Corrosion of copper canister in

repository conditions

KYT seminar 15.12.2016

Julkinen

Relevant publications

King et al., An Update of the State-of-the-art Report

on the Corrosion of Copper Under Expected

Conditions in a Deep Geologic Repository, Posiva

2011-01

Safety Case for the Disposal of Spent Nuclear Fuel

at Olkiluoto - Performance Assessment 2012, Posiva

2012-04

Safety Case for the Disposal of Spent Nuclear Fuel

at Olkiluoto - Models and data report for the

repository system, Posiva 2013-01

14.12.2016 Partanen Olli 2

Julkinen

Corrosion assessments

In Performance Assessment 2012, the canister

corrosion is assessed in three time phases

Operational

Post-closure: before and after buffer saturation (0 – 10 000

years)

Long-term evolution (>10 000 years)

14.12.2016 Partanen Olli 3

Julkinen

Canister corrosion modes

Atmospheric corrosion

Radiolysis induced corrosion (internal and external)

Stress corrosion cracking

High saline groundwater corrosion

Aerobic corrosion

Pitting corrosion

Sulphide corrosion

14.12.2016 Partanen Olli 4

Julkinen

Atmospheric corrosion

Oxidation of copper surface, above and below

ground

Oxide layer depth of tens to few hundreds of nm

expected

Corrosion less than 1 µm for storage times up to 2 years

14.12.2016 Partanen Olli 5

Julkinen

Corrosion in unsaturated buffer

Relative humidity high enough in the canister

interface to cause aqueous corrosion

Inhomogeneous water flow might cause uneven

swelling of bentonite

Increased aerobic corrosion rates at copper/bentonite/air

phase boundaries

14.12.2016 Partanen Olli 6

Julkinen

Corrosion in saturated buffer – post

closure Aerobic corrosion upper limit estimated from mass

balance

Less than 1 mm of uniform corrosion if all porosity of buffer

and backfill assumed to be air, all of which only reacts with

canister

Even less than this in reality, as oxygen is also consumed

by minerals and microbial activity

Corrosion in water containing Cl stops after anoxic

state is reached

14.12.2016 Partanen Olli 7

Julkinen

Corrosion in saturated buffer – post

closure Corrosion of copper in anoxic water has been

proposed since the 1980s

This mechanism has not been proven, and even if

possible, would not be relevant in repository contions

King, Critical review of the literature on the corrosion of

copper by water, TR-10-69

14.12.2016 Partanen Olli 8

Julkinen

Corrosion in saturated buffer – post

closure Sulphide will corrode copper in anoxic conditions

Sulphide sources: groundwater, buffer and backfill

Corrosion rate restricted by sulphide mass transfer

through buffer to canister surface

Buffer performance is important for canister performance, as

intact buffer limits the flow to and from canister surface

14.12.2016 Partanen Olli 9

Julkinen

Stress corrosion cracking

Stress corrosion cracking (SCC) cannot be

completely ruled out

Susceptible material, aggressive environment and tensile

stress required

Most aggressive SCC conditions in early repository

evolution (tens to hundreds of years), no premise for

SCC in long term evolution

14.12.2016 Partanen Olli 10

Julkinen

Stress corrosion cracking

14.12.2016 Partanen Olli 11

King & Kolář 2004, to be updated

Julkinen

Pitting corrosion

Canister does not suffer from traditional pitting in

anoxic conditions, but rather surface roughening

Oxidant required for classical pitting, only available

during early stages

14.12.2016 Partanen Olli 12

Julkinen

Radiolysis induced corrosion – internal

Internal corrosion should be limited by drying the fuel

assemblies and purging the canister with argon

before welding

But it is considered possible that some water could

be trapped inside fuel assemblies

All of the radiolysis products should react first with

the cast iron insert, then with the copper shell

14.12.2016 Partanen Olli 13

Julkinen

Radiolysis induced corrosion – external

Set dose rate limit on the copper canister surface is 1

Gy/h, with expected dose rates being lower

High dose rates (>100 Gy/h) increase copper

corrosion

10 – 100 Gy/h dose rates showed lower corrosion

rates in experiments

Gamma radiation penetrating the copper shell only

relevant for first 300 years after disposal

Alpha radiation radiolysis not an issue, as alpha

particles are stopped inside the canister

14.12.2016 Partanen Olli 14

Julkinen

Long-term corrosion – intact buffer

Uniform sulphide corrosion as the main corrosion

mechanism

Oxidising glacial meltwater not expected to reach repository

depth, canister surface to be anoxic indefinitely

Radiation doses at canister surface greatly decreased

Expected corrosion depths of few tenths of mm, even

with increased water flow from rock damage

Copper Sulphide Model (CSM) simulation model is

being developed, used to predict canister lifetime

14.12.2016 Partanen Olli 15

Julkinen

Long-term corrosion – intact buffer

14.12.2016 Partanen Olli 16

King 2007, to be updated

Julkinen

Long-term corrosion – eroded buffer

Corrosion rate increased due to increased sulphide

flow to canister

Hydraulic conductivity and sulphide concentration of

groundwater the most important parameters

14.12.2016 Partanen Olli 17

Julkinen

Current research projects

Cu corrosion and H2 formation in pure anoxic water,

VTT

Copper sulphide film formation, University of

Western, Ontario

Copper sulphide model, Integrity Corrosion

14.12.2016 Partanen Olli 18

Julkinen

Posiva’s future publications

Performance Assessment and Formulation of

Scenarios (PAFOS) for TURVA-2020

Safety case for operational license

Includes revised report on corrosion

Report on copper corrosion in high Cl- concentrations

14.12.2016 Partanen Olli 19

Kiitos

Thank you

14.12.2016 20Partanen Olli