MYRRHA mechanical design: The cooling systems

The cooling systems are designed to evacuate core power of 50-100 MWth plus additional heat produced in the spallation target, the decay heat in the in-vessel storage zones, the polonium decay heat and heat produced by all pumps.

The primary cooling system 

Lead-bismuth eutectic (LBE) core inlet and outlet temperatures at full power are 270 °C and 400 °C.

 Reactor internals

 Cooling system

Pump & heat exchangers arrangement, showing the LBE chicane flow path

In the reference MYRRHA design, there are two primary pumps for four primary heat exchangers installed in two casings at the periphery of the vessel (figure left, items 5 and 6; figure right, items 1 and 2). The pumps are vertical units with an impeller at the bottom end of a long shaft. The electric motor is located on the reactor cover, away from high neutron levels at the upper end of the shaft.

In order to enhance natural convection in the primary coolant during emergency situations (e.g. loss-of-flow conditions), the core hydraulic resistance is minimized to a level according to a pressure loss of less than 1 bar in nominal power conditions. For the same reason, the level difference between core and primary heat exchangers mid planes is fair-sized at 2 m. Consequently, there is no need to pressurize the LBE cold pool (in order to limit the vessel height) since the cold pool level is only ~ 1 m higher than the hot pool level according to the core pressure drop of ~ 1 bar. Furthermore, the LBE volume, which corresponds to this ~ 1 m level difference between hot and cold pool, offers an additional core cooling capacity at the beginning of transients from normal operation to loss-of-flow condition.

The secondary cooling system

The secondary cooling system uses water/vapour as coolant. The boiling water primary heat exchangers will be of the straight tubes, single pass and counter-current type, the LBE flowing downwards outside the tubes. They operate at lower pressure and lower water flow then pressurised water primary heat exchangers, which is an advantage since it mitigates the consequences of a primary heat exchanger tube rupture and it diminishes the risk of failure of whatever secondary cooling system component. Furthermore, the LBE flow path makes a chicane in the casing from the primary heat exchanger outlet towards the pump suction side which allows water and steam separation from the LBE main stream in the event of a primary heat exchanger tube rupture (figure right).

The secondary cooling system consists of two independent circuits (figure below). Each of the two secondary circuits is made of two primary heat exchangers (serving as evaporators), a steam separator of the water/steam mixture rising from the primary heat exchangers, a steam condenser, a cooling tower and interconnecting piping.

In each circuit saturated water enters the tube-sides of the two primary heat exchangers (operating in parallel) and leaves the primary heat exchangers as a steam-water mixture. Ferruling is provided at tube inlet in order to stabilize the flow. Steam rising from the separator is condensed into the air cooled condenser, from which the condensate flows into the separator where it adds to the water plenum and feeds the primary heat exchangers, thereby closing the water circuit.

 Cooling system schematic

The secondary cooling system is a passive system: the water flows in natural circulation in the loop (there are no pumps), no human intervention and no power operated valves. The driving force for the natural circulation is provided by the hydrostatic head difference between the hot leg and the cold leg of the circuit, achieved by arranging the separator at a sufficient elevation with respect to the primary heat exchangers.

The air cooled condenser performance is air-side controlled by air intake louvers and fans, which provide for forced circulation, thereby controlling the air flow rate at the inlet of the condenser. The condenser can be partially isolated in order not to overcool the condensate after a reactor trip.

Decay heat removal

Two systems are provided for the Decay Heat Removal function to increase diversity and redundancy.

  1. For normal decay heat removal  after a reactor shutdown: the secondary cooling system using the primary LBE/water & vapour heat exchangers and two independent and secured loops.
  2. In case of unavailability of the secondary cooling system the safety-grade Reactor Vault Cooling System is called upon. The reactor vault cooling system is placed between the guard vessel and the concrete pit. The system also keeps the concrete at acceptable temperature, even during accidental events.