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