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The Very High Temperature Reactor (VHTR) is a Generation-IV design in the conceptual pre-licensing phase for potential construction by 2030-2050. It is graphite moderated, helium cooled reactor that operates at an exit temperature of up to 1273 K, making it ideal for generating electricity at a plant thermal efficiency upwards of 48% and the co-generation of process heat for hydrogen production and other industrial uses. Extensive thermal-hydraulics and safety analyses of VHTRs are being conducted using Computational Fluid Dynamics (CFD) and heat transfer codes, in conjunction with experiments and prototype demonstrations. These analyses are challenging, largely due to the 3-D simulation of the helium flow in the 10 m long coolant channels in the reactor core and the need to examine the effects of helium bypass flow in the interstitial gaps between the core fuel elements. This research, performed at the UNM-ISNPS, developed an effective thermal-hydraulics analyses methodology that markedly reduces the numerical meshing requirements and computational time. It couples the heliums 1-D convective flow and heat transfer in the channels to 3-D heat conduction in graphite and fuel compacts of VHTR fuel elements. Besides the helium local bulk temperature, the heat transfer coefficient is calculated using a Nusselt number correlation, developed and validated in this work. In addition to omitting the numerical meshing in the coolant channels, the simplified analysis methodology effectively decreases the total computation time by a factor of ~ 33 - 40 with little effect on the calculated temperatures (< 5 K), compared to a full 3-D thermal-hydraulics analysis. The developed convective heat transfer correlation accounts for the effect of entrance mixing in the coolant channels, where z/D < 25. The correlation compares favorably, to within + 12%, with Taylor's (based on high temperature hydrogen heat transfer) and to within + 2% of the calculated results for full 3-D analyses of

The Institute of Space and Nuclear Power Studies at the University of New Mexico has developed a computer simulation of cylindrical geometry alkali metal thermal-to-electric converter cells using a standard Fortran 77 computer code. The objective and use of this code was to compare the experimental measurements with computer simulations, upgrade the model as appropriate, and conduct investigations of various methods to improve the design and performance of the devices for improved efficiency, durability, and longer operational life-time. The Institute of Space and Nuclear Power Studies participated in vacuum testing of PX series alkali metal thermal-to-electric converter cells and developed the alkali metal thermal-to-electric converter Performance Evaluation and Analysis Model. This computer model consisted of a sodium pressure loss model, a cell electrochemical and electric model, and a radiation/conduction heat transfer model. The code closely predicted the operation and performance of a wide variety of PX series cells which led to suggestions for improvements to both lifetime and performance. The code provides valuable insight into the operation of the cell, predicts parameters of components within the cell, and is a useful tool for predicting both the transient and steady state performance of systems of cells., See also Part 2 AD-A366 804.

The Institute of Space and Nuclear Power Studies at the University of New Mexico has developed a computer simulation of cylindrical geometry alkali metal thermal-to-electric converter cells using a standard Fortran 77 computer code. The objective and use of this code was to compare the experimental measurements with computer simulations, upgrade the model as appropriate, and conduct investigations of various methods to improve the design and performance of the devices for improved efficiency, durability, and longer operational life-time. The Institute of Space and Nuclear Power Studies participated in vacuum testing of PX series alkali metal thermal-to-electric converter cells and developed the alkali metal thermal-to-electric converter Performance Evaluation and Analysis Model. This computer model consisted of a sodium pressure loss model, a cell electrochemical and electric model, and a radiation/conduction heat transfer model. The code closely predicted the operation and performance of a wide variety of PX series cells which led to suggestions for improvements to both lifetime and performance. The code provides valuable insight into the operation of the cell, predicts parameters of components within the cell, and is a useful tool for predicting both the transient and steady state performance of systems of cells., ADA366805