To understand what occurred at the Fukushima Daiichi nuclear power plant (NPP), and its repercussions on the future of the nuclear industry, one needs a basic idea of how NPPs work. NPPs are built around a heavily-shielded reactor chamber or core, which is a concrete structure, surrounded by a steel containment vessel, designed to prevent radiation leaks.
Inside, fuel rods, (3.5 metres in length and 1 cm in width) are stacked up in fuel assemblies. The rods have zirconium casings, and contain uranium 235 (U-235), or mixes of U235 and plutonium Pt239, in concentrations of about three or four per cent.
When U-235 is hit by a neutron, it splits into lighter elements, releasing excess particles as energy. Fission is controlled by inserting or removing control rods from assemblies. The control rods contain cadmium or graphite to absorb free neutrons and they are used to shut down, or slow, fission.
The assemblies are covered by water, pumped at high pressure. The water absorbs heat, turning into steam, which drives turbines generating electricity, as in conventional thermal plants. The steam is continuously replaced by cool water and cooled in the massive towers characterising NPPs.
In some designs, the radioactive hot water in the reactor is cooled inside a heat-exchanger “jacket” of non-radioactive water, which receives heat but not the radiation. The non-radioactive water is used to drive turbines. Fukushima however, used Boiling Water Reactor (BWR) designs, which directly utilise radioactive water to drive turbines and can cause turbine contamination. Spent fuel, which is highly radioactive, is also stored under water to cool down, in shielded conditions before being moved out to landfills, which must be shielded for millennia.
The energy value of U-235 fission is immense. One kg of reactor fuel (containing 35 to 40 grams of U235) can generate as much electricity as 17,000 kg of coal. Even after shutdown, heat is generated by radioactive decay. Decay heat needs to be bled off for weeks after shutdown.
If heat isn’t removed, and temperatures rise higher than the fuel-rods’ melting point (about 1,000C for zirconium and 1,130C for uranium), the rods will melt. This is the so-called “China Syndrome” — a misleading description because meltdown cannot generate enough heat to burn through the Earth.
But if containment shields are busted, massive radiation may be released. This happened at Chernobyl in 1986 (the Soviet reactors had minimal containment shields) when radiation effects were seen across 140,000 sq kms. It nearly happened at Three-Mile Island, US in 1979.
This is what the Japanese are so desperately fighting to prevent. Damage-limitation after Chernobyl involved setting up an exclusion zone, depopulating 900 square kms permanently. Japan doesn’t have that sort of territory to spare though it has evacuated a 20x20 zone temporarily.
It’s imperative to locate NPPs near water sources and seawater is often used, as at Fukushima. At Fukushima, three of the six reactors were running on March 11, when the earthquake hit. Safety measures worked initially, with control rods inserted to shut down reactors as the quake was detected. Mains power failure occurred but backup generators kept coolant system working.
The tsunami hit some 30 minutes after the quake. The seven-metre waves destroyed supply pumps, and swamped the gensets and diesel tanks. The steam inside the reactors could no longer be cooled. Heat built up to a point where reactors 1, 2, 3 suffered meltdown damage according to the International Atomic Energy Agency. In addition, steam catalysed into hydrogen and oxygen, in a catalytic reaction with the zirconium. The hydrogen ignited to trigger at least two explosions. To prevent more explosions, containment vessels were deliberately breached to allow steam and hydrogen gas to escape.
The 1970s-commissioned Fukushima employed only active cooling systems. Newer designs use active-passive cooling designs, which are more effective if power is cut off. This disaster will surely lead to a global review of anti-earthquake and tsunami-safety measures for NPP. It will also lead to a review of spent fuel pool storage and landfill management.
This is all of particular relevance to India, which has 19 operational NPPs. In 2007-08, those supplied 1.6 GWh, operating at less than 50 per cent of total 3.7 Gw capacity due to lack of fuel. This was roughly two or three per cent of India’s grid-generated electricity.
The 2008 Indo-US nuclear agreement cleared the way for uranium imports as well as new NPP technology. There are five new reactors under construction. By 2009-10, India’s NPPs produced 22 billion KWh. In 2011-12, that may rise to 32 billion and by 2020, capacity is projected to increase to 20 Gw from 39 reactors, and by 2030, to 63 Gw.
These estimates are pre-Fukushima. After that, it’s going to be a hard set of choices for India’s policy-makers. Nuclear power presents a Faustian bargain. India has a huge, growing energy demand-supply gap and it’s fuel-deficient to boot.
If everything works, nuclear power is a lot cheaper, cleaner and more convenient than coal, or other fossils. That is, of course, not accounting for potential disaster management costs. If there is a problem, however, the potential for damage is exponentially higher.
