Alvin M. Weinberg

MOLTEN FLUORIDES AS POWER REACTOR FUELS

A high-temperature reactor using molten uranium salts (Aircraft Reactor Experiment) was operated for a short time at the Oak Ridge National Laboratory. The reactor was of the circulating-fuel type, with a BeO moderator. The maximum outlet temperature achieved was greater than 1500°F (1100 K). It is believed that with further development the ARE could be a prototype for an economical uranium burner. Two very different schools of reactor design have emerged since the first reactors were built. One approach, exemplified by solid fuel reactors, holds that a reactor is basically a mechanical plant; the ultimate rationalization is to be sought in simplifying the heat transfer machinery. The other approach, exemplified by liquid fuel reactors, holds that a reactor is basically a chemical plant; the ultimate rationalization is to be sought in simplifying the handling and reprocessing of fuel. At the Oak Ridge National Laboratory we have chosen to explore the second approach to reactor development. The ORNL aqueous homogeneous reactor is the best-known embodiment of the liquid reactor approach; in a sense it represents the natural culmination of the aqueous reactor systems.

The future of nuclear energy

In many ways nuclear energy is a fantastic success: a completely new source of energy now producing, or soon scheduled to produce, about 20 exajoules per year or almost 10 percent of all the energy man now produces. This energy will come from approximately 500 large reactors in 36 countries (see figure 1). These reactors, if replaced by oil‐fired power plants, would require about 107 barrels of oil per day—that is, about one‐seventh of all the oil produced in the world. Were the output of these plants used for electric resistive heating, in principle 5×106 barrels of oil per heating day could be displaced; if used to recharge electric vehicles, perhaps 10×106 barrels.

Current Status of Nuclear Reactor Theory

The mathematical theory of neutron chain reacting systems bears considerable similarity to the mathematical theory of quantum mechanics since the Schroedinger equation can be viewed as the equation for an infinitely large chain reactor in which the potential function is replaced by the distribution of neutron absorption cross sections. The chain reactor theory, not being self-adjoint, is somewhat more complicated than the Schroedinger equation theory. Nevertheless, perturbation theory, time dependent theory, and the theory of the stationary, i.e., just critical, state can be stated in much the same way as is done in quantum mechanics.

Inherently Safe Reactors and a Second Nuclear Era

The Swedish PIUS reactor and the German-American small modular high-temperature gas-cooled reactor are inherently safe—that is, their safety relies not upon intervention of humans or of electromechanical devices but on immutable principles of physics and chemistry. A second nuclear era may require commercialization and deployment of such inherently safe reactors, even though existing light-water reactors appear to be as safe as other well-accepted sources of central electricity, particularly hydroelectric dams.

Criteria for scientific choice

As science grows, its demands on our societys resources grow. It seems inevitable that sciences demands will eventually be limited by what society can allocate to it. We shall then have to make choices. These choices are of two kinds. We shall have to choose among different, often incommensurable, fields of science—between, for example, high‐energy physics and oceanography or between molecular biology and science of metals. We shall also have to choose among the different institutions that receive support for science from the government—among universities, governmental laboratories, and industry. The first choice I call scientific choice; the second, institutional choice. My purpose is to suggest criteria for making scientific choices—to formulate a scale of values which might help establish priorities among scientific fields whose only common characteristic is that they all derive support from the government.

A siting policy for an acceptable nuclear future.

A nuclear siting policy leading to a few, large concentrated sites, it is argued, is preferable in the long run to the present policy which could lead to many dispersed sites. Such a policy could be implemented incrementally if requirements for new nuclear generating capacity were met by adding reactors to the existing 100-odd sites. Such a concentrated nuclear siting policy would, to some extent, isolate nuclear activities while augmenting the strengths of the institutions responsible for managing them. Additionally, it would confer an element of permanence on the sites and thereby open new options fer managing low level wastes and reactor decommissioning. These actions may improve the public acceptability of nuclear energy in the United States as well as lead to a more rational contained nuclear system in the long run.

How long is coal's future?

Nearly all scenarios for future U.S. energy supply systems show heavy dependence on coal. The magnitude depends on assumptions as to reliance on nuclear fission, degree of electrification, and rate of GNP growth, and ranges from 700 million tons to 2300 million tons per year. However, potential climate change resulting from increasing atmospheric carbon dioxide concentrations may prevent coal from playing a major role. The carbon in the carbon dioxide produced from fossil fuels each year is about 1/10 the net primary production by terrestrial plants, but the fossil fuel production has been growing exponentially at 4.3% per year. Observed atmospheric CO2 concentrations have increased from 315 ppm in 1958 to 330 ppm in 1974 - in 1900, before much fossil fuel was burned, it was about 290–295 ppm. Slightly over one-half the CO2 released from fossil fuels is accounted for by the increase observed in the atmosphere; at present growth rates the quantities are doubling every 15–18 years. Atmospheric models suggest a global warming of about 2 K if the concentration were to rise to two times its pre-1900 value - enough to change the global climate in major (but largely unknown) ways. With the current rate of increase in fossil fuel use, the atmospheric concentration should reach these levels by about 2030. A shift to coal as a replacement for oil and gas gives more carbon dioxide per unit of energy; thus if energy growth continues with a concurrent shift toward coal, high concentrations can be reached somewhat earlier. Even projections with very heavy reliance on non-fossil energy (Neihaus) after 2000 show atmospheric carbon dioxide concentrations reaching 475 ppm.

Eugene Wigner, Nuclear Engineer

Wigner led the design of the Hanford nuclear reactors and founded a school to teach reactor physics to people working in industry.

‘Immortal’ energy systems and intergenerational justice

Some critics of our technological society have asserted that we are leaving a legacy of problems for our descendants — in the shape, for example, of CO2 pollution of the atmosphere and radioactive waste. The author argues that if some of our power generation systems turn out to be near ‘immortal’, with lives much longer than their book lives, we may, on the contrary, bequeth great benefits to our successors — in fully amortized plant with very low running costs. There are examples in history of similar benefits conferred by dams built hundreds of years ago but which still serve useful purposes today.

Science and Its Limits

William Ruckelshaus, in his beautiful essay “Risk, Science and Democracy,”1 has expressed very clearly what I shall call the regulator’s dilemma. “During the past 15 years there has been a shift in public emphasis from visible and demonstrable problems, such as smog from automobiles and raw sewage, to potential and largely invisible problems, such as the effects of low concentrations of toxic pollutants on human health. This shift is notable for two reasons. First, it has changed the way in which science is applied to practical questions of public health protection and environmental regulation. Second, it has raised difficult questions as to how to manage chronic risks within the context of free and democratic institutions.”1