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Alternative Nuclear PowerRod AdamsAdvances in South Africa and the Netherlands suggest that small-scale fission machines could become safe, reliable, and inexpensive sources of electricity and heat for ships, factories, and perhaps single-family homes.  rews on nuclear submarines spend months at a time underwater and are totally dependent on the constant,
reliable power output of a nuclear reactor. Similarly, some of NASA's long-distance space probes depend on
nuclear power. Yet the once-bright promise of nuclear energy is today tarnished by associations with nuclear
weapons, a few power station accidents, and concerns about wastes.
While most of the world continues to ignore the possibility of resurrecting nuclear energy sources, working groups in South Africa and the Netherlands are making
These smaller-scale applications employ a different kind of reactor design than the large commercial power plants, which use the reactor as a heat source for converting water to steam that drives steam turbines. In the smaller-scale reactors now being developed, the reactor heats a gas (like helium or nitrogen) that in a closed cycle directly drives a gas turbine. To understand the suite of options offered by the new breed of nuclear fission machines, we need to know how they work. From splitting atoms to electricity  achines that harness fission as a sustained source of power are called nuclear reactors because they are
designed to manage a key reaction affecting the nucleus in each of trillions of fuel atoms. This reaction is
fission, the splitting of an atomic nucleus into two main fragments, a process that is greatly enhanced if an
extra neutron joins the nucleus. Among the naturally occurring metals, fission occurs only with certain
isotopes of the heaviest metal, uranium. If these heavy metal atoms are packed together closely enough and
stimulated by neutrons from an outside source, neutrons released in the fission process can escape the initial
reaction, find another heavy metal nucleus, and cause a second reaction. If at least one neutron from each
fission causes another fission, the process is said to have reached criticality; the amount of material needed
for this condition is called critical mass.
Critical masses can be achieved with as little as one kilogram (2.2 pounds) of fuel. In a reactor, the fissionable material is normally carefully arranged in
Neutrons released in fission are initially moving so fast that they avoid capture by a heavy metal nucleus; hence, they cannot cause fission. To do that, they need to be slowed down. The best way to slow them is to assure that they pass through a lightweight material (one in which each component atom has a small, light nucleus). In this way the neutrons, in colliding with many of the nuclei, will be slowed, as in each collision they transfer some small part of their kinetic energy to one nucleus without being captured. Materials that slow the neutrons are called moderators. They reduce the quantity of fissionable materials needed to achieve a critical mass. The nuclei of some materials, called neutron absorbers, act as a sponge, absorbing a neutron without either undergoing fission or reacting in any way that releases neutrons. Neutron absorbers are used as needed to control the neutron population and fission rate. The energy released in nuclear fission comes in several forms, including the electromagnetic wave energy of gamma radiation and the kinetic energy of the two main fragments and such diverse particles as neutrons, electrons (called beta radiation), and alpha particles (helium nuclei consisting of two protons and two neutrons). The most penetrating and detrimental to
One tool used for managing fission is the neutron source. These devices combine a radioactive material with one of several light elements like lithium or beryllium, such that alpha particles emitted by the radioactive material strike the nucleus of the light element and cause the emission of a neutron. Just as an experienced outdoor cook can arrange charcoal, vents, a match, and lighter fluid to control a combustion heat source, nuclear reactor designers and operators can arrange fissionable materials, moderators, reflectors, absorbers, and neutron sources in various ways to control a fission heat source. ——————— SIDEBAR ———————
Harnessing fission heat
Minimizing Nuclear Wastes  n essence, a nuclear reactor, like any kind of combustion chamber or furnace, is a heat source that can be
connected to engines that convert the heat into a more usable energy form such as rotational kinetic or
electrical energy.
In the 1950s, when commercial fission reactors were developed, the most popular form of steady input heat engine involved a closed loop of water and steam, with the heat converting water to steam that drives a turbine. These engines were well understood, reliable, and clearly capable of converting heat from almost any source into useful work. Further, the industrial base supporting steam engines was extensive, and plenty of people were experienced in operating and maintaining them. The early designers and builders minimized the risk of their projects by combining the new nuclear fission--based heaters with the well-proven closed-cycle steam engines. They established the industry's prevailing and enduring wisdom that the best way to reduce unit costs was to build plants with steadily increasing capacity to take advantage of projected economies of scale. Thus, essentially all of today's commercial nuclear power plants are large, central stations that use steam engines to convert their heat output into a useful form of energy, usually electricity. Gas turbines heated by nuclear fission  skom, a large public utility in South Africa, has taken a serious look at nuclear fission technology and is
committed to the precommercial development of an alternative type of machine called the pebble bed
modular reactor (PBMR). The machine uses a high-temperature fission reactor as the heat source and helium,
an inert gas with valuable heat-transfer properties, as the working medium. In a closed cycle, the compressor
pushes the gas into the heat source, from which the heated and expanded gas advances to spin the turbine
before it enters the heat sink unit, whose function is to remove the gas's excess heat. From there the gas is
ready to be compressed again. A similar kind of cycle is used in a jet engine, with the atmosphere taking the
place of the heat sink by supplying fresh air and accepting the waste gases.
Since fission emerged as a promising heat source some 60 years ago, the alternative to steam-driven turbines--high-temperature, gas-driven turbines--has been dramatically improved. In parallel, nuclear reactors capable of reaching the high temperatures at which modern gas turbines are most efficient have been developed and proved in experimental programs. These developments culminated in the construction and operation of the PBMR's predecessors--several prototype reactors, one of which was operated for more than 20 years in Germany. Fuel for the PBMR is packaged inside "pebbles," billiard ball--sized spheres. Each pebble is a bit like a Russian doll, with the variant that inside the thick outer layers of silicon carbide steel a confined "sea" of graphite filler surrounds and enfolds about 10,000 fuel microspheres, each about 1 millimeter in diameter. Each microsphere consists of four protective layers
The core of a PBMR consists basically of a pile of pebbles held in a cylindrical container. The core is surrounded by a neutron reflector (made of graphite) that also contains channels for holding control rods. Space between the pebbles provides ample room for a flowing gas that removes the reactor's heat. Because higher temperatures naturally lead to higher efficiencies in heat engines, the PBMRs will achieve an overall thermal efficiency of about 45 percent compared to the 33 percent typical of conventional nuclear steam plants. The inert gas coolant eliminates the need for corrosion-resistant metal coatings to protect core materials. Safety features  ll the materials in the reactor are capable of withstanding elevated temperatures without melting, so there is
a large margin between the temperature used in normal operation
Eskom's researchers have determined that no operational accidents could result in public exposure to radioactive materials. Their plans provide a wide margin of safety with a 400-meter-radius safety zone. This means that the plants could be placed adjacent to populated areas, keeping transmission costs low. It also means that there will be no need to develop extensive emergency plans with a wide variety of local government agencies. In keeping with the utility industry's interest in being able to add generating capacity as demand grows, each PBMR will produce about 110 Mwe (million watts of electricity), roughly enough to power a city of 100,000. This size allows a utility to construct a 1,100-Mwe generating station over time, adding a generator or two until there are 10 on the same site to take advantage of common support systems. To allow the utility even more flexibility, the machines will automatically adjust their output to meet load variations. According to Eskom's projections, the final cost of electricity from the plants will be approximately U.S. 1.4--1.6 cents per kilowatt-hour with an installed base of 10 plants.
Future developments  he project points the way to other possibilities for applying fission technology to meet the needs of
additional customers. Downsizing and simplification do not have to stop at a 110-Mwe power plant module.
Though quite small compared to the 1,000--1,300 Mwe of the typical large nuclear station, South African
PBMRs will still be too big to use to push most ships or power an individual factory or industrial park.
Because most customers requiring heat for industrial purposes or environmental controls use far less than the
120 MW each PBMR will produce as a by-product of its electrical generation, these reactors are not well
suited to the market for cogeneration plants selling both heat and electricity.
In the Netherlands, a country that currently depends heavily on natural gas--fired gas turbines operating as cogeneration plants, a significant amount of research has been conducted on future power plants that can replace those units when the gas begins to run out. The country also has a centuries-old tradition as a maritime trading nation with a substantial shipping industry. The ships currently depend on diesel engines, which use expensive fuel and are subject to increasing emissions-control limits. One potential solution for both needs is the NEREUS concept, a pebble bed reactor that bears a close technological resemblance to the South African PBMR. That plant will produce about 8 Mwe, fill a cube less than 8 meters (26 feet) on a side, and be readily adaptable to the market for ships and cogeneration. The plants will use the same kind of fuel as the PBMR, allowing the project to leverage the investment made by the PBMR companies. Operation in the cogeneration mode will require the inclusion of optional heat exchangers that will make heat available for heating buildings, purifying water, or drying industrial products like paper. NEREUS project leaders plan a pool system of management, whereby the plant's owners will take care of routine operation and a specialized cadre of workers and facilities will do maintenance and repair work. This management system is already used with great success in fossil-fuel ship and aircraft engine programs. Using the financial assumptions standard in the small power plant market, NEREUS project leaders calculate a power cost of approximately U.S. 5 cents per kilowatt-hour. This would give them a strong competitive advantage against fossil-fuel machines, whose power often costs two or three times as much. Home power reactors n much the same way that microprocessors can provide the basis for computational machines ranging in size
from a pocket calculator to a mainframe computer, the high-temperature fuel microspheres could be used as
the basis for fission machines that are considerably smaller
There is nothing magic about the billiard ball--sized fuel element. It was originally chosen by German designers as a convenient size for a reactor using a continuous refueling concept. For smaller reactors, elements the size of golf balls or marbles might be needed. Making the smaller critical assemblies required for the very small reactors would also require fuel pebbles whose percentage of fissile material is higher than it is for pebbles in larger reactors. Though shielding can be expensive if space or weight is a limiting concern, it is also possible to put the machines at the bottom of a water tank the size of a swimming pool. Like their larger cousins, the very small reactors would probably use a gas in a closed cycle to harness the nuclear fission heat for driving an appropriately sized turbine. Microturbines using only a single moving part
Of course, the early adopters of such a technology will not be average homeowners. A likely initial customer might be the owner of an isolated tropical island or a remote mountain with a spectacular view. The machines could be designed as black boxes containing a decade or more of fuel and needing only a cooling supply and a place to put the output power. They would not spoil the view with an exhaust stack and could be buried to muffle all noise. The possibility of home-size cousins of the PBMR coming to a neighborhood might raise concerns. What if an external explosion, as from a ruptured natural gas main, shattered the tough shield surrounding the PBMR and scattered the pebbles? In such a scenario, the radioactive material would remain contained within the pebbles. Of course, the pebbles would be hot, in terms of both temperature and radioactivity. Residents would need to be evacuated until professionals collected all the pebbles, but then they could return safely to their homes. Given the current state of energy-industry politics, few people familiar with the field allow themselves to imagine that such a machine will ever exist. Engineers and scientists may acknowledge that the technology is simply an adaptation of existing machinery, but they pale when considering the difficulties involved in obtaining permission to build and sell the device. Perhaps this mind-set can change as machines such as the PBMR emerge as viable, attractive alternatives to the much-maligned conventional nuclear plants.  On the Internet Additional Reading: Ship-sized Reactors Adams Atomic Engines, Florida http://www.atomicengines.com NEREUS Project: Romawa, the Netherlands http://www.romawa.nl Town-sized Reactors PBMR Project: ESKOM, South Africa http://www.pbmr.com Rod Adams was a nuclear submarine officer for nearly 13 years. He holds an associate chair in the weapons and systems engineering department of the U.S. Naval Academy. |
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