The prodigious energy contained in the nucleus of the atom postulated by Albert Einstein’s epochal equation of energy and mass was thunderously demonstrated in the apocalyptic destruction of Hiroshima on August 6, 1945. That this energy could be harnessed and used for the beneficence of all mankind as intoned by President Eisenhower’s 1953 clarion call to use “atoms for peace” has always been one side of a two-edged sword, a sword of Damocles that points at the heart of civilization. The stark dichotomy of a technology that could destroy the earth and a technology that could produce electrical power that was “too cheap to meter” according a 1954 communiqué by Lewis Strauss, the head of the U. S. Atomic Energy Commission is at the core of the debate about the use of energy from nuclear power plants. The debate is further informed by the issues of radioactivity and the concomitant chimera of “the China syndrome” and the seemingly intractable issue of waste disposal. However, due to the environmental calamity of climate change that continued fossil fuel combustion could produce, it is necessary to reconsider nuclear energy as a possible alternative. This consideration must be based on some comprehension of nuclear fission with its attendant radiation, a knowledge of the history and development of nuclear technology, and an understanding of the waste disposal issue.
Nuclear Reactions and Radiation
In the most basic and simplistic model, an atom is made up of a nucleus of positively charged protons and uncharged neutrons that is orbited by a like number of negatively charged electrons so that the overall charge is zero. A chemical reaction is an interaction among atoms that involves the electrons. Sodium (Na) forms an electron sharing arrangement called a covalent bond with chlorine (Cl) to form table salt (NaCl). A nuclear reaction is an interaction with the protons and neutrons of the nucleus of an atom that results in the emission of radiation. From the practical macroscopic perspective, radiation consists of alpha and beta particles, gamma rays and neutrons. Alpha particles are the nuclei of helium atoms with two protons and two neutrons that are given off by the larger radioactive atoms such as radium and radon. Beta particles are essentially negatively charged electrons (rarely positively charged positrons) that originate in the region near the nucleus. Gamma rays are part of the electromagnetic spectrum that includes infrared, visible, ultraviolet and cosmic radiation. Beta particles and gamma rays are the principle types of radiation emitted by unstable nuclei as they decay over time until a stable nuclear composition is reached. The absorption of neutrons into the nucleus is the key to the nuclear fission process, and so, from the energy perspective, neutrons are the most important product of the nuclear reaction.
An element is defined by the number of protons in its nucleus which is also called its atomic number. Variations in the number of neutrons in the nucleus result in isotopes, some of which are stable and some of which are unstable. For example, the element carbon has two stable isotopes: the more common carbon 12 with 6 protons and 6 neutrons and the less common carbon 13 with 7 neutrons; and one unstable isotope, carbon 14 with 8 neutrons. Carbon 14 is the result of a nuclear reaction that occurs when neutrons in the upper atmosphere strike the nucleus of an atom of nitrogen, the main constituent of the atmosphere. Unstable isotopes reach a condition of stability through the emission of radioactivity – beta particles and gamma rays mostly. Carbon 14 becomes stable by beta decay, losing one half of its original activity by radiation over a period of 5,730 years. The time frame of the decay of carbon 14 decay, which is known as the half-life, is used by paleontologists and archaeologists to date fossils and other biogenic relics.
Nuclear Fission and Energy
Nuclear fission is simply one form of nuclear reaction, though a rather special one that involves the heavier elements, notably uranium and plutonium. It occurs when the nucleus of the atom is struck by a neutron. Depending on the isotope of the target atom and the energy of the incident neutron, the result will be either slow fission or it will be fast fission. The kind of fission that is most prevalent in the world’s 400 plus nuclear reactors is slow fission which is more typically called thermal fission . There are two naturally occurring isotopes of uranium: U238 with 92 protons and 146 neutrons comprises 99.3 percent; and U235 with 3 fewer neutrons is relatively rare at only 0.7 percent. Thermal fission occurs when a slow or thermal neutron is absorbed by the nucleus of a U235 atom. When this happens, U236 is formed. What is unusual about this isotope is that the sum of the masses of U235 and the absorbed neutron is greater than the mass of a normal U236 atom so that there is an excess of energy, enough to cause fission, the breaking apart of the nucleus. Thermal fission can also occur when a neutron is absorbed by Pu239 or U233, neither of which occur in nature. Under the influence of a fast neutron flux, Pu239 is produced from U238 and U233 is produced from thorium (Th232 ) which is a naturally occurring element more common than uranium. The products of fission are gamma rays, beta particles , neutrons as discussed above and two isotopic particles called fission fragments. The fission fragments are the result of the random separation of the uranium nucleus into two smaller segments that are comprised of protons and neutrons whose sum can range from 75 to 160; which is the atomic mass number of that isotope (an example of two fission fragments from a single fission would be krypton 90 and barium 144). The fission fragments are almost certain to be unstable as they will have either too many neutrons or too few protons; they will eventually become stable over time only through beta or gamma decay (for example krypton 90 decays to the stable zirconium 90 with approximately a 28 year half life). Fission fragments are the primary source of energy in the form of heat derived from the nuclear fission reaction.
The potential energy of a nuclear mass is implicit in the now familiar relationship E = mc2 due to the factor afforded by c, the speed of light (3 x 108 m/sec, which, when squared is a factor of about 100,000 trillion) . A kilogram of uranium will accordingly produce more than a million times more energy than a kilogram of a chemical fuel like petroleum. Thus a 1,000 MW generating station requires about 4 kilograms of U235 for each day of operation whereas a 1000 MW coal-fired plant requires 6 million kilograms of coal every day. The electron-volt (ev) is the unit utilized to measure energy on the small scale of the nucleus; it is that amount of energy imparted to an electron by a field of one volt. The electron volt is multiplied by one million to provide a convenient integer representation, i. e. Mev. Each fission of a uranium nucleus produces energy of about 200 Mev. Most of this (166Mev) is carried by the fission fragments which impart their energy as heat to the metallic structure of the reactor core. Water, known as coolant, flows through the core and is heated in the process to the point that it can be used to make steam as motive force for a steam turbine electrical generator. Each fission of a uranium nucleus also produces an average is 2.43 neutrons. This number is called the reproduction factor η which is crucial to the operation of the reactor because neutrons are needed to sustain the chain reaction. The fission neutrons have an average energy of 2 Mev and are called fast neutrons. However, for a neutron to by absorbed by U235 to cause fission, it must be at thermal energy, defined as less than .025 ev which corresponds to a velocity of less than 2200 m/sec. The slowing down or “moderating” of fast neutrons to thermal energies for reuse in the fission chain reaction is accomplished by what is appropriately known as the moderator. Water makes an excellent moderator as the two hydrogen atoms are about the same size as a neutron, and like two billiard balls, the energy of the incident (neutron) ball is absorbed by the target (hydrogen nucleus proton) ball. The reactor is said to be critical when there is one neutron left from the 2.43 neutrons that started out from each fission. This is the point that one fission creates the next fission, and the chain reaction is established.
Nuclear Reactor Technology
The nuclear age of the atomic bomb was conceptually and programmatically transformed into the age of the peaceful use of nuclear energy with the establishment of the Atomic Energy Commission on October 6, 1947. Early research and development evaluated a number of concepts for reactor design that were conducted by government established laboratories at Oak Ridge in Tennessee, Argonne in Illinois, Los Alamos in New Mexico, and Brookhaven in New York. The first program at Argonne was the conceptual design of a power reactor that could also make its own fuel (a type of reactor that has since been anthropomorphically christened as a breeder) by bombarding uranium with neutrons to make plutonium. Ironically, the first reactor to produce electric power was the Experimental Breeder Reactor (EBR) at the National Reactor Testing Station (NRTS) near Arco, Idaho in 1951. The government laboratory at Oak Ridge opted for a reactor core of enriched U235 alloyed with aluminum and cooled or moderated by water. It was this basic concept that formed the nexus with the U. S. Navy’s nuclear power program, which focused on a water reactor using zirconium instead of aluminum as the primary alloying element with enriched U235 as the fuel. The prototype of this design was built at the Idaho NRTS in 1953 and installed in the USS Nautilus (SSN 571) which went to sea on 17 January, 1955 “underway on nuclear power.” The pressurized water reactor (PWR) was perfected by the Westinghouse Corporation and the first commercial nuclear power plant in the United States was put into operation at Shippingport, Pennsylvania on December 2, 1957, reaching its full 60MW generating capacity about three weeks later. A third reactor concept used the reactor core to boil water directly instead of heating water under pressure and transferring the heat to make steam in a secondary system through a heat exchanger, as was the case with the PWR. The so-called boiling water reactor (BWR) was perfected by the General Electric Company and first demonstrated commercially at Dresden, Illinois in 1960.
There are several other approaches to the use of nuclear fission for the generation of electrical power that have reached the stage of commercialization in other countries. They vary according to the coolant used to convey the generated heat from the reactor to the generating station, the moderator used to slow the neutrons for reabsorption, , and the type of fuel used. Gas cooled reactors (GCR) use either helium or carbon dioxide to remove the heat from the reactor instead of water. Since there is no water to slow or moderate the neutrons, graphitic carbon is used within the structure as the moderator. The first reactor that was built and operated to supply commercial power in Europe was the 50 MW gas-cooled Magnox reactor at Calder Hall in the United Kingdom which went on line on 27 August, 1956. In addition to being cooled by carbon dioxide gas with graphite to moderate neutron energy, it used unenriched U238 as the fuel. Gas cooled reactors are still the predominant reactor type in Great Britain. Heavy water can also be used as the moderator and coolant in place of the regular “light” water. Heavy water (D2O) is comprised of two deuterium atoms and one oxygen atom (deuterium is hydrogen with a neutron and a proton in the nucleus). The advantage of heavy water is that it also allows for the use of unenriched U238 as the fuel. This is because the heavy water does not slow down the neutrons from fission as much as H2O; the faster neutrons induce some fission of the more common natural uranium, as long as the neutron population is augmented by some U235. The Canada – Deuterium – Uranium (CANDU) pressurized heavy water reactors (PHWR) are operating in 6 countries in addition to Canada . PWR and BWR reactors are sometimes called light water reactors to distinguish them from the CANDU heavy water reactors. A third basic type is the breeder reactor which converts natural U238 to Pu239 by bombarding it with fast neutrons; the plutonium is fissile. Since the neutrons are not slowed to thermal energies, there is no moderator per se. Liquid metals such as sodium are the preferred coolants since they have the appropriate thermal properties and do not slow down the neutrons. A number of breeder reactors have been built over the last several decades and a few remain in operation. A number of countries are touting breeder reactors as a key element to future energy policy.
Nuclear Waste Storage and Disposal
The 108 light water PWR and BWR reactors have been built of which 103 still remain in operation in the United States have been accumulating “nuclear waste,” which is really spent nuclear fuel for the last 50 years. Spent nuclear fuel is composed of three basic classes of materials: fission fragments which make up about 4 percent, the so-called actinides – elements heavier than uranium which make up about 1 percent, and U238, that is the uranium that did not fission which makes up the remaining 95 percent. Fission fragments, as discussed earlier, are the “real” waste products of fission, as they are random agglomerations of protons and neutrons that decay over time, the decay process results in the generation of heat. It is this decay heat that is central to the long-term storage issue. When the spent nuclear fuel is removed from the reactor after having been in service for anywhere from 3 to 6 years, it continues to generate heat in the range of 10 KW for some time. Because this heat would eventually melt the supporting metallic structure, the fuel must be stored in cooling pools to provide a heat exchange capability. The hue and cry for a permanent storage facility reached a crescendo in the 1970’s as these pools began to fill up. However, the fission fragment decay heat falls off exponentially until only 1 percent is left after a year. After several years in the pool, the fuel assembly can be removed, drained, dried and sealed in metal casks that are placed in concrete silos on-site, the natural circulation of air provides enough cooling for long term and reliable heat removal. This is a very stable arrangement that provides a viable waste storage alternative for decades at existing reactor sites, a 1,000 MW reactor generates about 33 tons of spent fuel in a year, a volume that is about the size of a refrigerator and which fits into about two dry storage casks.
The actinides pose a different problem, that of long term radioactive decay. The actinides are named for actinium, the first element in the series of elements that includes uranium and plutonium; also referred to as transuranics. Consisting primarily of isotopes of plutonium and americium, they have half lives that are measured in the tens of thousands of years. For example, the half life of Pu239 is 24,100 years and the half life of and Am243 is 7,370 years. The chimera of unprotected, unmonitored repositories of radioactive material over tens of thousands of years has proven to be a political if not a geological conundrum. When the nuclear waste issue became manifest, Congress passed the Nuclear Waste Policy Act in 1982. The law stated that the nuclear utility companies would be required to pay a tenth of a cent for every KW-hour of energy delivered and that the federal government would start accepting nuclear waste in 1998. The rise and fall of Yucca Mountain as the repository for this long term nuclear waste is the result of what may be euphemistically called Congressional geology. Based on the 1982 law, a search for possible long term storage sites was undertaken and three were considered: one in Texas, one in Washington State and Yucca Mountain in Nevada. The latter was rendered the site by a process of political elimination in 1987; Jim Wright of Texas was the Speaker of the House and Tom Foley of Washington State was the House majority leader. Not only was Yucca Mountain already on federal property and adjacent to a nuclear weapons test site, but it was in a state with only four electoral votes with limited political sway (Jim Reid of Nevada was in his first senatorial term). The deal fell apart in 2009 due to two key factors, one dealing with radiation, probabilities and geology and the other with politics. The Department of Energy originally sought to show that Yucca Mountain was safe for 10,000 years, all the while acknowledging that the peak radiation level for the intended waste volume would not be reached until about 300,000 years. In 2004, the U. S. Court of Appeals for the Federal Circuit ruled that any repository would have to show that the waste was safe for a million years. Meanwhile, on the political front, the Democrats won Nevada’s 4 electoral votes in the 2008 election and Harry Reid became the Senate Majority Leader. Yucca Mountain is out for the moment, but the political climate and the probabilistic geology may change. Thirty-nine of the fifty states have some sort of nuclear storage from either civilian or governmental provenance. According to one of the Barry Commoner’s seminal laws of ecology, “everything must go somewhere,”. The Nuclear Waste Policy Act is still in force. Nevada has 4 votes. Everyone else has 531. The spent nuclear fuel detritus will have to go somewhere, it cannot be thrown away.
The third component of spent nuclear fuel is the 95 percent of the original U238, which isn’t really spent at all and is only marginally radioactive. With the looming and potentially apocalyptic trajectory of world population, the inexorable depletion of fossil fuel and the buildup of greenhouse gases attendant therein, it is probably prudent to take another look at alternatives to a petroleum Armageddon. The fundamental debate concerns the isotope Pu239 . Unlike U235, it is not found in nature; it is made by bombarding U238 with neutrons. Like U235, it is fissile and can therefore maintain the quintessential chain reaction. The proliferation of man-made Pu239 is a serious stumbling block. It is technologically difficult and expensive to raise or enrich U238 with enough U235 to make it good for a nuclear reactor (about 4% enriched) or a nuclear bomb (about 85% enriched). The current process is to combine it with fluorine in a gas (UF6 or uranium hexafluoride) and use a series of gas centrifuges to separate the lighter U235 from the heavier U238 by centrifugal force. This takes a long time and a lot of centrifuges. This affords a technological limit to the proliferation of U235. When nuclear power was first incepted, it was not anticipated that the proliferation of Pu239 would be an issue. As mentioned above, EBR-1, the first reactor built in the United States was one that made (or bred) plutonium. It was intended that the U238 left over from thermal reactors would be used in breeder reactors to make Pu239 which would become the de facto nuclear fuel. A commercial plutonium reprocessing facility was opened in upstate New York in 1966 and operated for eight years, eventually producing 3,000 pounds of plutonium. It is not surprising that the public and the public policy makers would ultimately prove averse to the strategy of a plutonium based energy program; both the original Trinity bomb exploded at Alamogordo, New Mexico and the subsequent bomb dropped on Nagasaki on August 9, 1945 used plutonium as the fissile material. The arms race of the latter half of the 20th Century was fueled by plutonium, breeder reactors and plutonium processing facilities were built at Savannah River, South Carolina and Hanford, Washington to this end. The Nuclear Non-Proliferation Treaty of 1968 was an attempt to deal with the issue of plutonium as a commodity. The treaty specified that its signatories could avail themselves of nuclear power technologies only with a commitment agreement to eschew nuclear weapons development; however, it proved inadequate since it was voluntary and had no explicit mechanisms for enforcement. Due to the concomitant concerns of nuclear proliferation in light of the reality of an unenforceable treaty, President Jimmy Carter issued an executive order banning the civilian reprocessing of spent nuclear fuel in the United States in 1977. That ban is still in effect in the United States. In the rest of the world, reprocessing is the norm.
Reprocessing and Fast Fission Breeder Reactors
The reprocessing of spent nuclear fuel is common practice among the other industrialized countries that have a substantial commitment to nuclear energy. France’s electrical generating capacity is nearly 80 percent nuclear and the French are the recognized leaders in reprocessing technology; their process of plutonium-uranium extraction with the acronymic name PUREX extracts chemically pure Pu239 from the spent fuel, as it was originally developed for bomb grade plutonium. The Pu239 thus extracted is oxidized as plutonium dioxide and mixed with an oxide of U238 to produce a mixed oxide fuel called MOX that is used as reactor fuel. Russia, France and Great Britain have had reprocessing facilities for over ten years and Japan opened up a twenty billion dollar facility in 2006. However, given the relatively inexpensive (one tenth the cost) and low impact dry cask storage and the problems and costs attendant to the ultimate repository for high level nuclear waste, most countries are opting out of reprocessing. The poor economics of nuclear fuel reprocessing has resulted in a search for alternatives, and, ironically, the leading technology is a variant of the EBR breeder reactor that marked the advent of commercial nuclear energy in Idaho in 1951. This is the basis for the fast fission breeder nuclear reactors that have been touted as “new generation.” The basic idea is to take advantage of the fact that fast neutrons (i. e. those that are not slowed to thermal energies which is necessary when only U235 fission is employed) can be used to cause the much more common U238 to fission in addition to any Pu239 and U235 as they slow to thermal energies. This makes for a much more efficient energy producing system and one that can take advantage of the other fissile materials. The fact that the fast breeder reactor uses fast neutrons means that water cannot be used as the coolant, as it is a neutron moderator. The coolant of choice for this application is liquid sodium , which does not slow down or moderate neutrons so that they remain at high energies for fast fission, as previously discussed. One distinct advantage of the liquid metal reactors is that they operate at low pressures, contrary to the light water reactors which use high pressure water which can flash to steam if there is a piping leak with the attendant Three Mile Island implications. These new reactors are accordingly called Advanced Liquid Metal Reactors (ALMR). This is far from new – the Atomic Energy Commission in cooperation with the nuclear industry designed and started construction of a 400 MW Liquid Metal Fast Breeder Reactor (LMFBR) at Oak Ridge Tennessee with a target operational date of 1979. President Carter’s plutonium ban terminated the program.
Fast fission reactors have two very distinct advantages that should be considered in the debate over future energy policy: they would extend the available nuclear fuel supply by several orders of magnitude; and they would provide a viable alternative to the disposal of spent nuclear fuel. One type of fast fission reactor is the breeder. The term breeder means that the reactor makes more fuel than it uses. This is accomplished by surrounding the operating reactor core, fueled with any viable combination of U238, Pu239 , and U235 with a cylindrical bundle of vertical tubes of U238 which, when bombarded by fast neutrons, produces Pu239 to be used to refuel other ALMR reactors. It is estimated that there 5.5 million tonnes (a tonne, also known as a metric ton, is 1,000 kilograms or about 2,200 pounds – a standard U. S. ton is 2,000 pounds) of uranium reserves worldwide, enough to provide the current demand of 65,000 tonnes of 4 percent enriched U235 needed per year for 80 years. It is estimated that the use of the fast breeder reactor technology would increase the utilization of uranium, including U238 , by a factor of 50. Another way of stating this is that the fast breeder reactor provides about 50 times more energy per tonne of fuel than a similarly sized thermal reactor. In addition to the availability of U238 as an alternative fuel, there is a third fissile material that should be considered, U233 from thorium (Th) . Th232 is a naturally occurring element that is about three times more abundant that uranium. When it absorbs a neutron in a nuclear reaction, it is transformed into the fissile U233 . Though there has been no comprehensive survey of available reserves, it is estimated that there are about 4.4 million tonnes globally. It can be used as a fuel in the CANDU heavy water reactors and could readily be incorporated into advanced reactor designs. Fast fission type reactors also offer an alternative disposal scheme for spent nuclear fuel; they would burn it in what amounts to the nuclear version of the incinerator. The basic premise is to use a reprocessing method called pyrometallurgy that employs a high temperature chemical bath with an impressed electric current to attract the heavy metal transuranics including the actinides and most of the uranium to an electrode which can be removed. The electrode is then removed from the bath so that the reusable materials can be melted down and cast into an ingot for use as reactor fuel. Most of the highly radioactive fission fragments remain in the bath where they can be periodically processed into a glasslike brick for long term storage (in a place like Yucca Mountain). The remaining depleted uranium (mostly U238) could be stored for reuse in a breeder reactor. A 1,000 MW thermal reactor generates about 100 tons of spent fuel a year. It is estimated that a 1,000 MW fast reactor would generate about 1 ton a year.
The energy crisis facing the world mandates a new look at alternatives. Fossil fuel that cannot be replenished is being depleted at an increasing rate; renewable energy sources can only provide a fraction of the growing demand. Nuclear energy is the only clear alternative technology that has been demonstrated. The reuse of U238 from spent nuclear fuel would provide an energy source far into the future without the emission of greenhouse gases. In using the stockpiles of spent fuel that are already on the sites of the reactors where they would be recycled would obviate a large portion of the nuclear waste issue. These benefits must be weighed against the increased risk of bomb-grade plutonium proliferation and a higher probability of a radiological accident like Three Mile Island or Chernobyl. There is also a dollar cost, as the new reactors are projected to cost about $2 billion more than their conventional predecessors. But everything involves some risk and some cost. As Barry Commoner intoned in one of his four laws of ecology “there is no free lunch.”