Study on Tritium Behaviors in VHTR and Forward Osmosis Integration System

Abstract

Energy depletion and water scarcity are the two major global issues which are only intensifying due to global population growth. Efforts have been made to tackle the mentioned global issues and as an effective solution to this matter, the integration concept of the Very High Temperature Reactor (VHTR) and Forward Osmosis (FO) desalination is proposed. Through thermodynamic analysis calculations, the suggested VHTR-FO integration system is found to have significantly higher energy utilization rate than the existing nuclear desalination systems. The VHTR-integrated system has the potential of tritium migration from the VHTR core to the integrated industrial system through permeation in the heat exchangers. As this is a very serious safety problem, the tritium behavior in the VHTR-integrated system must be analyzed and understood. In order to understand and analyze tritium behavior in the VHTR-integrated system, the tritium code, Behavior of Tritium Analytic Code (BOTANIC) is developed using a chemical process code, gPROMS. The code involves tritium generation, sorption, leakage, purification, recombination, dissociation, permeation, trapping, release models. It is not only capable of tritium analysis but also chemical process analysis and system dynamics calculation. The developed code is verified using the analytical solutions and the benchmark code in step wise approach. As the migration mainly occurs through permeation in the heat exchanger, this mechanism is the most important phenomena in understanding tritium behavior. In this study, the printed circuit heat exchanger (PCHE) permeation model is developed which accounts the geometry and thermal distribution across the PCHE wall. The existing permeation model is found to significantly over predict or under predict permeation rate whereas the developed PCHE permeation model is found to accurately calculate permeation rate within ± 20 %. There is a lack of permeability data of hydrogen at low temperature region, the integration temperature region of VHTR and FO system. In order to resolve the error accounted with the lack of permeability at low temperature, hydrogen permeation experiment is conducted at low temperature region. Based on the permeability data, the effective permeability is defined and when compared with the permeability data it is found to be in good agreement. Using the developed BOTANIC code, PCHE permeation model and effective permeability the tritium behavior in the proposed VHTR-FO desalination system is analyzed. The tritium level in the final water product of VHTR-FO system is found to exceed the regulatory limit an order. This stresses the necessity of tritium mitigation, thus, sensitivity analysis is conducted in order to figure out the effective parameters in reducing tritium level in final product. Based on the sensitivity analysis results, mitigation concepts are suggested and investigated

PHX tritium barrier, PHX material substitution and PHX pre-heat treatment.