A case for modular nuclear reactors has been made in the U.S. with Energy Secretary Stephen Chu promoting it despite the accident in Japan. But the question of how best to use coal still remains since the fuel has several cost and availability advantages.
A new proposal by Hybrid Power Technologies LLC marries the helium gas reactor with the combustion turbine and coal. The system uses the helium gas cycle powered by a nuclear reactor to run the gas turbine. It allows the integration of solar and energy storage applications, too.
In this, the compressor of the combustion turbine is operated by helium gas, which means that the compressor can operate at various speeds providing air to multiple application, including gasification. This also means that gas turbine output is doubled since the turbine work is not used to compress air.
The reactor part of the hybrid nuclear-combined cycle plant reduces nuclear fuel required by 80%. This will make permitting easier, says Michael Keller of Hybrid Power.
The efficiency of the hybrid is over 50%. The reactor uses a vessel-within-vessel configuration which causes the reactor to be subjected lower pressure and temperatures than current concepts, thus enabling the use of materials essentially qualified to American Society of Mechanical Engineers (ASME) nuclear requirements.
Further, the vessel wall, with a thickness of roughly 80 mm, can be readily fabricated (as rolled plate) by a number of domestic manufacturing facilities – e.g. US shipyards and heavy vessel manufacturing facilities.
Field welding can be accomplished using existing, proven technologies. The vessel thickness of current gas reactor concepts is about 250 mm and is outside established manufacturing capabilities.2
The reactor core is essentially General Atomics’ 600 MW(t) prismatic design that consists of an annular core, with coated fuel particles formed into fuel rods that are inserted into graphite blocks stacked inside the reactor vessel – see www.ga.com.
The turbo-compressor is envisioned to be based on horizontal, heavy-frame combustion turbine designs routinely used in the power industry. These rugged designs have evolved into exceptionally reliable and powerful machines. Modern combustion turbines operate at temperatures exceeding 1340 C (2450 F), well above the 850 C (1562 F) inlet temperature of the hybrid’s helium turbine. Technology elements readily transferred from modern combustion turbines include: blades/airfoils, disks, blade tip sealing methods, shaft sealing methods (e.g. dry gas, brush and finger seals) and foil heat removal.
The turbo–compressor can also be rotated by a motor (located within the turbo-compressor vessel) or the main generator (acting as a motor) to support reactor decay heat removal. In both cases, clutches are used to engage the motors. These enhancements are primarily used to protect the financial asset, but overall reactor safety is improved as well.
The primary heat exchanger includes a recuperator and pre-cooler; these features being common to closed-system Brayton cycle designs. While confinement is potentially acceptable from a regulatory standpoint, the hybrid is envisioned to employ a full containment with passive heat removal features.
The hybrid’s full containment design is considered likely to significantly reduce possible public anxiety as well as potential licensing risks.
A novel hybrid design feature is that both the reactor vessel and primary heat exchanger are mounted on sliding bases, while the turbo-compressor is fixed. This design accommodates component and piping thermal expansions while recognizing that large, coupled rotating machines have only limited displacement capabilities.