General Atomics has determined that Standard TRIGA fuel elements may be safely operated up to 2 MW with natural convection cooling. Heat transferred from the fuel elements to the primary coolant in the tank is carried from the core by natural convection cooling with low power TRIGA reactors. These reactors may be operated for short periods with natural convection cooling up to a power level of approximately 100 kW but additional pool cooling may be required to maintain average bulk pool temperatures within permitted values if continuing operations are expected. Above a nominal power level of 100 kW, the secondary cooling system circulation pump should be turned on to remove reactor heat and, through the diffuser system flow, to interrupt the direct, upward flow of N-16. The primary coolant circuit is composed of the reactor tank, one or more primary pumps, a heat exchanger (plate type or tube type heat exchanger), filters and associated piping and valves. Two typical designs are shown in Figure 1. When forced cooling is used, the primary water is pumped from the reactor tank through a pipe (typically 100 mm stainless steel) and directed from the reactor platform downwards into the basement of the reactor building or into an adjacent room. Usually a water outlet temperature sensor (T_A) is installed in this section of the pipe for system evaluation.

The first major component in the primary loop is usually a filter with a nominal (approximately 25 μm) type filters sized to remove small suspended particles carried along with the primary water but not restrict flow. This filter acts to protect the primary pump and minimizes the fouling of the heat exchanger surfaces. An expansion bellows may be located in the subsequent piping to allow thermal expansion. The primary pump(s) could be made of aluminium but stainless steel pumps are now more frequent, typical horsepower is 10-15 kW. In some cases, the primary flow (and hence, pool temperature) can be regulated by an automatic control, normally the maximum flow is about 30 m³/h for a low power TRIGA reactor. A conductivity sensor (σ₁) and a pressure sensor (P₁) are sometimes installed after the pump and before the heat exchanger inlet. The heat exchanger for many facilities are tube and shell type in which the primary coolant passes through the heat exchanger’s small internal tubes and the secondary coolant flows around the tubes. Some facilities have replaced their traditional tube and shell type with plate-type heat exchangers. Plate-type heat exchangers have a larger heat transfer surface area and this results in a smaller heat exchanger for the same capacity. Additionally, the plate-type may have additional plates installed to increase the capacity further with only a small change in the overall size.

A second conductivity cell (σ₂) is installed after the heat exchanger, the conductivity cells before and after the heat exchanger are used to compare heat exchanger inlet- and outlet primary coolant conductivity and give operators an indication of a possible heat exchanger leak. A secondary water leak into the primary water is possible because the pressure in the secondary cooling system is always higher than on the primary side (this is to prevent an inadvertent release of radioactive coolant to the secondary). A defect or tube leak in the heat exchanger may be indicated by an increase in the conductivity difference across the heat exchanger. From the heat exchanger room the primary coolant pipe penetrates back into the reactor hall upwards to the reactor platform. A flow meter (Φ) and an inlet temperature (T_E) sensor are installed in the primary coolant pool return piping. In most cases the inlet temperature into the tank is between 20 and 25°C at 250 kW. This return temperature is dependent on the cooling capacity of the primary and secondary systems. Larger reactors may have difficulty maintaining a constant return temperature due to inherent system inefficiencies; e.g. hot, humid conditions limiting the cooling capacity of the evaporative cooling tower.

Modern primary coolant systems at research reactors may be completely made of stainless steel or a mixture of stainless steel and aluminium. Materials used to construct or repair the cooling system must be selected for corrosion resistance and compatibility with existing systems. Pump, pipes and heat exchanger may be obtained from milk- or soft drink industry suppliers at a lower cost if they have the same purity and quality requirements as the nuclear industry. Heat exchangers should be cleaned about every 10 years on the secondary side to remove deposits (mineral scale or biological fouling). These deposits depend on the secondary water quality control and reduce the heat transfer coefficient. Monitoring of heat exchanger fouling may be performed by careful evaluation of the inlet and outlet temperatures and pressures over time. Maintaining water chemistry on the secondary side may reduce the frequency of cleaning but improper or excessive chemical addition could potentially cause heat exchanger tube leakage. Replacing the gaskets or seals is recommended after the cleaning of a plate type heat exchanger to prevent leakage during subsequent operations.