Abstract: Grill gates respectively formed by oppositely moving, interfitting rod combs with gaps between the rods which are smaller than the particle diameter are so arranged so that each comb may be individually moved to the open position and shut again without allowing any of the even sized particles of the bulk material to pass through the other comb while testing the readiness of the comb operating system. Grooves of circular arc cross-section fitting the size of the spheres of a high-temperature reactor are provided in the upstream sides of the wall produced by the interfitting combs to enable close packing of the spheres against the closure wall in an ordered arrangement that allows either comb to be withdrawn and reinserted without breakage of the spherical elements. The grooves are oriented in the length dimension of the comb rods, and one set of comb rods provides the groove bottoms and the adjoining rods of the other set the cooperating groove flanks.