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An Elegant Solution

Small modular reactors address many long-standing nuclear safety concerns

Rising energy prices and concern over carbon emissions have helped breathe new life into prospects for U.S. nuclear power for the first time in decades. Along with this revival has come intense interest in a new generation of nuclear plants known as SMRs, or small modular reactors. SMRs address many long-standing nuclear safety concerns, while providing power companies with a cost-effective way to match generating capacity to customer demand.

By the 1970s, Weinberg and others had concluded that small reactors with primary systems inside their containment vessels would be a better long-term option for nuclear power.
By the 1970s, Weinberg and others had concluded that small reactors with primary systems inside their containment vessels would be a better long-term option for nuclear power.

To appreciate why these down-sized, self-contained units are such a vast improvement over traditional reactor technology, it's helpful to look back at the lengthy pedigree of nuclear power in the U.S.

Naval origins

The first power-producing nuclear reactors were pressed into service by the U.S. Navy just after World War II. The earliest units were developed to power ships and submarines in the early 1950s. Later in the decade, when utility companies adapted this technology to large-scale power generation, nuclear plants still retained their predecessors' basic design, but applied it on a much grander scale. Where naval reactors had produced tens of megawatts of electricity, their commercial cousins generated up to 1500 megawatts and could power entire cities.

Supersizing the basic naval reactor design turned out to be a mixed blessing for the power industry. While the larger plants were more efficient, their sheer size created more points of potential failure, resulting in a bewildering array of backup systems and other safeguards. "We learned to manage the complexity of this technology," says Dan Ingersoll, head of ORNL's Small Modular Reactor R&D Program, "but it was costly and difficult."

The proliferation of nuclear power plants continued through the 1970s, slowing somewhat as capacity began to exceed demand. "Then, after the Three Mile Island incident in 1979," Ingersoll recalls, "the industry pretty much came to a standstill. In terms of commercial nuclear power, very little has changed since then. Utilities are still operating complex megaplants that are basically scaled-up versions of early reactor designs."

Weinberg's legacy

Although support for nuclear power had all but evaporated by the end of the 1970s, the development of innovative reactor designs was alive and well. Years before Three Mile Island, former ORNL Director Alvin Weinberg left the laboratory and headed up first the U.S. Office of Energy Research and Development and later Oak Ridge Associated Universities' Institute for Energy Analysis. Initially, much of Weinberg's time was spent working with teams of scientists and policymakers considering what the next generation of nuclear power might look like, in terms of basic reactor technologies and power plant design.

In the wake of Three Mile Island and the antinuclear backlash that followed, Weinberg and his colleagues focused their attention specifically on what the nuclear industry would have to do to address its problems and make a strong comeback. "They conducted an in-depth study of the rapid growth in the size of power reactors," Ingersoll says. "Their conclusion was that very small reactors would be a better long-term option for nuclear power. In addition, the plants would have to be designed to eliminate vulnerable features by integrating all of the primary systems inside the containment vessel." This compact, integrated design, basically an SMR, was both elegant and robust. If electrical power were lost, the reactor could be safely shut down using self-contained coolant and gravity-driven natural circulation flows. This approach would eliminate both the long stretches of piping linking primary system vessels as well as the need for backup systems to guard against breaks in the pipes.

Unfortunately for proponents of this novel configuration, the 1980s saw orders for commercial power reactors in the U.S. plummet to zero. As a result, the innovative designs developed by Weinberg and his colleagues sat on the shelf for the next 20 years.

A broad resurgence

Today, with the broad resurgence of interest in nuclear energy in the U.S., interest in the small, integrated reactor concepts pioneered by Weinberg and his colleagues has taken off as well. "Several companies are developing designs for state-of-the-art SMRs," Ingersoll says. "They're betting that there will be a large market opportunity for these versatile, small-scale plants."

The big challenge for manufacturers is making small reactors that are as cost-effective as larger ones have been. These companies are calculating that their ability to build reactors on an assembly line, along with the reduced operating costs of SMRs, will enable modular reactors to match the economies of scale enjoyed by traditional nuclear plants.

Ingersoll notes that, in addition to matching the efficiency of larger plants, SMRs are also much more adaptable than their predecessors. "Because their demand on water and other infrastructure resources is comparable to that of coal or gas plants," he says, "modular reactors can be located in a much wider range of locations. This opens up nuclear energy as an option to many more applications, including large business and industrial customers."

In addition to their other benefits, SMRs offer power companies the ability to match their expansion to customer demand. If, for example, a community's need for electricity was growing at the rate of 200 megawatts every few years, the local power utility could add a 200-megawatt SMR to its modular nuclear plant cheaply and quickly. Ingersoll emphasizes that providing the ability to add capacity in relatively small increments is the true value of the modular reactor design.

Playing a key role

In addition to his responsibilities at ORNL, Ingersoll, who is also the Department of Energy's national technical director for its SMR program, says the laboratory will play a key role in developing the new generation of SMRs. One reason for this close cooperation is that ORNL is home to research programs that address the technical needs of SMR manufacturers in a number of areas, including sensor development, advanced materials, computer modeling and nuclear fuels research.

Several companies are developing designs for state-of-the-art small modular reactors.
Several companies are developing designs for state-of-the-art small modular reactors.

"We want to help the industry move the process of siting and manufacturing SMRs forward," Ingersoll says. "Having national labs provide research and development support to modular reactor manufacturers is part of DOE's plan to accelerate the deployment of commercial SMRs."

One of the biggest research and development challenges facing ORNL scientists on the hardware side of the SMR equation will be developing the specialized sensors and instrumentation needed for the new reactor designs. The SMR environment is a particular challenge because most sensors and other instruments will be located in the restricted space of the reactor vessel, and many of them will be exposed to high temperatures, high pressures and intense radiation.

SMR manufacturers will also be working with laboratory researchers at ORNL's Consortium for Advanced Simulation of Light Water Reactors facility, where they will be able to conduct detailed computer simulations of reactor operations. "We're in the process of creating new software that takes advantage of the huge computing capability available through the CASL program," Ingersoll says. "We will be able to model new SMR designs with very high fidelity and reliability." These detailed simulations will enable researchers to simulate the entire life cycle of reactor components and to predict when they will fail. "Knowing where components are in their life cycle is critical to safe and efficient reactor operation," he adds.

A promising start

Ingersoll says that, until the last few years, small reactor research didn't really attract much attention. Recently, however, there has been an explosion of interest in SMR technology from all quarters.

As evidence of this, Ingersoll notes that, later this year, DOE's SMR program will begin recruiting two companies who want to take on the challenge of designing, licensing and building working reactors. Six months later, they should have the contracts they need to begin the licensing process. Because DOE is shouldering half of the cost of developing these SMRs, the companies that are selected will have to convince DOE that they're worth the investment and that they can deliver results. The goal of the project is to deliver a working SMR by 2020.

While the first SMRs will represent a radical departure from traditional reactor designs, Ingersoll expects that the following generation of SMR power plants will include even more innovative technologies, such as high-temperature, salt-cooled reactors. These novel technologies are significant because they introduce new uses for nuclear energy—like providing both heat and power for nearby high-temperature industrial processes. "ORNL will be doing this kind of advanced reactor research for the foreseeable future," Ingersoll predicts. "We're in this for the long haul."—Jim Pearce