Metallic Fuel Technology
Lightbridge’s metal fuel technology came out of the research and development work for our thorium-based seed-and-blanket fuel assembly. The metallic seed rods used in our seed-and-blanket design are capable of operating safely at increased power density compared to standard uranium oxide fuel. Lightbridge determined that a fuel assembly comprised of only metallic fuel rods could provide significant benefits to a nuclear power plant.
Lightbridge’s metal fuel differs from the alloy fuel that has been historically evaluated for fast reactor applications. Previous investigations into metal nuclear fuels focused on low alloy compositions such as U-10Zr wherein the concentration of uranium is significantly higher than the alloy constituent. The Lightbridge Zr-U alloy is a high-alloy fuel comprised of U-50Zr. One of the primary differences, with respect to irradiation characteristics, of Lightbridge’s metal fuel compared to U-10Zr is a significant reduction in irradiation-induced swelling. The U-10Zr fuels exhibit high radiation-induced swelling (typically, ~30 volume percent within 2 atom percent burn) while swelling in the Lightbridge metal fuel is expected to be around 1 volume percent per atom percent burn.
Inherent design features of the Lightbridge metal fuel technology are expected to enhance safety characteristics under normal operation and certain off-normal events. A key part of the in-reactor and out-of-reactor testing planned by Lightbridge includes experiments to confirm and demonstrate these safety benefits.
Lower fuel operating temperature (380°C for the metallic fuel v. 1500°C for oxide fuel):
- Reduces the amount of heat that must be dissipated into the coolant at reactor shutdown and shortens the time required to do so.
- Although the metallic fuel has a lower melting temperature than oxide fuels, its low operating temperature and high thermal conductivity keep the fuel from approaching dangerous temperatures during accident scenarios.
Increased heat transfer between fuel and coolant that improves fuel coolability:
- Higher thermal conductivity of metal vs. oxide – Improves the speed at which heat from the fuel can be dissipated into the coolant;
- Increased fuel rod surface area (~35-40% greater); and
- Improved coolant mixing due to helical twist – Facilitates heat dissipation into the coolant and reduces local hot spots in fuel rods.
Improved cladding integrity due to metallurgical bonding of fuel to the cladding:
- Helps retain radioactive material inside the fuel rod in case of cladding breach.
To date Lightbridge has simulated a large break loss of coolant accident for a VVER-1000 with our metallic fuel. The peak fuel temperature observed for the metallic fuel was less than 500 degrees Celsius, well below the temperature required to initiate steam interactions with the zirconium cladding (i.e., above 900 degrees Celsius). The high thermal conductivity of the metallic fuel results in the fuel temperature decreasing to the temperature of the coolant water in less than 60 seconds with little to no increase during the blowdown phase of the accident. The same accident was modeled with conventional uranium dioxide fuel and showed a near instantaneous cladding temperature rise to above 1000 degrees Celsius. The fuel and cladding temperature continued to rise during the blowdown phase of the accident and did not reach a safe, stable temperature until nearly 8 minutes after the accident began. This simulation assumes a design basis accident wherein emergency core cooling systems and cooling water are available. Lightbridge intends to do a variety of accident response tests on both fresh and irradiated fuel to demonstrate the fuels behavior during worst-case accident scenarios. The superior thermal conductivity and increased surface area of the metallic fuel rod are expected to result in improved cooling via all mechanisms (e.g. steam environments) when compared to conventional fuel.
Power uprates are an efficient use of capital to increase revenue generation in nuclear power plants as they can be performed in a relatively short time frame (months versus years for new plant construction). No plant modifications need to be performed to utilize Lightbridge’s fuels at current plant power output. The extent of plant modifications that must be performed is dependent on the level of power uprate. The table below compares the projected incremental capital cost of plant modifications for Lightbridge’s power uprate fuels with the cost of a new build reactor.
The projected incremental annual net operating cash flows and return on investment for a nuclear power plant at various wholesale prices of electricity generated as a result of using Lightbridge’s power uprate fuels are shown in the table below.
The cash flow values shown above are incremental operating cash flows to a utility, net of incremental fuel costs due to increased power output and technology licensing fees payable to Lightbridge and the fuel vendor.
The total amount of plutonium at discharge in the Lightbridge-designed metallic fuel for a 10% power uprate and a 24-month fuel cycle is reduced by approx. 57% compared to conventional uranium oxide fuel.
The isotopic composition of plutonium contained in the used Lightbridge-designed metallic fuel shows a 64% reduction in the total amount of fissile plutonium isotopes (Pu-239 and Pu-241) per assembly. In addition to the reduced quantity of fissile plutonium isotopes, their weight fraction in the total amount of plutonium material is reduced from 68% in conventional spent uranium oxide fuel to 57% in the Lightbridge-designed used metallic fuel. At the same time, the weight fraction at discharge of Pu-238 isotope, which significantly degrades suitability of plutonium for weapons purposes, is increased from 2% to 9% in the Lightbridge-designed used metallic fuel.