Glossary
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The text of this glossary is everything you need to know to read the Gordian Knot News.
Acute Dose
ALARA
alpha particles
decay, becquerels(Bq)
biological half-life
decay, radioactive
DDSB, Double Double Strand Breaks
DSB, Double Strand Breaks
electrons, beta particles
fission
grays, Gy, mGy
half-life
ionizing
isotope
LNT, Linear No Threshold
photons, gamma rays
sieverts, Sv, mSv
SNT, Sigmoid No Threshold
UCert, Underwriter Certification
Uptake
isotope
Just about all ordinary matter is made up of about 100 elements. The elements in turn are made up of a tiny nucleus surrounded by a cloud of electrons. The nucleus is made up of protons and neutrons. Each element is distinguished by the number of protons in its nucleus; hydrogen has 1 proton, helium has 2 protons, and so on. But the number of neutrons in a hydrogen nucleus can be 0, 1, or 2. Most elements have the capability of accommodating differing numbers of neutrons in their nucleus, at least for a while.
A particular combination of protons and neutrons is called an isotope. Hydrogen nuclei with 0 or 1 or 2 neutrons are all isotopes of hydrogen. There are over a 3000 known isotopes, although most are very rare. A particular isotope is indicated by $^{n}{Xx}$ where the superscript tells us the total number of neutrons and protons, the isotope's mass, and the 1 or 2 letters tells us which element we are talking about (the number of protons). The three isotopes of hydrogen are $^{1}{H}$, $^2{H}$, and $^3H$. Sometimes we will just spell this out, e.g Hydrogen-2.
fission
A few isotopes will split into two much lighter isotopes when hit with a neutron. The only such isotope that occurs naturally in usable amounts is Uranium-235 (92 protons, 143 neutrons) or $^{235}U$. When such an isotope splits or fissions, it releases a remarkable amount of energy, about 50 million times more energy than that created by combining a carbon atom with oxygen to produce CO2. It also releases 2 or 3 neutrons. Under the right conditions, those neutrons can hit another fissionable nucleus producing a self-sustaining chain reaction. The job of a nuclear reactor is to maintain the right conditions for a chain reaction, known as controlling the reactivity, while capturing the energy that is released in the process.
decay, becquerels(Bq)
The lighter isotopes that result from this split are called fission products. Some of these fission products are unstable, combinations of protons and neutrons that cannot stay together for long. These unstable isotopes spontaneously decay to another isotope. We will sometimes talk about becquerels (Bq) which is the number of atoms that decay in one second. If the daughter isotope is also unstable, that isotope will decay to yet another isotope. This process continues until it reaches a stable daughter isotope.
half-life
Each unstable isotope decays at its own rate, which is measured by the isotope's half-life, the time it takes for half of the isotope to decay to something else. Some fission products decay extremely rapidly. They have half-lives that are a small fraction of a second. A few decay very slowly with half-lives of thousands of years. If an isotope has a half-life of 1 year, than half the isotope will have decayed in the first year after its creation, another half in the second year, and so on. Ten half-lives will reduce the isotope to one-thousandth of its original mass.
alpha particles, electrons, photons
When an isotope decays, it releases energy. In a nuclear power plant release, this energy can take one of three very different forms:
1) An alpha particle which is made up of two protons and two neutrons tightly bonded together. Alpha particles have nil penetrating power. They are stopped by the outer dead layer of our skin.
2. An electron similar to the electrons produced by old fashioned, cathode ray televisions. Sometimes referred to as a beta particle. Electrons have very little penetrating power. High energy electrons can damage skin. To do real, harm electron and alpha particles must be ingested or inhaled.
3. A high energy photon. This is the same particle that makes up sunshine, but much higher energy. Sometimes misleadingly called a gamma ray. A photon can penetrate all the way through a human body.
ionizing radiation
Living tissue is made up of cells. Cells are mostly water. If one of the particles created by radioactive decay enters a cell, it transfers a portion of its energy to the cell mainly by breaking the chemical bonds that hold the water molecule together. This is called ionization. Particles with enough energy to do this are called ionizing radiation. Ionization creates highly reactive molecules which can disrupt the cell's chemistry. These reactive molecules are also produced with the cell by our oxygen based metabolism. Nature had to develop mechanisms for handling this onslaught.
dose, grays(Gy), milligrays(mGy)
The amount of energy that a particle deposits in tissue, the dose, is measured in joules per kilogram of tissue. The shorthand for joules per kg (J/kg) is called a gray (Gy). Dose rate is our major concern. Dose rate is the product of the decay rate in Bq and the energy released per decay. The dose rates encountered in a nuclear power plant release are almost always a very small fraction of a gray per day. Therefore, we will usually work in units of mGy/d (one thousandth of a gray per day). The background dose rate on this planet from natural radioactive sources varies from about 0.003 mGy/d to more than 0.02 mGy/d.
sieverts(Sv), millisieverts(mSv)
The original assumption was that the amount of cell damage was proportional to the dose in grays. This proved untenable, so a modified dose was concocted which multiplies the dose in grays by a factor which depends on the type of particle and its energy. For photons and electrons the factor is 1.0. For alpha particles the factor is 20. This modified unit is called a sievert (Sv). If you receive a full body dose of 6 Sv over a short period of time, an hour or two, you will probably die due to bone marrow failure in a few weeks.
Double Strand Breaks (DSB), Double Double Strand Breaks (DDSB)
Radiation damage to our DNA is the big concern since unrepaired DNA damage can propagate to the entire organism in the form of cancer. DNA damage can be divided into single strand breaks (SSB) and double strand breaks (DSB), depending on whether the damage leaves one side of the double helix intact or not. Single strand breaks are repaired almost automatically using the intact strand as a template. SSB repair takes roughly a half hour. In a double strand break, this essentially error-less repair system may not be available. But Nature has endowed us with remarkably effective systems for handling isolated DSB's. DSB repair takes up to about 12 hours. The real problem is two or more closely spaced DSB's, known as Double Double Strand Breaks (DDSB). Some DDSB's may be misreconnected in a manner that is not repairable. Some of these mutated cells may escape our immune system, and some of those may develop into cancer.
Uptake, Biological Half-Life
If an isotope that emits radioactive particles is ingested or inhaled, then uptake, the fraction of that isotope which is absorbed into our organs, and the biological half-life which measures the time that fraction stays in our bodies before it is eliminated in the normal course of events, can become at least as important as the decay rate in Bq and the energy released in each decay.
Linear No Threshold (LNT) model of radiation harm.
LNT is the hypothesis that cancer incidence is proportional to cumulative dose in sieverts. Whether or not the dose was received over a very short time or spread evenly over years is irrelevant. Under LNT the only repair process that is possible is a repair that can be done in zero time.
Sigmoid No Threshold (SNT) model of radiation harm.
SNT is a model of radiation harm which assumes that what counts is the dose in each repair period. SNT further assumes that the repair period harm (increase in cancer incidence) is an S-shaped response to the repair period dose in sieverts. SNT further assumes that each repair period is independent, so the daily harms can be added up. Normally, the repair period is assumed to be a day.
acute, chronic dose
An acute dose is a dose received within a single repair period, such as an xray. A chronic dose is a dose received more or less evenly over many repair periods, such as the natural background dose,
ALARA
ALARA is the regulatory philosophy that nuclear plants must be designed to make all radioactive emissions As Low As Reasonably Achievable. There is no firm limit. The criteria is not whether the benefit of further reduction outweighs the cost. The criteria is: can the plant afford the reduction?
UCert, Underwriter Certification
UCert is a regulatory system in which the government specifies the compensation that will be awarded each person exposed to radiation and requires iron clad insurance for the resulting third party liability. The insurance market then sets the design and operating requirements. This is a variant of the system by which we regulate high pressure steam, ocean transportation, and other beneficial but hazardous activities.