The impact of changes in the pressure tube thickness on the expected discharged burnup in CANDU reactors

In Canada Deuterium Uranium or CANDU reactor design, the fuel bundles are placed horizontally inside fuel channels. Each of these fuel channels is comprised of concentric tubes called the pressure tube (PT) and the calandria tube (CT). As the reactor ages, due to continuous exposure to high neutron field and extreme thermal hydraulic conditions, the pressure tubes undergo geometrical changes such as diametral creep, sag, and axial elongation. These changes have an unfavorable impact on the operation of the reactor where eventually the overall reactor power output must be reduced in order to maintain the same safety margin as the new reactor. One of the options to mitigate this challenge is to utilize thicker pressure tubes. The impact of this proposed change from the neutron economy point-of-view, in terms of its impact on the expected discharged burnup value, is evaluated in this paper. Based on assessment performed at both the lattice and full-core levels, it is concluded that a percent increase in the thickness of the pressure tube leads to an approximately one-half-of-a-percent decrease in the expected discharged burnup value. (author)