The purpose of this paper is to review a number of design proposals and to summarize and assess the current state of the art in underground nuclear power plant design. In addition Some conclusions is drawn and a model of a safe nuclear power plant underground is suggested. 1. INTRODUCTION Underground siting of nuclear power stations has drawn increasing attention in various parts of the World. European countries took the early lead in this field because they have experience in the design and construction of underground hydroelectric stations and industrial plants, and because they have near-surface rock formations which are ideally suited for hosting large underground caverns. The world's first underground nuclear facilities were built in Norway (Halden) and was commissioned in 1959. Recent feasibility studies in a number of countries have proposed conceptual designs for full scale nuclear power stations sited in either mined-rock cavities or in shallow excavations covered with backfill (pit siting). These investigations, primarily in European and North American countries, have revealed a number of potential benefits of underground siting: improved containment of radionuclides under extreme accident conditions, increased station protection and security, reduction in seismic loading on nuclear equipment and structures. These benefits, however, could be realized only in combination with higher station construction costs and certain operational penalties. Because the above investigations were based on a number of different basic reactor designs and were geared to different national regulatory standards and different geological environments, a wide variety of underground design concepts have evolved. The purpose of this paper is to review a number of these design proposals and to summarize and assess the current state of the art in underground nuclear plant design. In addition, the paper attempts to weigh the advantages and disadvantages of this unique siting concept, as identified in the various investigations, and to draw some conclusions to assist energy planners in making decisions on the complex issue. 2.0 RECENT FEASIBILITY INVESTIGATIONS This section reviews the more recent and prominent feasibility studies in several North American and European countries. It is impossible, in the allocated space, to do more than highlight the major design features, economic implications, and general conclusions which were derived by these investigators and agencies. 2.1 United States The early feasibility studies of underground nuclear plant siting in the United States were carried out as joint efforts by the Aerospace Corporations/California Institute of Technology (1972) and by United Engineers/Acres American (1974). Both of these studies were preliminary first cut attempts at providing a broad but cursary evaluation of underground siting. Conceptual plant designs were not developed beyond the stage of a general plant arrangement and thus evaluations of the plant safety and economics were very qualitative. Nevertheless, the tone of these early reports was clearly optimistic and prompted other agencies to take a closer, more detailed look at the concept. In 1977 Sandia Laboratories, under contract with the US Nuclear Regulatory Commission, carried out a technical assessment of the potential benefits and penalties associated with underground siting of nuclear power plants.

Progress in mining technology and excavation methods has made subsurface space more readily accessible. This development has opened exciting opportunities for engineering science and technology. Among these are the use of the underground for the production, storage, conversion, transportation and final use of energy- including the ultimate disposal of radioactive waste products from nuclear power plants. Improved methods for the production of primary energy by conventional means such as drilling for petroleum and mining for coal are now supplemented with in situ processes. These mean enhanced recovery and will make currently non accessible sources of fossil fuel available for future exploitation. Oil importing countrie's desire to hedge against sudden supply shortages make huge underground cavities necessary for the storage of petroleum. With respect to natural gas the necessity to cope with large load variations calls for short term storage facilities. Here liquid natural gas in underground caverns may offer attractive solutions both from the economical point of view and when safety aspects are considered. Long term storage of solar energy as well as the tapping of geothermal heat flows implies large underground storage cavities as well as subsurface heat extraction methods. Medium to small thermal nuclear plants may be safely located nearby or even within urban centers to be used for district heating. It has also been suggested that nuclear reactors for power generation should be placed underground both for the sake of public safety and for economical reasons. Using water as an energy vector for fairly long distance transportation of heat in unlined rock tunnels is already a proven technology. This offers economically alternative means for the utilization of waste heat from nuclear power plants. The method applies to district heating and to consumers of low temperature heat such as green houses. For energy intensive industrial applications surface location may be excluded in built up areas. Here the subsurface space may provide unconventional siteing possibilities. The final disposal of radioactive waste presents a serious problem for which subsurface space at present seems to offer the only viable solution. In 1975 the World Energy Conference established a Conservation Commission with representatives from developed as well as developing nations, from centrally planned economies as well as from market economies, and with delegates from international organizations such as United Nations, The World Bank and the Economic Commission for Europe. According to its rather broad and general terms of reference the Conservation Commission should study the long range energy prospects for the world at large as well as Io r various different regions in the world. It was decided that the time horizon should stretch to the year 2020 and that the investigation should cover production potentials of all sorts of primary energy including new, unconventional alternatives, such as solar energy, fusion and geothermal. The Commission should explore the possibilities to conserve energy and improve the efficiency in its usage. The Conservation Commission was also asked to forecast future energy demands in various parts of the world and how these will develop with time.

In the safety assessment of the underground siting of a nuclear power plant, it is necessary to know the transport behavior of gaseous fission products in rock. A new in-situ experimental method has been developed in order to obtain the parameters governing the air movement in rock. The experimental results obtained with this method have made it possible to explain the transport behavior. The preliminary analyses has shown that the gaseous fission products, when leaked underground, would be kept and contained there to a fairly hi~h degree. INTRODUCTION Japan has an area of 377 thousand square kilometers covered by 70 percent of mountainous area and a population of 112 million. The flat parts are therefore densely populated and intensively utilized for various purposes. Under these circumstances, underground siting of nuclear power plant (NPP) is thought to be attractive from the view point of effective land use. Recently, a committee established by the Ministry of International Trade and Industry (MITI) pointed out that it is essential to comprehend the behavior of gaseous fission products in rock and seismic characteristics of underground structures in order to realize underground NPP construction. As for behavior of gaseous fission products in rock, however, there are few field data to be applicable for analyses. This paper describes a new method to determine the parameters governing the air movement in rock, the result of the in-situ measurements which, taken under the auspices of MITI, are the basis of the parameters, and the analytical result under the condition of a hypothetical accident of underground NPP. BEHAVIOR MODEL OF FISSION PRODUCTS IN ROCK. At a hypothetical accident such as a loss-of-coolant in an underground NPP, atmospheric pressure and temperature within the reactor cavern will increase and a part of fission products such as noble gas and iodine will be released from the reactor core. Therefore the gaseous fission products will leak into the surrounding rock from the reactor cavern. In this case it may be thought that these fission products will be transported to ground surface dominantly through gaseous phase rather than liquid phase. The behavior of gaseous fission products in rock can be generally expressed by Eqs. (1) and (2) (the Japan Society of Civil Engineers, 1974). EXPERIMENT CONCERNING CONTAINMENT OF GASEOUS FISSION PRODUCTS There are several reports concerning the flow of compressed air through rock (Binggeli, Verstraete and Sutter, 1964; Bernell and Lirdbof, 1965; DiBiageo and Myrvoll, 1972). However there still remain many problems to be solved regarding rock properties such as air permeability coefficient, effective porosity of rock, effective diffusion coefficient of fission products in rock, absorption and adsorption coefficients of rock for fission products, etc. In December 1978, an in-situ experiment was performed at Numappara Pumped-Storage Hydro Electric Power Station to examine those rock properties. Fig. 1 shows the experiment site and the geological profiles. Experiment Facility Testing holes. The experiment site is located about 100 m inward from the adit entrance and 50 m beneath the ground surface.

In connection with the geotechnical study of a proposed underground nuclear plant, a 300 m deep test hole was drilled at Ontario Hydro's Darlington site approximately 65 km east of Toronto. The hydrofracturing method was used to determine the state of stress in the Paleozoic limestones (0–220 m depth) and the Precambrian granitic gneiss (220–300 m). The results of these measurements indicated a state of high horizontal stress along the depth of boring and a consistent orientation of principal stresses in each geologic unit. These results were confirmed by a borehole TV camera survey, and by overcoring tests at shallow depths in the same general area. The measured stresses were used in an assessment of the stability of large reactor caverns for housing the nuclear components of a proposed 4 × 850 MWe underground nuclear power plant in Precambrian hard rock. It was found that under ambient temperature conditions the state of stress alone did not critically affect cavern stability. Next, the occurrence of thermal shock in the cavern under a loss-of-coolant accident condition was also analyzed. To aid in the design analysis of the thermomechanical stability, a series of tests were carried out on the thermal properties of Precambrian rocks at elevated temperatures, including thermal conductivity, thermal diffusivity and coefficient of thermal expansion. The test results on thermal properties are presented in the paper, along with the results of the finite element analysis and other pertinent rock mechanics tests performed. INTRODUCTION Underground siting of nuclear power plants has been receiving increased interest in various parts of the world owing to some potentially important benefits over equivalent surface installations. Reactors housed in deep rock provide an additional safety margin in containing the release of radionuclides, in reducing the damage due to earthquake in high seismic areas, in protecting against sabbotage, aircraft crashing, hurricanes, etc., in supplying high-pressure gravity-fed cooling water to the fuel core in emergency situations, and in minimizing the amount of land use and environmental impact (Oberth and Lee, 1979). These and other potential advantages are still being weighed in the United States against the major drawback of higher construction costs. Norway and Sweden, however, built their first underground power facilities some 20 years ago (McHugh, 1964; ENEA Report, 1962). In Canada, Ontario Hydro has recently embarked on an extensive investigation of the technical and economic implications of building a large 4 × 850 MWe underground CANDU (Canadian Deuterium Uranium) power station in Ontario. The location of the proposed power plant will have to satisfy several siting requirements including the proximity to a major load center to cut transmission costs, and the availability of an adjacent large body of cooling water. A third requirement, that of suitable host rock formation to ensure water tightness and cavern stability was the subject of a preliminary geotechnical investigation which included a geological surface study at the Darlington site, the drilling and coring of an NQ borehole (UN-l), and a number of borehole tests such as permeability and stress measurements.