In-Reactor Experiments (IRE)

One element of the Nuclear Engineering Division's support to nuclear reactor safety R&D has been its performance of accident simulation experiments on nuclear reactor fuels and materials in the Transient Reactor Test (TREAT) facility located at the Idaho National Laboratory (formerly at the Argonne-West site). Prior to the facility being placed into its current non-operational standby status in 1994, Argonne had been the principal user of TREAT and retains considerable expertise and experience in performing experiments in the facility.

In-reactor experiments were typically performed as part of advanced fuel development programs to investigate the response of prototypic and experimental fuels to off-normal and severe-accident conditions. Most of the tests were on fuel elements that were previously irradiated in a power reactor and thus had typical burnup-induced characteristics prior to the test. The test fuel was normally contained in a sealed capsule or flowing-coolant loop that was inserted into the TREAT reactor core. A programmed power transient in the TREAT core generated the prototypic accident-related fission heating in the test fuel. Generally, most of the phenomena of interest during the accident simulation occur as a result of the specified fuel over-heating transient. Thermal-hydraulic events taking place near the test fuel sample are monitored remotely, typically by thermocouples, flow meters, and pressure transducers. Motion of the over-heated test fuel is recorded real-time by a multi-channel fast-neutron hodoscope. Additional information is obtained after the test by studying both the macroscopic and microscopic changes in the fuel and surrounding structures with which the fuel may have physically and chemically interacted. This information provides a basis for evaluating the safety characteristics of the fuel.

Because of their ability to produce fission-generated heat in the test fuel, in-reactor transient experiments provide test conditions more prototypic than can be obtained in out-of-reactor experiments. As such, they complement less-complex separate-effects (or "phenomenological") tests performed in the laboratory or hot cell that use less prototypic materials or experiment conditions. The information provided by the in-reactor experiments guide the computational modeling of fuel transient behavior, validate model predictions, qualify new fuel designs for extended irradiation in lead test assemblies, provide an empirical basis for establishing fuel operational limits, and resolve key safety issues pertaining to advanced reactor fuel designs.

Because the test fuel in the TREAT experiments is usually highly radioactive, it must be examined and loaded into TREAT test hardware in the nearby Hot Fuel Examination Facility (HFEF) using remote handling capabilities of the hot cell. Close coordination among the experimenter, TREAT staff, and HFEF staff is essential in the performance of a TREAT experiment.

The planning, design, and interpretation of experiments in TREAT is synergistically aided by computer codes that predict the types of behavior anticipated in the experiment. For fast-reactor safety experiments, the SAS4A code system developed by NE is the primary analysis tool. Its analytical models apply to both the detailed conditions and phenomena generated in in-reactor transient tests and the corresponding conditions and phenomena in power reactors. Thus, the code provides a bridge for relating the experiments to the systems or conditions that the experiments are designed to simulate. The experiments themselves provide key information needed in the development of analytical models used in the codes.

Motion of the test fuel during the experiment is of key importance in many TREAT tests.  Test fuel motion is monitored using the fast neutron hodoscope at TREAT. The hodoscope provides a sequence of images of the test fuel using the higher-energy (“fast”) neutrons that are generated by fissions occurring in the test fuel and that penetrate the walls of the experiment vehicle.  Analysis and interpretation of the fuel-motion data from the hodoscope, thermal-hydraulic data from the test vehicle, neutronics data from the reactor, and metallographic data from fuel examinations together provide a basis for understanding the complex physical, chemical, and neutronic processes involved in reactor accident scenarios. TREAT hodoscope data reduction, analysis, and display techniques and software, developed by NE staff, will need to be modernized as part of reestablishing TREAT experiment capability.

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