In nuclear reprocessing plants, bulk liquid effluent is created containing low levels of radioactive species, from which it is desirable to remove radioactive species in order to enable the resultant liquid to be discharged. One process for removing radioactive species from such effluent is the Enhanced Actinide Removal Process ("EARP") operated at the Sellafield, UK site of British Nuclear Fuels plc. The process involves the treatment of liquor containing a significant quantity of iron by adding sodium hydroxide to increase the pH to 9-11. The raising of the pH precipitates ferric hydroxide in a flocculent form that carries down the majority of the alpha activity and also a number of beta/gamma active species and heavy metals. Adding small volumes of specific chemicals, such as nickel ferrocyanide, increases caesium removal. The resulting aqueous phase is virtually inactive.

The floc as first produced is voluminous, containing only a few hundred ppm of ferric hydroxide, and needs to be concentrated for economic long-term storage and disposal.

Ultrafiltration was chosen as it provides the most effective mutual separation between solid and surplus water. The ferric floc is therefore circulated through tubular graphite/zirconia membranes with pores typically a few hundredths of a u, m in diameter.

High concentration factors are achieved in two stages, the first reducing the volume, typically by 90-95%, without greatly increasing the viscosity, so that the final thixotropic product containing up to 100 g/1 iron or more can be produced in a smaller, specialised unit. The resulting sludge is suitable for encapsulation in cement, while the filtered solution is checked for radioactivity and discharged to sea. The washing is carried out using pH 10.5 water and at a temperature of around 40 °C. Washing of the sludge is indispensable and serves to remove nitrate which would otherwise have seriously deleterious affects upon the subsequent cementation process.

Although much of the radioactivity is removed from the liquid by the EARP process, a small amount of (3 activity remains, primarily due to the presence of the technetium-99 radionuclide. Typically, 87-88% the activity of the treated liquid arises from Tc-99, present as pertechnate [Tc (VII)], which is soluble in water. Similar problems of Tc-99 arise at other nuclear reprocessing installations.

Much effort has therefore been invested into research for methods of removing dissolved Tc-99 from solution. Thus, since 1980 or before British Nuclear Fuels plc has been studying the problem intensively and investigated at least the following techniques: <BR> <BR> <BR> <BR> <BR> <BR> Electrochemical ion-exchange<BR> <BR> <BR> <BR> <BR> Seeded ultra filtration<BR> <BR> <BR> <BR> <BR> Chemical reduction<BR> <BR> <BR> <BR> <BR> Electrochemical reductions<BR> <BR> <BR> <BR> <BR> Precipitation by tetraphenylphosphonium halide.

Precipitation by tetraphenylphosphonium (TPP) halide was published at least by March 1987, in US 4654173, which is included herein by reference. However, the process appears never to have been used commercially and insufficient Tc was removed from solution Despite the extensive research carried out into the problem, a commercially viable process for removing technetium from effluent has not been proved.

The present invention is based on the problem of providing a process for removing technetium from aqueous solution. More particularly, the invention is based on the problem of removing technetium from nuclear waste liquor in a process in which radioactive species are removed by precipitating a ferric floc and then washing the floc prior to cementation thereof.

The problem is solved by adding to the liquor a source of tetraphenylphosphonium ions to cause dissolved technetium to precipitate before the floc is washed and washing the floc at a reduced temperature. The reduced temperature (compare to those used in the prior art)

is suitably no more than about 25 °C and more normally no more than about 20 °C. In one class of processes, the washing is carried out at a temperature of about 20 °C.

The source of tetraphenylphosphonium ions is preferably a tetraphenylphosphonium halide, for example the chloride or, most preferably, the bromide. <BR> <BR> <BR> <BR> <BR> <BR> <P>As has been described, a current technique for removing a activity from bulk liquid effluent streams from nuclear reprocessing is that of the Enhanced Actinide Removal Plant (EARP) at the Sellafield, UK site of British Nuclear Fuels plc (described in The Nuclear Fuel Cycle from Ore to Waste, edited by P. D. Wilson, Chapter 9 by G. V.

Hutson). The EARP process uses the ferric floc precipitation technique described above followed by ultrafiltration. This excellent process removes the vast majority of a activity and also includes the use of precipitants to remove ß/y activity but, nonetheless, a small amount of P radiation remains in the permeate after flocculation; this residual ß activity is primarily due to the presence of the technetium-99 radionuclide.

According to the invention, a ferric floc precipitation process is modified by adding a source of tetraphenylphosphonium (TPP) ions to the liquor to precipitate technetium (which is as pertechnate) and subsequently washing the ferric floc and its associated TPP- Tc complex at a reduced temperature compared to conventional washing processes. It is possible in this way to reduce the percentage of the precipitated Tc lost during washing from about 50 % to about 10 %.

The concentration of TPP+ with which the liquor is dosed is not critical. Dosing with TPP+ to a concentration of I g/litre will result in the precipitation of 98 % of the Tc-99.

Over the TPP+ range of 0-1 g/l, the Tc-99 removal is effectively proportional to the TPP+ concentration (i. e. mol: mol removal up to 98 %). The TPP+ concentration required to give any % precipitation up to 98 % can therefore be predicted.

Experiments conducted on the effect of TPP treatment upon floc filtration and encapsulation indicate that no adverse effects on floc processing arise in the case of floes

having an iron concentration of up to 41g/l (although the maximum acceptable iron concentration has not been determined).

EXAMPLES Example 1 Comparison of tetraphenylphosphonium chloride (TPPC) and tetraphenyl- phosphonium bromide (TPPB) An experimental worksheet was produced containing four experimental runs (Table 1).

Table 1 Run Number Reagent Temp (°C) TPP+ conc. (g/1) 1 Br 20.0 0.37 2 Br 40. 0 0.67 3 Cl 40. 0 0. 37 4 Cl 20. 0 0. 67 Aliquots (10 ml) of Medium Active Concentrate (MAC) as treated in the EARP process were dosed with TPPB or TPPC to give TPP+ concentrations of 0.36 and 0.67 g/1. The liquor aliquots were then treated by the EARP process as described in the following paragraphs.

The pH of the liquor was adjusted to pH 4.5 by addition of 23 % NaOH whilst keeping the temperature to no more than 40 °C. For experiments at 40 °C the reagents were preheated to 40 °C, whilst for those at 20 °C they were cooled to 20 °C after the addition of the 23% NaOH. The pH of the liquor was then adjusted to 10.5 by addition of 0.1 M NaOH.

200ppm ferrocyanide was then added as a nickel ferrocyanide slurry and the liquor was stirred for a further 30 minutes. An aliquot (10 ml) of each neutralised liquor was then

filtered through an Amicon'cell and the permeates stabilised with an equal volume of 12N HNO3/0.1N HF prior to being analysed for Tc-99.

Results The results are shown in Table 2: Table 2 1 1 3 4 Volume of MAC (ml) 10.00 10. 00 10. 00 10. 00 TPP+ TPPB TPPB TPPC TPPC [TPP+] in final solution (g/1) 0.36 0. 67 0. 36 0.67 Vol. of 0. 1M NaOH (m/l) 12.00 11. 00 12. 00 11.40 Final Vol. (ml) 33. 17 32. 22 32. 87 32.82 Sample volume (ml) 10.00 10. 00 10. 00 10.00 Vol. of stabilised solution 10.00 10. 00 10. 00 10.00 DilutionFactor 6. 63 6. 44 6. 57 6.56 Sample Tc-99 (Bq/ml) 13E-01 l i2. 1OE+02 +1. 76E+02 +2. 88E+02 _1.63E+02 Tc-99DF 8. 00+1. 86 48. 90+27. 67 4. 12+0. 36-8364.95 47.12 MAC Tc-99 (Bq/ml) 5. 01E+04i 3.54E+03 Percentage Tc removal 87.51% 97.96% 75.74% 100% +2. 81% +2. 27% +3. 57% +2.12%

Discussion The results were mathematically analysed using to help identify the main effects on the percentage removal of Tc-99. Any interactions between these effects were not considered. Table 3 is taken from a printout obtained from the software used, showing the main effects on the percentage removal of Tc-99.

Table 3 Main Effects on Percentage Removal Predictor Settings Estimated effect Reagent Cl to Br 4.87 Temperature 20 to 40-6.91 Concentration 0.37 to 0.67 17.36

Example 2 Effects of temperature, concentration, time and wash conditions An experimental worksheet containing 14 runs was produced (see Table 4 below). From Table 4 it can be seen that run numbers 3 and 4 were replicates of run numbers 2 and 1 respectively. These replicates were carried out to give an indication of the variability of the percentage Tc-99 removal under the same conditions.

Table 4 Run number Concentration Temp. (°C) Treatment 1 0.36 40. 0 t3 2 0. 36 40. 0 tl 3 0. 36 40. 0 tl 4 0. 36 40. 0 t3 5 0.65 20.0 t1 6 0. 36 40. 0 t2 7 0. 36 20. 0 t2 8 0. 65 20. 0 t3 9 0. 65 20. 0 t2 10 0. 65 40. 0 t2 11 0. 36 20. 0 tt3 12 0. 36 20. 0 tl 13 0. 65 40. 0 t3 14 0.0| tel Aliquots (lOml) of MAC were dosed with a quantity of TPPB to give a TPP+ concentration of either 0.36 or 0.67 g/l. These were then treated by the EARP process as described in the following paragraphs.

The pH of the liquor was adjusted to pH 4.5 by addition of 23% NaOH. For experiments at 40 °C the reagents were preheated to 40 °C, whilst for those at 20 °C they were cooled to 20 °C after the addition of the 23% NaOH. The pH of the liquor was then adjusted to

pH 10.5 by the addition of 0.1 M NaOH. 200ppm ferrocyanide was then added as a nickel ferrocyanide slurry and the liquor was stirred for a further 40 minutes.

The liquor was then subjected to three different treatments, tl, t2 and t3: Treatment tl-The sample was stirred for 30 to 60 minutes and the pH and temperature rechecked. 10ml of the neutralised liquor was then filtered through an Amicon'cell and the permeate stabilised with an equal volume of 12N Han03/0. 1N HF prior to being analysed for technetium-99.

Treatment t2-The sample was allowed to stand for one week. The pH and temperature were then measured before filtration and stabilisation.

Treatment t3-The sample was allowed to stand for one week. The pH and temperature were then measured and the sample washed for 24-48 hours with pH 10.5 water. 10ml was then filtered and stabilised as in the previous treatments before technetium-99 analysis.

Results The neutralised volume, calculated DF, permeate Tc-99 level and calculated percentage Tc-99 removal for each of the experiments are presented in Table 5.

Table 5 POI P02 P03 P04 P05 P06 P07 P08 P09 Polo PO11 P012 P013 P014 Vol. of MAL 6/l 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 (ml) Vol. TPP+ (ml) 0.375 0. 374 0.374 0. 374 0.696 0. 374 0. 374 0.695 0.695 0.695 0.374 0. 373 0.695 0.695 Vol. of 23% 9.80 10.20 10.20 10.10 10.90 10.10 10.20 10.20 10.20 10.20 10.20 9.00 NaOH (ml) Vol. of 0.1M 0.70 4.00 3.60 5.00 3.50 4.10 3.80 4.00 4.80 4.40 3.30 3.80 3.50 3.20 NaOH (ml) Vol. FeCN (ml) 0.463 0.546 0.537 0.566 0.578 0.546 0.536 0.548 0.565 0.556 0.525 0.536 0.537 0.508 Vol. of washes 60.00 0.00 0.00 60.00 0.00 0.00 0.00 60.00 0.00 0.00 45.00 0.00 45.00 0.00 (ml) Final Vol. (ml) 81. 34 25.12 24.71 86.04 25.67 25.12 24.91 85.44 26.26 25.85 69.40 24.91 69.93 23.40 Vol. sample (ml) 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 Vol. of stabilised 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 solution(moi) DilutionFactor 16.27 5.02 4.94 17.21 5.13 5.02 4.98 17.09 5.25 5.17 13.88 4.98 13.99 4.68 Sample Tc-99 5.30E+3 1.18E4 1.26E+4 4.51E+3 2.74E+4 1.25E+4 1.24E+4 1.59E+3 1.59E+3 2.80E+3 2.06E+3 1.39E+4 5.15E+4 5.37E+3 (Bq/ml) 4.59E2 8.80E2 9.33E2 4.10E2 1.96E3 9.45E2 9.47E2 2.26E2 2.20E2 2.87E2 +2.58E2 1.05E3 4.56E2 4.63E2 Tc-99 DF 1. 540.07 2.09 4. 890. 36 13. 93 9. 190.50 4.65#0.30 1.92#0.08 5.290.24 il. l4 MAL6/1 Tc-99 (Bq/ml)1. 33E+053.54E+03 Percentage Tc-99 35.06% 55.56% 53.27% 41.52% 52.83% 53.49% 79.55% 93.72% 89.12% 78.49% 47.92% 45.95% 81.1% removal 2.84% 1.70% #1.89% 2.61% 1 1 il. 92% 1.87% 1.40% 0.42% #0.56% #1.31% 2.08% | _2.50% 0.82%

Discussion The results were mathematically analysed to help identify the main effects on the percentage removal of Tc-99. Interaction effects were taken into consideration and analysed. The DF of trial P05 was much lower than expected and it was therefore decided not to include it in the data analysed. Table 5 shows the main effects on the response with 95% confidence intervals.

Table 5 Main effects on percentage Tc-99 removal with 95% confidence intervals

Predictor Settings Lower Estimated Upper bound effect bound Concentration 0.36 to 16. 56 21. 88 27.19 0.65 Temperature 20 to 40-18.53-13.22-7.90 Treatment t3 to t2 3.42 12.11 20.79

Further analyses of the data were carried out but these are not presented here.

From the experimental work carried out and analysis of the resulting data, it appears that all of TPP concentration, temperature and treatment are important factors in determining percentage Tc removal from MAC. With respect to temperature, the results indicate that the percentage removal decreases with increasing temperature. The lowest percentage removal was produced under t3 conditions, indicating that washing re-dissolves the Tc- TPP complex.