Gas Generation & Radioactive Waste Disposal

Gas Generation & Radioactive Waste Disposal Paul Humphreys

Introduction • 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

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

Mechanisms MIC Hydrogen Generation Hydrogen Generation Polymer Degradation

Routes to Gas Generation Soluble Intermediates Polymeric Waste Components Gas (CH4, CO2, H2S) Microbial/ Chemical/ Radiolytic Degradation Cellulose IX Resins Microbial Metabolism Plastics/ Rubber Metals Corrosion H2

Disposal Facilities

Disposal Facilities • PCM • 14C • 222Rn

Geological Disposal • International agreement • Multi-barrier concept of disposal • LLW, ILW & HLW

Exposure Routes • Dose assessments calculated • Based on travel time back to surface • Scenario approach

Safety Cases • 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

Gas Generation Models • GRM • LLWR • GAMMON/SMOGG • UK NIREX/NDA • T2GGM • Canadian DGR

Model Components Soluble Intermediates Polymeric Waste Components Transport Gas (CH4, CO2, H2S) Microbial/ Chemical/ Radiolytic Degradation Cellulose IX Resins Microbial Metabolism Plastics/ Rubber Metals Corrosion H2

Hydrogen Processing • Processing of H2 has a major impact on model out puts • Access to CO2 key issue

Hydrogen Metabolism • 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

UK SMOGG Modelling • 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

Canadian T2GGM 16

Geological Setting DGR located in low permeability argillaceous limestone

Waste Disposal • 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 Normal Evolution

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 Results Pressure • Wide range of calculation cases considered • Including shaft failure cases 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

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

Key Assumption • 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

14C Story • 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 Reduced Dose Dose Calculation CH4 CO2 ` Geosphere Near-Field ? CH4 14C Gas ? ? Release Release Groundwater

Gas Generation & Radioactive Waste Disposal