Two methods are proposed for determining the effective radius of triple junctions of grain boundaries based on the analysis of the self-diffusion coefficient distribution in the junction region and the potential energy distribution of atoms. Both methods are considered using the example of triple junctions of high-angle tilt boundaries with and misorientation axes in nickel with varying contents of excess free volume. Triple junctions in the model were created by simulating the crystallization of a melt from three differently oriented crystalline seeds. Excess free volume was introduced into the triple junction by removing the corresponding number of atoms from the initial computational cell. During crystallization, the introduced free volume concentrated predominantly in the grain boundaries and triple junction regions, with the free volume fraction increasing toward the junction site. With increasing excess free volume in the triple junctions, self-diffusion proceeded more intensively. The diffusion intensity within the triple junction itself was generally distributed non-uniformly and increased toward the junction center. It is shown that the effective radius of the triple junction, determined using both methods based on the diffusion coefficient and atomic energy distributions, increases with the free volume content.