A catalytic reactor or heat exchanger includes a monolith defining a plurality of leaves, the monolith having a generally annular cross-section. The monolith is disposed within a generally cylindrical outer tube, and around a corrugated inner tube. The reactor includes a device for urging the monolith radially outward, so as to maintain contact between the monolith and the outer tube. Such device may include a coned washer, or it may be defined by a folded flap that is integral with the inner tube. In either case, the reactor compensates for metal creep, and virtually insures continued contact between the monolith and the outer tube.