Cyril R wrote:
You're going to need flux flattening to get the most power out of your reactor. A totally flat profile gets you the most power output for a given fuel thermal limit. A totally unflattened profile gets lower neutron leakage, but if it costs you many MWe it isn't going to be a good deal economically.
There would be some degree of core flattening in all practical designs thanks to the presence of a radial reflector around the core - in an unflattened core the outermost channels produce negligible power any way so there is little disadvantage to simply not having them and replacing them with more moderator water.
Bidirectional fueling gets a certain amount of axial power flattening (axial is along the channels right?) since the fuel at the insertion end will tend to be more active thhe fuel at the discharge end. [Atleast with an 8 bundle shift]
The question is whether you are willing to accept a core with only that level of flattening.
It appears that overburning the central fuel channels as is conducted at Bruce A is more efficient than the 15 milli-k reactivity loss associated with adjuster assemblies, but that it is still a significant reactivity loss as the Bruce A advantage over Bruce B is only about 1GWd/t - which is about 8 milli-k.
In the extreme entirely-unflattened case the average channel power would be ~27.5% the maximum channel power, which is obviously impractical, but this is improved with the reflectors. Apparently the cores at Pickering (with adjusters) manage something like 60% of maximum channel power on average. Bruce A appears to do better and obtain ~81% - probably thanks to the fine control possible with fuel burn induced reactivity control.
Perhaps a handful of thorium bundles in the centre positions of some of the highest power channels? That would reduce channel power and reactivity but not be truly parasitic absorption.