The effect of inner ring modification of standard 37-element fuel on CHF enhancement

• A standard 37-element fuel bundle for a CANDU reactor was considered to enhance the CHF.
• Modification of the inner ring radius is suggested to enhance the CHF of a standard 37-element fuel bundle.
• The subchannel analysis technique was applied to investigate the local CHF characteristics.
• The modification of the inner ring radius is very effective in enhancing the dryout power of a fuel bundle.

A CANDU-6 reactor has 380 fuel channels of a pressure tube type, which provides an independent flow passage, and each pressure tube contains twelve fuel bundles horizontally. The CHF of a CANDU fuel bundle in a horizontal fuel channel is one of the important parameters determining the thermalhydraulic safety margin as well as the trip set point of the Regional Overpower Protection (ROP) system. Hence, the CHF enhancement of a CANDU fuel bundle has been an issue for a long time and can be affected by the geometric configuration of the fuel elements as well as several appendages such as the end-plates, bearing pads, and spacers attached to the fuel elements. It is known that the first CHFs of a standard 37-element fuel bundle preferably occur at the peripheral subchannels of the center rod, owing to the relative small flow area or high flow resistance under high flow conditions or the normal operating conditions of a CANDU reactor.An increase of the inner ring radius of a standard 37-element fuel bundle without any changes in the fuel bundle or element sizes is introduced to enhance the CHF in consideration of the CHF characteristics of a standard 37-element fuel bundle for a CANDU reactor. Subchannel analysis techniques using the ASSERT-PV code were applied to investigate the local CHF characteristics according to the inner ring radius variation for the original diameter of the pressure tube. It was found that the modification of the inner ring radius is very effective in enhancing the dryout power of the fuel bundle through an enthalpy re-distribution of the subchannels and change in the local locations of the first CHF occurrences.