Verification of 235U Isotopic Abundance in Low Enriched UF6 Cylindersby a Low Resolution Gamma Spectroscopy Technique

摘要

Several codes for nondestructive (NDA) measurements have been developed over the past severalrnyears. These codes utilize peak fitting and peak ratio algorithms to determine relative isotopicrnabundances, activities and mass estimates for gamma emitting isotopes. The use of these codes hasrnsignificantly improved the capabilities of NDA based methodologies to perform quantitativerndetermination of nuclear materials with improved precision and accuracy. Depending upon thernapplication, it is not uncommon to see a larger number of destructive analysis (DA) methodologiesrnbeing replaced by faster and cheaper NDA approaches that are capable of meeting internationalrntarget performance standards for specific applications. A typical example of this evolution is thernaccountancy and control of uranium and plutonium inventories in nuclear facilities for safeguardsrnpurposes. MGAU?, NaIGEM? and PC-FRAM? are examples of various codes that have beenrnsuccessfully applied by national and international organizations in both laboratory and fieldrnMaterial Control and Accountability (MC&A) measurement activities.rnThe verification of 235U isotopic abundance in low enriched UF6 cylinders is a very commonrnmeasurement activity for conversion and fuel fabrication facilities. The measurement conditionsrnI.e., cylinder wall attenuation, high background continuum, energy resolution, etc.) generallyrnrequire use of a verification methodology that incorporates high-resolution gamma spectroscopyrnsystems (HRGS) into the measurement approach. However, the use of HRGS systems is some timesrnproblematic for field inspections because HRGS systems are dependent upon availability of liquidrnnitrogen. This paper presents some studies and field tests conducted by the Nuclear EnergyrnCommission of Brazil (CNEN) involving the verification of several 30B LEU cylinders in a fuelrnfabrication plant over the last 4 years. A low resolution gamma spectroscopy system composed by arnNaI(Tl) crystal, a portable mini-multichannel analyzer (MMCA) and the NaIGEM? code was usedrnto verify the 235U enrichment level of the cylinders. The results of the study are discussed,rndemonstrating how a low-resolution gamma spectroscopy system can provide a reliable, costrneffective, and efficient verification technique that is compliant with international standards andrnmeets the needs of the inspectorate.