The Modular Helium Reactor
for the Twenty-First Century
|Malcolm P. La Bar & Walter A. Simon|
A strong synergism exists between electricity production, economic activity, global warming and nuclear power. Electricity is needed for robust economic activity, but a large share of the world's electricity is produced by burning fossil fuels. Combustion of fossil fuels is causing increased concentrations of greenhouse gases in the atmosphere, which are suspected of causing global warming. Expanded use of nuclear power could provide the energy needed for robust economic activity without producing greenhouse gases.
There are, however, several factors impeding significant worldwide expansion of nuclear power for electricity generation. The main factors limiting its broader use can be summarised as being the public's perception of uncertain safety, marginal economics, and the disposition of wastes. However, an advanced nuclear power system has been conceptualised which addresses these issues. The system is under development in an international cooperative programme and has the promise of being commercially available for deployment in the next decade.
purpose of this paper is to:
Electricity and Economic Activity
A strong correlation exists between electricity production and economic activity. Figure 1 shows the correlation for the USA, using gross domestic product (GDP) as the measure of economic activity. Electricity demand and the economy have grown together, while non-electrical energy use has actually declined in some periods. Over the past 20 years, US conservation efforts have restrained the growth of total energy use to less that 20%, but electricity demand has increased more than 70%. The utility of electricity makes it a key energy resource for many industrial activities that drive the overall economy.
On a much broader scale, there is a strong correlation between electricity use and standard of living. Figure 2 shows, as a measure of standard of living, life expectancy versus electricity use. Electricity has the direct benefits of versatility and cleanliness in end-use, with well established safety standards, being able to power household, industrial, commercial, agricultural and medical facilities. Secondary benefits include communication and education. All of these contribute to high living standards, including longer life, lower infant mortality rates and less illiteracy.
The most notable negative aspect concerning electricity use is the waste of primary energy resources that electricity currently entails. Coal and nuclear plants generate most of the electricity in the USA. About 70% of the primary energy used for electricity generation is wasted, primarily due to the low thermal conversion efficiencies of coal and nuclear plants employing the Rankine cycle. The overwhelming benefits of electricity would be further enhanced if generation technologies were pursued with improved thermal conversion efficiencies.
Worldwide Demand for Electricity
The demand for electricity on a worldwide basis is projected to experience considerable growth in the next few decades. There are two basic factors driving this growth. The first of these is the growth needed in the developing countries for industrialisation and improvement of living standards. The second is a compounding effect due to the growth of the world's population, which is expected to increase by about 50% in the next thirty years.
The combination of these two factors is projected to result in the demand for electricity to about double by 2020. An impartial projection of the electricity generation technologies for meeting this demand is shown in Figure 3. Generation from coal, natural gas, nuclear and hydro are all projected to about double. Not surprisingly, very little growth of electricity generation from oil and renewables is projected.
Many other projections of future electricity generation do not indicate the substantial increase in nuclear generation shown in Figure 3. In fact, projections by the US Department of Energy (DOE) generally indicate that nuclear electricity generation will start to decline after 2000 because the number of nuclear plants that will be decommissioned will exceed the number of new nuclear plants coming on line.
If the DOE projection of declining nuclear electricity generation is valid, what will make up the difference to meet the electricity demand? The most likely answer is expanded use of coal. Figure 3 already projects that generation by coal will have to double. If nuclear actually decreases rather than doubling, coal will have to increase by substantially more than a factor of two. This would have an enormous impact on atmospheric CO2 concentration because coal plants emit about 1.5 kg of CO2 per kWh of electricity.
Atmospheric CO2 Concentrations
The concentration of CO2 in the atmosphere has been steadily increasing since the beginning of the industrial revolution, due to the combustion of fossil fuels. Figure 4 shows that the rate of increase of CO2 concentration has accelerated in the past few decades. In parallel with this, global temperatures have increased about 1°C. Most scientists attribute this global warming to the accumulation of greenhouse gases in the atmosphere and identify CO2 as the major greenhouse gas constituent.
The evidence suggests that efforts should be made on a global basis to curtail the rate of increase of CO2 concentration in the atmosphere. Electricity generation by burning coal is a major contributor to atmospheric CO2 concentrations. As such, efforts should be made to reduce the use of coal for electricity generation.
The Nuclear Power Solution
power is a practical alternative to coal for substantial electricity
generation capacity. So why is nuclear power not being broadly
embraced as a solution to help resolve the CO2 issue
and to provide the needed electricity generation capacity for
developing countries and a growing world population? The reason
is that there are problems with the current generation of nuclear
power, which have been described in detail by many experts. These
problems can be summarised as being:
To resolve these issues with current nuclear power plant designs, a revolutionary new nuclear power system has been conceptualised known as the Gas Turbine - Modular Helium Reactor (GT-MHR).
GT-MHR Design Overview
The GT-MHR is a second-generation nuclear power plant under development for deployment early in the next century. The plant design has evolved from the experience of operating more than 50 gas-cooled nuclear reactors worldwide and recent technological advances in fossil-fired Brayton (gas turbine) cycle systems.
Five helium-cooled reactors, which operated in the 1960s, 1970s and 1980s, demonstrated the inherent characteristics of helium cooled reactors. In the 1980s, modular helium reactor (MHR) designs were developed both in Germany and in the USA in response to the general public's safety concerns, with inherent passive characteristics for meeting stringent safety goals without relying on active safety systems or operator action. The major drawback of these passively safe designs, which employed the Rankine power conversion cycle, was they were evaluated to be non-competitive economically.
By the early 1990s, because of advances in industrial gas turbine technology, and in highly effective recuperators and related equipment, the potential emerged for coupling the unique high temperature capability of an MHR with a gas turbine in a closed Brayton cycle for the achievement of high efficiency and competitive economics.
The GT-MHR (Figure 5) couples an MHR, contained in one vessel, with a high efficiency gas turbine energy conversion system, contained in an adjacent vessel. The reactor and power conversion vessels are interconnected with a short cross-vessel and are located below grade in a cylindrical silo.
The MHR employs a graphite moderator and TRISO-coated particle fuel (Figure 6). TRISO fuel contains a spherical kernel of fissile or fertile material, as appropriate for the application, encapsulated in multiple coating layers. A low density carbon (buffer) layer surrounds the kernel to attenuate fission recoil atoms and provide void volume to accommodate fission gases. Surrounding the buffer is an inner pyrocarbon coating (IPyC), a silicon carbide (SiC) layer, and an outer pyrocarbon coating (OPyC). The IPyC, SiC and OPyC layers together form a miniature, highly corrosion resistant pressure vessel and an essentially impermeable barrier to the release of gaseous and metallic fission products. Extensive tests in the USA, Europe and Japan have proven the excellent performance characteristics of this fuel.
The overall diameter of standard TRISO-coated particles varies from about 650 microns to about 850 microns. For the MHR, TRISO-coated particles are bonded with a graphitic matrix to form cylindrical fuel compacts approximately 13 mm in diameter and 51 mm long. Approximately 3000 fuel compacts are loaded into a hexagonal graphite fuel element, 793 mm long by 360 mm across flats, the same type of fuel element which showed excellent performance at the Fort Saint Vrain reactor. One hundred and two columns of the hexagonal fuel elements are stacked 10 elements high to form an annular core (Figure 7). Reflector graphite blocks are provided inside and outside of the core.
TRISO-coated particle fuel remains stable to very high temperatures (Figure 8). The coatings do not start to thermally degrade until temperatures approaching 2000°C are reached. Normal operating temperatures do not exceed about 1250°C and worst case accident temperatures are maintained below 1600°C.
A simplified flow schematic for the GT-MHR is shown in Figure 9. Helium, heated in the reactor, expands through a gas turbine to generate electricity. From the turbine exhaust, the helium flows through the hot side of a recuperator, transferring residual heat energy to helium on the recuperator cold side (which is returning to the reactor). From the recuperator, the helium flows through a pre-cooler where it is further cooled. The cooled helium then passes through low and high pressure compressors with intercooling. From the compressor outlet, the helium flows through the cold, high pressure side of the recuperator where it is heated for return to the reactor. Nominal full power operating parameters are given in Table 1.
The gas turbine power conversion system has been made possible by key technology developments during the past decade in: large aircraft and industrial gas turbines; large active magnetic bearings; compact, highly effective plate-fin heat exchangers; and high strength, high temperature steel alloy vessels. Demonstrated gas turbine technology is available for turbines with power ratings that match the requirements of the passively safe MHR. Indeed, the high pressure (and density) helium working fluid actually provides higher output (ie the GT-MHR gas turbine is actually smaller that an equivalently rated fossil fired gas turbine).
The gas turbine system eliminates the extensive and expensive equipment required for the century-old Rankine steam cycle technology used by other nuclear power plants for conversion of thermal energy to electrical power. Not only does the elimination of steam plant equipment reduce capital and operating costs, but also the plant efficiency is markedly increased. The GT-MHR achieves a net thermal conversion efficiency of approximately 47%, as compared to current nuclear plants that have efficiencies of about 32% (Figure 10).
The GT-MHR design offers several advantageous performance characteristics, which are described below.
These safety design features result in a reactor that can withstand loss of coolant circulation or even loss of coolant inventory and maintain fuel temperatures below damage limits (ie the system is meltdown proof).
High Level Waste Form
The GT-MHR also provides important benefits for the destruction of plutonium, either weapons grade plutonium (WPu) or reactor grade plutonium (RPu).
As shown in Figure 12, this level of plutonium destruction is well beyond that achieved by other WPu disposition alternatives. By achieving this high level of plutonium destruction, the GT-MHR extracts a substantially higher portion of the useful energy content from the material than other reactor options without reprocessing and recycling. Because the plutonium-fuelled GT-MHR uses no fertile fuel material, all fissions in the core are plutonium fissions, and no new plutonium is produced by the operation of the reactor. Comparable results would apply to the use of reactor grade plutonium.
Furthermore, the discharged plutonium isotopic mixture is severely degraded (well beyond LWR spent fuel) making it particularly unattractive for use in weapons. In contrast, one weapons grade mixed oxide (MOX) fuelled PWR spent fuel assembly contains sufficient plutonium to fabricate more than one nuclear device.
summary, the GT-MHR is an advanced nuclear power system that
In addition to these attributes, the GT-MHR can be used to effectively disposition weapons grade plutonium. Over 90% of initially charged weapons desirable plutonium-239 and over 60% of the total plutonium can be destroyed in a once-through GT-MHR fuel cycle while achieving highly efficient energy production.
International Cooperative Programme
As a result of the GT-MHR's safety, efficiency and projected economics, General Atomics (GA) of the USA and the Russian Federation Ministry for Atomic Energy (Minatom) entered into a memorandum of understanding (MOU) in April 1993 to cooperate on the development of the GT-MHR. The MOU covered promotion, implementation and execution of a joint US/Russian GT-MHR design and development programme, and subsequent construction, testing and operation of a prototype in Russia.
The primary objective was to develop the GT-MHR as an export commodity for both the USA and Russia. Design and construction of the prototype was planned to be done in Russia in accordance with Russian, US and other international standards to ensure the suitability of the design for licensing in the USA and in other potential world markets. Disposition of WPu was also noted in the MOU to be a mission for which the GT-MHR was well suited.
The WPu disposition mission was addressed in the MOU because work performed by GA showed the GT-MHR to be very effective for this mission, and because an independent study completed in February 1993 at Russia's Kurchatov Institute confirmed the capabilities and advantages of the high burnup gas cooled reactor fuel cycle for the destruction of surplus WPu.
Subsequent to the March 1994 agreement between the USA and Russia to cease the production of WPu by 2000, Minatom proposed in June 1994 that the emphasis of the proposed cooperative GT-MHR program be shifted to focus on disposition of surplus WPu. Minatom proposed that the first GT-MHRs under the cooperative programme be built at Tomsk 7 (now known as Seversk), the site of some of Russia's plutonium production reactors, and that they be fuelled with surplus Russian WPu. This proposal was intended to not only reduce the Russian stockpile of surplus WPu, but also to provide alternative employment for former Russian nuclear weapons programme technical staff, and to generate power for use in the region surrounding Seversk.
In the summer of 1994, GA and Minatom agreed to initiate development of the GT-MHR for WPu destruction, with each party committing US$1 million for work performed in Russia. With this funding, the conceptual design of the GT-MHR for WPu destruction was planned to be performed in Russia by the OKBM institute. This work was begun in February 1995 and is scheduled to be completed and fully documented by October 1997.
In January 1996 Framatome of France joined the GA/Minatom cooperative programme, and in January 1997 Fuji Electric of Japan also joined the programme. Framatome and Fuji Electric have provided additional funding to support the conceptual design programme and Minatom has matched their contributions.
Programme Approach and Progress
Existing information on the US-sponsored GT-MHR design has been made available to the cooperative programme. The US GT-MHR design has served as a relatively mature starting point for the conceptual design effort. A major achievement to date resulting from the contribution of the OKBM designers has been significantly improved component conceptual designs, particularly in the power conversion system. The GT-MHR configuration now incorporates both US and Russian technology and is a clear demonstration of the advantages of the cooperative programme.
Following completion of the GT-MHR conceptual design, detailed design and component development testing will be initiated. Efforts are in progress to obtain broader international support from governments and industries. An international partnership is planned for management of the design, development and prototype construction activities. Following successful completion of the prototype and performance demonstration, the GT-MHR would be marketed to the worldwide electricity generation market.
GT-MHR is a second-generation nuclear power plant under development
in a cooperative programme involving General Atomics of the USA,
Minatom of Russia, Framatome of France and Fuji Electric of Japan.
The relevant conclusions are as follows:
An international team is actively working on the design and is pursuing the necessary support to achieve the programme goals, which are to complete the deployment of a prototype in Russia, with the an initial mission of plutonium disposition, and to market uranium fuelled GT-MHR plants to the world for electricity generation.
© copyright The Uranium Institute 1997 SYM9798