Yurii Karlin, Vadim Iljin, Olga Karlina, Vladimir Kropotov, Elena Kropotova,
Yurii Slastennikov., Igor Sobolev
Moscow state enterprise - MosNPO "Radon"
The 7-th Rostovsky Lane, 2/14, Moscow, 119121, Russia,
Tel. (095)2481680, Fax (095)2481941, Email: root@nporadon.msk.ru 

In decontamination of surface flow from factories exploiting open radiation sources not only technical difficulties emerge but economical as well. Significant amounts of secondary radioactive waste result from treatment of huge water bulk with extremely low levels of radioactivity with the use of water purification facilities. It should be emphasized that specific activity of such secondary waste does not usually allow to classify it as radioactive waste but in spite of this fact they require isolation from populated regions by radiation hygiene standards as they are carriers of raised radiation background. In present report based on operating experience obtained at MosNPO "Radon" amount of secondary radioactive waste resulting from surface flow decontamination by the various methods is evaluated. Difficulties emerging in development of low-level radioactive secondary waste management technology are described as well.

Moscow state enterprise (MosNPO "Radon") is a regional storage of radioactive waste (RW) [1-2]. Considering specifics of production cycle of similar enterprises, it is possible to select three main types of waters, subjecting for clean-up, as follows:

• low level liquid radioactive waste (LLLRW), entering into the storage from other organizations,
• radioactive water, formed while performing the technological operations with RW (from their transporting into MosNPO "Radon" to their placing in disposal site),
• surface flow waters, polluted by radionuclides in consequence of contact of atmospheric falls with solid radioactive waste (SRW) in storage or with radioactive soils at the disposal site of MosNPO "Radon". 

It is obvious that LLLRW volumes of the first and second type should be shorten by treatment in water-cleaning plants or changing the technological operations with RW, causing the LLLRW formation. It is more difficult to find economically acceptable ways, leading to shorten the volumes of the third type RW. 

In this paper a system for the deep cleaning of surface flows from radionuclides, characterized with the minimum volume of secondary RW is offered. This system was developed on the base of practical experience, received at MosNPO "Radon". 

Surface flows from the RW disposal site of Zagorsk branch of MosNPO "Radon" are characterized by the following factors:

• general annual overflow - up to 120-130 thousands of cubic meters (from them - up to 80% sewages from production premises),
• annual average chemical composition of surface flows: dry remainder - 460 mg/l, pH=7,3, Na⁺ - 36 mg/l, K⁺ - 12 mg/l, Ca²⁺ - 81 mg/l, Mg²⁺ - 48 mg/l, Fe³⁺ - 0,2 mg/l, Cl^- - 48 mg/l, HCO^3- - 408 mg/l, CO₃^2- - not detected, SO₄^2- - 27 mg/l, NO^3- - 3 mg/l, SiO₂ - 20 mg/l, COM (chemical oxygen minimum) - 75 mgO₂/l.
• concentration of main radionuclides and general specific activity: å b for ¹³⁷Cs = 1,4 - 1,8 Bk/l; å a for ²³⁹Pu = 0,6 Bk/l; A (¹³⁷Cs) = 1,6 Bk/l; A (⁹⁰Sr) = 0,2 Bk/l, besides traces of ⁶⁰Co, ²¹⁰Po, ²²⁶Ra, ²³⁸Pu, ²³⁹Pu, tritium and some other radionuclides are found. 

Given data show that the chemical composition of surface flows water corresponds to the natural one, in spite of that the share of ground infiltrating water in them does not exceed 20-30%. Contamination levels of water with radionuclides are much below the most-possible concentrations, defined by radiation safety rates [3], acting in Russia. So water of surface flows is not referred as liquid radwaste. 

However, even such low concentrations of radionuclides in the surface flows bring about their accumulation in natural objects of the coastal part drainage system (ground, vegetation, bestial organisms) [1]. Evaluation of polluted ground volume, depending on used radiation danger criteria, which are far from unambiguous, brings about values from 5000 m³ up to 30000 m³. Herewith, it should be noted that as for the specific contents of radionuclides the polluted grounds (with the exclusion of separate areas, which contribution does not exceed 1%) do not get under the category of solid radioactive waste, which are defined by the sanitary rules for radioactive waste treatment (SPORO-85) [4]. The single All-Russia legislative document, which recommendations following it is possible to classify a territory polluted with radionuclides, is Exhibit P-5 in NRB-96 [3] about criterion of interference on polluted territories. According to that document an interference level is determined by the radiation level influence more than 0,3 mZv/a (or excess of gamma-background rate above natural one for 3,4 mR/h). It is necessary to note that even these standards carry a reference nature. 

Problem of territory, adjoining to radwaste disposal site, pollution is solvable by the surface flows cleaning up to radionuclides concentrations close to natural ones. The concept of surface flows cleaning , which is realized in MosNPO "Radon" at present, is described in [2] and uses multibarriers principle:

• the first defensive barrier is drawn close to disposal tanks for RW. Its purpose is sharply to reduce the radionuclides outcome into the drainage system. It is planned to create a sorption barrier along some borders of RW disposal site and to develop a complex of mobile water cleaning modules, intended for express emergency purification of dispersed radioactive water on the strict access area,
• the second defensive barrier comprises a developed drainage system of gutters and cascade of two pond-settlers, designed for the free radionuclides inclusion in silts, algae, sediments and other, easy separated from water carriers,
• the third defensive barrier is connected to the region of output drainage system into the sanitary-defensive area MosNPO "Radon" and comprises some hardware for water cleaning, assembled in the stationary variant and intended for final post-cleaning of surface flows from radionuclides.

 The planned engineering constructions are already partly realized. Pond-settlers were put into usage in 1990. From 1995 a mobile module complex "Aqua-Express" is used, in which to autumn of 1997 more than 1000 m³ radioactive water were cleaned. From 1996 start-up and assembling works on intended for the surface flows after pond-settlers post-cleaning stationary plant "Crystal" are led. The first queue of plant "Crystal" consists of the unit for the surface flows cleaning from oils on a stringy hydrofobous material and floated loading of granulated polypropilen. 

The usage of plant "Aqua-Express" (Fig.1) has shown that cost of LLLRW cleaning (with initial total specific activity up to 2000 Bk/l of beta-nuclides and up to 100 Bk/l of alpha-nuclides) with the chemical composition, close to the surface flows, up to the levels, corresponding the radiation safety rates (in MosNPO "Radon" checking levels - 10 Bk/l for beta-nuclides and 1 Bk/l for alpha-nuclides), with the following overflow of purified water into the drainage system forms aproximately 70-80$ US per 1 m³ of LLLRW (Table.1). Herewith, the volume of secondary radwaste, subjecting to disposal, is aproximately 1%.

It is obvious that the surface flows cleaning in a similar plant with close technological parameters will cause formation of 1200-1300 m³ secondary radwaste per annum. This is a very great amount of secondary waste comparable to the volume of RW, enterring in MosNPO "Radon" for disposal from other organizations. However, it is characterized vastly more low contamination levels. 

Disposal cost of 1 m³ RW according to different evaluations forms from 3000$ US up to 5000$ US. It is not difficult to evaluate the total current expenses for the surface flows post-cleaning, which will form from expenses for water purification, expenses for the secondary radwaste disposal, and expenses for the stuff, conducting the specified works. For MosNPO "Radon" total expenses for the surface flows clearing are valued up to 20,000,000$ US per annum. 

Such great expenses require a sharp increasing in the enterprise budget, nearly reduplications of its production powers on the RW conditioning and significant increasing in the disposal site. In the current term with provision for limited financing of similar enterprises and as a whole negative public opinion to the expansion of areas, chosen for necessities of atomic energy, undertaking of the specified actions is presented impossible. 

Study of possibilities for surface flows clearing costs shortenning has shown that for this it is required to use sorbents in tens times cheaper, than synthetical ones (ion-exchange resins, selective inorganic sorbents), as well as sharply to reduce the volume of secondary waste, directed to the disposal site. The latter is reached by the increase of radionuclides concentrating in the final carrier. 

The studies directed on the revision of physicochemical forms, in which radionuclides migrate together with the surface flows from pond-settlers into the sanitary-defensive area of enterprise, have shown that a great part of radionuclides migrates on fine clay particles and particles of silts of submicro sizes. This is indicated by:

• absence of significant radionuclides sorption from surface flows water on sorbents of the most different types,
• specific activity radiometric measurements of fine sediments, separated from the surface flows water by ultrafiltration (specific activity of fine sediments is more than 3000 times above the one of surface flows ),
• identity of different radionuclides migration mechanism, detected in correlations of their contents in polluted ground of different degree distance from pond-settlers areas (Fig.2). 

Comparative data on some natural object sorption abilities (clay particles, water vegetation, microorganisms) and industrial synthetic sorbents (ion-exchange resins, inorganic selective sorbents), provided in Table II, have confirmed high sorption characteristics of clay particles and silts for radionuclides of the surface flows at MosNPO "Radon".

Table 2. Distribution factors of radionuclides between some sorbents and simulated solution, prepared from water of surface flows (ml/g) 

* - data received from [5] and ** - from [6]. 

Distribution factor of radionuclide K_d was calculated according to the equation:

where A₀ and A¥ - initial and final specific activities of simulated solution, V - simulated solution volume (ml), m - weight of air-dry sorbent or ground or burnt to ash biomass (g) at the temperature 400-500° C. 

Found particularities of the surface flows contamination by radionuclides allow to offer a technological scheme of plant for the surface flows after pond-settlers post-cleaning, in which high-priced synthetic sorbents are not used (Fig.3).

Fig. 3. Principle technological scheme of plant <Crystal>: 1 - pond-settlers, 2 - pump, 3 - filter with hydrofobous stringy material and granulated polypropilen, 4 - sand filter, 5 - plate separator, 6 - intermediate tank, 7 - pump, 8 - roll ultrafiltration devices. 

Preliminary cost estimations for the surface flows cleaning without using the sorbents bring about values from 2$ US up to 10$ US for 1 m³ of cleaned water that is in 7-40 times lower than in plants, using synthetic sorbents. On evaluations the amount of secondary waste from water purification only from clay particles will form from 30000 kg up to 50000 kg per annum or from 15 m³ up to 34 m³ by volume. Such low amounts of the secondary waste will not changed greatly the total volume of radwaste, disposed in MosNPO "Radon". Total expenses for the surface flows cleaning are decreased up to values not more than 1,500,000$ US per annum. 

In 1998 field tests of the pilot plant for the surface flows cleaning without using the sorbents with production up to 200 l/h are scheduled, which will allow to specify the done evaluations of cost for the surface flows post-cleaning at MosNPO "Radon". 

1. Karlin Y.V., Chuikov V.Y., Belianina N.G., Barinov A.S. The estimation possibility of cleaning from radionuclides for the coast parts of drainage system by the soil fractionating method. - // Abstracts of International Topical Meeting on Nuclear and Hazardous Waste Management SPECTRUM'96 (August 18-23, 1996, Seattle). Vol.2. P.956-962. 
2. Barinov A.S., Volkov A.S., Karlin Y.V., Kropotov V.N. Cleaning of the Moscow SIA "Radon" superficial flowing: the problems and the solutions - In Abstracts of 1995 Fall Meeting of Material Research Society (Nov 27 - Dec 1 1995, Boston). - P.584. 
3. Radiation Safety Rates (NRB-96): Hygienic rates. M.: Informational-publishing centre of Goskomsanepidnadzor, Russia, 1996.- 127p. 
4. Sanitary Rules for Radioactive Waste Treatment (SPORO-85)/ Minzdrav of the USSR. - M.: Minzdrav USSR, 1986. - 54p. 
5. Timofeeva-Resovskay E.A., Agafonov B.M., Timifeev-Resovskii N.V. The fate of radioisotopes in water basins. - Works of biology institute (Academy of sciences USSR, Ural branch), 1962, N22, pp.49-67. 
6. Titlianova A.A., Timofeeva N.A. Sorption of radioactive isotopes in soil. - Works of biology institute (Academy of sciences USSR, Ural branch), 1962, N22, pp.17-29. 

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