FAST NEUTRON REACTOR

PROBLEM TO BE SOLVED: To provide a fast neutron reactor capable of maintaining the safety and soundness of the unclear reactor even when a rotary plug is eliminated and an elevatable reactor core upper mechanism is installed. SOLUTION: An elevation mechanism 19 for a reactor core upper mechanism has an elevation column 21 connected at its lower end to the reactor core upper mechanism 8 and an elevation column drive mechanism to elevatably drive the elevation column 21, the elevation column 21 being inserted through an elevation column through-hole in an upper cover 10. A selector value operated failed fuel detector 20 has a unit body to guide a sampled coolant to the upper side and a body barrel composed of a thick cylindrical member to envelop the unit body. The reactor core upper mechanism 8 has a porous straightening structure 25 at the lower side to straighten the coolant 3 flowing out of a reactor core 2. The body barrel is joined at its lower side to the structure 25 and horizontally supported on its upper side by a supporting member 22.