Understanding Hydrogen/Tritium Behavior on Carbon to Predict and Control Tritium in Salt Reactors: Experiments, Modeling and Simulation

Report Date: August 2018
Appendices:  No

Abstract

Fluoride-salt-cooled High-temperature Reactors (FHRs) use solid fuel and salt coolants. Liquid salts, particularly flibe (Li2BeF4), generate significant quantities of tritium that must be captured to avoid its release to the environment. The primary emphasis of the MIT work on tritium is understanding its interactions with carbon to predict behavior and ultimately control tritium releases. The fuel contains large quantities of carbon and carbon beds can be used to remove tritium from the liquid salt. Carbon is a leading candidate for tritium removal from salt at 700°C because (1) it can adsorb significant amounts of tritium, (2) is compatible with salt at high temperatures and (3) is a component of the fuel. Because the system already contains carbon, its use elsewhere in the system such as for tritium removal in a carbon bed exterior to the core does not introduce new materials to the system. However, limited work has been done at the relevant reactor thermodynamic conditions. This report summarizes the work of the last three years, which consists of 1) understanding tritium transport in carbon and investigating different carbon-based materials for use as a tritium adsorber, and 2) modeling and simulating tritium transport in an FHR system.

Through this work, data has been collected at a temperature of 700oC and sub atmospheric pressures under 5 kPa. It was found that amorphous carbons show nearly fifty times the solubility of previously characterized tritium absorbers—including carbons used in the fuel. Different adsorption modes have been identified and it was found that a significant amount of this adsorption was reversible, which enables regeneration in an adsorption system. Moreover, it was found that various physical properties such as the material surface area, and pore size distribution are good predictors of hydrogen solubility. This provides a basis for further materials discovery and development with the ultimate goal to enable the reactor designer to choose carbon forms to match goals—either more or less tritium adsorption (less tritium in core and greater tritium adsorption in tritium removal systems).

With the new experimental data, system-level simulations were performed using TRIDENT and it was shown that the system can be optimized along various operating and design parameters to capture and control tritium. The coupled effects of generation, chemical speciation, adsorption and diffusion of tritium in the FHR system were simulated over 200 full-power days. It was found that an adsorption column using high-performance carbon-based catalyst adsorbed substantial amounts of tritium and reduced the peak release rate from 2400 Ci/day to 40 Ci/day for the 236 MWt FHR. Further, it was found that the total tritium inventory in the system can be decreased by more than 70%, from 68,400 Ci to 19,400 Ci. This demonstrates that adsorption technology has the potential to greatly reduce the risk of radiological release during normal operation and reactor transient events.

The modeling and characterization of carbon enables predicting tritium behavior under normal and accident (high-temperature) conditions. The TRIDENT system allows modeling of alternative tritium control systems.

Program:     ANP : Advanced Nuclear Power Program

Type:     TR 

RPT. No.: 177