The design of the HTGR endeavors to make the maximum use of inherent safety, and boasts the highest level of passive safety features possible. Safe shutdown of the reactor is ensured in an accident condition even without any operator’s emergency actions, by the low power density of the core, the inherent negative fuel temperature coefficient and the large graphite heat capacity. The core afterheat is removed by natural phenomena such as conduction, radiation and convection to the self-actuating reactor cavity cooling system. Such inherent safety features provide advantages in public acceptance.

There are two types of the HTGR designs, depending on the fuel type: the block and the pebble fuel type. General Atomics in USA developed a block fuel design, while Germany developed a pebble fuel design. Two fuel types shares the same basic technology of the coated particle fuel, TRISO, in which the fuel kernel of 0.5 mm diameter is multiply coated by silicon carbide layers. In the block type, TRISO particles are lumped together into compacts that are inserted into a hexagonal graphite fuel block. Fuel blocks are loaded into fixed positions in the core and reloaded periodically. In the pebble type reactors, TRISO particles are lumped together to make pebble fuels each of which is about as big as a tennis ball in dimension. Pebble fuels enter the core at the top, flow continuously downwards, and then exit the core at the bottom. The pebbles that leave the core are measured and then either rejected as waste fuel, or recirculated to be used again in the core. The failure probability of the TRISO particle is so low (~10-S per particle) that it is sometimes referred as a small containment.

15.4 Future trends

15.6.1 iPWR: SMART

15.6.1.1 A fully passive safety system for SMART

The hybrid safety system currently employed in the standard design of SMART is planned to be upgraded with fully passive safety system. The passive safety system will be developed to maintain the SMART plant in a safety shutdown condition following design base accidents such as LOCA and non-LOCA transient events without AC power or operator actions. The passive safety system consists mainly of a PRHRS and passive injection system (PIS). Based on the current SMART SDA design, the capacity of the PRHRS will be increased up to an operation period of at least 72 hours. All of the active safety features will be substituted with passive versions, eliminating the necessity for emergency diesel generator (EDG) or operator actions for at least 72 hours. A program to adopt a fully passive safety system in SMART began in March 2012, and the testing and verification are planned to be completed by the end of 2014.

Many countries that anticipate the development of nuclear technology by deploying NPPs try to develop the optimized projects to minimize the risks in financial and public acceptance. Then, the SMART will be the only viable solution among SMRs to fulfil the demands of such countries, and the KAERI and the Korean government together will create a worldwide strategy to support the construction of a prototype in Korea and global marketing. Middle East and North Africa (MENA), Southeast Asia and typical arid regions could be the potential owners.

15.6.1.2 Further issues for SMR development

The main focus of the Fukushima Daiichi event was the need to remove the decay heat in the reactor after successful shutdown. Although SMRs use the same fuel type and the same light-water cooling as large-scale light-water reactors, there are significant enhancements in the reactor design that contribute to the upgraded safety case. Safety of SMRs tended to rely on the same sorts of features to utilize the technology of large NPP as proven technology, but SMRs have characteristics that may result in significant safety improvements. First of all, there is less heat to remove because of the lower thermal output of the reactors. And the increased water inventory provides more coolant and therefore more time for compensatory actions to take place.

In spite of many safety improvements, some of the concepts being considered introduce new considerations to licensing and safety viewpoints. A skeptical viewpoint on SMRs is that SMRs work almost on the same principle as that of large reactors. Safety characteristics are not significantly better than large reactors due to the introduction of the same sorts of safety features. Some safety benefits also declined as reactor power approached the upper bound of the SMR category. Then, SMRs are expected to face increased regulatory oversight. The SMRs most likely to succeed are designs that use the same fuels and water-cooling systems as the large reactors in operation, because the regulatory authority might be accustomed to regulating those reactors.

The final challenge for SMRs is to guarantee the economic feasibility for potential owners. It concluded that SMRs would not be able to produce electricity more cheaply than large reactors but could fare well against non-nuclear power sources. Whereas the impacts of economies of scale vary according to the relative sizes of the reactors, there still are lots of benefits compensating the economic weakness.

Factors that influence SMRs economic analysis are such as considerably lower total capital cost than large NPPs, reduced on-site construction costs through factory — assembled delivery, less land area and cooling (BOP) requirements, passive designs and other features resulting in fewer staff, additional non-electricity applications providing new markets and income opportunities, and so on.

SMRs are targeting at utilities, companies and countries that are currently uninitiated in the world of nuclear power. To open this new marketplace, developers should provide cost-effective designs covering wide range of site specifics as well as an integrated model of financing, licensing, construction, manufacturing and fuel supply to reduce the risk of economic benefits.