A high pressure and temperature microwave digestion vessel assembly with enhanced safety features is disclosed. The assembly includes a membrane that is rupturable for pressure relief, and a secondary pressure relief mechanism including a seal cap with an annular stress relief region that shears or fractures when excess pressure is in the vessel interior. The membrane is positioned on a post extending from the seal cap into the inner liner. The diameter of the venting orifice may be adjusted to vary the desired pressure relief. The assembly also includes a piercing mechanism for venting the vessel after digestion is completed.