This article elaborates the advantages of underground nuclear parks over conventional nuclear power plant designs. Locating the reactors a few hundred feet underground in bedrock at a suitable site eliminates the need for containment structures, and the site would be largely impervious to physical attack from terrorists. (Indeed, it would be far easier to secure the few access points to an underground nuclear park than it is to protect the large perimeter of an isolated nuclear power plant.) A properly constructed underground facility would also be less subject to weather-related construction delays or the effects of hurricanes, tornadoes, flooding, or heat waves. Also, if designers were careful in the site selection, an underground nuclear park could virtually eliminate the transportation of hazardous nuclear waste material. Spent nuclear fuel could be moved via tunnel from the reactors to an array of storage tunnels; high-level waste could be permanently stored in another set of tunnels. When the reactors reach the end of their productive life, they can be decommissioned in place.

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Over the past decade, a so-called nuclear renaissance has been loudly proclaimed, but the fanfare has outpaced the actual accomplishment. The case for a greater exploitation of nuclear power is strong: it's a reliable, carbon emission-free energy source with a proven safety record. Some organizations have projected that the worldwide demand for energy will be the driving force behind the construction of as many as 1,000 nuclear power plants in the coming decades.

But the public perception of nuclear power is shaded by concerns over cost, safety, security, nuclear waste, and proliferation of material for weapons. For some people, the very sight of a nuclear power plant's cooling towers, which are not themselves a potential source of radiation, are symbols of dread. So precarious is the position of nuclear power in the public realm that the Three Mile Island accident, which released essentially no significant amount of radiation to the environment, still had the effect of halting the expansion of the nuclear industry in the United States for three decades. A serious accident or terrorist attack at any nuclear power plant in the world would have significant and long-lasting consequences on the worldwide nuclear industry.

There is an innovative solution to both the perception and safety problems—a solution that offers many other technical advantages. The concept is to place new nuclear reactors in integrated underground facilities. In addition to bringing substantial increases in safety and security, underground nuclear complexes would also greatly reduce the capital and operating costs and essentially eliminate the concerns of the public with high-level nuclear waste transportation. Co-locating reactors with reprocessing and fuel manufacturing facilities—all underground—would reduce proliferation risks associated with transportation of nuclear materials over long distances.

Changing the industry's conception of what a power plant should look like won’t be easy. And the biggest practical hurdle to building underground nuclear parks is the common perception that it would be excessively complicated and prohibitively expensive to construct something as complex as a nuclear power plant deep underground. We have looked into those objections, and have discovered that they are not as formidable as first thought.

A nuclear power plant can be thought of as three domains: the reactor area, the electricity generation area, and auxiliary areas. For example, assuming a GenIII light water reactor design, the reactor area includes all the equipment for generating steam, from the heat exchangers or moisture separators (depending on whether the water is pressurized or boiled) to the reactor vessels to the containment structure. This is an area that is unique to a nuclear power plant. It's also the domain that is under the most strict requirements for the arrangement of its components.

The electricity generation equipment includes high and low pressure turbines, condensers, a moisture separator heater, and the generator. The turbines and generator are typically aligned in a row with other equipment located in the surrounding space. The entire electricity generation equipment could probably be placed in a volume approximately 35 feet high by 35 feet wide by 200 feet long, with the exception of condensers, which take up significant space beneath low-pressure turbines.

Auxiliary areas of a nuclear power plant include such facilities as control rooms, emergency power systems, fuel storage, chemical and volume control systems, and waste heat removal systems. In general, the auxiliary facilities can be laid out in the manner that's most convenient, with the exception of the spent fuel pool, which must be accessible from the reactor room.

With nuclear power plants typically being sited on large parcels of land, designers have not had to concern themselves to a great extent with the volume of the various components. But to propose placing such facilities deep underground changes the calculus. Underground spaces are, by their very nature, limited in volume that can be economically excavated. As excavation volumes increase, the costs of construction and of support for the excavation increases rapidly.

It's important, then, to get a sense for the minimum volume a functional nuclear power plant could occupy. The pressure vessel for a boiling water reactor is typically around 80 feet tall and 24 feet in diameter; comparable dimensions for a pressurized water reactor are around 40 feet tall and 18 feet in diameter. The interior volumes of some containment structures can be more than 3 million cubic feet. Some of this volume is “empty space” that would not need to be incorporated into an underground nuclear plant. There are designs for very small, modular reactors that cut this volume down considerably: the NuScale reactor calls for a containment inner volume of just 11,000 cubic feet, about the size of an apartment.

Building several reactors in an underground nuclear park would add less than 1% to construction costs.

We have estimated that the volume needed for a single full-size (1,000 MWe) nuclear reactor together with all the generating and auxiliary equipment is approximately 2 million cubic feet. While that seems large—it's the volume of a 12-story cube—tunneling technology has advanced to make such spaces relatively routine to construct, especially when innovative excavation methods are employed.

Specialized construction companies use large tunnel boring machines that are capable of driving underground openings up to 47 feet in diameter through granite-like rock at rates of between 50 and 100 feet per day. (Tunnel lengths should be at least 2,000 feet to take full advantage of tunnel boring machines.) Costs for excavation by tunnel boring machines vary widely based on ground type, lining requirements, and project specifications. Boring through good ground that requires minimal support can cost about $2 per cubic foot, while more challenging conditions may cost upwards of$3.50 per cubic foot.

Such costs represent excavation and support alone, and do not include costs for tunnel lining, finishing, or contingency. These additional requirements may be expected to multiply the total cost of excavation by about a factor of three. It would be expected that an underground nuclear plant would be constructed in only the most favorable areas, so excavation may be accomplished for around $6 per cubic foot. So it would be expected that excavation for underground nuclear plants would add millions of dollars to the up-front cost of a nuclear power plant. Do the advantages outweigh those costs? Siting nuclear reactors underground is not a new idea. It can be argued that the first nuclear reactor—the sustained chain reaction devised by a team of scientists and engineers led by Enrico Fermi at the University of Chicago—was effectively underground: the bottom of the pile under the bleachers at Stagg Field was a few feet below grade. During the 1950s and 1960s special-purpose and small research reactors were built into excavated sites in Russia, Norway, Sweden, France, and Switzerland, and thus proved by demonstration the overall feasibility of underground reactor placement. However, studies in the 1970s that evaluated underground placement of a large power reactor suggested that the increase in safety and security would not compensate for the additional time and money needed to construct the required chambers, tunnels, and other openings. Instead of installing a single nuclear reactor and its attendant equipment underground, we propose something larger that can make the investment in excavation cost-effective. We propose building several reactors on one site, creating what we call an underground nuclear park that's analogous to a research or office park. Several reactors would be built into the bedrock some 300 to 1,000 feet below the surface. These reactors would share heat rejection systems and storage areas for spent fuel as well as long-term repositories for radioactive waste, which could be built on site. The co-location of so much infrastructure would more than compensate for the costs of excavation. Underground nuclear parks have many advantages over conventional nuclear power plant designs. Locating the reactors a few hundred feet underground in bedrock at a suitable site eliminates the need for containment structures, and the site would be largely impervious to physical attack from terrorists. (Indeed, it would be far easier to secure the few access points to an underground nuclear park than it is to protect the large perimeter of an isolated nuclear power plant.) A properly constructed underground facility would also be less subject to weather-related construction delays or the effects of hurricanes, tornadoes, flooding, or heat waves. Also, if designers were careful in the site selection, an underground nuclear park could virtually eliminate the transportation of hazardous nuclear waste material. Spent nuclear fuel could be moved via tunnel from the reactors to an array of storage tunnels; high-level waste could be permanently stored in another set of tunnels. What's more, when the reactors reach the end of their productive life, they can be decommissioned in place— essentially buried in their chamber along with the low-level waste produced by those reactors during their decades of operation. That solution would be safer and more cost-effective than conventional decontamination and decommissioning of a surface-sited reactor. There are many different ways an underground nuclear park could be built. Perhaps the most efficient method from an excavation and support standpoint would be to make a single pass with a tunnel boring machine, creating a circular passageway 50 feet wide and a few thousand feet long. Into this tunnel, several reactors and generator sets could be constructed, each one occupying one segment along one side of the rectangle. We expect that almost all the components of a nuclear power plant can travel through a tunnel of this diameter. To create more headroom for the reactors themselves, the bottom of the excavation could be lowered by drill and blast methods. Because drill and blast methods are more expensive and time-consuming per unit of rock volume than are tunnel boring machine methods, excavation beyond the initial tunnel should be kept to a minimum. A second smaller diameter tunnel is bored parallel to the nuclear chambers and connected to it with adits to provide independent access to the nuclear facilities. After excavation and the removal of the tunnel boring machines, the tunnel opening is further excavated by drill and blast methods to the required dimensions. Each straight-line section would then be subdivided into a minimum of three nuclear reactor chambers that would contain all the elements needed to generate electric power except for waste heat rejection. Removal of condenser waste heat would be accomplished at the surface by cooling towers or a surface water body such as a lake or river displaced laterally from the underground facility. At least 10 nuclear generating units could be built within the initial rectangular area. Tunnel segments could be used along with subsequently excavated spaces to install fuel reprocessing and manufacturing facilities, and for storage and disposal of low-level and high-level radioactive wastes. Using the expected cost for tunnels excavated by a tunnel boring machine, the main 1.6-mile tunnel for an underground nuclear park would cost approximately$100 million. The excavation of shafts and construction of systems for facility access and ventilation would add roughly an additional $100 million to the total excavation cost. Excavation of additional access tunnels and openings for auxiliary areas and power plant components that could not fit within the main tunnel would also cost roughly$100 million.

Several technical papers about the underground nuclear park proposal are available from the ASME Digital Library: www.asmedl.org

The total cost—$300 million—must be compared to the costs of constructing a dozen conventional nuclear plants on the surface. Nuclear power plant construction is estimated to cost$4,000 per kilowatt of electric capacity, so an underground nuclear park with 12 plants, each with a capacity of about 1,000 MWe, would cost about \$48 billion in power plants alone. The excavation of an underground facility would account for less than 1 percent of the total construction cost of the nuclear facilities.

The cost of siting a large nuclear complex underground is negligible compared to the overall cost of the facility. Couple that to the significant political, safety, and security advantages, and the case for underground nuclear parks is strong. To be sure, there are some issues left to be resolved—such as designing ventilation systems, allowing for access and egress under emergency conditions, and resolving electricity transmission issues for such a large generating facility—but it seems certain that those matters can be settled satisfactorily.

Perhaps the best opportunity for demonstrating the advantages of this concept will come with the introduction of the so-called Generation IV reactors. These designs, which are being researched by an international collaboration of nuclear engineers, mark a break with the types of reactors that have dominated nuclear power's first 50 years. The fresh approach for reactor design easily could be coupled with a new concept in power plant infrastructure, such as the underground nuclear park.

We strongly recommend that new program developments for Gen-IV reactors or advanced fuel cycle initiatives also include R&D concerning integrating these new technologies with the underground nuclear park concept. Also, since the long-term plan is to bring new fast spectrum reactors into the mix of reactors, so as to close the fuel cycle, and significantly reduce the volume of high-level waste, the design of underground nuclear parks should accommodate on-site facilities for reprocessing and fuel manufacturing, and for nuclear waste storage and disposal.

Nuclear energy plays an essential role in the electrical generating system, and it offers perhaps the greatest potential for large-scale emission-free power in the future. The best place for the reactors, however, may be tucked deep underground, well out of sight.