The fresh core of MYRRHA contains a lattice of 183 hexagonal channels of which 68 are loaded with fuel assemblies (this configuration considers a core composed of only fresh fuel). A space of 3 hexagons is cleared at the centre of the sub-critical core to house the spallation target module.
MYRRHA reference core configuration
Reactivity effects and safety parameters for the sub-critical mode operation
The adopted sub-criticality level (-4700 pcm for a keff of 0.955) assures a comfortable margin for safe operation taking into account foreseeable positive reactivity insertions due to fuel and coolant temperature effects, radial and axial expansion.
The Doppler constant, Tdk/dT, is -370 pcm, which is lower but of the same order of magnitude as for a Na-cooled fast reactor. The LBE-coolant temperature coefficient, dk/dT, is small but negative, -2.4 pcm/K, over the temperature range from 150 °C to 400 °C.
The effect of voiding the spallation target module has a negative reactivity contribution of about -1000 pcm. Voiding the group of 21 central assemblies for 30 cm of the active height has about +300 pcm positive reactivity contribution, while a full voiding has a zero or slightly negative reactivity contribution. Voiding all the assemblies in the core (both partial as total height) has a negative contribution of up to -2500 pcm. Flooding of the spallation target tube with lead-bismuth eutectic has a maximum of +144 pcm reactivity contribution.
The combined effect of radial and axial expansion of the structures going from the cold state (150 °C) to the hot state (Taverage = 350 °C) amounts to -490 pcm.
Cycle analysis: ten-step reshuffling scheme
A ten-step reshuffling scheme has been studied for a core power of 85 MWth in sub-critical mode operation. The total number of fuel assemblies is determined at every beginning of 90-day operation as to set keff to 0.955. The number of the fuel assemblies at the very beginning of the operation is 42 + 42/2 + 42/21. Then, the number grows up to 83 and fluctuates between 82 and 83. Because 10 fuel assemblies belong to a “batch”, every fuel assembly undergoes the operation 8.3 times on average, except for the outer fuel assemblies loaded in the initial core that are unloaded from the core before they are fully burned. On average, a keff drop of about 1500 pcm after 3 months (90 days) of operation is observed.
Three options are available to handle the reactor power drop during the operational cycle of 90 days.
- In the first option, no measures are taken and one has to live with the power gradient over 90 days.
- A second option would be gradually increase the beam current to compensate the power loss.
- A third option would be to use a burnable poison to keep keff as constant as possible.
All three of them have their advantages and disadvantages and a more detailed study (taking into account cost and plant availability) must be performed.
More information: R&D programme > Reactor physics