I. Core Performance: Why Must It Be Boron Carbide Pellets?

The core value of boron carbide pellets stems from their unique neutron capture mechanism. ¹⁰B absorbs neutrons through the (n,α)⁷Li reaction, producing no strong secondary radiation, and possesses a broad absorption energy spectrum suitable for various reactor types including thermal neutron reactors and fast neutron reactors. Compared with powder form, pellets offer higher density and structural strength, facilitating encapsulation in stainless steel or zirconium alloy cladding tubes to form standardized absorber rod assemblies.

However, boron carbide's stable lattice structure and extremely low self-diffusion coefficient make forming difficult and densification challenging. The sintering and preparation of high-density pellets (≥90% of theoretical density) has remained a technological challenge for decades—this is precisely where the technical barrier lies.

II. Main Application Scenarios

1. Control Rod Absorbers

Control rods are the "accelerator and brake" for regulating nuclear fission reaction rates. Boron carbide pellets are encapsulated in stainless steel cladding to manufacture control rods, safety rods, and regulating rods, widely used in light water reactors, heavy water reactors, high-temperature gas-cooled reactors, and fast neutron reactors.

Taking fast neutron reactors as an example, control rods consist of 61 stainless steel-clad boron carbide pellet rod bundles, achieving reactivity control through the neutron capture reaction of ¹⁰B. In TRIGA research reactors, 16 Incoloy cladding tubes filled with boron carbide pellets (70% theoretical density) are arranged in a square array, verified through neutronic analysis to possess complete compensation capability.

The China Nuclear Power Institute began developing boron carbide pellets in 1994. In 2024, it completed the batch delivery of 4.4 tons of high-density boron carbide pellets, with a density qualification rate exceeding 98%. The products will be used in shielding assemblies for nuclear power units under construction, marking the basic formation of a full-industry-chain independent supply model for high-density boron carbide pellets.

2. Burnable Poisons

In pressurized water reactors, boron carbide pellets also appear in the form of burnable poisons. Westinghouse's Wet Annular Burnable Absorber (WABA) uses Al₂O₃-B₄C annular pellets encapsulated in concentric zirconium alloy tubes. The role of burnable poisons is to compensate for excess reactivity during initial reactor loading, gradually being consumed as burnup progresses, thereby avoiding the issues associated with soluble boron.

Boron carbide-zirconium alloy burnable poisons offer excellent irradiation stability and corrosion resistance. The hollow pellet design reduces the "water displacement effect," enhances neutron moderation, and extends fuel service life.

3. Shielding Assemblies

Boron carbide pellets are also used in reactor shielding assemblies. The China Experimental Fast Reactor (CEFR) employs boron carbide shielding absorber pellets, which have been qualified through 383 effective days of irradiation testing, with fast neutron fluence reaching 3.8×10²² cm⁻², demonstrating high irradiation stability and compliance with service requirements.

III. Technical Challenges and Frontiers

The core challenge facing boron carbide pellets is irradiation-induced swelling. After ¹⁰B absorbs neutrons, helium gas is generated, and its accumulation within the pellet may cause cladding rupture. In fast reactor control rod designs, pellets typically use 70% theoretical density with reserved void space to accommodate helium gas.

Material behavior under accident conditions has been a research focus in recent years. In Core Disruptive Accident (CDA) scenarios of sodium-cooled fast reactors, boron carbide pellets and stainless steel cladding form eutectic melting at high temperatures, and boron migration and boron carbide melt relocation may introduce recriticality risks. Recent research, through radiant heating experiments, has for the first time directly observed eutectic melting, boron-carbon diffusion, and control rod failure mechanisms between B₄C pellets and SS-304 cladding under an inert argon atmosphere, providing critical data for accident mitigation measures.

From control rods to burnable poisons, from shielding assemblies to accident analysis research, boron carbide pellets continue to support the safe operation of nuclear energy systems through their irreplaceable neutron absorption performance. The batch production capability for high-density pellets is an important marker of self-reliance in the nuclear industry.