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Summary This research investigates the neutronic feasibility of a high‐performance soluble‐boron‐free (SBF) small modular reactor (SMR) core based on a new burnable absorber concept called the “Burnable Absorber‐Integrated Guide Thimble” (BigT). Three unique BigT designs were loaded into the core; each BigT design was judiciously ascertained from the core radial power profile to tailor the required reactivity depletion patterns for an SBF operation. The approach is demonstrated to work well as the SBF SMR design exceeds the targeted cycle length while successfully controlling its burnup reactivity swing between 634 and 800 pcm throughout most of its operation. This paper also describes the use of hafnium‐doped stainless steel as mechanical shim (MS) rods to attain the core criticality. Because the worth of the MS rods is relatively small, insertion and withdrawal of the rods during operation hardly alter the core radial power distributions. The resulting axial power profile, meanwhile, displays a more refined bottom‐skewed pattern during the early portion of the irradiation cycle due to partial top‐half insertion of the MS rods. This investigation further deliberates on a modified checker board control rod pattern to assure safe cold shutdown of the core. All calculations in this multiphysics assessment of the 3D SBF SMR core were completed by using a 2‐step Monte Carlo deterministic hybrid procedure based on the Monte Carlo Serpent and COREDAX diffusion codes with the ENDF/B‐7.1 nuclear data library.

Summary This paper presents a systematic review of the micro‐modular thorium‐fueled high temperature reactors (HTR) loaded with duplex fuel pellet design. Specifically, each unique criterion of the design is discussed separately in an attempt to understand the combined advantages of the proposed reactor. Micro modular HTRs have great potential as a source of electricity or industrial heat in the future. The use of thorium not only improves its economic performance and safety, but also prolongs its operation. Nevertheless, the traditional seed‐and‐blanket configuration would not be able to maximize the thorium fuel burnup. As such, a new tristructural‐isotopic (TRISO) fuel based on duplex pellet designs were proposed instead. It should be noted that these duplex pellets were not practical for pressurized water reactor (PWR) technology due to its uneven power distribution which possibly leads to a very high local pellet temperature. However, this unorthodox inherent characteristics of the duplex pellets may actually suit the HTR burnup profile better due to its substantially higher thermal margin ‐ thus possibly enabling a relatively long operation of the HTR. The paper shall thereby provide a reasonably sound justification for the detailed technical investigations of the proposed reactor design.

Summary This paper presents the neutronic analysis of a tristructural isotropic (TRISO)‐duplex fuel loaded into the high‐temperature reactor (HTR) prismatic fuel block designs for a seed‐and‐blanket (S&B) reactor model. The selected fuel block design is the simplified model of a representative fuel configuration for the gas turbine–module helium reactor (GT‐MHR) and high‐temperature engineering test reactor (HTTR). The duplex fuel pellet comprises of UO2 as the seed and ThO2 as the blanket and uses the TRISO fuels. Monte Carlo N‐particle extended (MCNPX) simulations were conducted to analyze the power and neutron flux distributions of the reference fuel block and that of the TRIOS‐loaded duplex fuel designs. Standard neutronic parameters were computed, such as burnup‐dependent infinite multiplication factors (kinf), average block spectrum, and fissile inventory. It was discovered that while the S&B model could attain the highest burnup and the longest cycle duration, its power peaking was more than threefolds than that of the reference model at the beginning of the cycle (BOC). The results also show that the “Duplex‐2” model could reduce its power peaking by 24% through the integration of S&B arrangement with the duplex rods. It is noted that spatial distribution of the UO2 seed and 235U enrichment are the chief determining factors in the power optimization of the heterogeneous HTR prismatic fuel blocks loaded with thorium.