Global energy consumption is growing rapidly. Current projections indicate that global consumption may increase by nearly 50% by 2030 (See Chart 1 below). Simultaneously, the average temperature of the earth has been steadily climbing in the last century, which has raised some speculation over man-made global warming (See Chart 2 below). These two issues coupled with the reality of a limited fuel source with volatile pricing sets the precedent for new sources of energy to be developed.
[Chart 1: Past and Future Global Energy Consumption]
[Chart 2: Historical Average Global Temperature].
The Nuclear Option
In recent years there has been a revived interest in procuring new light water nuclear reactors and developing advanced reactors for the future. In July 2001, the Generation IV International Forum (GIF) was commissioned to coordinate the world's most powerful countries in developing advanced reactor technology. Current members of GIF include Argentina, Brazil, Canada, China, Euratom, France, Japan, Republic of Korea, the Russian Federation, Republic of South Africa, Switzerland, the United Kingdom, and the United States. The main objectives of GIF and ultimately the Generation IV reactors itself are to improve safety, reduce the risk of proliferation, minimize natural resource consumption, and reduce the production and operating costs. Basically, the Generation IV reactors are to be safer, cheaper, more efficient, and more environmentally friendly the today's reactors.
Energy Cogeneration in Nuclear Reactors
From the technical point of view, nuclear reactors are basically heat generating devices. The result of the nuclear fission process is the generation of heat within the reactor. The heat is removed by the coolant circulating through the core, which can then be applied to the generation of electricity or used in providing hot water or steam for industrial or space heating purposes. The problem in nuclear energy cogeneration arises from the fact that the transport of heat is difficult and expensive. The need for a pipeline, thermal isolation, pumping, and the corresponding investments, heat losses, maintenance, and pumping energy requirements make it impractical to transport heat beyond some tens of kilometers. Current research has shown success in utilizing nuclear energy cogeneration for district heating and industrial process heat.
A potential market for district heating only appears in climatic zones with relatively long and cold winters. In western Europe, for example, Finland, Sweden, and Denmark are countries where district heating is widely used. In such countries, district heating networks generally have installed capacities in the range of 600 to 1200 megawatt-thermal (MWth) in large cities; this is not practical in small individual heating installations fueled by gas, oil, coal, or wood in smaller cities due to the limited power requirements of the heat market and the relatively low load factors.
However, the district heating market is expected to expand substantially. Not only because it can compete economically in densely populated areas with individual heating arrangements, but also because it offers the possibility of reducing air pollution in urban areas. Emissions resulting from the burning of fuel can be controlled and reduced up to a point in relatively large centralized plants.
Within the industrial sector, process heat is used for a very large variety of applications with different heat and temperature requirements that cover a very wide spectrum. Although there are some common features such as the need for minimal heat transport distance, the market for process heat is quite different from district heating. In energy intensive industries the energy input represents a considerable fraction of the final product cost. Many of the process heat users, in particular the large ones are usually located outside urban areas, often at considerable distances. This makes joint siting of nuclear reactors and industrial users of process heat not only viable, but also desirable in order to drastically reduce or even eliminate the heat transport costs.
Supercritical Water Cooled Reactor (SCWR)
The SCWR is similar to gen III and gen III+ reactors in terms of structure; however, by ramping up the design temperature and pressure of the coolant far greater thermal efficiency can be achieved. Supercriticality occurs when a fluid exists at temperatures and pressures above the critical point on the vapor dome(See T-s diagram below). A supercritical working fluid is very common in coal fired powerplants, but because of the poor thermal characteristics of uranium dioxide fuel this is not possible in nuclear plants. Basically, the temperatures required to take water supercritical would melt nuclear fuel.
The SCWR is modeled similiar to a current BWR in that there is only a primary loop. Also, becuase the coolant is far above the vapor dome, there are no phase changes in the cycle. This allows for a signficant reduction in the amount of equipment necessary to maintain the cycle.
Molten Salt Reactors (MSR)
Molten Salt Reactors are liquid-fueled reactors that can be used for production of electricity, production of hydrogen, and production of fissile fuels.
Electricity production and waste burndown are envisioned as the primary missions for the MSR. Fissile, fertile, and fission isotopes are dissolved in a high-temperature molten fluoride salt with a very high boiling point (1,400 C) that is both the reactor fuel and the coolant. The near-atmospheric-pressure molten fuel salt flows through the reactor core. The traditional MSR designs have a graphite core that results in a thermal to epithermal neutron spectrum.
In the core, fission occurs within the flowing fuel salt that is heated to ~700ºC, which then flows into a primary heat exchanger where the heat is transferred to a secondary molten salt coolant. The fuel salt then flows back to the reactor core. The clean salt in the secondary heat transport system transfers the heat from the primary heat exchanger to a high-temperature Brayton cycle that converts the heat to electricity. The Brayton cycle may use either nitrogen or helium as a working gas.
Very High Temperature Reactor (VHTR)
Perhaps the most interest has been generated for the Very High Temperature Reactor (VHTR). This is possibly because several concepts have been developed and studies indicate that the VHTR is likely feasible and safe. A VHTR is a gas-cooled, graphite moderated reactor in which a working fluid is subject to a brayton cycle. A brayton cycle is plotted on a T-s diagram below.
Several VHTR has been developed as research reactors. In 1960 germany began developing the Arbeitsgemeinschaft Versuchsreaktor (AVR). This reactor was a 17MW gas cooled, pebble design that operated from 1967 to 1988. In 2003, China completed the HTR-10, a 10MW, helium cooled reactor at Tsinghua University. The HTR was modeled after the german AVR. China plan's to impliment the HTR as a modular type plower plant in which multiple systems can be chained together to produce more power. Because of the extremely high temperatures at the reactors outlet it is possible to cogenerate hydrogen for powering fuel cells.