Development of tritium separation process by cryogenic distillation applicable to detritiation facilities of CANDU reactor and ITER fusion reactor

Funding contract no. 05-D10-33/05.10.2005

   The project's main objective is to promote the research regarding the design of a detritiation plant and more precisely the development of the most efficient "back-end" process - cryogenic distillation.
   The project has 6 phases, each phase consisting of more activities developed in a chronological order necessary for the fulfillment of the objectives.
   The first phases consisted in defining a data-base to contribute to the elaboration of a complex computational model for tritium separation through cryogenic distillation.
   This model could be considered an improvement of the bicomponent classical model used frequently until now, which neglected the presence of the third isotope, by considering it in a small concentration. The model reduces the differences between the theoretical and experimental results. This can be used both for the design and optimization of cryogenic distillation plants for hydrogen isotopes mixtures and for their simulation. The computational program is written in Pascal.    A very sensitive aspect regarding the research in the field of hydrogen cryogenic distillation is the design of a sampling and extraction system accompanied by an efficient automatization of the regulating system and its monitoring. The system is designed in such a way that it does not introduce disturbances into the cryogenic distillation system and leads to non-steady regimes difficult to control.
   During the cryogenic distillation of hydrogen isotopes, the heavy fraction made of complex molecules of hydrogen isotopes (HD, D2, DT, and T2) is accumulationg at the lower part of the distillation column, and the extraction takes place in the gaseous phase.
   For pumping and transportation of radioactive and explosive mixtures of hydrogen isotopes, several measures and special requests are to be taken into consideration. Therefore, there must be avoided the formation of tritiated waste and also there must be prevented the loss of inventory of both deuterium, which has a very high price and tritium (or its compounds) which is, above all, radioactive.
   Taking into consideration all these elements, a solution which meets the special safety conditions when manipulating radioactive and explosive products is the use of pumps which have as the operating principle the adsorption of the respective gas on zeolites. The design of the adoption system is based on data regarding the adsorption capacity and breaking time of the adsorbent used.
   In this way there was designed an extraction system based on the operating principle of a cryopump. The cryopump, which is actually the intermediate tritium storage vessel, has systems which provide advanced cooling (liquid N temperature) for adsorption phase and heating systems for the desorbtion phase. The design of this system meets the special operating requirements, cryogenic temperatures (up to liquid hydrogen temperature), quite frequent temperature variation, a special sealing system, high reliability and the possibility to integrate it with a sampling system. Operating conditions, and the type of processed fluid (tritium) require that the extraction system be completely automatized. Another condition which was taken into consideration when designing this system was the connection of this module for tritium storage, which is realized on metallic structures like spongious titanium or intermetallic compounds.
   From the point of view of automatization of the tritium extraction system, it has been realized a configuration of the system based on the introduction of a programmable automatic which commands the taps of the system. Using a particular software, it has been realized a logical closing and opening of the taps in order for the adsober to pass from the adsobtion phase into the desorbtion phase depending on the type of experiment. The digital inputs of the programmable automatic aquire signals from the limit switch (open-close) of the taps. The analogic inputs aquire process parameters of the system.
   A special attention was given to the most important element, the packing, which provides the isotopic separation during cryogenic distillation. The separation performance for two types of packing used for cryogenic distillation of hydrogen, ordered type, stainless steel B5 and B7, was experimentally determined. The tests were performed on a pilot plant at ICSI Rm. Valcea.
   The novelty of this project in the field of packing consists in the performance tests for the determination of the efficiency at the separation of hydrogen isotopes, in order to obtain a data base necessary for the design of the cryogenic distillation columns which will be part of the detritiation system of the tritiated water provided by a CANDU reactor, or fussion ITER.
   Also, the packings are made at ICSI Rm. Valcea and they are already patented. These ordered packings are used for water distillation and the results may be used in promoting these packings for water detritiation installations.
   In the last phase it is intended to realise a project of cascade cryogenic distillation for a detritiation plant which involves high levels of contamination.
   Cryogenic distillation cascade of hydrogen is designed to reach a concentration of tritium with at least three orders.
   When designing it is taken into account the mass transfer specific for tritium separation by cryogenic distillation in a distillation cascade made of two distillation columns and the heat transfer necessary for sizing the cryogenic cicles necessary for the operation of the condensers for the two columns of cryogenic distillation.
   The design took into consideration the conditions in the cryogenic laboratory. For the design there was used the soft developed during the project. The date obtained from the isotopic calculations of the cryogenic distillation cascade are presented as tables.
   To design the refrigerating systems necessary for the condensers to operate there have been analysed two possibilities: with under cooled helium and liquefied hydrogen.
   It has been observed from the thermodynamic analysis of the two cycles that in the case of the cycle with hydrogen a high degree of reversibility is obtained and also a high refrigeration efficiency.
   From the economical point of view the cicle with hydrogen has lower economical costs than those necessary for the cicle with helium. From the technological and maintenance point of view the cicle with hydrogen has advantages because it has maintenance and exploitation costs inferior to the cicle with helium.
   There are presented also the diagrams for the refrigerating installation of the cryogenic distillation columns.
   The researche regarding the development of sampling and extraction systems by extending to a cascade of cryogenic distillation made of two columns were carried on.
   As it was presented in the previous phases, these systems present a high degree of complexity because they are used at cryogenic temperatures and in tritiated environmnet. Also, there were developed monitoring systems of these systems in order to verify the efficiency of the sampling and extraction procedure.
    The extraction points of the samples which are to be analysed were positioned in such a way as to fully describe the isotopic transformation on the whole circuit from the feeding to the storage.
   The sampling circuits are connected to a gas-cromatograph-type of system and ionizing chamber.
   Automatization of the sampling and tritium extraction system is made of a control and automatization system which realises the sequences for closing and opening of the taps in order for the adsorber to pass from from the adsorbtion phase in desorbtion phase, and also hte sequence for sampling from points of interest. Also, the automatization provides the monitorization of the technological parameters of the distillation cascade.
   In order to extract the samples for analysis, on the sampling route electrovents are provided.
   The monitorisation of the sampling and extracting system and also thge monitorisation of main technological parameters is done configurating virtual screens.
   To realise the sampling and extraction system, there were used Programmable Automatic from Twido family, provided by Schneider Electric- France.
   To realise the system for monitoring of technological parametrs, there was used the system offered by National Instrument - USA.
   Modules for signal aquisition - Field Point type - were configured using own prrogramm FieldPoint Explorer.
   Also, to monitor the progress of the experiement there were realised "screens" for parameters visualisation, under the platform LabVIEW .
   During this project both the coordinator and the partners disseminated the results through scientific communications and published papers.
   The projects brings a major contribution to the development of the technology for tritium separation.