Anselmo T. Cisneros

Design Summary of the Mark-I Pebble-Bed, Fluoride Salt–Cooled, High-Temperature Reactor Commercial Power Plant

Abstract The University of California, Berkeley (UCB), has developed a preconceptual design for a commercial pebble-bed (PB), fluoride salt–cooled, high-temperature reactor (FHR) (PB-FHR). The baseline design for this Mark-I PB-FHR (Mk1) plant is a 236-MW(thermal) reactor. The Mk1 uses a fluoride salt coolant with solid, coated-particle pebble fuel. The Mk1 design differs from earlier FHR designs because it uses a nuclear air-Brayton combined cycle designed to produce 100 MW(electric) of base-load electricity using a modified General Electric 7FB gas turbine. For peak electricity generation, the Mk1 has the ability to boost power output up to 242 MW(electric) using natural gas co-firing. The Mk1 uses direct heating of the power conversion fluid (air) with the primary coolant salt rather than using an intermediate coolant loop. By combining results from computational neutronics, thermal hydraulics, and pebble dynamics, UCB has developed a detailed design of the annular core and other key functional features. Both an active normal shutdown cooling system and a passive, natural-circulation-driven emergency decay heat removal system are included. Computational models of the FHR—validated using experimental data from the literature and from scaled thermal-hydraulic facilities—have led to a set of design criteria and system requirements for the Mk1 to operate safely and reliably. Three-dimensional, computer-aided-design models derived from the Mk1 design criteria are presented.

Neutronics and Depletion Methods for Multibatch Fluoride Salt-Cooled High-Temperature Reactors with Slab Fuel Geometry

The Advanced High-Temperature Reactor (AHTR) is a 3400-MW(thermal) fluoride salt-cooled high-temperature reactor that uses coated particle fuel compacted into slabs rather than spherical or cylindrical fuel compacts. Simplified methods are required for parametric design studies to perform burnup analysis on the entire feasible design space. These simplifications include fuel homogenization techniques to increase the speed of neutron transport calculations and equilibrium depletion analysis methods to analyze systems with multibatch fuel management schemes. This paper presents three elements of significant novelty. First, the reactivity-equivalent physical transformation (RPT) methodology usually applied in systems with cylindrical and spherical geometries has been extended to slab geometries. Second, implementing this RPT homogenization, a Monte Carlo-based depletion methodology was developed to search for the maximum discharge burnup in a multibatch system by iteratively estimating the beginning of equilibrium cycle composition and sampling different discharge burnups. This iterative equilibrium depletion search method fully defines an equilibrium fuel cycle (keff, power, flux, and composition evolutions) but is computationally demanding. Therefore, an analytical method, the nonlinear reactivity model, was developed so that single-batch depletion results could be extrapolated to estimate the maximum discharge burnup in systems with multibatch fuel management schemes.

Feasibility of Once Through Subcritical Cores Driven by an Accelerator Spallation Neutron Source

The proliferation resistance of the nuclear fuel cycle would be increased if one could eliminate the need for both uranium enrichment and spent fuel reprocessing. Heavy-water and graphite moderated critical reactors can extract energy from natural uranium but offer a very low uranium utilization (low discharge burnup). The objective of the present study is to explore the feasibility of achieving high fuel utilization without resorting to enrichment and reprocessing using spallation neutron source driven subcritical reactors. Three different high burnup once through subcritical nuclear systems are investigated: a fluoride salt cooled high temperature reactor (FHR) with pebble fuel, a helium cooled core with sphere pack fuel based on General Atomics’ EM2 reactor concept, and a sodium cooled fast reactor that is loaded with fuel discharged from a high burnup Breed-and-Burn (B&B) fast reactor that is fed with depleted uranium, after removing the gaseous fission products and inserting the voided fuel rods into a new clad (without removing the old one). The pebble fuel design and fuel cycle for the FHR concept was optimized for maximum electric power multiplication using natural thorium fuelled subcritical core. The maximum attainable power multiplication was not high enough to merit future studies. The optimal discharge burnup of the fuel in the EM2 type subcritical core was found to be approximately 30% FIMA and the corresponding power multiplication was found higher than in the FHR but still not high enough for practical applications. Significantly better performance was obtained from the sodium-cooled source-driven core that is fed with metallic U-TRU-Zr fuel discharged at 20% FIMA from a critical B&B fast reactor that underwent recladding. The maximum attainable power multiplication was found to be close to 10 while fissioning an additional 20% of the loaded heavy metal.