Operation of existing 220 MWe PHWR at reduced power level

Reactor operates in the equilibrium core configuration for most of its life time. In equilibrium core configuration, the rate of refueling remains almost constant and the characteristics of the core remain unchanged with time. Thus in the design stage itself, the ideal equilibrium burnup distribution is finalized for a specified bundle shift scheme and the channel power distribution obtained is used for providing the coolant flow along the channels. In the middle of the year 2003, there was a proposal to operate the reactors at reduced power like 70 % FP or 90 % FP to meet the reduced requirement of the grid. Studies pertaining to equilibrium core have been carried out for 70 % FP and 90 % FP operation. For low power operation the degree of flux flattening can be reduced, there by increasing the average discharge burnup. Reduced xenon load and reduced fuel temperature also contribute to the improvement of average discharge burnup. Thus the operation at low power level will result in saving of natural uranium fuel. For equilibrium core when reactor operates at 100 % FP, the refueling rate is 8.86 natural uranium (NU) bundles per FPD (full power days). A study has been carried out at low power with new equilibrium burnup distribution, which shows the refueling rate becomes 6.55 and 4.50 NU bundles per FPD at 90 % FP and 70 % FP respectively. The analysis shows that by reducing the power level from 100 % FP to 90 % FP around 26 % NU bundles can be saved, and 49.2 % NU bundle savings is achieved by reducing the power level from 100 % FP to 70 % FP. (author)