becomes more difficult.

It was announced at a press conference  that a total of 11 specially-equipped vehicles will be used to spray  water on the crippled reactors at Fukushima-1 after an access path is  cleared using bulldozers. The big advantages of fire trucks over  helicopters is that their water cannons can be better aimed, from the  side rather than the top, and their operation is continuous rather than  in batches so they can deliver vastly more water. It is clearly  an appealing option. An additional 130 personnel have also been moved back on site to help with work.

Some additional key information from NEI:

Crews began aerial water spraying operations from  helicopters to cool reactor 3 at Fukushima Daiichi shortly before 9 p.m.  EDT on Wednesday, March 16. The operation was planned for the previous  day, but was postponed because of high radiation levels at the plant.  News sources said temperatures at reactor 3 were rising. Each helicopter  is capable of releasing 7.5 tons of water.

Spokesmen for TEPCO and Japan’s regulatory agency, Nuclear and  Industry Safety Agency, on March 17 Japan time refuted reports that  there was a complete loss of cooling water in the used fuel pool at  Fukushima Daiichi reactor 4.

The spokesmen said the situation at reactor 4 has changed little  during the day today and water remained in the fuel pool. However, both  officials said that the reactor had not been inspected in recent hours.

“We can’t get inside to check, but we’ve been carefully watching the  building’s environs, and there has not been any particular problem,”  said TEPCO spokesman Hajime Motojuku.

At about 7 p.m. EDT, NISA spokesman Takumi Koyamada said the  temperature reading from the used fuel pool on Wednesday was 84 degrees  Celsius and that no change had been reported since then. Typically, used  uranium fuel rods are stored in deep water pools at temperatures of  about 30 degrees Celsius.

Recent radiation levels measured at the boundary of the Fukushima  Daiichi plant have been dropping steadily over the past 12 hours,  Japan’s Nuclear and Industrial Safety Agency said on Wednesday night  (U.S. time).

At 4 a.m. EDT on Wednesday, a radiation level of 75 millirem per hour  was recorded at the plant’s main gate. At 4 p.m. EDT, the reading at  one plant site gate was 34 millirem per hour. By comparison, the Nuclear  Regulatory Commission’s annual radiation dose limit for the public is  100 millirem. Radiation readings are being taken every 30 minutes.

Japan’s Chief Cabinet Secretary, Yukio Edano, said earlier today a  radiation level of 33 millirem per hour was measured about 20 kilometers  from the Fukushima Daiichi plant earlier this morning. He said that  level does not pose an immediate health risk.

Edano said that TEPCO has resumed efforts to spray water into the used fuel pool at the damaged reactor 4.

TEPCO also continues efforts to restore offsite power to the plant,  with up to 40 workers seeking to restore electricity to essential plant  systems by Thursday morning, March 17.