Sunday, November 6, 2011

What Went Wrong at Fukishima? 24 Hours to Meltdown

Reactor 3 Explodes at Fukishima
A report by the IEEE (Institute of Electrical and Electronics Engineers) takes a close look at what went wrong at the Fukishima Nuclear Power Plant following the March 11, 2011 earthquake and tsunami.
With billions of dollars in research and technology invested in nuclear energy, the report identified six common-sense and seemly obvious lessons which could have minimized or prevented the impending meltdown.

A massive 9.0 quake struck at 2:46 pm on March 11, 2011 off the east coast of Japan. At the Fukushima Dai-ichi Nuclear Plant, operated by Tokyo Electric Power Co. (TEPCO), Unit numbers 1,2 and 3 of the six reactors were operating. #4, 5 and 6 were down for scheduled maintenance. The quake caused the plant to perform a routine auto-shutdown without incident.

Power outages caused by quake were widespread. Within 10 seconds, twelve diesel generators activated to power water pumps for cooling to the fuel rods. So far, all emergency procedures are working as planned.

At 2:52 pm Unit 1 using a non-electrical isolation condenser (IC) backup cooling system was cooling the reactor too quickly and a plant supervisor shut it down, per normal procedure.

Tsunami alerts predicted a 3 meter (9.8ft) high wave would strike the Fukushima prefecture. The Dai-ichi Plant is 10 meters above sea level (33 ft). For safety, non-essential personnel began evacuating plant.

The first wave struck at 3:27. With a second set of much bigger waves arriving at 3:35 pm. It has been approximately fifty minutes since the quake.

The second huge wave topped seawalls and surged through plant. It destroyed heat removal seawater pumps and inundated the control rooms controlling valves, pumps and other crucial equipment. Later, TEPCO employees would estimate the killer wave at 14 meters high (46 ft) from water stains on the walls.

Six generators located in basements were drowned and five more shut down when control rooms were flooded. Only one generator serving reactors 5 and 6, not located in a basement, continued operating. This lone functioning generator helped units 5 and 6 survive the disaster while the other reactors spiraled out of control.

Lessons from Fukishima
Even back up batteries failed and Reactor 1 suffered a complete power failure. The control room went dark and instrument panels stopped functioning. Cooling system pumps failed and the water which was supposed to be cooling radioactive fuel rods began to boil. Steam built inside the reactor building. Without working gauges and instruments, operators were not sure of how much water was left to cool the rods.

The non-electrical IC cooling system serving Reactor 1 had been shut down early in the crisis, due to it working too well. Now, without power,  plant operators were unable to reopen the valves, even manually.

Operators struggled to regain power at the plant.  They scavenged batteries out of cars in the parking lot and called out a small fleet of power-generating trucks. However,  the earthquake and tsunami ruined roads and mass evacuations clogged highways and these trucks promptly became stuck in traffic.

At 4:36 TEPCO finally officially alerted the Japanese government of the problem at Reactor 1.

Around 9 pm and working by flashlight, operators ingeniously powered up a few important instrument panels using the scavenged car batteries and were relieved to see that the water cooling the fuel rods in Reactor 1 seemed to holding up so far. Water levels were down, but the rods were not exposed.

Later, company analysis showed the instruments were incorrect. The water level had dropped so low the rods were completely exposed. Temperatures had topped 1300 °C (2372 °F) and the meltdown had already begun.

Around midnight, more instruments were brought online and showed that dangerous pressure inside the containment vessel had already exceeded its' maximum design and an explosion was a serious risk.

Teams struggled through the night and next day to vent the explosive pressure in the containment vessel and cool the rods. Power trucks finally arrived and prepared to restart pumps cooling the crippled reactor.

Fire hoses poured on fresh water until tanks were empty, then in desperation started using highly corrosive sea water. This was an tacit admission that saving the plant was no longer an option and now the focus was on preventing a massive nuclear disaster.

Unknown to operators, the meltdown was proceeding. Superheated fuel rods had begun to melt through the steel floor of the pressure vessel. Pressure built inside the reactor as residents within a 10 km area around the plant (6.2 miles) were evacuated.

Attempts to release the pressure from hydrogen gas inside the reactor continued but were not enough to prevent a catastrophic explosion almost exactly 24 hours after the tsunami hit the plant.

The explosion cut off power from the trucks and severed the fire hoses. The flow of cooling water ceased as radiation levels climbed and plant operators scrambled for safety.

The disaster continued to spiral out of control. The plant was now a radioactive hot spot and choked with debris from the tsunami and explosion of reactor 1. Workers struggled to cool Reactors 2 and 3, but without power or pumps, Reactor 3 exploded on March 14, followed by a possible explosion inside number two later that day. Later, another explosion tore the roof off building four.

As this slow-motion catastrophe unfolded, workers fought gallantly to contain it but efforts were continually hampered by the lack of power which caused pumps to fail and rendered safety controls useless. Japanese officials later admitted that three reactors suffered full meltdowns.

Certainly, lessons will be learned from this disaster. Nuclear Power Plant designers worldwide will be studying Fukishima for years to come and will develop better system designs and disaster plans.

But looking at how events unfolded, it was mainly a lack of planning in the common-sense, low tech processes which brought Fukishima to its' knees.

Article first published as What Went Wrong at Fukishima? 24 Hours to Meltdown on Technorati. 

Reactor 3 image:

Lessons background image:  

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