19.8 Nuclear Fission - ChemPRIME

# 19.8 Nuclear Fission

19.8 NUCLEAR FISSION

The first time that nuclear fission was achieved in the laboratory was by the Italian physicist Enrico Fermi (1901 to 1954) in 1934. Fermi was among the first to use the neutron in nuclear reactions, following its discovery by Chadwick in 1932. He hoped, by bombarding uranium with slow neutrons, to be able to prepare the first transuranium element. Instead he obtained a product which seemed to be a group II element which he identified incorrectly as radium. It remained for the experienced German radiochemist Otto Hahn to correct Fermi’s mistake. (In the meantime Fermi had been awarded the Nobel Prize.) Somewhat reluctantly, Hahn published a paper early in 1939 showing that the element produced by the bombardment of uranium was not radium at all but the very much lighter group II element barium, 36 places earlier in the periodic table! It then became clear that instead of knocking a small chip off the uranium nucleus as had been expected, the bombarding neutron had broken the nucleus into two large fragments, one of which was barium. We now know that the initial step in this process is the formation of an unstable isotope of uranium which then fissions in a variety of ways, some of which are shown below:

(19.36)

EXAMPLE 19.7 Using Fig. 19.5, make a rough estimate of the energy released by the fission of 1 g of uranium-235 according to the equation

${}_{\text{92}}^{\text{235}}\text{U + }{}_{\text{0}}^{\text{1}}n\text{ }\to \text{ }{}_{\text{56}}^{\text{140}}\text{Ba + }{}_{\text{36}}^{\text{93}}\text{Kr + 3}{}_{\text{0}}^{\text{1}}n$

Solution From Fig. 19.5 we can make the following estimates of the energies of formation per nucleon for the four species involved:

ΔHf (140Ba) = – 810 GJ mol–1ΔHf (235U) = – 730 GJ mol–1
ΔHf (93Kr) = – 810 GJ mol–1ΔHf (${}_{\text{0}}^{\text{1}}n$) = 0

Using these quantities in the same way as enthalpies of formation for chemical reactions, we obtain

ΔHm = [140(– 810) + 93(– 840) – 235(– 730)] GJ mol–1 = – 20 000 GJ mol–1

The enthalpy change per gram is then given by

ΔH = – 20 000 $\frac{\text{GJ}}{\text{mol}}$ × $\frac{\text{1 mol}}{\text{235 g}}$ = – 85 GJ g–1

Note: This is about the same quantity of heat energy as that produced by burning 3 tons of bituminous coal!

Calculations similar to that just performed soon persuaded scientists in 1939 that the fission of uranium was highly exothermic and could possibly be used in a super bomb. Adolph Hitler had been in power in Germany for 6 years, and Europe was teetering on the brink of World War II. The possibility that Nazi Germany might develop such a bomb and use it did not seem remote, especially to those scientists who had recently fled Nazi and Fascist Europe and come to the United States. Albert Einstein, himself one of these refugees, was persuaded to write a letter to President Franklin Roosevelt in August 1939 in which the alarming possibilities were outlined. Roosevelt heeded Einstein’s advice and established the so-called Manhattan Project, a super-secret research effort to develop an atomic bomb if at all possible. After 5 years of intense effort and the expenditure of more money than had ever been spent on a military-scientific project before, the first bomb was tested in New Mexico in July 1945. Shortly thereafter two atom bombs were dropped on the Japanese cities of Hiroshima and Nagasaki, and World War II ended almost immediately.

Some Features of Nuclear Fission

A crucial feature of the fission of uranium without which an atom bomb is impossible is that fission produces more neutrons than it consumes. As can be seen from Eqs. (19.36), for every neutron captured by a ${}_{\text{92}}^{\text{235}}\text{U}$ nucleus, between two and four neutrons are produced. Suppose now that we have a very large sample of the pure isotope ${}_{\text{92}}^{\text{235}}\text{U}$ and a stray neutron enters this sample. As soon as it hits a 235U nucleus, fission will take place and about three neutrons will be produced. These in turn will fission three more 235U nuclei, producing a total of nine neutrons. A third repetition will produce 27 neutrons. a fourth 81. and so on. This process (which is called a chain reaction) escalates very rapidly. Within a few microseconds a very large number of nuclei fission, with the release of a tremendous amount of energy, and an atomic explosion results.

There are two reasons why a normal sample of uranium metal does not spontaneously explode in this way. In the first place natural uranium consists mainly of the isotope ${}_{\text{92}}^{\text{238}}\text{U}$ while the fissionable isotope ${}_{\text{92}}^{\text{235}}\text{U}$ comprises only 0.7 percent of the total. Most of the neutrons produced in a given fission process are captured by ${}_{\text{92}}^{\text{238}}\text{U}$ nuclei without any further production of neutrons. The escalation of the fission process thus becomes impossible. However, even a sample of pure 235U will not always explode spontaneously. If it is sufficiently small, many of the neutrons will escape into the surroundings without causing further fission. The sample must exceed a critical mass before an explosion results. In an atomic bomb several pieces of fissionable material, all of which are below the critical mass, are held sufficiently far apart for no chain reaction to occur. When these are suddenly brought together, an atomic explosion results immediately.

A great deal of the 5 years of the Manhattan Project was spent in separating the 0.7 percent of 235U from the more abundant 238U. This was done by preparing the gaseous compound UF6 and allowing it to effuse through a porous screen. (Recall from Sec. 9.4 that the rate of effusion is inversely proportional to the square root of molar mass.) Each effusion resulted in a gas which was slightly richer in the lighter isotope. Repeating this process eventually produced a compound rich enough in 235U for the purposes of bomb manufacture.

Only the first bomb dropped on Japan used uranium. The second bomb used the artificial element plutonium, produced by the neutron bombardment of 238U [Eqs. (19.21) and (19.22)]:

${}_{\text{92}}^{\text{235}}\text{U + }{}_{\text{0}}^{\text{1}}n\text{ }\to \text{ }{}_{\text{94}}^{\text{239}}\text{Pu + 2}{}_{-\text{1}}^{\text{0}}p$

Fission of ${}_{\text{94}}^{\text{239}}\text{Pu}$ occurs in much the same way as for ${}_{\text{92}}^{\text{235}}\text{U}$, giving a variety of products; for example,

${}_{\text{94}}^{\text{239}}\text{Pu + }{}_{\text{0}}^{\text{1}}n\text{ }\to \text{ }{}_{\text{38}}^{\text{90}}\text{Sr + }{}_{\text{56}}^{\text{147}}\text{Ba + 3}{}_{\text{0}}^{\text{1}}n$      (19.37)

Again this is a highly exothermic reaction yielding about the same energy per mole (20 000 GJ mol–1) as 235U.

Nuclear Power Plants

Even before the atomic bomb had been produced, scientists and engineers had begun to think about the possibility of using the energy released by the fission process for the production of electrical energy. Shortly after World War II confident predictions were made that human beings would soon depend almost entirely on atomic energy for electricity. Alas, we are now 30 years into the future from then and no such miracle has occurred. In the United States only 4 percent of the electrical energy is currently produced by this method. The proportion is a little higher in some other countries, notably Great Britain, but nowhere is nuclear power even on the verge of replacing the fossil fuels. The unfortunate truth is that producing power from atomic fission has turned out to be much more expensive than was previously expected. Even in these days of high prices for the fossil fuels it is still only barely competitive.

A schematic diagram of a typical nuclear reactor is given in Fig. 19.6. The uranium is present in the form of pellets of the oxide U3O8 enclosed in long steel tubes about 2 cm in diameter. The uranium is mainly ${}_{\text{92}}^{\text{238}}\text{U}$ slightly enriched with the fissionable ${}_{\text{92}}^{\text{235}}\text{U}$. The rate of fission can be regulated by inserting or withdrawing control rods made of cadmium, which is a very efficient neutron absorber. In addition a moderator such as graphite or water must be present to slow down the neutrons, since slow neutrons are more efficient at causing fission than fast ones.

The energy released by the fisson of the uranium is carried off by a coolant, usually superheated steam at about 320°C. This steam cannot be used directly since it becomes slightly radioactive. Instead it is passed through a heat exchanger so as to produce further steam which can then be used to power a conventional steam turbine. The whole system is enclosed in a strong containment vessel (not shown in the figure). This vessel prevents the spread of radioactivity in case of a serious accident.

Nuclear power plants have two advantages over conventional power plants using fossil fuels. First, for a given energy output they consume much less fuel. Second, they produce far smaller quantities of toxic effluents. Fossil-fueled plants produce sulfur dioxide, oxides of nitrogen, and smoke particles, all of which are injurious to health.

Despite the much lower cost for fuel, nuclear power plants are very expensive to build. This is largely because of their chief disadvantage, the extremely dangerous nature of the radioactive products of nuclear fission. Fission products consist of a great many neutron-rich, unstable nuclei, ranging in atomic number from 25 to 60. Particularly dangerous are the long-lived isotopes ${}_{\text{38}}^{\text{90}}\text{Sr}$, ${}_{\text{55}}^{\text{137}}\text{Cs}$, and the shorter-lived ${}_{\text{53}}^{131}\text{I}$, all of which can be incorporated into the human body. Extreme precautions must be taken against accidental release of even traces of these materials into the environment. It should be realized in this connection that the possibility of a nuclear reactor running out of control and becoming a Hiroshima-type bomb is zero. The fuel used in nuclear reactors is not rich enough in ${}_{\text{92}}^{\text{235}}\text{U}$ for this to occur, and the worst possible accident is complete meltdown of the reactor core. Such an accident could be very serious because a great many highly radioactive isotopes would be scattered by the wind.

Even if fission products are handled successfully during normal operation of a nuclear plant, there still remains the difficulty of their eventual disposal. Although many of the unstable nuclei produced by fission are short-lived, some, like ${}_{\text{38}}^{\text{90}}\text{Sr}$ (25 years) and ${}_{\text{55}}^{\text{137}}\text{Cs}$ (30 years), have quite long half-lives. Accordingly these wastes must be stored for many hundreds of years before enough nuclei decompose to reduce their radioactivity to a safe level. At the present time, most of these wastes are stored as solutions in underground tanks near Richland, Washington. If the number of nuclear plants increases appreciably, the disposal of these wastes will become very difficult. Originally it was planned to store these wastes in solid form in underground salt deposits, but no entirely satisfactory site has yet been found.

Breeder Reactors

Because ${}_{\text{92}}^{\text{235}}\text{U}$ is only 0.7 percent of naturally occurring uranium, its supply is fairly limited and could well only last for about 50 years of full-scale use. The other 99 percent of the uranium can also be utilized if it is first converted into plutonium by neutron bombardment:

${}_{\text{92}}^{\text{238}}\text{U + }{}_{\text{0}}^{\text{1}}n\text{ }\to \text{ }{}_{\text{94}}^{\text{239}}\text{Pu + 2}{}_{-\text{1}}^{\text{0}}e$

As we have already seen, ${}_{\text{94}}^{\text{239}}\text{Pu}$ is also fissionable, and so it could be used in a nuclear reactor as well as ${}_{\text{92}}^{\text{235}}\text{U}$.

The production of plutonium can be carried out in a breeder reactor which not only produces energy like other reactors but is designed to allow some of the fast neutrons to bombard the ${}_{\text{92}}^{\text{235}}\text{U}$, producing plutonium at the same time. More fuel is then produced than is consumed.

Breeder reactors present additional safety hazards to those already outlined. They operate at higher temperatures and use very reactive liquid metals such as sodium in their cooling systems, and so the possibility of a serious accident is higher. In addition the large quantities of plutonium which would be produced in a breeder economy would have to be carefully safeguarded. Plutonium is an α emitter and is very dangerous if taken internally. Its half-life is 24 000 years, and so it will remain in the environment for a long time if dispersed. Moreover, ${}_{\text{94}}^{\text{239}}\text{Pu}$ can be separated chemically (not by the much more expensive gaseous diffusion used to concentrate ${}_{\text{92}}^{\text{235}}\text{U}$) from fission products and used to make bombs. Such a material will obviously be attractive to terrorist groups, as well as to countries which are not currently capable of producing their own atomic weapons.