In order to reduce heat and off-gas generation rates at early stage of the chemical denitration of high and medium level liquid
wastes from the reprocessing of nuclear fuel, a safety denitration method, Pre- and Mild-denitration technique, was originated.
In the Pre-denitration step, the formic acid of 0.06 to 0.3 times nitric acid concentration ([HCOOH]/[HNO3]=0.06 to 0.3) was poured into a nitric acid solution at 80°C and denitration was initiated (Pre-denitration). Then additional
formic acid was injected into the Pre-denitrated solution at a constant injection rate in 80°C and the solution mixture was
heated up to the boiling condition (Mild-denitration). In the Mild-denitration step, the denitration reaction was smoothly
initiated and a leak out of solution from a abrupt boiling has never occurred. The maximum heat and off-gas generation rates
were about 50 cal/s-l and about 1.01/min respectively even in the 10M nitric acid solution. These measured values were low
enough to ensure the safety operation of denitration.
The heat and off-gas generation behavior was experimentally examined during a safe chemical denitration, pre- and mild-denitration,
of simulated HLLW with a nitric acid concentration of 2 to 7.5 M. The maximum heat and off-gas generation were no more than
100 cal/s·1 and about 0.8 l/min, respectively. The solution temperature does not reach boiling temperature and no solution
was squirted out from the denitration vessel. The pre-and mild-denitration technique could be considered as one of safe methods
for removing nitric acid from the HLLW with various nitric acid concentrations. The pre- and mild-denitration also has an
advantage to improve the filtration characteristics of precipitates produced by the denitration of simulated HLLW. The denitration
of HLLW with 7.5M nitric acid concentration induced formation of “very easy-to-filter” solid. Moreover, a good filter cake
washing is possible.
The influence of urea on initiation and termination of the reaction between nitric and formic acids was experimentally examined.
The urea injection can terminate the denitration reaction in 2 to 10M salt-free nitric acid solutions and the simulated high
level liquid wastes (HLLWs) with a nitric acid concentration of 2 to 6M. An excess of urea can interrupt the initiation of
denitration in both simulated HLLW and salt-free nitric acid solutions. The initiation and termination of denitration are
in relation with nitrous acid formation and decomposition. Urea reacts with nitrous acid easily in the denitrating solution
and decomposes nitrous acid. As the urea concentration increases in the solution, the continuance of denitration become impossible
because the decomposition rate of nitrous acid exceeds the generation rate. In addition, the nitrous acid concentration can
not be high enough to initiate the denitration in the solution with an excess of urea because nitrous acid is decomposed by
The effect of phosphate ion on the filtration characteristics of solids generated in a high level liquid waste was experimentally
examined. Addition of phosphate ion into the simulated HLLW induced the formation of phosphate such as zirconium phosphate
and phosphomolybdic acid. The filtration rate of zirconium phosphate abruptly dropped in the midst of filtration because of
a gel-cake formation on the filter surface. The denitration of the simulated HLLW contained zirconium phosphate improved the
filterability of this gelatinous solid. The filtration rates of denitrated HLLW decreased with increase of the phosphate ion
concentration, since the solids formed by denitration had irregular particle size and configuration in the simulated HLLW
with phosphate ion. To increase the filtration rate of denitrated HLLW, a solid suspension filtration tester was designed.
The solid-suspension accelerated the filtration rate only in the simulated HLLW with more than 1500 ppm phosphate ion concentration.
Under this condition, the simple agitation can easily suspend the constituent solids of filter cake in the solution and a
much higher filtration rate can be obtained because the filter cake is continuously swept from the filter surface by rotation
Precipitate formation behavior in high-level liquid waste (HLLW) and its filtration characteristics were examined experimentally, using a simulated HLLW. The amount of precipitate formed by denitration became minimum, only at about 5% of Mo, Zr, Te and Ru, if the simulated HLLW was pre-heated until the total heat input exceeded 7.9·106 J/I HLLW before denitration or denitrated with the total heat input of more than 1.1·107 J/I HLLW. Under these conditions, a needle-shaped precipitate with 0.51.0 m diameter and 35 m length was formed. This precipitate can be separated easily by vacuum filtration. While, fine particles of about 0.1 m diameter were precipitated during denitration, if the simulated HLLW was denitrated under the conditions the amount of newly formed precipitate was not minimum. It was difficult to separate the fine particles by vacuum filtration.
The solids formation behavior in a simulated high level liquid waste (HLLW) was experimentally examined, when the simulated HLLW was treated in the ordinary way of actual HLLW treatment process. Solids formation conditions and mechanism were closely discussed. The solids formation during a concentration step can be explained by considering the formation of zirconium phosphate, phosphomolybdic acid and precipitation of strontium and barium nitrates and their solubilities. For the solids formation during the denitration step, at least four courses were observed; formation of an undissolved material by a chemical reaction with each other of solute elements (zirconium, molybdenum, tellurium) precipitation by reduction (platinum group metals) formation of hydroxide or carbonate compounds (chromium, neodymium, iron nickel, strontium, barium) and a physical adsorption to stable solid such as zirconium molybdate (nickel, strontium, barium).
The filtration characteristics of solids generated in a simulated high level liquid waste (HLLW) were experimentally examined, when the simulated HLLW was processed according to the ordinary way of actual HLLW treatment process. The filtration characteristics of solids depended on the particle size. The phosphomolybdic acid, which was very fine particle with about 0.1 m diameter, made slurry a difficult-to-filter slurry, if the phosphomolybdic acid content (wt%) to the whole solids in a slurry exceeded 50 wt%. On the contrary, the zirconium compounds (zirconium molybdate and zirconium telluride) had positive effect on filtration characteristics because of their relatively large particle size of about 3 to 5 m. When the zirconium compounds content was above 50 wt%, slurry became a easy-to-filter slurry. A centrifugal sedimentation was discussed as a solid/liquid separation technique for very fine particles such as phosphomolybdic acid. The theoretical feed flow rate corresponded to 0.1 m diameter particles was about 20 l/h at the centrifugal acceleration of about 8000 G.
Solid formation in a simulated high level liquid waste (HLLW) was experimentally examined at 2M and 0.5M nitric acid concentrations. The precipitation studies were conducted by refluxing the simulated HLLW around 100°C. Zr, Mo, Te and Ru were major precipitation elements in both 2M and 0.5M HNO3 solutions. The amount of precipitate in 2M HNO3 solution decreased with decreasing Zr concentration and no precipitation was found in the solution without Zr. Only about 10% of Zr, Mo and Te were precipitated, if the Mo/Zr ratio in the 0.5M HNO3 solution was kept below 0.5. Complete removal of Zr and Mo was the most effective way to prevent solid formation in the solution with 2M and 0.5M HNO3 concentrations.
The acidity change and solid formation in a simulated high level liquid waste (HLLW) containing precipitate were experimentally examined, when the acidity was reduced from 2M to 0.5M by denitration or simple dilution. The acidity of the simulated HLLW containing precipitate could be adjusted from 2M to around 0.5M by means of denitration or dilution, as well as the case of simulated HLLW without precipitate. The precipitation fractions of Zr, Mo and Te during denitration decreased with increasing amount of the precipitate already contained in the simulated HLLW. The amount of solid formed in the dilute simulated HLLW also decreased with increasing amount of precipitate in the simulated HLLW. Two process flow sheets for preparing HLLW for transuranic elements extraction were developed. One was a denitration process and the other a dilution process.