Process engineering and critical infrastructure planning failures were this week blamed for the catastrophic failures at Japan’s Fukushima nuclear power stations following last Friday’s earthquake and tsunami.
‘Failure to scrutinise risks’
Failure to properly scrutinise potential risks and take action to mitigate them is also thought to have contributed to a possible meltdown.
“There are three failures,” independent nuclear expert Tony Roulstone told NCE.
“The plant was not designed to contain against certain flood levels; the diesel back-up generators were located in one place; and it took a reported nine hours to fly in back-ups.”
“The plant was not designed to contain against certain flood levels; the diesel back-up generators were located in one place; and it took a reported nine hours to fly in back-ups”
Roulstone said that this was a process engineering fault and could have been reviewed.
However, failure to scrutinise potential risks could be down to a cultural issue within Japanese firms according another expert who asked not to be named but who is familiar with the country’s culture.
He highlighted an unwillingness to criticise senior executives in Japan as a factor which hindered a proper risk assessment.
“It’s a cultural issue. Too often staff will turn a blind eye rather than confronting the issue,” he said.
Mistrust between the Japanese public and the nuclear authorities could be further heightened following this week’s events, he added.
Japanese engineers were this week battling to prevent a meltdown at the plant in northern Japan after three major explosions ripped through it last weekend.
The three hydrogen explosions occurred at the Fukushima Daiichi One nuclear power plant, in Okuma, north eastern Japan.
A magnitude 9.0 earthquake 100km from the east coast of Japan at a depth of 24.4km triggered the crisis at the power plant. The quake launched a massive tsunami on eastern Japan which swamped infrastructure and killed at least 3,000.
The tsunami was 7.5m high when it reached Okuma and overtopped sea defences at the power plant, which news reports said, were to resist waves up to 6m high.
Bid to cool reactors
Japan’s authorities, led-by the Japanese Nuclear and Industrial Safety Agency (NISA), were attempting to cool three reactors with seawater in a desperate bid to avoid a full nuclear meltdown as NCE went to press.
Some radiation escaped into the atmosphere and 150 people were being monitored for radiation poisoning as NCE went to press. Around 170,000 residents living within 20km of the plant have been evacuated.
All three reactors that were operating when the tsunami struck shut down automatically, as designed. Three other reactors were not being used when the quake struck.
Critically, pumps circulating cooling water to the operating reactors failed when electricity supplies were cut and back-up diesel generators were flooded.
New generators were flown in, but these are reported to have taken nine hours to arrive.
Cooling water is critical to the safe operation of the boiling water reactors used at the Tokyo Electric Power Company (Tepco) Fukushima plant.
Extreme heat in reactor cores
The reactors have been in operation since the 1970s and rely on uranium nuclear fission to generate heat. In a closed system, steam from heated water surrounding the core drives turbines to generate electricity. This steam is then turned back into water by passing it through a condenser which uses seawater as the coolant. It is then pumped back into the reactor vessel to be reheated.
The reactor vessel itself is surrounded by a thick steel and concrete primary containment vessel, equipped with a water reservoir designed to prevent overheating.
When the quake struck, control rods were inserted into the reactor cores to stop fission, but due to the extreme heat in the reactor cores, water still had to be pumped around them to cool them.
Power ‘knocked out’
However, power supplied to the pumps from the electricty grid was knocked out by the earthquake and back-up diesel powered generators were inundated by floodwaters from the tsunami.
Without a cooling supply, the water in the reactor core rapidly turned into high pressure steam.
Roulstone said that to release pressure it is usual to vent the steam into the reservoir inside the containment vessel.
“Normally this pool is cooled by separate cooling circuits. But because of the loss of power, steam pressure rose in the containment vessel to a reported 800Kpa,” he said. “This pressure is above what the containment is designed for, and that is why they have vented the containment of reactors one, two and three − to prevent a major failure of the containment box.”
Explosions at all three reactors
Steam venting led to the explosions at all three reactors.
Roulstone said that it was likely that hydrogen gas was formed from a reaction between zirconium fuel cladding and water in the reactor vessel, and it was the presence of this hydrogen that caused the explosion.
Engineers were attempting to cool the cores by flooding the containment vessels with seawater as NCE went to press. If engineers fail to cool a reactor core, enriched uranium can melt into radioactive lava, which can burn its way out of the containment vessel or send radioactive steam out through pressure-induced cracks in the system.
Roulstone said that pumping in the sea water was likely to prevent a full scale meltdown. But he said the situation should never have been allowed to escalate this far.
In 2007, a magnitude 6.8 earthquake struck off the north west coast of Japan striking Tepco’s Kashiwasaki-kariwa plant in the Niigata region.
The earthquake exceeded the scale of event anticipated in the design codes for the plant. There was a radiation leak and a fire at an electrical substation.
“There was a perceived cover-up by the authorities,” said Bristol University civil engineering lecturer Katsu Goda.
“There will be a debate whether to rebuild or abandon this technology”
Katsu Goda, Bristol University
The authorities took over a year to restart the reactors and one is still inactive today, said Goda.
The cause of the leak has never been fully explained. Several years earlier, the plant was shut down following a discovery of deliberately falsified data.
“There will be a debate whether to rebuild or abandon this technology,” added Goda.
Abandoning nuclear power will be difficult. Japan’s 54 reactors provide 30% of the country’s electricity and this proportion is expected to increase to at least 40% by 2017.
How to control a nuclear reactor
1. SMOOTH OPERATION
Fukushima Daiichi One uses a boiling water reactor (BWR).
In a closed system, water surrounding the core boils into steam that drives turbines to generate electricity. This steam is then turned back into water by passing it through a condenser which uses seawater as the coolant. It is then pumped back into the reactor vessel to be reheated. The reactor vessel itself is surrounded by a thick steel and concrete primary containment vessel, equipped with a water reservoir designed to suppress overheating of the vessel.
2. DISASTER STRIKES
When the quake struck, control rods were inserted into the cores to stop fission, but due to the extreme heat in the reactor cores water still needed to be pumped around as if the reactors were still operating normally.
3. CORE MELTDOWN
If engineers cannot cool the reactor then the uranium can melt into a pool of radioactive lava which can burn its way out of the containment vessel and into the foundations.