Nuclear fission reactor, a vented nuclear fission fuel module, methods therefor and a vented nuclear fission fuel module system

A method of operating a nuclear fission reactor, comprising:

• generating a gaseous fission product with a plurality of nuclear fission fuel element bundles disposed in a reactor vessel;
• receiving the gaseous fission product into a plenum defined by at least one of a plurality of valve bodies associated with respective ones of the plurality of nuclear fission fuel element bundles disposed in the reactor vessel;
• controllably venting the gaseous fission product from the plenum by operating a valve disposed in the at least one of the plurality of valve bodies, the valve being in communication with the plenum;
• displacing the valve by allowing movement of a flexible diaphragm coupled to the valve;
• and
• threadably mounting a cap on the valve.