1 Introduction
The use of selective inorganic sorbents is one of the most up-to-date methods of radiochemical waste decontamination for aqueous low-level waste (LLW). Those of ‘Mayak PA’, coming from drainage system as well as water from spent-fuel assembly storage pool, are mainly contaminated with 137Cs. Water of storage reservoirs of the ‘Techa River Cascade’ is mainly contaminated with 90Sr [1]. Such multicomponent contaminated LLW predetermined the possibility of effective application of selective inorganic sorbents.
Mass distribution coefficients of 137Cs for some synthetic and natural materials were measured in order to define the most prospective type of Cs selective sorbents [2]. The results of experiments are given in Table 1. Full-scale dynamic laboratory tests were also conducted on real desalted water from the spent-fuel assembly storage pool in order to select the best sorbent among those offering the more perspectives: ferrocyanides, phosphates, and zeolites. Filtration rate in top–down direction was supported with about 20 column volumes per hour. Results of the experiments are given in Fig. 1. The effective filtration cycle with a 137Cs purification factor ≥ 10 was 30 thousand column volumes for zeolite CBF-10, and about 60 thousand column volumes for zirconium phosphate (T-3A). The effective filtration cycle on nickel ferrocyanides (NZA and T-35) was about 90 thousand column volumes. Thin-layer sorbent demonstrates better hydrolytic stability than composite sorbent.
Values of 137Cs distribution factors depending on the composition of the initial solution
| Sorbent | Caesium distribution factors (ml g–1) from the solution | |||
| Water | 0.1 M NaNO3 | 1 M NaNO3 | 0.05 M Ca(NO3)2 | |
| Vermiculite (Covdor) | 4900 ± 1600 | 770 ± 180 | 80 ± 16 | 2500 ± 800 |
| Clinoptilolite (Dzegvi) | 5900 ± 400 | 9700 ± 600 | 200 ± 10 | 8600 ± 1600 |
| Nickel ferrocyanide (NZA) a | 4000 ± 900 | 3300 ± 300 | 2800 ± 600 | 5200 ± 2100 |
| Nickel ferrocyanide (Seleks-CFN) a | 2800 ± 400 | 7400 ± 500 | 26 000 ± 2400 | 24 200 ± 5200 |
| Nickel ferrocyanide (T-35) b | 5700 ± 1300 | 8200 ± 1400 | 15 700 ± 3000 | 8600 ± 2000 |
| Titanium ferrocyanide (KNTZ) b | 2500 ± 900 | 5500 ± 1400 | 4400 ± 1100 | 16 300 ± 4800 |
| Iron ferrocyanide (FS-10) b | 890 ± 160 | 2200 ± 300 | 10 700 ± 600 | 4000 ± 400 |
| Zr phosphate (T-3A) | 3900 ± 400 | 1900 ± 300 | 460 ± 30 | 4400 ± 1600 |
| CFB-10 c | 3600 ± 1000 | 450 ± 20 | 82 ± 46 | 4600 ± 300 |
| CMP c | 2200 ± 500 | 440 ± 30 | 70 ± 12 | 1700 ± 300 |
| Sulfonic cation resin KU-2×8 | 8100 ± 800 | 180 ± 20 | 18 ± 5 | 16 ± 3 |
a Thin-layer ferrocyanides.
b Composite ferrocyanides.
c Zeolites.
Result curves for sorption of 137Cs on inorganic sorbents.
This paper concerns experimental tests on 137Cs and 90Sr removal from some aqueous LLW.
2 Experimental results
2.1 137Cs decontamination
The data obtained allowed us to perform experimental industrial tests of the process of LLW decontamination from 137Cs with water from the special drainage system of Mayak PA radiochemical facility, which annual water flow reaches 400 thousands m3 [3]. Composition of water used in experimental industrial tests was as follows: not stable; pH value in the range from 7.7 to 8.2, water hardness from 4.65 to 10.2 mg equiv l–1, calcium concentration from 46 to 117 mg l–1, magnesium from 29 to 53 mg l–1, sodium from 117 to 155 mg l–1, nitrite ion from 624 to 1232 mg l–1, 137Cs activity was (7.03–13.4) × 103 Bq l–1. Single synthesis of a large batch (about 8 m3) of ferrocyanide sorbent NZS, which is a close analogue to NZA, was performed. Water purification unit was installed for purification of special drainage water on the basis of two filters and the existing manifold pipelines. Filters F-1 and F-2 contained 3.85 m3 of NZS sorbent with size fraction from 0.2 to 0.3 mm per each filter. In F-2 NZS, sorbent was mixed with 0.77 m3 of spent resin KU-2×8 to make a composition with improved hydrodynamic properties. Experimental sorption filters were operated at water flow rate equal to 25–30 m3 h–1 or 7–8 column volumes per hour per each filter. The results of experimental industrial testing are given in Table 2. Data prove that in the process of application of sorption filters at selective caesium extraction units (with a purification factor ≥ 10), about 85 m3 of special drainage water was decontaminated. The best results were achieved using filter F-2 with added KU-2×8. About 1.96 TBq of 137Cs were extracted from LLW in total. 137Cs concentration in the filtrates exceeded only slightly the admissible level for discharged industrial water during the major part of the test. An additional purification factor was obtained on routine ion-exchange treatment. Thus, discharged water was practically not contaminated with 137Cs.
Results of experimental-industrial tests of drainage water decontamination from 137Cs with NZS sorbent
| Operation stage | Filters | Filtrate volume | Average 137Cs activity in the filtrates (Bq l–1) | Water decontamination factor from 137Cs | Amount of extracted 137Cs (GBq) | Radiation background (μSv s–1) | ||
| m3 | Column volume | Over resin | Near apparatus | |||||
| 1 | F-1 | 3266 | 848 | 347 | 15 | 15.9 | 0.60 | 0.30 |
| F-2 | 3266 | 848 | 151 | 35 | 16.6 | 0.14 | 0.30 | |
| 2 | F-1 | 17 470 | 4537 | 296 | 77 | 392.2 | 4.00 | 0.30 |
| F-2 | 17 470 | 4537 | 128 | 178 | 392.2 | 6.00 | 0.50 | |
| 3 | F-1 | 8649 | 2246 | 825 | 45 | 316.0 | 8.00 | 0.80 |
| F-2 | 8649 | 2246 | 181 | 206 | 321.5 | 10.0 | 1.00 | |
| 4 | F-1 | 14 011 | 3639 | 3922 | 4.2 | 175.8 | – | – |
| F-2 | 15 931 | 4138 | 403 | 37 | 256.4 | 18.0 | 1.60 | |
| 5 | F-2 | 11 740 | 3049 | 507 | 10.1 | 54.4 | – | – |
| Total result | F-1 | 43 396 | 11 272 | 1576 | 14.2 | 899.9 | 8.00 | 0.80 |
| F-2 | 57 056 | 14 820 | 292 | 64 | 1041.1 | 18.0 | 1.60 |
Experimental-industrial facility Seleks-CFN for the synthesis of ferrocyanide sorbent of NZS-type with an output up to 12 t/year was built up at Mayak PA with consideration of successful results of experimental industrial tests of synthesis and water purification process. Seleks-CFN sorbent synthesis is based on saturation of granular silica gel or other porous material, loaded into sorption column, with nickel and ferrocyanide ions. Water solutions of nickel ammoniate or potassium ferrocyanide are used. A sparingly soluble compound, nickel-potassium ferrocyanide, forms as a result of such a treatment in silica gel pores.
Seleks-CFN facility was commissioned in 1999 and it works in periodic mode when it is necessary. About 1420 kg of Seleks-CFN sorbent were synthesized in the course of the facility operation. The effective pH of contaminated water for Seleks-CFN sorbent is in the range 2–12. Statistic exchange capacity for caesium is up to 30 mg g–1. Half-exchange time is 2.1 min.
A process technology and a prototype of the industrial facility for decontamination of desalted water of spent-fuel assembly (SFA) storage pool were developed using this sorbent. A pilot sorption facility with sorbent volume of 65 l was mounted on the slotted covering of the pool. The column itself was placed at the depth about 3 m, which provided sufficient radiation shielding. In order to decontaminate the maximum volume of water in the storage pool, water was taken for treatment and purified water was discharged from the column in the opposite loci of the storage pool. Sorption column was used with bottom up flow rate from 2.5 m3 h–1 to 6.0 m3 h–1. The results of the experiment are presented in Table 3. They show that 22 885 m3 of water were filtered through the sorbent during the experiment, which is equal to about 253 thousand column volumes. High water purification efficiency and column hydrodynamic characteristics were stable during the whole filtration cycle. Activity of the initial water decreased 5 times for 137Cs. Preliminary calculation show that 2.07TBq of 137Cs were deposited in the column. The data show that Mayak PA developed an effective process of SFA storage-pool water decontamination from 137Cs.
Results of experimental/industrial tests of water treatment process in the storage pool using Seleks-CFN sorbent
| Operation stage | Filtrated water volume, m3 | Radionuclide activity in initial water (Bq l–1) | Radionuclide activity in filtrate, (Bq l–1) | Water decontamination factor from 137Cs | |
| 137Cs | 134Cs | 137Cs | |||
| 1 | 3706 | 1.49 × 105 | 1.73 × 103 | 2.53 × 103 | 59 |
| 2 | 3601 | 1.49 × 105 | 1.40 × 103 | 5.0 × 103 | 30 |
| 9.93 × 104 | 1.20 × 103 | 3.33 × 103 | 30 | ||
| 3 | 2990 | 1.12 × 105 | 1.07 × 103 | 3.27 × 103 | 34 |
| 4 | 2423 | 8.80 × 104 | – | 1.20 × 103 | 73 |
| 5 | 2901 | 7.47 × 104 | 600 | < 100 | > 700 |
| 6 | 2687 | 5.30 × 104 | 313 | 387 | 140 |
| 4.67 × 104 | 400 | 213 | 220 | ||
| 7 | 1239 | 5.90 × 104 | 667 | 333 | 180 |
| 8 | 2370 | 5.49 × 104 | 733 | 667 | 82 |
| 5.70 × 104 | 667 | 333 | 170 | ||
| 9 | 968 | 3.0 × 104 | 333 | 467 | 64 |
| Total | 22 885 |
2.2 90Sr decontamination
The most advanced oxy-hydrate sorbents whose composition is based on non-stoichiometric manganese dioxide were studied for purification of non-saline natural waters from 90Sr in static and dynamic conditions. The following sorbent types were used: ISM-S with composition (Na, K)xMnOy, where x = 0.25 to 0.3 and y = 1.9 to 2.1, as well as ISM-SP, which is the same sorbent, fixed in a matrix of inert medium (perchlorovinyl). Method was developed and sorbents were synthesized in Russia in the city of Perm under the leadership of Professor V. Volkhin [4]. The peculiar feature of these sorbents in comparison with well-known natural and synthetic samples on the basis of manganese oxy-hydrate is their ability for reversible sorption of strontium.
It was defined experimentally that the pH range from 6 to 10 with maximum at pH 6.9 is optimal for 90Sr sorption with ISM-S sorbent. Radionuclide desorption is possible in acid medium up to pH 1 to 2 [4]. The results of laboratory investigations allowed us to perform experimental industrial tests.
Three 3.5-l sorption columns (I to III) filled with the sorbent ISM-SP were used. One additional column was filled with a layer of quartz sand for mechanic filtration of the contaminated water (pH ~ 8, total beta-activity about 3.3 kBq l–1, 90Sr activity around 1.6 kBq l–1, water hardness –4.5 to 5.4 mg equiv l–1) before being directed toward the sorption columns. Contaminated water went top–down with a flow rate of 30 to 35 column volume h–1 trough two concatenated columns. Desorption was obtained with a 0.3-mol l–1 solution of HNO3 and regeneration with a 0.2 mol l–1 solution of NaOH. Output curves of strontium are presented in Fig. 2 depending of the columns engaged in the process. Optimum results were obtained for columns III–I. This shows that column performances are different depending on packaging. Over 7 m3 (~2000 column volumes) of radioactive contaminated natural water were decontaminated in the course of the third test cycle (engaging columns III–I) to the admissible level water discharge.
Output curves of 90Sr depending of columns engaging in the process.
The obtained results make it possible to conclude that the ISM-SP (ISM-S) sorbent is quite prospective for the decontamination of natural waters from Sr and it can serve as a basis for development of a respective process technology with a mode sorption–desorption–regeneration. Thus, Mayak PA experts have considerable experience in the use of inorganic selective sorbents for applied tasks of radiochemistry.
3 Conclusions
1. Thin-layer ferrocyanide sorbents are the most effective for low-level waste 137Cs decontamination.
2. ‘Mayak’ PA is experienced in synthesis of thin layer ferrocyanide sorbents; purification techniques for special drainage saline water and spent-fuel storage-pool non-saline water are used at the experimental industrial scale.
3. For 90Sr decontamination of natural contaminated water, sorbents based on non-stoichiometric manganese dioxide were tested and recommended for use.