Wednesday, January 22, 2014

An Incident at Fukushima Daiichi, Part 5, The Incident

Picture Legend
1. Chernobyl afterwards
2. Map of the Japanese nuclear plants in the Sendai region
3. Simplified cross-section sketch of a typical BWR Mark I containment as used in reactors 1 though 5.
RPV: reactor pressure vessel.
DW: dry well enclosing reactor pressure vessel.
WW: wet well - torus-shaped all around the base enclosing steam suppression pool. Excess steam from the dry well enters the wet well water pool via downcomer pipes.
SFP: spent fuel pool area.
SCSW: secondary concrete shield wall.

April 26th, 1986    Ukrainian Soviet Socialist Republic

   The engineers had began the gradual power down procedure the day before. 
   What was going to be tested was the ability to generate enough power to sustain cooling to a reactor after an emergency shut down when normal sources had been made unavailable. As we discovered in the last post, in an initial shut-down state a reactor produces approximately 7 percent of its total heat output and cooling is still required to avoid core damage. Cooling pumps require electricity to pump water through the reactor. What was being tested were ways to make up for a discrepancy  in the amount of time it took for emergency generators to start and the time it took them to get to full speed, or power output, which was about 60 to 75 seconds. 
   The test began with an automatic emergency shutdown. According to test protocol the reactor’s thermal output should have been no lower than 700 megawatts. Accordingly no detrimental effects on the safety of the reactor were anticipated, so the test program was not formally coordinated with either the chief designers of the reactor, or the scientific manager. Instead, it was approved only by the director of the plant (and even this approval was not consistent with established procedures).
   The experiment began at 1:23:04 Moscow Time. The steam to the turbines was shut off, beginning a run-down of the turbine generator. The water flow rate through the reactor decreased, leading to increased formation of steam voids (bubbles) in the core. This formation of steam voids reduced the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output. This caused yet more water to flash into steam, causing yet a further power increase. Inserting control rods into the core counteracted this energy increase, however the system being used had control of only 12 rods, the rest having already been removed manually. 
   It is believed that at 1:23:40 the EPS-5 button was manually pressed in what was probably a response to the core’s power increase. The pressing of the EPS-5 button (or else the initiation of automatic emergency procedures) caused a SCRAM (“safety cut rope axe man” I know, it’s weird, yet the term is believed to have been coined by physicist Enrico Fermi, the designer and builder of the first nuclear reactor, which happened to be located under the bleachers Stagg Field at the University of Chicago, and a precursor to the first atomic bombs) emergency reactor shut down, which engaged the drive mechanism to fully insert all of the control rods, including the manual rods that had been withdrawn earlier. A few seconds after the start of the SCRAM, the graphite control rod tips entered the core. A massive power spike occurred, and the core overheated, causing some of the fuel rods to fail, blocking the control rod columns and jamming the control rods at one-third insertion, with the graphite tips in the middle of the core. Within three seconds the reactor output rose above 530 megawatts.  The power spike caused an increase in fuel temperature and massive steam buildup. The reactor jumped to approximately 30,000 megawatt thermal, ten times the normal operational output. This caused a steam explosion which destroyed the reactor’s casing,  tearing off and lifting a 2,000-ton upper steel plate, to which the entire reactor assembly was attached, sending it flying through the roof of the reactor building. A second, more powerful explosion occurred about two or three seconds after the first; which dispersed the damaged reactor core, ejecting super heated lumps of the graphite moderator (carbon used as a neutron moderator). The ejected graphite and the demolished channels still in the remains of the reactor vessel caught fire on exposure to air, which heavily contributed to the spread of radioactive fallout and external contamination. 
   The citizens of the nearby city of Pripyat were not informed of the accident.  Within a few hours after the explosion, dozens of people fell ill, suffering headaches, and uncontrollable coughing and vomiting. The general Russian (at the time it was still the Soviet Union) population was not informed of the accident until April 28th, two days after the explosion. They were informed via a 20 second announcement on the evening news. Only after rising radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden, over 600 miles west of the Chernobyl Plant, did the Soviet Union admit that an accident had occurred.  

   “There has been an accident at the Chernobyl Nuclear Power Plant. One of the nuclear reactors was damaged. The effects of the accident are being remedied. Assistance has been provided for any affected people. An investigative commission has been set up."
—Vremya (the main evening newscast in Russia), 28 April 1986 (9:00PM)

   Four hundred times more radioactive material was ejected from Chernobyl than by the atomic bombing of Hiroshima, Japan. The accident released a total of one hundredth to one thousandth of the entire amount of radioactivity released by nuclear weapons testing during the 1950s and 1960s. About 38,610 square miles of land were significantly contaminated with fallout, the worst hit regions being in Belarus, Ukraine and Russia. Slighter levels of contamination were detected   over all of Europe, except for the Iberian Peninsula (a peninsula located in the extreme southwest of Europe that includes the  sovereign states of Spain, Portugal, Andorra, part of France, as well as the British Overseas Territory of Gibraltar).
    Some 135,000 people were evacuated from the area, including 50,000 from the city of Pripyat.
   After the explosion 237 people suffered from acute radiation sickness (ARS), of whom 31 died within the first three months.  Most of these were fire and rescue workers trying to bring the accident under control, and who were not aware of, or informed of, the latent  exposure to radiation such work would expose them to.  
   Isotopes of iodine (Iodine-131), caesium (Caesium-137) and strontium are responsible for most of the radiation exposure faced by the general population. 
   It was estimated that cancer deaths caused by Chernobyl could reach a total of 4,000 among the 5 million persons residing in the contaminated areas.
   Reactor No. 4  at Chernobyl is now encased in a large concrete sarcophagus, which was built quickly to allow continuing operation of the other reactors at the plant.
   After the explosion the other three reactors continued to operate. In 1991, Reactor No. 2 suffered a major fire, and was subsequently decommissioned. In November of 1996, Reactor No. 1 was shut down, followed by Reactor No. 3 on December 15, in 2000. The entire plant is now closed.
    It is likely that with no further decontamination efforts the gamma ray dosage at the site will return to background levels in about 300 years.
   The explosion at Chernobyl was the first level 7 event (the maximum classification) on the International Nuclear Event Scale, and is considered the worst nuclear accident ever.  
   There’s only been two level 7 events in the history of the nuclear power industry, the other being the ongoing incident at Fukushima Daiichi. Although not considered the worst nuclear accident in history (it  has been considered the second worst), Fukushima Daiichi has been considered the most complicated.
   But please keep in mind... the accident at Fukushima Daiichi is ongoing.

   On the Monday before the accident, March 7th, Tokyo Electric Power Company (TEPCO) submitted a report to Japanese nuclear safety officials which predicted the possibility of a tsunami reaching up to 34 feet at the Fukushima Daiichi nuclear facility in the event of a 7.2 magnitude earthquake similar to the Sanriku earthquake of 1896. TEPCO had actually made the prediction 3 years earlier, but had not felt the need to take immediate action. Considering the seawall surrounding the plant that was designed to protect it from such tsunami’s was only 19 feet high, one has to wonder about the quality of their judgment.
   When the March 11, 2011, 9.0 earthquake struck at 2:46 in the afternoon, reactors 1, 2 and 3 were operating at Fukushima Daiichi, but units 4, 5 and 6 had been shut down for periodic inspection (reactor 4 was defueled in November 2010. All of the fuel rods had been transferred to the spent fuel pool on an upper floor of the reactor building). The operating reactors underwent an automatic SCRAM emergency shutdown, as they were designed to do. When the reactors shut down, the plant stopped generating electricity. One of the two connections to off-site power for reactors 1, 2, and 3 also failed. On-site emergency diesel generators began providing power to continue pumping water to the reactor cores in order to dissipate the residual heat. Plant officials focused their attention on a damaged storage pool for spent nuclear fuel at the No. 2 reactor, the damage prompted the plant’s management to divert much of the attention and pumping capacity to that pool. 
      41 minutes after the mainshock quake the first tsunami hit Fukushima Daiichi. 3 minutes later the emergency condenser designed to cool the steam inside the pressure vessel of the No. 1 reactor failed.
   Approximately an hour after the earthquake a 46 foot tsunami wave flowed over the seawall as if it wasn’t there, inundating the facility and disabling the backup diesels, all of which but one  were housed underground. The resulting loss of electricity prompted failures of  the residual heat removal and low pressure coolant injection system's main pumps. It goes with out saying that the  automatic depressurization systems failed as well. Only the steam-powered pump systems (isolation condenser in reactor 1, high-pressure coolant injection and reactor core isolation cooling system in reactors 2 and 3) remained available. 
   TEPCO then notified authorities (“Tokyo, we’ve got a problem”) of a "first level emergency."
   Switching stations that provided power from the three backup generators located higher on a hillside failed due to not being high enough, when the building that housed them flooded. Power for control systems switched over to batteries that were designed to last about eight hours. A call for additional batteries was put out, but due to poor road conditions and other considerations, were late in arriving. 
   At 4:00 the Nuclear and Industrial Safety Agency ordered all 55 of Japan’s nuclear reactors shut down for precautionary reasons. At that time there were no reports of radiation being detected outside the borders of any of the nuclear power plants.    
   2 hours later the coolant water level in reactor 1 reached the top of the fuel, exposing it. The core temperature began to climb.
     At 7:03 Japan’s Prime Minister, Naoto Kan, declared a nuclear emergency. Officials told the Japanese people all of the proper safety procedures had been initiated, and that no radiation had leaked into the environment.
   By 7:30 the fuel in  Fukushima Daiichi number 1 reactor became fully exposed above the water line. Fuel damage in the central core started soon after
   An hour and a half later the evacuation began.

To be continued.

Addendum: 4-26-15: Chernobyl 29 years later

No comments:

Post a Comment