Although a considerable progress has been made in the evolutionary designs of LWRs, these are large reactors and many believe25 that development and demonstration of new, smaller, innovative designs with short construction and start­up times and low capital costs are necessary to usher a new era of nuclear power. Since the early 1990s, the interest of developing countries, mainly in Asia, has resulted in increased efforts on the design of small and medium sized power reactors. This is because in the next 50 years, electric demand is expected to be tripled, most of which will come from developing countries with small grid capacities. Also, in industrialized countries, electricity market deregulation is calling for power generation flexibility that smaller reactors may offer. Small and medium reactors (SMRs) are also of particular interest for non-electric applications such as seawater desalination and district heating, fuel synthesis, and, in the future, hydrogen production.

Small and medium sized reactors are, however, not new. We have currently 150 SMRs operational in the world, 41 of these with power levels less than 300 MWe and 109 having power levels between 300 and 700 MWe. The detailed breakdown show 32 gas cooled reactors in UK (AGR and GCR), 32 PWR, 24 BWR, 29 WWER and 27 HWRs.

Recent major drive for innovation in light water reactors has been toward integral reactors, where the core, pumps, pressurizers, and steam generators are contained inside a single reactor pressure vessel (RPV). They are of enhanced safety because there is no large break LOCA; they also endure less fluence on the reactor pressure vessel and employ passive safety systems. Three primary examples of these reactors are CAREM (Argentina), IRIS (USA), and SMART (Republic of Korea). Being small, they allow more shop-fabrication and hence improved quality. These are being designed primarily for sizes up to 700 MWe due to easy constructability of Reactor Pressure Vessels and to better match smaller electric grids.

SMR designs are also attempting to increase the fuel core life to enhance proliferation-resistant features and also to reduce the O&M costs. Eight to even 20 years of single core life has been envisioned. Another idea in this regard is to have refueling services provided by a central refueling organization, with crew dedicated to refueling, visiting each site as required. This would also improve efficiency. Similarly, barge mounted reactors could be returned to a central location for refueling.

Some designs have proposed to make extensive use of modularization, in which a significant portion of the plant is built as modules, which are fabricated outside of the principal buildings of the nuclear power plant. In some cases, the modules are fabricated off-site, to take advantage of existing fabrication facilities. Modularization serves to transfer a significant portion of the construction labor from the nuclear power plant to more easily controlled manufacturing environment. This reduces the site construction infrastructure and shortens the construction schedule, and hence the capital cost.

In order to improve economics, small reactor designs strive to minimize the manpower costs associated with the operation of the reactors. The inherent reactor shutdown and passive decay heat removal capability of some designs, in combination with modern advanced communication systems, may even facilitate remote operation with fewer operators, or even unattended, for some applications.

New research is underway to utilize the unique thermo-physical properties of supercritical water to enhance nuclear plant thermal efficiency to 40 — 45% from the current 33 -34%. This will also lead to considerable plant simplification. Because there will be no change of phase in the core, the need for steam separators and dryers as well as for BWR-type recirculation pumps is eliminated, which will lead to smaller reactor vessels. In a direct cycle steam generators are not needed. However, to make this possible, advances are required in high temperature materials to improve corrosion, stress corrosion cracking, and wear resistance.

Major innovative reactors in the world26 are tabulated in Table VII. Key features of SMRs include simplification and streamlining of designs as well as emphasis placed on safety features avoiding off-site impacts in case of accident. Such characteristics should facilitate their acceptability by local communities. However, none of these reactors have been built; only recently announcements have been made for beginning the preparatory phase for construction of KLT-40 in Severodvinsk in Russia and of a 65 MWt pilot version of SMART in KAERI, Republic of Korea. Two KLT-40 nuclear submarine reactors will be built on a floating barge with a displacement capacity of 20,000 tonnes. It is expected that the floating nuclear plant in Russia will produce power in 2006 and the pilot plant in Korea in 2008.

TABLE VII. MAJOR INNOVATIVE REACTOR DESIGNS UNDER DEVELOPMENT AROUND THE WORLD

5. UTILIZATION OF THORIUM FUEL

There has been a recent renewed interest in thorium fuel cycles. The reasons for this are to (1) burn excess weapons Pu without creating more, (2) generate less long-lived radioactive waste, (3) design reactors to operate in a safer mode, (4) reduce U-235 enrichment, (5) go to higher temperatures, and finally having large thorium deposits.

Thorium-232 is three times more abundant than uranium and available in India, Brazil, USA, Turkey and China. It is not a fissile material but it can produce U-233 in a reactor, which, from a neutronic standpoint, is an excellent nuclear fuel among the three nuclear fuels — U-235, Pu-239 and U-233. It also produces much less minor actinides from fission. Thorium dioxide is the only stable oxide of thorium, which accounts for its greater stability compared to uranium dioxide. It is also much more resistant to chemical interactions and has a high thermal conductivity. The melting point of thorium dioxide is 3050 degree centigrade. Thorium contains naturally up to about 100 ppm of Th-230; this and other neutron reactions of Th-232 and U-233 produces U-232, which decays with emission of hard gamma rays. Thorium fuel fabrication is similar to U-fuel but it requires remote operation because of the gamma emission from U-232 decay chains. In addition high chemical inertness of thorium dioxide makes it very difficult to be dissolved and reprocessed. Because of these drawbacks the thorium fuel cycle is considered a more proliferation-resistant fuel.

Thorium fuel cycles have been studied in the past in several countries on a smaller scale but its importance has increased in recent years as a non-proliferating fuel and also for reducing the inventory of Pu. Germany had used Thorium fuels for several years on the AVR, a pebble-bed high temperature research reactor, and on the THTR, Thorium High Temerature Reactor. Both in Germany and the US the fuel fabrication technology has been developed under high temperature reactor programs to a well proven, industrial process. The coated fuel particles for the HTGRs have shown excellent performance under irradiation and reactor operation. In Russia also tests of thorium-based fuels for WWER and LMFBRs have shown an excellent irradiation behavior.

The US has shown new interest in thorium fuel and has initiated four projects under the Nuclear Energy Research Initiative. Their primary motive is to develop an advanced proliferation-resistant, low cost uranium-thorium dioxide fuel. The Radkowski Thorium Reactor (RTR), being investigated in the US, Russia and Israel, revives the seed-blanket concept of the US Light Water Breeder Reactor design that operated in Shippingport in the late 50s. The concept assumes a once-through fuel cycle with no reprocessing; U-233 is bred and mostly burned in the reactor.

Most prominently, India has been pursuing a strong program on thorium fuel cycle activities. India has a closed fuel cycle strategy, which calls for using U-Pu fuel cycle for fast breeder reactors and a closed Th-U-233 fuel cycle in the next stage with advanced heavy water reactors. The Advanced Heavy Water Reactor (AHWR), currently under design, plans to use thorium for 75% of the power. Utilization of thorium is their focal point for development. All aspects of the fuel cycle including the back end are being studied in India. Activities for Thorium fuel development in India include studying: (1) dissolution of irradiated thorium fuel, (2) effective utilization of recovered fissile and fertile material, and (3) thorium fuel fabrication.