CHEMICAL PROBLEMS ASSOCIATED WITH THE DEVELOPMENT OF NUCLEAR REACTORS1 WILLIAM F. KIEPFER2 The College of Wooster, Wooster, Ohio
ATOMIC energy has been making the newspaper front pages into science classrooms and vice versa for over eight years. I n recent months many of the promises rather than the horrors have held the headlines. More and more laymen are coming t o understand what the scientist means when he predicts that a genuinely peaceful world will be able t o witness a "beating of swords into plowshares" on a scale never before dreamed of. It is hoped that the present discussion can serve the dual purpose of making available to chemistry teachers some of the pertinent information about reactors and about the role that chemists and chemical engineers play in the research that is needed to make industrial nuclear power a reality. The United States Atomic Energy Commission's concern with the development of ultimate commercial power reactors has been widely publicized. Industrial teams have made feasibility studies of the economics of a variety of reactor One after another of the experimental reactors have operated successf~ly. Breeding new fissionable material concurrent with power production is no longer a mere theoretical prediction. The respectable power level of 1000 kilowatts has been attained by One of a homogeneous reactor. Security regulations do not allow any treatment of this topic t o be amplified with many specific illustrations. Much of the information about reactors is empirical, gained by expensive and time-consuming research. It is very diicult a t this time to say what is unimportant to the weapons program. However, it is possible to consolidate much pertinent qualitative information from the open literature. Chemists are called upon to do a great variety of re-
search in the development of reactors. The theoretical physicist, and after him the design engineer, keeps up a constant clamor for new types of materials and accurate knowledge about existing substances which the chemist must provide. The ultimate success of the whole reactor program will certainly depend upon its economic feasibility. These economies, both "neutron" and "dollar" economies, as Pitzer has stated, are the paramount problems for chemistry to solve. The implications of the term "dollar" economy are the old familiar ones, akin to those surmounted by any chemical industry in profitable existence. The term "neutron economy" is much less familiar t o chemists. It is appropriate, then, t o begin this discussion with a brief glance a t some of the physical phenomena which are pertinent background. EUNDAMENTALS OF REACTOR OPERATION
A nuclear reactor is an engine which consumes fuel
to deliver energy, ~h~ primary source of energy is the
highly unconventional fission of nuclei, The delivery of the energy is essentially the conventional process, since current designs for reactors provide onlyfor the removal of heat. It is conceivable that the reactor of the future may use some of the tremendous amounts of gamma radiation directly to produce chemical reaction, power however, involves only heat-exchanger devices, Their and maintenance is greatly complicated by the presence of other intense radiation. The anatomy of the fission process need not be expounded here; a brief outline with the aid of Table will suffice. The three currently consideled fuels are U3s p u 2 3 2 , and U233, ~ h ,first is obtained from Presented a t the 15th Summer Conference of the New Eng- natural uranium by isotope separation processes, the land Association of Chemistry Teachers a t Bowdoin College, last two from the ''breedingn process of neutron capture Brunswick, Maine, August, 1953. and ThZ3%, respectively. ' Research Participant in the Chemistry Division of the Oak byEach fission act produces approximately 200 million Ridge National Laboratory, 1951-52. Nucleonics, 11, NO. 6, 49 (1953). electron volts of energy. I t may be enlightening
SEPTEMBER, 1954
489
to recall that the creation of an electron-positron pair represents the conversion into mass of about one m. e. v. of energy and that this in turn is about 200 times the energy of chemical bonding. The bulk of the fission energy appears as kinetic energy of the massive fission fragments. Because the fission fragments usually have more neutrons than the natural isotopes of equal atomic number, their nuclei tend to readjust their protonneutron ratios by radioactive decay. As soon as fission fragments are formed, some eject "prompt" neutrons almost immediately, others by delayed mechanisms. The same is true of gamma emissions. Beta decay contributes probably only about 5 m. e. v. of the 20 m. e. v. indicated. The 10 m. e. v. labeled "absorbed neutrons" comes from the miscellaneous nuclear reactions involving stray neutrons with the atoms of the reactor core and structnre.
The disposition of the two or three neutrons set free by the average fission process is the biggest concern of the reactor designer. Obviously, one must produce for each fission another fission in order for the reactor to be "critical" and remain in operation. If, however, the initiation of fission were the only fate for neutrons, the reactor would explode. Criticality for a reactor implies a carefully regulated balance to avoid such bomb-like behavior. It is obvious that the "neutron economy" referred to above involves the distribution of the remaining one or two neutrons between the other two fates: useful capture to breed new fissionable nuclei, and loss by escape from the zone of reaction or by parasitic capture in the machine itself. The escape problem may be solved by geometrical considerations or by modifying the reactor design to include a "reflector" or "breeder blanket." The parasitic-capture
Materiel. Testing Reactor
View of the mssaive shielding showing the ports for experimental Durposes. This water-cooled heterogeneous reactor uses enriched uranium fuel. develops 30,000 kiloaatts of power from a neutron flux of 2 X in an active volume of only 100 litera.
It
JOURNAL OF CHEMICAL EDUCATION
"breeder" reactor t o make plutonium without having to incorporate a means of slowing down neutrons. Outline of the Fisnion Process Space will not allow more than a very cursory re160 m. e. v. Kinetic energy view of the essential features of reactor construction. of fission fragments 5 m. e. v. Prompt neu- There is a great variety of designs differing in purpose, trons cost allowance, fuel type, etc. The "core" where fise. v. Prompt =amj 5 m.mas sion occurs may be the size of a football or of a house; 20 m. e. v. Radioactive de- it may utilize natural uranium (0.7 per cent P5) or m. e. v.) highly enriched fuel. Usually the core also contains a 10 m.cay e. series v. Absorbed neu"moderator" which serves to slow down neutrons to trons > 40 different pairs possible speeds a t which capture t o cause fission is most likely. Decay to stable nuclei by 75 I n homogeneous reactors the medium in which the fuel fraament different chains Propagatmn of fission chain is dissolved can also perform the moderating function; reaction Breeding of new fissionable heterogeneous reactors require an additional component. Heat is removed by circulating a "coolant" 11338 P -n"B I 9 (gas or liquid) through the core and through external Th231 U?JS heat exchangers; for a homogeneous reactor this can be Losses by capture or escape the fuel solution itself. The bulkiest part of any installation, of course, is the shielding to protect against losses are more difficult t o combat. Keeping them neutrons and gamma rays. Not only must the opersmall dictates stringent requirements in nuclear prop- ating core be shielded, but also the radiations generated erties for any material which can satisfy the extreme by fission products. The remainder of the installation demands of high temperature, corrosive environment, is conventional but dependent upon the use to which and mechanical stress. Not only is the fate of the the reactor is to be put. Huge steam turbines may neutron itself a problem, but if it should be captured by generate electricity for a power station; smaller mobile a nucleus which would then decay with a long half-life, engines will be designed for more concentrated power a complicating radiation hazard would exist even after delivery. the reactor had ceased to operate. MATERIALS FOR REACTOR CONSTRUCTION The physicist designates the probability that a nucleus can capture a bombarding neutron as its "cross It can be seen from the foregoing discussion that section." If a neutron flux is described as a number of "neutron economy" considerations prompt much of the neutrons crossing a plane of unit area in a unit time, chemical and metallurgical research on materials to he then the number of reactions with nuclei in that plane used for reactor components. One point m y be will be proportional to the area they effectively present emphasized in passing. This is the unusual importance to the neutron beam. Cross sections are generally ex- of the isotope-separation problem in connection with pressed in "barns," a unit having the dimension of such materials. Chemists ordinarily are content to 1 X 1 0 P 4cm.l Present nuclear theory gives no simple isolate an element t o a reasonable degree of purity. picture for predicting the likelihood of reaction, but This may not be enough when isotopes of the same the concept of nuclear energy states does suggest that element differ in the all-important nuclear properties. the probability of capture will depend upon both the Many different methods are being employed for isoconstitution of the struck nucleus and the energy of the tope separation. The isotopes of heavier elements are incident neutron. I n addition to being captured, handled best by physical methods of separation; neutrons can be scattered by interaction with nuclei. lighter isotopes are more efficiently fractionated by exProbabilities for scattering are generally smaller, yet change reactions. For convenience, reactor components will be classilight nuclei, for which the probability of capture is very low, can be used t o perform the very useful func- fied as fuels, structural materials, moderators, coolants, tion of slowing down or "moderating" neutrons within and shielding materials. the reactor without removing them. Elements of Fuels. Most of the details of the chemical procesintermediate and high atomic weights tend to have sing of reactor fuels are still classified. The general larger capture cross sections for slower neutrons. For neutron energies up t o about 0.1 e. v. these cross secTABLE 2 tions are inversely proportional t o neutron velocity. Sources of Fissionable Materials This range includes "thermal" neutrons (about 0.025 e. Eslimted r e s e w v.). From 0.1 to 1.0 e. v. cross sections vary greatly, Ore (Ibs. U and Th) often being very large for particular neutron velocities High grade >10'%U and Th) 10' which correspond to resonance with nuclear energy pitchblen e, thonte states. Most nuclei have very low cross sections for 108 Medium erade (1.0-0.1%) ,-. oarnotice, monazite high-energy neutrons, yet a few important exceptions grade (
