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Gas Generation & Radioactive Waste
Disposal
Paul Humphreys
• Gas generation is a fundamental issue in radioactive waste disposal
• Direct impact on:– Waste processing and packaging – Facility design – Radionuclide release
• Nature and extent of gas generation depends on type of waste and the facility
Introduction
Gas Generation Issues
Gas Generation
Release of Radioactive
Gases
Groundwater Impacts
Engineering Impacts
Methylated Gases
14C & 3H labelled Gases
Microbial Activity
Radiolysis/ Radiation/Decay Corrosion
Mechanisms
MICHydrogenGeneration
Hydrogen GenerationPolymer Degradation
Polymeric Waste Components
Routes to Gas Generation
Cellulose
IX ResinsPlastics/ Rubber
Soluble Intermediates
Microbial/ Chemical/ Radiolytic
Degradation
Microbial Metabolism
Metals
Gas(CH4, CO2, H2S)
Corrosion H2
Disposal Facilities
Disposal Facilities•PCM•14C•222Rn
Geological Disposal • International agreement – Multi-barrier concept of disposal• LLW, ILW & HLW
• Dose assessments calculated
• Based on travel time back to surface
• Scenario approach
Exposure Routes
• Radioactive waste disposal sites are evaluated via a safety case– Includes risk assessment modelling based
on exposed dose• 10-6 yr-1
• Safety cases produced throughout the lifetime of a repository
• Gas generation issues need to be integrated into a safety case. – Gas generation modelling
Safety Cases
• GRM– LLWR
• GAMMON/SMOGG– UK NIREX/NDA
• T2GGM– Canadian DGR
Gas Generation Models
Polymeric Waste Components
Model Components
Cellulose
IX ResinsPlastics/ Rubber
Soluble Intermediates
Microbial/ Chemical/ Radiolytic
Degradation
Microbial Metabolism
Metals
Gas(CH4, CO2, H2S)
Corrosion H2
Transport
• Processing of H2 has a major impact on model out puts
• Access to CO2 key issue
Hydrogen Processing
• Controlled by corrosion rate• 3 TEA processes – H2 + 2Fe(III) 2Fe(II) + 2H+
– 4H2 + SO42‑ + 2H+ H2S + 4H2O
– CO2 + 4H2 → CH4 + 2H2O
• Hydrogen metabolism key process in controlling repository pressure– 4H2 = 1H2S or
– 4H2 + 1CO2 =1CH4
Hydrogen Metabolism
• Illustrative calculated results for net rates of gas generation from UILW in higher strength rocks for the 2004 Inventory
• H2 dominates
• CO2 assumed to be unavailable due to cement carbonation
UK SMOGG Modelling
Canadian T2GGM
16
Geological Setting
DGR located in low permeability argillaceous limestone
• 200,000 m3 of LLW & ILW • No HLW or spent fuel
Waste Disposal
19
Normal Evolution• Oxygen consumed (in a few years)• Water starts to seep into repository• Water aids corrosion and degradation of
wastes• Gas pressure increases• Water is forced out into surrounding rock
mass• Bulk and dissolved gases slowly migrate out
into shaft and rock mass • Small quantities of dissolved gas (and no bulk
gases) reach biosphere over 1 Ma timescales
• Wide range of calculation cases considered
• Including shaft failure cases
Results• Peak pressure 7 – 10 MPa
(Repository horizon: 7.5 MPa, Lithostatic 17 MPa)
• Methane is the dominant gas• Repository does not saturate over 1 Ma
timescale
•Peak pressure 7 – 10 MPa(Repository horizon: 7.5 MPa, Lithostatic 17 MPa)•Methane dominant gas•Repository does not saturate over 1 Ma timescale
Saturation
Pressure
Water Limitation and Humidity
Seepage Gas Pressure
Saturated
UnsaturatedTOUGH 2
Corrosion and microbial processes slow as humidity decreases from 80% to 60%
Geosphere
Corrosion and microbial processes stop <60%
• Availability of CO2 in a cementitious repository – Major impact on overall gas volumes – Fate of waste derived carbon dioxide
• Fate and transport of 14C another area of uncertainty
Key Assumption
• Substantial quantities of 14C generated in nuclear power reactors
• Present in irradiated metal and graphite– Chemical form and chemical evolution
major impact on transport. • The release of volatile 14C is assumed
to be in the form of methane
14C Story
`
Release Release GroundwaterG
as
CH4
CH4 CO2
? ?
?14C
Dose Calculation
Near-Field
Geosphere
Biosphere
ReducedDose