A high-pressure cylindrical reactor suitable for a high-pressure process using supercritical ammonia to form bulk crystals of group III nitride or transition metal nitride is disclosed. In one instance, the reactor has a reactor body and lid formed of precipitation hardenable Ni-Cr superalloy and is sealed by a gasket made of Ni-based metal. Ni content of the gasket is greater than Ni content of both the reactor body and lid. The gasket is tapered so that its thickest part is at or near the gasket's inner radius or circumference, and the thinnest part of the gasket is more than 0.2 inch thick and is at or near the gasket's outer radius or circumference. The gasket's surfaces are compressed at 60,000 psi or higher. This construction provides a consistent seal of the reactor for repeated use.
