IAEAInternational Atomic Energy Agency

Passive Neutron Assay of Uranium Mass Measurements in Scraps and Wastes

V. Nizhnik, M. Pickrell SGTS/TND

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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 CaseUF4 + 2 Mg Umetal + 2 MgF2

Product: Single U metal piece and burnt matrix material; both of known mass

In realityUF4 + 2 Mg X·Uchunks + Y·UF4 + Z·MgF2 + W·MgProduct: Number of U metal chunks, un-burnt 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

Modified Passive MultiplicityAssumptions 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 non-multiplying media due to low density (M = 1)

Alpha value for UF4 is known, based on U isotopic information

Long Crocs

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Modified Passive Multiplicity

M – multiplication in the metal chunks

x – U metal fraction in total U mass in item

mU – total U mass in measured item

U metal term UF4 term

• 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 CounterCounter:

•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.

Good Cycle

Good Cycle

Cycle-by-Cycle Multiplicity Distribution

Bad Cycle

Rejected by RSD based rejection logic

Rejected by Multiplicity Distribution analysis logic

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Analysis with Modified PM

Conventional vs Modified Method ResultsMeasurement 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

UF4 Alpha value a=3.85

Bias of Total U mass in the measured batch

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

Relative content of UF4(1-x)

Theoretical Bias