An inverted hydride-fueled pressurized water reactor concept

Abstract

Previous studies conducted at MIT showed that power performance of typical pin geometry PWRs are limited by three main constraints: core pressure drop, critical heat flux (CHF) and fretting phenomena of the fuel rods against grid spacers. The present work investigates the possibility to reduce the limiting effect exerted by these constraints by radically changing the core geometry, rather than by only taking measures to address specific constraints. The geometry modification consists of inverting the relative position of fuel and coolant, thus generating the so-called inverted geometry. An inverted assembly consists of a fuel prism perforated with cylindrical, vertically oriented, cooling channels, arranged in a triangular lattice. A pin vs inverted comparison, performed at cell level, shows that an inverted geometry can attain the same fuel volume fraction of the pin geometry but with a much lower pressure drop and fuel temperature. Also, CHF performance can be enhanced, relative to the pin geometry, by inserting multiple short-length twisted tapes (MSLTTs) inside the cooling channels, and fretting concerns do not apply since spacer grids are not needed. When the pin vs inverted comparison is performed at whole-core level, the same conclusion on pressure drop and fuel temperature apply to reactor types in which, thanks to low operating pressure and/or fuel-coolant chemical compatibility, the inverted core can be designed as to closely resemble a modular repetition of the inverted unit cell, i.e. the so-called continuous inverted geometry.

(cont.) However, the high operating pressure characterizing a PWR, together with the need of avoiding fuel-water interaction, require the inverted PWR (IPWR) to be provided with particularly thick ducts enclosing the fuel prisms. These ducts, together with the wide inter-assembly water gaps needed for control rod insertion, cause the inverted geometry to become discontinuous, and to lose part of the pressure drop and fuel temperature advantages characterizing a continuous inverted geometry. A U-Th-Zr-hydride fuel was selected for the IPWR. The main reasons that led to its choice were the negligible fission gas release which is compatible with the need to enclose the fuel in a large duct, and the pre-hydriding metal structure of the fuel which allows an effective drilling. A detailed study was performed to maximize the performance of a hydride-fueled IPWR, accounting for structural mechanics, thermal hydraulics, neutronics and manufacturing-related constraints. The analysis was performed over a wide spectrum of lattice geometries, each characterized by specific values of the cooling channel diameter and pitch. Three cooling channel designs were examined: MSLTT-provided channels, channels provided with a long twisted tape inserted in the top half of the core, and empty channels. Two duct designs were examined: collapsible and non-collapsible. The former, about 210 mm wide and with ~9 mm thick walls, is designed to collapse onto the fuel prism upon primary system pressurization.

(cont.) The latter, about 100 mm wide and with -6 mm thick walls, is internally pressurized: its small size together with the reduced differential pressure across its walls, allow preventing duct-fuel contact, but significantly penalize the reactor power performance due to the reduced volume available for fuel and coolant. As a consequence of these design options, a total of six IPWR designs were examined. Because of the scarcity of pressure drop data referred to MSLTT designs, pressure drop tests were performed and results entered in the IPWR computational analysis model. Besides usefulness for the IPWR study, the wide range of MSLTT designs that were tested allowed supplementing the literature with valuable experimental data. It was found that pressure drop is the most limiting IPWR design constraint, followed by CHF and, only marginally, fuel temperature. The fuel web thickness, i.e. the minimum thickness of fuel meat between adjacent cooling channels, was also found to significantly affect the attainable power. Specifically, the smaller this thickness, the higher is the power. To allow fuel prism manufacturability, fuel web thicknesses as low as 2 mm were examined. The IPWR provided with collapsible ducts and empty cooling channels was verified to outperform all the other IPWR designs examined.

(cont.) Conclusions on the competitiveness, from the attainable power viewpoint, of this IPWR design against typical pin geometry PWRs depend on the IPWR considered (maximum powered, but provided with a very small web thickness, or a "selected design", having lower power but larger fuel web thickness) and on the PWR relative to which the comparison is performed (maximum powered, but with thin 6.5 mm OD fuel rods, or reference geometry 9.5 mm OD rods). If the maximum powered IPWR is considered, maximum power gains are of 13% and 48% with respect to the maximum powered PWR and to the reference PWR respectively. If the selected IPWR design is considered, no power gain is possible relative to the maximum powered PWR, while a power gain of 19% is achievable relative to the reference PWR. A comprehensive analysis, including LBLOCA modeling and neutronics, was performed on the selected IPWR design. This reactor was demonstrated to be able to deliver a thermal power of 4078 MW, corresponding to a 19% gain with respect to the reference PWR analyzed with the same pressure drop limit. Power density and specific power are 119 MW/m3 and 73.6 kW/kgHM respectively. Required fuel enrichment to achieve a 17.2 month fuel cycle is 15%. Although a net power gain was demonstrated, the economic competitiveness of the IPWR concept is penalized by the higher enrichment required and, eventually, by higher manufacture costs of the inverted assemblies relative to pin assemblies. A complete economic analysis, not performed in this work, would be needed to assess the benefits of the IPWR design.