Fuel assembly for FBR type reactor

In the center of a reactor core liable to cause Bundle-Duct-Interaction (BDI), the face-to-face distance at the inside of the wrapper tube is made greater than that of other portions, and material softer than other portions is lined to that portion. With such a constitution, when BDI occurs due to the expansion of fuel cans and wire spacers, a contact pressure of about 10 kgf/mm2 is caused between the inner surface of the wrapper tube and the spacer wire at the outer circumference of the fuel element bundle. As a result, since the soft material appended to the inside of the wrapper tube is abrased to increase the face-to-face distance at the inside of the wrapper tube to reduce BDI. Accordingly, the time to reach the BDI limit value is delayed, to extend the life time of fuel assemblies. (T.M.)