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
14C
& 3H
labelled
Gases
Release of
Radioactive
Gases
Methylated
Gases
Gas
Generation
Groundwater
Impacts
Engineering
Impacts
Microbial Activity
MIC
Hydrogen
Generation
Corrosion
Hydrogen Generation
Polymer Degradation
Radiolysis/
Radiation/Decay
Soluble
Intermediates
Polymeric Waste
Components
Cellulose
IX Resins
Plastics/
Rubber
Metals
Microbial/
Chemical/
Radiolytic
Degradation
Corrosion
H2
Microbial
Metabolism
Gas
(CH4,
CO2,
H2S)
•PCM
•14C
•222Rn
• International agreement
– Multi-barrier concept of disposal
• LLW, ILW & HLW
• Dose assessments
calculated
• Based on travel
time back to
surface
• Scenario approach
• 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
• GRM
– LLWR
• GAMMON/SMOGG
– UK NIREX/NDA
• T2GGM
– Canadian DGR
Cellulose
IX Resins
Plastics/
Rubber
Metals
Microbial/
Chemical/
Radiolytic
Degradation
Corrosion
H2
Microbial
Metabolism
Gas
(CH4,
CO2,
H2S)
Transport
Soluble
Intermediates
Polymeric Waste
Components
• Processing
of H2 has a
major
impact on
model out
puts
• Access to
CO2 key
issue
• 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
• 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
16
DGR located in low
permeability
argillaceous
limestone
• 200,000 m3 of LLW & ILW
• No HLW or spent fuel
•
•
•
•
•
•
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
19
• Wide range of
calculation cases
considered
• Including shaft
failure cases
Pressure
• 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
Saturation
•Peak pressure 7 – 10 MPa
(Repository horizon: 7.5
MPa, Lithostatic 17 MPa)
•Methane dominant gas
•Repository does not
saturate over 1 Ma timescale
Geosphere
TOUGH 2
Seepage
Unsaturated
Gas
Pressure
Saturated
Corrosion and
microbial
processes slow
as humidity
decreases
from 80% to
60%
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
• 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
Biosphere
Dose Calculation
CH4  CO2
`
Geosphere
Near-Field
CH4
Gas
14C
Release
Release
Groundwater