Denitration of Radioactive Liquid Waste

In recent years varying emphasis has been dedicated to the denitration of high active waste (HAW) and medium active waste (MAW) solutions generated in the nuclear fuel cycle, particularly in fuel reprocessing. These solutions contain free nitric acid and considerable amounts of nitrate salts. Denitration can be performed either thermally or in solution by chemical means. Various organic reductants such as formic acid, formaldehyde and sugar have been successfully applied. Yet another alternative is electrolytic denitration.

The main incentive of chemical denitration is reducing the acid content and in turn the salt load of the wastes formed by the neutralization of the acid with caustic soda. The ruthenium volatilization in the course of calcination and vitrification is also reduced. Further, the corrosion of waste storage tanks is minimized.

Conflicting opinions exist on whether or not the benefits arising from a chemical denitration of HAW concentrates are worthwhile the extra expenditures required. In case of MAW conditioning the situation is different if an incorporation into an organic matrix, e.g. bitumen or platics, is foreseen. Safety considerations may well justify a pre-treatment step.

Denitration reactions with HCOOH and HCHO proceed according to several reactions depending on the acidity and/or the redox potential of the reacting mixture. The parameters acting on the induction time — Inherent to these reactions — and the denitration rate have been identified and their possible effects discussed. These mainly deal with the concentration of the reacting mixture components, the pressure in the reactor and the nitrous acid concentration. The basic outcome of this study is that gaseous compounds in equilibrium with nitrous acid catalyze the denitration reaction. As a consequence, every action which could remove these products from the reacting mixture extends the induction time and are detrimental to the denitration rate.

The conditions whereby the noble metals can affect the denitration process by modifying the reactions stoichiometry and by entailing oscillations of the reacting mixture are also presented and discussed. These phenomena were showed to occur in presence of rather high amounts of noble metals (those existing in HLLW) and iron. At last, based on the various experimental data observed, a tentative reaction mechanism is proposed involving a reaction between NO₂ on HCHO/HCOOH as an intermediate reaction between HCHO/HCOOH and HNO₃.

Besides formic acid and formaldehyde several organic reagents are used to decompose nitric acid. Denitration by use of sugar was first carried out at HANFORD, USA. More than 12 moles nitric acid can be decomposed per mole sucrose. The decomposition is linerarly dependent on the concentration of sugar and the presence of iron increases the reaction rate. On account of the slow reaction, which does not give rise to foaming, a simple uncomplicated design of reaction vessels is possible. Moreover the reagent is very cheap. At the Karlsruhe Nuclear Research Centre the denitration by use of ethanol was investigated. The nitric acid waste solution can be mixed at room temperature and after heating up to about 80°C the decomposition reaction starts without induction time. In a simulated medium level liquid waste solutiom more than 90% of the initial nitric acid are decomposed within 2 h. The reaction rate depends on the concentration of nitric acid and nitrate salts. The molar ratio of alcohol to nitric acid is between 0.5 and 1. The off gas contains mainly of CO₂ and N₂O. A selective precipitation of transuranium elements during denitration is possible by use of diethyl oxalate. In acid medium this reagent hydrolyzes to give ethanol and oxalic acid, which serve as denitration and precipitation reagents, respectively.

The DWK-concept for the reprocessing plant at Wackersdorf uses the HCHO-process. The gaseous reaction products are recombined to nitric acid and recycled to the high tritium acid recovery system. Denitration of MAWC is not mandatory but offers the chance of reducing the volume of solidified waste by 340 concrete shielded drums (400 1) per year. It provides flexibility in the processing of MAW by not being limited to the maximum salt concentration rather to the activity limits. For MAWC the HCOOH-process is used.

The denitration of high and medium active liquid waste with formic acid is analyzed with respect to the risk of an explosion of the reaction vessel. The topics which are discussed in this context, are the formation and reaction of explosive gases, the uncontrolled reaction of nitric acid with formic acid because of false operation conditions, the formation and reaction of an organic phase, and the catalytic effect of traces of rare earths or noble metals. Quantitative results were exemplified. The analysis is based on the reaction models of Holze et al. [3] and [4].

The denitration of highly radioactive liquid waste and intermediate level liquid waste with formaldehyde have been investigated. A number of different simulated solutions as well as real waste from fuel reprocessing have been used in cold laboratory experiments and in hot experiments performed with the FIPS II denitrator. Partial or exhaustive reduction of nitric acid can be achieved in a simple and safe way feeding formaldehyde into the boiling acidic batch or by simultaneous feeding of acidic waste and formaldehyde generating mainly NO.

Denitration in a formaldehyde batch with intent to increase N₂O generation is not straight away recommendable for the following reason. Aqueous formaldehyde usually contains 10 – 14 % methanol which will react with NO₂ and HNOp producing methylnitrite. The concentration of methylnitrite in the off gas during the first periode of denitration following the above mentioned procedure might reach ignition range.

Experiments at ambiant temperature supplied the investigations of the reaction mechanism during the induction periode.

A batch process has been developed to destroy nitric acid present in waste solutions from reprocessing plants by formic acid. The main reaction products N₂O and CO₂ may be released into the atmosphere.

The process was subjected to testing on lab scale involving simulated and real medium-level waste concentrate, on semitechnical scale involving 50 1, and on pilot scale involving 200 1 batch volumes of simulated medium-level waste solutions. In the denitration runs between 81 and 99 % of the nitric acid could be destroyed. The final nitric acid concentration ranged from 0.2 to 0.02 mol/l. The aerosole drag-out with the reaction gases lies between 1 and 5 ppm related to the reaction vessel contents.

The amount of formic acid dragged out ranges from 0.2 to 0.3 %.The formic acid dragged out can be conveniently separated in quantitative terms in a scrubbing column.

Corrosion tests performed on material specimens and on the reaction vessel itself have shown that Incoloy 825 is a suitable construction material for the denitration vessel.

The explosion limits and pressures of formic acid vapour/water vapour/air mixtures were determined. The presence of an explosive mixture can be excluded downstream of the condenser.

A restart of the reaction after extended periods of interruption in metering which are necessary, e.g., in case of technical disturbances, was tested on the pilot facility under various conditions and does not cause problems. The associated induction period can be calculated in advance with sufficient accuracy using a mathematical approach developed from laboratory scale experiments. If the solution is cooled down immediately when metering is interrupted, the reaction restarts already during reheating, even before starting to meter the feed solution.

The PETRA hot-cell facility has been developed to study HAW conditioning alternatives. The first strategy to be assessed includes the denitration of high active waste (HAW) and the contemporaneous oxalate precipitation of the rare earths and actinides. A “cold” pilot plant was constructed to establish design and operating conditions which could be successfully applied within the facility. All experiments were based on feeding simulated HAW into the denitrator containing formic acid. The problem of induction period may be overcome by feeding the HAW locally, rather than well distributed, into the reactor. However, the presence of strong nitrous acid scavengers must be eliminated as these could block the denitration reaction. In this context, iron plays an intermediate role, partially protecting the oxalic acid from nitrous acid, while allowing the formic/nitric acid destruction to proceed. Off-gas and pressure measurements reliably detect the commencement and continuation of the process while electrical conductivity indicates the prevailing nitric acid concentration. After HAW addition, a reflux period of several hours is necessary to attain sufficiently low nitric/formic acid concentrations. Following supernatant removal, the recovery and destruction of the oxalate precipitate entail its dissolution in dilute nitric acid and subsequent concentration.

In the treatment and solidification of medium and high level radioactive waste concentrates, large amounts of NOx are produced, which are normally scrubbed with nitric acid or alkaline solutions. In this way big volumes of secondary wastes containing high nitrate concentrations, which cannot be easily discharged, are obtained. The NOx catalytic abatement with selective ammonia reduction, which has been studied at the COMB/MEPIS-RIFIU Laboratory, would permit to avoid this problem, with large cost savings. The NOx catalytic abatement has been experimentally studied at laboratory scale taking into consideration the following operational parameters: the catalyst bed temperature; the gas residence time; the vapour concentration; the NOx concentration; the gas velocity; the catalyst grain size distribution; the catalyst time-life. The best operational conditions have been selected for obtaining abatement yields of the order of 99.5%.

Actinide partitioning from high level liquid waste has been subject to experimental investigations at the JRC-Ispra and CEA-Fontenay-aux-Roses. In this context, two main flow-sheets based on solvent extraction were drawn up which both involved a denitration step by means of formic acid. In the first process named HDEHPl, denitration was intended to lower the HLLW acidity down to pH 2 while keeping actinides and mainly plutonium in a soluble and extractible form which was proved to be quite effective through secondary reactions between noble metals and formic acid. As for the second process -the TBP2 process — denitration was performed during and after the concentration of high level waste in order to reach a final acidity of about 1 M HNO₃. On the whole, the results of denitration experiments carried out on real waste at the technical scale agreed quite well with those previously achieved on simulates in demonstrating that plutonium can be quantitatively removed from denitration precipitate under certain operating conditions.

This paper summarises the work carried out in the laboratory and on the Full Scale Inactive Facility at Sellafield aimed at optimising the calcination stage of the Windscale Vitrification Plant process. The effects of the calcination conditions and the calcination additives, sucrose and lithium nitrate, on denitration and off-gas behaviour are discussed.

Vitrification processes for high level wastes in nitric acid solutions require drying, calcination and melting operations that are conducive to the formation of oxidized compounds of certain elements. These include ruthenium and technetium with the volatile oxides RuO₄ and TC₂O₇ corresponding to a high degree of oxidation. In addition to selective traps, various methods are available for limiting this volatilization to reduce the radioactive and technological hazards. One of these methods, predenitration, has been investigated for many years, as a means of significantly reducing the oxidation of nitrosyl ruthenium compounds produced above 130°C during calcination. Another method involves the addition of reducing agents directly to the HLLW solution prior to calcination. Extensive testing on lab-scale equipment and semi-industrial pilot facilities has made it possible to define the optimum experimental conditions capable of reducing the total ruthenium and technetium volatility in industrial high level waste vitrification facilities to 1.5% and 8%, respectively, of the feed solution activity.

Concentration of HLLW is an interim step between reprocessing and solidification process. The process using denitration by means of HCHO is composed of three main steps: receiving of solutions and feeding, concentration in a kettle type evaporator, and absorption of NOx. This process is operated semi-continuously: concentration is performed at constant level (with denitration and NOx absorption) and evaporator is emptied by batch to the storage. The industrial experience has been gained for nearly 30 years in three reprocessing plants. The safety of the chemical process reaches very high standard thanks to the appropriated operating mode and efficient process control. Only very few incidents have been recorded. No alternative to this process is envisaged today.

Basically denitration of radioactive liquid waste is performed batchwise by feeding formic acid or formaldehyde into a nitric waste solution at boiling temperature or vice-versa. Although sugar and ethanol have been proposed as alternatives, their respective advantages over formic acid and formaldehyde (lower cost for sugar and better efficiency for ethanol) do not appear, in most cases high enough to counteract their inherent drawbacks (longer reaction time for sugar and high flammability in case of ethanol). The only application for which sugar is definitely superior deals with denitration of high level waste during vitrification (in the rotary kiln according to the AVM process) which enables a significant reduction of Ru and Tc volatilities. However, for all the other applications, so far, formic acid and formaldehyde remain the most suitable reagents for denitrating adioactive liquid waste.