Discovery of Fission

Prior to the actual discovery of the neutron in 1932, it had long been suspected, in part due to a suggestion by Ernest Rutherford in 1920, that a heavy neutral particle existed as a constituent of the atomic nucleus.1 The discovery of the neutron, as with many of the early discoveries in nuclear physics, was accidental. In the bombardment of beryllium by alpha particles from a naturally radioactive source, a very penetrating radiation had been observed in experiments begun in 1928. In view of...

References

David Halliday, Robert Resnick, and Kenneth S. Krane, Physics, Volume Two, Extended Version, 4th edition (New York Wiley, 1992). 2. J.R. Stehn, M.D. Goldberg, R. Wiener-Chasman, S.F. Mughabghab, B.A. Magurno, and V.M. May, Neutron Cross Sections, Volume III, Z 88 to 98, Report No. BNL-325, 2nd edition, supplement No. 2 (Upton, NY Brookhaven National Laboratory, 1965). 14 Note At 0.0253 eV, T En k 0.0253(1.602 x 10 19) 1.381 x 10 23 293 K 20 C, where k is Boltzmann's constant. 15 We are speaking...

Thermal Neutrons

In many reactors, a succession of elastic collisions with the nuclei of the so-called moderator (see Section 7.2) reduces the neutron energies to lower and lower values as the neutrons impart some of their kinetic energy to the target nuclei. Ultimately, the thermal motion of the nuclei in the reactor cannot be neglected, and the neutron energies then approach a Maxwell-Boltzmann distribution (qualitatively, a bell-shaped curve), characteristic of the temperature of the fuel and moderator. This...

Low Energy Region and the v

For slow neutrons, below 1 eV, one is interested in variations of cross section with energy over a very small energy region. For example, from 0.1 to 0.025 eV the neutron velocity drops by a factor of 2, but the energy change may be small compared to the width of a resonance, typically of the order of 0.1 eV.12 The behavior of the cross section over so narrow a region is often dominated by the tail of a single resonance. If the denominator in Eq. (5.5) does not change appreciably over this...

Competition Between Capture and Fission

For neutrons in a nuclear fuel, such as uranium or plutonium, the most interesting reactions are fission and capture. (Elastic scattering in the fuel changes the neutron energy only slightly, due to the high mass of the target nuclei, and has little importance.) If the goal is fission, as it is with 235U or 239Pu, capture has the effect of wasting neutrons. The ratio of the capture cross section to the fission cross section is therefore an important parameter. It is commonly denoted by the...

Mean Free Path

Neutrons do not travel a well-defined distance through material before undergoing interactions. They are removed exponentially, at separate rates for each of the possible reactions.5 The average distance traversed by a neutron before undergoing a reaction of the type specified is the 'mean free path X for the reaction. Numerically, it is equal to the reciprocal of SP Sx in Eq. (5.1), namely Thus, the mean free path is inversely proportional to the magnitude of the cross section. Although the...

Total Cross Section

The total cross section aT for neutron reactions is the sum of the individual cross sections. If we limit consideration to the reactions considered earlier, then ar aei + a a aei + ain + a y + af, (5.2) where aT is the sum of the elastic scattering cross section (ael) and the absorption cross section (aa), and the absorption cross section is the sum of the cross sections for inelastic scattering (aln), capture (aY), and fission (af ).

Reaction Cross Sections Definition of Cross Section

The neutrons in a reactor may interact with a variety of target nuclei. Possible reactions are elastic scattering, capture, and, in some cases, fission or inelastic scattering. Which of these reactions occurs for an individual neutron is a matter of chance. The probability of each outcome is commonly couched in terms of the reaction cross section a for the event. The term cross section suggests an area, and the reaction cross section for a given reaction can be thought of as the effective...

Nuclear Fission

Energy production in a nuclear reactor derives from fission. In a typical fission reaction, the excited compound nucleus divides into two main fragments plus several neutrons. The fission fragments have high kinetic energies, and it is this energy quickly converted into heat that accounts for most of the energy production in nuclear reactors or bombs (see Section 6.4 for more details on the energy release). A typical fission reaction, here illustrated for a 235U target, is of the general form n...

Neutron Capture

In the first stage of many reactions, the neutron combines with the target nucleus to form an excited compound nucleus. The term neutron capture is usually restricted to those cases where the excited compound nucleus decays by the emission of gamma rays. For example, again taking 238U as the target nucleus, neutron capture leads to the formation of 239U Here, the compound nucleus is 239U*, where the asterisk again indicates an excited state. The number of gamma rays emitted in the de-excitation...

Inelastic Scattering

Inelastic scattering differs from elastic scattering in that the target nucleus is left in an excited state. It decays, usually very quickly, to the ground state, with the emission of one or more gamma rays. An example, the inelastic scattering of neutrons on 238U, is n +238 U n +238 U* n +238 U + 7's, where the asterisk indicates an excited state of 238U. The total kinetic energy of the incident neutron equals the sum of the kinetic energies of the neutron and 238U nucleus after the scattering...

Elastic Scattering

In elastic scattering, a neutron and nucleus collide with no change in the structure of the target nucleus (or of the neutron). An example, the elastic scattering of neutrons on carbon-12 (12C), is Although the structure of the 12C nucleus is unchanged, the neutron changes direction and speed, and the 12C nucleus recoils. The total kinetic energy of 1 See Section A.2.4 of Appendix A for the definition of the electron volt (eV) and the related terms keV (equal to 103 eV) and MeV (equal to 106...

Neutron Reactions of Importance in Reactors

The term nuclear reaction is used very broadly to describe any of a wide array of interactions involving nuclei. Innumerable types of nuclear reactions can occur in the laboratory or in stars, but in considering energy from nuclear fission interest is limited almost entirely to reactions initiated by neutrons. Here, the important reactions are those that occur at relatively low energies, several million electron volts (MeV) or less, characteristic of neutrons produced in nuclear fission.1 These...

Neptunium

The difficulties in accurately establishing the dose caused by the ingestion of radionuclides is illustrated by the case of 237Np.29 It is an extreme case because the changes over time in the assessment of 237Np hazards have been exceptionally large. It is of particular interest because in current calculations for the Yucca Mountain nuclear waste repository, 237Np is the largest contributor to the potential radiation dose after 100,000 years. The dose from the ingestion of a given amount of a...

Radon Spas

For nearly a century, some people have believed that radiation has beneficial health effects and radon spas have flourished. In an interchange in the December 2001 issue of Health Physics, one author termed the notion of a radon health spa an oxymoron, while another brief paper suggested that radon may be helpful in treating rheumatoid arthritis and that the dangers of exposures to radon at low doses are greatly exaggerated in calculations that assume the validity of the linearity hypothesis.

Effects of Radon Exposure in the General Population

The discrepancy discussed in the previous subsection is of lesser immediate importance than the disagreement between the results of two approaches to relating lung cancer rates in the general population to residential radon concentrations. One method for studying the relationship is through case-control epidemiological studies. A case group, composed of people who have been diagnosed as having lung cancer, is compared to a control group of randomly selected people from the same general area....

Radon Radiation Doses from Radon

As discussed in Section 3.5.1, indoor radon is the largest contributor to the radiation dose that the average person receives from natural or other sources. The UNSCEAR estimate of 1.3 mSv yr (the lower of the two estimates quoted in Table 3.5) corresponds to an annual collective dose of about 8 million person-Sv for the world population of 6 billion people. This is more than 10 times the estimated global dose commitment from the Chernobyl accident of roughly 600,000 person-Sv over a 70-year...

Radionuclides of Special Interest Radium

In the early studies of the radioactive decay series in nature, it became obvious that radium was a uniquely valuable radionuclide. The chemical separation of radium from uranium ore yields almost pure 226Ra, with no significant admixture of other isotopes of radium. The half-life of 226Ra is 1600 years, which means that it has a much higher specific activity than, say, 238U or 232Th and yet the activity of a sample of radium will remain almost constant for many decades. Thus, radium became the...

Radiation Standards and Health Criteria Standards for the General Public

From about 1960 to 1990, the standard established in the United States and internationally was that the average additional exposure for members of the general population should not exceed 1.7 mSv yr, and for any individual, it should not exceed 5 mSv yr (excluding radiation workers). This was the maximum permitted dose, over and above the dose received from natural and medical sources. Not coincidentally, this limit for additional dose was about equal to the background from natural and medical...

Genetic Effects

There is no evidence that directly demonstrates genetic damage to the offspring of people exposed to radiation, although in the 1950s and earlier, there was considerable emphasis on the potential genetic effects of radiation, based on animal studies. Since then, in the words of NCRP Report No. 116 the genetic risks were found to be smaller and cancer risks larger than were thought at the time 6, p. 12 . A 2001 UNSCEAR Report summarizes the current evidence as follows No radiation-induced...

Regulatory Guidelines and Scientific Assessments

As emphasized earlier, risks calculated on the basis of the linearity hypothesis do not rest on a firmly established scientific foundation. Instead, they represent estimates that, as thought appropriate for guidelines to be used in radiation protection, may err on the side of caution. If the effects of low-level radiation are not well known, it is appropriate for an advisory body that will influence regulations to take a conservative approach.22 This position was enunciated, for example, in...

Summary of Estimates Relating to Cancer Induction at Low Doses

Table 4.1 summarizes recent estimates for radiation risk coefficients. The overall consensus of these estimates suggests that the risk of fatal cancer at a dose of 1 Sv is 0.10 for the general population and that it is appropriate to use a DDREF of 2 at low doses and low dose rates, at least for low-LET radiation. This leads to an overall estimate for the risk to the general population when exposed to low, protracted doses Risk of eventual fatal cancer 0.05 per sievert (0.0005 per rem). The EPA...

Hormesis and Adaptive Response

The simplest criticism of the LNT hypothesis is that it has not been proven, namely, that there is little evidence from studies of exposed populations that directly demonstrates that low doses of radiation cause cancer. In fact, the converse has also been argued that there are reasons to believe that low doses of radiation do not cause cancer. One argument that is advanced in support of this thesis is based on the existence of biological mechanisms to repair the cellular damage that occurs at...

Difficulties in Obtaining Observational Evidence of Low Dose Effects

In general, there are major difficulties in trying to identify a small number of radiation-induced cancers in the presence of the background of many natural cancers. One difficulty is statistical, especially if the data are subdivided to study cancers for particular age groups or in a particular organ. For example, if a population is divided into 6 age groups and one separately considers 5 cancer types and both genders, there are a total of 60 categories. On average, there should be three cases...

Nuclear Industry Workers

A comprehensive study of cancer mortality workers in the nuclear industry analyzed aggregated data from seven laboratories three in the United States, three in the United Kingdom, and one in Canada 20 . Among the 95,673 workers included, there were 15,825 deaths of which 3976 were from cancer. The population was subdivided by dose and an expected number of deaths was calculated for each group, based on its size, with adjustments for factors such as gender and age. Most of the workers (60 ) had...

Indoor Radon

Studies of the effects of indoor radon are important in their own right, as a guide to establishing sensible protective measures against the possible hazards of the source of most of the collective human dose from natural radiation sources. Because the doses vary substantially from place to place, it might be expected that studies of the relation between indoor radon and lung cancer levels would shed light on the general question of the effects of low-dose exposures. However, as discussed...

Observational Evidence for Cancer at Low Dose Rates Range of Studies

The atomic bomb survivor data, while the best available for the effects of radiation at high doses and high dose rates, become ambiguous for doses below 50 or 100 mSv, and may not be applicable to doses delivered over a protracted period of time. For that reason, there have been extensive studies of the cancer rates in populations other than the atomic bomb survivors that have been exposed to radiation at above-average rates. These include populations living in regions of elevated levels of...

Effects of Low Radiation Doses Importance of Low Doses

From a policy standpoint, it is unfortunate that the available information on the effects of low doses is as inadequate as it is. Virtually all of our direct evidence on the harmful stochastic effects of radiation, for studies in both animals and humans, comes from observations at high doses or high dose rates, most notably the Hiroshima-Nagasaki studies. However, most of the exposures of concern, such as those from nuclear weapon tests, nuclear energy, and indoor radon, involve much lower...

Threshold for Unambiguously Observed Effects

It is convenient to apply a single radiation risk coefficient to broad populations. However, as already noted, this coefficient is based on observations of effects for high doses delivered over a short period of time. To estimate the effects at lower doses, it is necessary to make an extrapolation from the high-dose information, or, in other terminology, to establish the shape of dose-response curve. This topic is discussed again in Section 4.3.3, but here we consider a basic preliminary issue...

Risk Models Absolute and Relative Risk

As suggested earlier, the cancer risk from a given exposure depends on a variety of factors other than the dose itself. For instance, the BEIR V report cites sex, attained age, age-at-exposure, and time-since-exposure as relevant parameters in determining risk 11, p. 166 . Further, the risk varies from organ to organ. The risk can be related to dose using either an absolute or relative risk model. In the absolute risk model, the risk depends on the dose. In the relative risk model, the risk...

Risk Coefficient

The most serious long-term effect of exposures at doses below the 1 Gy (or 1 Sv) region is an increased risk of cancer. This is a stochastic effect because the chance of cancer depends on the magnitude of the dose. There is strong evidence of increased risk down to 0.1 or 0.2 Sv, but there is considerable uncertainty about the effects of smaller doses as discussed at length below. To fill in the picture, extensive efforts have been made to determine the cancer rate as a function of radiation...

Effects of High Radiation Doses Deterministic Effects

It is well established that high radiation doses are fatal. For a dose in the neighborhood of 3-5 Gy received over a short period of time, there is approx 4 See, e.g., Refs. 5 , p. 4, and 6 , p. 8. imately a 50 chance of death within 60 days, although the probability of death is influenced by the prior health of the individual and the treatment administered.5 For Hiroshima victims, 50 lethality was produced at about 3 Gy. For Chernobyl victims receiving doses between about 4 and 6 Gy, 7 of 23...

Types of Effects Deterministic and Stochastic

In radiation studies, a distinction is made between deterministic and stochastic effects.4 Deterministic effects depend on the killing of many cells over a relatively short period of time. They are induced by intense exposures, and the outcome of this exposure is reasonably well defined. The magnitude of the dose determines the intensity of the effect. The most obvious deterministic outcome is the death of the victim within a short time (a few months, or less) after the exposure. Stochastic...

Types of Studies

Much has been learned from the long array of studies and analyses, but as discussed here, some crucial questions remain inadequately resolved. The most direct means of study is the observation of the effects of radiation on humans. There have also been numerous experiments on animals. Although the human and animal studies have provided considerable information on radiation damage at reasonably high doses, they have not led to clear-cut conclusions at low doses. The lower the dose, the smaller...

Agencies and Groups Carrying out Radiation Studies

In recognition of the importance of determining the dangers that ionizing radiation may pose for humans, the health consequences of radiation exposures have been studied almost since the first discovery of X-rays and radioactivity. The studies greatly intensified during and after World War II, with contributions from many individuals and groups throughout the world. Official advice as to radiation protection is provided internationally by the International Commission on Radiological Protection...

Summary

Radioactivity is ubiquitously present on Earth, primarily from natural ra-dionuclides. As a result, the average person receives an effective dose in the neighborhood of 3 mSv yr, with wide geographic variations. Radon and its progeny, particularly indoor radon, are the largest contributors to this dose. The only human activities that produces large doses are medical diagnosis and treatment (primarily from X-rays, not radionuclides), which give an average dose of about 0.4 mSv yr with very large...

Occupational Exposures

The above discussion has focussed on radiation exposures for the general population. Occupational exposures can add appreciably to the dose for some individuals. Here, one can distinguish between past exposures that are of mainly historical interest, present-day exposures that are viewed as ordinary risks of the workplace, and accidents that serve as warnings for the future. In the early history of work with radioactive and fissionable materials, high radiation doses were, on occasion, incurred...

Nuclear Accidents

Two important accidents have occurred in civilian nuclear power plants the 1979 Three Mile Island accident and the 1986 Chernobyl accident. Neither is included in the NCRP tabulation because the total release at Three Mile Island was very small and the releases from the accident at Chernobyl had, in the words of the NCRP, virtually no impact on the population of the United States 3, p. 28 . However, the Chernobyl accident had an impact in parts of Europe. The total exposures, primarily in...

Nuclear Fuel Cycle

The civilian nuclear power fuel cycle, involving mining, fuel fabrication, and reactor operation, contributes a negligible dose to the general public. If calculated on the basis of 1980s practices, as in the NCRP estimate in Table 3.5, it averages about 0.0005 mSv yr for the United States.29 The world average for the 1990s reported in Table 3.5 is even lower. The largest contributions to the dose are from uranium mining and processing operations, including the 27 The United States, USSR, and...

Other Sources of Radiation Consumer Products

A variety of other human activities involve radiation exposures. One broad group is listed in Table 3.5 as consumer products. These are considered by the NCRP but not included in the UNSCEAR compilation. Sources of exposure in this category most notably include natural radionuclides in the water supply and in building materials 3, p. 31 . Another large but uncertain source of exposure is tobacco. Tobacco leaves collect lead-210 (210Pb) from the air and this radionuclide and its immediate decay...

Radiation Doses from Medical Procedures

X-rays have been used for medical diagnosis for almost 100 years. The trend with time has been to more examinations per capita and, with better defined beams and more sensitive detection, smaller doses per examination. The world average dose per capita for 1991-1996 was 0.4 mSv yr. Variations among countries were great, with the more prosperous countries in general having higher level of health care, including a greater per capita frequency of X-ray examinations and a higher average radiation...

Cosmogenic Radionuclides

Cosmogenic radionuclides are continuously produced in the atmosphere by incident cosmic rays, most importantly carbon-14 (14C) from the interaction of cosmic-ray neutrons with nitrogen-14 atoms. Actually, tritium (3H) is produced at a still higher rate, but tritium (T 12.3 years) has a much shorter half-life than 14C (T 5730 years) and, therefore, its inventory in the atmosphere is much less. As a result, almost all of the effective dose of 0.01 mSv yr from cosmogenic radionuclides is due to...

Cosmic Rays

Cosmic rays impinge on the top of the Earth's atmosphere, both from the Sun and from outside the solar system. The cosmic rays reaching sea level are primarily secondary particles produced when energetic galactic cosmic rays collide with molecules in the atmosphere. They include muons, neutrons, gamma rays, and electrons. The total flux (and dose) varies greatly with altitude and, to some extent, with latitude. Averaging over latitude, the population-weighted mean outdoor dose at sea level is...

Terrestrial Radiation

Many radionuclides in the ground in particular, 40K and members of the uranium and thorium series emit gamma rays and produce an appreciable radiation exposure. This terrestrial dose includes doses from both the soil and building materials, with substantial variations in the magnitude of the dose from location to location. For example, limestone rocks and wood buildings have much lower radionuclide content than granite rocks and buildings of brick or granite. The outdoor dose is due to the...

Survey of Radiation Exposures Natural Sources of Radiation Overview

Average radiation exposures resulting from natural sources are far greater than those that result from human technology (especially if one omits medical exposures), although the latter usually attract greater attention. The important natural sources include inhaled radon and its progeny, radionuclides in the Earth's crust, radionuclides in the body, and cosmic rays. The resulting doses are summarized in Table 3.5, in terms of averages for the United States and the world.18 The two sets of...

Radionuclides in the Oceans

Over the billions of years since the Earth was formed, there has been a substantial interchange of material between the land and oceans. For example, flowing water erodes rocks and carries material from the rocks into the oceans. Therefore, it is to be expected that the same elements that are present in the Earth's crust will also be present, to one degree or another, in the oceans. However, a high concentration of an element in the Earth's crust does not necessarily mean a high concentration...

Concentrations of Radionuclides in the Environment Radionuclides in the Earths Crust

There are substantial variations in the concentrations of chemical elements and, hence, of radionuclides in different parts of the Earth's crust. For example, for the crust as a whole, the uranium abundance by mass is about 3 parts per million (ppm) (i.e., 0.0003 ), with common rocks having ura nium concentrations ranging from 0.5 to 5 ppm 14, p. 61 . Concentrations in uranium ores commonly exceed several hundred ppm, and there are deposits that are reported to have concentrations as high as 65...

The Linearity Hypothesis

According to the linearity hypothesis, the cancer risk (above the normal rate) is proportional to the magnitude of the excess radiation dose, over the full range from zero dose to high dose, independent of the dose rate.14 Thus, if a dose of 1 Sv in a brief period corresponds to a 10 risk of a radiation-induced cancer, then a dose of 1 mSv (gradual or sudden) corresponds to a 0.01 chance. More generally, each increase of 1 mSv in dose would add 0.01 to the cancer risk. Sometimes, to be...

Status of Reprocessing Programs

Until the late 1970s, reprocessing had been planned as part of the U.S. nuclear power program. A reprocessing facility at West Valley, New York was in operation from 1966 to 1972, with a capacity of 300 MTHM yr. This is enough, roughly speaking, for the output of 10 large reactors. There were plans for further facilities at Morris (Illinois) and Barnwell (South Carolina) which would have substantially increased the reprocessing capacity. However, all these plans have been abandoned.27 In part,...

Extraction of Plutonium and Uranium

The alternative to disposing of the spent fuel is to reprocess it and extract at least the uranium and plutonium. In reprocessing, the spent fuel is dissolved in acid and the plutonium and uranium are chemically extracted into separate streams, for use in new fuel. The most widely used method for this is the suggestively named PUREX process. Most early U.S. plans for reprocessing assumed that 99.5 of the U and Pu would be removed. The remainder constitutes the high-level waste. In the...

Disposal or Storage of Spent Fuel

For many years, it had been assumed that all U.S. civilian nuclear waste would be reprocessed, but U.S. reprocessing plans have been abandoned. Instead, 25 An annual refueling shutdown of 2 months would mean a maximum capacity factor of 83 , which is well below the present U.S. average. official plans now call for disposing of the spent fuel directly, while retaining for many decades the option of retrieving it. The fuel is to remain in solid form and the fuel assemblies eventually placed in...

Back End of Fuel Cycle Handling of Spent Fuel Initial Handling of Reactor Fuel

Periodically, a portion of the fuel in the reactor is removed and replaced by fresh fuel. In typical past practice, an average sample of fuel remained in the reactor for 3 years, and approximately one-third of the fuel was removed each year, with a shutdown time for refueling and maintenance of up to about 2 months. The trend is now to extend the interval between refueling operations and to reduce the time for refueling.25 Currently, time intervals of 18 months and shutdowns of 1 month are...

Energy per Unit Mass of Fuel

The discussion in the preceding subsection is incomplete, because it omits many crucial factors that significantly modify the amount of 235 U required by a reactor. These include the following 1. Not all of the 235U is consumed in the reactor. For example, for the case described in Table 9.2, the 235U content per MTHM is 37.5 kg in the fresh fuel and 8.6 kg in the spent fuel (i.e., a consumption of only 77 of the 235U). 2. About 14 of the thermal neutron-absorption reactions in 235U result in...

Burnup as a Measure of Fuel Utilization Thermal Efficiency of US Reactors

The thermal efficiency of a reactor is the ratio of the electrical energy produced to the total heat energy produced. Since 1973, the average thermal efficiency of U.S. reactors has ranged between 30.6 and 32.1 , according to DOE compilations 23, Table A6 . There has been a gradual improvement with time, and since 1985 it has been above 31.5 , reaching 32.1 for the years 1996-2002. We will use the approximate figure of 32 as the nominal average efficiency of LWRs. 19 For a discussion of the...

Other Fuel Types

The focus here has been on UO2, which is the usual fuel for LWRs. Other fuel types are of interest, however, even if not widely used at present (see, e.g., Ref. 1 ). Possibilities include the following Mixed-oxide fuel (MOX). MOX fuel, a mixture of uranium and plutonium oxides, uses plutonium in order to exploit its energy content, reduce the stocks of potential weapons materials, or both (see Section 9.4.2). Metal alloy fuels. Metallic fuel, in the form of alloys of uranium, provide an...

Separative Work

In a 235U enrichment process, there are three streams of material the input or feed, the output or product, and the residue or tails. The system operates with a cascade of steps, with the enrichment of the product increasing successively in each step.17 As the enrichment cascade progresses, the tails from an intermediate stage have a higher 235U concentration than the original feed material, and these tails can profitably be returned to the cascade. There are different strategies for reusing...

Adopted Enrichment Practices

During World War II, not knowing which method would be the most effective, the United States embarked on both diffusion and electromagnetic separation, as well as still another method that was later discarded (namely thermal diffusion, which exploits temperature gradients). The electromagnetic separation technique was abandoned in the United States after World War II and was widely considered to be obsolete. However, it was found in 1991, after the Gulf War, that Iraq had been secretly using...

Methods for Enrichment

The leading enrichment methods in terms of past or anticipated future use are as follows 13 Gaseous diffusion. The average kinetic energy of the molecules in a gas is independent of the molecular weight M of the gas and depends only on the temperature. At the same temperature, the average velocities are therefore inversely proportional to M. For uranium in the form of UFg, the ratio of the velocities of the two isotopic species is 1.0043.14 If a gas sample streams past a barrier with small...

Degrees of Enrichment

Natural uranium has an isotopic abundance by number of atoms of 0.0055 234U, 0.720 235U and 99.275 238U.9 In the remainder of the discussion of uranium isotopic enrichment, we will follow the standard practice of describing the 235U fraction in terms of 'mass rather than, as is common in many other scientific applications, of number of atoms.10 For natural uranium, the 235U abundance by mass is 0.711 . The presence of the small amount of 234U is often ignored, because corrections on the order...

Radon Exposures from Uranium Mining and Mill Tailings

In the early days of uranium mining, little attention was paid to radiation safety. In the Middle Ages, long before uranium had been identified as an element, metal miners in southern Germany and Czechoslovakia contracted lung ailments, called Bergkrankheit (mountain sickness). Modern scientists have attributed the ailment to lung cancer caused by a high uranium concentration 4 U3O8 is not yellow in its pure form. Yellowcake is about 85 U3O8 5, p. 241 , and the yellow color results from another...

Front End of the Fuel Cycle Uranium Mining and Milling Uranium Deposits in the Earths Crust

The concentration of uranium varies greatly among geological formations. The average concentration in the Earth's crust is about 3 parts per million (ppm) by weight, but extremes extend from under 1 ppm to something in the neighborhood of 500,000 ppm.3 Uranium resources are widely distributed, with substantial uranium production in many countries, including Australia, Canada, Kazakhstan, Namibia, 3 For example, one deposit in Canada is identified as having zones of over 50 U3O8, which...

Steps in the Nuclear Fuel Cycle

A schematic picture of the fuel cycle is shown in Figure 9.1, which indicates alternative paths, with and without reprocessing 2 . The steps in the fuel cycle that precede the introduction of the fuel into the reactor are referred to as the front end of the fuel cycle. Those that follow the removal of the fuel from the reactor comprise the back end of the fuel cycle. At present, there is only a truncated back end to the fuel cycle in the United States, as virtually all commercial spent fuel is...

Characteristics of the Nuclear Fuel Cycle Types of Fuel Cycle

The nuclear fuel cycle is the progression of steps in the utilization of fissile materials, from the initial mining of the uranium (or thorium) through the final disposition of the material removed from the reactor. It is called a cycle because in the general case, some of the material taken from a reactor may be used again, or recycled. Fuel cycles differ in the nature of the fuel used, the fuel's history in the reactor, and the manner of handling the fuel that is removed from the reactor at...

The Natural Reactor at Oklo

A remarkable discovery was made in 1972 by French scientists analyzing uranium extracted from the Oklo uranium mine in Gabon. The uranium was depleted in 235U, sometimes by large amounts, although, normally, the isotope ratios in uranium are nearly constant over the surface of the Earth. It was soon suspected and then demonstrated that this isotopic anomaly was due to a natural uranium chain reaction occurring more than a billion years ago. Conclusive evidence in support of this explanation was...

High Conversion Ratios Without Breeding

Before turning to fast breeder reactors, it may be noted that even if breeding is not achieved with thermal reactors, a high conversion ratio can still be desirable. One motivation could be plutonium production. Another motivation is the extension of fuel resources. As the conversion ratio increases, the energy output increases for a given original 235U content. A high conversion ratio means a high ratio of capture in 238U to absorption in 235U. This must be accomplished without losing...

Potential of U for a Thermal Breeder Reactor

The number of neutrons produced is significantly higher for 233U (no 2.296) than for 235U, and there have been serious suggestions for developing 233U thermal breeders. These date to as early as 1945, in work done by Eugene Wigner's group in Chicago 3, Chapter 6 . A cycle is envisaged in which 233U is produced initially in a reactor with 235U as the fissile fuel and 232Th as the fertile fuel. Subsequently, a 233U-232Th cycle could, in principle, be self-sustaining. Not only is no higher for...

Difficulty of Reaching a Conversion Ratio of Unity with U

As discussed in Section 7.4, the limiting condition for a breeder reactor is that the conversion ratio, C, be at least 1. This means that the number no of neutrons produced for each neutron absorbed in 235U, must be two or more.15 For thermal neutrons absorbed in 235U, no 2.075 (see Table 7.1). Were there no losses, this would suffice for breeding 1 neutron for continuing the 14 Sometimes, a distinction is made in terminology, with conversion ratio used when C < 1 and breeding ratio used when...

Characterization of Reactors

As discussed in Section 7.1.1, the condition for a chain reaction is that for every neutron initiating fission in one generation, one or more neutrons initiate fission in the next generation. If, in addition, another fissile nucleus is produced for every 235U atom consumed, then there is no decrease in the amount of nuclear fuel available. This is the principle of the breeder reactor. The conversion ratio C (or breeding ratio B) is defined as the ratio of the rate of production of fissile...

PWR Reactor Cores

We consider here the specific characteristics of a reactor core based on a Westinghouse Corporation PWR design, but the gross features are similar for all large LWRs.13 The reactor fuel is in the form of cylindrical uranium oxide (UO2) pellets, about 0.8 cm in diameter and 1.35 cm in length. The pellets are placed in tubes called fuel rods or fuel pins made of zircaloy, a zirconium alloy (98 Zr, 1.5 Sn, and small amounts of other metals 16, p. 234 ) selected on the basis of structural strength...

Components of a Light Water Reactor

The containment structure and enclosed components for a typical PWR and a typical BWR are shown schematically in Figures 8.2 and 8.3.12 The most conspicuous difference between them is the absence of a steam generator in the BWR. At the heart of the reactors, literally and figuratively, is the reactor core, contained within the reactor pressure vessel. The pressure vessel encloses three vital components The fuel itself, contained in many small fuel rods comprising the reactor core. The...

PWRs and BWRs

The two types of LWR in use in the world are the pressurized water reactor (PWR) and the boiling water reactor (BWR). The difference between them, as embodied in the names, is in the condition of the water used as coolant and moderator. In the PWR, the water in the reactor vessel is maintained in liquid form by high pressure. Steam to drive the turbine is developed in a separate steam generator. In the BWR, steam is provided directly from the reactor. These differences are brought out in the...

History of Commercial Reactor Development

After World War II, the leading countries in nuclear reactor development were the United States, the United Kingdom, Canada, and the Soviet Union. Each went in a different direction. The first U.S. power reactors, beyond plutonium-producing or experimental reactors, were built for submarines, not for civilian electricity generation. The earliest were a PWR for the submarine Nautilus, commissioned in 1955, and a sodium-cooled reactor for the submarine Seawolf. The Seawolf reactor had...

Types of Reactors

A variety of different reactors are in use in the world today, although there was greater diversity in the early days of reactor design. Table 8.1 lists the types of nuclear power plant in operation in late 2003 as well as those reported to be under construction or on order 10 . The dominant reactor is the light water reactor (LWR), which uses ordinary water as both the coolant and moderator and enriched uranium in UO2 pellets as the fuel. There are two types of light water reactor the...

World Inventory of Reactor Types Reactor Sizes

The earliest reactors had generating capacities well below 100 MWe, but there was a rapid transition to 1000-MWe reactors and larger. The move to a larger size was motivated by the desire to capture economies of scale. Some analysts suggest that this escalation proceeded too rapidly, especially in the United States, and was responsible for some of the difficulties encountered in achieving short construction times and reliable operation. The mean capacity of all reactors in operation worldwide...

Control Materials

As discussed in Section 7.5.2, control materials are needed to regulate reactor operation and provide a means for rapid shutdown. Boron and cadmium are particularly good control materials because of their high cross sections for the absorption of thermal neutrons. These control materials are usually used in the form of rods. Control rods for pressurized water reactors (PWRs) commonly use boron in the form of boron carbide (B4C) or cadmium in a silver-indium alloy containing 5 cadmium. Control...

Coolants

The main function of the coolant in an electricity generating plant is to transfer energy from the hot fuel to the electrical turbine, either directly or through intermediate steps. During power plant operation, cooling is an intrinsic aspect of energy transfer. However, in a nuclear reactor, cooling has a special additional importance, because radioactive decay causes heat production to continue even after the reactor is shut down and electricity generation has stopped. It is still essential...

Moderators

As discussed in Section 7.2, a moderator is required if the reactor is to operate at thermal neutron energies. This means that most operating reactors use moderators, with the fast breeder reactor the exception. The options for moderating materials are limited Hydrogen (Z 1 . The isotopes 1H and 2H are widely used as moderators, in the form of light (ordinary) water and heavy water, respectively. 4 In particular, this fuel is for high-temperature gas-cooled reactors (see Section 16.4.3). Helium...

Fuels

There are few nuclides that can be used as reactor fuels. The paucity of possible candidates can be seen by examining the properties of the naturally occurring heavy elements 2 The fast breeder reactor program was subsequently sharply reduced, with the centerpiece of the U.S. fast breeder reactor program, the Clinch River Breeder Reactor, abandoned in 1975. 3 There has been speculation about a quite different sort of molten-salt reactor, driven by a proton accelerator. If pursued, this would...

Nuclear Weapon Tests

Extensive aboveground nuclear weapons tests were carried out by the United States, the Soviet Union, and other countries, particularly in the period from 1952 to 1962.27 The resulting worldwide average effective dose peaked at 0.11 mSv yr in 1963 and dropped to 0.0055 mSv yr by 1999 7, pp. 228-230 . The average cumulative dose from 1945 to 1999 (i.e., the sum of the annual average doses over the 55-year period) was 1.1 mSv in the northern hemisphere, 0.3 mSv in the southern hemisphere, and 1.0...

Components of Conventional Reactors Overall

Any generating plant consists of an array of structural components and a system of mechanical and electrical controls. In a nuclear plant, there are special demands on structural integrity and reliability. In addition, a nuclear reactor is characterized by the use of specialized materials, some aspects of which were already discussed in Chapter 7. In standard reactors, these are the fuel itself, the coolant, the moderator, and neutron-absorbing materials used to control the power level. A main...

Western Hemisphere Other Than the United States

Canada embarked on a large program of nuclear construction, mostly in the late 1970s, and in 1995 had 22 operating reactors, all based on the CANDU reactors, which have made Canada a leading country in nuclear capacity and generation. However, in the 1990s, 8 of the 22 reactors in Canada were put into a temporary laid-up status, one in 1995 and 7 in late 1997 and early 1998. These eight were the oldest ones in operation, all having started up in the 1970s. This step was taken at the behest of...

Measures of Waste Magnitudes

The inventories of wastes have been described earlier in terms of mass. However, several different sorts of measure can be used to describe the amount of nuclear wastes Mass. The most common mass measure for nuclear waste is the mass of the uranium in the initial fuel, more broadly designated as metric tonnes of initial heavy metal (MTIHM or just MTHM) (see Section 9.3.1). The fuel is held in cylindrical fuel rods, usually made of zircaloy, which are grouped in assemblies. The total mass of an...

Inventories of US Nuclear Wastes

Inventories of U.S. commercial and military wastes are summarized in Table 10.1. They include the wastes that are planned for the Yucca Mountain repository as well as further amounts expected to be produced. The commercial wastes constitute 90 of the total slated for Yucca Mountain, measured in terms of the mass of the initial uranium used to generate the wastes (in MTHM). In general, they are the hottest wastes, so they contain more than 90 of the total activity 3, p. A-9 . When the...

High and Low Level Wastes

Nuclear wastes are sometimes divided into high-level and low-level waste, along with a separate category of transuranic waste High-level waste (HLW) is the highly radioactive fission and neutron-capture product of the nuclear fuel cycle. It may be in the form of either spent fuel or liquid and solid products from the reprocessing of spent fuel.1 (In some alternative definitions, spent fuel is in a category by itself. Many DOE tabulations reserve the term HLW for reprocessed wastes alone, and...

Military and Civilian Wastes

As discussed in Chapter 2, the first nuclear reactors were those built during World War II to produce plutonium for weapons. In order to extract the plutonium, it is necessary to reprocess the spent fuel, first converting it to liquid form. The residue remaining after the plutonium and uranium (and sometimes other elements) are extracted constitutes the reprocessed wastes. Reprocessing increases the volume of the residue and puts it in a form that can more readily escape into the environment....

Categories of Nuclear Waste The Nature of the Problem

The term nuclear waste embraces all residues from the use of radioactive materials, including uses in medicine and industry. The most highly radioactive of these are the spent fuel or reprocessed wastes from commercial nuclear reactors and reactors that produced plutonium for nuclear weapons. The other wastes have much lower levels of activity and have far less potential to cause harm, although the establishment of sites for the disposal of these wastes has encountered public opposition in many...

Impact of Fuel Cycle Changes and Breeder Reactors

The question of changes in the fuel cycle and particularly the question of breeder reactors is closely connected to the issue of resources. For the most part, we have assumed a once-through fuel cycle using uranium. If, instead, a comprehensive reprocessing plan is implemented, such as the full actinide recycle option discussed in Section 9.4.3, the situation would be somewhat relieved. In this cycle, the uranium required for a given electricity output is roughly halved, which effectively...

Uranium from Seawater

Uranium from seawater represents a very large potential additional resource, but at a cost that may be considerably higher than that of any uranium resource considered earlier. The volume of seawater in the oceans is about 1.4 x 1021 L, with an average uranium concentration of 3.2 parts per billion (see Section 3.4.3), corresponding to 4 billion tonnes of uranium. However, the energy content is very dilute, and vast amounts of water would have to be processed to extract uranium. The cost of...

Adequacy of Terrestrial Uranium Resources

Adopting the probably conservative resource estimate of 20 million tonnes, uranium resources would suffice for the needs of LWRs for about 100,000 GWyr at a rate of 200 tonnes of U per gigawatt-year. Present world generation from nuclear power is roughly 300 GWyr yr. Thus, a resource of this magnitude could sustain four times the present rate of generation for 80 years. Such an increase does not appear to be imminent and, therefore, there is little pressure on uranium resources at this time....

Estimates of Uranium Resources Classification of Resources

An extensive description of the world uranium industry is provided by the Red Book series, published by the OECD since the 1960s under the title Uranium Resources, Production and Demand (see, e.g., Ref. 4 ). It is common to classify resources in terms of the degree of knowledge of the location and extent of identified or expected uranium deposits. So-called conventional resources are divided in the Red Book into four groups reasonably assured, estimated additional (in two categories of...

Uranium Prices and Electricity Costs

Uranium prices are now very low compared to those projected several decades ago. They dropped markedly in recent years, in large measure due to the lag in the expansion of nuclear power. U.S. prices peaked in 1978 at an average of 43 lb of U3O8 47 . In 2001, the average price paid by U.S. utilities was 10 lb U3O8 ( 26 kg U) 17, p. 11 . The relationship between uranium price and the contribution of uranium fuel costs to electricity costs depends on the effectiveness of fuel utilization. As was...

Conventional Units for Amounts of Uranium

The magnitude of uranium resources can be specified in terms of the amount of uranium oxide (U3O8) or the amount of natural uranium (U). Commonly, U.S. organizations expressed the resources in short tons of U3O8, whereas international organizations, such as the OECD, use tonnes of uranium. The units are related by the equivalence 30 In terms of the units in which uranium prices are usually couched, 1 kg of U is equivalent to 2.60 lbs of U3O8 therefore, a price of 100 lb of U3O8 is equivalent to...

Waste Disposal

All countries with announced plans for disposing of high-level radioactive wastes are planning on eventual disposal in deep geologic repositories, typically made by excavating caverns or holes in favorable environments. Many of the plans for these permanent disposal facilities include a period during which the waste could still be retrieved. Deep geologic disposal has been the favored course in U.S. thinking since the first attempts to formulate plans. There have been continuing efforts to...

General Features of Reprocessing Options

Any fuel cycle that recycles the fissile components of the spent fuel (mainly the remaining 235U and the plutonium isotopes 239Pu and 241 Pu), increases the energy obtained from the existing uranium resources. If the minor actinides are included with the uranium and plutonium in the new fuel, the wastes will have much less long-term radioactivity than wastes in the once-through fuel cycle. The mass of the spent fuel is greatly reduced if the uranium is either returned to the reactor or is...

Pyroprocessing

The above-discussed chemical reprocessing processes are known as aqueous processes. An alternative approach, under active exploration for use in conjunction with future reactors, is the pyroprocess or electrorefining process. In this method, the spent fuel is dissolved at very high temperatures in molten cadmium, creating an electrolytic bath. Groups of chemical elements are separately extracted on the basis of differences in the potentials at which they dissolve and ionize. In particular, ions...

The UREX Process

An alternative to the advanced aqueous process is the uranium extraction process (UREX and UREX+). It differs in the means of separating out the uranium. Several output streams are specifically identified in this process 45, p. II-3 1. Uranium. The uranium is extracted in very pure form (at purity levels of 99.999 percent). The leaves it free of highly radioactive contaminants and makes it easy to handle for disposal or reuse in a reactor. 2. Plutonium and minor actinides. Neptunium, americium,...

Alternative Reprocessing and Fuel Cycle Candidates Advanced Aqueous Process

In the widely used PUREX process, the plutonium and uranium are extracted and the fission products and minor actinides constitute the wastes. The advanced aqueous process is a modification of the PUREX process in which the minor actinides are recovered as well. Uranium is crystallized out at an early stage to reduce the bulk of the material that must be dealt with in the further chemical processing 44, p. 60 . The two product streams, one of uranium and the other of plutonium and the minor...

Use of Mixed Oxide Fuel

The fuel manufactured from the output of the reprocessing phase is generally a mixture of plutonium oxides and uranium oxides, with 3 to 7 PuO2 and the remainder UO2. It is called a mixed-oxide fuel or MOX. At the higher 239Pu enrichments, a burnable poison would be added to the fuel to reduce its initial reactivity. Due to differences in the nuclear properties of 239Pu and 235 U, most LWRs are limited to using only about a one-third fraction of MOX Table 9.4. Reprocessing plants for commercial...

Isotopes and Elements

It is necessary to keep in mind the distinction between the atomic mass ME of the element, as it occurs with its natural mixture of isotopes, and the mass Mi of any particular isotope. The atomic mass of an element is given by where fi is the fractional abundance of the isotope, by number of atoms. For example, carbon has two stable isotopes 12C and 13C, with atomic masses Mi 4 In this expression, although A is ordinarily defined as a dimensionless integer, we attach to it the same units as the...

Atomic Masses and Energy Release A Atomic Mass and Atomic Mass Number

In describing a nuclear species for example, 238U one could specify either the mass of the nucleus or the mass of the atom (the nucleus plus the electrons). It is virtually universal practice to specify atomic mass. The mass M of an atom (expressed in atomic mass units) is not exactly equal to the mass number A of the atom, except for 12 C, where the equality is a matter of definition. However, the numerical difference between M and A is small. This is because the constituents of the atom the...