Research – Page 2 – Advanced Reactors Group

2- and 3-tier advanced fuel cycles

Guanheng Zhang (alumnus), Gang Wang, Lucas David, Sai Vadlamudi, Massimiliano Fratoni, Ehud Greenspan

The Seed and Blanket (S&B) concept is able to support several new 2- and 3-tier advanced fuel cycle systems to improve nuclear fuel cycle sustainability. Instead of using the thorium blanket of the S&B reactor in a once-through mode, accumulating a large inventory of Trans-Th elements, this study assesses the feasibility of using the discharged Trans-Th to feed PWRs and molten salt reactors (MSRs) that operate on a closed fuel cycle (their fuel is recycled). This fuel cycle option provides a possible solution to the large amount of U-233 bred in the S&B core whose decay daughters are the major contributors to long-term radioactivity and radiotoxicity.

The specific fuel cycle considered here is a three-stage energy system PWR(LEU)-S&B-PWR(Trans-Th) illustrated below. Stage 1 contains once-through low enriched uranium fueled LWRs, Stage-2 uses S&B reactors having TRU-transmuting seed and thorium blankets, and Stage-3 has LWRs that operate on a closed U-233/Th fuel cycle. The recovered Trans-thorium from Stage 2 is mixed with some fresh thorium to serve as the makeup fuel of the Stage-3 PWR. All the discharged fuel is reprocessed and recycled except for the uranium recovered from Stage-1 discharged fuel and a fraction of the thorium discharged from Stage-2 blankets. The seed of Stage-2 S&B reactors is fed with TRU separated from Stage-1 PWRs. This system may offer the fastest and possibly most cost-effective way to get rid of the High Level Waste from the nuclear industry. It is found that one S&B core can support 3.3 PWRs in this 3-tier advanced fuel cycle system.

Ternary lithium-alloys for fusion blankets

Lawrence Livermore National Laboratory (LLNL) is attempting to develop a lithium-based alloy—most likely a ternary alloy—that maintains the beneficial properties of lithium (e.g. high tritium breeding and solubility) while reducing overall flammability concerns for use in the blanket of the Inertial Fusion Energy (IFE) power plant.

Alejandra Jolodosky, Alan Bolind, and Massimiliano Fratoni

The goal of this work is improved safety and performance for fusion energy. Lithium is often the preferred choice as breeder and coolant in fusion blankets as it offers excellent heat transfer and corrosion properties and, most importantly, has a very high tritium solubility that results in very low levels of tritium permeation throughout the facility infrastructure. However, lithium metal vigorously reacts with air and water, exacerbating plant safety concerns. Consequently, Lawrence Livermore National Laboratory (LLNL) is attempting to develop a lithium-based alloy—most likely a ternary alloy—that maintains the beneficial properties of lithium (e.g. high tritium breeding and solubility) while reducing overall flammability concerns for use in the blanket of the Inertial Fusion Energy (IFE) power plant.

The IFE power plant being studied employs inertial confinement fusion (ICF) through the use of lasers aimed at an indirect-driven target composed of deuterium-tritium fuel. The fusion driver/target design implements the same physics as the National Ignition Facility (NIF). The IFE power plant uses lithium in both the primary coolant and blanket; therefore, lithium-related hazards are of primary concern. Although reducing chemical reactivity is the primary motivation for the development of new lithium alloys, the successful candidates will have to guarantee acceptable performance in all their functions. Our focus is to evaluate the neutronics performance of a large number of lithium-based alloys in the blanket of the IFE engine. In particular, parameters determining alloy selection are the tritium breeding ratio (TBR) and energy multiplication factor (EMF). Activation analysis is performed on the selected alloys to assess specific safety and environmental properties, including evaluation of decay heat, contact dose rate, accident dose, and waste disposal rating.

Neutronic analysis found that the best performing alloys (higher TBR and higher EMF) combine elements that exhibit low absorption cross sections and high q-values, such as tin, barium, strontium, or zinc, with elements with high neutron multiplying cross sections, like lead or bismuth. A large number of alloys (e.g. LiPbZn and LiSnZn) met TBR constraints greater than or equal to 1.05 and an EMF constrain greater than or equal to 1.1 for a wide range of lithium concentrations. When the EMF constraint was increased to 1.2, the additional power demand was too high for alloys not containing tin. Additionally, it was found that when an alloy already contains a high amount of lithium (greater than 50%), doubling the 6Li content from 7.5% to 15% increases the TBR by 13%. After a certain percent of enriched 6Li, the lack of tritium and additional neutrons produced from 7Li(n,n’T) reactions end up reducing the TBR. At lower total lithium concentrations (<50%), the TBR will continue to increase to higher 6Li enrichments since the 7Li(n,n’T) reactions will not be as significant.

Activation calculations were performed for a series of elements that exhibited good TBR and EMF properties. This analysis revealed bismuth as a poor choice as it performed worst for all of the criteria evaluated. Alloys containing zinc and tin also showed some of the highest decay heats, contact dose rates, and accident doses. Most of the alloys examined can be stored in dry containers at an estimated one year after shutdown. Additionally, if necessary, the entire volume of the blanket for every alloy except LiPbBa and LiBaBi could be remotely handled. Accident doses were high in alloys containing zinc, copper, or gallium, but were not high enough to pose a major safety concern. With the exception of LiBaBi, activation analysis demonstrated that all the alloys could be used as blankets of the IFE reactor without posing major environmental or safety concerns.

Future neutronics work will focus on the optimization of lithium-ternary alloy concentrations for a given TBR and EMF.

Multi-physics modeling of fluoride-cooled high-temperature reactors (FHRs)

Xin Wang, Dan Shen, Katy Huff, Manuele Aufiero, Massimiliano Fratoni, April Novak

To improve understanding of coupled physics in FHRs, this work involves the development of tools and methods for coupling at thermal hydraulics and neutronics within the context of FHRs. Low-dimensional models relying on simplified neutron kinetics and heat transfer have been implemented in a python package, PyRK. Higher dimensional models that couple these physics in finite element frameworks (including both MOOSE and COMSOL) are also being developed. Finally, models which coupled monte carlo simulation with CFD tools are also being iterated upon.

WARP (“Weaving All the Random Particles”)

Ryan Bergmann (alumnus), Kelly Rowland, Rachel Slaybaugh, Jasmina Vujic

To improve reactor design and operation, fast and accurate neutron transport calculations are needed. Today’s supercomputers are comprised of heterogeneous architectures designed to reduce power consumption, and new algorithms are required to use these hardwares. WARP, which can stand for “Weaving All the Random Particles”, is a three-dimensional (3D), continuous energy, Monte Carlo neutron transport code developed to efficiently execute on a CPU/GPU platform. WARP is able to calculate multiplication factors, flux tallies, and fission source distributions for time-independent problems and can run in both criticality or fixed-source modes. WARP currently transports neutrons in unrestricted arrangements of spheres, cylinders, parallelpipeds, and hexagonal prisms and is able to entertain both vacuum and reflecting (specular) boundary conditions.

What sets WARP apart from previous, somewhat similar endeavors is its breadth of scope and novel adaptation of the event-based Monte Carlo algorithm. Previous codes have been limited to restricted nuclear data or simplified geometry models, where WARP instead loads standard data files and uses a flexible, scalable, optimized geometry representation. WARP uses a suite of highly-parallelized algorithms and employs a modified version of the original event-based algorithm that is better suited to GPU execution.

Targeted Modification of Neutron Energy Spectra

James Bevins, Rachel Slaybaugh

The goal of this project is to modify properties of existing neutron sources to make them more desirable for high-impact applications. Neutron sources are broadly classified by their intensity and energy distribution. For many applications, such as medical treatments, radiation damage studies in epi-thermal or fast reactors, neutron radiation effects studies on semiconducting devices, or nuclear forensics, no current neutron source has the characteristic intensity and energy distribution required to meet many of test or operational objectives. Historically, surrogate methods and equivalencies were developed to provide calibration metrics that enabled the use of existing sources even though they did not have the correct intensity and energy characteristics. However, these methods often result in large design margins and experimental uncertainties. Fundamentally, many applications do not have the ability to test across the complete range desired and there is a high-order mismatch between the experiment and the desired physics and conditions.

This research proposes instead to use existing, high intensity sources with a custom energy tuning assembly (ETA) to tailor the neutron spectrum to address neutron source capability. Neutron filters, screens, and moderation have been used in the past to alter a neutron source’s spectral characteristics, but these tended to be simple in objective and construction. To generically tailor a spectrum, many different materials and geometric configurations have to be explored rapidly. Because of the sheer size of the possible phase space, designing by hand will only explore a small subset of that space and is unlikely to arrive at consistently valid solutions. This research is developing a metaheuristic optimization tool to customize ETAs for any application. The ETA design will be tested using D-T neutron generators and the LBNL 88” Cyclotron facilities. The goal will be to demonstrate the ability to model, measure, and experimentally validate the design of an ETA to achieve a desired spectrum.

Advanced Burner Reactor with Breed-and-Burn Thorium Blankets for Improved Economics and Resource Utilization

Guanheng Zhang (alumnus), Chris Keckler, Alejandra Jolodosky (former), Massimiliano Fratoni, Jasmina Vujic, Ehud Greenspan

This study assesses the feasibility of designing a Seed and Blanket (S&B) Sodium-cooled Fast Reactor (SFR) to generate a significant fraction of the core power from radial thorium-fueled blankets. The goals of this project support sustainability of the nuclear fuel cycle. The blanket operates in a Breed-and-Burn mode without exceeding currently-verified experimental radiation damage limits. The S&B core is designed to maximize the fraction of neutrons that radially leak into the subcritical blanket. The blanket makes beneficial use of the leaking neutrons for improved economics and resource utilization. Since the blanket fuel requires no reprocessing or remote fuel fabrication, a larger fraction of power from the blanket will result in a lower fuel cycle cost per unit of electricity generated. A unique synergism is found between a low conversion ratio seed and the breed-and-burn thorium blanket. The benefits of this synergism are maximized when using an annular seed surrounded by inner and outer thorium blankets.

Fuel cycle analysis of the S&B design, including basic fuel cycle parameters, nuclear waste characteristics (radioactivity, inhalation/ingestion toxicity), proliferation resistance, fuel cycle cost, and resource utilization, is conducted and compared with a reference Advanced Burner Reactor and Pressurized Water Reactors. The S&B cores can utilize 7% of thorium’s energy value without the need to develop irradiated thorium reprocessing capabilities. This is ~12 times the amount of energy that LWRs generate per unit weight of natural uranium mined, showing vast improvement in resource utilization over current systems.

Preliminary studies have found that the S&B core could establish several new fuel cycle options. Currently under investigation is the option of using the S&B design to burn minor actinides and plutonium from LWR spent fuel, which would greatly benefit the situation for geologic disposition of spent nuclear fuel and increasing the sustainability of nuclear power.

Thorium-fueled Resource-renewable BWRs (RBWR-Th)

Phillip Gorman, Sandra Bogetic (former), Jeffrey Seifried (alumnus), Christopher Varela (former), Guanheng Zhang (alumnus), Massimiliano Fratoni, Ehud Greenspan

RBWRs are intermediate-spectrum light water reactors (LWRs) that achieve missions normally reserved for sodium fast reactors—fuel sustainability or TRU transmutation with unlimited recycling—while using LWR technology. The spectrum in an RBWR is much harder than in a typical BWR because the fuel is arranged in a tight triangular lattice and the core has a very high exit void fraction. Hitachi developed several designs to achieve these goals using depleted uranium (DU) as the fertile fuel. In order to mitigate the positive void feedback that occurs in high Pu content fuels (such as DU), the Hitachi RBWR designs featured a parfait-style axial design in which the fissile material was loaded into short “seed” sections that were separated by fertile blanket regions. This design led to several safety concerns stemming from the high linear heat rates. At Berkeley, we are investigating using thorium instead of DU as the primary fertile fuel since the number of neutrons emitted per neutron absorbed does not increase with increasing absorbed neutron energy as quickly in U-233 as it does in Pu-239. The change in neutron emissions with energy allows negative void feedback while using a single elongated seed region, which can reduce the linear heat rates.

Results to date have been encouraging. The thorium-fed RBWRs can nearly match the burnup and cycle length of the uranium-fed RBWRs, while featuring much lower linear heat rates and better safety margins. This project is a collaboration between MIT, University of Michigan, and UCB.

Breed-and-Burn (B&B) reactor design and optimization

Jason Hou (alumnus), Staffan Qvist (alumnus), and Ehud Greenspan

Breed-and-burn (B&B) reactors are a special class of fast reactors that have the potential of significantly improving the sustainability of the nuclear fuel cycle. They are designed to use low grade fuel (e.g. depleted uranium) without fuel reprocessing. One of the most challenging practical design feasibility issues of B&B reactors is the high level of radiation damage their fuel cladding must withstand—more than twice the maximum radiation damage cladding materials in fast reactors have previously been exposed to. This work investigates the feasibility of reducing the peak minimum required radiation damage level by introducing a three-dimensional (3D) in-core fuel management strategy.

A new conceptual design of a B&B core made of axially segmented fuel assemblies was adopted to facilitate the 3D shuffling. The assemblies of the 3D shuffled system are each made of two to four subassemblies that are axially stacked. The subassemblies can be disconnected from one another and then stacked together in a different order and/or combination to constitute new assemblies. Each subassembly is made of short vented fuel pins, requiring a few centimeters of fuel-free space on the top of the pins to accommodate the venting device and the axial fuel swelling.

A combinatorial search methodology has been developed and implemented for 3D shuffling pattern (SP) optimization based on the Simulated Annealing (SA) algorithm. The primary objective of the SA optimization is to minimize the peak radiation damage and its secondary objective is to minimize burnup reactivity swing, the radial power peaking factor, and the variation in power level experienced by a fuel assembly over the cycle.

Compared with the optimal conventional 2D fuel shuffling, the optimal 3D SP offers (1) a 34% reduction of the peak radiation damage level, down to ~350 dpa, (2) a 45% increase in the average fuel discharge burnup, and hence the uranium utilization, and (3) does not violate any major neutronics or thermal-hydraulics constraints. For the same peak dpa level, the average discharge burnup of the 3D shuffled core is 2.23 times that of the 2D shuffled core; this corresponds to a ~120% relative increase in the fuel utilization. These significant improvements may enable nearer-term commercialization of B&B reactors. In the long term, the successful deployment of the B&B core along with optimal 3D shuffling of fuel could provide at least a 30-fold increase in uranium utilization compared to current once-through LWRs, and hence significantly improve the sustainability of the once-through nuclear fuel cycle.