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.