Matthew J. Memmott

The Use of Flashing Drums and Microchannel Heat Exchangers to Generate Steam in Large Integral Light Water Reactors

Abstract Integral pressurized water reactors are innovative reactors in which all of the components typically associated with the nuclear steam supply system of a nuclear power station are located within the reactor pressure vessel. In order to facilitate this modification in large [~1000-MW(electric)] light water reactors (LWRs), compact heat exchangers such as microchannel heat exchangers must be used. Previous attempts at using microchannel heat exchangers were unsuccessful since they are prone to vapor locking and crud blockage when the primary coolant boils. Therefore, the authors propose the use of a flashing drum to facilitate boiling in conjunction with a primary microchannel heat exchanger for a large integral LWR. The integral inherently safe light water reactor (I2S-LWR) is used as a basis for the implementation of this novel concept. The high-temperature, high-pressure secondary water generated in the secondary loop through heating in the microchannel primary heat exchanger of the I2S-LWR is sent to a flashing drum where 99.9% pure vapor is extracted and sent to the turbines. This prevents boiling in the primary heat exchanger that in turn reduces crud deposition, flow instabilities, and the potential for channel blockage or vapor locking in the small channel sizes of microchannel heat exchangers. The benefits and disadvantages of this approach are presented in this paper. Unfortunately, this innovative approach to nuclear steam generation for integral LWRs is challenged by a potential decrease in thermodynamic efficiency. Therefore, a sensitivity study is presented that explores the impact of several design variables on the thermodynamic efficiency of the plant. As part of this study, a simple and a complex Rankine cycle were modeled in order to determine the impact that system design modifications can play in recovering thermodynamic efficiency lost by the steam drum. Both cycles utilize turbines, condensers, and condensate/recirculation pumps, while the complex Rankine cycle utilizes a four-stage turbine with subsequent separation and open feedwater heaters. The optimized efficiencies for the simple and complex Rankine cycles are 31% and 33%, respectively, indicating that additional system enhancements to the power conversion system could compensate for the inclusion of a flashing drum.

An Evaluation of the Annular Fuel and Bottle-Shaped Fuel Concepts for Sodium Fast Reactors

Abstract Two innovative fuel concepts, the internally and externally cooled annular fuel and the bottle-shaped fuel, were investigated with the goal of increasing the power density and reducing the pressure drop in the sodium-cooled fast reactor, respectively. The concepts were explored for both high- and low-conversion core configurations and for metal and oxide fuels. The annular fuel concept is best suited for low-conversion metal-fueled cores, where it can enable a power uprate of ~20%; the magnitude of the uprate is limited by the fuel-clad chemical interaction temperature constraint during a hypothetical flow blockage of the inner annular channel. The bottle-shaped fuel concept is best suited for tight high–conversion ratio cores, where it can reduce the overall core pressure drop in the fuel channels by >30%, with a corresponding increase in core height between 15 and 18%. A full-plant RELAP5-3D model was created to evaluate the transient performance of the innovative fuel configurations during the unprotected transient overpower and station blackout. The transient analysis confirmed the good thermal-hydraulic performance of the annular and bottle-shaped fuel designs with respect to the reference case with traditional solid fuel pins.

I 2 S-LWR Concept Update

Pressurized water reactor of integral configuration (iPWR) offers inherent safety features, such as the possibility to completely eliminate large-break LOCA and control rod ejection. However, integral configuration implemented using the current PWR technology leads to a larger reactor vessel, which in turn, due to the vessel manufacturability and transportability restrictions, limits the reactor power. It is reflected in the fact that there are many proposed iPWR SMR concepts, with power levels up to approximately 300 MWe, but not many iPWR concepts with power level corresponding to that of large traditional PWR NPPs (900 MWe or higher). While SMRs offer certain advantages, they also have specific challenges. Moreover, large energy markets tend to prefer NPPs with larger power. The Integral Inherently Safe Light Water Reactor (I2S-LWR) concept is an integral PWR, of larger power level (1000 MWe), that at the same time features integral configurations, and inherent safety features typically found only in iPWR SMRs. This is achieved by employing novel, more compact, technologies that simultaneously enable integral configuration, large power, and acceptable size reactor vessel. This concept is being developed since 2013 through a DOE-supported Integrated Research Project (IRP) in Nuclear Engineering University Programs (NEUP). The project led by Georgia Tech includes thirteen other national and international organizations from academia (University of Michigan, University of Tennessee, University of Idaho, Virginia Tech, Florida Institute of Technology, Brigham Young University, Morehouse College, University of Cambridge, Politecnico di Milano, and University of Zagreb), industry (Westinghouse Electric Company and Southern Nuclear), and Idaho National Laboratory. This concept introduces and integrates several novel technologies, including high power density core, silicide fuel, fuel/cladding system with enhanced accident tolerance, and primary micro-channel heat exchangers integrated with flashing drums into innovative power conversion system. Many inherent safety features are implemented as well, based on all passive safety systems, enhancing its safety performance parameters. The concept aims to provide both the enhanced safety and economics and offers the next evolutionary step beyond the Generation III + systems. This paper presents some details on the concept design and its safety systems and features, together with an update of the project progress.