Nuclear power plant as Ghana’s next green baseload solution
Ghana considered nuclear power dating back to 1960s but has been unable to achieve this long-existing agenda of producing electricity from nuclear energy. Despite being wrecked at various stages, recent developments around Ghana’s nuclear agenda gives hope, serving as a shining example to West African sub-region.
However, I must anchor the point that there are outstanding issues to be addressed to deliver Ghana’s first nuclear power. These include nuclear safety, public perception, management, legislative framework, safeguards, stakeholders’ involvement, licensing/regulatory framework, funding and finance.
Prof. Samuel Dampare, Director-General of Ghana Atomic Energy Commission (GAEC)
The rests are radiation protection, electrical grid, environmental protection, human resource development, emergency planning, nuclear fuel cycle, radioactive waste disposal, industrial involvement, procurement, security and physical protection, site and supporting facilities.
Prof Seth Kofi Deborah, Director of Nuclear Power Institute at Ghana Atomic Energy
Public perception in this discourse implies that fake news and misconceptions about nuclear power affect public acceptance and policy making. Apart from this, licensing/regulatory framework that fuels enabling environment for operations needs to be fully resolved, understanding stakeholders opinions coupled with human resource development. This implies acquiring retaining skilled labour to ensure competent workforce for all phases among the top priorities. Nuclear safety requires strict compliance with International Atomic Energy Agency (IAEA) standards to achieve nuclear as Ghana’s green baseload solution.
Mr. William Ahiataku-Togobo of Bui Power Authority Executive Office
It presupposes that to fully address outstanding issues to deliver Ghana’s first nuclear power, burning questions such as what kind of human resources are needed?, how much will it cost, can Ghana find access to finance?, what will Nuclear Power Ghana do with the waste?, is it safe to fully go for nuclear?, and can accident be managed ?
Dr. Stephen Yamoah, Executive Director of Nuclear Power Ghana
Apart from that, developing relevant institutions, building capabilities, expertise whilst liaising with stakeholders, developing regulatory framework, electrical grid upgrade, procurement, site preparations before contracting.
Another outstanding issues include ensuring safety construction towards preparations for construction, getting ready for fuel arrival on site, human resources and building competence, regulator ready for review of the construction license application and inspection, project management, control of cost and scheduling during the time of construction.
Despite the notion that nuclear is an out-dated technology, the cost of finance, market design, political changes, perceived competition with renewable energy and the public’s misconceptions about radioactive waste disposal, Ghana should be fully ready to make knowledgeable commitment, build and negotiate a contract for the first nuclear power plant to commission and operate the first nuclear power plant as Ghana’s next green baseload solution.
Other areas of nuclear power technology in focus
In this discourse, I will recommend for the next workshops other wide-scale deployment of nuclear power technology that offers substantial advantages over other energy sources. These next-generation systems are based on three general classes of reactors including gascooled, water-cooled and fast-spectrum.
Gas-cooled reactors implies nuclear reactors using gas (usually helium or carbon dioxide) as a core coolant have been built and operated successfully but have achieved only limited use to date. An especially exciting prospect known as the pebble-bed modular reactor possesses many design features that go a good way toward meeting power generation goals. This gas-cooled system is being pursued by engineering teams in China, South Africa and the U.S. South Africa plans to build a full-size prototype and begin operation in 2006.
The pebble-bed reactor design is based on a fundamental fuel element, called a pebble, that is a billiard-ball-size graphite sphere containing about 15,000 uranium oxide particles with the diameter of poppy seeds. The evenly dispersed particles each have several high-density coatings on them. One of the layers, composed of tough silicon carbide ceramic, serves as a pressure vessel to retain the products of nuclear fission during reactor operation or accidental temperature excursions. About 330,000 of these spherical fuel pebbles are placed into a metal vessel surrounded by a shield of graphite blocks. In addition, as many as 100,000 unfueled graphite pebbles are loaded into the core to shape its power and temperature distribution by spacing out the hot fuel pebbles.
Heat-resistant refractory materials are used throughout the core to allow the pebble-bed system to operate much hotter than the 300 degree Celsius temperatures typically produced in today’s light-water-cooled (Generation II) designs. The helium working fluid, exiting the core at 900 degrees C, is fed directly into a gas turbine/generator system that generates electricity at a comparatively high 40 percent thermal efficiency level, one quarter better than current lightwater reactors.
The comparatively small size and the general simplicity of pebble-bed reactor designs add to their economic feasibility. Each power module, producing 120 megawatts of electrical output, can be deployed in a unit one tenth the size of today’s central station plants, which permits the development of more flexible, modest-scale projects that may offer more favorable economic results. For example, modular systems can be manufactured in the factory and then shipped to the construction site.
The pebble-bed system’s relative simplicity compared with current designs is dramatic: these units have only about two dozen major plant subsystems, compared with about 200 in light-water reactors. Significantly, the operation of these plants can be extended into a temperature range that makes possible the low emissions production of hydrogen from water or other feedstocks for use in fuel cells and clean-burning transportation engines, technologies on which a sustainable hydrogen-based energy economy could be based.
These next-generation reactors incorporate several important safety features as well. Being a noble gas, the helium coolant will not react with other materials, even at high temperatures.
Furthermore, because the fuel elements and reactor core are made of refractory materials, they cannot melt and will degrade only at the extremely high temperatures encountered in accidents (more than 1,600 degrees C), a characteristic that affords a considerable margin of operating safety.
Yet other safety benefits accrue from the continuous, on-line fashion in which the core is refueled during operation, one pebble is removed from the bottom of the core about once a minute as a replacement is placed on top. In this way, all the pebbles gradually move down through the core like gumballs in a dispensing machine, taking about six months to do so. This feature means that the system contains the optimum amount of fuel for operation, with little extra fissile reactivity. It eliminates an entire class of excess-reactivity accidents that can occur in current water-cooled reactors. Also, the steady movement of pebbles through regions of high and low power production means that each experiences less extreme operating conditions on average than do fixed fuel con-figurations, again adding to the unit’s safety margin. After use, the spent pebbles must be placed in long-term storage repositories, the same way that used-up fuel rods are handled today.
With water-cooled reactors, even standard water-cooled nuclear reactor technology has a new look for the future. Aiming to overcome the possibility of accidents resulting from loss of coolant (which occurred at Three Mile Island) and to simplify the overall plant, a novel class of systems has arisen in which all the primary components are contained in a single vessel. An American design in this class is the international reactor innovative and secure (IRIS) concept developed by Westinghouse Electric.
Housing the entire coolant system inside a damage-resistant pressure vessel means that the primary system cannot suffer a major loss of coolant even if one of its large pipes breaks. Because the pressure vessel will not allow fluids to escape, any resulting accident is limited to a much more moderate drop in pressure than could occur in previous designs.
To accomplish this compact configuration, several important simplifications are incorporated in these reactors. The subsystems within the vessel are stacked to enable passive heat transfer by natural circulation during accidents. In addition, the control rod drives are located in the vessel, eliminating the chance that they could be ejected from the core. These units can also be built as small power modules, thereby allowing more flexible and lower-cost deployment.
Designers of these reactors are also exploring the potential of operating plants at high temperature and pressure (more than 374 degrees C and 221 atmospheres), a condition known as the critical point of water, at which the distinction between liquid and vapor blurs. Beyond its critical point, water behaves as a continuous fluid with exceptional specific heat (thermal storage capacity) and superior heat transfer (thermal conductance) properties. It also does not boil as it heats up or flash to steam if it undergoes rapid depressurization. The primary advantage to operating above the critical point is that the system’s thermal efficiency can reach as high as 45 percent and approach the elevated temperature regime at which hydrogen fuel production can become viable.
Although reactors based on supercritical water appear very similar to standard Generation II designs at first glance, the differences are many. For instance, the cores of the former are considerably smaller, which helps to economize on the pressure vessel and the surrounding plant. Next, the associated steam-cycle equipment is substantially simplified because it operates with a single-phase working fluid. In addition, the smaller core and the low coolant density reduce the volume of water that must be held within the containment vessel in the event of an accident. Because the low-density coolant does not moderate the energy of the neutrons, fast-spectrum reactor designs, with their associated sustainability benefits, can be contemplated. The chief downside to supercritical water systems is that the coolant becomes increasingly corrosive. This means that new materials and methods to control corrosion and erosion must be developed. Supercritical water reactor research is ongoing in Canada, France, Japan, South Korea and the U.S.
With Fast-spectrum reactors, a design approach for the longer term is the fast-spectrum (or high-energy neutron) reactor, another type of system. An example of this class of reactor is being pursued by design teams in France, Japan, Russia, South Korea and elsewhere. The American fast-reactor development program was canceled in 1995, but U.S. interest might be revived under the Generation IV initiative.
Most nuclear reactors employ a thermal, or relatively low energy, neutron-emissions spectrum. In a thermal reactor the fast (high-energy) neutrons generated in the fission reaction are slowed down to “thermal” energy levels as they collide with the hydrogen in water or other light nuclides. Although these reactors are economical for generating electricity, they are not very effective in producing nuclear fuel (in breeder reactors) or recycling it.
Most fast-spectrum reactors built to date have used liquid sodium as the coolant. Future versions of this reactor class may utilize sodium, lead, a lead-bismuth alloy or inert gases such as helium or carbon dioxide. The higher-energy neutrons in a fast reactor can be used to make new fuel or to destroy long-lived wastes from thermal reactors and plutonium from dismantled weapons. By recycling the fuel from fast reactors, they can deliver much more energy from uranium while reducing the amount of waste that must be disposed of for the long term. These breeder-reactor designs are one of the keys to increasing the sustainability of future nuclear energy systems, especially if the use of nuclear energy is to grow significantly.
Beyond supporting the use of a fast-neutron spectrum, metal coolants have several attractive qualities. First, they possess exceptional heat-transfer properties, which allows metal-cooled reactors to withstand accidents like the ones that happened at Three Mile Island and Chernobyl. Second, some (but not all) liquid metals are considerably less corrosive to components than water is, thereby extending the operating life of reactor vessels and other critical subsystems. Third, these high-temperature systems can operate near atmospheric pressure, greatly simplifying system design and reducing potential industrial hazards in the plant.
More than a dozen sodium-cooled reactors have been operated around the world. This experience has called attention to two principal difficulties that must be overcome. Sodium reacts with water to generate high heat, a possible accident source. This characteristic has led sodium-cooled reactor designers to include a secondary sodium system to isolate the primary coolant in the reactor core from the water in the electricity- producing steam system. Some new designs focus on novel heat-exchanger technologies that guard against leaks.
The second challenge concerns economics. Because sodium-cooled reactors require two heat-transfer steps between the core and the turbine, capital costs are increased and thermal efficiencies are lower than those of the most advanced gas- and water-cooled concepts (about 38 percent in an advanced sodium-cooled reactor compared with 45 percent in a supercritical water reactor). Moreover, liquid metals are opaque, making inspection and maintenance of components more difficult.
Next-generation fast-spectrum reactor designs attempt to capitalize on the advantages of earlier configurations while addressing their shortcomings. The technology has advanced to the point at which it is possible to envision fast-spectrum reactors that engineers believe will pose little chance of a meltdown. Further, nonreactive coolants such as inert gases, lead or lead-bismuth alloys may eliminate the need for a secondary coolant system and improve the approach’s economic viability.
Nuclear energy has arrived at a crucial stage in its development. The economic success of the current generation of plants in country like United States of America, has been based on improved management techniques and careful practices, leading to growing interest in the purchase of new plants. Novel reactor designs can dramatically improve the safety, sustainability and economics of nuclear energy systems in the long term, opening the way to their widespread deployment.
Nuclear Power Primer
Most of the world’s nuclear power plants are pressurized water reactors. In these systems, water placed under high pressure (155 atmospheres) to suppress boiling serves as both the coolant and the working fluid. Initially developed in the U.S. based on experience gained from the American naval reactor program, the first commercial pressurized light-water reactor commenced operation in 1957.
The reactor core of a pressurized water reactor is made up of arrays of zirconium alloy–clad fuel rods composed of small cylinders (pellets) of mildly enriched uranium oxide with the diameter of a dime. A typical 17-by-17-square array of fuel rods constitutes a fuel assembly, and about 200 fuel assemblies are arranged to form a reactor core. Cores, which are typically approximately 3.5 meters in diameter and 3.5 meters high, are contained within steel pressure vessels that are 15 to 20 centimeters thick.
The nuclear fission reactions produce heat that is removed by circulating water. The coolant is pumped into the core at about 290 degrees Celsius and exits the core at about 325 degrees C. To control the power level, control rods are inserted into the fuel arrays. Control rods are made of materials that moderate the fission reaction by absorbing the slow (thermal) neutrons emitted during fission. They are raised out of or lowered into the core to control the rate of the nuclear reaction. To change the fuel or in the case of an accident, the rods are lowered all the way into the core to shut down the reaction.
In the primary reactor coolant loop, the hot water exits the reactor core and flows through a heat exchanger (called a steam generator), where it gives up its heat to a secondary steam loop that operates at a lower pressure level. The steam produced in the heat exchanger is then expanded through a steam turbine, which in turn spins a generator to produce electricity (typically 900 to 1,100 megawatts). The steam is then condensed and pumped back into the heat exchanger to complete the loop. Aside from the source of heat, nuclear power plants are generally similar to coal- or fuel-fired electrical generating facilities.
There are several variants of the light-water-cooled reactor, most notably boiling-water reactors, which operate at lower pressure (usually 70 atmospheres) and generate steam directly in the reactor core, thus eliminating the need for the intermediate heat exchanger. In a smaller number of nuclear power plants, the reactor coolant fluid is heavy water (containing the hydrogen isotope deuterium), carbon dioxide gas or a liquid metal such as sodium.
The reactor pressure vessel is commonly housed inside a concrete citadel that acts as a radiation shield. The citadel is in turn enclosed within a steel-reinforced concrete containment building. The containment building is designed to prevent leakage of radioactive gases or fluids in an accident.
Central to this discourse, I strongly recommend that attempts should be made in the near future workshops by Nuclear Power Ghana to equip journalists on other areas of nuclear technology for generating power including gas-cooled, water-cooled and fast -spectrum reactors broaden their knowledge.
Source: Jerry John Akornor/Raymond Karvi