UC Berkeley

This study assesses the feasibility of designing Seed and Blanket (S&B) Sodium-cooled Fast Reactor (SFR) to generate a significant fraction of the core power from radial thorium fueled blankets that operate on the Breed-and-Burn (B&B) mode without exceeding the 200 Displacements per Atom (DPA) radiation damage constraint of presently verified cladding materials. The S&B core is designed to have an elongated seed (or “driver”) to maximize the fraction of neutrons that radially leak into the subcritical blanket and reduce neutron loss via axial leakage. The blanket in the S&B core makes beneficial use of the leaking neutrons for improved economics and resource utilization. A specific objective of this study is to maximize the fraction of core power that can be generated by the blanket without violating the thermal hydraulic and material constraints. Since the blanket fuel requires no reprocessing along with 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 (CR) seed and a B&B blanket fueled by thorium. Among several benefits, this synergism enables the very low leakage S&B cores to have small positive coolant voiding reactivity coefficient and large enough negative Doppler coefficient even when using inert matrix fuel for the seed. The benefits of this synergism are maximized when using an annular seed surrounded by an inner and outer thorium blankets. Two high-performance S&B cores were designed to benefit from this unique synergism: (1) the ultra-long cycle core that features a cycle length of ~7 years; (2) the high-transmutation rate core where the seed fuel features a TRU CR of 0.0. Its TRU transmutation rate is comparable to that of the reference Advanced Burner Reactor (ABR) with CR of 0.5 and the thorium blanket can generate close to 60% of the core power; but requires only one sixth of the reprocessing and fabrication capacity per unit of core power.

Nevertheless, these reference cores were designed to set upper bounds on the S&B core performance by using larger height and pressure drop than those of typical SFR design. A study was subsequently undertaken to quantify the tradeoff between S&B core design variables and the core performance. This study concludes that a viable S&B core can be designed without significant deviation from SFR core design practices. For example, the S&B core with 120cm active height will be comparable in volume, HM mass and specific power with the S-PRISM core and could fit within the S-PRISM reactor vessel. 43.1% of this core power will be generated by the once-through thorium blanket; the required capacity for reprocessing and remote fuel fabrication per unit of electricity generated will be approximately one fifth of that for a comparable ABR. The sodium void worth of this 120cm tall S&B core is significantly less positive than that of the reference ABR and the Doppler coefficient is only slightly smaller even though the seed uses a fertile-free fuel. The seed in the high transmutation core requires inert matrix fuel (TRU-40Zr) that has been successfully irradiated by the Fuel Cycle Research & Development program. An additional sensitivity analysis was later conducted to remove the bias introduced by the discrepancy between radiation damage constraints -- 200 DPA applied for S&B cores and fast fluence of 4x1023 n(>0.1MeV)/cm2 applied for ABR core design. Although the performance characteristics of the S&B cores are sensitive to the radiation damage constraint applied, the S&B cores offer very significant performance improvements relative to the conventional ABR core design when using identical constraint.

Fuel cycle characteristics of the S&B cores were compared with those of the reference ABR, and a Pressurized Water Reactor (PWR). The fuel cycle cost of the S&B reactor with same LWR TRU transmutation rate as the reference (CR=0.5) ABR is 0.53 cents/kWe-h versus 0.73 cents/kWe-h for the ABR – about 27% lower; it is even lower than that of contemporary PWRs. The longer cycle may enable the S&B cores to operate at a ~10% higher capacity factor and thereby further improve their economic viability. The S&B cores can utilize at least 7% of thorium energy value without a need to develop irradiated thorium reprocessing capability. This is ~12 times the amount of energy that the LWRs generate per unit of natural uranium mined. By softening the blanket spectrum the thorium utilization can increase by a factor of at least three when using thorium hydride rather than metallic fuel; Fully Ceramic Encapsulated (FCM) fueled blanket can achieve the discharge burnup of 481.5 MWd/kg if the FCM fuel keeps its integrity up to such burnup – this is over 80 times the energy extracted by present PWR per unit mass of natural uranium.

If reprocessed, the Trans-Th fuel bred in the S&B core can enable to support new fleets of 233U-Th fuel self-sustaining energy systems that use thermal and epithermal reactors such as Molten Salt Reactors (MSR) and Reduced-moderation Boiling Water Reactors (RBWR). Alternatively, the S&B reactors can be used to close the LWR fuel cycle using a 3-tier system: TRU extracted from Tier-1 LWR is used for fueling the seed of Tier-2 S&B cores while the 233U (Trans-Th) extracted from Tier-2 S&B blanket is used as the fissile feed for Tier-3 LWR. It is estimated that in such a 3-tier energy system 1GWe of S&B SFRs can support 3.3 GWe of PWRs versus ~1.7 GWe of PWRs that can be supported by 1GWe of CR=0.5 ABR.

In summary, the Seed-and-Blanket core concept studied in this project is found highly promising as it offers:

• Improvement in the economic viability of Sodium-cooled Fast Reactors due to the significant reduction in the fuel cycle cost and possibly increase in the capacity factor that may be enabled by the longer cycles. The improved economics may justify earlier commercialization of SFRs.

• Significantly smaller investment in the construction of the fuel reprocessing and the remote fuel fabrication infrastructure required for a given capacity of SFRs.

• Several new promising fuel cycle options feature substantial increase of the thorium resource utilization without fuel reprocessing

• Supporting a large number of LWRs by a given capacity of SFRs on the S&B configuration. Thus, it enables to close the nuclear fuel cycle faster and with smaller investment.

In conclusion, the S&B reactor concept we proposed is feasible and potential to significantly improve the economic viability of fast reactors and of LWR TRU transmuting system using existing structural materials. It enables significant utilization of thorium resource without reprocessing.