Precise characterization of the fast-neutron energy spectrum constitutes a fundamental requirement for evaluating radiation damage in reactor pressure vessels (RPV). Conventional MLEM iterative adjustment algorithms applied to high-precision spectrum adjustment often result in potential spectral artifacts due to the inability to rigorously propagate off-diagonal covariance terms. A novel Bayesian Regularized Maximum Likelihood Expectation Maximization (BR-MLEM) framework is proposed to address these challenges by integrating hierarchical Bayesian priors with adaptive regularization. This methodology systematically incorporates covariance matrices associated with the prior spectrum, response functions, and measurements, employing singular value decomposition (SVD)-based Monte Carlo sampling to mitigate noise amplification. Application to the H. B. Robinson Unit 2 benchmark demonstrates the algorithm’s capability to produce physically consistent spectra while effectively suppressing non-physical distortions observed in standard MLEM iterations. The adjusted calculated specific activity exhibits excellent agreement with experimental dosimetry data, achieving consistency comparable to generalized least-squares methods but with superior stability in high-energy regions. Posterior uncertainty analysis reveals a significant reduction in relative standard deviations across primary fast-neutron energy groups. These findings confirm that the BR-MLEM framework enhances the fidelity of fluence estimation and provides a robust tool for RPV lifetime management.
This study presents a combined experimental and Geant4-based assessment of the radiation field and shielding performance of the Iranian Inertial Electrostatic Confinement fusion device (IR-IECF) operating in deuterium–deuterium (D–D) mode and producing 2.45 MeV neutrons. A detailed Geant4 model of the IR-IECF device, its surrounding borated-polyethylene shielding, and the relevant dosimetry locations was developed to calculate neutron and gamma dose distributions. Experimental gamma doses were measured using GR-200 (LiF:Mg,Cu,P) thermoluminescent dosimeters, whereas neutron ambient dose-equivalent rates were measured using a Berthold LB6411 rem counter. In the Geant4 model, neutron dose scoring was performed using LiF-based virtual dosimeters positioned at the corresponding monitoring locations, allowing the numerical results to be compared with the experimental neutron measurements. The simulated neutron and gamma dose values showed good agreement with the corresponding experimental data, supporting the reliability of the developed Geant4 model for radiation-field characterization. In addition, a dedicated secondary-particle scoring analysis was performed to characterize the secondary radiation generated during neutron transport. Electrons, protons, and photons were the dominant recorded secondary particles, while secondary neutrons accounted for approximately 1.69% of the recorded secondary tracks. Overall, the combined experimental and numerical approach provides a practical framework for radiation-field characterization and shielding assessment of IECF-based neutron sources.
Single-metal interlayer coating systems reliably protect standard Zircaloy tubes, making them suitable candidates for accident-tolerant fuel (ATF) cladding in nuclear service environments. However, they also deplete during interfacial metallic interactions (e.g., during Nb-Zr interdiffusion) and co-precipitate (e.g., when Mo interlays between Cr and Zr), thereby losing their availability as protective coatings for long-term use. In the quest to address these shortcomings, the present study explores a multiple-metal-engineered high-entropy alloy (HEA) coating (CrFeNiMoAl), proposed as a reliable alternative, particularly based on Mo as a refractory metal, for testing under high-thermal conditions. Four coating variants with Mo content ranging from 0 to 30 % (HEAMo0-3) were deposited on Zircaloy-4 via magnetron sputtering. To examine their corrosion behavior, these coatings were comparatively tested under conditions simulating Pressurized Water Reactor (PWR) cooling systems via electrochemistry in LiOH/H3BO3, by accelerated annealing (1200 °C at atmospheric pressure for 5 min), and subcritical corrosion tests at 360 °C in 18.6 MPa steam. In addition to material strengthening, improved barrier performance is attributed to the optimal Mo content, which supports the presence of predominantly body-centered cubic (BCC) nanocrystalline multiphase structure. Barrier performance depends largely on Mo content in the following order of increasing strengths: HEAMo1 (with 10.1 % Mo) ˃ HEAMo2 (with 20.7 % Mo) ˃ HEAMo3 (with 30 % Mo) ˃ HEAMo0 (with 0 % Mo). Higher Mo content oversaturated the internal coating structure, leading to coating degradation, increased Mo-ion release, and the formation of additional pathways for corrosive ions to reach the Zr surface. This work does not aim to investigate interfacial elemental diffusion, as ample research already addresses this topic. The essential insight from the present study concerns the impact of coolant chemistry and coating content on the performance and durability of ATF-cladding materials for next-generation nuclear reactor fuel systems.
