Integral reactors have been adopted in nuclear-powered submarines; it is not well known, however, that the first, and so far the only, ‘commercial’ iPWR was operational as early as 1964. It was the nuclear ship Otto Hahn, a German nuclear-powered freighter and research facility, which was launched in 1964 and commissioned in 1968. She sailed 650 000 nautical miles in 10 years without any technical problems, but was eventually docked because of nuclear hysteria, with ports and harbors refusing entry to a nuclear ship. The Otto Hahn featured helical steam generators, a solution favored by several current designs.

As far as terrestrial power reactors are concerned, the first iPWR type design to be proposed was the PIUS (Process Inherent Ultimate Safety) reactor by the Swedish ASEA-Atom in the early 1980s. It was conceived in response to the Three Mile Island accident with the objective of replacing the active safety approach with inherent safety.

Essentially, the reactor was placed in a large pool of borated water in a concrete pressure vessel located underground. Core cooling was by natural circulation with upper and lower density locks to prevent mixing between the circulated hot reactor coolant and the cold pool water. PIUS got quite a bit of attention, but never real traction. Initially ASEA-Atom had a 500 MWe integral design with steam generators inside the vessel, but later switched to a more conventional 640 MWe with steam generators outside.1 PIUS was first and foremost a natural circulation reactor, with the integral configuration being a mean of implementation, which was eventually dropped.

A true iPWR was proposed in the mid-1980s by US Combustion Engineering. This was the MAP (Minimum Attention Plant), a 900 MWt self-pressurized, full natural circulation design with multiple once-through steam generators located inside the vessel.2 MAP was eventually shelved by Combustion Engineering (which had become ABB-CE) in favor of the 320 MWe SIR (Safe Integral Reactor), developed in the late 1980s in collaboration with Rolls-Royce, Stone and Webster and the United Kingdom Atomic Energy Authority (UKAEA).3 The SIR core design, based on the CE System 80 commercial PWR, had a 55 kW/liter core power density, about half that of traditional PWRs, 24 month refueling cycle, 12 once-through integral steam generators arranged in an annular space above the core and an integral pressurizer in the vessel head. The six wet-winding glandless coolant pumps were mounted around the upper circumference of the vessel.

SIR was typical of iPWR designs which, starting in the early 1990s, were developed, and still continue to be, across the world, most notably in Russia, Argentina, Republic of Korea, Japan, China, as discussed in Part IV of this Handbook. In the USA the first significant effort was the IRIS (International Reactor Innovative and Secure), developed from the late 1990s to the end of 2009, when it was terminated.4,5 The IRIS project was led by Westinghouse with a major part of the work being performed by the international partners, which included industry, laboratories and academia. While in other countries the current effort on iPWR designs has continued on the concepts developed earlier, the current US effort has basically started anew, spurred by the Department of Energy (DOE) solicitation of SMR designs, which was finalized in 2011.

Going back to SIR, a very significant contribution to advancing the state of the art was a seminal paper by the UKAEA partner investigating the cost benefits of smaller reactors.6 It showed that the traditional capital cost economy of scale does not hold when other factors typical of smaller plants are taken into consideration. The paper listed and discussed quite a number of these factors: increased factory fabrication, more replication, multiple units at a single site, improved availability, faster progression along the learning curve, bulk ordering, better match to demand, smaller front-end investment, reduced construction time, increased lifetime, design appropriate to site and, elimination/downgrading of some safety systems. Most of these factors will be discussed in Chapter 10. The last factor, about the safety systems, is of momentous importance and will be fully explored in Section 3.4.

The next two sections, dealing with the safety and economics imperatives, are based on work performed during the 10 years of development of the IRIS design.

There are many reasons for this choice, aside from the familiarity of this author. The key one is that IRIS has systematically sought both safety excellence beyond the generally accepted limits and the synergism between safety and economics. Practical reasons are that IRIS is no longer being pursued and its work has been fully documented in over 500 open literature publications which provide a jumping point for iPWR designs, both present and future.