The Physics of Chemical Shims: Why Advanced Reactors Are Struggling to Eliminate Soluble Boron

Advocates for next-generation small modular reactors frequently frame the elimination of soluble boron as a pure simplification of plant chemistry, removing corrosive acids and complex pumping systems from the primary coolant loop. The pitch is highly appealing: take the acid out of the water, and the reactor becomes cheaper to build, easier to maintain, and less prone to chemical degradation over its multi-decade lifespan. But reactor physics dictates that the immense excess reactivity loaded into fresh nuclear fuel must be suppressed somewhere. As researchers analyzing soluble-boron-free cores note, removing the chemical shim does not eliminate the physics problem; it merely relocates it. It forces the entire burden of reactivity control onto mechanical control rods, fundamentally altering the reactor's safety profile and creating new vulnerabilities that engineers are still working to resolve.[4]

The core challenge of any nuclear reactor is managing the immense potential energy loaded into fresh fuel. At the beginning of a fuel cycle, a pressurized water reactor contains significantly more fissile material than is strictly necessary to maintain a critical chain reaction. This surplus, known as excess reactivity, ensures the reactor can continue operating for 12 to 18 months without needing to be refueled. However, if left unchecked, this excess reactivity would cause the chain reaction to accelerate uncontrollably. Therefore, it must be continuously and precisely absorbed to keep the reactor operating at a steady state.[1]

Commercial nuclear power has long relied on two distinct systems working in tandem to manage this delicate balance: mechanical control rods for rapid, localized adjustments, and a 'chemical shim' for slow, whole-core suppression. The chemical shim consists of boric acid dissolved directly into the primary coolant water that flows over the hot fuel rods. Because the boron is spread evenly through the water, it turns the whole reaction up or down uniformly, without distorting the core's power shape the way a solid control rod does.[1][6]

Boron-10 is an exceptional neutron poison, meaning it has a massive appetite for absorbing the thermal neutrons that sustain the nuclear chain reaction. Currently, boric acid is maintained in reactor coolant at concentrations ranging from 0 to 2000 parts per million, depending on how far along the fuel is in its burnup cycle. At the start of a cycle, when the fuel is fresh and highly reactive, the boron concentration is at its highest. This homogeneous liquid absorber acts as a invisible blanket, keeping the core calm and stable.[5][6]

As the uranium fuel depletes over a typical operating cycle, operators gradually dilute the boric acid concentration by injecting pure water into the primary loop. This steady removal of boron from the coolant compensates for the loss of fissile material, keeping the reactor critical without requiring constant mechanical intervention. Because the chemical shim handles the heavy lifting of long-term reactivity control, the mechanical control rods can remain mostly withdrawn, held in reserve for emergency shutdowns, known as scrams, or for making minor, rapid power adjustments when grid demand fluctuates.[1][5]

However, dissolving a neutron poison into the primary coolant introduces cascading systemic complexities. Boric acid is inherently corrosive to the steel and alloy components inside the reactor vessel. Managing this corrosion requires specialized piping, highly resilient pumps, and elaborate chemical volume control systems that add significant capital cost and physical footprint to the plant. Furthermore, the constant adjustment of the boron concentration generates a continuous stream of low-level radioactive wastewater that must be carefully processed, stored, and eventually disposed of.[3]

Beyond corrosion and wastewater, the chemical shim is the primary driver of radioactive tritium generation in standard pressurized water reactors. When Boron-10 absorbs a neutron, it occasionally undergoes a reaction that produces tritium, a radioactive isotope of hydrogen. According to a 2014 design report from the University of California, Berkeley, 'Approximately 90% of the total tritium in PWR reactor coolant is produced in the coolant by the soluble boric acid reactivity shim.' This forces operators to implement complex tritium diffusion barriers and stringent containment protocols to prevent the isotope from escaping into the environment.[2]

These compounding drawbacks have driven a surge of interest in soluble-boron-free reactor designs, particularly among developers of small modular reactors. By eliminating the chemical shim entirely, engineers can remove entire classes of accidents related to inadvertent boron dilution, shrink the plant's physical footprint, and drastically reduce the corrosion of internal components. For a small modular reactor designed to be factory-built and shipped on a truck to a remote location, eliminating the bulky chemical volume control system represents a massive engineering victory.[3][4]

But removing the chemical shim means the mechanical control rods must absorb the entirety of the core's excess reactivity. In a standard pressurized water reactor, control rods are primarily used for rapid shutdowns and fine-tuning, with their overall 'worth'—the amount of reactivity they can suppress—intentionally diluted by the presence of the boric acid. In a soluble-boron-free reactor, they become the sole active mechanism for long-term reactivity control, forcing them to carry the full weight of the reactor's physics from the first day of the fuel cycle to the last.[4]

This shift creates a severe spatial imbalance within the reactor core. Control rods are discrete, solid absorbers inserted from the top of the vessel. When they are used to suppress large amounts of reactivity, they severely depress the neutron flux in their immediate vicinity while allowing power to peak in unrodded areas. Unlike the homogeneous chemical shim, which suppresses the reaction evenly, deeply inserted control rods distort the axial power profile, creating hot spots that can degrade the fuel cladding over time.[1]

To compensate for the lack of soluble boron, a boron-free reactor requires a much higher total control rod worth. This means each individual rod must be engineered to absorb a massive number of neutrons. This high individual worth introduces a distinct and severe vulnerability during a hypothetical rod ejection accident, a scenario where a mechanical failure of the drive mechanism housing causes the highly pressurized reactor coolant to violently blow a control rod assembly out of the core.[4]

Because the individual rod holds back so much reactivity, its sudden removal acts like a compressed spring being released, triggering an immediate and violent localized power spike. A 2026 study published in the journal MDPI modeled this exact scenario to compare the safety margins of different reactor architectures. In traditional boron-based small modular reactors like the SMART and NuScale designs, a rod ejection accident resulted in peak power levels reaching 104% to 212% of the reactor's nominal power, a manageable transient that existing safety systems can easily handle.[4]

In stark contrast, when the same rod ejection accident was modeled in a soluble-boron-free core, the highly concentrated nature of the mechanical reactivity control caused a massive and instantaneous spike. The researchers found that the normalized reactivity drove the total core power to a staggering peak of 5000% of its nominal value. While the study concluded that fuel-cladding integrity was not ultimately threatened due to the rapid thermal feedback mechanisms inherent to the fuel, the sheer magnitude of the excursion highlights the unforgiving margins involved.[4]

To mitigate these localized spikes and reduce the overall burden on the control rods, boron-free designs must rely heavily on fixed burnable poisons. These are solid neutron absorbers, such as gadolinium or erbium, that are baked directly into the uranium fuel pellets during the manufacturing process. These poisons burn away at a highly predictable rate, matching the steady depletion of the uranium fuel. This provides a baseline level of reactivity suppression without the need for moving parts, corrosive chemicals, or active operator intervention.[2]

Yet burnable poisons are entirely passive; they cannot adjust to dynamic changes in power demand, nor can they compensate for unexpected xenon gas transients that occur during normal load-following operations. Safety frameworks generally require at least two independent and diverse systems for reactor shutdown to ensure defense-in-depth. Without soluble boron acting as the ultimate backup, achieving this mandated diversity requires highly complex control rod algorithms, overlapping mechanical rod banks, and flawless drive mechanisms to ensure the core can be safely secured under any conceivable condition.[4]

The transition away from chemical shims represents a fundamental trade-off in modern nuclear engineering. Simplifying the fluid chemistry inevitably complicates the mechanical physics, shifting the risk from fluid corrosion to solid-state mechanical failure. As the industry moves toward commercializing the next generation of small modular reactors, the deciding factor will be whether advanced burnable poisons can sufficiently buffer the core's excess reactivity, or if the unforgiving demands placed on physical control rods prove too steep a price to pay for the operational convenience of clean water.[4]