Measurements of UF$sub 6$ cylinders with portable instruments Page: 4 of 39
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available space, and ease of placement. It was anticipated that the L/2
measurement would provide a measure of the total mass of 235U according
to the expression
5 = MUF6 [a + I235J MUF6 [a + b 1235] (2)
MUF6 = mass of UF6
r = 235U/234U ratio
a, c = constants
b = constant if r is fixed.
Not only was good correlation achieved for the L/2 geometry, but the con-
tact measurements also gave very good results using the same relation-
ship. Because of the larger uncertainty resulting from the evaluation of
the background from nearby cylinders for the L/2 measurement, it was
abandoned--in favor of the faster contact measurements--after about 40
cylinders were measured. Nevertheless, the L/2 measurement results
indicate that the L/2 geometry is very favorable for assay if cylinders are
well isolated from each other.
In order to better understand the passive neutron assay
method, and, in particular, the fortuitous contact measurement results
several calculations were performed with the Monte Carlo code, MCN.
Since the 19F(a., n) thick target energy spectrum is not well known, two
source neutron spectra were used, one with single energy neutrons at 1 MeV
and the other a fission spectrum. Figure 5 shows the neutron leakage spec-
trum from a 5000 lb cylinder (3%) assuming a uniform spatial distribution
of 1 MeV neutrons, and the corresponding results for the fission spectrum
source are shown in Fig. 6. The neutron moderation by the tF6 is quite
marked for both source spectra; thus, it is apparent that a 2j-ton cylinder
is a distributed source of neutrons with energies pArmarily in the range
100 eV -- 100 keV.
The neutron leakage factor, defined as the fraction of the source
neutrons which escape from the cylinder, was also calculated as a function
of weight and enrichment of UF6 in 21-ton cylinders. For a fixed weight
of 5000 lb, the leakage factor changes from about 0. 80 to 0. 84 as the enrich-
ment varies from 1% to 4%. The leakage factor is a stronger function
of cylinder weight, varying from about 0. 93 to 0. 82 as the weight of 2j%
enriched UF6 is increased from 2000 to 5000 lb. The overall effect of weight
variation on the passive neutron assay is reduced because the neutron leakage
spectrum is harder for lower weights and the detection efficiency decreases
with increasing neutron energy.
The spatial distribution of the neutron leakage was also calcu-
lated to determine whether the average of the 4 and 8 o'clock contact meas-
urements (cf. Fig. 4) is a direct measure of UF6 weight. The neutron
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Walton, R.B.; Reilly, T.D.; Parker, J.L.; Menzel, J.H.; Marshall, E.D. & Fields, L.W. Measurements of UF$sub 6$ cylinders with portable instruments, report, January 1, 1972; United States. (digital.library.unt.edu/ark:/67531/metadc1032400/m1/4/: accessed September 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.