HTGR U-235 MEASUREMENT CONTROL

Uranium-235 measurement control for high-temperature gas-cooled reactor (HTGR) fuel manufacture can involve more than 20 categories of material with special nuclear material (SNM) content, which may be on hand at any given inventory. Each of these categories has its own random error contribution and utilizes 10 or more measurement techniques, each with its own systematic error and bias. Three of these techniques, which are major contributors to the overall SNM accountability in HTGR fuel production, are discussed in this paper: 1. Davies-Gray titration for the assay of final, TRISO-coated (U,Th)C2 particles. 2. Nondestructive assay of in-process particles by active neutron interrogation using a random driver. 3. Passive gamma-ray assay of scrap and waste material using a segmented gamma scanner. Secondary reference standards, with values traceable to the National Bureau of Standards (NBS) and in forms representative of the material being assayed, are measured at predetermined frequencies to obtain the measurement error components for each measurement technique. These error components are obtained as random, short-term systematic, and long-term systematic errors (including a calibration error) for each of the three measurement methods and are applied to the physical inventory to obtain the limit of error (LE) for an inventory difference (ID).