The CNRS Research Program on the Thorium cycle ... - Pacen - IN2P3

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Thorium Cycle – Molten Salt Reactors June 2008 Both thermodynamic and kinetic aspects have to be considered to evaluate a reprocessing scheme. Fundamental data (redox potentials, activity coefficients) determined either by experimental measurements [LCB05, WPF05, FPS05, BRL07, DPF08, CMN07, CDL05, LEB93] or by molecular dynamic simulations [SST08], yield efficiencies for each reaction in its equilibrium state. Such types of calculations and the analytical equations they rely on, have already been published [DEL08, DPI05, DSS08, DMH08]. <strong>The</strong> proposed reference scheme used for the processing of TMSR-NM fuel salt is given in figure 9. <strong>The</strong> first step involves an on-line gaseous extraction with helium bubbling (green upper lefthand corner box) to remove gaseous fission products, Xe and Kr and noble metals by a flotation process. Presently it is assumed that this bubbling is performed on the entire reactor volume salt over a period of time of the order of the minute. On the other hand, batch process acting at a much smaller speed (few dozens litre per day) separates the actinides which are returned to the reactor salt from the harmful fission products (mostly lanthanides). V.B.1 On-line extraction of gaseous fission products Fission leads to the formation of many poorly soluble neutron poisons which have thus to be eliminated. Among the most harmful one finds 135 Xe. Each 233 U fission atom produces 0.75 noble gas atom (Xe and Kr). <strong>The</strong>ir subsequent radioactive decay generates non volatile residues in the reactor core or along the piping and within the heat exchangers. A fraction (about 15%) of these gases is short-lived and naturally decays within the molten salt. <strong>The</strong> remaining 85% must be evacuated. Based on the positive experience acquired with the MSRE [CEA76, WMC70, GRI70, SEA70, HEN70], the method envisaged involves an extraction by means of He bubbling. Since gases have a very low solubility in molten salts (approximately 10 -8 moles/(salt cm -3 )/atm -1 ) they tend to spontaneously migrate towards the interface between the gas cover and the salt interface. <strong>The</strong> injection of He bubbles inside the salt is thus expected to drag to the surface all the insoluble gaseous FP. This first step in the purification process before returning the salt to the reactor involves the further He gas circulation in decay tanks where specific active carbon trap solid FP such as Ba, La, Cs, Rb, Sr, Y and Zr. Fig. 9 TMSR-NM reference fuel salt processing. It is effected both on line as “He bubbling” or in batch mode (within the zone circumscribed by the orange thick dash line). <strong>The</strong> thick solid line follows the fuel salt progression along the four stages of batch processing. <strong>The</strong> evolution of its colour from blue to red symbolizes the improved cleanliness state. Purple thin lines correspond to actinide circulation within batch processing. Dark exiting lines indicate waste fluxes, while dark input lines for ThF4 and H2O corresponds to replenishing of fuel by addition of fertile material and input chemical for batch processing. Finally the blue lines show the internal return circulation of liquid Bi-Th melt. 14/29
Thorium Cycle – Molten Salt Reactors June 2008 It is also expected that fission products such as Nb, Ru, Ta, Te, … will be swept away by the He gas. Indeed, because of the low value of the molten salt redox potential these so called noble metals are in their metallic state. Thus they are not solvated in the salt as are most ionic species. Instead, they remain as a suspension with a weak binding to the liquid medium. It is believed that via a flotation mechanism He bubbles will be adsorbed to the noble gas particles allowing them to be carried away by the gas flow. This assumption is supported by the MSRE experiment where a significant part of the noble metals was retrieved inside the decay tanks. A fraction of tritium (T) produced by fission is also extracted by the He flux. This tritium would then be separated from He by a chemical reaction on copper oxide. It is envisaged to store T, Xe and Kr waste in pressurized canisters for a length of time allowing a later safe release (typically 100 years). ORNL experiments have already provided relevant data. Still given the present evolutions both in reactor design and in salt specifications, some updating is necessary. When operational at Grenoble, the loop described in Fig.8 should provide a unique testing facility. In the mean time, <strong>CNRS</strong> has decided to support the construction of a dedicated system (Fig. 10) to begin the exploration of the physical properties of the He bubbling extraction process and to evaluate its performance. <strong>The</strong> first experiments are planned in 2008. V.B.2 Batch reprocessing Batch reprocessing involves the daily (or weekly) removal of a fraction of the salt in order to separate actinides from harmful FP produced during the reactor operation. <strong>The</strong> volume that can be handled and the reprocessing time are crucial parameters for the overall viability of the MSR concept. Neutron reactor simulations have specified the minimum acceptable level of reprocessing (see Sect. III.B). As compared to the MSBR project [CEA76], TMSR-NM reactor physics requires batch reprocessing rates smaller by about two orders of magnitude. <strong>The</strong> reference scheme depicted in Fig.8 involves four stages. <strong>The</strong> physics and chemistry for the last one “Stage 4: salt composition and redox control” and the corresponding research effort has been touched on in Sect. V.A. We now describe stages 1 to 3 and their impact on the organization of the <strong>CNRS</strong> effort. Filter H, He Injection Graphite Collumn & Salt Injection Nozzle Support Safety tank Steel vessel Fig.10 Temperature controlled He and H bubbling experimental column built at <strong>CNRS</strong>-Grenoble for the evaluation of the extraction efficacy from a molten salt of rare gases and noble metals. V.B.2.a Stage 1 : Uranium extraction by fluorination This method has been well tested at ORNL. <strong>The</strong> extraction yield of Uranium has been shown to exceed 99%. <strong>The</strong> method proceeds according to the following chemical reaction: UF4 + F2(g)→ UF6(g). It is performed in a chemical reactor in which the UF4 laden salt is injected against contrary flowing fluorine (in 50% excess). At the same time, some other FPs are oxidized by F (I, Np, Te, Ru, Nb) as well as some alloying elements of the reactor structure materials (Cr, Mo). All chemical species are then extracted as gases which are circulated on a sequence of selective traps (NaF). <strong>The</strong> 15/29
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The CNRS Research Program on the Thorium cycle ... - Pacen - IN2P3

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