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7-1
RFSS: Lecture 13 Neptunium Chemistry• From: Chemistry of actinides
à http://radchem.nevada.edu/classes/rdch710/lectures%20and%20chapters.html
§ Nuclear properties and isotope production
§ Aqueous phase chemistry§ Separation and Purification§ Metallic state§ Compounds § Structure and coordination
Neptunium nuclear properties• 22 known Np isotopes
§ 237Np longest livedà Neutron irradiation of U
* Consecutive neutron capture on 235U* 238U(n,2n)237U237Np + b-
* Alpha decay of 241Amà Used at target for 238Pu production by neutron irradiationà Reaction with 23 MeV and 30 MeV electrons to produce 236Puà Critical mass is 73 kgà 2500 kg in environment from fallout
§ 238,239Npà Short half-life, useful radiotracers
* From neutron irradiation of 237Np and 238U§ 235,236Np
à Cyclotron irradiation of 235U* 235U(d,n)236Np* 235U(p,n)235Np
• Np isotopes formed in Earth’s crust§ Dynamic equilibrium established
à U, Pu, Np, Am in most stable oxidation statesà Np(V) is not extractedà Oxidized to Np(VI) then extractedà Reduced to Np(V) and back extracted into 0.1 M HNO3
• Tri-n-octylamine§ Used for separation of Np from environmental samples
à Extracted from 10 M HClà Back extracted with 1 M HCl+0.1 M HF
HDEHP
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Metallic Np
• First synthesis from NpF3 with Ba at 1473 K• Current methods
§ NpF4 with excess Ca
§ NpO2 in a molten salt processà Can also use Cs2NpO2Cl4 and Cs3NpO2Cl4
à LiCl/KCl as electrolyte at 723 K§ NpC reduction with Ta followed by volatilization of
Np§ Electrodepostion from aqueous solution
à Amalgamation with Hg from 1 M CH3COOH and 0.3 M CH3COONa at pH 3.5
à Distillation to remove Hg
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Metallic Np data• Melting point 912 K
§ Boiling point estimated at 4447 K• Density 19.38 g/mL• Three metallic forms
§ Enthalpies and entropies of transitions à ab
* Transition T 553 K* ΔS=10.1 JK-1mol-1
* ΔH=5.607 kJmol-1
à bg* Transition T 856 K* ΔS=6.23 JK-1mol-1
* ΔH=5.272 kJmol-1
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Neptunium oxides• Two known anhydrous oxides
§ Np2O5 and NpO2
• NpO2
§ From thermal decomposition of a range of Np compounds
§ Isostructural with other actinides
§ Fluorite lattice parameter§ Stable over a range of
temperatures§ Phase change from fcc to
orthorhombic at 33 GPaà Stable to 2.84 MPa and
673 K• Np2O5
§ From thermal decomposition of NpO3
.H2O or NpO2OH(am)
§ Np2O5 decomposes to NpO2 from 693 K to 970 K
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Np halides• Fluorides
§ NpF3, NpF4, NpF5, and NpF6§ Prepared from reactions with HF at 773 K