Passive Neutron Assay of Uranium Mass
Measurements in Scraps and Wastes
V. Nizhnik, M. Pickrell
IAEA
International Atomic Energy Agency
SGTS/TND
Conventional Passive Multiplicity
• Passive multiplicity is based on a theoretical
hierarchy called the Point Model
• So named, because it is a zero dimensional.
• Therefore, spontaneous fission, multiplication,
and alpha-n production are UNIFORM across
the sample.
• Therefore, the sample nuclear material is evenly
distributed in a homogenous matrix.
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The Modification Necessary to
Measure Uranium Wastes
• What happens when the material is not
uniform and evenly distributed?
• The conventional method develops a bias error.
• This work developed a modification of the
Point Model / Passive Multiplicity equations
to analyze non-uniform, inhomogeneous
samples.
• Applied to uranium wastes with metal ingots
mixed with diffuse oxide powder.
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Passive Multiplicity (PM) Assumptions Kill this
slide
• Homogeneous distribution of nuclear material through an
item matrix material
• The same chemical form of Uranium material in the item, i.e.
uniform (a,n)-neutron production rate in the item volume
• Uniform physical property of uranium material (mostly
concerns material density)
• If item properties are in compliance with listed statement:
• all the neutrons are equal in the term of multiplication
• Alpha parameter is the same for any point of item material
Then Passive Multiplicity Analysis can be applied for precise U
mass analysis
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Wastes: “Close Look” Example
Uranium Metal Production Process
Ideal Case
UF4 + 2 Mg  Umetal + 2 MgF2
In reality
UF4 + 2 Mg 
Product: Single U metal piece and
burnt matrix material; both of known
mass
X·Uchunks + Y·UF4 + Z·MgF2 + W·Mg
Product: Number of U metal chunks, unburnt UF4 in un-burnt Mg+MgF2 matrix.
Their mas ratios are unknown.
Impact on Neutron Multiplicity:
•
•
•
•
•
Non-homogeneous distribution of U material
Multi-component chemical composition
Unknown mass ratio for uranium material components
Non-uniform (a,n)-neutron production rate
Spatial dependent neutron multiplication
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Passive Multiplicity
Conventional Passive Multiplicity Equations
Long
Crocs
Modified Passive Multiplicity
Assumptions and known parameters:
Assign:
x
–U metal fraction in total U mass in item
Then:
(1-x) – fraction of UF4 in total U mass in item
U metal chunks are pure: no (a,n)-neutrons production (a = 0)
Dispersed un-burnt UF4 material in Mg+MgF2 matrix is nonmultiplying media due to low density (M = 1)
Alpha value for UF4 is known, based on U isotopic information
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Modified Passive Multiplicity
U metal term
M–
x–
mU –
UF4 term
multiplication in the metal chunks
U metal fraction in total U mass in item
total U mass in measured item
• Corrects for non-uniform distribution of multi-component Uranium
material
• Determines total Uranium mass in measured item
• Determines U mass in metal and UF4 form
• Determines neutron Multiplication in metal phase (gives information on
average metal chunks size)
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Example of U Waste Measurements
Plutonium Scrap
Multiplicity Counter
Counter:
• Efficiency 55%: allows fast acquisition of
neutron multiplicity data with good statistics
• Size of the measurement cavity: d=20cm,
h=42cm
Items:
• 34 Uranium waste items with various Gross
Weight (up-to 30kg)
• Packed in Poly bags to fit in the measurement
cavity
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Target total uncertainty:
• Not more than 2.3% for 1 Sigma confidential
interval
Rejection of Cosmic Rays Bursts
Cosmic rays prompt neutron “bursts” when interacting with High-Z
(Uranium) material nuclei; they produce high multiplicities of detected
neutron and affect counting rates.
Cycle-by-Cycle Multiplicity Distribution
Good
Cycle
Rejected by RSD based
rejection logic
35
30
25
20
Bad
Cycle
15
10
5
Good
Cycle
0
1
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11
21
31
Rejected by Multiplicity
Distribution analysis logic
41
51
61
71
81
Analysis with Modified PM
Conventional vs Modified Method Results
Measurement of a batch of uranium waste items
(individual item U mass and Total U mass)
Theoretical bias of Conventional vs
Modified PM for NU
U metal average multiplication:
M=1.05
Relative
UF4 Alpha value a=3.85
content of
UF4(1-x)
Bias of Total U mass in the
measured batch
Theoretical
Bias
The bias is higher in case of analysis of enriched Uranium since values of Multiplication
in metal phase and Alpha in UF4 phase are higher then for natural Uranium
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