A soluble-boron-free (SBF) pressurized water reactor (PWR) core potentially offers significant operational improvements over the commercial designs. This dissertation investigates the neutronics feasibility of two SBF PWR core designs, namely (1) a single-batch 200 MWth small modular reactor (SMR), and (2) a 3-batch 3400 MWth AP1000 equilibrium core. The research specifically pursues the SBF core designs with a new burnable absorber (BA) concept named “Burnable absorber-Integrated Guide Thimble” (BigT).
Neutronic sensitivities and practical design considerations of the BigT concept are points of highlight in the first half of the thesis. The BigT concepts are particularly characterized in view of its BA material and spatial self-shielding variations. It is demonstrated that the BigT is sensitive to the BA’s geometrical aspect ratio (i.e., thickness and azimuthal span), especially when gadolinium is used as the BA. Furthermore, the work also deliberates major conceptual details of the BigT absorber design, such as the assembly top-head replaceability requirement, dashpot-compliant bottom-end and possible thermal-mechanical characteristics of the BigT design. Moreover, the BigTs are also benchmarked against the conventional BA technologies (e.g., Westinghouse’s IFBA and WABA rod designs) in representative 17×17 and 16×16 fuel assembly lattices. The BigT absorbers are shown to perform reasonably well in comparison with the commercial BA technologies, especially in terms of reactivity depletion and power distribution managements. It is also shown that sufficiently high control rod worth can be obtained with the BigT in place. All assembly-level depletion calculations in the BigT sensitivity studies were completed using the Monte Carlo Serpent code with ENDF-B7.0 nuclear data library.
The second half of the dissertation meanwhile focuses on the strategic loading of the BigT to enable SBF PWR operation in the two aforementioned 3-D core applications. In the SBF SMR core, the BigTs were designed region-wise and each BigT design was judiciously crafted to tailor the required reactivity depletion patterns ascertained from the core radial power profile. In the AP1000 equilibrium core, the BigT absorbers were designed batch-wise. In particular, BigTs in fresh feed assemblies were paired with the commercial IFBA fuel rods to attain an upward reactivity depletion pattern. Meanwhile, BigTs in once-burned assemblies were devised so as to obtain a downward reactivity trend which possibly cancels the aforementioned reactivity upswing of the fresh assemblies.
These unique approaches of strategically loading the BigT absorbers in the cores have been demonstrated to work very well to potentially enable the SBF PWR operations. In fact, the SBF SMR design exceeds the targeted cycle length while successfully navigating the burnup reactivity swing between 360~990 pcm throughout the operation. The work also introduces the use of hafnium-doped stainless steel mechanical shim (MS) rods in order to attain the core criticality. Since worth of the collective MS rods is relatively small (< 600 ppm), it was clearly demonstrated that insertion and withdrawal of the rods during the SBF operation negligibly alter the core radial power distributions. Nonetheless, the core axial power profile displays a more refined bottom-skewed pattern during the early part of the irradiation cycle due to the partial top-half insertion of the MS rods. The work also deliberates on a modified checker-board control rod pattern utilizing 95 w/o enriched boron carbide absorber to assure a safe cold shutdown condition of the SMR core. All calculations in the multi-physics performance assessment of the 3-D SBF SMR core were completed using the conventional 2-step Serpent-Coredax reactor analysis procedure. Meanwhile, the preliminary investigations on the SBF AP1000 equilibrium core suggest that our proposed approach can be quite promising as the burnup reactivity swing over the 490-day equilibrium cycle was quite small (< 2,000 pcm). In addition, it was shown that sufficiently high hot shutdown margin with reasonable radial power distribution can also be attained with the conventional AP1000 control rod pattern.