Abstract: In the combination of a reactor vessel which includes a quench ring and a dip tube surrounding a bottom outlet, there is an improved quench ring structure. It includes a first annular cooling conduit mounted against the floor of the reactor chamber and surrounding the outlet. That floor supports a refractory lining. There is a second annular cooling conduit mounted beneath the first conduit. And, there is a high temperature resistant seal between the two conduits. The result maintains a tight seal against escape of high temperature reactor gas, in spite of any warping of the reactor floor.