Gap Measurement Systems for Nuclear Fuel Rod Assemblies

Manufacturers of fuel rod assemblies for Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR) are required to measure the gaps between each of the individual fuel rod rows in a number of locations over the height of the fuel rod assembly.

Starting in the mid 1980's Capacitec began the process of replacing Strain Gauge contact type measurement sensor wands with one back-to-back pair of non-contact capacitive gap sensors installed onto a flexible wand design. These flexible wands were able to function in a background radiation environments up to 10⁷ RADS accumulated dosage while still providing an overall accuracy of better than 10 microns.

The gap between individual fuel rods in a nuclear fuel rod assembly is critical to the safe and efficient functioning of a nuclear power plant. Fuel rod assemblies are typically manufactured with  up to 17 x 17 rows of 10 mm diameter fuel rods measuring 6 meters in height. The gaps between fuel rods typically run from 1 to 3 mm.

Capacitec supplies non-contact gap measurement sabers with overall size of 0.9 mm thick by 350 mm long, and 20 mm wide. Each saber contains dual non-contact displacement sensors located near the tip. The sensors' individual gap outputs are summed along with the fixed thickness of the saber to provide total air gap. An automated system horizontally advances and retracts the sabers 300mm between each fuel rod row at approximately 1 meter vertical height intervals. The dual capacitance sensors take highly precise position compensated gap measurements between rods pairs with an accuracy of 0.25% of full scale or better.