MEMBRANE TECHNOLOGY OF LIQUID RADIOACTIVE WASTE (LRW) PURIFICATION

MEMBRANE  TECHNOLOGY  OF  LIQUID  RADIOACTIVE  WASTE  (LRW) PURIFICATION
 
A.A.Svittsov 1,  B.E.Riabchikov 2,  S.B.Hubetsov 3
 
1.        Limited Liability Company Scientific and Production Firm Gella-TECO
127055, Moscow, ul. Novoslobodskaja, 54, building 3.
(E-mail: tecoas@ yandex.ru).
2.        Closed Corporation Mediana-Filter
111116, Moscow, Energeticheskiy proezd, 6.
(E-mail: riabchikov-45@ yandex.ru).
3.        All-Russian Scientific and Research Institute of Atomic Power Plants.
Moscow, ul. Ferganskaja, 25. 
(E-mail: khub@ mail.ru).
 
 Annotation
This report presents technology of liquid radioactive waste processing using the method of micellary-enhanced ultrafiltration. It ensures extraction of radioactive components only from waste not affecting saline ballast.  Such a technology sharply decreases the quantity of radioactive concentrates planned for long-term storage and enables reuse of purified water and chemical reagents.
 
Key words
 liquid radioactive waste,  micellary-enhanced ultrafiltration, associating additives, selective extraction of radionuclides.
 
 
 
Introduction. Substances and materials with radionuclide content exceeding normative levels are considered to be radioactive waste, not subjected to further usage. The main peculiarity of this type of industrial waste is the fact that it is  impossible to use them any longer because any chemical and physico-chemical changes cant  provide for biological safety of these substances. As radioactivity can not be destructed technological LRW purification processes can realize only, maximal possible waste concentration resulting in purified up to maximum permissible concentration (MPC) water.
Further handling of concentrates means their hardening and practically everlasting storage in special mortuaries. Besides the cost of mortuaries maintenance is the largest sum in the budget of the process of radioactive waste treatment. The cost is directly proportional to waste volumes for storage, the last ones in their turn depend on volumes of LRW, planned to be treated, their content and technologies of treatment and hardening.
The following productions and operations are considered to be sources of radioactive waste:
-         mining and processing of radioactive ores, nuclear fuel production;
-         operation and withdrawal of atomic power stations;
-         processing of irradiated nuclear fuel;
-         weapon material production and processes of disarmament;
-         operation and withdrawal of ships with transport nuclear power plants;
-         carrying out scientific and research works using radioactive and fissile materials;
-         application of radionuclides in medicine, science and technique.
According to Russian standards there are three groups of radioactive waste depending on their activity levels [1]:
low active - <10-5 Cu/l;
medium active 10-5 1.0 Cu/l;
highly active - > 1.0 Cu/l.
At present there are 600 millions m3 of radioactive waste of total activity about 1.5 billions Cu. More than 90% of this activity are due to military works for mass production of  nuclear weapon materials. Their main part is liquid waste but in non-conditioned state, i.e. non-hardened ones, that is why they are dangerous for people and environment.
LRW also contain (depending on nature), g/l:
dissolved salts 0.1-40.0;
dissolved organic substances 0.1-5.0;
microsuspensions and colloids 0.01-0.1;
petroleum derivatives 0.0-1.0.
So, mass content of radioisotopes is negligible i.e. thousandth of a percent in total mass of dry residue while being highly radioactive. The main disadvantage of traditional LRW processing technologies is the fact, that necessary purification  coefficients 10-5-1010 can be achieved only at solution demineralization of the same rate. Then practically all system components go into concentrates and further in hardened blocks. These processes require huge volumes of storage rooms, because mass of waste planned for storage substantially increases, financial costs increase as well.
Distillation goes first in LRW processing chains, it ensures concentration of practically all components with high purity of condensate, coagulation follows distillation, using sedimentation or filtration. Recently there appeared centrifugal precipitating separators, but common sand filters are mostly used. Reagent softening is carried out if there are salts of hardness.
Traditional ion exchange is used for low active waste, today thousands tons of spent ionites are accumulated in APS areas, they are secondary dry radioactive waste.
Sorption on activated carbon is not so often in use, because carbon as a whole will be the same secondary waste.
The last progress in technology of LRW processing is the development of selective inorganic sorbents linking up definite radionuclides from solutions complicated in content [2]:
 
s+ = Ni2[Fe(CN)6 ]à Cs4[Fe(CN)6]+ 2Ni2+.                                                                     
 
The tendency of ions of alkaline metals to become a part of  ferrocyanides of transition metals increases as ion radius of alkaline metal increases. Due to this reason caesium sorption selectivity even against of NaCl background reaches the value of several thousands.
Attempts to use membrane separation processes for LRW processing have been often undertaken in the past. First of all electrodialysis and reverse osmosis were used in the technology for high-ohmic water production. The sequence of operations is presented in fig. 1.
It is clear that part of water can be purified up to maximum permissible concentration (MPC), but the  quantity of high saline waste (up to 50% of initial volume) is too voluminal to use such processes in industry. Mass of introduced chemical reagents can double and even triple total salt content of initial solution, i.e. of is good for super pure water production, when concentrate for disposal is not dangerous for ecology, it is extremely bad to answer the problems of atomic industry.

Methods.This problem can be solved using micellary-enhanced ultrafiltration. The idea of the method is the following: initial solution is subjected to some chemical or physico-chemical modification, resulting in conversion of toxic components (radionuclides in our case) into associated form, which exists as a rule as colloids (it explains the name of the method). Then  modified solution goes for membrane separation on a macroporous ultra-  or microfiltration membrane which arrests only toxic components in associated form, but lets ballast salts and organic substances pass through.

At present there are several methods of such modification due to some additives to the solution [3]:
-         water insoluble organic compound, reacting with necessary component,  according to extraction mechanism. Emulsion  formed is separated by the porous membrane;
-         surface active substances (SAS): at concentrations higher than critical concentration of micelle formation they  require additional purification due to physical adsorption of pollutant on SAS micelles;
-         water soluble polyelectrolytes with such functional groups which can attach needed components to themselves according to mechanisms of ion exchange, complexation etc. Solution remains homogeneous, its separation goes on in compliance with rules of ultrafiltration of high molecular compound (HMC) solutions;
-         fresh prepared sol or fine dispersed sorbent, necessary compound is constrainted on their particles due to physical adsorption;
-         due to conversion of dissolved ion particles of the compound into molecular phase and then into colloidal one at their hydrolysis when alkali was added into solution;
-         chemical reagents: component in need is conversed into insoluble form while reacting with them. Sedimentation should be stopped at the stage of colloidal phase formation.
It should be pointed out that there is no strict differentiation between mentioned above variants, i.e. mutual addition of variants is possible to solve the problem.
Let us consider two last variants in detail, because they are promising for high active waste.
Sedimentation being at system supersaturation, the whole process can be divided into three stages as precipitant is being added:
formation of gems of critical size;
germ aggregation into particles;
precipitation of particles and  structurization of precipitation.
Different ways can stop the process in colloidal field of dispersity. They are: introduction of stabilizers (SAS, polymers), sorbing on surfaces of colloidal particles they prevent further aggregation; changes of temperature, pH etc. The choice depends upon the nature of reacting substances. It is interesting to consider the typical view of curves  optical density of solution D and membrane purification rate of specific component φ via solution pH from this point of  view (fig.2).

Fig.2: Dependence of purification rate φ and optical density  D  on acidity rate     of  solutions containing chromium (r) ions:
1 500; 2 350; 3 100 mg/l

 
All three stages of sedimentation are clearly seen here. Effective solution purification from
chromium using membranes is possible at pH ~ 5.0; but precipitating operations are carried out at pH > 9.0, i.e. alkali consumption is several tens more.
Naturally, association of toxic compounds using hydrolysis or precipitants is a complicated physico-chemical process, investigators control reaction speeding up, particle sizes, aggregative stability of the system. But all these problems can be solved and have been solved in many cases. Undoubtedly one good chemist in the team will ensure any problem solution, including other types of waste.
Such a variant of micelle-enhanced ultrafiltration method, when fine dispersed adsorbent is introduced into the initial solution  is highly promising for LRW processing. Technology of adsorbent itself is a little bit changed, because colloidal dust of product but not granules is the object-matter, and increase in sorption capacity is secondary positive result. It is important that radionuclides constrainted on colloidal sorbent particles do not desorb further, but they are hardened waste already.
Results and discussion. Practical application of this method is technology of processing of waters from special laundries.
Deactivation of special cloth by washing requires about 100g of washing substances and 30 kg of water per 1 kg of dry cloth; out of 30 kg of water 12 kg are spent for washing and about 18 kg for rinsing. According to several data 3-4 g of usual domestic pollutants are washed out from 1 kg of cloth: proteins, microsuspensions, grease and petroleum derivatives, microorganisms and insignificantly small immeasurable quantity of radionuclides in mass units.
When two flows washing waters and rinsing waters are mixed together liquid low active laundry waste are for med, their average content is the following one (fig.1):
-         dry residue 3.5 g/l;
-         out of them organic (oxidizable) substances 1.7 g/l;
-         out of them washing substances 3.3 g/l;
-         specific activity 110-7 1 10-8 Cu/l.
It should be pointed out, that washing substances are not  practically, their function is to increase pollutant solubility in water due to surface tension decrease.
Ultrafiltration membrane separation process for liquid mixtures guarantees practically total absence of high molecular, emulsified and microsuspended components in solutions, in our case domestic pollutants being washed out. Only washing substances with low molecular mass and, unfortunately, radionuclides remain in filtrate. But thanks to the fact, that radionuclides are chemical elements of medium and lower parts of the periodic table and they are of rich chemical properties, it is possible to carry out their association, i.e. conversion them in such a form of existing in solutions, which is effectively arrested by membranes. And then filtrate is water solution of washing substances. The choice of additives depends on isotopic LRW content. Mass of additives should not exceed 2% of the quantity of washing substances being introduced.
Process flow sheet of LRW processing from special laundry of atomic power station is presented in fig.3.
Two waste flows washing and rinsing waters go along their technological chains, only concentrate of washing water ultrafiltration and bottom product after evaporation of reverse osmosic concentrate of rinsing waters are directed for  hardening.

         

Preliminary technical and economic estimation of LRW processing from one of atomic power stations according to new technology is presented in the table.
 
 
Table. Approximate calculated indices of treatment process of LRW from special laundry (according to the data of Khmelnitskaja atomic power station (APS)
 
/
Indices name
Current technology of special purification-7
Reconstruction according to the circulation flow sheet
1.
 
Fresh water consumption, t/year
7300
4700
2.
 
Washing substances consumption, t/year
38.3
3.2
3.
 
, 3/
6900
440
4.
Volume of LRW, being evaporated, m3/year
 
132.0
18.2
5.
Mass of dry residue, incoming liquid waste storage, t/year
44.2
5.5
6.
Mass of dry substances incoming domestic-fecal sewerage, t/year
132.0
20.0
7.
Cost of LRW processing, thousands of c.u./year (conventional unit)
178.6
105.6
 
 
List of literary sources used:
 
[1] Nikiforov A.S., Kulichenko V.V., Zhiharev M.I. Deactivation of Liquid Radioactive Waste M: Energoatomizdat, 1985 183p.
[2] Patent 2054316 RF. Method of Inorganic Sorbent Preparation. Korchagin U.P., Hubetzov S.B.
[3] Svittsov A.A., Abylgaziev T.Zh. Micellary-enhanced (reagent) Ultrafiltration. Progress in Chemistry. M, 1991,  Issue 11, V.60, p.p. 2463-2468.
 

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