A Reactor Physics Study of the Economic Penalty Associated with L VRF and Increased Pressure Tube Thickness

Abstract

Both the pressure tube (P/T) life and coolant void reactivity are essential factors in the design of CANDU reactors. In this paper, we report on a reactor physics study of the economic penalty associated with solutions related to these factors, i.e. increased pressure tube thickness and the use of burnable poisons. Natural uranium (NU), 0.9% and 1.2% slightly enriched uranium (SEU), and mixed-oxide (MOX) fuels were compared for an advanced 43-element CANFLEX geometry bundle design. Our calculations show that the burnup penalty associated with a 25% increase in P/T thickness for NU is of the order of 14%, which represents well over one million dollars every year in a CANDU 6 reactor. On the other hand, the same increase in tube thickness for a CANDU 6 fuelled with 1.2 w/o SEU fuel (using a 2 bundle-shift) yields a penalty of only 3.1 %, or a few 100k$'s per year. This very large reduction in the burnup penalty for the same pressure tube thickness increase illustrates the fact that slightly enriched fuel in CANDU is superior to natural uranium in terms of neutron economy. Another significant advantage of using enriched fuel in CANDU is the possible development of Low Void-Reactivity Fuel (LVRF). Previous studies have shown that void reactivity in a CANDU reactor can be reduced or even eliminated by adding an appropriate amount of neutron poisons mixed with depleted uranium in the inner elements of the CANDU fuel bundles. However, this can only be achieved with enriched fuel. In order to estimate the cost associated with the introduction of LVRF in CANDU, we compared the effect on void reactivity and discharge burnup of various quantities of a burnable poison (Gd) and a of a more permanent poison (Dy). The burnup penalty was defined relative to the performance of unpoisoned fuel with the same fissile content. We found a burnup penalty of approximately 1000 MWD/t(U) for each mk reduction in core void reactivity, regardless of the type of poison used in the design. All the calculations in this study were carried out by the DRAGON/DONJON chain of codes with Winfrith 69 groups library.