The Worst Nuclear Accidents: What Really Happened at Chernobyl and Fukushima

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Fukushima

Ranking nuclear accidents, Chernobyl and Fukushima were, on a scale of 1 to 7, a 7 in severity. But these accidents were not the same. With different site designs, the level of radiation released and the degree of harm to workers and the general public differed significantly.

Chernobyl Nuclear Plant Design

Chernobyl site and plant: The Chernobyl complex consists of four nuclear reactors, with two additional reactors under construction at the time of the accident in 1986. The Soviet-designed Chernobyl reactor, RBMK-1000, is a graphite-moderated boiling water reactor.

How the RBMK-1000 Works

Put simply, the fuel rods are zirconium alloy clad and contain uranium dioxide fuel. Inserted into a graphite moderator, water is pumped from the bottom through the fuel channels as it heats, boils, and turns to steam. The steam then drives the turbines.

Control rods, in case of emergency or energy control, can be lowered into the core to control the power output, or shut the reactor down. The RBMK design does not have a primary containment, which is typically a structure of steel and concrete around the reactor vessel built to withstand a pressure increase, and contain radioactive release in the event of an accident. After the accident at Unit-4, Units 1-3 continued operation. The last one was shut down in 2000. There are 11 other RBMK’s still operating in Russia.

Fukushima by Lincun

Fukushima Nuclear Power Plant Design

The complex consists of Fukushima Dai-ichi 1, 2, 3, 4, 5, 6, and Fukushima Daini 1, 2, 3, 4, one of the worlds largest reactor complexes. Fukushima Dai-ichi are GE boiling water reactors (BWR) designed in the 1960s, and built in the 1970s.

How the BWR Works

Each fuel assembly is zirconium clad with uranium dioxide fuel. As water is pumped up through the core, it is heated and turned to steam, thus driving the turbines. The design protecting against radiation release is control rod insertion, in case of emergency, to control the fission of the core. The second defense is a primary containment vessel. Built of reinforced steel in a concrete shell, it surrounds the reactor pressure vessel containing the fuel rods. A suppression pool, filled with water, located below the reactor vessel and part of the primary containment vessel, acts as a heat sink in case of an accident. Dai-ichi units 1, 2, 3, and 4 were involved in the accident described below.

The Chernobyl Accident

On April 26, 1986, Unit 4 of the nuclear power station at Chernobyl had a severe and life-threatening accident that released massive amounts of radiation into the environment. During a test of the turbines, an operator made errors in disabling some automatic shutdown options, causing the rapidly unstable fuel to be impossible to control. The operator tried to control the core reaction by inserting control rods, but due to the hot-fuel-and-water reaction, the control rods jammed the channels halfway down. Steam generation then spread throughout the core, causing an explosion and releasing fission products into the air. A few seconds later, a second explosion threw fuel fragments and hot graphite skyward. A number of fires started, and burned for days, releasing large amounts of radioactive elements into the environment. With no primary containment the release of radioactivity was staggering.

Fukishima Accident

On March 11, 2011, Japan and subsequently, Fukushima, was hit by a 9.0 earthquake and its resulting tsunami. Fukushima Dai-ichi 1, 2, 3, and 4 were involved in  a severe accident as a result.

The plants shut down effectively after the earthquake, but were damaged by the tsunamis that followed. The first tsunami wave hit approximately 41 minutes later, with a second wave which caused the submergence of the emergency diesel generators, electrical switchgear, and batteries. This plunged the station into an electrical blackout, isolating the reactors from their ultimate heat sink. In shutdown mode, with no way to achieve heat removal from the core, steam production increased, reactor vessel pressure rose significantly, and the water level covering the core lowered.

  • The core started melt down, resulting in the generation and release of large amounts of hydrogen, which escaped from the primary containment.
  • Units 1, 2, and 3 had hydrogen explosions which blew off the roof and walls of the top part of the reactor buildings surrounding the primary containment.
  • Unit 2 also experienced a rupture in the suppression pool.
  • The fuel for Unit 4 was stored in a fuel pool where a fire started. Volatile and easily airborne radionuclides , fission products, were carried out with the steam and hydrogen.

Radioactivity at Chernobyl and Fukushima

Both plants released significant amounts of radionuclides. For Fukushima, these were limited to outgased products with the most significant being I-131 (half-life, 8 days), Cs-137 (half life, 30 years) and Cs- 134 (half life, 2 years). The radiation at the point of the accident became 1000 times the normal on-site dose rates but has decreased significantly over time. Cs- 137, the problem, travels easily in the environment and very difficult to clean up. No radiation illness or death caused by the Fukushima accident has occurred.

Chernobyl vs Fukushima

Chernobyl’s accident spewed core material into the air. With no primary containment to block the release, many more fission products and core material became airborne; approximately 100+ times that of the Fukushima release. Among the 600 workers present, 2 died within hours and 28 died within the first few months, from radiation sickness. Exposure of  106 more to high levels have yet not resulted in death. The spread of radionuclides affected millions of people across Europe. Today, an increase of thyroid cancers have been detected.

Differences Between Fukushima and Chernobyl

Comparison of the two level 7 accidents shows that they are quite different. The power plants and radionuclide differences caused different dispersion patterns. The Fukushima reactors, protected by the primary containment vessel, released far less radioactivity than Chernobyl, which had no primary containment. Couple the Chernobyl design flaws with poorly-trained operators, and it becomes obvious why this was truly a nuclear disaster, the long-term effects of which are still being studied.

Sources

NRC. Backgrounder on Chernobyl Nuclear Power Plant Accident. Accessed Nov. 12, 2011.

BBC News. How does Fukushima differ from Chernobyl? (2011). Accessed Nov. 12, 2011.

World Nuclear Organization. Fukushima Accident 2011. (2011). Accessed Nov. 12, 2011.

© Copyright 2011 Judy Haar, All rights Reserved. Written For: Decoded Science

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