Commercial electric power began with small reactors of light-water-cooled design. Key examples are the Shippingport, 60 MWe reactor designed by the Westinghouse — operated Bettis Naval Atomic Power Laboratory, which started operation in 1958; the Yankee Rowe reactor, 185 MWe (Westinghouse) in 1960; the Indian Point One reactor, 275 MWe (B&W) in 1962 (all pressurized water reactor [PWR] designs); and Dresden 210 MWe (General Electric) in 1960 (a boiling-water reactor (BWR) design).

The eight military reactors for terrestrial application developed by the US Army Nuclear Power Program included (1) the stationary plants operated at Fort Belvoir, Virginia, which started operation in April, 1957, seven months before Shippingport and five years before criticality of the Ft. Greely, Alaska reactor; (2) the portable reactor operated at McMurdo Sound at the South Pole in 1962; and (3) a barge — mounted reactor operated off the coast of Panama City, Panama, in 1967. These plants ranged from 1.75 to 10 MWe and performed either a heating or desalinization function in addition to the generation of electricity. Another example of a portable reactor is the Russian PAMIR reactor designed primarily to power remote military radar outposts. The first was the TES-3, a 2 MWe nuclear plant completed in 1961. The design was modified in the 1980s to a smaller, more mobile 630 kW reactor.

The much larger US naval program, which pioneered the application of nuclear power for the propulsion of submarines and surface ships, has produced multiple pressurized water reactors and one sodium-cooled reactor of small ratings. Additionally, several countries have followed suit with naval propulsion — most notably Russia, which expanded its development of water-cooled submarine reactors to submarines using lead-bismuth coolant and has also built nuclear powered naval surface ships and ice-breakers.

Commercial (merchant marine) propulsion has also been exploited through the development of ocean freighters and icebreakers. Four freighters, all with reactors of light-water design, have been built and operated albeit without commercial success: (1) the US Savannah, 74 MWt, in effective service starting 1962; (2) the German Otto Hahn, 38 MWt, 1968; (3) the Japanese Mutsu, 36 MWt, 1972; and (4) the only vessel still in operation under nuclear power, the Russian Sevmorput, 135 MWt, delivered in 1988, which also has ice-breaking capability.

The Otto Hahn reactor design is of special interest since its integral design characteristic is the typical configuration being exploited by several modern PWR SMR vendors. As extensively elaborated in Chapter 3, the term integral design means the co-location of all components and piping of the primary coolant system in the single pressure vessel. By contrast, the typical large-rated PWRs are loop systems with the primary system components, e. g., the steam generators, primary coolant pumps and pressurizer connected by piping to each other, and the pressure vessel which houses the reactor core and the control elements.

To date Russia alone has constructed and operated nine nuclear-powered ice­breakers, starting in 1959 with the Lenin. Two vessel classes have been built: the Arktika class, each vessel with two OK-900A reactors each of 171 MWt; and the Taymy class, each vessel with a single KLT-40M reactor of 135 MWt. (NB: All reactors of the ocean vessels noted above drive propulsion shafts, thus their ratings are only in MWt.) Russia is also constructing a non-self-propelled floating nuclear power station, the Akademik Lomonosov, to provide power supply to remote coastal towns. The reactor station scheduled for delivery in 2016 is powered by two modified ice-breaker reactors, each a KLT-40S reactor of 35 MWe. With these reactors the station can provide either 70 MWe of power, 300 MWt of district heating or 240 000 m3/day of fresh water.

The development of a nuclear propulsion system for military aircraft was initiated in 1946 as the US Nuclear Energy for the Propulsion of Aircraft (NEPA) project and continued under the name of the Aircraft Nuclear Propulsion (ANP) program. Two different systems for nuclear-powered jet engines were pursued — a direct air cycle concept developed by General Electric and an indirect air cycle by Pratt & Whitney. Only the direct air cycle program advanced sufficiently to produce reactors. The first product of the GE program was the Aircraft Reactor Experiment (ARE) which operated for 1000 hours in 1954. It was a 2.5 MWt nuclear reactor experiment using molten fluoride salt (NaF-ZrF4-UF4) as fuel, a beryllium oxide (BeO) moderator, and liquid sodium as a secondary coolant. In 1955, this program produced the successful X-39 engine with heat supplied by the Heat Transfer Reactor Experiment-1 (HTRE-1). The HTRE-1 was replaced by the HTRE-2 and eventually the HTRE-3 unit powering the two jet turbines. Additionally, an operating reactor named the aircraft shield test reactor (ASTR), was flown aboard a modified B-36 bomber to test shielding rather than powering the plane. The HTRE-3 used a shield system of flight-type design but was not taken to power before the program was canceled in 1961.

All these earlier reactors led to the current explicit offering of reduced size modular power plants. These current SMRs, listed in Table 1.1, encompass all coolant technologies being exploited for larger nuclear reactors. Table 1.1 lists only

those reactors offered by international industries. Additional reactors not included in Table 1.1 are under development by national research institutions but have not yet reached the commercialization stage. For example, the fluoride-salt-cooled high — temperature reactor (FHR) (Forsberg et al., 2013) is a 180 MWe reactor with 700 °C peak operating temperature coupled to an air-Brayton combined cycle system.