Subcritical PWR with Sr-90 Core

Conventional Pressurized Water Reactors (PWRs) are constrained by the necessity of maintaining a critical state (k_eff = 1.0). To sustain criticality against geometric neutron leakage and burnup, small modular PWRs require disproportionately large uranium fuel inventories.

This article introduces the Subcritical Pressurized Water Reactor (Sr-PWR), a 10 MW_th reactor architecture that decouples power generation from criticality by maintaining k_eff = 0.95. A central, retractile Strontium-90 (Sr-90) photoneutron driver core acts as an external neutron source, supplying a baseline flux of ≈ 6.4 × 10¹⁵ n/s. By combining this driver with standard 5% Low-Enriched Uranium (LEU) fuel, an internal graphite reflector matrix, and classical 150 bar PWR thermal-hydraulics, the Sr-PWR achieves an 80% reduction in initial uranium mass, complete immunity to prompt criticality, and extended in-situ waste transmutation over a 28-year continuous operational cycle.

1. Introduction & The Criticality Penalty

In a conventional PWR, loading 1,200 to 1,800 kg of fuel for a 10 MW_th core is not driven by thermal output demands, but by the physics of neutron multiplication. A significant fraction of the fissile inventory exists purely to overcome parasitic absorption in control mechanisms and boundary leakage.

By shifting the reactor kinetics to a subcritical regime (k_eff = 0.95), the thermal power is governed directly by subcritical multiplication:

Because the continuous background flux is supplied externally, the fuel lattice no longer needs to achieve self-sustained criticality on its own. This eliminates the core volume penalty, allowing a 10 MW_th system to operate on a drastically reduced fuel footprint.

2. Sr-PWR Core Architecture

The Sr-PWR preserves standard PWR balance-of-plant components—utilizing light water coolant at 150 bar (315°C outlet) and commercial UO₂ fuel pin manufacturing—while redesigning the interior core layout around three concentric functional zones:

1. Central Driver Core (Primary Neutron Source): A retractile assembly consisting of ∼ 50 kg of Sr-90 (emitting high-energy β⁻ via Y-90), housed within a tungsten Bremsstrahlung converter sleeve and an annular Beryllium target shell. High-energy gammas (> 1.666 MeV) induce (γ, n) interactions, emitting photoneutrons directly into the core.

2. Subcritical Fuel Matrix: A compact array of standard 5% LEU fuel rods submerged in light water, holding a total mass of only ∼ 250 kg (containing ∼ 12.5 kg of U-235).

3. Internal Graphite Reflector: Blocks of high-density nuclear graphite (IG-110), clad in oxidation-resistant Silicon Carbide (SiC) or Zircaloy, surround the fuel grid inside the vessel to ensure > 90% boundary neutron albedo.

3. Kinetic Control & Single-Axis Safety

Reactivity control in the Sr-PWR is consolidated along a single mechanical axis: the insertion depth of the central Sr-90 driver rod.

Power Modulation: Adjusting the axial position of the driver rod modulates the photoneutron coupling to the fuel matrix, allowing fine thermal throttling.

Shutdown Mechanism: Fission is halted by retracting the central driver pin into an external, passively cooled storage cask. Without S_driver, the subcritical core drops to zero fission power within milliseconds.

Passive Safety: Because k_eff is locked at 0.95, prompt-critical runaway is physically impossible. If a thermal excursion occurs, steam formation (voiding) degrades moderation, instantly driving k_eff down toward 0.70.

4. Long-Dwell Physics: In-Situ Transmutation & Waste Evolution

Because the driver continuously forces neutrons into the core over the 28.8-year half-life of Sr-90, the Sr-PWR operates on a 28-year unbroken fuel cycle:

Elimination of Xenon Lockout: High-cross-section poisons like Xenon-135 (Xe-135) rapidly capture driver photoneutrons, transmuting into Xe-136—a stable isotope with a near-zero capture cross-section (0.26 barns). The core converts neutron poisons into neutronic glass.

Actinide Fission Cascade: Non-fissile U-238 captures neutrons to breed Pu-239. Over 28 years, sequential capture produces heavier actinides (Pu-241, Cm-245), which possess massive thermal fission cross-sections (1,000-2,000 barns) and split in-situ.

Waste Vector Transformation: By Year 28, over 90% of the initial U-235 and bred Plutonium are fissioned into light elements. The residual fuel contains highly denatured Plutonium (∼ 64% non-fissile Pu-240 / Pu-242) and fission products that decay to background levels within 300 years.

5. Specifications Summary

6. Conclusion

The Subcritical PWR with a Sr-90 Driver Core resolves the fuel mass penalty of small reactors while maintaining full compatibility with commercial 150 bar light-water manufacturing. By converting high-level nuclear waste (Sr-90) into a long-life neutron driver, the Sr-PWR achieves ultra-high fuel burnup, simplified single-axis kinetic control, and a self-cleansing waste stream—offering a pragmatic path toward multi-decade, maintenance-free power generation.