The invention discloses a method for calculating an axial swelling effect of a
fast neutron reactor assembly. The method comprises the following steps of 1, separating a transport computation grid from a
burnup computation grid, wherein the transport computation grid is kept unchanged in the whole computation, and the
burnup computation grid keeps growth of a same proportion along with axial swelling of a fuel
assembly; 2, based on a variational
nodal method, establishing a weak form of a
neutron transport equation solution, wherein a section in a node is a function related to a spatial position, so that appearance of various materials is allowed in a transport computation node; 3, expanding a flux by utilizing a spherical
harmonic function and a spatial orthogonal polynomial, and solving a flux expansion moment by adopting a response
matrix method so as to obtain
flux distribution of each node; and 4, calculating a
burnup level of the node according to average power of a burnup node, obtaining a homogenized section of the burnup node according to burnup interpolation, and adjusting the homogenized section in the burnup node according to
axial elongation of the
assembly to obtain a required homogenized section in the computation node.