Principles of U and Pu composition measurements by MEST

1. Principles of U and Pu composition measurements by MEST

2. Outline • Characteristics of Uranium and Plutonium Isotopes • Principle techniques to determine the Uranium composition (enrichment) • Assay if Uranium is reprocessed • Principle techniques to determine the Plutonium composition (burn up grade) • Am-241 as Plutonium daughter product

3. Characteristics of Uranium and Plutonium Isotopes

4. Categories of Uranium Isotopes Natural Uranium U-234 Trace U-235 0.720% U-238 99.27% Low-Enriched Uranium (LEU) 0.720% < E < 20% High-Enriched Uranium (HEU) E > 20% Weapon grade > 90% Enriched Uranium (commonly used) U-234 Trace U-235 3% to >90% U-238 <10% to 97% Depleted Uranium (example) U-234 Trace U-235 0.20% U-238 99.8%

5. Composition of a Plutonium Sample • In a reactor, Plutonium isotopes accumulate with increasing burn up • Pu sample contains: • 5 important Pu • Np-237 • Am-241 (intensive gamma peak)

6. Composition of a Plutonium Sample • Typical Plutonium isotopic composition as a function of burn up • Common used definitions (not IAEA): • Weapons-Grade < 7% 240Pu or >93% 239Pu • Reactor-Grade >7% 240Pu or <93% 239Pu

7. Identification of U & Pu by MEST equipment • MEST equipment • IdentiFINDER with neutron detection capability • Portable HPGe detector U & Pu radiation Alpha & Beta Gamma Neutron

8. Uranium identification by FLOs • FLOs using the identiFINDER in easy mode • During the primary detection and identification of Uranium, the enrichment grade is totally unclear • DU, NU, LEU, HEU are displayed as Uranium • Reprocessed Uranium? • More details could be seen by • IdentiFINDER in expert-mode • HPGe measurements • Manual evaluation of the gamma spectrum • Multi Group Analysis

9. IdentiFINDER in expert mode

10. Manual evaluation of the gamma spectrum: • Peak-Ratio Techniques: • Significant peaks for Uranium and their daughter products (in equilibrium) • U-235: 186, 140 keV • U-238: 1001, 766 keV • Possible ratios: • Ratio 186 keV to 766 and1001 keV • Ratio 186 keV to 98 keV (U X-ray)

11. Manual evaluation of the gamma spectrum:

12. HPGe spectrum 90-200 keV OPEN: legend, x-axis, peak positions

13. NaI(Tl) spectrum 90-200 keV for Low Enriched U

14. NaI(Tl) spectrum 90-200 keV for High Enriched U

15. Multi Group Analyzis - Uranium • Evaluation of the spectrum within the energy range 90-100 keV is called Multi Group Analysis • Lines of U-238, U-235 and U X-rays • Measurement with specific detectors and detector setup: • Planar HPGe detector • Resolution < 630 keV for Co-57 • Gain: 75 eV per channel • Evaluation with specific software: • MGAU or MGA++

16. MGAU HEU measurement DU measurement

17. Reprocessed uranium • Reprocessed Uranium has additional U-232 • U-232 is not showing significant lines in the spectrum, but the decay chain is similar to Th-232 decay chain (both have Th-228 as daughter + Th-228 decay chain) • Presence of Thorium-daughter products is first indication for reprocessed Uranium • Peaks at 911, 969 and 338 keV are significant for Ac-228 as part of the Th-232,but not for the U-232 decay chain • Missing Ac-228 peaks in Uranium measurements with ‘Thorium’  reprocessed Uranium

18. Reprocessed Uranium

19. Reprocessed Uranium HEU with Thorium daughter products 911 keV 969 keV Th-232 with daughter products

20. Plutonium identification by FLOs • FLOs using the identiFINDER in easy mode only • The burn-up grade of Plutonium is not displayed • Plutonium is a mixture of Am-241 and Plutonium isotopes  Am-241 identification shown as Am(Pu?) • Easy mode identification results could be: Plutonium, Am(Pu?) and neutron alarm • More details could be seen by • IdentiFINDER in expert-mode • HPGe measurements • Manual evaluation of the gamma spectrum • Multi Group Analysis

21. IdentiFINDER in expert mode

22. Spectrum evaluation • Complex spectrum, with high density of gamma lines • Peak ratio evaluation complicated, due to overlapping lines of different isotopes • Pu-240 line without overlaps from other isotopes is missing • Some peak ratio methods working fine for low burn-up but fail for high burn-up and vice versa.

23. Typical Pu spectrum with HPGe

24. Typical Pu spectrum with HPGe

25. Multi Group Analysis - Plutonium • Evaluation of the spectrum within the energy range 93-104 keV is called Multi Group Analysis • Lines of Plutonium isotopes, Np-237, Am-241 and U-237 • Measurement with specific detectors and detector setup: • Planar HPGe detector • Resolution < 630 keV for Co-57 • Gain: 75 eV or 250 eV per channel • Evaluation with specific software: • MGA low, MGA high or MGA++

26. MGA User Interface

27. MGA - evaluation

28. MGA result list

29. Am-241 (Plutonium?) • Am-241 in Plutonium sample • Pu-241 (HL=14.4y) decays to Am-241 (HL=433y)) • The older the Pu sample the more accumulated Am-241 • IdentiFINDER’s Template method sees dominant Am-241 signature but not the Pu any more  each Am-241 and unshielded (filtered) Pu identification is displayed as Am (Pu?) • Manual evaluation of the spectrum necessary: no significant lines for Am-241 at energies > 125 keV • Use software with peak search algorithm, or • Display the Am(Pu?) spectrum in logarithmic scale

30. Am-241 (Plutonium?) results Pu in logarithmic scale: 150 – 750 keV range with peaks Am-241 and Pu in linear scale look similar Am-241 in logarithmic scale: 150 – 750 keV range without peaks

31. Summary • Manual evaluation of Uranium composition possible (for unshielded or low shielded samples) • Determination if Uranium is reprocessed possible • Manual evaluation of the Plutonium composition difficult, but identiFINDER can already distinguish between RGPu and WGPu • Determination if Am(Pu?) is Am-241 or Pu mixed with Am-241 is possible • Programs for the evaluation of the Pu and U composition exist (for unshielded or low shielded samples)