Fuel Reliability August 2016

IN USE: DEVELOPMENT OF ACCIDENT TOLERANT FUEL USING MOLYBDENUM ALLOY CLADDING

ISSUE STATEMENT

The accident at Fukushima Daiichi in 2011 illustrated the vulnerability of plants in extended station blackout (SBO) accidents. Explosion of hydrogen gas, primarily from rapid reaction of zirconium-alloy cladding with high temperature, high pressure steam, led to damage of the primary system. Melting of the fuel rods accelerated the release and disper-sion of fission products outside of the plant boundary.

Preliminary evaluations indicate that maintaining availabil-ity of passive cooling to remove the decay heat is of utmost importance in preventing rapid fuel rod heat-up in severe accidents. An additional defense is to replace the zirconium alloy cladding with an advanced material which can signifi-cantly reduce the steam reaction rate and maintain the mechanical integrity of the fuel cladding at high tempera-tures during a severe accident. An accident tolerant fuel (ATF) cladding can provide additional coping time for plant operators to restore emergency cooling.

DRIVERS

Nuclear SafetyRapid production and accumulation of hydrogen inside the pressure vessel and containment need to be avoided under any conditions including severe accidents to reduce the risk of hydrogen explosion and subsequent release of harmful fis-sion products to the public.

Radiological FactorsMaintaining fuel cladding integrity, reducing the rate of fuel temperature increase, and avoiding breach of the primary system in any situations are keys to preventing wide-spread release of harmful levels of fission products.

Industry CommitmentA major fallout from the Fukushima Daiichi accident is waning public confidence of nuclear energy in several coun-tries. The ability of industry to design and operate accident tolerant nuclear systems, including safer fuel designs, is criti-cal to the long term viability of nuclear as a reliable energy source.

RESULTS IMPLEMENTATION

The planned scope of work aims to demonstrate fabricability of molybdenum (Mo)-based alloys into thin-wall tubes and composite tubes with coatings, and obtain key engineering properties of the tubes, including corrosion, oxidation, weld-ing, mechanical and irradiation properties. Research on advanced molybdenum alloys with improved corrosion and oxidation resistance will be performed. Limited irradiation study of the material will be conducted in collaboration with the Halden Reactor project and national laboratories.

This work represents the first effort to adopt Mo-alloys for light water reactor fuel applications, and as such engineering data will need to be generated to ensure its suitability for operation in light water reactor environments, including heat flux, irradiation, neutron capturing, water chemistry variables, stringent dimensional stability requirements, as well as long service time. Additionally, the fuel cladding needs to be demonstrated to tolerate accident conditions in hydrogen-enriched, high pressure steam at temperatures exceeding 1000°C.

Fabrication of long, thin-wall Mo tubes and formation of a surface protective coating with metallurgical bonding using physical vapor deposition have been successfully demon-strated through 2013–14. During 2014–16, laboratory char-acterization tests have proven that the coated Mo alloy clad-ding can (1) survive steam at 1000°C for more than 7 days, and 1200°C for 12–24 hours, (2) achieve high creep strength superior to the current Zr-alloy cladding at the operating temperature (300–400°C).

For consideration of Mo-alloy cladding, the protective coat-ing on long thin wall tube need to be fabricated via an indus-trially qualified thermal-mechanical reduction process in order to achieve high cladding quality and economics in fab-rication. Production of such thin protective liner on the outer surface of a dissimilar metal (Mo) of substantially higher strength and hardness has not been contemplated. Development effort though 2015–16 has demonstrated the feasibility of forming high quality lined tubes via hot iso-static pressuring (HIPing), and HIPing plus hot co-extru-sion has also been demonstrated to be capable of forming bonded 2” OD rod from 6” OD billet. Additional effort is needed to fully demonstrated this feasibility of fabricating lined tubes using direct HIPing and HIPing + Extrusion + Drawing. Tubes fabricated by the thermo-mechanical

EPRI | Nuclear Sector Roadmaps August 2016

reduction methods are believed to be suitable for fuel clad-ding application, and vigorous laboratory characterizations tests and test reactor irradiation will be performed to com-plete the EPRI effort.

Research results will be made available to the Department of Energy in collaboration with fuel vendor, Areva, for consid-eration of commercial reactor irradiation and in-plant dem-onstration. Furthermore, inquiries for collaboration on Mo-alloy cladding technology have been received from organizations committed to development of advanced reac-tors, including Molten Salt and Gas Cooled Reactors, which will be operated at temperatures exceeding 600°C.

PROJECT PLAN

The project is divided into five parts.

Cladding Behavior and Transient License Analysis• Analyses of behavior of nuclear fuel rods under severe

accidents using EPRI’s MAAP code and definition of requirements for accident tolerant fuel cladding.

• Transient licensing behavior of Mo-based cladding, i.e., power ramps under normal operations and reactivity ini-tiated accidents (RIA), using the FALCON code.

Tube FabricationFabrication of composite molybdenum-zirconium and molybdenum-advanced stainless steel tubes using coating and mechanical reduction technologies: • Scoping study has been performed in 2012 and suitable

technologies including vacuum plasma spray, high veloc-ity oxi-field (HVOF), and physical vapor deposition have been defined. Tube samples have been fabricated for test-ing and characterization.

• Plan for mechanical reduction to produce triplex cladding will be pursued following successful demonstration of the feasibility of the composite Mo tube concepts, as dis-cussed below.

Material Characterization and New Molybdenum AlloysCharacterization of composite Mo tubes and advanced Mo alloys for improved corrosion and oxidation resistance:• The following five properties of composite Mo-alloy tubes

will be tested and characterized to determine the feasibil-ity of the Mo-alloy concept for LWR fuel cladding:

– Long term corrosion behavior under PWR and BWR water chemistry conditions.

– Resistance to steam and hydrogen enriched oxidation at temperatures of 1000–1700°C.

– Mechanical integrity and chemical stability of coated Mo-alloy tube via burst test and annealing experi-ments for temperatures up to 1000–1200°C.

– Welding of tube to endplug. – Mechanical properties for fuel rod designs

• Theoretical and experimental alloy mechanical design is ongoing to provide guidance for exploring new Mo alloy systems for corrosion and oxidation resistance and improved ductility with an ultimate goal of developing a Mo-alloy without needing coating for protection.

Fabrication of Lined Mo Tubes via a Thermo-Mechanical Process to Achieve High Tube Quality and Economical Production in Industrial Scale

IrradiationIrradiation of composite molybdenum tubes for composite stability and basic embrittlement properties• Irradiation will be contemplated as a part of DOE-

funded program at the ATR and membership-funded program at the Halden Project. Additional irradiated in a fast flux reactor will be explored to achieve high irradiation dose simulating that of high burnup fuel is also planned.

Feasibility Analysisfor Expanded Irradiation and In-Plant DemonstrationFeasibility analysis of all test results will be performed to determine suitability for additional irradiation and in-plant demonstration.

RISK

Risks associated with the research include:• Molybdenum is known to react with oxygen and water

vapor at elevated temperatures to form volatile MoO3. This concern has decreased some as our initial test data have indicated that Mo tubes of 8 mil (180 um) thickness will survive exposure to 1000°C steam at atmospheric pressure for up to 6 days, and maybe much longer if there is 1% hydrogen present in the steam, where as a 22 mil Zr-alloy will survive not more than 6 hours. Additional tests will be performed.

• Inherent to any composite material, mechanical and chemical integrity at the material interface is very impor-tant, and there is a lack of data to support design analysis. If test results demonstrate inadequate stability and integ-rity, then the design may not be suitable for the intended applications.

Fuel Reliability August 2016

• Molybdenum has higher neutron absorption cross sec-tions than zirconium, hence will add an economic pen-alty. In certain high energy cores, enrichment greater than 5% may be required to gain nuclear reactivity similar to the current Zr-UO2 design and become unattractive to utilities. Analysis of utilizing Mo-95 depleted molybde-num may eliminate this disadvantage. The costs to extract Mo-95 using newly developed technologies will be ana-lyzed, but the prospects are not yet known.

• Due to the large expense of introducing new fuel materi-als to commercial reactors, financial support from govern-ments is necessary to sustain the development process. To date, the U.S. Department of Energy has provided finan-cial and laboratory support for accident tolerant fuel development. The Organization for Economic Co-opera-tion and Development (OECD) - Nuclear Energy Agency (NEA) is in the planning phase to offer global support for this effort. Should support wane from the governments, program continuation will be at risk.

RECORD OF REVISION

This record of revision will provide a high level summary of the major changes in the document and identify the Road-map Owner.

revision description of change

0 Original Issue: December 2012 Roadmap Owner: Bo Cheng

1 Revision Issued: December 2013 Roadmap Owner: Bo Cheng

Changes: Expanded description in the Results Implementation section. Updated detail provided in the Material characterization and new molybdenum alloys section. Risks section updated.

2 Revision: December 2014 Roadmap Owner: Bo Cheng

Changes: Update in the Material characterization and new molybdenum alloys and Risks sections.

3 Revision: August 2016 Roadmap Owner: Bo Cheng

Changes: Updated tasks in Project Plan, Swim lanes and schedule. Modified Results Implementation section.

EPRI | Nuclear Sector Roadmaps August 2016