Utilisation of waste heat from GT-MHR and PBMR reactors for nuclear desalination

The gas turbine-modular helium cooled reactor (GT-MHR) is currently being developed by an international consortium; the pebble bed modular reactor (PBMR) is to be constructed in South Africa. In both these reactors, circulating helium that has to be compressed in two successive stages cools the reactor core. For thermodynamic reasons, these compression stages require pre-cooling of the helium to about 26 degrees C through the use of pre-cooler and intercooler helium-water heat exchangers. Considerable thermal power (approximate to 300 MWth) is thus dissipated in the pre-cooler and the intercooler. This thermal power is then evacuated to the heat sink. Depending upon the specific designs, the temperature ranges of the water in these exchangers could be between 80 and 130 degrees C. This is an ideal range for desalination in a multiple-effect distillation (MED) plant, which can be coupled between a mixer (of the flows from the pre-cooler and the intercooler) and the switch-cooling unit, evacuating the heat to the heat sink (sea or river). It is thus interesting to evaluate the desalination costs of such a system, utilising virtually free heat. The usual code for desalination cost evaluation is the DEEP software, developed by the International Atomic Energy Agency. Actual versions of DEEP do not have models for GT-MHR and the PBMR providing heat for desalination. This paper describes the successive steps that led CEA to the development of these models from basic thermodynamic considerations and their integration in the new, CEA version of the DEEP code. The models are then applied to a realistic case study based on the TUNDESAL project [1]. It is shown that the desalination cost of a GT-MHR + MED system is 34% lower than that of a gas turbine, combined cycle plant + MED system, for a fossil fuel price of about 21 $/bbl and a discount rate of 8%. Under the same conditions, this cost is 2% lower for the PBMR + MED systems.