Safety layers stopped nuclear meltdown
April 2nd, 2011
Nuclear core ‘meltdown’ was averted at Fukushima power plant, thanks to multiple redundant safety layers and cooling systems, despite a massive quake and tsunami.
Saving of the Japanese nuclear power plant, in a series of incidents starting on March 12, after the 11 March earthquake in Japan, was explained by Dr Josef Oehmen, researcher at MIT in Boston, USA.
Nuclear plants at Fukushima are Boiling Water Reactors (BWRs) that produce electricity by spinning a turbine with steam. Nuclear fuel heats the water to 285°Centigrade.
Nuclear fuel used here is uranium oxide in a ceramic matrix with a very high melting point, about 2800°C. The fuel is manufactured in pellets, shaped like cylinders about 1cm tall and 1cm in diameter.
Pellets are placed into a long tube of zirconium alloy, with an intended failure temperature of 1200°C, to be caused by auto catalytic oxidation of water.
Tubes, called fuel rods, are placed together in assemblies, of which several hundred make up a reactor core.
Five barriers to fuel release
The solid fuel pellet, a ceramic oxide matrix, is the first barrier that retains many of the radioactive fission products.
Zircaloy casing is the second barrier between radioactive fuel and the reactor.
The core is placed in a pressure vessel of thick steel, at about 7MPa (1000psi), the third barrier to radioactive material release.
The primary loop of pressure vessel, pipes and pumps for water coolant, are in a containment structure, the fourth barrier to radioactive material release in the event of a meltdown. Containment is by air tight seal of very thick steel and concrete.
A large, thick concrete structure is poured around the containment structure, named ‘secondary containment’, forming a fifth barrier. The reactor building outer shell serves merely as weathering protection.
During normal, full-power operation, the neutron population in a core is stable and the reactor is in a ‘critical’ or working state. Nuclear fuel in a reactor can not cause a nuclear explosion as in a nuclear bomb.
Hydrogen caused Chernobyl disaster
The Chernobyl disaster was caused by excessive pressure buildup, a hydrogen explosion and rupture of all structures, propelling molten core material into the environment. Chernobyl did not have a containment structure.
Control rods are made of boron that absorbs neutrons. During normal operation in a BWR, control rods maintain the chain reaction at a ‘critical’ state, or shut the reactor down from 100% power to about 7% power, at residual or ‘decay’ heat that must be removed by cooling systems to prevent fuel rods from overheating and failing.
Water cooling is primary barrier
Maintaining enough cooling is the main challenge in damaged reactors. Many fission products decay (produce heat) extremely quickly, within seconds, others decay more slowly, like some cesium, iodine, strontium, and argon.
The earthquake that hit Japan was several times more powerful than the worst earthquake the nuclear power plant was built for.
When the earthquake hit, the nuclear reactors all automatically shut down. Control rods were inserted into the core and the nuclear chain reaction stopped. The cooling system has to carry away residual heat, about 7% of full power heat load.
The earthquake destroyed external power supply. The reactor and its backup systems are designed to handle this. For the first hour, the first set of multiple emergency diesel power generators had started, but thee tsunami flooded the diesel generators.
Water v residual heat
Reactor operators switched to emergency battery power lasting 8 hours. Residual heat remained and could lead to fuel failure or ‘core meltdown’ after several days.
Reactors have a number of independent and diverse cooling systems; water cleanup system, decay heat removal, reactor core isolating cooling, standby liquid cooling, emergency core cooling. Which had failed is not clear yet.
They had to keep temperature below 1200°C, and keep pressure manageable. Steam and other gases in the reactor had to be released from time to time. Some of these gases are radioactive fission products in small quantities, through filters and scrubbers, with little consequence to health.
Mobile generators were transported to the site and some power was restored, but more water was boiling off and being vented than was added, initiating a reaction between zircaloy and water, producing hydrogen gas which is extremely combustible in contact with air, and during venting, an explosion occurred.
Outer shell damage only
The explosion took place outside containment, but inside the thin ‘weather’ shell. Subsequent similar explosion occurred at Unit 3 reactor that destroyed the top and some of the sides of the reactor’s outer thin shell.
Some fuel rod cladding exceeded 1200°C and some fuel damage occurred. Engineers decided to inject sea water mixed with boric acid, a neutron absorber, to ensure the rods remained in water. Boric acid also traps some of the remaining iodine in the water.
Pressure in the plant stabilised.
Radiation levels at Fukushima plant have fallen to 231 micro sieverts (23.1millirem) by March 14.
Water used in the cooling system is purified and demineralised to limit corrosion. Seawater requires more cleanup after the event, but could be used in emergencies in this type of plant.
PHOTO; Boiling water reactor containment includes a pressure vessel of thick steel as thrid barrier, a containment structure of very thick steel and concrete as fourth barrier, and a large, thick concrete structure as fifth barrier.
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