Current commercial nuclear reactors are ~ 30% efficient thermal-to-electric, since they operate on a steam cycle, the heat rejection temperature is generally ~ ambient temperatures which makes heat rejection problematic, and generally requires substantial water. One of the ways to make micro- reactors more cost effective is to substantively increase their efficiency, however, the efficiency improvements cannot be incremental, ~ few%; the improvements must hold sufficient advancement to warrant a transition to a new conversion system. LPS proposes a very unique combined cycle system which promises a thermal-to-electric efficiency improvement ~ 2X, which would establish micro-reactors with efficiencies on the order of combined cycle natural gas turbine plants, but with no CO2 exhaust. Generally speaking, efficiency improvements require new state points in a Carnot cycle, either raising the top-end usable temperature or lowering the bottom end temperature, or both. Lowering the heat rejection temperature compared to a steam cycle is impractical. There are several micro reactor concepts under development which have reasonably high operating temperatures. To take advantages of these higher operating temperatures, LPS has been investigating a combined cycle concept with high efficiency which has a heat rejection temperature ~60-100C. This means that waste heat can be rejected directly to the atmosphere using forced air convection with modest sized heat exchangers. Due to its relatively high heat rejection temperatures and its high efficiencies, this conversion system is also ideal for space. For this reason, LPS is responding to a NASA 2021 SBIR call looking for Nuclear Thermal Propulsion (NTP) or Nuclear Electric Propulsion (NEP) reactors with high temperatures (~2600 C) for NTP and relatively high > 927C reactor temperatures for NEP. LPS is proposing a bi-modal system which combines NTP and NEP in the same reactor, and has indicated that this is the preferred cycle for generating ~ MWe power. LPS specifically mentioned that it planned to propose to this solicitation in order to advance the thermal-to-electric conversion technology for both terrestrial and space applications. Consequently, there is at least one additional very high value proposition use for a micro- reactor using this technology. The proposed cycle diagram is shown below: The high temperature cycle is a He recuperated Brayton cycle, with the output of the recuperator transferred to a boiler (HRVG) which drives a isobutane (R-600a)Rankine cycle. As a note, these state points and efficiencies are not optimized, but represent data from prior analyses. It should be noted that the 16.9% enthalpy extraction efficiency of the R-600a cycle is not added to the overall efficiency, because it is extracting energy from the residuals of the Brayton cycle. The total gross efficiency is ~ 54%. This proposed system is extremely versatile. First, as a power system, it exhibits feasible efficiencies unheard of in a reactor system. Second, because it will be using high-speed rotating machinery ~ 35,000 rpm, the wild AC output is converted to 60 Hz AC through highly efficient digital power electronics, the turbomachines and alternators will be quite small. ~1% of the size of an equivalent steam plant. Third, the system will be compact enough that it can be located at sites where process heat is required. The present design incorporates a recuperator between the turbine exit and the HRVG. The turbine outlet temperature is ~883 Kelvins, which is very high quality heat. A non-recuperated system will be ~ 25% efficient thermal-to-electric, but recent studies have shown that the need for process heat is twice the demand for electrical power. Consequently, a system with the same reactor and conversion system can meet a wide range of needs. If the process heat temperature requirement is lower, then a small recuperator can be used to reduce the process heat temperature while increasing the electrical power output. In this SBIR, out team plans to carefully analyze the combined cycle system and it components. Since affordability is an important function, we will analyze where 3D printing can replace conventional fabrication methods. This could be particularly true for the recuperator (a high temperature heat exchanger and the HRVG). In the past LPS also investigated producing a 3D printed SiN turbine/ compressor/shaft system for a small 7 kWe closed Brayton cycle. This system would be considerably larger, but the technology may have production cost advantages. At the conclusion of Phase I, we will have a candidate design for a 20 MWth micro-reactor system, and development plans for a sub-scale demonstrator to be developed in Phase II. As we now envision it, the Phase II demonstrator would incorporate a heat source to simulate the recuperator output. This thermal energy stream would go to the HRVG and demonstrate the closed R-600a bottoming cycle. Sufficient closed Brayton cycle systems have been demonstrated so very accurate modeling is available based on experiments. We cannot afford to demonstrate both the closed Brayton and the R-600a Rankine cycle in Phase II without augmented funding. The Rankine cycle has not been demonstrated in this form, so demonstrating this portion of the overall cycle would reduce overall risk of the combined cycle system.
