WASTE CRATE ASSAY SYSTEM (WCAS): ASSAY SOLUTIONS FOR VERY LARGE REMOTE HANDLED CRATES Page: 4 of 16
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between fission and cosmic-ray events. This can be accomplished through truncated multiplicity (TM)
Because there were no plutonium standards at the Albuquerque fabrication site, the initial
calibration of the WCAS-A was accomplished using a 252Cf sample of known mass and relating its
counting rate to a plutonium sample. Calibration is performed in an empty container where the
calibration is in terms of doubles counts per second versus 240Pueff mass. The corrected doubles rate in an
actual waste container is the measured coincidence rate multiplied by the AAS-derived correction factor.
2. PLUTONIUM-TO-CURIUM RATIO METHOD
For the high gamma-dose samples, we measure the associated 244Cm neutrons to calculate the
plutonium based on the stream-averaged ratio of curium to plutonium supplied by the operator. The
waste generation is from sites that have known Cm/Pu ratios. Table I gives the neutron yields from 244Cm
and Pu for both spontaneous fission (SF) and (a,n) reactions in oxides. Prior to the first separation stage,
the ratio of curium to plutonium is about 0.003. However, the SF emission rate from 244Cm is about 3.2 x
104 times that of plutonium (with 24% 240Pu). Therefore the emission from 244Cm dominates that from
plutonium by a factor of ~100. This factor increases to about 6000 in the waste after plutonium
Table I. Neutron Emission Rates
SF Emission Oxide (a,n)
Half Life SF Rate Rate Rate
Isotope (years) (fissions/seg) (n/seg) (n/seg)
238U 4.47 x 109 0.00677 0.0136 8.26 x 10-5
240Pu 6.55 x 103 472.2 1020 1.41 x 102
244Cm 1.81 x 101 4.02 x 106 1.08 x 107 7.73 x 104
We see from Table I that the (a,n) source neutrons from 244Cm are only a small fraction (~1%) of
the SF neutrons, so both the singles and doubles rates are directly proportional to the 244Cm content. The
244Cm spontaneous fissions dominate all other sources of neutrons, and the neutron multiplication is
negligible so the 244Cm mass can be determined from this simple singles (S) equation:
S = s mcm Y (1 + a),
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MENLOVE, H. O.; RINARD, P. M. & AL, ET. WASTE CRATE ASSAY SYSTEM (WCAS): ASSAY SOLUTIONS FOR VERY LARGE REMOTE HANDLED CRATES, article, October 1, 2001; New Mexico. (digital.library.unt.edu/ark:/67531/metadc719682/m1/4/: accessed September 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.