Traceable determination of the absolute neutron emission yields of UO<inf>2</inf>F<inf>2</inf> working reference materials

Abstract

The nuclear material contained in the process equipment of a uranium enrichment plant (referred to as holdup) is an important component of the overall nuclear material inventory for the plant. Accurate quantification and verification of holdup is needed to improve international safeguards and nuclear material accountancy. This is also needed for criticality safety and waste disposition. Passive neutron and gamma-ray nondestructive assay (NDA) methods are used to measure the holdup in process equipment. A key advantage of neutron measurements is that neutrons are highly penetrating and can be measured through thick walled equipment. The dominant source of neutrons in the UO2F2 holdup is from the 19 F(α,n) 22 Na reaction resulting from 234 U alpha decay when uranium is enriched. There is a considerable spread between different historic determinations of the 19 F(α,n) yield from uranium which limits the accuracy of modeling and the calibration of NDA instruments. Furthermore, the compound form and presence of water also significantly affects the neutron emission rate from the holdup. This paper describes a series of experimental measurements performed at Los Alamos National Laboratory (LANL) to determine the absolute neutron emission yield from 10 different UO2F2 working reference materials (WRMs) fabricated at the Portsmouth Gaseous Diffusion Plant (PGDP). The Mini Epithermal Neutron Multiplicity Counter (Mini ENMC) and a NIST certified 252 Cf neutron source were used for these measurements. The high efficiency and short die-away time of the Mini ENMC provides the high measurement precision needed to certify the neutron emission yield. The experiment was designed to achieve sub 1% accuracy in the net counting rate on each item and to provide assurance that important factors such as instrument stability, item placement and background were well understood. The traceable neutron yields measured from the WRMs were used to determine a more accurate neutron yield for the UO2F2 material. The results were compared to historical neutron emission rates. The F(α,n) data obtained from these A. M. LaFleur is with Los

Neutron triples counting data for uranium

“…The singles count rate is dominated by the yield of F(α,n) reactions and has been discussed in detail elsewhere [9]. Consequently we cannot use the singles rate to directly quantify 238 U for these UO 2 F 2 samples.…”

“…Ten tubes of well characterized UO 2 F 2 were studied [9]. Each aluminum tube was 2.54 cm in outer diameter, 0.124 cm in wall thickness, and filled to about 21 cm with 230 g of anhydrous powder.…”

“…The right-hand products of ‫ܯ‬ raised to a power and the square brackets represent the enhancement in the detected correlated neutron rates due to self-multiplication. Based on previous work [9] we estimate for our UO 2 F 2 samples the average ߙ value to be to be about 7.2 with a range of ±0.2, and ‫ܯ‬ to be about 1.0027 with a range of <0.0002 and independent of neutron source reaction with roughly 20% of the induced fission taking place in 235 U and the rest in 238 U. For fast (1 MeV) fission we estimate, using the tables of Zucker and Holden [17], the values of ߭ ூଵ , ߭ ூଶ , and ߭ ூଷ to be about 2.449, 4.937, and 8.112 respectively.…”

“…The detection efficiency for fission neutrons varies with mean energy and over the range of interest here (1.9 MeV for 238 U SF to 2 MeV for induced fission) the range is 0.636 to 0.628 with an absolute relative standard deviation of about 1.3%. Detailed sensitivity calculations have been undertaken using Monet Carlo methods [9]. The mid-range value is adopted with half the spread being added to the uncertainty, namely (0.632±0.012) counts per neutron.…”