Higher powered TRIGA research reactors ( ≥ 2 MW) have more elaborate primary cooling systems. Additionally, because the forced cooling flow is required for the higher power operation, monitoring of the coolant flow and temperatures are required and become an integral part of the reactor protection system. Standard TRIGA fuel may be operated up to 3 MW with forced convection cooling, but TRIGA fuel designed for higher power (one difference from Standard fuel is the smaller fuel element diameter) will also have these same systems.

1. A primary cooling system which transfers the heat generated in the reactor core by forced convection but also permits natural convection cooling for low power operation or decay heat removal
2. A decay tank or hold-up system that permits the decay of N-16, a neutron activation product, in a shielded area before returning the coolant to the tank
3. Emergency core cooling system (ECCS), which supplies water to the reactor core in the event of a tank draining or loss of coolant accident (LOCA) and thus protects the integrity of the fuel cladding by removing the decay heat

Primary Cooling System

Due to the flow restrictions and losses added to the system from the additional forced cooling piping around the reactor, the forced/natural cooled reactor can only be operated in a natural circulation mode at power levels up to 500 kW or in a forced down-flow mode at higher power levels. The heat generated in the reactor core is transferred to a secondary water-cooling in a similar fashion to the bulk cooling system described in Section 2.1 above. The secondary system transfers the heat to a cooling tower or other heat sink, where the heat is removed. The reactor heat is transferred directly to the bulk water in the reactor tank in the natural convection cooling mode until temperature limits require outside forced cooling of the pool. The primary cooling system and connected systems of a typical high power (in this case, 3 MW) TRIGA facility are shown in Figure 2. As noted before, the higher power TRIGA cooling systems actually have two modes of operation; forced and natural convection cooling.

Natural Convection (Low Power Operations) Cooling

In this mode of operation, the primary pumps remain turned off and the reactor core is cooled by the pool water flowing down past the outer fuel elements and up past the inner fuel elements. This pattern is established by the available coolant and the radial power profile across the reactor. This flow pattern has been verified by several tests conducted at the TRIGA reactor at the University of Illinois. A mode switch on the reactor control console limits the reactor power and sets the safety system settings for the fuel temperature at values permissible under forced convection flow mode of operation. The operating time of the reactor is limited by the rise in bulk pool temperature. The dose rate at the top of the reactor should be monitored since the N-16 produced in the core will rise directly to the pool surface in the natural convection mode. An additional diffuser pump may be considered if dose rates are determined to be an operational problem in natural convection mode

Forced Convection Cooling

In the forced convection mode, the coolant is pulled down through the core by the suction of one or more primary coolant pumps. The primary flow rate (approx. 13 m3/min) must be achieved and stabilized before the reactor is turned on or the reactor will scram immediately as a result of a “low flow” signal.

In the forced convection mode, the primary coolant enters the reactor tank through a 25.4 cm line near the top of the pool. The coolant is drawn by pump suction down through the core upper shroud and through the reactor core into the core lower plenum. The coolant enters a 30.48 cm diameter pipe from the lower plenum and travels in a shielded trench to the N-16 decay tank. Two pumps operating in parallel pump the coolant from the N-16 decay tank, through the heat exchanger and back to the reactor tank. The reactor suction line is positioned within the tank to flow up above the reactor core before exiting the tank. This pipe has an anti-siphon line provided that allows air into the suction line if the water level in the tank drops below 4.42 m from the normal pool level. This prevents the primary pumps from pumping the reactor core dry in the event of a break in the primary cooling system downstream from the pumps. The anti-siphon line is a backup to the float level switches which normally turn the primary pumps off when tank level drops below a nominal value