By Earl F. Glynn
Background information on the science of radiation and nuclear energy to help understand the nuclear crisis in Japan
Twenty years ago Mr. Nobuo Asai, a public information manager at a Japanese nuclear power plant, said the subject of nuclear energy and radiation was not part of the school curriculum in Japan and “nobody understands the quantitative concepts of radiation and most people consider that radiation is dangerous” at any level.
But is radiation and nuclear power understood any better in the U.S. today at the time of the problems in the nuclear reactors in Japan?
How many understand the detection of radiation does not necessarily mean the level is harmful? How many understand there is natural radioactivity in almost everything and all radiation cannot be avoided?
Information about radiation and nuclear energy can improve the public’s understanding of the matters in Japan to help everyone decide what the appropriate level of concern should be. Responses should be based on facts and science instead of fear of the unknown.
Atoms of Matter
Matter is composed of atoms of elements.
An atom is composed of a nucleus consisting of protons and neutrons with electrons orbiting about the nucleus.
The “center” of the hydrogen atom, called the nucleus, normally consists of one proton, but in nature sometimes a hydrogen nucleus can have one or two neutrons in addition to the proton.
The number of protons in the nucleus is known as the atomic number of an atom. The total number of protons and neutrons in the nucleus is known as the atomic weight.
Often an atom is denoted with the name of the element following by a dash and its atomic weight.
“Regular” hydrogen is denoted as Hydogen-1.
“Heavy hydrogen” with one neutron in the nucleus is denoted as Hydrogen-2 and is often called deuterium.
An even heavier hydrogen atom with 2 neutrons is named tritium and is denoted Hydrogen-3.
These three kinds of hydrogen, called isotopes, occur in nature, with 99.98% being Hydrogen-1. Regular hydrogen and deuterium are stable, but tritium is radioactive.
Uranium will be mentioned next since it is the most used fuel for nuclear reactors.
The diagram below shows a schematic representation of 2 of the 23 known isotopes of uranium. All isotopes of uranium have 92 protons but differ in the number of neutrons in the nucleus.
All of the isotopes of uranium that occur in nature are radioactive because they are not stable.
Through a complicated radiation decay chain radioactive uranium will decay into stable isotopes of lead. For example, Uranium-238 will decay into Lead-206 over billions of years.
Radiation Types and Protection
Some atoms are not stable and spontaneously decay into another form with the release of energy.
There are three primary forms of radiation with different physical protection safeguards for each:
Alpha decay is through the emission of alpha particles, each of which is the nucleus of a helium atom consisting of two protons and two neutrons. Because these particles are relatively large, and have an electric charge, they are very likely to interact with other atoms as they lose energy. Alpha particles can travel at up to one-twentieth the speed of light, but can be effectively stopped within a few centimeters in air. Alpha particles don’t get very far in the environment. A single sheet of paper will stop most alphas.
Beta decay is through the emission of beta particles (an electron or a positron). Depending on their energy level, beta particles can be stopped by approx. 30 feet of air, half-an-inch of water, or about an eighth of an inch of aluminum.
Gamma decay. Unlike alpha and beta particles, gamma rays are not charged particles but are a wavelength of light similar to X-rays. The main difference between X-rays and gamma rays is their origin. Gamma ray photons can have about 10,000 times more energy than photons in visible light spectrum. Time, distance and shielding are the three main physical protections from gamma radiation. Limiting the time of exposure is an obvious solution. Doubling the distance from a gamma emitter will result in one-fourth the exposure. Shielding from gamma radiation can be complicated.
See also: Understanding Radiation Its Effects and Benefits, Nuclear Energy Institute, 2007.
See also: Potential Health Effects of Radiation Exposure from MIT’s Technology Review.
Any radiation health concern would need to address the specific type of radiation threat.
The health effects of alpha particles depend heavily upon how exposure takes place. External exposure (external to the body) is of far less concern than internal exposure, because alpha particles lack the energy to penetrate the outer dead layer of skin.
However, if alpha emitters have been inhaled, ingested (swallowed), or absorbed into the blood stream, sensitive living tissue can be exposed to alpha radiation. The resulting biological damage increases the risk of cancer; in particular, alpha radiation is known to cause lung cancer in humans when alpha emitters are inhaled.
The greatest exposures to alpha radiation for average citizens comes from the inhalation of radon and its decay products, several of which also emit potent alpha radiation.
Beta radiation can cause both acute and chronic health effects. Acute exposures are uncommon. Contact with a strong beta source from an abandoned industrial instrument is the type of circumstance in which acute exposure could occur. Chronic effects are much more common.
Chronic effects result from fairly low-level exposures over a along period of time. They develop relatively slowly (5 to 30 years for example). The main chronic health effect from radiation is cancer. When taken internally beta emitters can cause tissue damage and increase the risk of cancer. The risk of cancer increases with increasing dose.
Some beta-emitters, such as carbon-14, distribute widely throughout the body. Others accumulate in specific organs and cause chronic exposures:
Because of the gamma ray’s penetrating power and ability to travel great distances, it is considered the primary hazard to the general population during most radiological emergencies. In fact, when the term “radiation sickness” is used to describe the effects of large exposures in short time periods, the most severe damage almost certainly results from gamma radiation.
- Radiation Dose – Radiation Projection
- Ionizing Radiation “Quantities and Units”
- Ionizing Radiation “Harmful effects of radiation”
Splitting the atom in a controlled way is how nuclear energy is produced.
The diagram below shows a neutron colliding with a Uranium-235 atom, which briefly becomes a highly excited Uranium-236 atom. This Uranium-236 atom fissions into two fragments and three neutrons, while releasing energy.
The three neutrons could trigger additional fission reactions if they interact with additional Uranium-235 atoms.
In a nuclear reactor such a chain reaction must be carefully controlled through geometric design and the control rods. The geometric design ensures continuous operation but inhibits run-away exponential growth in fission reactions.
The two fission fragments above, Krypton-92 and Barium-141, are both radioactive and have separate decay paths.
The activity of a radioactive substance decreases over time and this is known as “decay.”
Given a certain level of activity, the time it takes for the activity to reduce to 50 percent of the original amount is known as the half-life. The passage of about 7 half-lives results in a reduction to less than 1 percent of a previous radioactivity level.
Half-lives of various fission decay products can vary from a fraction of a second to millions of years depending on the isotope.
Most fission products go through a number of beta decays in succession.
The beta decay path with half-life time for the Krypton-92 fission product is as follows:
Kr-92 1.8 seconds
Rb-92 4.5 seconds
Sr-92 2.71 hours
Y-92 3.54 hours
Krypton-92 quickly decays to Rubidium-92,which quickly decays to Strontium-92 within minutes. But the decay from Strontium-92 to Yttrium-92 to the stable Zirconium-92 takes days.
The beta decay path for Barium-141 is as follows:
Ba-141 18.27 minutes
La-141 3.92 hours
Ce-141 32.5 days
With the longer half-lives in its decay chain, Barium-141 takes much longer than the Krypton-92 chain to complete its decay.
Fukushima’s No. 3 reactor uses a plutonium-uranium fuel mix, which would result in more decay paths. In addition to being radioactive, Plutonium would create chemical toxicity concerns.
The decay products create additional energy in the form of heat while the fission chain reaction is maintained. Once the fission is shut down, it takes time for the heat created by the decay chains to become negligible.
The problem reactors in Japan are boiling water reactors, while most reactors in the U.S. are pressure water reactors.
In either type, heat from the reactor core is used to create steam to turn a turbine connected to an electric generator.
The core of a nuclear reactor consists of a number of assemblies with uranium or possibly other fuels.
In a boiling water reactor control rods are inserted from underneath the core. These control rods absorb neutrons as a way to control the fission chain reaction process.
Boron is a common material used in fuel rods to absorb neutrons and slow down or halt fission reactions.
Control rods can be partially removed to allow the fission chain reaction to proceed at the desired level.
On Thursday the Korean government said it was sending 56 tons of boric acid to Japan to help with the unfolding nuclear crises.
A Reuters source on Wednesday said the Tokyo Electric Power was considering dispensing boric acid from a helicopter as a fire retardant, but did not mention it also had nuclear properties to retard fission reactions.
Video: Nuclear Power Generator
Shut down. When a nuclear reactor is shut down and the fission chain reaction stops, a significant amount of heat will be produced for some time due to the decay of the fission products.
Over time a reactor core will consist of new and old fuel assemblies. At some point the uranium fuel will be at a level to no longer be effective and will be removed from the core.
In some countries when a fuel assembly has been in a reactor for three to six years, the spent fuel assemblies may be stored under water for 10 to 20 years before being sent for reprocessing or dry cask storage. The pool water acts as a coolant for decay heat and for shielding against the radiation.
See Used Nuclear Fuel Handled with Care from the Nuclear Energy Institute.
Crisis in Japan
The crisis in Japan is still unfolding.
The New York Times reported on Wednesday that “no harmful levels of radioactivity” would travel from Japan to the U.S. according to current estimates from the Nuclear Regulatory Commission.
The Los Angeles Times on Thursday reported that EPA’s “Radnet” is a network of radiation monitors keeping close watch in case radioactive isotopes from Japan reach California soon. The radioactivity levels are expected to be “very low levels” and “well within safe limits.”
President Obama on Thursday said “we do not expect harmful levels of radiation to reach the West Coast, Hawaii, Alaska, or U.S. Territories in the Pacific.” (See video: Obama: Harmful Radiation Not Expected in U.S.)
Public responses in the U.S. to the crisis in Japan should be based on scientific knowledge about what is happening and good engineering practices and judgment, and not fear since many do not understand radiation.
We hope this article contributes to the public understanding of the science behind the events in Japan.
- Japan Radiation Map by Prefecture. Note the “Under Survey” areas are reportedly being censored.
- Six Ways Fukushima is Not Chernobyl, ProPublica, March 18, 2011.
- Small amounts of radiation headed for California, but no health risk seen, Los Angeles Times, March 17, 2011.
- Scientists Project Path of Radiation Plume, New York Times, March 16, 2011.
- Nuclear Energy Institute’s Frequently Asked Questions: Japanese Nuclear Energy Situation, March 16, 2011.
- Fukushima Nuclear Power Plant, Wikipedia
- Nuclear Energy: Just the Facts, Nuclear Energy Institute.
- Radiation Protection from EPA: Alpha Particles, Beta Particles, Gamma Rays, Decay Chains
- Energy from Matter, Nobelprize.org.
- Are the products of nuclear fission radioactive?, Answers.com.
- Nuclear Matters: A Practical Guide, Office of the Under Secretary of Defense.
- Guidelines for Iodine Prophylaxis following Nuclear Accidents, World Health Organization, 1999.
- Nuclear energy information centres in Japan, IAEA Bulletin, Feb. 1990.
- Coal Ash Is More Radioactive than Nuclear Waste, Scientific American, Dec. 2007.
Contact: Earl F Glynn, [email protected], KansasWatchdog.org
Earl Glynn has a BS degree in nuclear engineering.