Nuclear Disasters

Exactly five years ago today on March 11th, 2011, Japan’s Fukushima Dai-ichi Nuclear Plant went through a series of equipment failures, nuclear meltdowns and releases of radioactive materials at the Fukushima, Nuclear Power Plant, following the Tohoku Tsunami on 11 March, 2011.  It is the largest nuclear disaster since the Chernobyl disaster of 1986 and only the second disaster (along with Chernobyl) to measure Level 7 on the International Nuclear Events Scale (INES).

Japanese officials are still trying to understand all the factors that contributed to the meltdowns at the Fukushima Dai-ichi nuclear power plant.  Officials came to the conclusion that the plant was not designed to withstand the 40-foot tsunami that hit it which was caused by a magnitude 9.0 earthquake.  Within days, three of the plant’s six reactors had suffered severe fuel damage—and possibly even melted down—raising fears of radiation dispersal in Japan and around the world.  When the tremors rattled the plant, control rods automatically scrammed the reactor as they were designed to do, cutting off the fission process. Then the plant lost electricity from the grid and the diesel generators kicked on, only to be swamped and disabled by a 30-foot tsunami within the hour.

With no power to keep coolant flowing, the energy from radioactive decay began to build up, raising the pressure within the reactor vessel. Tokyo Electric Power Company (TEPCO) reported on March 12 that safety valves had been triggered in the reactor vessel, and pressure inside the containment structure had increased to double the design limits. Fearing that the containment structure itself might fail, the utility made the calculated decision to vent it through filters and out to the environment (beyond the support building), albeit at the risk of releasing small amounts of radioactivity—mainly the isotopes created by the decay, including iodine-131 and cesium-137.

Over the next few days, it became obvious that the fuel was damaged. The question became whether it would melt, and if it did, whether it would melt through the reactor vessel and into the containment structure. While all of the specifics are not yet known, the fuel certainly suffered severe damage, and at least part of it likely melted. During this time, the spent fuel stored in pools in the support building surrounding the containment structure was also overheating. This presented a grave dilemma: If that spike in temperature wasn’t stopped, the spent fuel, which wasn’t surrounded by a safety barrier, could release radioactivity directly into the environment.

Despite the failure of the first and second barriers and the venting of radioactive water and steam, a truly major release of radioactivity has been averted—a “major release” being a Chernobyl-style accident in which a large fraction of the fission products escape the plant.

The Chernobyl Nuclear disaster is widely considered to have been the worst power plant accident in history, and along with Fukushima Dai-ichi, is one of only two classified as a level 7 event on the International Nuclear Event Scale.The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.

The Chernobyl disaster was attributed to a flawed system along with human error.  The operating crew on the nuclear plant was planning to test whether the plant’s turbines could produce sufficient energy to keep the coolant pumps running in the event of a loss of power until the emergency diesel generator was activated.

To prevent any interruptions to the power of the reactor, the safety systems were deliberately switched off. To conduct the test, the reactor had to be powered down to 25 percent of its capacity. This procedure did not go according to plan and the reactor power level fell to less than 1 percent. The power therefore had to be slowly increased. But 30 seconds after the start of the test, there was an unexpected power surge. The reactor’s emergency shutdown (which should have halted a chain reaction) failed.

The reactor’s fuel elements ruptured and there was a violent explosion. The 1000-tonne sealing cap on the reactor building was blown off. At temperatures of over 2000°C, the fuel rods melted. The graphite covering of the reactor then ignited. The graphite burned for nine days, churning huge quantities of radiation into the environment. The accident released more radiation than the deliberate dropping of a nuclear bomb on Hiroshima, Japan in August 1945.

With events such as these, society will demand increased safety standards, more rigorous planning, careful checklists, and increased transparency in the whole nuclear political system.

Nuclear security is the most essential element of safe nuclear. According to the International Atomic Energy Agency (IAEA) nuclear security plan can be achieved through “prevention, detection of and response to malicious acts, and Information coordination and analysis.  The Fukushima incident has called for increased transparency in the public and private sector, as the plant’s operator Tokyo Electric Power (TEPCO) received severe scrutiny from the international community because of the problems at the reactor.  Aside from these two important measures, it is also important to have specific protocols and fail proof designs that will hedge or completely eliminate the risk of nuclear disasters occurring from malfunction or unexpected natural disasters.

Sources:

http://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/safety-of-nuclear-power-reactors.aspx

http://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/chernobyl-accident.aspx

http://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/fukushima-accident.aspx