1. Why not connect the nuclear turbine directly to one end of the generator, with a conventional combustion turbine/air-compressor attached to the other end of the generator (i.e. a “single shaft” configuration)?

The “single-shaft” operates at the fixed speed of the generator -- typically, 3600 revolutions per minute (rpm) for the 60 hertz frequency US electrical system and 3000 rpm for the 50 Hertz European electrical system. The hybrid nuclear turbines and compressors rotate at different speeds, as optimally established using key safety, output, performance, and efficiency considerations, as further explained below.

1. Safety  One of the more severe events for a power plant is the “loss-of-electrical-load” event (i.e. the generator trips off-line). The impact on a reactor can be quite challenging and potentially severe. By de-coupling the reactor from electrical grid disturbances, safety is significantly improved. During a “loss-of-load” event for a hybrid-nuclear unit, compressed air is vented from the combustion turbine power block while the combustion turbine-generator spins down. The reactor block is essentially unaffected by the event, with reactor output reduced as needed in a non-critical fashion.
2. Output/Performance  In order to optimize performance while using proven designs, the hybrid-nuclear helium turbine that drives the helium compressors rotates at approximately 6000 rpm. A 2nd “split-shaft” helium turbine drives the air compressor at approximately 3000 rpm in order to use 50 hertz style combustion turbine air compressors -- the 50 hertz compressors are about 20% larger than their 60 hertz US counterparts. This feature enables the hybrid design to maximize the output of the combustion turbine at all times.
   Also, the hybrid’s variable speed large 50 hertz style compressors significantly improve the performance of coal gasification processes. Conventional Integrated Gasification Combined-Cycle (IGCC) coal gasification plants typically employ combustion turbines that supply only part of the air needed for gasification process, with this air provided by the combustion turbine’s fixed speed compressor. Accordingly, additional and relatively inefficient stand-alone air compressors must also be provided. This arrangement adversely impacts the effectiveness of the process. The hybrid-nuclear units, with their larger 3000 rpm compressors, supply all the air required for the gasification of coal. Thus, the overall efficiency of the plant is significantly improved, as is the electrical output of the plant.
3. Efficiency  In order to reduce power, conventional gas turbines must choke back air flow because the air compressor is attached to a fixed speed electrical generator. This is an inherently inefficient process. However, the hybrid-nuclear configuration does not employ a fixed speed air compressor. Further, the “split-shaft” configuration of the helium turbines allows optimum maneuvering capabilities. Also, closed-cycle gas turbine cycles such as the hybrid-nuclear design rely on removing the working fluid (helium, in this case) to reduce pressure, thereby very effectively reducing power. These features collectively cause the hybrid-nuclear design to retain a high efficiency in all load ranges.
   

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2. Does the hybrid-nuclear plant use the basic designs of the Next Generation Nuclear Plant (NGNP) being developed by the Department of Energy (DOE)?  - see www.nuclear.energy.gov

Generally no, although some the proven developments of the NGNP program may be employed. The simpler hybrid-nuclear technology does not need to utilize the riskier “cutting-edge” features of the NGNP designs. Specific differences include:

1. The lower reactor temperature of the hybrid-nuclear design promotes the use of proven materials as well as component designs – e.g. see item “c.” below. The +900 C (+1650 F) reactor outlet temperature of the NGNP is not needed.
2. The hybrid-nuclear turbo-compressor is a conventional stand-alone, split-casing horizontal design to facilitate maintenance and simplify the design. Further, proven conventional gas turbine oil lubricated bearings are used in conjunction with conventional gas turbine labyrinth seals, brush seals and oil seals to limit helium leakage. The NGNP “cutting edge” magnetic bearing system is not needed.
3. The recuperator heat exchanger used by the hybrid-nuclear technology is a simple and straightforward design while being located in a stand-alone vessel. This configuration simplifies maintenance, inspections and repair efforts while reducing costs. While the NGNP has not fully settled on specifics, the emphasis is currently being placed on designs that combine the turbo-compressor, generator and recuperator in a single vessel. Also, the NGNP is developing high-temperature designs for the recuperator. None of these technically challenging features are required for the hybrid-nuclear configuration.
4. As noted in the response to FAQ #1, the hybrid-nuclear turbo-compressor is not attached to a constant speed generator and is thus largely immune to electrical grid problems. The direct-cycle NGNP design uses a generator attached to the constant speed helium compressor and helium turbine.

The hybrid-nuclear technology heavily emphasizes the use of well proven elements from conventional nuclear, gas turbine and fossil power plants. This enables the hybrid-nuclear technology to be much more quickly deployed than the NGNP (apparently targeted for the year 2021). Further, the hybrid-nuclear design is much larger, safer and more efficient than the NGNP. Thus, the hybrid-nuclear technology is inherently more competitive and more commercially viable than the NGNP.

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3. Does the presence of the nuclear reactor overly complicate the operation of the power plant owing to government (Nuclear Regulatory Commission or NRC) regulations?  

Not anywhere near the degree to which conventional nuclear plants are impacted, as the majority of the regulations that govern conventional nuclear units are not applicable to the hybrid-nuclear design. This occurs because the hybrid-nuclear plant has considerably fewer safety systems relative to conventional nuclear power plants. Further, the key accident concerns (basically melting the nuclear fuel) that complicate conventional nuclear plant compliance with NRC requirements are not particularly germane to the hybrid design as the core can not melt. Thus, the overall complexity of regulatory compliance effort is significantly reduced.

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4. The reactor block and combustion turbine are coupled to one another, as is the gasification block used with the coal variant of the hybrid-nuclear plant. Will not such configurations noticeably reduce the overall reliability of the power plant?  

Not necessarily because the blocks can be decoupled from one another. Equipment redundancies are also present, as discussed below.

1. The hybrid-nuclear block is essentially indirectly coupled to the power block by way of the air compressor. Physically, the nuclear turbine is attached to the combustion turbine’s air compressor by way of a flexible coupling that can be unbolted. With a properly sized start-up clutch, the combustion turbine could temporarily drive the air compressor and generator without operating the reactor. However, plant’s output would be cut approximately in half until the reactor was brought back into service. The ability to run the combustion turbine power block without the reactor has more than just operational advantages. For instance, the power block can constructed in roughly 18 months and then brought into service, with the more involved reactor block (and gasification block) brought on line when completed. The ability to so quickly establish a source of revenue is a significant financial advantage, particularly relative to conventional nuclear plants with construction time frames on the order of roughly 5 years or longer.
2. The gasification block supplies fuel to the combustion turbine. However, most Integrated Gasification Combined-cycle (IGCC) plants are also designed to use natural gas. Thus, with the gasification plant out-of-service, the power plant can run on natural gas.
3. The gasification units of IGCC plants typically include redundant operating trains, thereby allowing continued operation if equipment problems occur. In passing, early gasification plants did suffer from reliability problems. But that same statement can also be made about early combustion turbines and nuclear reactors. Both of these technologies now routinely operate at capacity factors well over 90%. The maturing gasification technologies are now achieving similar high levels of reliability.

The hybrid-nuclear technology’s inherent high level of flexibility will insure that the facilities will be both profitable and dependable.