Developing Fuels with Enhanced Accident Tolerance. Fiona Rayment and Dave Goddard

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1 Developing Fuels with Enhanced Accident Tolerance Fiona Rayment and Dave Goddard NNL Technical Conference 30 th April 2015

2 UK Fuel Ambition: Development of Fuels with Enhanced Safety, Economic & sustainability Benefits using Indigenous UK R&D Skill & Facility Base Enhanced Economics Better Burn Ups Better Operational Flexibility Better Manufacturability Enhanced Safety during Accident Conditions Enhanced Coolant Containment Enhanced Fuel Retention within Cladding Enhanced Sustainability replace Unat with Urep reduce repository burden Level of Benefit / Ambition LWRs Accident Tolerant Fuels HTRs Coated Particle Fuels Fast Reactors Pu / Minor Actinide / Metal Fuels SMR Track Timescale for Industrial Deployment

3 Challenges for fuel development Advanced fuel and cladding material and chemical properties not fully understood R&D required to understand effect of these on neutron economy, production of activation products and how properties alter under irradiation / high temperature conditions Steps needed: Further investigation and development of new materials Industrial prototypes through existing/new fabrication technology New data measurements and evaluations through irradiation tests and modelling - especially for industrial prototypical fuels

4 Nuclear Fuel Centre of Excellence Accident Tolerant Fuels Basic U / MOx Fuels NFCE BASIC CAPABILITY Coated Particle Fuels Fast Reactor Fuels

5 The ATF Challenge Fukushima revealed vulnerabilities of the established UO 2 /Zr alloy fuels to a LOCA (loss of coolant accident). The challenge facing the international nuclear fuels community is to develop improved fuel/cladding materials that are more resilient and could be used in existing or new build reactors.

6 Economics of ATF Nuclear Plant Accident Scenario Fission products contained and plant potentially reclaimed Estimated cost $2Bn Comparison of potential ATF claddings during cooling loss scenario Fission products escape to containment and plant cannot be reclaimed but cooling restored after short time Cooling not restored for long time and fission products escape containment $10.6Bn $34Bn Data from Lahoda et al, What should be the objective of accident tolerant fuel RT-TR-14-6, [2014] Key ATF attributes Tolerate higher temperatures (up to 1700 C) Reduce hydrogen generation Increase grace period from minutes hours days.

7 Overview of different ATF options (1) Apply a coating to the Zr alloy cladding material to improve oxidation resistance Smallest change to existing manufacturing processes. Candidates include Cr, MAX phases, SiC (2) Replace the cladding with a better high temperature material SiC composites - for GenIV high temperature gas cooled reactors. Advanced steels (e.g. FeCrAl) (3) Replace both fuel and cladding Doping UO 2 could improve thermal conductivity. Higher density fuel compounds (e.g. nitride or silicide) could improve thermal conductivity but water reactivity is a concern.

8 Why change the fuel material? UO 2 has poor thermal conductivity UN and U 3 Si 2 have higher thermal conductivity Higher density fuels have same power output for a lower enrichment These economic benefits can offset the development costs of the new claddings and fuels.

9 Handover

10 High Density Fuel Options Material Theoretical density (TD) /g.cm -3 Difference in heavy metal TD compared to UO 2 Thermal conductivity at 1100 C /Wm -1 K -1 Melting Point / C Thermal expansion coefficient /x10-6 K -1 However. UO UN % U 3 Si % UN would need to be enriched in 15 N to avoid 14 C production in reactor and subsequent issue for storage/re-cycle/disposal. Both UN and U 3 Si 2 are reactive to some extent with water. Need to understand water reaction under PWR conditions and potential consequences of a burst pin. Irradiation induced swelling is slightly worse than UO 2, however more testing is required under PWR operating and transient conditions.

11 USDoE ATF programme USDoE have set out a timetable to have Lead Test Assemblies (LTAs) ready by NNL are supporting a Westinghouse led consortium to develop a new manufacturing route for U 3 Si 2 fuel and deliver fuel for test irradiations in From LWR Accident Tolerant Fuel Performance Metrics, INL/EXT [2014]

12 Manufacture of high density fuels High density fuels were considered in the early days of the industry. U 3 Si 2 -Al dispersion fuels are also commonly used as research and test reactor fuels. Manufacturing routes have been developed to fabricate U 3 Si 2 powder but not for large scale production. All current and historical routes combine U (metal) with Si.

13 Options for U 3 Si 2 fuel manufacture Direct reaction with Si or SiH 4 Heat treatment? Crush Mill Press Sinter Grind U UF 6 U x Si y U 3 Si 3 Si 2 2 powder U 3 Si 2 pellet Hydrogen reduction UF 4 Direct reaction with Si or SiH 4 Arc melt U + Si Proposed UF 6 +Si Proposed UF 4 +Si Current melt processing route Metallothermic reduction with Mg Homogenise with Si powder and precompact U metal Hydride/ de-hydride U powder Previous work: UF 6 + SiH 4 + Li reaction at 1000 C (Robinson et al, US Patent , 1967) UF 6 + Si at C (Lessing and Kong, US Patent , 2000) No reports of UF 4 + Si or SiH 4 reactions

14 Thermodynamic assessment of UF 6 + Si Many possible reactions (ΔG is negative), but we don t know the kinetics. Undesirable competing reaction forming UF 4. Higher Si containing USi x phases have a more negative ΔG.

15 Thermodynamic assessment of UF 6 + SiH 4 Undesirable competing reaction forming UF 4. Lower Si containing USi x phases have a more negative ΔG. Need T>~700 C for reaction forming U 3 Si 2 to become favourable.

16 Thermodynamic assessment of UF 4 + Si Need T>~850 C for any reaction forming a silicide to become favourable. Formation of U 3 Si 2 is not favourable at any temperature

17 Experimental plans Small scale tests of the UF 4 +Si reaction using a TGA. UF 6 + Si + H 2 reaction rig to investigate kinetics of reactions. Nuclear Fuels Centre of Excellence (NFCE) equipment being installed to support this work. Arc-melter to develop conventional melt processing route. Inert glovebox line to develop pelleting process. Scale up considerations, e.g. off gas (SiF 4 ) treatment or reuse and recycle routes. UF 6 + Si (+ H 2 ) reaction rig design

18 NNL international and academic links on ATF EPSRC funded CAFFE (Cambridge/Manchester/ Imperial) MAX phases EPSRC funded XMat project (Imperial/QMUL/Birmingham) Materials for extreme environments including MAX phases and SiC composites NNL icase PhD (Sheffield Univ) MAX phases EPSRC funded PACIFIC (Manchester/Bristol/Imperial) Advanced fuels NNL icase PhD (Bristol) Water interaction with U 3 Si 2 NNL strategic project on ATF EU Horizon 2020 FALSTAFF project on UN fuels and SiC cladding unsuccessful but possible proposal for next call fkts project (Nottingham) UN fabrication routes International consortia and working groups, e.g. CARAT, OECD-NEA, IAEA, NUGENIA USDoE Westinghouse led ATF consortium Potential fuel/cladding irradiation tests via UK input to Halden reactor project

19 Acknowledgements NNL Emma Johnston, Alex Brooke, Henry Foxhall, James Paul, Pete Morris, Dan Shepherd, Pete Durham, Simon Marshall, Jon-Paul Hurn, Dave Graham, Chris Rhodes, Marc Thomas, Alison Graham, Dave Checkley + NNL Preston facilities team. Springfields Fuels Ltd Bob McKenzie, David Eaves, Barry Livingstone. Westinghouse (US/Sweden) Ed Lahoda, Peng Xu, Sumit Ray, Lars Hallstadius

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