The Fire of Prometheus: Nuclear Fission and Fusion

James Richard Fromm

Nuclear Fission Reactions

One of the two types of highly energetic nuclear reactions is nuclear fission, in which a single nucleus of an element of high atomic weight absorbs a neutron. Some such nuclei become unstable when they absorb a neutron and emit a nuclear particle or radiation to regain stability, while others split into two nuclei (each about half the weight of the original nucleus) and perhaps several additional neutrons. Nuclear fission reactions useful in atomic weapons or power generation are all chain reactions, in which the unstable nuclei split and release several neutrons which in turn trigger additional fissions. When chain reactions occur in nuclear fission weapons ("atomic bombs") all of the fissions and energy release occurs in an extremely short time and the energy release is uncontrolled, but when they occur in nuclear reactors the energy release is controlled by moderators and neutron absorbers so that the energy appears as controlled heat in the reactor core. The source of the energy is the very small difference in mass between the fission products and the fissioning nucleus, which is converted into heat energy (and other forms of energy including radiation) according to the Einstein relationship E = mc2. The first self-sustaining chain reaction was carried out in 1942 at an experimental nuclear reactor in the stands of Stagg Field at the University of Chicago in the United States.

Comparatively few nuclei can participate in a self-sustaining chain reaction, and of these only one exists in nature: the isotope 235U. Naturally occurring uranium is a mixture of two isotopes, 235U (99.3%) and 235U (0.7%); only the isotope 235U fissions when it absorbs a slow neutron. Three types of nuclear reactors exist, all of which are based on the fission of 235U: the natural uranium reactors, the enriched uranium reactors, and the breeder reactors.

Natural uranium reactors are fuelled by the mixture of 238U and 235U which constitutes natural uranium. The only successful type is the CANDU reactor developed in Canada. This type basically consists of clad natural uranium fuel rods immersed in a moderating pool of heavy water (deuterium oxide). Commercial power stations based on this design are in operation and under construction in Ontario and Quebec. The advantage of the design is its ability to use natural uranium; the disadvantage of the design is the requirement of moderation by heavy water, which must be obtained from the naturally occurring small percentage of deuterium oxide in water. Large extraction plants have been built to produce the many tons of heavy water needed for these reactors.

Enriched uranium reactors are fuelled by mixtures of 238U and 235U which have had a significant fraction of the 238U removed so that the mixture is enriched in 235U. The United States has led in the development of such reactors, although other countries have also built them. Enrichment to anywhere between 2% and nearly 100% 235U is possible; the higher enrichments are used in nuclear weapons and the lower in enriched uranium power reactors. Preparation of the enriched uranium is done in gas-separation plants, of which the largest is at Oak Ridge, Tennessee, U.S.A. The process used is gaseous diffusion and takes advantage of the small differences in velocity of 235UF6 and the slightly heavier 238UF6; uranium hexafluoride is a highly corrosive gas. This process requires large amounts of electrical power to drive the pumps and other apparatus.

Efforts have been made to develop other separative methods but thus far non have proceeded beyond pilot-plant or laboratory-scale operations. The first commercial power reactor, producing 90 MW, began operation in 1957 (Shipping port Unit 1, Duquesne Light Company, Shipping port, Pennsylvania, U.S.A.). Modern nuclear generating stations average perhaps 500 MW, and with the exception of the CANDU type are all moderated by ordinary water, often pressurized, and use enriched uranium fuel.

Nuclear reactors are capable of operation on isotopes other than 235U, such as 239Pu and 233U, but these isotopes do not exist in nature. They can, however, be produced in nuclear reactors from the naturally occurring 238U and 232Th respectively. Nuclear reactors carrying out this process can actually produce more nuclear fuel than they consume, and so they are called breeder reactors. In breeder reactors, some of the neutrons produced are absorbed by 238U which then undergoes the following chain of reactions:

238U + 1n rarrow.gif (63 bytes) 239U
239U rarrow.gif (63 bytes) 239Np + e-(half-life 23.5 minutes)
239Np rarrow.gif (63 bytes) 239Pu + e-(half-life 2.35 days)

The half-life of 239Pu is long enough (24,360 years) to permit its accumulation, and it undergoes slow-neutron fission similar to that of 235U. Breeder reactors based on this cycle operate best without a moderator and thus with fast-moving neutrons, and are called fast breeder reactors; most of the breeders designed thus far are fast breeders.

Fast breeder reactors have higher power densities and smaller cores than slow-neutron reactors, and water is not an effective enough heat-transfer mechanism to move heat from the core to the exterior steam turbine; moreover, water would itself act as a moderator. Hence water is replaced by liquid sodium or other liquid metals, so that these reactors are often referred to as LMFBRs (liquid metal fast breeder reactors). Experimental breeder reactors have been built in Europe, the United States, and the U.S.S.R., but only the 350 MW BR350 Soviet facility (Shevchenko, U.S.S.R., on the Caspian Sea) provided power to a commercial grid. At the present time, breeder reactors must still be considered experimental.

The alternative breeder cycle, using the only naturally occurring isotope of thorium, is:

232Th + 1n rarrow.gif (63 bytes) 233Th
233Th rarrow.gif (63 bytes) 233Pa + e-
233Pa rarrow.gif (63 bytes) 233U + e-

The half-life of 233U is long enough (162,000 years) to permit its accumulation, and like 235U and 239Pu it undergoes slow-neutron fission. No breeder power reactors based on this cycle have yet been constructed.

Promise in Nuclear Fusion

One of the most energetic processes we know is nuclear fusion. Nuclear fusion occurs when lighter elements, which have a lower binding energy per nucleon, fuse together to form a larger nucleus with a greater binding energy per nucleon. The difference in binding energy per nucleon appears as energy, and the nucleus formed has greater nuclear stability than the nuclei which fused to form it. All stars produce energy from the fusion of hydrogen to helium. The simplified overall reaction is 41H --> 4He, and it can occur in stars once their internal temperature reaches about 10 MK due to gravitational collapse. Other nuclear fusion reactions can proceed in stars if their internal temperature is higher. Carbon formation or "helium burning" begins at about 100 MK. This reaction may proceed through an unstable beryllium intermediate:

24He rarrow.gif (63 bytes) 8Be, 8Be + 4He rarrow.gif (63 bytes) 12C

Beryllium is far less common in the universe than is hydrogen, helium, or carbon.

The stellar nuclear fusion reactions require temperatures beyond our ability to produce and sustain on earth. However, the energy released by a nuclear fission bomb can provide sufficient energy to effect the easier fusion reactions of the heavier isotopes of hydrogen 2H (deuterium) and 3H (tritium) to helium:

22H rarrow.gif (63 bytes) 4He, 2H + 3H rarrow.gif (63 bytes) 4He + 1n

These reactions have been used in thermonuclear weapons (hydrogen bombs). Under the conditions reached in a nuclear fission explosion these fusion reactions can take place and release vast amounts of energy. Attempts have been made for many years to construct fusion generators capable of carrying out these reactions continuously or at least under less destructive conditions. Despite major efforts, especially by the U.S.A. and U.S.S.R., no self-sustaining nuclear fusion reactions have been carried out on earth other than in thermonuclear weapons.

Copyright 1997 James R. Fromm