Nuclear Physics

Understanding nuclear physics is an important task for an informed citizenry. Concepts surrounding radioactivity, nuclear power and nuclear weapons frequently arise in political or social discourse. Misunderstandings and irrational fears can get in the way of recognizing the significant opportunities or threats associated with nuclear technology.

Furthermore, the implications for nuclear physics are all around us. Hospitals use radiation therapy to destroy tumors. Scientists use radiometric dating to measure the age of dinosaur fossils. Around 20% of the electricity in the US is generated in nuclear reactors. We are living in a nuclear world and you need to understand it.

Atomic Structure

An atom consists of electrons, protons, and neutrons, but remarkably is made up of almost entirely empty space. If the nucleus were the size of a marble then the entire atom (orbit of electrons) would be the size of a sports stadium. The number of protons in the nucleus (positive charge) determines the number of electrons (negative charge) in orbit, defining the atom’s chemical identity regardless of the number of neutrons. Nuclei with the same number of protons yet different numbers of neutrons are called isotopes, like “carbon-12” or “carbon-14”. However, while electrons determine chemical bonding, the nucleus is independently responsible for nuclear energy and radioactivity.

Radioactivity and Nuclear Decay

Most nuclei are stable, but some are subject to radioactive decay. Plainly speaking, radioactivity is the explosion of the nucleus. It takes place suddenly and randomly like popcorn popping, and releases a million times more energy per atom than TNT. The debris or shrapnel of this explosion is called radiation and it flies out at great speeds, sometimes approaching the speed of light.

The damage to your cells and molecules caused by this debris is measured in rem. If you get a dose of 100 rem your body will likely repair most of the damage. If you  are hit with 200 rem or more you’ll get sick (radiation poisoning) and your hair might fall out as your body diverts effort to repair the damage. Although truly intense radiation can kill you very quickly, low levels are part of nature. For example, the typical American is exposed to about 0.2 rem per year in nature and people living in Denver are exposed to 0.1 rem more than people in NYC.

Don’t be too scared about radiation though. Did you know that estimates hold that fewer than 2% of Hiroshima victims died of radiation-induced cancer? About 20% of people die of cancers from unknown causes so keep in mind that radioactivity causes excess cancers. A 1% cancer dose (2,500 rem) will have a 1% of change of eventually triggering cancer. So if you are exposed to 100 rem of radiation (4% dose) your chance of getting cancer increases from 20% to 24%.

The odds for an individual, however, get worse when spread over an entire population. For example, if you exposed the entire US population with 0.1 rem then the cancer probability would increased by 0.004% (0.1/2500) or among a 300,000,000 people: 12,000 additional cancers.

Half-life and Radiometric Dating

Polonium-215: 0.0018 second
Bismuth-212: 1 hour
Sodium-24: 15 hours
Iodine-131: 8 days
Iron-59: 1.5 months
Cobalt-60: 5 years
Strontium-90: 30 years
Radium-226: 1,620 years
Carbon-14: 5,730 years
Plutonium-239: 24,000 years
Chlorine-36: 400,000 years
Uranium-235: 710 million years
Potassium-40: 1.3 billion years
Uranium-238: 4.5 billion years

Once a nucleus explodes it is gone, so radioactivity does eventually go away. The time it takes for the radioactivity of a substance to decay to half of its original level is called the half-life. The remaining nuclei then have a 50% chance of decaying during the next half-life. Most radiation is undetectable and harmless because atoms with short half-lives vanish rapidly and those with longer half-lives decay slowly. The most dangerous material is often the intermediate half-life since you are only affected by the decays during your lifetime.

These predictable rates of decay for various materials are very useful in science as natural clocks. One common example is carbon-14 dating. This isotope makes its way into organic material, including humans, and by measuring the rate of decay we can determine when something dies like a tree or ancient leather artifact. The measurement is useful to about 10 half-lives or 57,300 years. Beyond that, scientists will use methods like potassium-40 dating to measure when a rock formed. These realities about radioactive decay can prove facts about geology, biology and archaeology that may otherwise remain elusive.

It is important to note that radioactivity is natural and isn’t contagious. It has existed as a natural phenomenon throughout history and the human body has evolved to deal with low levels. What you have to worry about is the number of rem and keeping it reasonably low.

Nuclear Weapons

Nuclear weapons operate by a chain reaction of neutrons hitting the nucleus of an atom. Since the nucleus is so small they typically pass right through, but with a critical mass of material there is at least one unlucky target. When that nucleus explodes it repeats the process releasing massive amounts of energy. Let’s take a look at the different types of nuclear weapons.

Uranium bombs (used at Hiroshima) are made from purified uranium-235 with a relatively simple design. The material is the problem since the process of enriching uranium (separating uranium-235 from uranium-238 which absorbs neutrons) requires high tech equipment like centrifuges or gas diffusion plants. To detonate the bomb typically a cannon shoots a piece of uranium-235 into another piece at high velocity to reach the critical mass. For uranium bombs it is around 33 pounds of near 100% uranium-235. This why you hear about international concern surrounding rogue state enriching uranium.

Plutonium bombs (used at Nagasaki) are made from plutonium-239 which is easily available as the by-product of nuclear reactors. The bomb, however, is very complex as it requires implosion. To combat premature detonation that stops the chain reaction, this bomb uses a shell of spread-out material surrounded by explosives that compress the plutonium uniformly. Hard? Think of trying to squeeze a water balloon in your hands uniformly. Even though the critical mass is only 13 pounds and the material is easier to attain the technology makes it unlikely that rogue states or terrorist would use this type of bomb.

Hydrogen bombs (most of US arsenal) can be 1,000 times more powerful, but are the toughest to make. They require a uranium or plutonium bomb simply to trigger the hydrogen fusion rapidly before blowing apart. At very high temperatures deuterium and tritium (types of heavy hydrogen) will fuse into helium and release a lot of energy. This is the process going on in the sun that essentially powers our world. As you can imagine the design is very difficult and some of the details are still classified.

Mushroom Cloud

The mushroom shape is induced by the high temperatures in the nuclear explosion.

Nuclear “fallout” can kill more people than the blast itself if detonated near the ground. If a lot of dirt mixes into the fission fragments from the uranium or plutonium then the added weight can bring the radioactivity back to the ground before the short half-life material decays away. This is why people built fallout shelters in the 1950s to wait out the decay to fall below the threshold for radiation illness.

In 2007 the United States disclosed possession of 5,866 warheads and Russia disclosed 4,162 which is where the term mutually assured destruction arose. This is also one of the reasons the 2010 S.T.A.R.T. treaty was considered so important with so many weapons still pointed at each other. Currently nine countries have nuclear capabilities: the United States, Russia, United Kingdom, France, China, India, Pakistan, North Korea and Israel (undeclared.)

Radiological weapons, or “dirty bombs”, use ordinary explosives to spread already radioactive material. The actual low-levels of radioactivity, however, make dirty bombs an unattractive tool to terrorists. While the explosion itself can kill people, a terrorist doesn’t want to sit around for years waiting for people to develop cancer.

Nuclear Power

Similar to a nuclear bomb, a nuclear reactor uses a chain reaction of fission to gain energy from uranium-235 and/or plutonium-239. The difference is that the rate of fission is kept low to create a sustained chain reaction. This involves slowing down the neutrons with a moderator like graphite or water. The constant energy released then heats water, which releases steam that runs a turbine creating electricity. This steam process (even used on nuclear submarines) is similar to coal power plants, but just with a different fuel.

Nuclear Power by Rahm EmanueaelIt is important to know that a nuclear reactor cannot explode like a nuclear bomb. Due to physics, the chain reaction will stop if the neutrons are not slowed down since they will be absorbed by the uranium-238 at higher rates. Remember that reactor-grade uranium is 97% uranium-238, unlike the enriched bomb-grade material. Even if it did explode (like Chernobyl) it runs out of neutrons and breaks up the reactor before more energy can be released. The blast is not as concerning as the radioactive waste released.

The U.S. has just more than 100 nuclear power plants that generate around 20% of our electricity. Why not build more? The answer has more to do with waste than safety. The Chernobyl and Three Mile Island incidents were the result of poor design and human error. The new pebble bed reactor is actually designed so that if everything fails the reaction will slow down by overheating. The primary issue of public concern is how to handle the nuclear waste byproduct of the reactor process.

Nuclear Waste

As I mentioned earlier, plutonium-239 is produced in reactors along with the fission material like strontium-90. Essentially when uranium-238 absorbs a slow neutron, instead of fissioning, it becomes uranium-239, which then decays over time into plutonium-239. This can then be extracted from the waste material for weapons or additional energy in breeder reactors.

It takes 10,000 years for waste to decay back to the radioactive level of mined uranium. This illustrates the problem: how do you store waste for 10,000 years? Thankfully, the radioactivity decreases by 10 fold within 300 years and low probabilities and quantities of leakage make it less concerning. Further, the natural radiation in the Colorado River would be more dangerous than leakage from the U.S. storage facility at Yucca Mountain, Nevada. The conclusion is that while nuclear waste is concerning it is an exaggerated problem.

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