The In-Core Real-Time Mechanical Testing of Structural Materials (INCREASE-I) project was launched under the Second Framework for Irradiation Experiments (FIDES-II), an OECD Nuclear Energy Agency (NEA) collaborative research programme that enables member countries to jointly advance nuclear fuels and materials research by making effective use of international irradiation facilities and collective expertise.
INCREASE-I project aims to develop capabilities for performing a variety of in-core mechanical testing under neutron irradiation. Unlike conventional reactor experiments, where the design and analysis are tailored to a single reactor and a specific experiment, INCREASE-I employs a generalised design and analysis framework that can be adapted to the requirements of testing multiple materials. The INCREASE-I project achieved a major milestone by successfully transitioning from design, fabrication and qualification into full system integration and irradiation readiness. This accomplishment represents the culmination of several years of collaboration among the US Department of Energy (DOE), including Idaho National Laboratory (INL), the US Nuclear Regulatory Commission (NRC), Electric Power Research Institute (EPRI), the French Alternative Energies and Atomic Energy Commission (CEA), NRG PALLAS, the Joint Research Centre - European Commission (JRC), and Research Centre Rez in the Czech Republic (CVR). Together, these organisations established foundations for a first-of-a-kind experiment. This in-core experiment generates unique stress relation data on material behaviour under coupled irradiation, thermal and mechanical loading conditions.
A major achievement of the project has been the successful fabrication, qualification and verification of both the active and passive irradiation capsule systems. This culminates in a fully integrated and instrumented experimental assembly. To ensure system performance and reliability prior to reactor insertion, extensive quality assurance and validation activities were completed. These efforts included instrumentation verification, weld development and qualification, leak testing, autoclave testing, sensor validation and bench-scale functional testing of critical components. Completing these activities significantly reduced technical risk and demonstrated readiness for deployment within the Massachusetts Institute of Technology Reactor (MITR).
Figure 1: Fabrication and assembly of active irradiation capsules. (a) Active tensile specimen; (b)Type-K thermocouples; (c) Bellow encapsulation; (d) Active capsule before encapsulation; (e) Final active capsule.
Figures 1 and 2 illustrate key stages in the fabrication, assembly and verification of active irradiation capsules, fabricated at the Institute for Energy Technology (IFE), that will provide in-situ stress relaxation measurements during irradiation.
Figure 2: (a) Verification of active irradiation capsules and (b) Shipment preparation.
Significant progress was also achieved on the passive irradiation capsules, which contain both stress relaxation specimens and static specimens for post-irradiation microstructural characterisation. In addition, passive temperature sensors and passive neutron fluence monitors were incorporated to measure irradiation temperatures and neutron flux reached during irradiation. Standardised specimen preparation, inspection, handling, loading and assembly procedures were developed to minimise uncertainties associated with specimen geometry and surface condition while improving experimental reliability and repeatability.
Another major accomplishment was the successful integration of the active and passive irradiation capsules into a unified experimental platform. Close co-ordination among INL, MIT and other partners enabled resolution of challenges associated with fabrication, assembly, instrumentation integration and system interfaces. The final INCREASE-I assembly demonstrated compatibility among the mechanical, thermal, pressure and instrumentation subsystems, positioning the experiment for deployment and operation within the reactor environment.
Figure 3: Passive irradiation capsule preparation and assembly. (a) Passive capsule before assembly and (b) fully finished passive capsule.
Figures 3 and 4 show the passive capsule assembly and fully integrated INCREASE-I experiments ready for MITR irradiation.
Figure 4: Final integrated INCREASE-I assembly ready for MITR irradiation. (a) Assembly of all 8 irradiation capsules (4 passive, 4 active); (b) traceability of all mineral insulated (MI) cables coming from each sensor; (c) Integration of INCREAS-I assembly into MITR irradiation position; (d) Conversion of all MI cables coming from each sensor to soft extention cables with traceability that connect to data acquition system (DAQ).
Beyond enabling the upcoming irradiation campaign, INCREASE-I directly supports the development of INCREASE-II. The two projects share common design concepts. In fact, the INCREASE-I capsule was intentionally developed using a modular and adaptable architecture to facilitate deployment in multiple international research reactors. The lessons learnt in fabrication, instrumentation integration, system qualification, operational procedures and risk management are being incorporated into the design and planning of INCREASE-II.
Collectively, these efforts reflect the strength of international partnerships in addressing complex scientific and technological challenges. They establish a pathway for expanded international irradiation testing capabilities to support structural material qualification, model validation, and the advancement of next-generation nuclear energy technologies.
