Koroush Shirvan

Increasing the fuel discharge burnup of current light water reactors (LWRs) promises reductions in fuel cycle and/or operations costs. By assuming a constant core power density, the economic gain is enabled by better fuel utilization and/or increased capacity factor. In this effort to investigate greater than 62 MWd/kgU maximum rod average burnup for 110+ kW/l core power density, two core designs have been developed for a standard 17x17, 193 fuel assemblies pressurized water reactor (PWR). The levelized unit cost methodology is employed to evaluate the operation and maintenance and capital costs of the fuel cycle and examine the economic viability of both core design pathways. Core design and optimization are performed using the commercial STUDSVIK code package. Fuel performance analysis is realized in full core configuration via auditing FRAPCON4.1 and the high-fidelity code BISON. To provide a realistic assessment, the core design process takes into consideration best practices in current PWR core design. It features acceptable performance in terms of various core design constraints on maximum allowable peaking and boron concentration. Gadolinia (Gd2O3) is used as a burnable poison with a maximum of 9 wt% concentration while feeding 89 or 77 fuel assemblies in a 3-batch refueling scheme. Full core fuel performance simulation that allows for characterization of relevant fuel temperatures, plenum pressures, stresses, and strains is performed with respect to two bounding burnup levels. Such performance is potentially licensable for the 18-month high burnup core (<68 MWd/kgU peak pin) while it is more challenging for the 24-month high burnup core design pathway (<75 MWd/kgU peak pin). Maximum rod plenum pressure is identified to be the most limiting fuel performance parameter. While the scope of the present study focuses on the steady state plus overpower conditions, the acceptability of the new discharge burnup has to be further assessed by considering uncertainties and impact under accident scenarios in the future.

Increasing the core power density of nuclear reactors allows higher power generation from the same volume, reducing the cost of the plant. The lower power rating and higher degree of inherent safety for small modular reactors motivate exploring the highest achievable power density to improve their economics and overcome lack of economy of scale. Given the urgency to deploy carbon free energy sources in the near term, the current nuclear fuel supply must be adopted. Assuming standard 17x17 pressurized water reactor (PWR) fuel assemblies with enrichment below 5%, a parametric study on core power density of an integral PWR (iPWR) is performed. The reference core design is rated at 60 kW/L and composed of 77 reduced-height fuel assemblies. The reactivity control during normal operation is assumed to utilize soluble boron and fuel burnable poisons. The commercial STUDSVIK code package is used to design cores ranging from 20 kW/L to 170 kW/L. This range results in respective output powers between 50 to 420 MWe for an assumed 32% plant thermal efficiency. Core safety parameters, such as peaking factors, Minimum Departure from Nucleate Boiling Ratio and maximum fuel temperatures are evaluated for all designs. Simplified economic evaluation of lifetime-levelized unit costs for the fuel cycle, operation and maintenance and capital expenditures was performed as a function of power density. The relatively low core discharge burnup (32 MWD/kgU) leads to high lifetime-levelized fuel cycle unit cost for the reference core. However, increase in power density allows significant reduction in operation, maintenance and capital unit costs. Doubling power density resulted in 80% reduction in total levelized cost of electricity. The unique features of a forced circulation iPWR technology combined with compact steam generator technology could allow achieving high power densities (>125 kW/L). Specific examples on relationship of cost and safety margins are also discussed.

The desire to improve the economic competitiveness and deployment pace of nuclear energy through modularization, manufacturing, and series production had led to the development of smaller size reactors. As the standard 17x17 fuel technology is mainly maintained in the pressurized water reactors (PWRs) category, this translates into a lower number of fuel assemblies in the core and sometimes a reduced fuel height. To assess the impact of such scale change in core design on fuel cycle cost and spent fuel volume, a scoping analysis is performed based on infinite lattice calculations, leakage, fuel management reduced models, and levelized unit cost of electricity (LCOE) estimate. As such, cost dynamics driven by fuel specific power, burnup, core leakage, feed, cycle length, fuel assembly height as well as uranium market data are captured with consistent set of assumptions and analysis methods. A selection of 5 reactor designs representative of leading PWR developers is assessed and compared. Pursuing higher specific powers and optimal burnups are highlighted as the main fuel cost reduction drivers, nevertheless, practical limitations and opportunities must be evaluated to establish the feasibility of such enhanced fuel operation. In consequence, a detailed core design is performed using SIMULATE3 code for 5 PWR variations including natural and forced coolant circulation modes, two reactor scales, power densities of 73, 112, and 123 kW/l and higher discharge burnups. Design and optimization are performed at the lattice level, for the reflector, and at the core loading level. Satisfactory steady-state operation including power distribution, coolant operating limits, and reactivity requirements are analyzed and reported in this paper. The fuel economics of the detailed core designs confirm the scoping analysis findings. Despite the unlocked power uprates in small PWRs, the achievable burnup for a given fuel specific power requires more enrichment and shorter fuel height results in higher fabrication costs per mass of fuel, which makes scaling down core size a more expensive endeavor on the fuel cycle front. Spent fuel volumes are reported for the PWRs designed in this paper. These volumes are driven by the core average discharge burnup regardless of the scale in consideration. Additional cost and core performance aspects related to heavy reflector gains, fuel-reflector substitution, and disposal cost policy in the U.S. are examined.

Nuclear energy produces more carbon-free electricity than any other source, accounting for ~20% of U.S. electricity generation. With the net-zero emission goals of the upcoming decades, there is a consensus and recognition of the vital importance in continual operation of existing reactor fleet. Over the last 25 years, the U.S. NRC has approved over 130 power-uprates for the existing fleet ranging from ~0.5-20%. Given that the rise of cheap natural gas has challenged the economic viability of adding new nuclear power plants, further exploration on increasing the value of the existing fleet is critical. An additional uprate of 50% to existing nuclear generation will mostly meet the carbon-free electricity needs of the next 10 years. In 2008, MIT and Westinghouse studied the feasibility of a 50% power uprate, and the major barriers to its adoption were the lack of availability for greater than 5% enriched fuel and uncertainty regarding feasible plant lifetimes to motivate the needed refurbishment investments. The advent of LEU+ (5-10% enrichment) and Accident Tolerant Fuels (ATFs) in a carbon-constrained economy as well as approval of an 80-year life by U.S. NRC for selected plants, provides an opportune time for revisiting the extend for an achievable power-uprate. This paper will outline and quantitatively support additional extended power-uprate for a generic PWR and BWR by leveraging LEU+ and ATF based on neutronics, thermal-hydraulics, fuel performance and economic assessments. The maximum allowable uprate is achieved by fuel geometry optimization while constraining the fuel materials to near term options (e.g., UO2/Zr). With adequate refurbishment of plant equipment, an economical 50% power uprate can be possible while shortening the fuel cycle to 12-month. A fleet-wide power uprate of such magnitude will not only enable meeting the U.S. emission-free electricity generation targets by 2030s, but it will also provide a much-needed bridge for advanced reactor deployment timelines and revitalizes U.S. utilities interest to invest in nuclear energy at a scale needed to achieve future net-zero targets.

Recent nuclear construction project schedules in the US and Europe were 2.5x their original estimates due to productivity misses, supply chain challenges, hiring constraints, and other issues. Small modular reactors are proposed as a remedy. Smaller reactors will require one fourth to one third the person-hours of labor and leverage more common supply chains, putting less strain on each step of the construction process. This study presents a methodology for estimating the construction duration of nuclear projects, so the sensitivity of proposed concepts to staffing and labor constraints can be analyzed. The methodology builds on a previous overnight capital cost estimation tool and builds a Gantt chart out of 226 tasks. The genetic algorithm assigns resources to each task to minimize construction time subject to peak staffing and hiring rate constraints. The results were consistently within 15% of seven benchmark durations. Under tighter labor constraints, the construction duration of large reactors increased 42% on average and only 3% for small reactors. Modularization of systems and structures reduced construction durations 60%, but it increased the sensitivity to labor constraints. Small reactors were more sensitive to the modularization assumptions because of the compact structures and layout, and the power specific cost sensitivity was higher, but the total financial impact was smaller because the total costs were lower.