Mulberry (uranium alloy)

Mulberry is a uranium alloy.

It is used as a non-corroding[1] or 'stainless'[2] uranium alloy.[3] It has been put forward as a structural material for the casings of the physics package in nuclear weapons, including those of North Korea.[4]

The composition is a ternary alloy,[5][6] of 7.5% niobium, 2.5% zirconium, 90% uranium.[3]

Mulberry was developed in the 1960s at UCRL.[6][7] Binary alloy compositions were first studied to avoid the mechanical problems of pure uranium: corrosion, dimensional instability, inability to improve its mechanical properties by heat treatment.[8] Uranium-molybdenum alloys were found susceptible to stress-corrosion cracking, uranium-niobium alloys to be weak, and uranium-zirconium alloys to be brittle.[8] Ternary alloys were next studied to try to avoid these drawbacks. Uranium-niobium-zirconium was found to be corrosion resistant and to permit age hardening, which could increase its hardness from 110 to 270 ksi.[8][9]

Multiple crystal phases were observed, with a critical temperature of 650°C. Above this the body-centered cubic γ phase was stable. Water quenching to room temperature produces a γs transition phase and with aging this transforms to a tetragonal γo phase. Further aging produces a monoclinic ɑ″ phase that is observed metallographically as a Widmanstätten pattern.[10][11] The crystal structure of the alloy has been studied, particularly the γ phase.[6][7][12][13] Uranium inclusions have been observed within the alloy although, unlike the binary alloys, niobium-rich inclusions were not.[14] Early studies were uncertain as to whether these were inherent behaviours, or artifacts of their processing.


  1. ^ Larson, D.T. (1971). "Oxidation of a Ternary Uranium Alloy". Journal of Vacuum Science and Technology. 8 (1): 80. Bibcode:1971JVST....8...80L. doi:10.1116/1.1316361.
  2. ^ Henry, Charles R. (February 26, 1965). "Plutonium and Uranium as Engineering Materials". UCRL: 7–8. AEC Contract No. W-7405-eng-48. {{cite journal}}: Cite journal requires |journal= (help)
  3. ^ a b "North Korea bargains with nuclear diplomacy" (PDF). Jane's. p. 10.
  4. ^ Jane's, North Korea, p. 9.
  5. ^ Williams, R.O. (June 1979). "Stability of the body-centered cubic gamma phase in the uranium-zirconium-niobium system". J. Nucl. Mater. 82 (1): 184–192. Bibcode:1979JNuM...82..184W. doi:10.1016/0022-3115(79)90052-7.
  6. ^ a b c Vandervoort, R.R.; Peterson, C.A.W. (1964). "Mechanical Properties of Some Uranium Alloys". UCRL. UCRL-7771. {{cite journal}}: Cite journal requires |journal= (help)
  7. ^ a b Vandervoort, R.R.; Peterson, C.A.W. (1964). "The Properties of a Metastable Gamma Phase Uranium-base Alloy: U-7.5 Nb 2.5 Zr". UCRL. UCRL-7869. {{cite journal}}: Cite journal requires |journal= (help)
  8. ^ a b c Hoge, K.G; Kuhn, B.A.; Reshenk, V.L. (1973). "Flow behavior of mulberry uranium" (PDF). 4 (18). California Univ., Livermore (USA). Lawrence Livermore Lab. UCRL--51346. {{cite journal}}: Cite journal requires |journal= (help)
  9. ^ Erickson, W. C.; Jaynes, G. E.; Sandstrcsn, D. J.; Seegmiller, R.; Taub, J. M. (September 1972). "Evaluation of Uranium Alloys". LASL: 8–13. {{cite journal}}: Cite journal requires |journal= (help)
  10. ^ Dean, C.W. (24 October 1969). "A Study of the Time-Temperature Transformation Behavior of a Uranium=7.5 weight per cent Niobium-2.5 weight per cent Zirconium Alloy" (PDF). Union Carbide Corporation, Y-12 Plant, Oak Ridge National Laboratory. Oak Ridge Report Y-1694. {{cite journal}}: Cite journal requires |journal= (help)
  11. ^ Vandermeer, R.A. (1973). "Recent Observations of Phase Transformations in a U-Nb-Zr Alloy". Metals and Ceramics Division, Oak Ridge National Laboratory. {{cite journal}}: Cite journal requires |journal= (help)
  12. ^ Abey, A.E.; Joslyn, E.D. (April 1972). "The elastic constants of uranium-7.5% niobium-2.5% zirconium as a function of pressure". J. Less Common Met. 27 (1): 9–15. doi:10.1016/0022-5088(72)90099-9.
  13. ^ Lopes, Denise Adorno; Restivo, Thomaz Augusto Guisard; de Lima, Nelson Batista; Padilha, Angelo Fernando (2014). "Gamma-phase homogenization and texture in U–7.5Nb–2.5Zr (Mulberry) alloy". Journal of Nuclear Materials. 449, Issues 1–3, June 2014, Pages 23-30 (1–3): 23–30. Bibcode:2014JNuM..449...23L. doi:10.1016/j.jnucmat.2014.02.030.
  14. ^ Gareth Thomas; Richard M. Fulrath; Robert M. Fisher, eds. (1972). Electron Microscopy and Structure of Materials. University of California Press. pp. 340–345. ISBN 0520021142.