The Production of the Isotope 15 N by Isotopic Exchange in Nitrox System at Pressure

heavy metal needs to be fabricated into ADS fuel demanding 4.2 t of 15 N, which cost The operation of 15 N production plant at pressure of 0.7 +/- 0.05 bar in stable conditions at total reflux and in production is demonstrated. The 10M HNO 3 flows used in the primary column were 6.6 L/h and 6.0 L/h, 47, respectively 33% higher than that of atmospheric pressure (4.5 L/h) and corresponding flow rates of the same column were 3.12 mL/cm 2 . min, respectively 2.83 mL/cm 2 .min for operation at pressure, compared to 2.12 mL/cm 2 ’.min at atmospheric pressure. The 10M HNO 3 flows used in the final column were 324 mL/ h and 264 mL/h, 62, respectively 32 % higher than that of the atmospheric pressure (200 mL/h) and flow rates of that column were: 3.05 mL/cm 2 .min, respectively 2.49 mL/cm 2 .min, compared to 1.86 mL/cm 2 ’.min for atmospheric pressure operation. The HETP values are lower for both separation columns operated at pressure and higher 10M HNO 3 flow rates, that being an argument in the favour of operation the 15 N production plant by isotopic exchange in Nitrox system at pressure. The nitrogen enriched in 15 N losses in the waste sulphuric acid, evacuated at the bottom of product refluxer stage II, were : 5.18 -10.58 ppm of feeding with 10M HNO 3 of the plant. Any losses of nitrogen enriched in 15 N represent an additional uncontrolled production, which diminishes the production of 15 N plant. In the case of operation at pressure the flows of H 15 NO 3 extracted as plant product are higher than those corresponding to atmospheric pressure because the nitric acid solution is chemically equilibrated with nitrogen oxides at the operation pressure and temperature. At a pressure of 1 atm. in the 15 N production plant the nitrogen content of nitric acid solution would be 10.919M instead of 10M.

The isotope 15 N, with a natural abundance of 0.365 at. %, has a cross section for neutron absorption of 8x10 -5 barns, compared to the other stable isotope of nitrogen, 14 N, with a natural abundance of 99.635 at. %, which has a cross section for neutron absorption of 1.78 barns. According to that property the isotope 15 N is utilized in nuclear equipments, which cannot have natural abundance nitrogen in composition.
In nuclear energetics it is considered the utilization of nitride type nuclear fuels as U 15 N, Pu 15 N, etc., instead of oxidic fuels for generation IV nuclear reactors and Accelerator Driven Systems (ADS) for transmutation of the radioactive actinide of nuclear waste. It has to mention that for nitride fuel preparation it cannot use the most abundant nitrogen isotope, 14 N, because it gives radioactive 14 C and 3 H by participation to the reactions: 14 N(n,p) 14 C, 14 N(n,t) 12 C, [1 -4] Nitride type nuclear fuel has important advantages compared to oxidic fuel according its higher melting point, excellent thermal conductivity, higher fissile density, lower fission gas release and good radiation tolerance. For preparation of nitride nuclear fuel will be utilized the isotope 15 N at 99 at. % 15 N concentration, for the above mentioned reasons, but also to diminish the number of separation work unities (SWU) in the plant for 235 U enrichment for nuclear fuels.
Taking into account that the world 15 N production is estimated at 20 -40 Kg/year and the use of that isotope for production of nitride nuclear fuels for nuclear reactors and ADS, 1000 times increase of its production is estimated. In a 1000 TWhe nuclear park LWRs and fast neutron CAPRA reactors use oxidic fuels and ADS is assumed to operate on nitride fuel, where the nitrogen is enriched in 15 N. For that nuclear park 33.2 t/year of heavy metal needs to be fabricated into ADS fuel demanding 4.2 t of 15 N, which cost The operation of 15 N production plant at pressure of 0.7 +/-0.05 bar in stable conditions at total reflux and in production is demonstrated. The 10M HNO 3 flows used in the primary column were 6.6 L/h and 6.0 L/h, 47, respectively 33% higher than that of atmospheric pressure (4.5 L/h) and corresponding flow rates of the same column were 3.12 mL/cm 2 . min, respectively 2.83 mL/cm 2 .min for operation at pressure, compared to 2.12 mL/cm 2 '.min at atmospheric pressure. The 10M HNO 3 flows used in the final column were 324 mL/ h and 264 mL/h, 62, respectively 32 % higher than that of the atmospheric pressure (200 mL/h) and flow rates of that column were: 3.05 mL/cm 2 .min, respectively 2.49 mL/cm 2 .min, compared to 1.86 mL/cm 2 '.min for atmospheric pressure operation. The HETP values are lower for both separation columns operated at pressure and higher 10M HNO 3 flow rates, that being an argument in the favour of operation the 15 N production plant by isotopic exchange in Nitrox system at pressure. The nitrogen enriched in 15 N losses in the waste sulphuric acid, evacuated at the bottom of product refluxer stage II, were : 5.18 -10.58 ppm of feeding with 10M HNO 3 of the plant. Any losses of nitrogen enriched in 15 N represent an additional uncontrolled production, which diminishes the production of 15 N plant. In the case of operation at pressure the flows of H 15 NO 3 extracted as plant product are higher than those corresponding to atmospheric pressure because the nitric acid solution is chemically equilibrated with nitrogen oxides at the operation pressure and temperature. At a pressure of 1 atm. in the 15 N production plant the nitrogen content of nitric acid solution would be 10.919M instead of 10M.
Keywords: 15 N production, isotopic exchange, nitrox system, packed columns, pressure is optimistic estimated at 50 million Euro/year (the price of 15 N 99 at. % was considered 10 Euro/g). Implementation of 15 N recovery in the fuel reprocessing could decrease the penalty due to 15 N loss. The use of a closed gas cycle in the nuclear fuel fabrication stage is mandatory [5,6].

Experimental part
The experimental test of 15 N separation by isotopic exchange in the system of nitrogen oxides in gaseous phase -10M nitric acid solution, Nitrox system [7]: After passing through the primary column the 10M HNO 3 solution arrives in the stage I of the product refluxer where it reacts with gaseous sulphur dioxide, fed at the bottom of it. The gaseous mixture of NO, NO 2 produced in stage I of the refluxer, enters in the stage II of the refluxer, fed with a supplementary flow of sulphur dioxide, so that the most important part of NO 2 is reduced to NO. The both stages of the product refluxer are Hastelloy C-276 made chemical reactors packed column type, resistant to the corrosion due to sulphuric acid, nitric acid, NO x , and SO 2 . The refluxers were packed with 6x6 mm glass Raschig rings, chemical and thermic resistant. The refluxer of the final column [8] is a Hastelloy C-276 column type reactor, having an interior heat exchanger and a glass Raschig ring packing, 5x5 mm.
In all experiments presented in this paper the pressure of sulphur dioxide, feeding the plant refluxers, was 0.9 bar.
The sulphuric acid solution, produced in the refluxers of 15 N plant, is evacuated at the bottom of it. The sulphur dioxide flows were automatically controlled in order to maintain the reaction zone at constant height in the refluxer. If the reaction zone goes up to the top of the refluxer there is the possibility that sulphur dioxide enters in the isotope separation column damaging the stainless steel packing, by attack of sulphuric acid and sulphur dioxide.
If the reaction zone goes down to the refluxer bottom there is the possibility to loss nitrogen enriched in 15 N in the discarded sulphuric acid.

Results and discussions
A test of the final separation column included the following steps: the filling of the separation column with 10M HNO 3 solution to ensure a proper wetting of the packing, draining slowly the column with a flow of 10M HNO 3 , feeding with sulphur dioxide of the product refluxer. In table 1 is presented the results of 5 tests of the final column (F.C.), among them the run (2) has been done independently of the primary column (P.C.) and in the runs (3,9,11,12) the final column was fed with 10M nitric acid extracted from the bottom of the primary column.
The flows utilized in the final column were 324 -330 mL/h, the flow rates being: 3.056, respectively 3.11 mL/ cm 2 .min, 1.5 times higher than the flow rates used when the feeding sulphur dioxide pressure was lower. Table 1 does not include data on isotope separation, which was measured in the run (13) and the results will be presented later, in this paper. Except the run (13) in which the isotopic equilibrium of both separation columns was achieved, the other tests have been accomplished in 10 -12 h, being experimentally followed: the fluids flowing, the automatic controlled feeding of sulphur dioxide, sulphuric acid evacuation at the bottom of the refluxers and the maintaining of the stable position of the HNO 3 -SO 2 reaction zone.
The test of the primary separation column (P.C.) were done in the same way as it was presented for the final column; that column has been fed with 10M nitric acid solution at 4.5 -6.6 L/h., the flow rates being between 2.12 and 3.12 mL/cm 2 .min (table 1). The flow rates increasing were possible according to the increasing of the sulphur dioxide pressure feeding the refluxers at 0.9 bar [9].
In the run (13) table 1, the variation of the 15 N separation in time, measured on the production plant, the SO 2 feeding pressure being 0.9 bar during 73 days of the experiment, has been determined. In the figure 1 are presented the isotopic equilibrium curves (isotope separation dynamics) of both columns of the plant. During run (13) the flows of 10M HNO 3 feeding the primary column were 6.6 L/h and 6.0 L/h and the flow rates of 3.12, respectively 2.83 mL/ cm 2 .min, compared to the flow 4.5 L/h and 2.12 mL/ cm 2 .min at lower pressure. In the run (13) the final column was fed with 10M HNO 3 , enriched in 15 N, from the bottom of the primar y column, the flows being 324 mL/h, respectively 264 mL/h and flow rates 3.056 and respectively 2.49 mL/cm 2 '"min, compared to 200 mL/h and a flow rate of 1.88 mL/cm 2 '" min used at lower pressure. After about 12 days of continuous operation the 15 N production was started with a 10M H 15 NO 3 . 1), marks the increasing of the operation pressure in the separation plant to 0.7 +/-0.05 bar, by partially closing of the valve on the line by which the gaseous mixture NO, NO 2 leaves the top of the primary column to nitric acid refluxer [7].
Because of the pressure variations on the SO 2 line, feeding product refluxers of the plant and the fact that the most part of SO 2 flow, entering in the stage I of the primary column refluxer was not automatically controlled, a cyclic changing of HNO 3 -SO 2 reaction zone position to the bottom, respectively to the top of the refluxer has been observed. In the first situation occurred losses of nitrogen enriched in 15 N in the waste sulphuric acid. In order to determine that losses the nitrogen from sulphuric acid was reduced to ammonia, which gives a yellow coloured compound by reaction with Nessler reagent, which was analysed by spectrophotometer at 425 nm. The nitrogen contents of 3 sulphuric samples were: 5.18, 6.71,  The operation of the separation plant at 0.7 +/-0.05 bar was tested. A stable operation of the 15 N separation plant at total reflux (plateau 7) and in production (plateau 8 -11) has been observed. In table 2 are presented the experimental data obtained in 73 days of operation of 15 N separation plant.
The calculus of the isotopic separation (S) and of the height equivalent to theoretical plate (HETP) [10] for each column was done by using the mean values of the 15 N concentration of the plateaus 1 -12, marked on the graph of figure 1. The HETP values were calculated not only in the case of operation at total reflux but also in production, in order to have comparison elements with the data of separation plant operated at atmospheric pressure, (table  3). It has to mention that in the case of operation at pressure the flows of H 15 NO 3 , extracted as plant production, are higher than those corresponding to atmospheric pressure because the nitric acid solution is chemically equilibrated with nitrogen oxides at the operation pressure and temperature. For instance if 10M HNO 3 solution is equilibrated with nitrogen oxides at 1 atm. and 198M thus the total nitrogen concentration of the liquid phase will be 10.919M [11]. In this situation, if the flow of H 15 NO 3 extraction is 1 mL/h, at 1 atm. pressure it corresponds to 1.0919 mL/h.
The HETP values of both columns are lower in the case of operation at pressure (plateaus 8 -10, fig. 1) or actually at atmospheric pressure (plateaus 1-6, fig. 1) in both situation being used 10M HNO 3 flows (flow rates) 33, respectively 47% higher than those presented in table 3 for atmospheric pressure. The fact that have been used higher flows (flow rates) in the 15 N separation columns and HETP values are lower, than those determined at atmospheric pressure, represents an important argument in the favour of plant operation at pressure.
The mean production flow in the case of the three production periods at atmospheric pressure presented in table 3 has been 0.697 mL/h 10M H 15 NO 3 , at a mean isotopic concentration of 92.85 at. % 15 N.  In the case of operation of 15 N plant at pressure a production flow of 0.9 mL/h of 10M H 15 NO 3 at 99 at. % 15 N would be 27 % higher than that achieved at atmospheric pressure. That increasing is considered important taking into account that it was achieved on the same plant, without any additional energy consumption, because the plant pressure will be increased based only on the liquid sulphur dioxide vapour pressure of the cylinder.

Conclusions
The operation of 15 N production plant (two separation columns connected in series) at pressure of 0.7 +/-0.05 bar in stable conditions at total reflux and in production is demonstrated.
10M HNO 3 flows used in the primary column were 6.6 L/h and 6.0 L/h, 47, respectively 33% higher than that of atmospheric pressure (4.5 L/h).
The flow rates of the primary column were 3.12 mL/ cm 2 .min, respectively 2.83 mL/cm 2 .min for operation at pressure, compared to 2.12 mL/cm 2 .min at atmospheric pressure.
10M HNO 3 flows used in the final column were 324 mL/ h and 264 mL/h, 62, respectively 32 % higher than that of the atmospheric pressure (200 mL/h).
The flow rates of the final column were: 3.05 mL/ cm 2 '"min, respectively 2.49 mL/cm 2 .min, compared to 1.86 mL/cm 2 .min for atmospheric pressure operation.
HETP values are lower for both separation columns operated at pressure and higher 10M HNO 3 flow rates, that being an argument in the favour of operation the 15 N production plant by isotopic exchange in Nitrox system at pressure.
The nitrogen enriched in 15 N losses in the waste sulphuric acid, evacuated at the bottom of product refluxer stage II, were : 5.18 -10.58 ppm of feeding with 10M HNO 3 of the plant. Any losses of nitrogen enriched in 15 N represent an additional uncontrolled production, which diminishes the production of 15 N plant.