Consideration of Subchannel Area of a 37-element Fuel to Enhance CHF

Abstract

A CANDU-6 reactor has 380 fuel channels of a pressure tube type which provides an independent flow passage and the fuel bundles rest on horizontally. Most of the aging effects for a CANDU® operating performance are originated from a crept horizontal pressure tube. As the operating years of a CANDU® reactor is proceeded, a pressure tube has been experienced a high neutron irradiation damage under a high temperature and pressure. It is expanded radially as well as axially during its life time. It results in a crept of the pressure tube which allows a by-pass flow on the top section inside of a pressure tube due to a low flow resistance on more open space in its top section than the bottom section. Hence, the crept pressure tube deteriorates the CHF (Critical Heat Flux) of the fuel channel and finally makes the reactor operating performance and thermal margin be worse. Recently, one of the advanced CANDU® fuels which is called as a modified 37-element fuel (M37 Fuel) was suggested by Ontario Power Generation at Canada. The main purpose of the M37 Fuel is to overcome power degradation due to pressure tube creep by enhancing the CHF without any big changes of a fuel bundle configuration except reducing the diameter of the center element. However, the diameter reduction of the center element of a 37-element fuel can make the reactor safety be worsened even small by increasing the coolant void reactivity during LBLOCA because of larger volume of coolant in the fuel channels. Also, the reduction of the power density of the center element due to its small diameter would contribute to redistribute the power density of the remaining 36 elements fuel. If assuming the same power of the fuel bundle or the same reactor core power, the power density of outer elements can be increased because of the power density decrease of the center element. It affects to increase the maximum temperature of the outer fuel elements or increase the fission gas releases which are strong dependent on the fuel temperature or linear element power. The present paper introduces the configuration changes of the standard 37-element fuel such as the ring diameter change of a fuel bundle to enhance CHF without any impact on the reactor safety. And, it will show how much CHF or Dryout power of a fuel channel is increased compared to the standard 37-element fuel bundle.