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Category: Nuclear Power, Devices, and Accidents — Nuclear Devices

The following question was answered by an expert in the appropriate field:

Q

How can people build and live in the two cities in Japan that had atomic bombs exploded there? The power plant in Japan and in Russia caused everyone there to move.

A

Good question. And I can see how this might seem a bit odd, but what's happening is that there are different circumstances—nuclear weapons vs. nuclear reactors—that make all the difference. So first let me give a little background, and then I'll see if I can explain the different circumstances that make the difference between these two types of events.

First, some background. Nuclear fission involves splitting an atom and releasing energy. But when we split an atom, it forms two atoms (called fission fragments) that are both radioactive. This is where, for example, cesium-137 (137Cs) and iodine-131 (131I) come from, as well as a host of other fission fragments you might have read about following the Fukushima and Chernobyl accidents. This is where the radioactivity comes from in both nuclear reactors and in nuclear weapons.

We know how much energy is released by a single fission, and we know that each fission produces two fission fragments. We also know that not every fission produces the exact same fission fragments—about 6% of the time, for example, we'll see something with a mass of 137 protons and neutrons, and about 5% of the time one of the fission fragments will have a mass of around 90 protons and neutrons. So we can calculate how much 137Cs (and any other fission product) from pretty much any size of nuclear fission weapon as well as from nuclear reactors (based on their power history).

There's one other piece of information that's necessary to be able to compare nuclear weapons to nuclear reactors: the amount of energy they produce. In a nuclear reactor, the energy is generated over a period of time (weeks, months, and years), and fission fragments are building up the entire time a reactor is operating. In a nuclear weapon, all of the energy comes out in an instant. So a nuclear reactor can produce radioactive fission fragments continuously over a long period of time while a nuclear weapon produces them in one shot. And in the "for what it's worth" category, a 20 kiloton (kT) nuclear weapon produces as much energy as a 1,000 megawatt (MW) nuclear reactor that's operating for one day. A 1,000 MW reactor that's operating for a longer period of time will be producing more fission products.

All of this is common to both nuclear reactors and weapons; from here, however, things diverge a little bit.

Nuclear reactors use the energy from fissioning atoms to heat water (or gas or some other heat-transfer medium). This hot fluid then transfers the heat to ultimately produce electricity. In a water-cooled reactor, for example, the hot water will be used to produce steam, and the steam then spins turbines in order to produce electricity. As I mentioned earlier, we know how much energy is released from a single fission, and so we can easily calculate how many fissions it takes, for example, to run a 1,000 MW reactor for a given amount of time to produce electricity. There are other factors to consider also, but addressing them starts to add to the complexity of the answer, and they're not essential to the final answer. Let it suffice to say that we have a pretty good idea how much radioactivity is produced, no matter how long a reactor has been operating, the power it was operating at, and so forth. This is the amount of radioactivity that's available to cause contamination in the event of a reactor accident.

In a nuclear weapon, all of the fissions take place in the blink of an eye—a few microseconds. In that time, we get all of the energy and all of the fission products that are going to be produced. This is one of the differences between nuclear weapons and nuclear reactors—nuclear reactors can build up fission fragments over a much longer period of time so they will have more radioactivity in the core than a nuclear weapon will produce.

Another factor is the altitude at which the radioactive fission products are released. The weapons used in Japan were deliberately set off more than 300 meters (m) above ground in order to maximize the damage from the blast. This meant that most of the fission products from the explosion stayed in the atmosphere where they were blown away by the winds—they settled to the ground over many kilometers, and a lot of them ended up coming down over the ocean. So in the case of the nuclear weapons there was less radioactivity, and the fission products came to earth over a large area. In the case of the nuclear reactor accidents, there was more radioactivity from fission products that came to earth fairly close to the reactors.

The last major factor is the half-life of the radioactive fission products that are produced. Most of the fission products that are produced immediately after fission are short lived so they decay away rapidly. In a reactor, on the other hand, the short-lived radioactivity has been decaying away the whole time the reactor has been operating—more is produced all the time, but the older fission products (the ones produced months or years ago) are longer lived, and they're going to be around for a while. This means that after a nuclear explosion, the radioactive fission products that settle to the ground decay away fairly quickly, while after a reactor accident, the radioactive fission products will be around for a longer period of time.

One final thing to consider: our standards have changed over the last 72 years. I really don’t know how much contamination there was in Hiroshima and Nagasaki after the bombs went off. But I DO know that, at that time, there were few (if any) accepted standards for reoccupying contaminated territory, and what radiation standards did exist at that time were far less stringent than those we have today. It's not that people were cavalier back then, it's just that we had very little experience working with radiation and radioactivity. What was acceptable in a devastated Japan in 1945 might not be acceptable in today's Japan (or Ukraine or anywhere else).

So, here's a quick summary:

• Nuclear reactors produce more radioactive fission products than nuclear weapons.
• More of the fission products will settle to the ground following a reactor accident.
• The radioactive fission products produced in a reactor tend to be longer lived by the time they're released into the environment.
• The regulatory standards for living in contaminated areas are far different today than they were in 1945.

OK, here's one last thing. These factors help to explain why it was possible to move back into Hiroshima and Nagasaki but not back into the area around Chernobyl and Fukushima. But even though people are still not permitted to live in these areas, this doesn't mean that radiation levels are dangerous. I have not been to the Chernobyl area, but there are plenty of reports about the thriving ecosystem in the evacuated area, and there have been plenty of people working in the area for years or even a few decades. Radiation dose rates, while elevated, are clearly not life threatening. On the other hand, I have been to the area around Fukushima—just a month after the accident there. The dose rates I measured were elevated, but they also posed no risk in either the short or the long term. Here, too, there have been people working and returning for personal property with no observed health effects. This doesn't mean that there's no risk associated with living in these areas, but the presence of thriving wildlife and the lack of apparent health effects on the people who enter these areas suggests that the risks might not be as high as implied by the evacuation.

P. Andrew Karam, PhD, CHP

Answer posted on 13 November 2017. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.