A PC-based system for simulation of processes in the isotopic exchange column of a cryogenic pilot plant

The technology developed at the Institute of Cryogenics and Isotope Separations is based on catalytic isotope exchange between water and hydrogen gas both carrying various isotopes of hydrogen: normal hydrogen, deuterium, and tritium. This isotope exchange is followed by cryogenic distillation to separate the various isotopes of hydrogen gas. The detritiation process was simulated using water as working fluid, with a small content of deuterium and a gaseous mixture of hydrogen and deuterium. Isotopic separation of hydrogen is very important for nuclear plants utilizing the CANDU reactor concept. Tritiated water is extracted from the moderator of the reactor and the tritium is removed in order to reduce the radiation levels from the reactor. One of the most important steps in this process is the catalytic isotope exchange between tritiated water and hydrogen gas. In the 'Catalytic isotope exchange' system, tritium transfer occurs from the liquid phase (tritiated heavy water) into the gaseous phase (hydrogen) in an isotope exchange column. The column is packed with alternating layers of two types of catalysts: an ordered B7 type of phosphorous bronze to catalyze the equilibration between liquid and gaseous water and a PT/C/PTFE catalyst to catalyze the reaction between water vapor and hydrogen gas. The corresponding reactions are (DTO)L+ (D2O)V ↔ (D2O)L + (DTO)V and (DTO)V + (D2)G ↔ (D2O)V + (DT)G. The design of the process requires a constant catalytic exchange temperature that must be maintained at a value of 90 deg.C. To achieve this, the plant is equipped with electrical heat exchangers for heating the gas (hydrogen) and the heavy water. The control is achieved using an array of sensors and controllers. The sensors used for monitoring the process are type J thermocouples. The temperature control is achieved by controlling the electrical power fed to the heaters. Besides sensing the fluid temperatures at various points, we also monitor the vapor pressure and the heavy water level in the column. The level is controlled via two dosing pumps, which work alternately to feed water to the column. In addition, we employ two monitoring systems for security. The first one monitors the temperature of hydrogen inside the heat exchanger and at the inlet of column and ensures that these are lower than the maximum safe temperatures (90 deg.C and 150 deg.C, respectively). The second security system monitors the pressure of the nitrogen used for cooling the electrical resistance rods inside the heaters. We created a front panel user interface with LabViewTM, which gave us interactive control of the system and a faster way to program instrumentation and data acquisition systems. By using LabViewTM in design, test and implementation we reduced the system development time and increased the productivity. Because LabViewTM has the same development tools and language capabilities as a standard computer language such as C - looping and case structure - it is well suited for modeling and simulation. (authors)