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

Gas Generation Release of Radioactive Gases Groundwater Impacts Engineering Impacts Methylated Gases 14 C & 3 H labelled Gases

Microbial Activity Radiolysis/ Radiation/Decay Corrosion MIC Hydrogen Generation Hydrogen Generation Polymer Degradation

Polymeric Waste Components Cellulose IX Resins Plastics/ Rubber Soluble Intermediates Microbial/ Chemical/ Radiolytic Degradation Microbial Metabolism Metals Gas (CH 4, CO 2, H 2 S) Corrosion H2H2

PCM 14 C 222 Rn

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 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

Polymeric Waste Components Cellulose IX Resins Plastics/ Rubber Soluble Intermediates Microbial/ Chemical/ Radiolytic Degradation Microbial Metabolism Metals Gas (CH 4, CO 2, H 2 S) Corrosion H2H2 Transport

Processing of H 2 has a major impact on model out puts Access to CO 2 key issue

Controlled by corrosion rate 3 TEA processes – H 2 + 2Fe(III) 2Fe(II) + 2H + – 4H 2 + SO H + H 2 S + 4H 2 O – CO 2 + 4H 2 CH 4 + 2H 2 O Hydrogen metabolism key process in controlling repository pressure – 4H 2 = 1H 2 S or – 4H 2 + 1CO 2 =1CH 4

Illustrative calculated results for net rates of gas generation from UILW in higher strength rocks for the 2004 Inventory H 2 dominates CO 2 assumed to be unavailable due to cement carbonation

16

DGR located in low permeability argillaceous limestone

200,000 m 3 of LLW & ILW No HLW or spent fuel

19 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 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

Seepage Gas Pressure Saturated Unsaturated TOUGH 2 Corrosion and microbial processes slow as humidity decreases from 80% to 60% Geosphere Corrosion and microbial processes stop <60%

Availability of CO 2 in a cementitious repository – Major impact on overall gas volumes – Fate of waste derived carbon dioxide Fate and transport of 14 C another area of uncertainty

Substantial quantities of 14 C generated in nuclear power reactors Present in irradiated metal and graphite – Chemical form and chemical evolution major impact on transport. The release of volatile 14 C is assumed to be in the form of methane

` Release Groundwater Gas CH 4 CH 4 CO 2 14 C Dose Calculation Near-Field Geosphere Biosphere