Nuclear power currently only serves the market segment of large scale base load electricity generation. Other energy markets, like cogeneration and heat production or market segments like the smaller scale (but still industrial) electricity production are entirely served with fossil fuels (and hydropower). When these fuels at acceptable prices are being depleted and if actively marketed, and if an inherently safe small-scale nuclear plant could be developed and marketed, a huge market could emerge for this new form of nuclear power. Pebble Bed High Temperature Reactor technology is most suitable for designing small inherently safe nuclear reactors. The oldest small designs were meant for application as district heating plants, making full use of the self-controlling features of nuclear reactors. The ACACIA concept (AdvanCed Atomic Cogenerator for Industrial Applications) is a design for industrial cogeneration, producing 13.6 MW of electricity and 17 tons of industrial quality steam per hour, with a total efficiency of 63%. In case the electricity production would be maximized at the expense of the steam quality, an electrical output of 16.5 MW could be achieved, and the plant efficiency would rise to 86% (electric efficiency 41%). The heat source is a pebble bed reactor with 40 MW of thermal power. The energy conversion system is a direct recuperated helium cycle with a radial compressor and an axial helium turbine. A number of operational and safety related transients have been calculated with two different simulation codes. The safety related transient analyses show the reactor power and the fuel temperature behaviour after a full loss of coolant accident, and illustrate the inherently safe nature of the plant. The operational transient simulations show the suitability of the system for an industrial user. Furthermore, the transport of radioactive fission products within the primary circuit has been analyzed. A cost study shows high kWh-costs compared to large scale generating plants, but the treatment of scaling factors for this particular case needs continued attention. However, for those areas in the world without fossil fuel supply networks and with only small-scale demand, ACACIA will still be an economic option. To improve matching with non-utility market needs, the current ACACIA design will be adapted from a direct cycle system to an indirect cycle system, where primary cycle will be strictly separated from the remainder of the plant. A conceptual comparison with the direct cycle system will be discussed.
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