Nuclear power generation technology has undergone an evolution from fuel rods and heavy water to newer designs of reactors that can be cooled by light water and more recently by gas. There is much research underway in China and the USA involving gas-cooled, high-temperature reactors (HTR) based on the continually evolving pebble bed modular reactor (PBMR) technology. Advanced research into PBMR technology in China is focused on both uranium-fueled and thorium-fueled versions that involve cooling the reactors using pressurized helium circulating in a closed-cycle turbine.
Helium has the advantage of having 5.1708-times the heat capacity or specific heat of air based on equal weight and temperature (BTU/lb- R or KJ/Kg- K). It has a specific heat ratio of 1.66 as compared to 1.4 for air. That higher specific heat ratio allows a power turbine to extract more work from an equivalent mass flow-rate of helium (Kg/sec) as compared to air. It also has a gas constant that is 7.23707-times that of air.
That gas constant offers an advantage in applications like airships as it translates to air having 7.23707-times the density of helium at equal pressure and temperature, causing the entrapped helium inside the airship envelope to produce great buoyancy. When the higher density of air is divided by its lower heat capacity when compared to helium, the result is (7.23707)/(5.1708) = 1.3996 which means that for equal pressure, temperature and volume flow rate, air will have 39.96 percent greater heat capacity than helium with which to remove heat from a high-temperature reactor.
The design evolution of nuclear reactors suggests that gas-cooled PBMR-HTR reactors could eventually be cooled by air, nitrogen or carbon dioxide. Researchers have been seeking methods by which to resolve the flammability problem of pebble bed reactors, an advance that opens the door to cool them using pressurized air. A high-temperature nuclear reactor may be cooled by the equivalent volume of helium, air, nitrogen or even carbon dioxide being pumped through the reactor at the same pressure. The equivalent volume of air will have 7.23707-times the density as helium with up to 39.96 percent greater thermal capacity with which to cool the reactor. Nitrogen can be extracted from atmospheric air and provide comparable performance as air.
Single-stage Closed-Cycle Turbine:
It is possible to compare air (or nitrogen) to helium a theoretical gas-cooled nuclear reactor which operates up to 950 C (1223 K) with the gas being heated to 1170 K with heat been transferred to the gas at 95 percent-effectiveness. For the basis of comparison a single-stage closed-cycle turbine is used with final cooling being done by ocean water with gas cooled to 47 C or 320 K. The single axial-flow compressor and single turbine rotate on a common shaft and both operate at a pressure ratio of seven-to-one with 87 percent-isentropic efficiency.
A single-stage closed-cycle turbine that extracts heat from a gas-cooled, high-temperature reactor (HTR) can operate using air (or nitrogen) at the same temperature, pressure and volume flow rate as helium. The air (or nitrogen) can cool the reactor that can operate a turbine engine with exhaust heat supporting the operation of a thermal desalination facility. The final exhaust of the turbine will be cooled by ocean water prior to the pressurized air or gas being re-compresses through the power producing cycle. Closed-cycle turbine engines can offer a wide range of output at high efficiency by varying system pressure to many times above or below atmospheric between LP turbine exhaust and LP compressor intake.
Multistage Closed-Cycle turbine:
Air or nitrogen-cooled HTR technology could offer higher efficiency using twin-spool turbine engine operation that includes after-cooling between the low-pressure and high-pressure compressors, reheating the gas between the high-pressure and low-pressure turbines and recovering exhaust heat in a recuperative heat exchanger. The system pressure ratio of nine-to-one may operate with 2-stage compression using compounded compressors and turbines with three-to-one pressure ratio and 90 percent isentropic efficiency. The theoretical performance of such a system is provided in the following table.
Helium has an efficiency advantage over air in single stage, simple-cycle turbine operating without a recuperative heat exchanger. The addition of such a heat exchanger increases the efficiency of a closed-cycle turbine that operates on air or nitrogen and that rejects enough exhaust heat to sustain the operation of a thermal desalination plant. Air (or nitrogen) can operate as efficiently as the working medium in closed-cycle multi-stage turbine. Such a turbine would use an after cooler between its compressors; reheat the gas between its turbines and use a recuperative heat exchanger to improve efficiency. The exhaust would still be hot enough to sustain a thermal desalination plant.
Purified and filtered atmospheric air or nitrogen at varying pressures would re-circulate through the closed-cycle turbo-machinery of an air-cooled high-temperature reactor (HTR). A large proportion of the air or nitrogen would remain within the system with a reserve supply stored on-site in large tanks. Using gas from a storage system to cool a reactor reduces the risk of damaging the turbo-machinery or fouling the heat exchange surfaces. Steam turbines in a steam driven nuclear power station can sustain damage and be eroded by from tiny high-speed droplets of saturated steam eroding surface material from the turbine blades.
The evolving development of gas-cooled, high-temperature reactors may eventually allow pressurized air or nitrogen to be used as the cooling medium and also as the working fluid that drives the turbines. Air and nitrogen have very similar thermal properties and heat ratios and could be used to cool nuclear reactors once the flammability of the fuel for pebble bed modular reactors is resolved. Such a development would allow the operation of gas-cooled nuclear reactors in countries that have a shortage of helium. The exhaust heat from such power plants can sustain the operation of thermal desalination plants in nations that need additional electric power and that have a shortage of potable water for their populations.
Research into and development into gas-cooled (helium), pebble-bed modular reactors is underway in China with similar research being undertaken in the USA. Advances in that research may eventually lead to the development of reactors that may be cooled by air, nitrogen, or even pressurized carbon dioxide. In all cases, extra gas may be stored in underground caverns and be transferred to and from the reactor as needed. Pebble-bed modular reactor technology can operate on either uranium or thorium with thorium being more plentiful and the spent product less volatile.