Department of Nuclear Engineering, University of California, Berkeley, CA 94720-1730
NE-161 Project Fall 1994
Due to its high temperature, the MHTGR has a 38% plant efficiency, which is 6% higher than the average commercial Light Water Reactors Power Plant (LWR). The inherent, passive safety design of the MHR makes it virtually impossible for severe accident to happen. The reactor core is designed that fission products are kept in the fuel under all postulated events. Even if all the coolant is lost, the fuel can still cool itself down. In fact, it is not possible to melt the MHR core. Thus, the safety characteristics of the MHR lead to the fact that, no public evacuation is needed under all accidents conditions.
More recently in the early 90's, several technologies breakthrough suggested an improvement considerations in the MHR advanced reactor program. This leads to the development of the Gas Turbine Modular Helium Reactor (GT-MHR), which evolved from the MHTGR. Instead of using the steam cycle as the ways of generating electricity, the GT-MHR directly couples the reactor to a gas turbine. In using the one phase Brayton cycle, the MHR dramatically leaps from a 38% steam cycle plant efficiency to a world record of 48% plant efficiency. In addition to the high efficiency of generating electricity, the high temperature reactor also offers the unique application as a high temperature heat source for industry applications, such as the desalination of sea water, which can not be accomplished by the LWRs. The high temperature, simplicity and inherent safety of the design offers the GT-MHR the safety, economics and environmental advantages over other sources of generating electricity. Still under development, the GT-MHR design emphasize on the inherent, passive safety, economic competitiveness and high availability.
Each module includes the reactor vessels for heat generation, the power conversion vessels for energy conversion from heat to electricity, and a cross vessel connecting the reactor vessel to the conversion vessel as shown in figure 1. The whole unit is placed in underground silo surrounded by the Reactor Cavity Cooling system which enables a decay heat to be removed passively. The high strength steel reactor vessel is about 24m tall and 8.5m in diameter. The 600 MW(t) annulur core, core supports, control rod drives and a shutdown cooling system are located in the reactor vessel. The control rod drives and reserve shutdown systems are at the top of reactor vessel so they can provide a gravity driven shutdown in case of a loss of total electrical power and serve as access for refueling. The shutdown cooling system provides a forced circulation of cooling for shutdown, such as refueling and maintenance. The GT-MHR core reactivity is controlled by two systems, the control rod drives and the completely separate reserve shutdown system which employs boron pellets in hoppers in the core to control reactivity. This redundant design makes sure the reactor will shut down under any conditions. The annular core is made up of graphite fuel columns in hexagonal shape. The geometry provides a large surface-to-volume ratio. The reactor is fueled with 20% enriched uranium oxycarbide and fertile particles of natural uranium in microspheres, and are lined together to form fuel rods. The ceramic coated fuel particles are placed inside the column, with the hexagonal graphite reflector blocks inside and outside of the annulus.
Fig. 1 GT-MHR reactor vessel and power conversion vessel. Courtesy of General Atomics.
Electricity is produced in the power conversion vessel which house the gas turbine, a generator, two compressors and heat exchangers necessary for conversion. The vessel is about 7.5m in diameter and 23m tall. High temperature helium gas is expanded, cooled down and compressed in the power conversion vessel to generate electricity. The interconnected cross vessel provides the path for helium to move from one vessel to another.
Unlike other nuclear power plant, the GT-MHR runs on the gas Brayton cycle instead of the steam Rayleigh cycle. The GT-MHR directly connects the heated coolant Helium into a gas turbine for electricity generation. This direct coupling method eliminates the need for the conventional large energy conversion facility, which leads to the high efficiency of the GT-MHR. The process flow diagram is shown in figure 2. Helium gas is heated up to about 850 degrees at 7.02 MPa in the core. It leaves the reactor through the central duct of the cross vessel and flows into the power conversion vessel. The gas is then expanded in the turbine to drive the electrical generator to produce electricity in either 50 or 60 MHz. At 510 degree Celsius and 2.65 MPa, the coolant exits the turbine. The exhausted helium, still hot, enters the low pressure side of the recuperator to heat the cold helium from the cooling leg. back to the reactor. The coolant then flows through a series of heat sinks and compressors for cooling and compressing to 7.24 MPa at 112 degree Celcius. The cold Helium are then reheated in the recuuperrator in the high pressure side. The helium leaves the recuuperrator at 490 degree celsius at 7.07 MPa. Through the outer annulus of the cross vessel, the Helium flows back to the reactor for reheat to complete the cycle.
Fig. 2 GT-MHR Helium cycle diagram. Courtesy of General Atomics.
Besides the unique features of the fuel design, the GT-MHR also uses helium gas as coolant. There are several advantages in using Helium as coolant. The helium is in single phase, which does not boil and gives certain pressure measurement, thus simplifying the reactivity calculations so more accurate results can be obtained. Due to its neutron transparent property, the primary coolant circuit is less radioactive which keeps the dose for the maintenance workers low. Further, helium is chemically inert and does not react with core materials, such as graphite, cladding, and minimizes the effect of corrosion and fission products release.
Fig. 3 Maximum fuel temperature during conduction cooldown.
Courtesy of General Atomics
Fig. 4 Fractional Gas release from fuel. Courtesy of General Atomics
In the case of loss of coolant accident (LOCA), when the helium coolant is not available to cool the reactor, the control rod mechanism would shutdown the reactor, and then the passive heat removal system starts automatically to cool the reactor. Accidents scenarios are postulated, and tested by computer coded simulations and experiments to assess the capability of the GT- MHR passive system. The most severe accidents in the reactor system is the complete loss of coolant accident (LOCA), which all the active cooling systems are unavailable under this conditions. In the GT- MHR, this is the case of depressurization and the loss of forced cooling Helium, which is called the d "conduction cooldowns." Without the connective heat removal system, radiation and conduction heat transfer is the only way for cooling. During an accident, the core temperature rises slowly over several hours due to the decay heat generation. The core has high enough temperature which the decay heat can be radially transfer to the reactor vessel. Heat is then removed from its uninsulated reactor vessel surface to the reactor cavity cooling system (RCCS). Arrays of the RCCS riser panels are placed surrounded the reactor vessel. Decay heat transferred to the reactor vessel wall is then transfer through the reactor support structures to the cooling panels of the RCCS. Heat is then removed to the ambient from the cooling panels through natural convection. This conduction cool down process is shown in figure 5. The safety features of the GT-MHR is designed to withstand this depressurized, loss of coolant conditions, without the release of fission products from the first barrier, ceramic coated layer of the fuel. This passive heat removal system response of this accidents conditions, assuming no operators intervention, have been extensively studied analytically and using different computer codes, such as the COMMIX code developed by the Argonne National Laboratory. Results confirmed that the RCCS adequately maintains the maximum reactor vessel temperature within design criteria in conduction cool down[Berkoe]. Since the GT-MHR can shut and cool itself down safely under the most severe accident conditions, the GT-MHR is capable of safe shut down with other accidents such as the loss of offsite power and the minor loss of coolant.
Fig.5 Decay heat flow during conduction cooldown. Courtesy of Jonathan M. Berkoe of Bechtel
The slow temperature transient of the reactor is obtained because of the use of Graphite in the reactor core. Graphite serves as the moderator and reflector for the GT-MHR. The high thermal capacity of the graphite and the low power density of the core tends to reduce temperature transients caused by power changes. That is why during a LOCA accident, the fuel temperature rises very gradually. The high strength and stability of the graphite at high temperature makes it suitable for core support and protection in the high temperature reactor. The core is configured to allow a large core surface to volume ratio and the low power density makes passive heat removal possible, even under the worst accident conditions. The maximum fuel temperature is well below the integrity limit of the ceramic coated fuel. Moreover, the passive heat removal system is built-in to the system and does not require any operator action to activate the system, which minimizes operator errors. Thus under this condition, it is impossible for the GT-MHR core to melt, and fission products are retained in the fuel.
The modular design of the GT-MHR contributes importantly to the economic factor as well. Due to its modular design, plants components can be manufactured in factories in relatively large quantity, transported to the plant site and welded together economically. This offers advantages in shortening the plant construction time and keeps the capital cost down.
The simplicity of the design makes the GT-MHR a very economical and safe options in power generation. The direct conversion of energy in the power conversion vessel reduced the construction cost significantly by eliminating the huge facility for energy conversion in conventional steam turbine plant. Because of the number of the generic components, such as valves, pumps, and pipes, are significantly decreased, the maintenance and operating costs are also reduced. The built-in inherent safety features and the passive heat removal system of the plant reduces the number of active components for coolant circulation such as feed pump and feed water heater in the system. In addition to savings of the one time capital cost of the active system, the passive heat removal system is projected to be virtually maintenance free. The selection of Helium as coolant also helps keeping maintenance cost down due to the properties of Helium gas. Helium is chemically and neutronically inert. It does not react with core material and piping system, which reduces the maintenance cost related to corrosion, and the risk of a LOCA. Due to the neutron transparent properties of Helium, the amount of radioactivity in the primary coolant circuit is decreased, thus reduced the maintenance cost, and worker doses.
The high plant efficiency and passive safety characteristics of the plant makes the GT-MHR a very competitive and attractive options of power generation in the next decade. In fact, an economic feasibility of the GT-MHR is conducted in 1993. Studies show that this option of nuclear power is very competitive with all other sources of electricity generation. The GT-MHR power generating cost can compete directly with the combined cycle combustion gas turbine at the time of service, even if the natural gas price is assumed to grow slowly.
6. General Atomics. "The New Reactors," Nuclear News, Sept 1992.
