I only have an understand of how PWR's do nuclear fission, looking at the diagram for the salt reactors it's a bit confusing. Where is the fission taking place if the salt is cycling throughout the entire plant? How is flux controlled? How is neutron capture handled while cycling is taking place?
The fission is happening everywhere there is salt with fuel in it. Most of the fissions are occurring in the "main" part of the core, where the geometry is favorable for neutron multiplication, but there are fissions occurring everywhere in the primary loop. This is one of the challenges of molten salt reactors.
In addition to the prompt neutrons, some fissions will produce a "neutron precursor" that will eventually release a neutron after a slight delay. These neutron precursors are generated in the main part of the core, then transported into the primary loop, where they release their neutron and produce additional fissions in the primary loop. It gets interesting....
To account for the (very) radioactive primary loop, there is an intermediate coolant loop that will isolate the primary loop from the final coolant loop.