Passive Neutron Assay of Uranium Mass Measurements in Scraps and Wastes

V. Nizhnik, M. Pickrell SGTS/TND

IAEA International Atomic Energy Agency

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 UF 4 + 2 Mg

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U metal + 2 MgF 2 Product: Single U metal piece and burnt matrix material; both of known mass In reality UF 4 + 2 Mg

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X · U chunks + Y · UF 4 + Z · MgF 2 + W · Mg Product: Number of U metal chunks, un burnt UF 4 in un-burnt Mg+MgF 2 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 UF 4 in total U mass in item U metal chunks are pure: no (a,n)-neutrons production (a = 0) Dispersed un-burnt UF 4 material in Mg+MgF multiplying media due to low density (M = 1) 2 matrix is non Alpha value for UF 4 is known, based on U isotopic information

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Modified Passive Multiplicity

M – x – m U – • • • • U metal term multiplication in the metal chunks U metal fraction in total U mass in item total U mass in measured item UF 4 term Corrects for non-uniform distribution of multi-component Uranium material Determines total Uranium mass in measured item Determines U mass in metal and UF 4 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 • Target total uncertainty: Not more than 2.3% for 1 Sigma confidential interval

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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

35 30 25

Bad Cycle Good Cycle IAEA

Rejected by RSD based rejection logic 20 15 10 5 0 1 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

UF 4 Alpha value a=3.85

Relative content of UF 4 (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 UF 4 phase are higher then for natural Uranium

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