Pressure vessel doesn't work like that.My hypothesis is that the reactor they are testing is so tall that it exceeds the draft of the submarine and a hump is needed to clear the top of it. Given the presence of a hump, and the potential water flow turbulence it would bring, the reactor and the hump would have to behind the sail and then streamlined to minimize the flow effects.
Not a problem for the 094 which can hide the height with its hump.
Next generation PLAN SSN would have to call for a taller draft on the sub in order to sufficiently contain the natural circulation reactor.
Creating a non-circular cross section pressure vessel will decrease to the fraction of the pressure rating .
That is more likely something on the outside of the pressure vessel, in the double wall section of the boat, and it is more likely unrelated to the reactor.
Ad absurdum it can be just an additional ballast tank, to increase the underwater stability.