Keywords: nuclear desalination, HTGR breyton cycle, Eskom, PBMR, helium, pebble bed modular reactor, direct cycle gas turbine, waste heat, water circuits cooling, safety, developing countries, South Africa, reverse osmosis, desalination plant backup, nuclear energy, seawater desalination
The pebble bed modular reactor, desalination challenges and options
The Pebble Bed Modular Reactor (PBMR) is a development lead by Eskom, the South African state power utility. The technology used is based on the previous German HTGR work linked to a direct cycle gas turbine (or Breyton Cycle) being developed in conjunction with Mitsubishi Heavy Industry. The initial commercial plant design has a thermal output of 400 MW with an electrical output (nett) of over 165 MW. The interesting feature of desalination is that the nature of the inter-cooled closed cycle is the rejection of waste heat (about 200 MW) at temperatures of up to 120°C to cooling water circuits. The options that could be considered include a reverse osmosis plant using a sea water inlet temp of 25°C with an outlet from the reactor's coolers of 40°C. This would result in a power consumption of some 14 MW from the reactor with a water production of 78,000 m³/day per reactor. If the evaporative approach is to be used, the current design can yield 400kg/s of water @ 102°C, or (with minor modifications to the coolers and some increased limits on the operating flexibility of the reactor) 342 kg/s @ 115°C. In both these last two cases, there would be no reduction in the electrical power dispatched to the grid. The advantages of such a system to desalination applications are several. The size of the reactor means that even in reasonably small electrical grids (as small as 1000 MW total) a number of PBMRs could be grouped together. This would avoid the problem of backing up the desalination system with a fossil fuel source when the reactor is in maintenance. An effective installation could be four PBMRs linked through a common cooling water system to two desalination plants (each to be supported by one reactor). Since the PBMR uses on-load fuelling systems, the plant does not have the problem of short operating cycles (12–18 months) but can operate for six years between 30-day maintenance periods. It is also designed, due to the inherent safety features of small HTGRs, to be operated with a far smaller nuclear infrastructure, using extensive turnkey vendor support (as with current gas fired power stations). These features make the PBMR far more suitable for developing countries, where the application of existing nuclear designs (e.g. a. 1000 MW PWR) would be problematic.