Reactor core for nuclear reactor, nuclear reactor and method of operating nuclear reactor

A plurality of fuel assemblies are formed in such a state that the criticality set value at a low temperature state upon start of the operation is lower than a critical value. The set value includes buckling based on the shape of the reactor core and the infinite multiplication factor and the neutron transport area in a low temperature state are determined such that the value of the effective multiplication factor is less than 1. In addition, the reactor core has a columnar shape, and the height and the diameter are determined so that the buckling satisfies the subcriticality condition. In a small-sized nuclear reactor, the effective multiplication factor at low temperature state tends to be increased compared with that in the large-sized reactor, but since the reactor core is determined small with the set value being corresponded to the subcriticality state, the reactor core can reliably be made to the subcriticality state even upon reactor shut down. Then, the operation of the reactor can reliably be facilitated, as well as the structure of the reactor core can be constructed easily thereby enabling to reduce the construction cost greatly. (N.H.)