The problem with nuclear reactions is that unless a complex set of conditions is maintained, they get out of control and start melting their way to the centre of the earth, generating dangerous gaseous build-ups and subsequent explosions on the way.
All that is required to stop a nuclear reaction
is to de-criticalise the reactive mass.
So, at the heart of your reactor, create a circular pool. At the centre, build a concrete cone, with the point in the middle. At equal distances around the cone, have a series of 5 or more grooves leading from the top of the cone in the middle, down and out to the perimeter of the cone out at the edges. Now create a ceiling that is an inverse of the cone-shape, leaving a large upended V-shaped void in cross-section.
Now place a series of buoyant heat-resistant sledges at each terminal point of each groove, and place a bit of nuclear fuel into each.
The idea is that as the V-crevice is flooded, the sledges float upwards, and are drawn together at the peak of the cone to form a critical mass. If there is an accident and coolant levels drop, then the critical mass is naturally separated and stops reacting.
As long as the thermal mass of the coolant in your reactor is sufficient to support cooling of the constituant fuel-sections post-criticality - you should have a passive negative-feedback that ensures it is impossible to undergo meltdown.