Answered by Kevin Claver – ESER / Health Physicist
When comparing the events of the bombing of Nagasaki and the Chernobyl accident there are many factors that play a role in the resulting contamination of each area. The following discussion will not be exhaustive; however, it will mention two of the big factors.
Before we talk about the why we need to understand the how. Both nuclear bombs and nuclear reactors undergo a nuclear reaction called fission. Fission involves splitting an atom and releasing energy. When an atom is split it forms two additional atoms (called fission fragments), which are radioactive.
A nuclear bomb is designed to produce a large amount of energy in a very short amount of time (microseconds). All of the energy and all of the fission products are produced in that instant of time. Compare this to a nuclear reactor that is designed to generate energy over a greater period of time (months or years), and fission fragments can build up over the entire time the reactor is operating. Since a reactor can continue to produce fission fragments over a longer period, more radioactivity is contained within the reactor than a nuclear bomb can produce.
The first factor is the nuclear bomb dropped on Nagasaki was detonated at an altitude of approximately 1,650 feet. This was done to maximize the amount of damage from the blast. This allowed for the fission products from the explosion to remain in the atmosphere and be carried away by the wind, and therefore the fission products eventually settled toward the ground over many kilometers. Since the explosions from Chernobyl occurred at ground level, greater amounts of radioactivity from fission products settled close to the reactors. This caused the surrounding area to have high levels of contamination.
The second factor is the half-life of the fission products. A half-life is the time it takes for radioactivity to decay to half of its original value. A large portion of the fission products produced by a nuclear weapon have very short half-lives (minutes or seconds) and decay away very quickly. This is compared to a nuclear reactor, which operates over a long period of time and its fission products with very short half-lives would decay away while the reactor was still operating. Fission products with long half-lives (months or years) would not have enough time to decay away and would build up during the time the reactor was operated. Therefore, fission products, which settle on the ground because of a nuclear weapon would decay away relatively quickly, whereas fission products from a reactor accident would be present for a longer period of time.
Since the radioactive material resulting from the reactor accident was greater in quantity, spread over a smaller area and included radionuclides with long half-lives, thus people are not allowed to live within that area as it may increase the risk of health effects. Currently, people are not allowed to live in the area, however, there are reports describing an ecosystem that is thriving in the evacuated area.
Go outside in the snow and grab two handfuls of loose soil from a flower pot. Drop one handful where you are standing. Throw the other handful as high in the air as you can!
The soil represents the fission products from each of the incidents. The handful dumped on the ground represents Chernobyl. The handful thrown up in the air represents the atomic bomb. Look at the dirt on the snow. Where and how did the thrown snow land? Which handful has more remaining in one place? Come back next week and compare. Even if the dirt is covered by snow it’s still there. If it were fission products it would still have a negative impact on humans.