Nuclear Power

 

 

Nuclear energy is energy in the nucleus (core) of an atom. Atoms are tiny particles that make up every object in the universe. There is enormous energy in the bonds that hold atoms together. Nuclear energy can be used to make electricity. But first the energy must be released. It can be released from atoms in two ways: nuclear fusion and nuclear fission.

In nuclear fusion, energy is released when atoms are combined or fused together to form a larger atom. This is how the sun produces energy.

In nuclear fission, atoms are split apart to form smaller atoms, releasing energy. Nuclear power plants use nuclear fission to produce electricity.

Nuclear power plants are very clean and efficient to operate. However, nuclear power plants have some major environmental risks. Nuclear power plants produce radioactive gases. These gases are to be contained in the operation of the plant. If these gases are released into the air, major health risks can occur. Nuclear plants use uranium as a fuel to produce power. The mining and handling of uranium is very risky and radiation leaks can occur. The third concern of nuclear power is the permanent storage of spent radioactive fuel. This fuel is toxic for centuries, handling and disposal is an ongoing environmental issue.

Nuclear Structure

An atom consists of an extremely small, positively charged nucleus surrounded by a cloud of negatively charged electrons. Although typically the nucleus is less than one ten-thousandth the size of the atom, the nucleus contains more than 99.9% of the mass of the atom! Nuclei consist of positively charged protons and electrically neutral neutrons held together by the so-called strong or nuclear force. This force is much stronger than the familiar electrostatic force that binds the electrons to the nucleus, but its range is limited to distances on the order of a few x10-15 meters.

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     The number of protons in the nucleus, Z, is called the atomic number. This determines what chemical element the atom is. The number of neutrons in the nucleus is denoted by N. Theatomic mass of the nucleus, A, is equal to Z + N. A given element can have many different isotopes, which differ from one another by the number of neutrons contained in the nuclei. In a neutral atom, the number of electrons orbiting the nucleus equals the number of protons in the nucleus. Since the electric charges of the proton and the electron are +1 and -1 respectively (in units of the proton charge), the net charge of the atom is zero. At present, there are 112 known elements which range from the lightest, hydrogen, to the recently discovered and yet to-be-named element 112. All of the elements heavier than uranium are man made. Among the elements are approximately 270 stable isotopes, and more than 2000 unstable isotopes.

 

The Nuclear Fuel Cycle

The nuclear fuel cycle consists of "front end" steps that lead to the preparation of uranium for use as fuel for reactor operation and "back end" steps that are necessary to safely manage, prepare, and dispose of the highly radioactive spent nuclear fuel.

Exploration. A deposit of uranium, discovered by geophysical techniques, is evaluated and sampled to determine the amounts of uranium materials that are extractable at specified costs from the deposit. Uranium reserves are the amounts of ore that are estimated to be recoverable at stated costs.

Mining. Uranium ore can be extracted through conventional mining in open pit and underground methods similar to those used for mining other metals. In situ leach mining methods also are used to mine uranium in the United States. In this technology, uranium is leached from the in-place ore through an array of regularly spaced wells and is then recovered from the leach solution at a surface plant. Uranium ores in the United States typically range from about 0.05 to 0.3 percent uranium oxide (U3O8). Some uranium deposits developed in other countries are of higher grade and are also larger than deposits mined in the United States. Uranium is also present in very low grade amounts (50 to 200 parts per million) in some domestic phosphate-bearing deposits of marine origin. Because very large quantities of phosphate-bearing rock are mined for the production of wet-process phosphoric acid used in high analysis fertilizers and other phosphate chemicals, at some phosphate processing plants the uranium, although present in very low concentrations, can be economically recovered from the process stream.

Milling. Mined uranium ores normally are processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching. The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake," which is sold on the uranium market as U3O8.

Uranium conversion. Milled uranium oxide, U3O8, must be converted to uranium hexafluoride, UF6, which is the form required by most commercial uranium enrichment facilities currently in use. A solid at room temperature, UF6 can be changed to a gaseous form at moderately higher temperatures. The UF6 conversion product contains only natural, not enriched, uranium.

Enrichment. The concentration of the fissionable isotope, 235U (0.71 percent in natural uranium) is less than that required to sustain a nuclear chain reaction in light water reactor cores. Natural UF6 thus must be "enriched" in the fissionable isotope for it to be used as nuclear fuel. The different levels of enrichment required for a particular nuclear fuel application are specified by the customer: light-water reactor fuel normally is enriched up to about 4 percent 235U, but uranium enriched to lower concentrations also is required. Gaseous diffusion and gas centrifuge are the commonly used uranium enrichment technologies. The gaseous diffusion process consists of passing the natural UF6 gas feed under high pressure through a series of diffusion barriers (semiporous membranes) that permit passage of the lighter 235UF6 atoms at a faster rate than the heavier 238UF6 atoms. This differential treatment, applied across a large number of diffusion "stages," progressively raises the product stream concentration of 235U relative to 238U. In the gaseous diffusion technology, the separation achieved per diffusion stage is relatively low, and a large number of stages is required to achieve the desired level of isotope enrichment. Because this technology requires a large capital outlay for facilities and it consumes large amounts of electrical energy, it is relatively cost intensive. In the gas centrifuge process, the natural UF6 gas is spun at high speed in a series of cylinders. This acts to separate the 235UF6 and 238UF6 atoms based on their slightly different atomic masses. Gas centrifuge technology involves relatively high capital costs for the specialized equipment required, but its power costs are below those for the gaseous diffusion technology. New enrichment technologies currently being developed are the atomic vapor laser isotope separation (AVLIS) and the molecular laser isotope separation (MLIS). Each laser-based enrichment process can achieve higher initial enrichment (isotope separation) factors than the diffusion or centrifuge processes can achieve. Both AVLIS and MLIS will be capable of operating at high material throughput rates.

Fabrication. For use as nuclear fuel, enriched UF6 is converted into uranium dioxide (UO2) powder which is then processed into pellet form. The pellets are then fired in a high temperature sintering furnace to create hard, ceramic pellets of enriched uranium. The cylindrical pellets then undergo a grinding process to achieve a uniform pellet size. The pellets are stacked, according to each nuclear core's design specifications, into tubes of corrosion-resistant metal alloy. The tubes are sealed to contain the fuel pellets: these tubes are called fuel rods. The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of a power reactor.

The back end of the cycle is divided into the following steps:

Interim Storage. After its operating cycle, the reactor is shut down for refueling. The fuel discharged at that time (spent fuel) is stored either at the reactor site or, potentially, in a common facility away from reactor sites. If on-site pool storage capacity is exceeded, it may be desirable to store aged fuel in modular dry storage facilities known as Independent Spent Fuel Storage Installations (ISFSI) at the reactor site or at a facility away from the site. The spent fuel rods are usually stored in water, which provides both cooling (the spent fuel continues to generate heat as a result of residual radioactive decay) and shielding (to protect the environment from residual ionizing radiation).

Reprocessing. Spent fuel discharged from light-water reactors contains appreciable quantities of fissile (U-235, Pu-239), fertile (U-238), and other radioactive materials. These fissile and fertile materials can be chemically separated and recovered from the spent fuel. The recovered uranium and plutonium can, if economic and institutional conditions permit, be recycled for use as nuclear fuel. Currently, plants in Europe are reprocessing spent fuel from utilities in Europe and Japan.

Waste Disposal. A current concern in the nuclear power field is the safe disposal and isolation of either spent fuel from reactors or, if the reprocessing option is used, wastes from reprocessing plants. These materials must be isolated from the biosphere until the radioactivity contained in them has diminished to a safe level. Under the Nuclear Waste Policy Act of 1982, as amended, the Department of Energy has responsibility for the development of the waste disposal system for spent nuclear fuel and high-level radioactive waste. Current plans call for the ultimate disposal of the wastes in solid form in licensed deep, stable geologic structures

Nuclear Reaction

Inside the reactor of an atomic power plant, uranium atoms are split apart in a controlled chain reaction. In a chain reaction, particles released by the splitting of the atom go off and strike other uranium atoms splitting those. Those particles given off split still other atoms in a chain reaction. In nuclear power plants, control rods are used to keep the splitting regulated so it doesn't go too fast. If the reaction is not controlled, you could have an atomic bomb. But in atomic bombs, almost pure pieces of the element Uranium-235 or Plutonium, of a precise mass and shape, must be brought together and held together, with great force. These conditions are not present in a nuclear reactor. The reaction also creates radioactive material. This material could hurt people if released, so it is kept in a solid form. 

Nuclear Fuel Rods

This view looks down on the fuel rods at Penn State's Breazeale Reactor. The reactor is a TRIGA model manufactured by General Atomics. The blue light surrounding the fuel is known as Cherenkov radiation, produced when charged particles travel through matter (in this case, water) at speeds greater than light. Penn State University is the site of the first licensed reactor. Sources: the Penn State Radiation Science and Engineering Center

 

This chain reaction gives off heat energy. This heat energy is used to boil water in the core of the reactor. So, instead of burning a fuel, nuclear power plants use the chain reaction of atoms splitting to change the energy of atoms into heat energy. This water from around the nuclear core is sent to another section of the power plant. Here it heats another set of pipes filled with water to make steam. The steam in this second set of pipes powers a turbine to generate electricity.

 

Nuclear Fission

Nuclear Fission

Fission is a nuclear process in which a heavy nucleus splits into two smaller nuclei. An example of a fission reaction that was used in the first atomic bomb and is still used in nuclear reactors is.

 

An atom's nucleus can be split apart. When this is done, a tremendous amount of energy is released. The energy is both heat and light energy. This energy, when let out slowly, can be harnessed to generate electricity. When it is let out all at once, it makes a tremendous explosion in an atomic bomb. The word fission means to split apart. 

235U + n ----> 134Xe + 100Sr + 2n

     The products shown in the above equation are only one set of many possible product nuclei. Fission reactions can produce any combination of lighter nuclei so long as the number of protons and neutrons in the products sum up to those in the initial fissioning nucleus. As with fusion, a great amount of energy can be released in fission because for heavy nuclei, the summed masses of the lighter product nuclei is less than the mass of the fissioning nucleus.
     Fission occurs because of the electrostatic repulsion created by the large number of positively charged protons contained in a heavy nucleus. Two smaller nuclei have less internal electrostatic repulsion than one larger nucleus. So, once the larger nucleus can overcome the strong nuclear force which holds it together, it can fission. Fission can be seen as a "tug-of-war" between the strong attractive nuclear force and the repulsive electrostatic force. In fission reactions, electrostatic repulsion wins.
     Fission is a process that has been occurring in the universe for billions of years. As mentioned above, we have not only used fission to produce energy for nuclear bombs, but we also use fission peacefully everyday to produce energy in nuclear power plants.

The following are essential components/systems of a thermal nuclear fission reactor:

  1. The fuel - the fissile material (U-235), either as found in natural uranium or enriched. In some cases plutonium is added. The fuel is produced in the form of metal or oxide pellets.
  2. Fuel cladding - a metal shell in which the fuel pellets are contained. It protects the fuel from corrosion and prevents fission products from escaping.
  3. A moderator - made of light elements, it slows down the fission neutrons to thermal levels without unduly absorbing them.
  4. A coolant - to transport the heat generated from the core to the steam generator for driving the turbine.
  5. Control rods - made of neutron absorbing material, these can be moved in or out of the core to control the reaction and maintain it at a critical level or to stop the reaction during shutdown.
  6. A pressure vessel - to prevent radioactive material from escaping in case of excessive internal pressure.
  7. A containment structure or neutron shield - (concrete or other material) to protect operators and the public from radiation.

 

Nuclear Fusion

Nuclear Fusion

University of California, Berkeley Graphic

 

Another form of nuclear energy is called fusion. Fusion means joining smaller nuclei (the plural of nucleus) to make a larger nucleus. 

Fusion is a nuclear process in which two light nuclei combine to form a single heavier nucleus. An example of a fusion reaction important in thermonuclear weapons and in future nuclear reactors is the reaction between two different hydrogen isotopes to form an isotope of helium:

  2H + 3H ----> 4He + n

 

This reaction liberates an amount of energy more than a million times greater than one gets from a typical chemical reaction. Such a large amount of energy is released in fusion reactions because when two light nuclei fuse, the sum of the masses of the product nuclei is less than the sum of the masses of the initial fusing nuclei. Once again, Einstein's equation, E=mc2, explains that the mass that is lost it converted into energy carried away by the fusion products.


     Even though fusion n is an energetically favorable reaction for light nuclei, it does not occur under standard conditions here on Earth because of the large energy investment that is required. Because the reacting nuclei are both positively charged, there is a large electrostatic repulsion between them as they come together. Only when they are squeezed very close to one another do they feel the strong nuclear force, which can overcome the electrostatic repulsion and cause them to fuse.


     Fusion reactions have been going on for billions of years in our universe. In fact, nuclear fusion reactions are responsible for the energy output of most stars, including our own Sun. Scientists on Earth have been able to produce fusion reactions for only about the last sixty years. At first, there were small scale studies in which only a few fusion reactions actually occurred. However, these first experiments later lead to the development of thermonuclear fusion weapons (hydrogen bombs).


     Fusion is the process that takes place in stars like our Sun. Whenever we feel the warmth of the Sun and see by its light, we are observing the products of fusion. We know that all life on Earth exists because the light generated by the Sun produces food and warms our planet. Therefore, we can say that fusion is the basis for our life.

When a star is formed, it initially consists of hydrogen and helium created in the Big Bang, the process that created our universe. Hydrogen isotopes collide in a star and fuse forming a helium nucleus. Later, the helium nuclei collide and form heavier elements. Fusion is a nuclear reaction in which nuclei combine to form a heavier nucleus. It is the basic reaction which drives the Sun. Lighter elements fuse and form heavier elements. These reactions continue until the nuclei reach iron (around mass sixty), the nucleus with the most binding energy. When a nucleus reaches mass sixty, no more fusion occurs in a star because it is energetically unfavorable to produce higher masses. Once a star has converted a large fraction of its core's mass to iron, it has almost reached the end of its life.

     The fusion chain cannot continue so its fuel is reduced. Some stars keep shrinking until they become a cooling ember made up of iron. However, if a star is sufficiently massive, a tremendous, violent, brilliant explosion can happen. A star will suddenly expand and produce, in a very short time, more energy than our Sun will produce in a lifetime. When this happens, we say that a star has become a supernova.

     While a star is in the supernova phase, many important reactions occur. The nuclei are accelerated to much higher velocities than can occur in a fusing star. With the added energy caused by their speed, nuclei can fuse and produce elements higher in mass than iron. The extra energy in the explosion is necessary to over come the energy barrier of a higher mass element. Elements such as lead, gold, and silver found on Earth were once the debris of a supernova explosion. The element iron that we find all through the Earth and in its center is directly derived from both super novae and dead stars.

There are Two Types of Reactors in the United States

The Pressurized Water Reactor (PWR)
Nuclear Power Pressurized Water Reactor

PWRs keep water under pressure so that it heats, but does not boil. Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.

 

The Boiling Water Reactor (BWR)
Nuclear Power Boiling Water Reactor

Nuclear Regulatory Commission Graphic

 

BWRs actually boil the water. In both types, water is converted to steam, and then recycled back into water by a part called the condenser, to be used again in the heat process. Since radioactive materials can be dangerous, nuclear power plants have many safety systems to protect workers, the public, and the environment. These safety systems include shutting the reactor down quickly and stopping the fission process, systems to cool the reactor down and carry heat away from it, and barriers to contain the radioactivity and prevent it from escaping into the environment. Radioactive materials, if not used properly, can damage human cells or even cause cancer over long periods of time.

Nuclear power (% of total primary energy supply)

IEA (International Energy Agency) 2007

France 42.6
Sweden 36.2
Lithuania 31.9
Armenia 27.7
Slovakia 24.8
Bulgaria 24.3
Switzerland 22.5
Belgium 21.9
Slovenia 21
Korea (Republic of) 17.9
Finland 17.3
Ukraine 16.1
Japan 15
Czech Republic 14.3
Hungary 13
Germany 12.3
Spain 10.3
United Kingdom 9.1
United States 9
Canada 8.8
Russian Federation 6.1
Romania 3.8
Argentina 2.8
South Africa 2.3
Mexico 1.6
Netherlands 1.3
Brazil 1.2
China 0.8
India 0.8
Pakistan 0.8
Glossary of Nuclear Science Terms
Absorber
Any material that stops ionizing radiation. Lead, concrete, and steel attenuate gamma rays. A thin sheet of paper or metal will stop or absorb alpha particles and most beta particles.
Alpha particle (alpha radiation, alpha ray)
A positively charged particle (a Helium-4 nucleus) made up of two neutrons and two protons. It is the least penetrating of the three common forms of radiation, being stopped by a sheet of paper. It is not dangerous to living things unless the alpha-emitting substance is inhaled or ingested or comes into contact with the lens of the eye.
Atom
A particle of matter indivisible by chemical means. It is the fundamental building block of elements.
Atomic number
The number assigned to each element on the basis of the number of protons found in the element's nucleus.
Atomic weight (atomic mass)
Approximately the sum of the number of protons and neutrons found in the nucleus of an atom.

- B -

Background radiation
The radiation of man's natural environment originating primarily from the naturally radioactive elements of the earth and from the cosmic rays. The term may also mean radiation extraneous to an experiment.
Beta particle (beta radiation, beta ray)
An electron of either positive charge (+) or negative charge (-), which has been emitted by an atomic nucleus or neutron in the process of a transformation. Beta particles are more penetrating than alpha particles but less than gamma rays or x-rays.

- C -

Contamination
Radioactive material deposited or dispersed in materials or places where it is not wanted.
Cow
A radioisotope generator system.
Curie (Ci)
The basic unit used to describe the intensity of radioactivity in a sample of material. One curie equals thirty-seven billion disintegrations per second, or approximately the radioactivity of one gram of radium.

- D -

Daughter
A nucleus formed by the radioactive decay of a different (parent) nuclide.
Decay (radioactive)
The change of one radioactive nuclide into a different nuclide by the spontaneous emission of alpha, beta, or gamma rays, or by electron capture. The end product is a less energetic, more stable nucleus. Each decay process has a definite half-life.
Decontamination
The removal of radioactive contaminants by cleaning and washing with chemicals.
Density
That property of a substance which is expressed by the ratio of its mass to its volume.
Dose
A general term denoting the quantity of radiation or energy absorbed in a specific mass.

- E -

Electromagnetic radiation
Radiation consisting of electric and magnetic waves that travel at the speed of light. Examples: light, radio waves, gamma rays, x-rays.
Electron
An elementary particle with a unit electrical charge and a mass 1/1837 that of the proton. Electrons surround the atom's positively charged nucleus and determine the atom's chemical properties.
Electron capture
A radioactive decay process in which an orbital electron is captured by and merges with the nucleus. The mass number is unchanged, but the atomic number is decreased by one.
Eluant
Washing solution (The solution that is introduced into the cow).
Eluate
The washings obtained by elution (the solution that comes out of the cow).
Elute
To separate by washing (to milk).
Excited state
The state of an atom or nucleus when it possesses more than its normal energy. The excess energy is usually released eventually as a gamma ray.

- F -

Fission
The splitting of a heavy nucleus into two roughly equal parts (which are nuclei of lighter elements), accompanied by the release of a relatively large amount of energy in the form of kinetic energy of the two parts and in the form of emission of neutrons and gamma rays.
Fission products
Nuclei formed by the fission of heavy elements. They are of medium atomic weight and almost all are radioactive. Examples: strontium-90, cesium-137.

- G -

Gamma ray
A highly penetrating type of nuclear radiation, similar to x-radiation, except that it comes from within the nucleus of an atom, and, in general, has a shorter wavelength.
Geiger counter
A Geiger-Müller detector and measuring instrument. It contains a gas-filled tube which discharges electrically when ionizing radiation passes through it and a device that records the events.
Generator
A cow-a system containing a parent-daughter set of radioisotopes in which the parent decays through a daughter to a stable isotope. The daughter is a different element from that of the parent, and, hence, can be separated from the parent by elution (milking).

- H -

Half-life
The time in which half the atoms of a particular radioactive nuclide disintegrate. The half-life is a characteristic property of each radioactive isotope.
Health physics
That science devoted to recognition, evaluation, and control of all health hazards from ionizing radiation.

- I -

Induced radioactivity
Radioactivity that is created by bombarding a substance with neutrons in a reactor or with charged particles produced by particle accelerators.
Ion
An atomic particle that is electrically charged, either negative or positive.
Ionizing radiation
Radiation that is capable of producing ions either directly or indirectly.
Irradiate
To expose to some form of radiation.
Isomer
One of several nuclides with the same number of neutrons and protons capable of existing for a measurable time in different nuclear energy states.
Isometric transition
A mode of radioactive decay where a nucleus goes from a higher to a lower energy state. The mass number and the atomic number are unchanged.
Isotope
Isotopes of a given element have the same atomic number (same number of protons in their nuclei) but different atomic weights (different number of neutrons in their nuclei). Uranium-238 and uranium-235 are isotopes of uranium.

- K -

K-capture
The capture by an atom's nucleus of an orbital electron from the first K-shell surrounding the nucleus.
keV
One thousand electron volts.

- M -

MeV
One million electron volts.
Microcurie (Ci)
One millionth of a curie (3.7 x 104 disintegrations per second).
Milk
To elute a cow.
Minigenerator
A trademark of Union Carbide Corporation that is used to identify radioisotope generator systems for educational use.

- N -

Neutrino
An electrically neutral particle with negligible mass. It is produced in many nuclear reactions such as in beta decay.
Neutron
One of the basic particles which make up an atom. A neutron and a proton have about the same weight, but the neutron has no electrical charge.
Nuclear reactor
A device in which a fission chain reaction can be initiated, maintained, and controlled. Its essential components are fissionable fuel, moderator, shielding, control rods, and coolant.
Nucleon
A constituent of the nucleus; that is, a proton or a neutron.
Nucleonics
The science, technology, and application of nuclear energy.
Nucleus
The core of the atom, where most of its mass and all of its positive charge is concentrated. Except for hydrogen, it consists of protons and neutrons.
Nuclide
Any species of atom that exists for a measurable length of time. A nuclide can be distinguished by its atomic weight, atomic number, and energy state.

- P -

Parent
A radionuclide that decays to another nuclide which may be either radioactive or stable.
Photon
A quantity of electromagnetic energy. Photons have momentum but no mass or electrical charge.
Proton
One of the basic particles which makes up an atom. The proton is found in the nucleus and has a positive electrical charge equivalent to the negative charge of an electron and a mass similar to that of a neutron: a hydrogen nucleus.

- R -

Rad
Radiation Absorbed Dose. The basic unit of an absorbed dose of ionizing radiation. One rad is equal to the absorption of 100 ergs of radiation energy per gram of matter.
Radioactive dating
A technique for estimating the age of an object by measuring the amounts of various radioisotopes in it.
Radioactive waste
Materials which are radioactive and for which there is no further use.
Radioactivity
The spontaneous decay of disintegration of an unstable atomic nucleus accompanied by the emission of radiation.
Radioisotope
A radioactive isotope. A common term for a radionuclide.
Radionuclide
A radioactive nuclide. An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation.
Rate meter
An electronic instrument that indicates, on a meter, the number of radiation induced pulses per minute from radiation detectors such as a Geiger-Muller tube.

- S -

Scaler
An electronic instrument for counting radiation induced pulses from radiation detectors such as a Geiger-Muller tube.
Scintillation counter
An instrument that detects and measures gamma radiation by counting the light flashes (scintillations) induced by the radiation.
Secular equilibrium
A state of parent-daughter equilibrium which is achieved when the half-life of the parent is much longer than the half-life of the daughter. In this case, if the two are not separated, the daughter will eventually be decaying at the same rate at which it is being produced. At this point, both parent and daughter will decay at the same rate until the parent is essentially exhausted.
Shielding
A protective barrier, usually a dense material, which reduces the passage of radiation from radioactive materials to the surroundings.
Source
A radioactive material that produces radiation for experimental or industrial use.
Spill
The accidental release of radioactive materials.
Stable
Non-radioactive.

- T -

Tracer
A small amount of radioactive isotope introduced into a system in order to follow the behavior of some component of that system.
Transmutation
The transformation of one element into another by a nuclear reaction
    

Credit: U.S. Department of Energy, International Atomic Energy Agency, U.S. Nuclear Regulatory Commission, Nuclear Science Division ---- Lawrence Berkeley National Laboratory, Penn State Radiation Science and Engineering Center, American Nuclear Society, European Commision



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