Appendices

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A

absorbed dose

amount of energy deposited in a unit mass of some material by ionising radiation. Measured in grays or rads.

accelerator driven system

a reactor, which use a proton accelerator to produce neutrons to fission the fuel. Because the reactor does not depend on neutrons from fission to keep the reaction going, it is technically subcritical. This means it can take a wide variety of fuels, including isotopes extracted from spent fuel, which would be otherwise unuseable.

ACR

see advanced CANDU reactor.

actinide

a series of heavy metals ranging from atomic number 89 to 103 including thorium (90), uranium (92) and plutonium (94).

activation energy

energy required by reactants before the can undergo a given reaction.

activation product

in the context of nuclear power, previously stable isotopes that have been transmuted into radioisotopes through neutron capture.

activity

rate of nuclear decay. The more nuclei of a sample that decay in a shorter time, the higher the activity. Activity depends on the stability of an isotope but also on the size of the sample. Halving the size of a sample of a given isotope, will halve the activity. It measured in becquerels (SI) or curies (English).

advanced CANDU reactor

These are the generation III+ contributions from Atomic Energy of Canada Limited building upon and improving their innovatve and successful CANDU designs.

advanced gas cooled reactor

This generation II British reactor is graphite moderated and carbon dioxide cooled. They are currently the mainstay of the British fleet, but are inferior to LWRs and such will not be developed further.

ADS

see accelerator driven system.

AECL

Atomic Energy of Canada Ltd. They specialise in a family of heavy water reactor known as the CANDU.

AGR

see advanced gas cooled reactor.

ALARA

As Low As Reasonably Achievable. Principle currently governing radiological presentation to the public and workers, which is based upon the Linear No Threshold hypothesis rather a Threshold hypothesis reflecting the latest understanding of the effects of radiation dosage.

alpha particle

helium-4 nuclei expelled from an unstable nucleus during nuclear decay. The alpha particle is the most ionising kind of nuclear radiation but because of this, it will be absorbed by a sheet of paper or a few centimetres of air.

atomic mass

rest mass of an atom relative to a measurement that gives carbon-12 a rest mass of 12.

atomic number

number of protons in the nucleus of an element. It is atomic number that determines the chemical identity of an atom. It is also known as the proton number.

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B

back end (fuel cycle)

the portion of the fuel cycle, which happens after irradiated fuel has been removed from the reactor, which involves processing and disposal of waste, in whatever form that may be.

becquerel

unit of activity, symbol Bq. 1 Bq is the activity of a sample when one nucleus decays each second.

beta particle

light weight, ionising particle released during nuclear decay. The beta negative particle is a negatively charged electron, while the beta positive particle is a positively charged positron. In terms of ionisation, they are equivalent and differ only in the sign of the charge, hence why they could not at first be distinguished from each other. Beta particles will be absorbed by a plank of wood or twenty centimetres of air.

binding energy

equivalent energy of the mass defect under the formula E=mc². The binding energy is the energy released when the nucleons are brought together to form a nucleus and hence is the energy that must be put into the nucleus to separate the nucleons again.

boiling water reactor

This common design uses water as both coolant and moderator but without the high pressures of a PWR. The water is allowed to boil and is used to drive the turbines in a direct cycle.

BWR

see boiling water reactor.

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C

calandria

component of the CANDU reactor design. It is the tank, which contains heavy water for moderation. The pressure tubes containing the fuel run through the calandria.

CANDU

Canadian-Deuterium-Uranium. It is a series of pressurised heavy water reactors reactors designed by Atomic Energy of Canada Limited featuring on-the-fly refuelling and a high degree of fuel cycle flexibility.

capacity factor

average percentage of time when the reactor is online.

centrifuge enrichment

modern enrichment process which seperates isotopes of gasified uranium in a centrifuge. By virtue of uranium-238 being heavier than uranium-235, the former will tend to settle towards the outside more than the latter. It is generally 50 times more energy efficient than the old gas diffusion enrichment method.

closed cycle (cooling)

ccoling system which uses cooling towers to dissipate waste heat without constantly drawing and discharging water to and from a local sink such as a river or sea.

closed cycle (fuel)

fuel cycle, which involves the reprocessing of spent fuel to extract any useful material and sending it back in some form into the reactor.

confinement (fusion)

the process of containing the reacting plasma to ensure the nuclei can move close enough to each other to fuse.

containment structure

steel reinforced concrete suprastructure enclosing the parts of a nuclear reactor containing radioactive material. It is designed to maintain containment, even in the event of a total core meltdown and to protect its contents in the event of extreme external events such as large aircraft collisions.

critical

description of a reactor that is in a state of criticality.

criticality

the condition in a reactor where for every neutron that causes a fission, one neutron released from that fission will cause another fission leading to a constant power level.

curie

US unit of activity, symbol Ci. It is defined such that 12 Ci is the activity of 12g of radium-226. 1 Ci is equivalent to 37 billion Bq.

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D

daughter nucleus

product of a nuclear reaction where a nucleus changes its form.

decay

see radioactive decay.

defense-in-depth

engineering principle of nuclear safety engineering, which specifies the use of multiple redundand and indepedent barriers between the outside environment and the hazardous materials within the reactor.

delayed neutrons

neutrons released from the decay of neutron rich fission products. This tends to happen around 80s after the fission, which created the products in the first place. Although delayed neutrons account for less than 1% of neutrons produced from fission, they are essential to controllability.

depleted uranium

uranium metal composed of purely uranium-238 having had the uranium-235 removed.

deuterium

hydrogen-2, symbol D. Isotope of hydrogen (atomic number 1) with one neutron. It is also known as heavy hydrogen.

diffusion enrichment

old enrichment process, which uses the diffusion of gasified uranium through a porous membrane. Because uranium-235 is lighter than uranium-238, the former will be more likely to diffuse through the membrane than the latter.

dirty bomb

colloquial name for radiological dispersal device.

DoE

US Department of Energy.

dose equivalent

absorbed dose of ionising in human tissue corrected for the quality factor of the particle responsible. Measured in sieverts or rems.

DU

see depleted uranium.

DUPIC

Direct Use of spent PWR fuel In CANDU. It is the process of physically converting spent PWR fuel into fuel bundles for use in CANDUs in order to reduce HLW. Though a PWR is unable to sustain criticality on this fuel, hence why it is termed "spent", the heavy water moderation of the CANDU allows for criticality on the remaining trans-uranics.

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E

EBR

Experimental Breeder Reactor. This was the first nuclear reactor to power an electrical appliance, specifically four light bulbs.

electrical power output

rate of useful electricity production. For example, a 1GWe reactor will produce useful electricity to grid at a rate of one gigawatt.

electron

light, negatively charged particle. Electrons in the atom occupy the majority of the volume of the atom, but contribute only a miniscule amount of mass. The interaction of electrons in an atom with those in other atoms is the basis for chemical reactions.

electron-volt

unit of energy defined as the energy one electron gains when accelerated through one volt of potential difference. There are around six billion billion electron volts in a joule.

engineering breakeven

the circumstance when the energy produced in a reactor exceeds the energy required to make the reaction work in the first place.

enrichment

the process of increasing the proportion of a particular isotope in a batch of an element. It is most commonly applied to uranium, which is naturally low in the -235 isotope (0.7%), but is enriched to increase the proportion to around 3-5%.

ESBWR

Economic and Simplifed Boiling Water Reactor. Generation III+ BWR design from General Electric. Most notable advanced features is a core, which is able to safely transfer heat during operation purely by the natural process of convection rather than requiring the coolant to pumped by engineered components.

EPR

European Power Reactor. A Generation III+ 1600MWe PWR designed by Franco-German company Areva. The first under construction is Olkiluoto-3 in Finland.

eV

see electron-volt.

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F

fast breeder reactor

fission reactor that depends solely on fast neutrons for sustaining the reaction. It is configured so that these fast neutrons breed more fissile material in a fertile blanket than is consumed in the core.

fast burner reactor

fission reactor that depends solely on fast neutrons for sustaining the reaction. It lacks the fertile blanket of a fast breeder reactor and so consumes more fissile material than it breeds.

fast neutrons

neutrons with an energy of the order of millions of electron volts. Fast neutrons in the reactor are released directly from fissioning nuclei and neutron rich fission products.

FBR

see fast breeder reactor.

fertile

in the context of nuclear power, the description of an isotope that can be bred, through neutron capture, into a fissile isotope.

fissile

description of an isotope that will undergo fission upon absorption of a suitable neutron.

fission

nuclear process where a heavy, unstable nucleus splits into two daughter nuclei releasing neutrons and a large amount of energy.

fission product

nuclide produced from the fission of a heavier nuclide.

front end (fuel cycle)

portion of the fuel cycle, which happens before fuel is burnt in the reactor, which involves the extraction of any raw materials, any chemical or physical processing, enrichment and fabrication.

FUD

fear, uncertainty and doubt. It is the rhetorical technique of eroding faith in something through the use of hypothetical or exaggerated concerns.

fusion

nuclear process where two light nuclei combine together to form a heavier nucleus, releasing an enormous amount of energy.

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G

gamma particle

very high energy photon released in nuclear processes. Gamma radiation is electromagnetic radiation like radio or visible light, but of a much higher energy. It is not very ionising compared to other nuclear radiation and as such requires large quantities of concrete or lead to attenuate it.

geiger counter

device to measure activity. It consists of a geiger-muller tube that sparks whenever nuclear radiation enters it, and a counter that counts every time the G-M tube has a spark. Of course, the geiger counter only records how much nuclear radiation it itself receives and so must be used with thought.

Generation I

Early commercial nuclear reactors, built in the 1950s and 1960s. The only ones currently still operating are the British Magnox reactors.

Generation II

Large scale, economic commercial nuclear reactors built in the 1960s, 70s and 80s. They comprise most of the reactors currently operating in the West.

Generation III

Evolution of Generation II reactors, built in the 1990s with improvements in all areas such as waste reduction, economics and safety.

Generation III+

Further evolution of the Generation III. Generation III+ are currently being licensed around the world. Most notably, they include the AECL ACR-1000 PHWR, the GE ESBWR, the Westinghouse AP-1000 PWR and the Areva EPR PWR.

Generation IV

Revolutionary new designs of nuclear reactors expected to come on stream in the next several decades. Many of them are fast and incorporate recycling as part of their design.

gigawatt-year

the energy produced by a 1GW reactor operating for one year or equivalent product of power and time. A 1GW reactor represents large size of reactor in service so a gigawatt-year represents a large amount of energy on a commercial scale. When referring to, for example, waste per gigawatt-year, it is meant the waste produced by the operation of a 1GW reactor over the course of a year or equivalent.

gray

unit of absorbed dose. 1 Gy is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of a substance.

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H

half-life

the interval of time in which half a sample of an isotope will decay, and hence the activity of the sample will halve (ignoring activity from unstable daughter nuclei).

heavy water

water that has deuterium in place of normal hydrogen. Chemically, heavy water behaves exactly the same as normal water, if marginally heavier, but the deuterium can act as a better moderator than light water.

heavy water reactor

This type uses heavy water for the moderator like the CANDU.

HEU

see high-enriched uranium.

high-enriched uranium

uranium enriched to more than 20% uranium-235, the rest being uranium-238.

high level waste

very radioactive waste requiring shielding and cooling, generally in the form of spent fuel and fission products. It is 3% of the total volume of radwaste but has 95% of the total radioactivity.

high temperature reactor

This type uses helium coolant and graphite moderation to achieve much higher temperatures and hence thermodynamic efficiency. The fuel is in the form of sphere or prisms rather than rods. The PBMR is an example.

HLW

see high level waste

hormesis

biological theory that organisms are made more resilient by low level exposure to a substance, which is toxic in larger doses.

HTR

see high temperature reactor.

HWR

see heavy water reactor.

hydrogen

lightest element, atomic number 1, symbol H. It comprises 70% of all visible matter in the known universe and is a major constituent of water and hydrocarbons.

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I

IAEA

International Atomic Energy Agency.

ICRP

International Commission on Radiological Protection.

ILW

see intermediate level waste

INES

see International Nuclear Event Scale.

intermediate level waste

radwaste of moderately high activity requiring some shielding. It is generally in the form of some chemical by-products from other stages of fuel cycle, activated metal cladding from spent fuel and activated reactor materials from decommissioning. It is 7% of the total volume of radwaste but only 4% of the total activity.

International Nuclear Event Scale

scale used to measure the severity of a nuclear accident, malfunction or anomaly. It rates an event from 0, meaning no safety significance , to 7, meaning major accident with widespread contamination.

ionisation

removal of one or more electrons from an atom. It can disrupt the structure of complex molecules like DNA. It can be caused by the nuclear radiation among other things.

ionising radiation

radiation of sufficient energy to ionise matter.

isotope

nucleus of an element with varying amounts of neutrons. Carbon-12 and carbon-14 are different isotopes of carbon, one with six neutrons, one with eight. The suffix number is the mass number.

ITER

International Thermonuclear Experimental Reactor. International project to build a large fusion reactor, which will sustain a fusion reaction for at least 10 minutes and achieve engineering breakeven.

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J

JET

Joint European Torus. An experimental Tokamak fusion reactor based at Oxford, which achieved a fusion reaction for a split second. It will have been the largest fusion project until ITER.

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L

laser enrichment

experimental enrichment technique, not yet in commercial use, but offers great saving in energy over centrifuge enrichment in the future when it is commercialised.

LEU

see low-enriched uranium.

light water

water, whose hydrogen is the most common isotope, hydrogen-1. The vast majority of water is light water.

light water reactor

general term for PWRs and BWRs, which use light water as coolant and moderator.

linear no-threshold hypothesis

the mathematical hypothesis that a toxic substance is harmful at all dosages big and small and that the harmful effects are directly proportional to the dosage.

LLW

see low level waste

LNT

see linear no-threshold hypothesis

low-enriched uranium

uranium enriched in -235 above the natural 0.7% but to below 20%, the rest being uranium-238.

low level waste

generally common materials contaminated with small quantities of radioactive material such as tools, clothing, filters, not just from nuclear power stations but also from hospitals and industry. It is 90% of the total volume of radwaste but only 1% of the total radioactivity.

LWR

see light water reactor.

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M

magnox

Generation I British reactor and the only generation I reactor design still in service. It is graphite moderated and carbon dioxide cooled and uses natural uranium. It was a descendent of reactors originally intended for the production of nuclear weapons.

mass defect

difference in mass of a nucleus and the total mass of the unbound consituent nucleons.

mass number

total number of nucleons in the nucleus of an atom. Mass number of a nucleus of an element describes the isotope. It is also known as the nucleon number.

moderator

material that can slow down neutrons. Moderator is used to slow neutrons in a thermal fission reactor so that they can cause fission in the fissile nuclei.

MOX

mixed oxide fuel. The actinide composition is 93% uranium and 7% plutonium.

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N

NDA

Nuclear Decommissioning Authority (UK).

NER

see nuclear electric rocket

neutrino

very light, electrically neutral particle. Neutrinos or anti-neutrinos are routinely released in nuclear processes, but are usually only important for making the calculations work out.

neutron

heavy, electrically neutral particle that helps to make up the nucleus. Depending on conditions and their energy, they can be absorbed by susceptible nuclei and cause fissile nuclei to fission.

non-proliferation treaty

international agreement, first signed in 1968 and since developed and expanded, to stop the spread of nuclear weapons by restricting the trade of all nuclear technology and materials to signatory nations, which agree to full compliance with international safeguards.

NPT

see non-proliferation treaty.

NRC

Nuclear Regulatory Commission (USA).

NTR

see nuclear thermal rocket

nuclear electric rocket

propulsion technology, which uses a nuclear reactor to provide electric power to an ion drive.

nuclear radiation

ionising radiation released through nuclear processes. Principally, this includes alpha, beta, gamma and neutron radiation.

nuclear thermal rocket

propulsion technology, which uses a nuclear fission reactor to superheat an inert gas to povide large thrusts with low fuel consumption.

nuclei

plural of nucleus.

nucleon

constituent particles of a nucleus. Protons and neutrons are nucleons.

nucleus

central component of the atom. It is about 1% the size of the atom but accounts for almost all of its mass. It composed of predominantly of protons and neutrons.

nuclide

term for a nucleus of specific atomic and mass number. There is a somewhat debateable distinction between this term and the isotope, and often used interchangeably.

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O

open cycle (cooling)

cooling system which takes water directly from a large sink such as a river or sea, uses it to absorb the waste heat and then discharges the heated water back to the sink.

open cycle (fuel)

fuel cycle, which uses fresh uranium once through the reactor and then treats the spent fuel as waste.

Orion

nuclear propulsion concept. It involved propelling a spacecraft by detonating nuclear explosives behind a shield at the base of the spacecraft.

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P

PBMR

see pebble bed modular reactor.

pebble bed modular reactor

HTR being developed in South Africa. It uses fuel encased in spheres of graphite and silicon carbide to ensure fission product containment and meltdown resistance.

PHWR

see pressurised heavy water reactor.

plutonium

heavy metal, atomic number 94, symbol Pu. It is not found naturally, but is bred from uranium, mainly as -239, but also as -240 and -241 with -238 for special applications.

positron

anti-particle of the electron. It has similar properties, except most notably, equal and opposite electric charge.

pre-ignition

the process of a nuclear bomb blowing itself apart from the initial energy buildup before it can release the vast amounts of heat typically expected from them.

pressurised heavy water reactor

PWR, which uses heavy water instead of light water, such as the CANDU.

pressurised water reactor

This is the most common design of power reactor. It uses water at very high pressure to serve as both coolant and moderator. By keeping the water at high pressure, it ensures that it remain liquid even at high temperatures.

primary cooling system

first stage of reactor cooling, involving coolant passing through the core taking heat directly from the fuel.

prompt criticality

the condition where a reactor is critical purely due to prompt neutrons without needing the help of delayed neutrons. A reactor in a state of prompt criticality is uncontrollable.

prompt neutrons

neutrons released directly from fission of a nucleus. Because they are released immediately, the time between the fission of the original nucleus and the subsequent fissioning of other nuclei due to the neutrons released, is very small, a small fraction of a second. A reactor critical on prompt neutrons alone changes power too quickly to be controllable.

proton

heavy, positively charged particle that helps to make up the nucleus. The number of protons determines the structure of the electrons in the atom and hence determines the element.

proton number

alternative name for atomic number.

PWR

see pressurised water reactor.

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R

rad

US unit of absorbed dose. 100 rads is equivalent to 1 Gy.

radwaste

waste produced by nuclear activities classified by regulatory agencies as radioactive and requiring handling accordingly. These are divided into low level waste, intermediate level waste and high level waste.

radiation

see ionising radiation or nuclear radiation.

radioactive decay

nuclear process where an unstable nucleus changes to another nucleus by the release of nuclear radiation.

radioisotope

radioactively unstable isotope.

radioisotopic heating unit

small heat device run by the decay of a radioactive element commonly used to keep electronic components within operating temperatures in cold conditions.

radioisotopic thermoelectric generator

This is a power source for small, long term and remote devices like deep spacecraft. It uses the heat produced by the decay of a radioactive sample to generate a voltage that can be used to power electrical devices.

radiological dispersal device

conventional explosive laced with radioactive material. The chemical explosion is intended to disperse the radioactive material over a wide area.

radionuclide

radioactive nuclide.

RBMK

Soviet reactor design. It was light water cooled and graphite moderated. It was known for being a wreckless design, a fear which was proven when the Chernobyl 4 RBMK suffered a total core meltdown.

RCS

see reactor cooling system

RDD

see radiological dispersal device.

reactor cooling system

coolant loop, which is directly responsible for carrying heat away from the fissioning fuel. It is also known as the primary cooling system.

rem

US unit of dose equivalent of ionising radiation in human tissue. 100 rems is equivalent to 1 Sv.

RHU

see radiological heating unit.

RTG

see radioisotopic thermoelectric generator.

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S

SCRAM

Emergency reactor shutdown. It originally stood for Security Control Rod Axe Man referring to the practise in the prototype reactors of performing an emergency shutdown by physically cutting the rope suspending the control rods. Nuclear technology has come some way since then.

secondary cooling system

in two loop reactors such as PWRs, it is the cooling system, which carries heat from the primary cooling system, passed through a heat exchanger to the turbines to generate electricity.

SI

Systeme Internationale. The internation standard system of measurements used by scientists and engineers worldwide. It shares many units with the metric system.

sievert

unit of dose equivalent of ionising radiation in human tissue, symbol Sv. 1 Sv is the amount of radiation required to dump 1 joule of energy in 1 kilogram of human tissue.

spent fuel

fuel, which has been irradiated in the reactor for long enough that it is no longer capable of sustaining criticality in it.

steam generator

component of a two loop reactor design where heat carried from the fissioning fuel through the RCS is used to boil water to drive the electricity generating turbines.

subcritical

description of a reactor in a condition of subcriticality.

subcriticality

the condition in a reactor where for every neutron that causes a fission, less than one neutron released from that fission will go on to causes another fission on average, leading to a dropping power level.

supercritical (reactor)

description of a reactor that is in a condition of supercriticality.

supercritical (liquid or gas)

material that is under very high pressure and temperature so that it exists as both liquid and gas at the same time. Supercritical water is the coolant proposed for a type of Generation IV reactor design.

supercriticality

the condition in a reactor where for every neutron that causes a fission, more than one neutron released from that fission will go on to cause another on average, leading to an increasing power level.

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T

temperature coefficient

Propert of a reactor, which determines how it will react to an change in temperature. A positive temperature coefficient results in reactivity increasing as a result of a temperature rise, generally a bad thing. A negative temperature coefficient results in reactivity decreasing as a result of a temperature rise, generally a good thing.

tertiary cooling system

term sometimes used for the cooling system, which removes waste heat from the coolant after passing through the turbines and delivers it to a cold sink such as the cooling towers or a river or sea.

thermal efficiency

the percentage of heat generated in a reactor that is turned into useful electricity. For any thermodynamic heat engine, be it nuclear powered or fossil fuelled, this is generally between 25 and 40%. A thermal efficiency of 40% will mean a 3GWt reactor will generate 1.2GWe.

thermal neutrons

neutrons with an energy of the order of tenths or hundredths of an electron volt. Thermals neutrons in the reactor were originally fast, but were slowed down by the moderator.

thermal power output

the raw rate of heat production in the reactor. For example, 3GWt means the reactor will be generating heat at a rate of three gigawatts.

thermal reactor

fission reactor, which depends on thermal neutrons to sustain the reaction.

thermonuclear

description of a reaction or device that uses nuclear fusion.

thorium

heavy metal, atomic number 90, symbol Th. It is three time more abundant than uranium and an important future nuclear fuel. Natural thorium is almost entirely made from the -232 isotope.

threshold hypothesis

model of exposure to a toxic substance, which says that damaging effects only occur above a threshold dose and exposures under that threshold do not cause harm.

TMI

Three Mile Island. Power station in Pennsylvania. Unit 2 suffered a partial core meltdown in 1979.

Tokamak

type of fusion reactor with a toroidal shape, which uses magnetic confinement.

tritium

hydrogen-3, symbol T. Isotope of hydrogen (atomic number 1) with two neutrons.

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U

UNSCEAR

United Nation Scientific Committee on the Effects of Atomic Radiation.

uranium

heavy metal, atomic number 92, symbol U, the primary fuel in current nuclear reactors. It is found naturally as 99.3% -238 and 0.7% -235.

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V

vitrification

The process of incorporating hazardous materials in blocks of glass where it will remain immobile and contained for long periods of time.

void coefficient

Property of a reactor, which determines how it will react to the formations of voids in the coolant. A positive void coefficient results in reactivity increasing as a result of the formation of voids, generally a bad thing. A negative void coefficient results in reactivity decreasing as a result of the formation of voids, generally a good thing. It is similar in appearance to the temperature coefficient although the origins of the two differ.

VVER

Russian designation for a PWR.

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W

waste

see radwaste.

WNA

World Nuclear Association

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Y

yellowcake

powder of uranium concentrate produced from the milling and processing of uranium ore.

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Abbreviations for units

The table below lists the standard abbreviations for units.

The SI defines one unit per quantity. By attaching prefixes to these units, their size is changed to something more appropriate for the particular measurement. The table below lists these prefixes. For example, 2kBq (2 kilobecquerels) is 2,000 becquerels, 45nSv (45 nanosieverts) is 45 billionths of a sievert.

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Periodic table of elements

Below is the Periodic Table of Elements containing the 117 known elements. The atomic number and the chemical symbol for each element are displayed in the cells. Move the cursor over the cell for the full name of the element.

Copyright © 2007 Josh Baxter

Last updated: 2007/06/30