**2.3 Current testing for NTP materials**

Although historical experience in NTP design has provided a wealth of valuable data, recent advances in materials research have somewhat altered approaches to the design of NTP fuel, especially with respect to fuel material compositions, fabrication, and testing. Programs described earlier used HEU; current design concepts are based on high assay low enrichment uranium (HALEU) with a 235U enrichment of less than 20% (often also referred to as simply LEU). Working with LEU greatly reduces security concerns and allows existing NASA facilities to work with fuel samples with minor modifications to address radiological concerns. HALEU would also be available at a significantly lower cost than that of HEU, and is much easier to transport. At the time of this writing, NASA is working with existing feed stock for test specimens as the U.S. cannot currently produce HALEU fuel. However, in June 2021, the U.S. Nuclear Regulatory Commission (NRC) approved a request from Centrus Energy to produce HALEU fuel at its enrichment facility in Piketon, Ohio. Centrus has already built 16 centrifuge machines for uranium enrichment, expecting to begin HALEU production by early 2022 [28]. This fuel will be used by both NASA and a number of advanced reactor prototypes under development in the U.S.

Current research and development efforts are organized within NASA's Space Technology Mission Directorate and are focused on both fabrication and performance under prototypical conditions. Although no NTP engine prototypes have been developed since the earlier work in Rover and NERVA programs, other facilities have been used for materials testing under reactor conditions. In early 2015, the first partial-length fuel elements were tested in the Nuclear Thermal Rocket Element Environmental Simulator (NTREES) located at NASA's Marshall Space Flight Center (MSFC) [29]. NTREES has been designed to provide up to 1.2 MW of heating to simulate an NTP thermal environment by capturing exposure to hydrogen heated to temperatures up to more than 3000 K. Numerous tests have been completed in NTREES; however, the facility is non-nuclear and unable to produce the intense neutron and gamma fluxes that will be present in an NTP engine. To that end, a number of tests have been completed or are planned for highpower transient tests in TREAT. In June 2019, the experiment designated as SIRIUS-CAL was the first test of an NTP-type fuel specimen. As with NTREES, a number of tests with representative fuel specimens have been completed and are ongoing.

To date, tests have been performed using CERMET fuel specimens based on fabrication experience gained in earlier ANL and GE CERMET tests, along with other facility tests. About 200 CERMET samples were tested in the various programs by thermally cycling to high temperatures in hydrogen [6], providing valuable data for performance and fabrication. CERMET fuel also allows for considerable control in fabrication due to the unique structure of the material itself.

Building on the earlier experience with natural tungsten as a matrix material, new materials have been evaluated:


CERMETs can be used in LEU designs as discussed in Refs. [23, 32]. However, parasitic absorption of tungsten and 238U, as well as reduction in fissile content, make it impossible to build a CERMET-based fast spectrum LEU core using natural tungsten [33]. Thus, the current focus of CERMET LEU cores is on thermal spectrum systems [23, 32]. For thermal reactors, CERMET offers the advantage of a higher fuel density as compared with composite GMWDF. However, the neutronic behavior of a CERMET compact is challenging because of parasitic absorption, the lack of moderating ability, and a short mean free path for the thermal neutrons [23]. These challenges result in difficulty adding reactivity to the core, requiring large fuel loadings and effective reflectors. They also exhibit significant self-shielding across fuel elements (with NERVA dimensions), leading to intra-element peaking and non-uniform burnup distributions after several tens of hours of operation [23].

NASA has been pursuing a parallel path in evaluation of CERMET- and CERCER (CERamic–CERamic)-based fuel forms. In 2021, NASA decided to place more emphasis on CERCER-based fuel concepts moving forward, although a number of CERMET-based fuel experiments are in the testing pipeline for the next few years. As opposed to CERMET, CERCER fuel requires approximately seven times less HALEU, has lower maximum fuel meat stresses, and is lighter [34]. CERCER fuels with coated fuel particles also offer the potential for increased margins with respect to fuel matrix melting compared to CERMET systems, but are at a lower level of technological and fabrication maturity. CERMET fabrication and testing began in the 1960s and 1970s for NTP applications, while CERCER (in NTP applications) is a relative newcomer [35]. The fabrication processes of CERCER fuels is currently based on relatively simple compression and sinter methods.

Both CERMET and CERCER fuels are being tested at both the TREAT and NTREES facilities. **Figure 2** illustrates the current plan for the experiments at TREAT with both CERCER and CERMET for the next several years. The CERMET tests have served as a technology pathfinder for CERCER fabrication and testing methods. The figure also shows the current plan for the testing program at TREAT, with experimental configurations becoming more complex, as well as plans to migrate from CERMET to CERCER fuels.

**Figure 2.**

*Current high power neutronic testing plans; picture on the right shows the SIRIUS-1 test specimen being prepared for irradiation testing.*
