Development of an Alternative Corrosion Inhibitor for the Storage of Advance Gas Reactor Fuel

1. Development of an Alternative Corrosion Inhibitor for the Storage of Advance Gas Reactor Fuel P Standring (Sellafield Ltd) B Hands, S Morgan & A Brooks (National Nuclear Laboratory)

2. Contents • Background • Advanced Gas Reactor Fuel • Beginning of storage properties • Storage Issue • Technical Underpinning • Out-line process • Inhibitor Selection • Active Studies • Forward Fuel Corrosion Studies • Lead Container Study

3. Advanced Gas Reactor – Fuel Element • Dimensions • ~1m long by 0.24m diameter • Composition • Graphite sleeve, Fuel Pins, Grid, Guide Tube & Braces • AGR Fuel Pins • 36 fuel pins (~1m long by 15mm diameter) • Thin walled 20Cr/25Ni/Nb stainless steel tubes • Ribbed tubes to improve heat transfer • 64 hollow UO2 pellets per pin • Anti stacking grooves • Grid/Guide Tube/Braces • 20Cr/25Ni/Ti stainless steel

4. Beginning of Storage Properties • At the beginning of storage some irradiated AGR fuel pins and structural components are left sensitised • In wet storage sensitised pins are susceptible to corrosion through inter-granular attack (iga) • Pre-requisites for iga • Must have a sensitised microstructure (through wall to lead to failure) • Radiation Induced Segregation (RIS) is observed to occur on 20Cr/25Ni/Nb stainless steel cladding in the temperature range 350ºC to 520ºC; peak effect at 420ºC • Some elements of a 7-8 element stringer affected • Linked to an applied mechanical stress • Failure sites normally associated with areas of stress • Must be exposed to a corrosive environment • For example Chloride • Thresholds • Burn-up ~15GWd/t string mean irradiation (SMI) c.f. current discharges >30Gwd/t SMI • Unclear if threshold for chloride exists (failures observed ~0.2ppm)

5. AGR Fuel Storage at Sellafield • To prevent the potential for AGR fuel to corrode during wet storage, the corrosion inhibitor sodium hydroxide is deployed at Sellafield where practicable • Sodium Hydroxide was selected in the 1980s after a programme of materials studies • The exception is Thorp Receipt & Storage (TR&S) where a reprocessing buffer is stored in demineralised water

6. Why Investigate an Alternative Corrosion Inhibitor? • The AGR reprocessing buffer has had to be increased to sustain fuel receipts from reactors – as a result of the recent extended shutdown of Thorp • Net impact: • Storage durations has increased • Current storage regime is not passivated for AGR fuel • Whilst sodium hydroxide is the preferred corrosion inhibitor for AGR fuel, deployment is not feasible due to compatibility issues with containers used for LWR storage

7. Technical Underpinning – Two Strands Plant & Process Compatibility Fuel Corrosion Initial Assessment Feasibility assessment (inactive) HAZAP Demonstrate inhibits propagating iga and look at operating envelope (active) Full Audit of plant materials Confirmatory Studies Confirmation at plant-scale Lead containers (active) Safety Case Development

8. Alternative Corrosion Inhibitors – Inactive Studies • Hydroxide, borate, chromate, molybdate, nitrite and pertechnetate (all known to inhibit pitting corrosion of stainless steel) were excluded: • React with aluminium • Toxicology • Others ranked by their ability to inhibit propagating attack of a simulated sensitised grain boundary/crevice, stability to radiation, toxicology and aluminium corrosion rate • Demineralised water was evaluated, but was ruled out as corrosion could be detected in simulated pond water using a metal loss technique

9. Inhibition of Propagating Attack – Active Studies • Active Studies – Material Selection • AGR Pins or Braces • Brace material chosen as previous studies for sodium hydroxide used braces • Braces receive a higher dose than fuel pins for the same irradiation temperature • Historical Post Storage Examination of braces had shown them to be more susceptible to corrosion than fuel pins • Difficult to select right pin material/need to do whole pin testing • Brace selection • Cross section of braces collected that would have been irradiated in the 350- 520ºCregion • Material ranked by initiating corrosion and measuring current • Ability to sustain corrosion over a 60 day period

10. Inhibition of Propagating Attack – Active Studies • Test Method • Zero Resistance Ammetry • An electrochemical method for distinguishing between a system which is passive and one undergoing corrosion • Test Sequence adopted to prove principle • Corrosion initiated in 2ppm chloride • Corrosion inhibitor (10ppm nitrate) introduced after corrosion had been sustained (~13 days) • Corrosion re-initiated by removing nitrate and increasing chloride concentration from 2ppm until initiation observed.

11. Active Testing of AGR Brace Material Undertaken by NNL on behalf of Sellafield Ltd

12. Active Studies- example of ZRA output

13. Inhibition of Propagating Attack by Sodium Nitrate Corrosion initiated (water + 2ppm chloride) Corrosion inhibited (10ppm nitrate + 2ppm chloride) Corrosion re-initiated (water+ 2ppm chloride; increased to 50ppm)

14. Additional Active Studies • Evaluation of the operating envelope • Constant inhibitor level with increasing chloride concentration • Constant chloride concentration at decreasing inhibitor concentration • Materials Studies of Brace Samples • Strauss Testing & SEM examination • Grain Boundary Composition (undertaken by Studsvik Nuclear on behalf of Sellafield Ltd)

15. Forward Studies – Fuel Corrosion • Fuel Corrosion studies to date have provided the basis and confidence that sodium nitrate would act as an efficient corrosion inhibitor for AGR fuel and is unlikely to impact on its integrity if deployed • Because of the differences between AGR braces and fuel pins it is necessary to confirm active study findings through a Lead Container Study utilising whole AGR pins before full pond dosing could be considered. • 20:25:Ti Stainless steel c.f. 20:25:Nb Stainless Steel • Braces have crevices c.f. pins no crevices • Braces levels of stress are minor c.f. pins

16. Forward Studies – Lead Container Study • The main objectives of the study are to: • Confirm that the introduction of sodium nitrate to 10ppm has no impact of AGR fuel integrity • Confirm that the introduction of sodium nitrate to 10ppm has no impact on known failed AGR fuel • Confirm that under nitrate dosing to 10ppm a chloride excursion up to 2ppm does not initiate fuel corrosion • Confirm that fuel that is undergoing propagating attack (fuel corrosion) is inhibited by the addition of sodium nitrate to 10ppm • Confirm that the overall impact of establishing the recommended 10ppm sodium nitrate passive regime has minimal impact on fuel that is under going propagating attack • In studying these objectives a staged approach will be applied. Procedure to the next stage being dependent upon satisfactory demonstration of the previous

17. Lead Container Set-up

18. Summary • Out-lined the process being adopted to establish a new corrosion inhibitor for AGR fuel • Provided examples of the findings to date • Out-lined the forward Fuel Corrosion studies required to support the full deployment of sodium nitrate