Nuclear chemistry and radiochemistry

The ordinary chemical properties of the elements depend on the occupancies, energies, and shapes of the electron orbitals that account for almost the entire volumes of atoms. Ultimately, of course, it is the nuclear charge that determines the electronic structure of the atom, and virtually all the mass of the atom is carried in the nucleus. Generally, however, we tend to regard the nucleus as something that is "just there", and whose detailed study belongs more in the realm of physics than chemistry.

But there are a number of properties and proesses involving the nucleus that have direct chemical consequences, and these will be the subject of this unit. Foremost among these is radioactive decay; this process not only transforms the nucleus of one atoms into that of another, but the particles emitted can cause further chemical changes when they strike nearby molecules.

Structure of the nucleus

The chemist's working model of the nucleus envisages a collection of protons and neutrons, each having a mass of about 1 atomic mass unit, and all collected together into an extremely compact region of unimaginably high density.

A given nuclide (a particular kind of nucleus) is characterized by its atomic number which gives both the nuclear charge and the number of protons, and its neutron number. The sum of the atomic and neutron numbers is the mass number.

A nuclide is designated by writing its chemical symbol (determined by its atomic number) together with a superscript denoting the mass number. For e xample, the nuclide with atomic number 16 and neutron number 18 would be 34S; if we want to remind ourselves that the atomic number of sulfur in 16, we could write 16S34.

What holds the nucleus together?

Since the only electrostatic charges in the nucleus are positive, it is clear that the electrostatic (electromagnetic) force, which is the sole force governing the behavior of the electrons, cannot be responsible for the stability of the nucleus. On the contrary, electrostatic repulsion between the p rotons actually promotes the decay of nuclei. There must then be some other force between the nucleons (the particles within the nucleus) that is stronger than the electrostatic repulsion and thus capable of overcoming it.

Even the strong force has its limits, however. This can be seen by noting that all stable nuclides heavier than hydrogen contain neutrons as well as protons, and the ratio of neutorns to protons increases as the atomic number increases. This implies that neutrons are reqired to "dilute" the electrostatic repulsion of the protons so as to allow the strong force to predominate.

Binding energy of the nucleus

For most practical purposes, chemists are able to regard the proton and neutron as having identical masses, each being associated with a mass number of unity on the atomic mass scale. In nuclear chemistry, howev er, we must be more exact. The most precise mass measurements of the two kinds of nuclear particles y ield the following values:

proton mass = 1.007825 amu = 1.67265E–27 kg

neutron mass = 1.008665 amu = 1.67495 E–27 kg

It would appear from this that a nucleus of 2He4, being composed of two protons and two neutrons, should have a mass of just twice the sum of the above two numbers, or 4.032980 amu. The measured mass of the helium nucleus turns out to be slightly less than this, 4.002604 amu. The difference is known as the mass defect.

The mass defect represents the mass that is lost when two protons and two neutrons combine to form a helium-4 nucleus. Where did the mass disappear to? If you recall Einstein's mass-energy relationship E = mc2, it should be apparent that the "lost" mass was simply converted into energy. The formation of the helium nucleus can thus be represented as the exothermic reaction

2p + 2n = 2He4 + (energy equivalent to 0.030376 amu)

Nuclear energy units: mass and MEV

Everybody is familiar with the formula E = mc2, but the units in which this formula expresses energy are kg m2 s–2. Does this make sense? It does if you recall that kinetic energy is mv2/2, and thus has the very same mass units. A mass of 1 kg moving at a velocity of 1 km/s possesses a kinetic energy of 1 J; this is in fact the definition of the joule, so the Einstein formula gives the energy directly in joules.

The joule is quite appropriate for expressing the energy changes of ordinary chemical reactions, but for nuclear processes that involve the conversion of mass into energy, a much larger unit would be more convenient. Logic would suggest the use of megajoules or gigajoules, but the traidition is to use another unit, the electron-volt. The kinetic energy acquired by one mole of electrons that have been accelerated through a potential difference of 1 volt is 96.49 kJ. Thus the electron-volt is roughly 100 kJ/mole — about the energy of a weak chemical bond. Nuclear reaction energies are most commonly expressed in millions of electron-volts, or MEVs.

Of course, if we turn the Einstein equation around, wqe see that mass is proportional to energy (m = Ec2), so in large-scale nuclear reactions such as occur in stars, energies are frequently expressed indirectly as masses: kg, grams, or daltons (atomic mass units).

Binding energy and nuclear structure

The binding energy of the He4 nucleus is 0.030376 d, or 28.294 MeV. (Binding energies are always epxressed in terms of one mole of nuclei.) Similar highly exohermic energies are associated with the formaiton of other stable nuclei. In order to compare the binding energies of different nuclides, it is more useful to divide this total binding energy by the number of protons and neutrons in the nucleus. This quantity is known as the binding energy per nucleon.

The binding energy per nucleon is a function of mass number, as illustrated in the plot.

**** binding energy plot 2-1 ****

The relationship depicted above is the basis for all forms of "atomic energy", so it is important that you know its general form and thoroughly understand its significance. The most important feature of this plot is the maximum that occurs at mass number 56. For this particular mass number, the most stable arrangement of nucleons occurs when they are in groups of 26 protons and 30 neutrons, corresponding to the iron nucleus 26 Fe56.

All other possible arrangements of neutrons and protons are less stable and are exothermically convertible into Fe56 . Iron is therefore the heaviest element that can be synthesized in normal stars.

Nuclear structure, stability, and isotopic abundances

The stability of a nucleus depends both on the mass number (there are no stable nuclei heavier than Bi83) and on the ratio of neutrons to protons. Of the 280 known stable nuclei, all except 1H1 have at least as many neutrons as protons. Moreover, it is clear from the plot that as the mass number increases, the relative number of neutrons needed to maintain stability increases.

*** relative atomic abundances Fig 2-3 ***

The greater the binding energy of the nucleus, the greater the probability will be that once formed, it will resist destruction. This means that the more stable nuclei should be more abundant in the universe. On this basis, i appears that nuclides having neutron numbers of 2, 8, 20, 28, 40, 50, and 126 are especially stable, as shown in the plot

*** Fig 2.3 ***

This sequence of "magic numbers" is reminiscent of the similar series 2, 8, 18, etc., that describes the occupancy of the atomic electron shells, and it forms the basis of a similar shell model of the nucleu. According to this model, which we shall not describe in detail, nuclei containing even numbers of nucleons are more favored (notice that the magic numbers are all even). As shown below, most stable nuclei have even N and even Z, and thus even mass numbers:

*** table *** pg 5

= = = = =

Radioactive decay

Nuclear chemistry had its beginning with the discovery of Becquerel in 1896 that uranium salts produce radiation which, like X-radiation, could be regorded photographicall, and could also produce ions when passed through air.

Further study revealed that different radioactive substances produce different types of "rays" (particles of radiation). Each is characterized by the manner in which a collated beam of the ray is deflected by a magnetic or electostatic field. By this means, the two of the three types of radiation were shown to consist of positive particles (alpha rays) and negative particles (beta rays) The neutral radiation was identified as a form of electromagnetic radiation that became known as gamma rays.

**** table ***

Radioactive decay is a consequence of nuclear instability. This instability arises from the very strong electrostatic repulsion between the positively-charged protons crowded into the very small confines of the nucleus. Nuclear radii vary from about 1E-5 to 8E-5 Å, giving rise to proton-proton repulsive energies of the order of 108 kJ/mol. Although the nuclear binding force (the "strong force") is much more powerful, it falls off with distance much more rapidly. As the number of particles within the nucleus increases, the binding forces beween the more distant nucleons diminished much more rapidly than does the electrostatic repulsion; the latter therefore becomes more sigificant in heaver nuclei.

Modes of nuclear decay

An unstable ucleus will decay into a more stable one, emitting (or occasionally, capturing) an appropriate particle in the process. The particular kind of particle emitted (and thus the type of radioactivity associated with the nuclide) will depend in part on its location on the Z/N plot of Fig. 2.2.

If the proton-to-neutron ratio is too low, the nucleus falls below the "band of stability" and it may decay by emitting a negative electron (beta-decay); the product nucleus will have the same mass number, but its charge will be greater by one. Perhaps the most well known example of a beta-emitter is carbon-14, which is widely used to data archeological materials:

6C14 = 7N14 + –1e0

In effect, a neutron has been converted into a proton, and the carbon has been transmuted into nitrogen.

Nuclei which fall above the band of stability possess too many protons; these can be converted into neutrons by loss of positive charge. There are three possible ways of accomplishing this:

@ @ @

The nucleus can emit a positive electron, known as a positron:

8O15 = 7N15 + 1b0

If the nucleus is quite heavy, it can simultaneously lose mass and positive charge by emitting alpha-particles, which are just helium nuclei:

92U238 = 90Th234 + 2He4

The nucleus can capture an atomic electron, usually from the 1s shell. This process, known as electron capture of K-capture, is relatively uncommon. In effect, and electron "falls into" the nucleus, so there is no real "decay" of the nucleus. The only radiation produced is electromagnetic radiation (X-ray, ultraviolet, or visible) corresponding to the energy losses as the outer electrons fall into the series o vacancies created by the temporary vacancy in the 1s orbital.

- - -

A few nuclei exhibit more than one decay mode. For example, the decay of Cu64 is distributed as follows: 19% beta emission to Zn64, ?% positron emission and ?% electron capture, both of the latter processes yielding Ni64.

Multiple decay products are most commonly encountered when a new, unstable nucleus is formed in a nuclear reaction.

Nuclei which are simply too heavy to be stable (i.e., beyond Bi) usually lose mass by alpha-emission. The 2He4 nucleus is so tightly bound that it can almost be considered a "fundamental particle" in its own right.

Some nuclei decay by emitting gamma-radiation. Such nuclei may possess a stable Z/N ratio (which of course remains unchanged), but their nucleons are in an unstable arrangement; such a nucleus is said to be in an excited state.

Radioactive decay rates

A single unstable nucleus may decay almost as soon as it is formed, or it may last for millions of years before disintegrating. It is no more possible to predict when a particular nucleus will decay than to predict when an individual person will die. Radioactive decay, like mortality, is a stochastic process which must be treated statistically; using statistics and a sufficiently large sample, we can predict to any desired accuracy the fraction of atoms that will have decayed (or the fraction of a population that will no longer be alive) after a given time.

The activity of a sample of nuclei is the number of disintegrations per unit time. The activity is directly proportional to the number of undecayed nuclei present at any given time:

rate = –dN/dT = kN

In other words, the activity will fall off with time (hence the negative sign) at a decreasing rate (note the k[N] term; n, of course is continually decreasing as more nuclei decay.)

The activity is an example of a quantity whose rate of change depends on the instantaneous magnitude of that quantity. This very common functional relation is known as an exponential or first-order decay law; the rates of many chemical reactions follow such a law. The same law (but without the minus sign) describes the unrestricted growth of a population of bacteria, or of a compound-interest bank account.

The constant k in the equation is the decay constant; its value is a characteristic property of a given kind of nucleus. The more unstable the nucleus, the larger the value of k.

If the activity of a sample is A0 at time t=0 and A at time t, the ratio of these activities is given by the integrated form of Eq ?:

ln(A0/A) = ln N/N = -kt

After a certain time t1/2, half of the original sample of nuclei will have decayed. This time is known as the half-life

ln (1/2) = -kr = 0.693

The half-life is more commonly used than k to characterize the instability of nuclear species. The relation beetween these two quantities can be seen by rearranging the above equation:

r = 0.693/k

Problem example

The alpha-emitting americium isotope 95Am241 is used as an ion source in many kinds of residential smoke detectors. The half-life of this nuclide is 458 years. How long will it take for the activity to fall by five percent?

Solution

ln (A/A) = ln(0.95) = –kt = –0.693 (t/0.693 t /458 y

t = (458 y) (–0.051) / .693 = 34 y

- - - - -

Natural radioisotopes and decay series

There are around 75 radioactive nuclides that occur naturally in the earth and its atmosphere. Many of these have such long half lifes that they are considered to have been present when the earth was formed. Among these are the most common isotopies of all the natural elements beyond 83Bi, as well as a number of other isotopes of lighter elements. Among this latter group is 19K40, which constitutes about 0.01% of natural potassium and is believed to be the major source of heat within the earth.

Any short-lived nuclides present when the earth was formed will of course have decayed a long time ago. This is the reason that the elements technetium (43Tc) and the halogen astatine (85At) are not found in nature. Tc is the only element with an atomic number less than 83 that possesses no stable isotopes. As for At, its most stable (i.e., least unstable) isotope has a half life of only a few hours. The holes in the periodic table corresponding to these elements were not filled until the elements were prepared synthetically around 1940.

The natural radionuclides whose half lifes are much less than the age of the earth (about4.5E9 y) must be presumed to be formed continuously from some other source. In most cases, the source is the decay of a longer-lived nuclide; if a long-lived nuclide decays into a short-lived specis, the concentration of the latter will tend to be almost constant with time.

Most of the natural radioisotopes are the products of the decay of uranium (the heaviest naturelly-occurring element) or thorium. It was from pitchblende, an ore of uranium, that Marie and Pierre Curie first isolated other radioactive elements with half-lifes much shorter than that of uranium. This led to their discoveries of polonium (r=138 d), then radium (r = 1620 y) and also the noble gas radon (r = 3.8 d)

Three natural radioactive decay series have been identified. Each is known by the name of its parent nuclide: U238 (shown here), Th232, and U236. (A fourth series, based on Np237 with r=106 y has long since disappeared.) In each series, a sequence of alpha-decays eventually leads to a different stable isotope of lead. This is the cause of the considerable variation in the composition and average atomic weight of naturel lead ore deposits, and is the reason that the atomic weight of Pb cannot be speciied to more than four significant figures.

Nuclear reactions

If a partice strikes a nucleus with sufficient force to penetrate it, the unstable configuration that results will decay into one or more products. The difficulty lies in imparting the required velocity to the projectile particle. In conventional chemical reactions, the fraction of collisions that are sufficient energetic to disrupt a molecule can always be increased by raising the temperature. Nuclear energies are so great, however, that the temperature has little effect, unless one is taling about increases of millions of degrees. Thermally -induced nuclear reactions do take place in stars, but here on earth we must find some other way of obtaining fast-moving particles.

One source of such particles is an unstable disintegrating nucleus. For example, the alpha rays from the decay of polonium served to induce the first artificial nuclear reaction

2He4 + 7N14 = 9F18 = 1H1 + 8O17

In 1915, Ernest Rutherford found that protons and an isotope of oxygen are released withn alpha particles are passed through a simple of nitrogen gas. He assumed that these came from the decomposition of the unstable 9F18 nucleus that he believed to be the initial product of the reaction.

Artificial readioactivity

Shortly thereafter, in 1934, Frederic and Irene Joliot-Curie fond that boron, aluminum, and magnesium could be made radioactive by bombardment with alpha-particles, the radioactivity continuing after the alpha source was removed.

*Irene Curie was the daughter of Marie and Pierre Curie; she married her mother's assistant Frederic Joliot.

This "artificially-induced radioactivity" showed that radioactive isotopes not found in nature had been produced:

5B10 + 2He4 = 7 N13 + 0n1

13Al + 2He4 = 25P30 + 0n1

12Mg24 + 2He4 = 14Si27 + 0n1

In the 1930's, E.O. Lawrence of the University of California invented the cyclotron, a device that could accerate protons, deuterons (1H2 nuclei) and alpha-particles to very high velociies. This quickly led to the synthesis of the first "missing" element, technetium, in 1937. A molybdenum plate containing a mixture of Mo isotopes was bombarded by a deuteron beam in a cyclotron for several months, resuling in reactions of the type

42Mo99 + 1H2 = 43Tc100 + 0n1

More recent accelerators have been developed which can produce hightly energetic beams of virtually any charged particle.

Types of nuclear reactions

The reaction in Eq. ?? is known as a (d,n) reaction. The designation (d,n) means that the nucleus in question is bombarded by a deuteron and that the ensuing reaction releases a neutron. This is just one of many types of nuclear reactions that have been observed.

type example
(al,n) 33As75 + 2He4 = 35Br73 + 0n1
(al,p) 46Pd106 + 2He4 = 47Ag109 + 1H1
(p,n) 32Li7 + 1H1 = 4Be7 + 0n1
(p,g) 7N14 + 1H1 = 8O15 + g
(p,al) 4Be9 + 1H1 = 3Li6 + 2He4
(d,p) 15P31 + 1H2 = 15P32 +1H1
(d,n) 83Bi209 + 1H2 = 84Po210 + 0n1
(n,g) 27Co59 + 0n1 = 27Co60 + 1B1
(n,p) 21Sc45 + 0n1 = 20C45 + 1H1
(n,al) 13Al27 + 0n1 = 11Na24 + 1H2

 Many nuclear reactions result in more than one product. In such cases, the initial collision between the bombarding particle and the target nucleus is believed to prouce an unstable agglomerate known as a compound nucleu. For example, the following two reactions are known to produce identical distributions of decay products:

29Cu63 + 1H1 = [30Zn64] = (products)

28Ni60 + 2He4 = [30Zn64] + (products)

This observation is explained by proposing that the compound nucleus Zn64 is the immediate product of each reaction.

Neutron-induced reactions

Neutrons, being uncharged, are not repelled by the positive charge within a nucleus, so they are useful projectiles for inducing nuclear rections. Of course, their very lack of charge prevents them from being accelerated, and they cannot be stored; free neutraons are unstable, decaying with a half life of 12 min:

0n1 = 1H1 = –1e0

This means that the neutrons required for such studies must be obtained from nuclear reactions. One such reaction is

7Be9 + 2H4 = 6C12 + 0n1

which led to the discovery of the neutron in 1924. By mixing a natural alpha-emitter such as Po or Ra with beryllium, a fairly compact source of neutrons can be obtained;

A more commonly used method is a (d,n) reaction carried out on tritium gas adsorbed on a solid substrate:

1H2 + 1H3 = 2He4 + 0n1

The advantage of this method is that much of the energy released by this reaction is imparted directly to the nucleus.

Nowadays, the primary source of neutrons for the large-scale production of radioisotopes is a nuclear reactor, in which the neutraons are produced as byproducs of nuclear fission.

Transuranium and actinide elements

The discovery of rhenium in 1925 left four gaps in the periodic table, corresponding to elements 43, 61, 85 and 87. As mentioned p reviously, 43Tc was the first to be synthesized, in 1934. Astatine was prepared in 1940 by bombardment of Bi209 in a cyclotron:

84Bi209 + 2He4 =85At211 + 2 0n1

Elements 61 (prometghium) and 87 (francium) defied attempts to produce them in idenifiable amounts by cyclotron bombardment; they were eventually isolated as decomposition products of U235 and Ac227, respectively. No stable isotopes of any of these four elemens are believed to exist.

In 1934, Enrico Fermi, then working in his natie Italy, attempted to prepare the first transuranium element (i.e., an element with z < 92) by irradiating uranium with neutrons. The preduct showed new radioactivity not present in the uranium, but five years later Hahn and Strassman reported that this radioactivity was associated with isotopes of elements much lighter than uraium; Fermi had failed to produce a new element, but had unwittingly discovered nuclear fission.

Using more carefully conrolled conditions, other workers succeeded in getting uranium to capture neutrons, and prepared Np in 1940 and Pu in 1941. Gradually, by the mid-1950's, elements up through 100 had been prepared. Elements beyond 100 have been prepared by bombardment of high-Z nuclei by more massive particles, such as boron, carbon, or neon ions. These elements are all highly unstable; the half-lifes of elements beyond 101 are no more than a few seconds; for element 106, the most recently reported, t is less than 0.01 sec.

It has been predicted that element 114, if it could ever be prepared, would be relatively stable and sufficiently long-lkived to allow it to be isolated and characterized. All attempts to prepare such a "superheavy" element have so far failed.

Nuclear fission

After Fermi had induced a new radioactivity in a simple of uranium by irradiating it with neutrons, Otto Hahn and Fritz Strassman in Germany discovered that one of the p roducts is Ba136. In 1936, Lise Meitner suggested that the isotope U235 was being split into two parts:

92U235 + 0n1 = 56Ba139 + 36Kr94 + 3 0n1

Notice that three neutrons are produced for every one that is absorbed. If the three product neutrans can then induce fission in three more nuclei, a self-stustaining chair reaction can be achieved.

From Fig??? it is apparent that the binding energy per nucleon drops off slowly above Z = 55-60. This means that if a heavy n ucleus could be split into two lighter ones, energy will be released. Although such a process would be exothermic, it is not spontaneous; in practice, nuclear fission must be induced by absorption of a neutron (as above) or an alpha particle. The particle is captured by the target nucleus, forming an excited compound nucleus which then decomposes into two or more fragments.

For most ptactical applications of nuclear fission, it is necessary to carry out an isotopic enrichment process which raises the U235 fraction to an acceptable level. This is accomplished by converting the uranium into the gaseous hexafluoride UF6, and then taking advantage of the fact that the lighter isotope U235F6 diffuses at a rate that is 1.0043 times as fast as the heaver isotope (Graham's law).

Another fissionable nucleus suitable for practical use is 92Pu239. This nuclide, with a half life of 24,300 years, is made by irradiating the non-fissionable U236 with slow neutrons:

92U236 + 0n1 = 92U239

The short-lived U239 (24 min) undergoes beta decay into Np239, which then uields Pu239 by a second beta decay:

92U239 = 93Np239 + –1e0

93Np239 = 94Pu239 + –1e0

For nuclei such as U235 and Pu239, fission is induced by the absorption of slow (low kinetic-energy) neutrons; fast neutrons do not get absorbed, and are thus ineffective in inducing fission. The size and shape of the fissionable fuel source are both important. If there is too little fuel, or if it is too spread out, many of tthe product neutrons will be lostand the reaction will not become self-sustaining. The critical mass is the amount of material required to ensure that every fission reaction induces at least one more fission.

The value of the critical mass depends on the isotopic purity of the fuel, on its shape (spherical is best), and on the ability of the container walls to reflect neutrons back into the fuel source. For U235, the critical mass is 6  kg. A supercritical mass exposed to fissioning conditions will explode in 10–6 seconds, producing a temperature of 107 K. The explosive power of nuclear devices is usually expressed in megatons of the chemical explosive trinitrotoluene (TNT); one megaton corresponds to the fissioning of 50 kg of U235 or Pu239.

The actual nuclear reaction associated with fission is far ore complicated than Eq ??? suggests. For one thing, not all of the uranium or plutonium nuclei actually undergoes fission; in the case of U235, about 71 percent of the nuclei simply ratain the camptured neutron, forming U236. Of the nuclei that do fission, splitting occurs in a wide variety of ways, resulting ino more than sixty different nuclides in the range of Z = 95 - 139. Since these are mostly neutron-rich and therefore radioactive, these p oducts themselves liberate energy (an in some cases neutrons) as they decay. Energetically, the reaction can be summarized as

92U235 + 0n1 = 2.5 0n1 + 200 MEV

Controlled nuclear fission

In order to serve as a practical source of power, nuclear fission must be carried out slowly and steadily in a controlled manner. This is accomplished in a device known as a nuclear reactor. The fission process in a reactor must be self-sustaining. This requires that it must operate in a critical mode (in which each fission induces a new fission), while at the same time it must be protected from becoming supercritical.

This fine line between sub-criticality, criticality, and disaster is maintained by having the fuel dispersed within a large volume. The The most common arrangement is to make the fuel into pellets of UO2 or UC2. The pellets are packed into stainless steel tubes that are inserted into the reactor body at regular intervals. In between the fuel rods are boron control rods. Boron is a highly efficient absorber of neurons. If the rods are completely inserted into the reactor, the neutron flux between fuel rods is so low that very little energy is produced. As ore energy is required, and control rods are withdrawn.

Another important component of a reactor is a moderator. This i a substance that slows down fast neutrons to they can efficiently induced fission. Water (both H2O and D2O), and graphite care commonly-used moderators.

In this schematic diagram of one common type of nuclear reactor, you will notice that there are three isolated circulating fluid systems. The heat produced by the nuclear reaction itself is transmitted directly to the primary coolant, which in this type of reactor is water held at high pressure to keep it from vaporizing. (In this particular reactor, the water also acts as a moderator.) Another fluid sometimes used as a primary collent is molten sodium.

The high neutron flux within the reactor induces radioactivity in the coolant, so it is important to retain the coolant in a closed cycle. Its only contact with the outside world is supposed to be in the heat exchanger, in which water in a second closed cycle is vaporized into steam which is directed into the turbine that drives the generator. The steam emerging from the turbine exhaust is condensed for reclycing within a second heat exchanger. Only the cooling water from this exchanger comes into contact with the environment.

After a reactor has been in operation for some time, the fuel is partially depleted and the fuel elements become contaminated with radioactive fission producs. These products often have lower densities (and thus larger volumes) than their parent substances; this can lead to expansion and creacking of the fuel elements. Radiation damage produced by neutron-induced transformations within the fuel element tubes can encourage corrosion and failure of the tube walls. Before this point is reached, the fuel elemens must be removed and chemically processed to recover any unspent fuel. The unusable residues most b e converted into a chemically stable form such as a glass, and stored in a safe place. Just what constitutes a permanent "safe place" is currently highly conroversial.

Breeder Reactors

The process in which the U235 content of natural uranium is enriched accounts for a major portion of the expense of nuclear fuel. (Bear in mind that U235, which accounts for 993 out of 1000 atoms in natural uranium, is not fissionable.) A breeder reactor makes use of some of the fast neutrons generated during sission to convert non-fissionable isotopes such as U238 or thorium-232 into fissionable nuclides:

92U238 + 0n1 (fast) = 94Pu239 + 2 –1e0

90Th232 + 0n1 (fast) = 92U233 + 2 –1e0

Thus a breeder reactor can generate new fuel as the old fuel is consumed.

Ideally, each fission should produce two neutrons — one slow neutron to induce a second fission, and the other (a fast neutron) to generate a new atom of fuel. This means that, in order to compensate for lost neutrons and other inefficiencies, an everage of considerably more than two neutrons must be produced per fission. It takes 7-10 years for a typical breeder reactor to produce as much fuel as was initially present.

Because water acts as a moderator, slowing down the fast neutrons required for the reactions shown above, it is not practical to use water to cool a breeder reaction. This limits the design to reactor types colled by gass or liquid metals.

The U238 or Th252 is usally not incorporated into the fuel elements themselves, but surrounds the reactor core. Thus in order to utilize the newly-generated fuel, the entire reactor must be disassembled and the fuel elements reprocessed. This leads to one of the major drawbacks to the U238-fueled breeder reactor: plutonium is one of the most toxic substances known, and there are real concerns about its accidental dispersal into the environment, both during shipment to a reprocessing plant, or its diversion into weapons production (for which plutonium is the preferred fuel.)

Natural uranium-fueld reactors

The costs and dangers associated with the produion of enriched uranium fuel have encouraged the development of reactors that use natural uranium. Because of the reduced U238 content the neutron flux in natural uranium is smaller, and moderators that absorb neutrons less efficiently than ordinary water must be used. The graphite-moderated gas-cooled reactor is one type, widely used in the UK and Europe, but not in North America.

Heavy water, 2D2O, absorbs neutrons less efficiently than 1H2O, and this moderator forms the basis of the Canadian Deuterium-Uranium ("CANDU") reactor, whose design also permits replacement of spent fuel rods without shutting down the reactor.

Nuclear fusion

If you look at Fig ???, you will notce that the binding energy per nucleon climbs very rapidly at the left end of the plot where the mass numbers are small. This means that reactions such as the following will be highly exothermic, capable of producing far more energy than can be obtained from a fission reaction.

1H1 + 1H1 = 1H2 + –1e0

1H2 + 1H2 = 2He3 + gamma

2He3 + 2He32 = 2He4 + 2 1H1

Fusion reactions such as these are far "cleaner" than fission; the products are usually not radioactive, although neurons released in some of the side reactions will induce radioactivity in nearby materials.

Before two nuclei can fuse, they most be squeezed together. Owing to the coulombic repulsion between their positive charges, this is very difficult, and requires that the reacing nuclei possess very hgh kinetic energies which are achieved by heating the fuel nuclei to around 107 K. This means that in order to release the huge amount of energy potentially available from fusion, a very great amount of energy must first be expended in order to start the reaction. For weapons applications, the required ignition temperature is achieved by means of a fission bomb. The fusion fuel itself is typically solid lithium deuteride.

Controlled nuclear fusion would be the most powerful and inexhaustible source of energy, and a relaively clean one. Achievement of this nuclear El Dorado is proving highly elusive, however. Althoug laboratory-scale fusion of small amounts of material has been achieved (for example, by using the highly concentrated beam of light from a powerful laser as the ignition source), the amount of fusion energy generated has not yet exceeded the amount required to operate the apparatus. One major problem with controlled fusion is that no conainer can withstand the high temperatures involved. One answer is magnetic containment; a powerful magnetic field confines the plasma (ionized gas) to a small volume. Research on controlled fusion commenced in the 1950's, and will likely continue for many years before a practical source of fusion energy is developed.

5. Some applications of nuclear chemistry

From the time that the first natural isotopes were isolated and the first artificial ones created in the late 1930's, almost every major field of edeavor has been profoundly affected by the techniques and results of nuclear chemistry. What follows is a brief survey of some of the uses to which nuclear transformations and individual isotopes have been applied.

Radiotracers in chemistry

Different isotopes of the same element exhibit about the same chemical behavior, so by substituting a radioactive isotope for the natural isotopic mixture of an element, the presence or absence of that element can be easily determined by measuring the radioactivity. This can serve as a powerful tool for elucidaing the structures of substances and the mechanism of reactions.

For exaple, consider the structure of the thiosulfate ion S2O32– .. Using conventional Lewis electron-dot construction, two possible structures can be written for this ion. In one, the two sulfur atoms are equivalent, while in the other, they are not. To determine which is correct, the following reaction was carried out:

SO32– + *S = S2O32–

(The asterisk indicates that the sulfur ws isotopically "labeled".) The resulting thiosulfate ion was then treated with acid, liberating the elemental sulfur by disproportionation:

S2O32– .= SO32– + *S

The conclusion is that the two sulfur atoms in the ion are not equivalent; if they were half the radioactivity would end up in the sulfite ion.

Another important radiotracer experiient set out to answer the question of whether the dioxygen produced in photosynthesis comes from the CO2, the H2O, or both:

6 CO2 = 6 H2O = C6H12O6 + 6 O2

When H2*O (water labeled with O16) was used, all the radioactivity ended up in the O2, suggesting that the crucial step in the reaction is the oxidation of water:

2 H2O + Hnu = O2 + 4H+ + 4e-

Dating of fossils, archeological samples, and rocks

Carbon-14 dating

The upper atmosphere is bathed in a continual shower of energetic particles from outer space, known as cosmic rays. These particles act as a source of neutrons, which in turn collide with atmospheric nitrogen atoms to form C14:

7N14 + 0n1 = 6C14 + 1H1

Carbon-14 is radioactive, decaying back to N14 wqith a half-life of 5730 y:

7C14 = 7N14 +

Because the half-life of this decay is very short compared to the age of the earth, the rates of formation and decay of C14 are identical; over each square centimeter of the earth's surface, about 100 C14 atoms are created, and an equal number decay, each minute. This C14 gets oxidized to C14O2, where it forms about 10–10 percent of atmospheric CO2. Plants take up CO2 during photoshnthesis, so plants, and animals (all of which either eat plants or eat animals which eat plants) will contain C14 in the same ratio.

When an organism dies, it stopes taking up carbon, and the C14 is no longer renewed. At this time, "the meter starts running"; the raio of C14/C12 begins to decrease, falling by50 percent every 5230 years. By comparing the radioactivity of the CO2 produced by burning a sample of the fossilized material, this ratio can be measured by mass spectroscopy, and the time of the organism's death can be estimated.

Potassium-argon dating

Potassum-40, with a half-life of around 1019 y, decays mostly into X40, but about 10 percent of K40 atoms decay into A40. It is the ar gon that is more important because being a gas, it is rapidly excluded from the molten material from which igneou rocks are formed. Once the material solidifies, however, the new argon that forms as the K40 continues to decay is trapped within the crystal lattices of the rock. By measuring the ration of K40 to Ar40 in a ground-up sample of such a rock, the time at which it crystallized can be determined.

Actimation analysis

The determination of whch elements are present in a sample of a substance, and in what relative amounts, has always been a difficult chemical problem, particularly when the sample is a very small one. Activation analysis is now widely used for this purpose. It consists in irradiating the sample to be analyzed with slow neutrons from a suitable source. The neutrons strike the atoms in the sample and by means of (n,p), (n,gamma) and similar reactions, produce patterns of radioactivity that are characteristic of each element. Especially revealing are the gamma rays, whose energies can efficiently fingerprint a given decay process.

Unlike most other forms of chemical analysis, activation analysis is non-destructive; this makes it a fvorite method for analyzing the color pigments in paintings and other works of art. Activation analysis is widely used in archaeological research and in forensic work.

Chemical effects of nuclear rections

When a nucleus decays, emitting a particle, most of the energy released in the decay ends up as kinetic energy of the departing particle. Conservation of momentum, however, reuires that tome of the kinetic energy be alloted to the nucleus itself. This effect is analogous to the recoil of a gun when it fires a bullet.

The recoil energy of a nucleus which decays or is transformed by a (n,gamma) reaction is often many times geater than the energy required to break the chemical bond between the atom containing that nucleus, and a neighboring atom. In this way the nuclear reaction can initiate a chemical reaction.

This effect was first observed when ethyl iodide was irradiated with neutrons. The recoil of the I127 nucleus following the (n,gamma) reaction breakes the I–CH2-CH3 bond; the resulting iodide ions can be detected by formation of a precipitate of AgI when a soluble silver salt is added. This is known as a Szilard-Chalmers reaction, after the workers who discovered the process in 1934. The major practical use of this effect is to chemically separate synthetic (and thus radioactive) isotopes created by a neuron beam from the target material.

When a nucleus emits a gamma-ray, the energy of the radiation will be reduced by the amount of recoil energy imparted to the nucleus. This energy shift can be compensated for by moving the sample toward the detector at a constant velocity, typically a few millimeters per second. This gives rise to a Doppler shift which raises the gamma-ray energy by a small amount. This method, known as Mossbauer spectroscopy, is an extraordinarily sensitive means of measuring recoil energies. The importance of this is that recoil energy is affected by the local chemical environment surrounding the decaying nucleus. Mossbauer studies have helped resolve questions such as the degree of ionic character of the bond in SnCl4. Mossbauer spectroscopy is now a widely employed tool.

5.5 Industrial applications

Radiography

This refers to the use of gamma radiation, obtained from Co60 or some other artificial isotope, as a means of obtaining images of otherwise opaque bodies, much as is done with X-rays. The major use of radiography is examination of metal castings for internal flaws, and to check the integrity of welds. X-rays are not sufficiently penetrating for these applications.

Cross-linking of polymers

Polymeric substances such as polyethylene are normally quite flexible and they also soften when warmed. Both of these characteristics are due to the weak van der Waals forces between adjacent molecules, which allows them to slide past one another. By irradiating with such a material with gamma rays ("gamma radiololysis), ions are formed at various points along the polymer chains. When oppositely charged ions on adjacent chains are close to each other, new bonds are created between them. This cross-linking, as it is called, results in a far more rigid structure, transforming the solid into a harder one when can withstand higher temperatures.

Food irradiation

THe normal way to preserve food from bacterial action is to seal it into an impervious container such as a jar or can, and then heat it sufficiently to kill all the bacteria. Unfortunately, the time and temperature required to accomplish this can often render the food unpalatable, so the variety of foodstuffs that can be preserved in this way is quite limited.

An alternative approach, first developed for the U.S. Army in the 1940s, is to pass the food through a strong flux of gamma rays. Such radiation is quite lethal to living organisms at levels that have little significant effect on the taste or composition of the food itself.

As this method of food preservation has become more popular, some members of the uninformed public have confused "irradiation" with "radioactivity", and believe that the food itself is somehow made radioactive. This is of course not correct; gamma rays cannot transform a stable nucleus into one that is radioactive, and virtually all foods already contain natural radioactive nuclei anyway. Others poit out that if the ionization produced by gamma rays can kill bacteria, it can also produce unknown, and possibly dangerous, new substances within the food.

The latter argument could better be applied to ordinary cooking, which in may ways is a far more chemically violent process, and one which produces huge numbers of new substances (many of which are poorly characterized), , some of which are demonstrably carcinogenic.

6. Environmental aspects of radioactivity

Radioactivity in the environment, and its effects on human health, are subjects of intense interest to everyone. Unfortunately, widespread ignorance of the technical aspects of these subjects on the part of the general public has led to much suspicion, confusion, and misinformation.

In order to intelligently discuss environmental radioactivity, it is first necessary to understand how radioactive decay affects nearby matter, particularly the cells and tissues of organisms. Also needed is some means of expressing the quantity of radiation and its effects on matter in a quantitative manner.

  1. Interaction of decay products with matter

Radioactive decay generates high-velocity charged particles such as electrons or helium ions (beta- and alpha-decay), and high-energy electromagnetic radiation (gamma ray photons). When any of these particles strikes a molecule, the effect is the same as if the molecule (or a particular part of it) had been raised to a very high temperature. This localized excitation manifests itself as a very vigorous vibration of nearby atoms, often leading to breaking of chemical bonds. Bond cleavage can result in the formation of a pair of ions or of radicals, depending on whether the bonding electrons ramain together on one atom, or end up divided between them:

(A:B)* = A+ + B (ion-pair formation)

(A:B)* = A. + B. (radical formation)

(The asterisks here indicatet that the species is in an excited state.) Another possible result of radiation-induced excitation is direct ionization:

H. + CH3I = .CH3 + HI

In summary, the general effect of radiation is to disrupt chemical bonds; this initial bond cleavage is then followed by a variety of chemical processes that lead to the formation of new substances. In solids exposed to very high radiation fluxes (as occurs in the structural parts of a nuclear reactor), this may lead to disruption of the crystal lattices and physical deterioration of the material itself.

In aqueous systems, including biological materials, the main effect is to produce free radicals which act to disrupt the metabolic processes in the cell. Even if the cell is not killed outright, the DNA molecles that carry its genetic information may be disrupted, making it impossible for the cell to divide and reproduce itself properly. In high animals the cell may continue to divide, but the division process may proceed without any control, resulting in the type of malignant g rowth associated with cancer.

Quantitative measurement of radioactivity and its effects

Radioactivity is characterized by the nature of the particle (alpha, beta or gamma), and by the rate of production of the particle, or its activity, expressed in disintegrations per second (dps). The SI unit of activity is the becquerel, corresponding to 1 dps. An older unit, the curie, is widely used to characterize higher-activity sources:

1 curie = 3.7E10 disintegrations per second = 37 Gb

The activity measures the strength of the source of radioactivity, but it conveys no information about its effects on matter. For radiation to have any effect, it must first strike or penetrate the object in question, and it must then be absorbed.

Alpha particles, being both highly ionized and relatively massive, are very strongly absorbed by all kinds of matter. For this reason, they are not very penetrating; even a sheet of paper or a few centimeters of air will absorb an alpha particle.

Beta particles (electrons) are much more penetrating: they will pass through air without much loss, and will even pass through thin pieces of metal. On the other hand, the amount of ionization they produce in materials is much less than that due to alpha paricles.

Gamma radiation is extremely penetrating; effective shielding against gama rays requires thick shielding by high-density materials such as concrete or lead. Once a gamma photon enters a substance, it creates roughly as much ionization as a beta particle.

Neutrons (which are products of fission rather than decay products) are quite penetrating; their lack of electric charge makes them immune to electrostatic interactions with atoms and nuclei, they interact only by direct collision. In doing so, however, they carry sufficient momentum to result in considerable radiation damage.

Dosimetry

The activity of a sample tells us nothing about the quantity of radiation actually absorbed by a material exposed to it, so other units are employed to convey this dosimetry information. There are two parallel sets of these units, one expressing the quantity of radiation absorbed by a material (the dose), and the other giving the dose equivalent, which is a measure of the biological effect of radiation on the human body.

The SI unit of radiation dose is the gray (Gy); one gray corresponds to the absorption of one joule of radiation energy per kilogram of material. An older unit, the rad (radiation absorbed dose) expresses the same thing in units of ergs absorbed per gram of material; its use is now strongly discouraged.

-- Radiation exposures in medical procedures are commonly measured in milligrays. Typical values are about 1.4 mGy for an X-ray, and up to about 30 mGy for a CT scan. An adult whole-body exposure of 5 gray commonly leads to death in 14 days. But much greater levels (up to about 80 Gy) are employed in cancer treatments in which the radiation is directed specifically at the tissues to be destroyed.

Dose equivalent

Radiation dose, discussed above, refers to the total quantity of energy absorbed by material that has been subjected to ionizing radiation, but it does not take into account the actual amount of damage this radiation causes to the material. Because different kinds of radiation have different penetrating- and ionizing-powers in different matarials, a quantity called the dose equivaent

[ see http://en.wikipedia.org/wiki/Radiation_dose_equivalent]

Dose equivalents are defined as the product of the dose and a quality factor Q, also known as the relative biological effectiveness of the radiation.. The latter quantity reflects the relative penetrating- and ionizing-powers of the different kinds of radiation:

X-ray, beta-, positrons and gamma readiation - Q=1

alpha radiation - Q = 20

neutrons: Q = 5 to 20, depending on kinetic energy

In the case of radiation exposure to specific kinds of tissues (as opposed to whole-body exposure), an additional weighting fraction

The SI unit of dose equivalent is the sievert (Sv) whose value is defined relative to the damage produced by a dose of one joule of gamma radiation absorbed in one kilogram of tissue. Thus if exposure of 1 kg of tissue produces 3.5 times this amount of damage, the dose equivalent is 3.5.

In North America, the older term rem (roentgen eqivalent man) is still widely employed to express dose equivalent, although its use is officially deprecated.

The deletorious effects of ionizing radiation on the human body have made dose equivalents especially important in assessing environmental effects on health.

[table ]

6.3 Natural sources of radiation

There is radioactivity all around us; our hoses, the food we eat, the air we breathe, and our very bones are all slightly radioactive. This natural background radiation contributes a does of 1-5 rems per year to most individuals, depending on the particular surroundings and food preferences, and also on whether one smokes.

It is important to understand the sources and significance of this when assessing the dangers of artificial sources.

The natural background comes from the decay of about 75 different radionuclides present in the environment. Some of these are terrestrial radionuclides and their decay products, while others are due to the action of cosmic rays on atmospheric gases.

Primordial radionuclides

There are a number of natural radionuclides whose half-lifes are of the order of the age of the earth (4.5E9 y), and which are therefore still present in the earth. This includes, of course, all of the natural elements heavier than Bi. The most abundant of these are uranium, thorium, and radium. In addition to these, K40 is especially significant; this isotope accounts for 0.01% of natural potassium, which is itself one of the more abundant element in the earth's crust.

Although the primordial radionuclides are all found in the crust, rocks and soil, many of them (or their radioactive decay products) can be taken up by plants. Thus they enter our bodies through those plants that are grown for food or forage. Other radionuclides enter the environment through combusion or mining.

One important product of natural radioactive decay is the noble gas radon. All isotopes of radon are radioactive; one common one is Rn222, whose half-life is 3.8 days. Because it is a gas, radon can diffuse out of the soil and accumulate in enclosed spaces such as basements. Radon is an alpha-emitter, as are its daughter products, and all are known to cause an increased incidence of lung cancer. Some years ago, very high radon concentrations were found in the air within private homes in some parts of North America. In many cases the levels were far above what are allowed for industrial exposure. Most of these houses were built in areas where the levels of natural uraium and radium are higher, and modern energy-efficient construction practices that reduce internal ventillation have exacerbated the problem.

The cosmic ray background

In addition to terrestrial sources of radiation, the earth is bathed in a continuous flux of cosmogenic radiation which consists of the primary cosmic rays themselves, and the secondary rays produced by the action of the primary particles on atmospheric gases.

The primary cosmic rays consist mostly (78%) of protons, with about 20% He4 atoms or ions, and the remainder heavier nuclei and electrons. The kinetic energies of thse particles range from merely high to stupendous — up to 1014 Mev. About one cosmic particle strikes every square centimeter of the outer atmosphere every second.

These primary particles collide with atoms of oxygen or nitrogen in the air with so much force that they shatter them into a variety of debris, including neutrons (by p,n reaction), electrons, positrons, and gamma photons. The neutrons in turn strike other nuclei, where they create new radioactive species by (n,p) and similar reactions.

Carbon-14, as already explained, is produced in this way, and a (n,t) reaction on the hydrogen atoms within water vapor is the source of natural tritium, 1H3. In all, about twenty short-lived radionuclides are produced by the action of cosmic rays.

The amount of radiation introduced into the body by cosmic rays and their secondary products depends on where one lives.The flux of cosmic particles approximately doubles for each 1800-meter increase in altitude near the surface of the earth. The average sea-level dose is 30-60 mrem/y. Owing to the way in which the earth's magnetic field focuses the charge components of this radiation, the dose will be slightly greater at high latitudes (> 50°) . In North America, typical exposures attributable to cosmic rays vary from 30-120 mrem/y. Travelers on cross-country aircraft flights typically receive doses of about 5-8 mrem.

Some common sources of background radiation

Cosmogenic radioactivity

Most of the natural background radiation in buildings and food comes from K40 and from uranium and its decay products, particularly radium and radon. All plants (and thus all foodstuffs) contain potassium, and therefore K40; in addition, the K40 in wooden building walls typically contributes 40 rem/y. Brick and stone, which contain both K4 and uranium, can contribute dosages of 70-100 rem/y.

Phosphate rock, consisting mainly of CaHPO4, is used to manufacture fertilizers and also the gypsum-board widely used in "drywall" construction. Any natural substance containing calcium or its cation will also contain the corresponding form of radium, which is in the same periodic g roup. Thus the mining of phosphate rock begins a process that introduces significant quantities of radium and its decay products into the environment. (The Ra activity in drywall is about 25 picocuries/g.)

Combustion of fossil fuels is another major input of radioactivity into the environment. Mention was made above of he K40 content of plant materials; coal, whch is derived from this source, contains K40 as well as as uranium and its daughter radionuclides. Natural gas, which is in most respects a "cleaner" fuel than coal, always contains a certain amount of radon, which contributes an activiy of 20-30 picocuries per liter, depending on its source.

Two of the radioactive daughters of radon are Pb210 and Po210. These adsorb strongly to particulate material such as smoke and dust, and in this form can be a major source of airborne radioactivity. It has been found that the activity of Pb210 decay products is 104 times greater in tobacco smoke than in the dried leaves.

Drinking water and foodstuffs constitute another source of radiation dosage. Water from springs and wells, having leached through rocks that contain K40 and uraniumm, is generally more radioactive than water from streams or that is collected directly from rainfall. The activity of foods varies widely, and is higher in those such as fish and meat that contain more potassium.

A "standard" 70-kg adult contains about 1200 g of calcium, mostly in the bones. Along with this comes 10-100 picograms of radium, contributing an internal dosage of about 10 rem/y. Other internal sources are 20 rem/y from K40 (the boy contains about 140 g of potassium), and 40 mrem/y from Po210 in soft tissues.