Brazilian Multipurpose Reactor (RMB)

• The Brazilian Multipurpose Reactor (RMB) is a versatile nuclear facility designed to deliver 30 MW thermal power and a core neutron flux of 4×10^14 n/cm²·s.
• It supports applications in radioisotope production, material irradiation, and neutron imaging through simulation-driven guide and instrument design.
• Its modular structure and optimized neutron transport strategies position Brazil as an emerging leader in advanced nuclear research.

The Brazilian Multipurpose Reactor (RMB) is a large-scale, high-flux nuclear research facility under development in Brazil with a comprehensive suite of capabilities spanning radioisotope production, nuclear fuel and material irradiation, neutron beam applications, and supporting advanced instrumentation for neutron science. The RMB is designed to achieve a thermal power of 30 MW, deliver a core neutron flux of approximately $4 \times 10^{14}$ $n/\mathrm{cm}^2\,s$, and maintain a power density of 312.5 $\mathrm{W/cm}^3$ (Oliveira et al., 12 Oct 2025). It represents a major infrastructural advancement, positioning Brazil among international leaders in nuclear research reactor technology.

1. Facility Overview and Technical Specifications

The RMB’s primary specifications serve multiple objectives in science and industry:

• Thermal Power: $30\;\mathrm{MW}$ guarantees sufficient neutron production capacity for both radioisotope generation and material research.
• Neutron Flux: Core flux of $4\times 10^{14}\;n/\mathrm{cm}^2\,s$ enables efficient activation analyses, high-yield isotope production, and in-depth material irradiation studies.
• Power Density: $312.5\;\mathrm{W/cm}^3$ sustains high local neutron fluxes even at distant irradiation positions or beamline exits.

The operational design ensures that the minimum required thermal neutron flux for key activities (e.g., Mo-99 production) is $$\phi_\text{min}=5.1\times 10^{12}\;n/\mathrm{cm}^2\,s$$, maintained even under heavy irradiation scenarios including perturbations from highly absorbing materials such as Gd-157 (Oliveira et al., 12 Oct 2025). The reactor core employs U$_3$Si$_2$ fuel plates and features a modular structure allowing flexible deployment of both irradiation and beamline instruments (Oliveira et al., 2022).

2. Neutron Source and Instrumentation

The RMB is configured to support advanced neutron science:

• Neutron Guide Building (N02): Houses three cold and three thermal neutron guides delivering beams to a suite of state-of-the-art instruments, critical for materials characterization and fundamental research (Souza et al., 2020).
• Flagship Instruments:
  • Sabiá (SANS): Small-Angle Neutron Scattering instrument (cold guide CG1), optimized for nanostructure characterization with operational wavelengths spanning $1-30\;\text{Å}$ and typical flux parameters compatible with high-resolution international standards (Souza et al., 2020).
  • Araponga (HRPND): High-Resolution Powder Neutron Diffractometer (thermal guide TG1), engineered for crystallographic and magnetic studies. Achieves sample fluxes $\sim 1.13 \times 10^7\,n/\mathrm{cm}^2\,s$ in simulations, with $\Delta d/d \approx 10^{-4}$ resolution (Souza et al., 2022).

Instrument	Neutron Type	Typical Flux ($n/\mathrm{cm}^2\,s$)	Application Area
Sabiá	Cold	($\sim10^7$ at sample)	Nanostructures, soft matter
Araponga	Thermal	($\sim10^7$ at sample)	Crystallography, magnetism

Monte Carlo ray-tracing via McStas and MCNP6 has informed instrument and neutron transport design, supplying quantitative predictions for guide efficiency, optimization of collimation, and spectral matching (Souza et al., 2020, Souza et al., 2022, Oliveira et al., 2022).

3. Neutron Transport Optimization

Critical technical advances in neutron transport underpin RMB performance:

• Guide Design: Utilizes both simple (transmitted) and split guide configurations. The split guide approach—dividing primary guides horizontally or vertically to simultaneously feed multiple instruments—demonstrates up to 20% improved flux efficiency relative to singular transmission, particularly when instrument collimation would otherwise significantly truncate flux (Souza et al., 2020).
• Supermirror Coating Selection: Theoretical optimizations rely on the critical angle formula for reflection:

$\theta_c \approx 1.73 \times 10^{-3} m \lambda$

where $m$ is the coating index and $\lambda$ the neutron wavelength. Asymmetric supermirror coatings ($m_{\mathrm{out}} > m_{\mathrm{in}}$) on curved guides enhance transport efficiency and spectral selectivity.

• Simulation-Driven Configuration: Multiple source profiles and transport geometries (straight vs. curved, split vs. transmitted guides) have been quantitatively benchmarked, with split/horizontally split channels proving best in most cases—especially when paired with advanced collimation and monochromator systems (Souza et al., 2020).

4. Applications in Nuclear Science, Medicine, and Industry

RMB’s multipurpose architecture supports an array of high-impact research and industrial functions:

• Radioisotope Production: Domestically meets Brazil’s demand for Mo-99, with robust neutron flux stability confirmed by simulation, even under local flux perturbations from absorptive targets (Oliveira et al., 12 Oct 2025).
  • Mo-99 Production Calculation: Activity after 7 days ($t$) irradiation:

  $$A(t) = N \cdot \phi \cdot \sigma_f \cdot (1 - e^{-\lambda t})$$

  where $$\sigma_f \approx 5.85 \times 10^{-24}\;\mathrm{cm}^2$$ (Oliveira et al., 12 Oct 2025).

• Material and Fuel Irradiation: Supports nuclear and materials research via controlled high-flux environments for irradiation, enabling studies on radiation damage, activation products, and performance of advanced alloys/composites.
• Neutron Imaging (Neinei Instrument): High-resolution radiography and tomography, competitive in neutron flux and imaging resolution with international standards, with tunable $L/D$ ratios and Monte Carlo-driven optimization of beam quality (Oliveira et al., 2022).
• Neutrino Physics: The RMB is expected to produce up to $3,500$ antineutrino events per day, enabling precision experiments on oscillation phenomena (parameter $\theta_{13}$), inverse beta reactions, and serving nuclear safeguards via online monitoring of fuel burnup and isotope ratios (Oliveira, 26 Mar 2024).
• Terbium Isotope Production: Feasibility for nuclear medicine use (e.g., Tb-161, Tb-155) via direct irradiation or neutron activation of gadolinium targets (Oliveira et al., 12 Oct 2025).

5. Simulation Methodologies and Instrument Benchmarking

Sophisticated modeling strategies guide instrument configuration and benchmark performance:

• Multi-code Workflow: MCNP6 simulates core, reflector, and neutron moderator; PTRAC outputs directly feed McStas ray-tracing to quantify neutron beam delivery, spectral distribution, and collimation effects (Souza et al., 2022, Oliveira et al., 2022).
• Instrument Comparison: Simulated instrument parameters (e.g., Neinei neutron flux) are directly compared to leading facilities such as DINGO (OPAL), Neutra (PSI), ANTARES (FRM-II), and BT2 (NIST), confirming RMB’s competitive edge in high-flux imaging and scattering applications (Oliveira et al., 2022).
• Performance Metrics: Key operational measures include neutron beam intensity at the sample/detector, spectral purity, beam divergence as a function of $L/D$ ratio, and spatial uniformity (Oliveira et al., 2022, Souza et al., 2022).

6. Future Directions, Technical Challenges, and Impact

While RMB’s design and simulated performance indicate readiness for multi-disciplinary applications, several avenues for further enhancement and investigation are recognized:

• Beam Homogeneity Optimization: Ongoing work involves refining guide and collimator arrangements to minimize spatial non-uniformity at the detector plane—an important factor for imaging and SANS experiments (Oliveira et al., 2022).
• Gamma Shielding and Dose Management: Simulation methodologies incorporating dose analysis and shielding design (especially for CCD-based imaging) using MCNP or equivalent codes are under consideration (Oliveira et al., 2022).
• Detector Technology Advancements: Modernization could include adoption of skipper-CCD or ultra-low noise detectors to improve antineutrino signal-to-noise ratio for neutrino physics experiments (Oliveira, 26 Mar 2024).
• Robustness to Irradiation Perturbations: Via Monte Carlo analysis, the reactor exhibits stability in neutron flux even when loaded with strongly absorbing materials—ensuring reliability for critical radioisotope production and irradiation tasks (Oliveira et al., 12 Oct 2025).
• Expanded Instrument Suite: The integration of complementary diffractometers (e.g., Flautim for high-intensity powder diffraction) and advanced neutron imaging supports broadening of scientific reach and throughput (Souza et al., 2022).

The RMB’s comprehensive instrument set, high flux levels, and rigorous simulation-driven design position it as a pivotal platform for radioisotope production, advanced materials research, nuclear fuel performance studies, imaging, and fundamental physics investigations. The facility is expected to catalyze major advances across nuclear medicine, industrial radiography, materials science, and reactor safeguards, situating Brazil within the foreground of global neutron science and multipurpose reactor operation.