hydrogen yields from plutonium alpha-radiolysis of nitrate ... · • hydrogen production from...
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Hydrogen yields from plutonium alpha-radiolysis of nitrate solutions
NNL: Robin Orr, Howard Sims, Robin Taylor, Kevin Webb, Colin Gregson
University ofManchester: Simon Pimblott, Greg Horne
SACSESS International Workshop24th April 2015
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Background
• Nitric acid is used widely within the nuclear industry:• Dissolution of spent nuclear fuel• Reprocessing facilities• Analytical processes• POCO and decommissioning operations
• Hydrogen production from radiolysis of aqueous solutions is a potential hazard:• Flammable gas mixtures• Pressurisation
• Relevant to safety studies of reprocessing flowsheets at higher plutonium concentrations e.g. GANEX and mixed oxide fuel reprocessing
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Water radiolysis: Hydrogen production
• H• + H• H2
• eaq− + eaq
− (+2H2O) H2 + 2OH−
• eaq− + H• + H2O H2 + 2OH−
• Removal of hydrogen by OH• OH + H2 H + H2O
H2O
H2O+ + e−pre
Ionizing radiation
H2O*
H2 + O•OH• + H• eaq−
nH2O
OH• + H3O+
OH• + H-
H2 + OH−H2 + O•−
H2O−
H2O
H2O
H•H3O+
H2O
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Water radiolysis: Hydrogen production and the effect of nitrate
• H• + H• H2
• eaq− + eaq
− (+2H2O) H2 + 2OH−
• eaq− + H• + H2O H2 + 2OH−
• Scavenging of the precursors of H2• e-
pre/aq + NO3- NO3
2- H• + NO3- NO3
2- + H+
H2O
H2O+ + e−pre
Ionizing radiation
H2O*
H2 + O•OH• + H• eaq−
nH2O
OH• + H3O+
OH• + H-
H2 + OH−H2 + O•−
H2O−
H2O
H2O
H•H3O+
H2O
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Literature: Gamma radiolysis of nitrate and nitric acid solutions
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Literature: Pu alpha radiolysis of nitric acid
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Literature: Alpha radiolysis of nitric acid
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Literature: Alpha radiolysis of nitric acid
• Results for Pu fall below those for Cm and Po
• Kuno observed a small effect of Pu concentration on G(H2) but Bibler did not for addition of Pu to Cm solution
• Kuno saw an effect of solution depth while Bibler recorded an effect from solution agitation ([NO3
-] 0.7 – 10 mol.L-1)
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Experimental method
• Solutions• Pu concentration 12 g.L-1 (0.05 mol.L-1)• 0.75 mol.L-1 H2SO4• 0.0001-6 mol.L-1 HNO3
• Hydrogen yield• Typically 0.5 ml solution in ~60 ml glass vessel
• Vessel sealed and periodically headspace sampled by gas syringe (~10 ml gas) with stirring prior to sampling
• Sampled gas volume replaced with air• Hydrogen concentration determined using µ-GC (Mol-sieve 5A column, Arcarrier, TCD detector)
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Experimental method: Example hydrogen production rates
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Initial results: plutonium nitrate
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Effect of plutonium concentration?
• Pu(IV) is a scavenger of hydrogen precursors• Pu4+ + e-
aq → Pu3+ k=3x1010 L.mol-1.s-1
• Analogous effect well established for Cu2+
• Cu2+ + e-aq → Cu+ k=3.3x1010 L.mol-1.s-1
• Cu2+ + e-pre → Cu+ k~1x1013 L.mol-1.s-1
• Compare with NO3-
• e-aq + NO3
- NO32- k=9.7x109 L.mol-1.s-1
• e-pre + NO3
- NO32- k~2x1013 L.mol-1.s-1
• Scavenging capacity = kSx[S]
• Further experiments performed with a range of Pu concentration: 1-38 g.L-1 (0.0004-0.16 mol.L-1)
Pastina et al., J. Phys. Chem. A, 103 (1999) 5841
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Results: Effect of plutonium concentration
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Effect of plutonium concentration?
• Evidence of an effect from Pu concentration at low nitrate concentration
• Unlikely the concentration dependence seen by Kuno is due to Pu(IV) scavenging precursors of H2
• Further measurement made using Am in nitric acid
• Am(III) does not scavenge precursors of hydrogen
• Two test methods used:• Same as previous method• Sealed “Single shot” vessels only sampled once but using 5 ml solution
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Results: Americium nitric acid solutions
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Conclusions/summary
• New measurements of the molecular yield from nitric and sulphuric acid solutions reported, particularly at low concentrations of nitrate
• Results are consistent with previously reported data for alpha radiolysis at high nitrate concentrations
• Evidence of an effect of Pu concentration at low nitrate concentrations
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Acknowledgements
• SACSESS project
• Sellafield Ltd.
• UK Nuclear Decommissioning Authority