Robert C. Petroski

TERRAPOWER, LLC TRAVELING WAVE REACTOR DEVELOPMENT PROGRAM OVERVIEW

Energy security is a topic of high importance to many countries throughout the world. Countries with access to vast energy supplies enjoy all of the economic and political benefits that come with controlling a highly sought after commodity. Given the desire to diversify away from fossil fuels due to rising environmental and economic concerns, there are limited technology options available for baseload electricity generation. Further complicating this issue is the desire for energy sources to be sustainable and globally scalable in addition to being economic and environmentally benign. Nuclear energy in its current form meets many but not all of these attributes. In order to address these limitations, TerraPower, LLC has developed the Traveling Wave Reactor (TWR) which is a near-term deployable and truly sustainable energy solution that is globally scalable for the indefinite future. The fast neutron spectrum allows up to a ~30-fold gain in fuel utilization efficiency when compared to conventional light water reactors utilizing enriched fuel. When compared to other fast reactors, TWRs represent the lowest cost alternative to enjoy the energy security benefits of an advanced nuclear fuel cycle without the associated proliferation concerns of chemical reprocessing. On a country level, this represents a significant savings in the energy generation infrastructure for several reasons 1) no reprocessing plants need to be built, 2) a reduced number of enrichment plants need to be built, 3) reduced waste production results in a lower repository capacity requirement and reduced waste transportation costs and 4) less uranium ore needs to be mined or purchased since natural or depleted uranium can be used directly as fuel. With advanced technological development and added cost, TWRs are also capable of reusing both their own used fuel and used fuel from LWRs, thereby eliminating the need for enrichment in the longer term and reducing the overall societal waste burden. This paper describes the origins and current status of the TWR development program at TerraPower, LLC. Some of the areas covered include the key TWR design challenges and brief descriptions of TWR-Prototype (TWR-P) reactor. Selected information on the TWR-P core designs are also provided in the areas of neutronic, thermal hydraulic and fuel performance. The TWR-P plant design is also described in such areas as; system design descriptions, mechanical design, and safety performance.

Using the Neutron Excess Concept to Determine Starting Fuel Requirements for Minimum Burnup Breed-and-Burn Reactors

Abstract In a breed-and-burn (B&B) reactor, the reactor is first started with enriched uranium or other fissile material but thereafter can be refueled with natural or depleted uranium. B&B reactors have the potential to achieve >10% uranium utilization in a once-through fuel cycle versus <1% for light water reactors. A newly developed method for analyzing B&B reactors—the “neutron excess” concept—is used to determine the minimum amount of startup fuel needed to establish a desired equilibrium cycle in a minimum burnup B&B reactor. Here, a minimum burnup B&B reactor is defined as one in which neutron leakage is minimized and feed fuel can be discharged at uniform burnup. The neutron excess concept reformulates the k-effective of a system in terms of material depletion quantities: the total number of neutrons absorbed and produced by a given volume of fuel, which are termed “neutron excess quantities.” This concept is useful because neutron excess quantities are straightforward to estimate using simple one-dimensional (1-D) and zero-dimensional (0-D) models. A set of equations is developed that allows the quantity of starter fuel needed to establish a given B&B equilibrium cycle to be expressed in terms of neutron excess quantities. A simple 1-D example of a sodium-cooled, metal fuel reactor with a startup enrichment of 15% is used to illustrate how the method is applied. An estimate for the required amount of starter fuel based on a 0-D depletion model is found to differ by only 3% from the actual amount computed using the 1-D example model.

Characterizing Limited-Separations Fuel Cycles Using Breed-and-Burn Reactors

A fuel cycle option is evaluated in which fuel bred in breed-and-burn (B&B) reactors is used to start up additional B&B reactors, with the fuel being recycled using limited-separations processes instead of full actinide reprocessing. This fuel cycle aims to minimize processing requirements and proliferation risk while still being able to achieve exponential growth and high uranium utilization. The neutron excess concept is applied to compute the starting fuel requirements of new B&B reactors, allowing fleet doubling times to be estimated. A simple analytic expression for doubling time is derived, which is applied to example B&B reactors using a hypothetical core composition. It is found that larger reactors are able to achieve shorter doubling times because of their smaller starter fuel requirements per unit power. Several variant fuel cycle configurations are examined, and their doubling times are computed.

Using Traveling Wave Reactor (TWR) Technology to Provide Globally Scalable and Sustainable, Carbon-Free Energy

Energy security, reducing air pollution, and carbon emissions are topics of high importance to many countries throughout the world, particularly in Asia where energy use is expected to grow at 3.7 % per year, the highest growth rate in the world. According to the International Energy Agency (IEA), China alone is expected to account for almost one-fourth of world energy demand in the next 20 years. Although low-carbon options like wind and solar have seen large strides in deployment, growing by double and triple digits, the building of new coal plants still outpaces them all by orders of magnitude. In addition, most intermittent sources currently use fossil fuel generators as back up, lowering the potential gains that can be made in emission/carbon reduction goals. To further exacerbate this issue, worldwide electricity production is expected to double by 2040 to meet global needs, where coal is expected to play a major role in supplying that electricity unless an alternative can be found. Given the need to reduce the use of fossil fuels due to emissions/pollution/carbon concerns, and a desire for sustainable and globally scalable energy sources, an “all of the above” strategy for electricity generation has become an imperative. Nuclear power meets the requirements of a non-emitting source, and thus will need to be considered as part of the global energy strategy. However, nuclear energy in its current form has limitations, both perceived and real, regarding economics, waste, proliferation, and safety. In order to further improve on the current generation of reactors, TerraPower has developed the Traveling Wave Reactor (TWR), a near-term deployable and truly sustainable energy solution that is globally scalable for the indefinite future. As a fast reactor, the TWR allows up to a ~35-fold gain in uranium utilization when compared to conventional light water reactors (LWRs) using enriched fuel. Compared to other fast reactors, TWRs represent the lowest cost and lowest risk alternative: (1) they provide the energy security benefits of an advanced nuclear fuel cycle without the associated proliferation and cost concerns of fuel reprocessing; (2) they require less lifetime enrichment than LWRs, translating to a reduced number of enrichment plants that need to be built; (3) they produce less waste by volume than an LWR, resulting in less needed waste capacity requirements and reduced waste transportation costs; and (4) they require less uranium ore to be mined or purchased since natural or depleted uranium can be used directly as fuel. In addition to the benefits described above, the paper also describes the origins and current status of the TWR engineering, design, development, and test programs at TerraPower. Areas covered include the key TWR design challenges, and brief a description of the TWR-Prototype (TWR-P) reactor.

Fast Reactor Design Using the Advanced Reactor Modeling Interface

The Advanced Reactor Modeling Interface (ARMI) code system has been developed at TerraPower to enable rapid and robust core design. ARMI is a modular modeling framework that loosely couples nuclear reactor simulations to provide high-fidelity system analysis in a highly automated fashion. Using a unified description of the reactor as input, a wide variety of independent modules run sequentially within ARMI. Some directly calculate results, while others write inputs for external simulation tools, execute them, and then process the results and update the state of the ARMI model. By using a standardized framework, a single design change, such as the modification of the fuel pin diameter, is seamlessly translated to every module involved in the full analysis; bypassing error-prone multi-analyst, multi-code approaches. Incorporating global flux and depletion solvers, subchannel thermal-hydraulics codes, pin-level power and flux reconstruction methods, detailed fuel cycle and history tracking systems, finite element-based fuel performance coupling, reactivity coefficient generation, SASSYS-1/SAS4A transient modeling, control rod worth routines, and multi-objective optimization engines, ARMI allows “one click” steady-state and transient assessments throughout the reactor lifetime by a single user. This capability allows a user to work on the full-system design iterations required for reactor performance optimizations that has traditionally required the close attention of a multi-disciplinary team. Through the ARMI framework, a single user can quickly explore a design concept and then consult the multi-disciplinary team for model validation and design improvements. This system is in full production use for reactor design at TerraPower, and some of its capabilities are demonstrated in this paper by looking at how design perturbations in fast reactor core assemblies affect steady-state performance at equilibrium as well as transient performance. Additionally, the pin-power profile is examined in the high flux gradient portion of the core to show the impact of the perturbations on pin peaking factors.Copyright