Nuclear

Structure

The nucleus, being the largest sub-cellular compartment, varies in diameter from 10 to 20 micrometres. It is surrounded by a double membrane forming the nuclear envelope, about 30 nm wide. This selectively allows molecules to enter and leave the nucleus, and separates chemical reactions taking place in cytoplasm from reactions happening within the nucleus. The outer membrane has ribosomes. The inner and outer membrane fuse at regular spaces, forming nuclear pores.

Similar to the cytoplasm of a cell, the nucleus contains nucleoplasm - a highly viscous solid containing the chromosomes and nucleoli. Chromosones contain information encoded in DNA attached to proteins called histones and are usually arranged in to a dense network called chromatin. Nucleoli are granular structures which make ribonucleic DNA (rDNA) and assemble it with proteins.

• Thermal (slow) reactors are composed of fuel (fissionable material), moderating materials to slow neutrons to low velocities (to prevent capture by U238), heavy-walled pressure vessels to house reactor components, shielding to protect personnel, systems to conduct heat away from the reactor, and instrumentation for monitoring and controlling the reactor's systems. Most nuclear reactors used for electric power generation are of this type. The first plutonium production reactors were thermal reactors using graphite as the moderator.
• Fast reactors require highly enriched fuel (sometimes Weapons Grade), but no moderating material (the enrichment process removes most of the U238 that captures fast neutrons). This type of reactor is used in mobile applications, where space constraints are a major concern, as well as for the production of plutonium (see fast breeder).

Designs for fast power reactors to date have all been cooled by liquid metal. They have also been of two types, called pool and loop reactors.

To provide the power for a dynamo-electric machine, or electric generator, nuclear power plants rely on the process of nuclear fission. In this process, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits into two smaller atoms. The fission process for uranium atoms yields two smaller atoms, one to three fast-moving free neutrons, plus an amount of energy. Because more free neutrons are released from a uranium fission event than are required to initiate the event, the reaction can become self sustaining--a chain reaction --under controlled conditions, thus producing a tremendous amount of energy. The newly released fast neutrons must be slowed down (moderated) before they can be absorbed by the next fuel atom. This slowing down process is caused by collisions of the neutrons with atoms of an introduced substance called a moderator.

In the vast majority of the world's nuclear power plants, heat energy generated by burning uranium fuel is collected in ordinary water and is carried away from the reactor's core either as steam in boiling water reactors or as superheated water in pressurized-water reactors. In a pressurized-water reactor, the superheated water in the primary cooling loop is used to transfer heat energy to a secondary loop for the creation of steam. In either a boiling-water or pressurized-water installation, steam under high pressure is the medium used to transfer the nuclear reactor's heat energy to a turbine that mechanically turns a dynamo- electric machine, or electric generator. Boiling-water and pressurized-water reactors are called light-water reactors, because they utilize ordinary water as the moderator. In all light-water reactors to date this water is also used to transfer the heat energy from reactor to turbine in the electricity generation process. In other reactor designs the heat energy may be transferred by light water, pressurized heavy water, gas, or another cooling substance.

The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full power days in a reactor's operating cycle (between refueling outage times) is related to the amount of fissile 235U contained in the fuel assemblies at the beginning of the cycle. A higher percentage of 235U in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent," and it is discharged and replaced with new (fresh) fuel assemblies. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor.

The amount of energy extracted from nuclear fuel is called its "burn up," which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burn up is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.

• Pressurized water reactor (PWR)
• Boiling water reactor (BWR)
• Pressurised Heavy Water Reactor (PHWR or CANDU)
• Advanced gas-cooled Reactor (AGR)
• Light water cooled graphite moderated reactor (RBMK)
• D2G reactor

Advanced reactors

More than a dozen advanced reactor designs are in various stages of development. Some are evolutionary from the PWR, BWR and CANDU designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor, two of which are now operating with others under construction.

The best-known radical new design is the Pebble Bed Modular Reactor, discussed below.

More could be added about advanced reactor designs the PBMR has a web page for example.

• Pebble Bed Reactor

Nuclear fuel cycle

See nuclear fuel cycle.

The first nuclear reactors were used to generate plutonium for nuclear weapons. Additional reactors were used in the navy (United States Naval reactor ) In the mid-1950s, both the Soviet Union and western countries were expanding their nuclear research to include non-military uses of the atom. However, as with the military program, much of the non-military work was done in secret. On June 27, 1954, the world's first nuclear power plant generated electricity but no headlines--at least, not in the West. According to the Uranium Institute (London, England), the first reactor to generate electricity for commercial use was at Obninsk, Russia. The Shippingport reactor (in Pennsylvania) was the first commercial nuclear generator to become operational in the United States. The Shippingport reactor was ordered in 1953 and began commercial operation in 1957.

Lots of construction in 60s and 70s (oil crisis influenced) - need some numbers here

In the aftermath of the 1979 Three Mile Island accident, the U.S. nuclear market was the first to deteriorate. No new nuclear plants have been ordered in the USA since then.

Negative influence of Chernobyl increasing regulations increased costs.

need dates, declining construction numbers, reference to legislation in US

In 1997, a total of 78 reactors were either under construction, planned, or indefinitely deferred. These units have a combined capacity of 67,484 MWe, approximately 25 percent of the total capacity already in existence. However, only 45 reactors were under construction worldwide. The remaining 33 units are either being planned or indefinitely deferred. Three U.S. units are not projected to come on-line. Some experts have predicted that Watts Bar 1, which came on-line in 1997, will be the last U.S. commercial nuclear reactor to go on-line. Other experts, however, predict that electricity shortages will revamp the demand for nuclear power plants.

need more recent figures

As of 2003, the immediate future of the industry in many countries still appeared uncertain, the most notable exceptions being Japan, China and India, all actively developing both fast and thermal technology, South Korea, developing thermal technology only, and South Africa, developing the Pebble Bed Modular Reactor (PBMR).

Statistics

In 2000, there were 438 commercial nuclear generating units throughout the world, with a total capacity of about 351 gigawatts.

In 2001, there were 104 (69 pressurized water reactors, 35 boiling water reactors) commercial nuclear generating units that are licensed to operate in the United States, producing 32,300 net megawatts (electric), which is approximately 20 percent of the nation's total electric energy consumption. The United States is the world's largest supplier of commercial nuclear power.

In France, 80% of all electric power comes from nuclear reactors.

• MinAtom (Russia)

References and Links:

• MIRV -- Multiple Independent Re-entry Vehicles, nuclear devices carried, usually ten or twelve at a time on a single ICBM, allowing a single launched missile to strike a handful of targets, and allowing a few missiles to strike several targets redundantly.
• SALT I -- Strategic Arms Limitation Treaty. A treaty signed by Richard Nixon and Leonid Brezhnev in 1972, limiting the growth of US and Soviet missile arsenals.
• SALT II -- A treaty designed to further limit the growth of US and Soviet missile arsenals.
• START -- STrategic Arms Reductions Treaty -- A treaty proposed by Ronald Reagan to reduce the numbers of missiles and warheads.
• INF -- Intermediate Nuclear Forces Treaty, signed in 1987, which eliminated tactical ("battlefield") nuclear devices and GLCMs from Europe.
• GLCM -- Ground Launched Cruise Missile.
• ALCM -- Air Launched Cruise Missile.
• ICBM --Intercontinental Ballistic Missile
• SLCM -- Submarine Launched Cruise Missile.
• SLBM -- Submarine Launched Ballistic Missile.
• Ballistic missile -- A missile using a ballistic trajectory involving a significant ascent and descent including suborbital and partial orbital trajectories.
• Cruise missile -- A missile using a low altitude trajectory intended to avoid detection by radar systems. Cruise missiles have shorter range and lower payloads than ballistic missiles, usually, and are not known to carry MIRVs.
• MAD -- Mutual assured destruction. The doctrine of preventing nuclear war by creating a situation in which any use of nuclear weapons would result in the certain destruction of both the attacker and the defender.
• ABM -- Anti-Ballistic Missile. Missiles designed to intercept and destroy ballistic missiles. Can also refer to the ABM treaty, signed by Richard Nixon and Leonid Brezhnev, which halted the development and use of such systems due to fears that such systems could counter the MAD scenario and, thereby, increase the likelihood that an ABM protected country would use their nuclear weapons aggressively.
• SDI -- Strategic Defense Initiative, more commonly known as Star Wars. A system proposed by Ronald Reagan to use space-based systems to detect, intercept and destroy ICBMs and MIRVs. Criticized for its costs, doubts that it would be effective, and concerns that it would violate the ABM treaty and offset MAD, it was not supported by the US Congress at that time.

External Links:

The largest modern weapons include a fissionable outer shell of uranium. The intense fast neutrons from the fusion stage of the weapon will cause even natural (that is unenriched) uranium to fission, increasing the yield of the weapon many times.

The cobalt bomb uses cobalt in the shell, and the fusion neutrons convert the cobalt into cobalt-60, a powerful long-term (5 years) emitter of gamma rays. In general this type of weapon is a salted bomb and variable fallout effects can be obtained by using different salting isotopes. Gold has been proposed for short-term fallout (days), tantalum and zinc for fallout of intermediate duration (months), and cobalt for long term contamination (years). The primary purpose of this weapon is to create extremely radioactive fallout making a large region uninhabitable. No cobalt or other salted bomb has been built or tested publicly.

A final variant of the thermonuclear weapons is the enhanced radiation weapon, or neutron bomb which are small thermonuclear weapons in which the burst of neutrons generated by the fusion reaction is intentionally not absorbed inside the weapon, but allowed to escape. The X-ray mirrors and shell of the weapon are made of chromium or nickel so that the neutrons are permitted to escape. This intense burst of high-energy neutrons is the principle destructive mechanism. Neutrons are more penetrating than other types of radiation so many shielding materials that work well against gamma rays are rendered less effective. The term "enhanced radiation" refers only to the burst of ionizing radiation released at the moment of detonation, not to any enhancement of residual radiation in fallout (as in the salted bombs discussed above).

For more technical details see: Nuclear weapon design

The dominant effects of a nuclear weapon (the blast and thermal radiation) are the same physical damage mechanisms as conventional explosives. The primary difference is that nuclear weapons are capable of releasing much larger amounts of energy at once. Most of the damage caused by a nuclear weapon is not directly related to the nuclear process of energy release, but would be present for any explosion of the same magnitude.

The damage done by each of the three initial forms of energy release differs with the size of the weapon. Thermal radiation drops off the slowest with distance, so the larger the weapon the more important this effect becomes. Ionizing radiation is strongly absorbed by air, so it is only dangerous by iteself for smaller weapons. Blast damage falls off more quickly than thermal radiation but more slowly than ionizing radiation.

When a nuclear weapon explodes, the bomb's material comes to an equilibrium temperature in about a microsecond. At this time about 75% of the energy is emitted as primary thermal radiation, mostly soft X-rays. Almost all of the rest of the energy is kinetic energy in rapidly-moving weapon debris. The interaction of the x-rays and debris with the surroundings determines how much energy is produced as blast and how much as light. In general, the denser the medium around the bomb, the more it will absorb, and the more powerful the shockwave will be.

In a burst at high altitudes, where the air density is low, the soft X rays travel long distances before they are absorbed. The energy is so diluted that the blast wave may be half as strong or less. The rest of the energy is dissipated as a more powerful thermal pulse.

Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations. In detonations near a water surface, the particles tend to be lighter and smaller and produce less local fallout but will extend over a greater area. The particles contain mostly sea salts with some water; these can have a cloud seeding affect causing local rainout and areas of high local fallout.

The radiobiological hazard of worldwide fallout is essentially a long-term one due to the potential accumulation of long-lived radioisotopes, such as strontium-90 and cesium-137, in the body as a result of ingestion of foods incorporating these radioactive materials. The hazard of worldwide fallout is much less serious than the hazards which are associated with local fallout.

Blast and thermal injuries in many cases will far outnumber radiation injuries. However, radiation effects are considerably more complex and varied than are blast or thermal effects and are subject to considerable misunderstanding. A wide range of biological changes may follow the irradiation of animals, ranging from rapid death following high doses of penetrating whole-body radiation to essentially normal lives for a variable period of time until the development of delayed radiation effects, in a portion of the exposed population, following low dose exposures.

For more technical details see: nuclear explosion

Nuclear weapons in culture

Nuclear weaponry has become a part of our culture, the decades post-WW II being can be termed the atomic age. The stunning power and the astonishing visual effects are a strong influence on art, from Andy Warhol's silkscreen Atomic Bomb (1965) and James Rosenquist's F-111 (1964-65) to Gregory Green's constructions and the efforts of artist James Acord to use uranium in his sculptures.

Films featuring nuclear war or the threat of it include Dr. Strangelove or, How I Learned to Stop Worrying and Love the Bomb, On The Beach, The Day After, The War Game (1966), Threads (1985), WarGames (1983); as well as less-famous films such as Miracle Mile and Broken Arrow (1996). Also the series of movies Planet of the Apes finish with the launching of cobalt bombs. Godzilla is considered by some to be an analogy to the nuclear weapons dropped on Japan.

A memorable episode of The Bionic Woman featured the threat of a cobalt bomb. A main character in Repo Man was a designer of the neutron bomb.

Nuclear weapons are a staple element in science fiction novels. The so-called dirty bomb was predicted in a 1943 article by Robert A. Heinlein titled "Solution Unsatisfactory" which caused him to be investigated by the FBI, concerned that there had been a breach of security on the Manhattan Project.

• More Technical Details
  • nuclear weapon design
  • nuclear explosion
• History
  • History of nuclear weapons
  • Manhattan Project
  • Los Alamos National Laboratory
  • Nuclear test explosion
• Related Technology and Science
  • nuclear physics
  • nuclear fission
  • nuclear fusion
  • nuclear reactor
  • nuclear engineering
• Military Strategy
  • nuclear warfare
  • nuclear strategy
  • Mutual Assured Destruction
• Proliferation and Politics
  • nuclear proliferation
  • Nuclear Non-Proliferation Treaty
  • Comprehensive Test Ban Treaty
  • nuclear disarmament
• General
  • Weapons of mass destruction

References

External links

Low level Waste (LLW) is generated from hospitals and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters etc which contain small amounts of mostly short-lived radioactivity. It does not require shielding during handling and transport and is suitable for shallow land burial. To reduce its volume, it is often compacted or incinerated before disposal.

Intermediate level Waste (ILW) contains higher amounts of radioactivity and some requires shielding. It typically comprises resins, chemical sludges and metal fuel cladding, as well as contaminated materials from reactor decommissioning. It may be solidified in concrete or bitumen for disposal. Generally short lived waste (mainly from reactors) is buried in a shallow repository, while long lived waste (from fuel reprocessing) will be disposed of deep underground.

High level Waste (HLW) arises from the use of uranium fuel in a nuclear reactor and nuclear weapons processing. It contains the fission products and transuranic elements generated in the reactor core. It is highly radioactive and hot. It can be considered the "ash" from "burning" uranium. HLW accounts for over 95% of the total radioactivity produced in the process of nuclear electricity generation.

The Effects of Radiation on People

We tend to think of biological effects of radiation in terms of their effect on living cells. For low levels of radiation exposure, the biological effects are so small they may not be detected. The body has defense mechanisms against many types of damage induced by radiation as well as by chemical carcinogens. Consequently, biological effects of radiation on living cells may result in three outcomes:

1. injured or damaged cells repair themselves, resulting in no residual damage
2. cells die, much like millions of body cells do every day, being replaced through normal biological processes
3. cells incorrectly repair themselves resulting in a biophysical change.

The period of time between radiation exposure and the detection of cancer is known as the latent period. Those cancers that may develop as a result of radiation exposure are indistinguishable from those that occur naturally or as a result of exposure to other chemical carcinogens. Furthermore, National Cancer Institute literature indicates that other chemical and physical hazards and lifestyle factors (e.g., smoking, alcohol consumption, and diet) significantly contribute to many of these same diseases.

Although radiation may cause cancer at high doses and high dose rates, public health data do not unequivocally establish the occurrence of cancer following exposure to low doses and dose rates -- below about 10,000 mrem (100 mSv). Studies of occupational workers exposed to chronic low-levels of radiation above normal background have shown no adverse biological effects. Even so, the radiation protection community conservatively assumes that any amount of radiation may pose some risk for causing cancer and hereditary effect, and that the risk is higher for higher radiation exposures. A linear, no-threshold (LNT) dose response relationship is used to describe the relationship between radiation dose and the occurrence of cancer. This dose-response model suggests that any increase in dose, no matter how small, results in an incremental increase in risk. The LNT hypothesis is accepted by the NRC as a conservative model for estimating radiation risk.

High radiation doses tend to kill cells, while low doses tend to damage or alter the genetic code (DNA) of irradiated cells. High doses can kill so many cells that tissues and organs are damaged immediately. This in turn may cause a rapid whole body response often called Acute Radiation Syndrome. The higher the radiation dose, the sooner the effects of radiation will appear, and the higher the probability of death. This syndrome was observed in many atomic bomb survivors in 1945 and emergency workers responding to the 1986 Chernobyl nuclear power plant accident. Approximately 134 plant workers and firefighters battling the fire at the Chernobyl power plant received high radiation doses (70,000 to 1,340,000 mrem or 700 to 13,400 mSv) and suffered from acute radiation sickness. Of these, 28 died from their radiation injuries.