A fuel assembly includes: a square cylinder channel box having a square cross-section; and fuel rods disposed in a square lattice shape and filled with a nuclear fuel material. The fuel rods includes: a first rod disposed in an outer layer in the box and a second rod disposed in an inner layer in the box and having a larger diameter than the first rod. An absolute value of a difference between a hydraulic equivalent diameter around the first rod and a hydraulic equivalent diameter between a surface in the box and the first rod is more than or equal to a value obtained by multiplying an absolute value of a difference between the hydraulic equivalent diameter around the first rod and the hydraulic equivalent diameter around the second rod by a ratio of a cross-sectional area of the inner layer to a cross-sectional area of the entire box.

A fuel assembly includes: a square cylinder channel box having a square cross-section; and fuel rods disposed in a square lattice shape and filled with a nuclear fuel material. The fuel rods includes: a first rod disposed in an outer layer in the box and a second rod disposed in an inner layer in the box and having a larger diameter than the first rod. An absolute value of a difference between a hydraulic equivalent diameter around the first rod and a hydraulic equivalent diameter between a surface in the box and the first rod is more than or equal to a value obtained by multiplying an absolute value of a difference between the hydraulic equivalent diameter around the first rod and the hydraulic equivalent diameter around the second rod by a ratio of a cross-sectional area of the inner layer to a cross-sectional area of the entire box.

The present invention relates to a small modular reactor including small fuel assemblies. The small modular reactor comprises: a reactor body; and a nuclear fuel material which is accommodated in the reactor body, includes a plurality of fuel assemblies, and has a section arrangement region. The section arrangement region includes a first arrangement region and a plurality of second arrangement regions that are located outside the first arrangement region and spaced apart from each other. The fuel assemblies include: first nuclear fuel assemblies that each have a first size and are closely arranged in the first arrangement region; and second nuclear fuel assemblies that each have a second size smaller than the first size and are arranged in the second arrangement region.

The invention relates to a combined cold and thermal neutron moderator (10) having a longitudinal axis (t) and at least one neutron exit window (50) at a first end (11, 12), and a cold moderator (20) arranged along the longitudinal axis (t), a thermal pre-moderator (30) and a vacuum space (40), said thermal pre-moderator (30) having a pre-moderator mantle (32) surrounding the cold moderator (20) along the longitudinal axis (t) and separated from the cold moderator (20) by the vacuum space (40), characterized in that the pre-moderator (30) comprises a premoderator collar (34) extending at least at the first end (11, 12) from the premoderator jacket (32) towards the longitudinal axis (t), which substantially covers the portion of the vacuum space (40) between the cold moderator (20) and the premoderator mantle (32) from the direction of the neutron exit window (50), leaving substantially free at least one end (21, 22) of the cold moderator (20) at the end (11, 12).

WO

The invention relates to a combined cold and thermal neutron moderator (10) having a longitudinal axis (t) and at least one neutron exit window (50) at a first end (11, 12), and a cold moderator (20) arranged along the longitudinal axis (t), a thermal pre-moderator (30) and a vacuum space (40), said thermal pre-moderator (30) having a pre-moderator mantle (32) surrounding the cold moderator (20) along the longitudinal axis (t) and separated from the cold moderator (20) by the vacuum space (40), characterized in that the pre-moderator (30) comprises a premoderator collar (34) extending at least at the first end (11, 12) from the premoderator jacket (32) towards the longitudinal axis (t), which substantially covers the portion of the vacuum space (40) between the cold moderator (20) and the premoderator mantle (32) from the direction of the neutron exit window (50), leaving substantially free at least one end (21, 22) of the cold moderator (20) at the end (11, 12).

WO