[Korea`s nuclear technology ( 10 )] SMART: A solution for nuclear newcomers

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This is the 10th in a series of articles that highlight the challenges and opportunities facing Korea`s nuclear power industry. - Ed.



By Kim Hark-rho



From the 1950s, when nuclear power plants began to be built to generate electricity, they were dominated by small reactors with an electric power capacity of less than 300 MW and medium-sized reactors with a range between 300 MW and 700 MW. The following two decades witnessed a shift toward large reactors (with electric power above 700 MW) that took advantage of economies of scale. However, since the early 1990s, there has been renewed interest in the development and application of small- and medium-sized reactors (SMRs) in developing and developed countries.

Small- and medium-sized reactors are suitable for developing countries with a low electrical grid capacity, insufficient infrastructure and limited investment capability. According to the International Atomic Energy Agency, 20 of the 54 countries planning to construct nuclear power plants for the first time will have to build small nuclear reactors with a capacity of less than 300 MWe due to the lack of infrastructure required for large-scale reactors.

There has also been a growing interest in small- and medium-sized reactors in developed countries that have deregulated their electricity market, calling for flexibility in power generation. The modular design of SMRs provides for an incremental capacity increase. SMRs have also attracted interest because they can be used for non-electrical purposes, including seawater desalination, district heating, hydrogen production, coal liquefaction and other process heat applications.

The global market for SMRs is forecast to grow, given the global efforts to reduce reliance on fossil fuels and a rapid growth in electricity demand in developing countries. The Central Research Institute of Electric Power Industry in Japan forecast in a 2006 report that the global demand for SMRs would reach 400-850 units. The U.S. Department of Energy estimated in a 2007 report that the global demand for SMRs to be 500 to 1,000 units by 2050. In a 2005 report, the Science and Technology Policy Institute in Korea said the global market for SMRs would reach $350 billion in the future - $250 billion for small-scale power generation and $100 billion for seawater desalination.



According to statistics compiled by the International Atomic Energy Agency, small reactors with a capacity of less than 300 MWe accounted for 7 percent of the total nuclear power plants under construction in 2007, a surge from 1.2 percent in 2000. This proportion is expected to further increase as more developing countries are set to go nuclear.

The prospect of a booming demand for SMRs drove many countries to increase research and development efforts on SMR designs that are reliable and safe, sustainable and economic. Currently, over 50 concepts and designs of innovative SMRs are under development worldwide, with several advanced countries ready to commercialize their designs. Korea is one of the countries competing to for early-mover status in the emerging global market.

The Korea Atomic Energy Research Institute launched a project to develop an SMR in 1996. Based on numerous preceding studies, the Korean government decided to develop an integral type pressurized water reactor (PWR) with a rated thermal power of 330 MWt and electric power of 100 MWe. This reactor is called System-Integrated Modular Advanced Reactor or SMART.

The SMART project was part of Korea`s ambitious plan to foster the nuclear power industry as one of its new growth engines. SMART was intended for developing countries for which small reactors are the best option, either because their power grids are small, or because their power grids need to be geographically scattered. To facilitate SMART exports, Korea sought to develop core technologies on its own and secure design capabilities.

One advantage of SMRs is that they can easily accommodate advanced design concepts and technology. Designers can achieve drastic safety enhancement for SMRs by adopting intrinsic safety features and passive safety systems. They can also negate economies of scale enjoyed by large-scale reactors by pursuing innovative approaches that lower costs - system simplification, component modularization, on-site fabrication and reduction of the construction time.

The SMART reactor is characterized by a drastically enhanced safety standard and its capability to undertake diverse functions - electricity generation, seawater desalination and district heating. One SMART reactor can supply power and water to a city with a population of 100,000.

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One defining characteristic of SMART is its integral layout. The single reactor pressure vessel contains all primary components such as core, steam generators, reactor coolant pumps and a pressurizer. This integral arrangement of the reactor vessel assembly makes it possible to remove the large-size pipe connections between major components, thus essentially excluding the occurrence of large break loss of coolant accidents. The in-vessel pressurizer is designed to control the system pressure at a nearly constant level over the entire design basis events.

In addition, another important design feature in SMART is the introduction of simplified and improved safety systems. Most notably, SMART employs passive safety systems such as passive residual heat removal system (PRHRS) to accomplish the inherent safety functions and mitigate the consequences of postulated accidents. PRHRS prevents overheating and over-pressurization of the primary system in case of emergency events by removing the core decay heat only through natural circulation.

The low power density design with slightly enriched (<5 w/o) UO2-fueled core has proven to provide a thermal margin of higher than 15 percent to accommodate any anticipated transients with regard to the critical heat flux.

This feature ensures the core thermal reliability under normal operation. Reactivity control during normal operation is achieved by soluble boron and control rods. Burnable poison rods are introduced for the flat radial and axial power profile, which results in the increased thermal margin of the core. The nearly constant reactor coolant average temperature program in the reactor regulating system improves load follow operation performance in view of a stable pressure and water level within the pressurizer.

The modular type once-through steam generator cassette consists of helically coiled heat transfer tubes to produce superheated steam at 30 degrees Celsius in normal operating conditions. The small inventory of the steam generator secondary side water prohibits the water`s return-to-power following a steam line break accident.

Other improved design features include the canned motor reactor coolant pump, which has no pump seals, thus preventing loss of coolant associated with pump seal failure. Four channel control rod position indicators contribute to the simplification of the core protection system and to the enhancement of the system reliability.

Furthermore, an advanced man-machine interface system using digital techniques and equipment reduces the human error factors and consequently improves the plant reliability.

Engineered safety systems designed to function automatically on demand consist of a reactor shutdown system, safety injection system, passive residual heat removal system, shutdown cooling system and containment spray system. Additional safety systems include a reactor overpressure protection system and severe-accident mitigation system. Under any circumstances, the reactor can be shut down by inserting control rods or boron injection.

The core is maintained undamaged for 72 hours without any corrective actions by the operator. The reactor overpressure at any design basis events can be reduced through the opening of the pressurizer safety valve.

Preliminary safety analyses of SMART show that the reactor remains in the safe condition for all the design basis events. The detailed safety analyses will be carried out in the course of standard design.

A small-sized reactor is known to be economically less competitive than a large-capacity commercial power reactor, but aforementioned simplified features contribute to the reduction of construction costs.

The emergence of SMART as an energy source with multiple functions promises a new era of nuclear energy utilization. SMART will become one of the first nuclear reactors in the range of 100 MWe that can be used for various applications. The basic design information of SMART now under development is given in Table 1.

At present, preparations are underway to obtain the standard design approval on SMART from the Korean licensing authority. Developers are currently working on experimental verification of SMART technology. They plan to obtain design approval by 2011. For this, they intend to complete documentation needed for the approval application this year. To achieve this target, major players of the Korean nuclear industry, including Korea Power Engineering Company, Korea Nuclear Fuel, and Doosan Heavy Industries, are working together with KAERI.

The promoters of the SMART project, once they have the design approval, will enter the budding global small nuclear reactor market. Several countries have already expressed keen interest in building SMART plants. For instance Kazakhstan has agreed with the Korean government to undertake a joint safety study on SMART as part of its program to introduce nuclear power generation. The country`s electric grids are divided into three regions and the western and northern regions suffer an electricity shortfall. A feasibility study with Indonesia has shown that two SMART plants are an optimal choice for Madura to provide tap water as well as electricity to the island`s residents. SMART is also a good option for Libya whose electricity grid is too small to accommodate large-scale nuclear power plants. Furthermore, the country needs to build seawater desalination plants.

The SMART project is widely open to investment by interested domestic organizations. International cooperative work is underway to produce mutual benefits and the SMART developers are sure that it is the best localization approach for the development of an advanced nuclear reactor system.