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Non-destructive techniques for the assay of nuclear materials

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### Non-destructive techniques for the assay of nuclear materials

Techniques developed at the Institute of Isotopesof the Hungarian Academy of Sciences

J. Zsigrai, N. C. Tam, L. Lakosi, J. Bagi

Institute of Isotopes, Budapest, Hungary

Overview materials

Novel methods for safeguardsand combating illicit trafficking

- Determining the matrix of Uranium samples using HRGS
- Gamma-spectrometric Uranium age dating
- Quantitative assay of PuBe neutron sources
- using HRGS
- using neutron counting
A routine method

- Portable Spent Fuel Attribute Tester using MRGS
Summary

Relevant properties of nuclear material materials

1. Elemental and isotopic composition

235U, 238U, 239Pu

categorization

nuclear process

232U, 236U

2. Total NM content (mass)

3. Matrix

4. “Age” of the sample

All these properties give indications about the origin of an unknown NM.

Determining the matrix of materialsUranium samples

Based on gamma-spectrometric determination of U-mass

Measuring the 1MeV gamma-line of 234mPa (daughter of 238U)

- correcting accurately for self-absorption

- [corrected intensity at 1MeV] ~ [mass of 238U]

- mass of 238U + enrichment

total U mass

- calibrating the system with a reference U-set

Homogeneous, pellet- or powder-form material:

accuracy <2%

1.0 materials

0.9

0.8

0.7

0,5613

U-

acetate

0.6

U-nitrate 0,474

0.5

0.4

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Determining the matrix of Uranium samplesWhat is the matrix?

- based on the accuracy of the Uranium-mass measurement

UO

0,881

2

mU/mtotal

U

0,848

O

3

8

Measurement number

Fig. 1. (U-mass)/(Total mass) ratios for different Uranium compounds

Uranium age dating using HRGS materials

“Age” : time since last chemical separation / enrichment

initially no daughters of U are present in the sample

First use of a gamma-spectrometricmethod for uranium age datingrelevant to safeguards and combating illicit trafficking.

- Advantages of gamma-spectrometry:
- non-destructive
- no special sample preparation needed
- relatively simple equipment

Based on the ratio of the activity of 226Ra (214Bi) to 234U

234 materialsU

230Th

226Ra

222Rd

218Po

214Pb

214Bi

Uranium age dating using HRGS- Measuring the activity of 214Bi
- By a coaxial Ge detector
- In a low-background iron chamber
- 609keV line of 214Bi
- Relative to 238U (intrinsic efficiency-calibration)

- Radon: not a real problem
- further studies to improve precision

Uranium age dating using HRGS materials

- Measuring the activity of 234U
- By a planar LEGe detector
- Standard methods are not satisfactory
- MGAU, U235
- large systematic errors, especially for smaller amounts of NM

- Our method: variant of intrinsic efficiency-calibration
- Using: 121keV of 234U 143keV, 163kev, 186keV, 205keV of 235U

Uranium age dating using HRGS materials

“Age”

- Age of a HEU (~90%) sample (inter-comparison exercise, 2001)

Uranium age dating using HRGS materials

Sensitivity of the present equipment

Detector efficiency

at 609keV ~ 0.5%

- difficulties with LEU samples

Calculated intensity of the 609keV line of 214Bi in our 150cm3 coaxialdetector, coming from 10g of UO2 pellets of various enrichments

Quantitative Assay of Plutonium-Beryllium Neutron Sources materials

- Large number (~200) of PuBe neutron sources leftfrom industrial applications
- The Pu-content has to be
- accounted for, reported to and inspected by IAEA

- Unreliable data: declared neutron outputundeclared Pu-content

- Determining the Pu content: two independent methods
- Gamma-spectrometry
- Neutron coincidence counting

Quantitative Assay of Plutonium-Beryllium Neutron Sources materials

1. Gamma-spectrometry

0 - 600 keV

0 – 5 MeV

Gamma-spectra of a PuBe source

Quantitative Assay of Plutonium-Beryllium Neutron Sources materials

1. Gamma-spectrometry

- Isotopic composition
- Intrinsic calibration method (MGA code)
- Insensitive to the neutron-induced background

- Total Pu-content
- Evaluating 375keV and 413keV peaks of 239Pu
- absorption correction method
- accuracy: ~ 5% in favorable case ~ 15% in worst case

- Evaluating 375keV and 413keV peaks of 239Pu
- + 129keV: effective geometry of the source
- Sufficient for safeguards purposes

Quantitative Assay of Plutonium-Beryllium Neutron Sources materials

Results of the gamma-spectrometric measurements

“Nominal”: from the declared neutron yield, assuming pure 239Pu (basis of present bookkeeping)

Neutron detectors materials

(3He tubes)

PuBe sampleholder

Quantitative Assay of Plutonium-Beryllium Neutron Sources2. Neutron coincidence counting

- 14 3He tubes around the source
- home-made electronics, commercial shift register JSR11

Moderator

Coincidence neutrons from

- n-induced fission of Pu
- 9Be(n, 2n)8Be reaction
- spontaneous fissionof Pu (negligible)

Quantitative Assay of Plutonium-Beryllium Neutron Sources materials

2. Neutron coincidence counting

- Total Pu-content:
- Calibration is required (coincidence counts)/(total counts) Pu-mass

- Isotopic ratios of 239Pu and 240Pu
(Total counts) = T =

= T(R/T,239Pu/Pu,240Pu/Pu)

Fig.: Pu-mass as a functionof the ratio of coincidencecounts (R) to total counts (T)

Portable Spent Fuel Attribute Tester materials

In the spent fuel pond of Paks NPP

Cable to

MMCA 166

Steel wire

- Detection of unreported irradiation when CVD is not usable
- Assistance to IAEA inspectors
- Attributes for SF:Cs-134, Cs-137,Zr-Nb-95, Ce-Pr-144

Container with

CdZnTe detector

Modular collimator tubefilled with air

?

Assemblies

(spent fuel, Co-containers,

absorbers)

Summary materials

Novel methods

- Determining the matrix of Uranium samples using HRGS
- Based on high-precision measurement of total U-content

- Uranium age dating using HRGS
- The first use of a gamma-spectrometric method

- Quantitative assay of PuBe neutron sources using
- HRGS
- neutron counting
A standard method

- Portable Spent Fuel Attribute Tester using MRGS
- Hungarian implementation of a widely used concept
- Routinely used at the nuclear power plant Paks

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