METHOD FOR PURIFYING LIQUID RADIOACTIVE WASTE

Abstract

 The invention relates to techniques of processing radioactively contaminated materials and may be used in cleaning liquid radioactive waste (LRW) which accumulate and form during the operation of nuclear power plants and other facilities of the nuclear cycle. The technical result of the claimed invention consists in decreasing the dose load on service personnel during the process of cleaning liquid radioactive waste, simplifying the liquid radioactive waste cleaning process, and enhancing the reliability and safety of the liquid radioactive waste conditioning process. In order to achieve said technical result, a method is proposed for cleaning liquid radioactive waste, which includes feeding liquid radioactive waste into an storage tank, adding sorbents into the stated storage tank, mixing the liquid radioactive waste and sorbents in the tank, separating the used-up sorbent from the solution, wherein the sorbent is fed into the storage tank in a package made of materials which are soluble in an aqueous medium.

Description

[0001] The invention relates to techniques of processing radioactively contaminated materials and may be used in cleaning liquid radioactive waste (LRW) which accumulate and form during the operation of nuclear power plants and other facilities of the nuclear cycle.

[0002] There is a method of cleaning radioactive waste liquid (RWL) (Patent

º RU2560837 dated November 19, 2013) which involves dosing of hydrogen peroxide into the distillation residue of RWL, exposure of the said residue to a xenon lamp UV radiation, microfiltration to separate radioactive sludge containing radioactive cobalt, iron and manganese, and adsorption for removal of the radioactive caesium. Prior to the above the RWL distillation residue was filtered through a filtering mesh and ozonized in a counterflow contact chamber; the processing with a xenon lamp UV pulses is done by impulses whose duration is 10500ms, wherein continuous UV radiation within the range of 190300 nm with the intensity of at least 1·107 W/m2 was used and sorption is carried out at or ferrocyanide ion-selective sorbents.

[0003] There is a method of liquid radioactive waste neutralization (Patent

º RU 2189650 dated September 26, 2000), comprising preparing a starting liquid radioactive waste solution, including adjustment of its pH to a value from 8 to 12, the sorption of radionuclides on natural sorbents by exposing natural sorbent in liquid radioactive waste prepared solution with preselection natural sorbent fraction. This method is different in that salinity amounts of inorganic and organic substances in the liquid radioactive waste solution that is prepared equals not more than 25 g/l, natural sorbent fraction with particle size of not less than 0.1 microns is selected, separation of the resulting radioactive sorption sorbent from the solution is carried out by pressure filtration through ultra or microporous membrane filter with plasma coating at a pressure drop on the filter of at least 4atm, while radioactive sorbent is discarded from the plasma coating and sent for recycling.

[0004] As a prototype we can examine a LRW processing method described in a patent for useful model

ºRU40817 dated April 22, 2004, which includes feeding sorbent into LRW cleaning node, feeding LRW for cleaning, separating purified LRW from the sorbent. The implementation of the abovementioned method requires using a line for purifying liquid radioactive wastes, comprising means for feeding and removing LRW, means for feeding, movement and removal of sorbent; monitoring and control means, characterized in that the LRW feeding means are a choke input of LRW placed in the upper part of the moving pipe; means for removing LRW are executed as a drain node; means for feeding, movement and removal of the sorbent are executed as a feeding pipe with a conveyor screw placed inside and a fresh sorbent storage unit placed at the top of the feeding pipe sequentially with a feeding pipe of a LRW purification unit, comprising a moving pipe with a conveyor screw placed inside it, which is connected with the moving pipe at the top and joined with the used-up sorbent drain node pipe at an angle that is placed under the bottom part of the drain node pipe; a container for removal of the used-up sorbent, wherein the fresh sorbent storage unit, the moving pipe and the used-up sorbent drain node pipe are primarily aligned vertically, which provides an opportunity to move LRW and sorbent under the influence of gravity and/or of mechanical action.

[0005] These technical methods, however, have shortcomings, such as a requirement to use specialized equipment for feeding sorbent into the liquid radioactive waste, namely conveyor screws, batchers, devices for feeding the sorbent, storage nodes, conduits and others. Such complex and expensive equipment requires regular maintenance and repairing: cleaning conduits which feed sorbents, nodes for loading sorbents from possible clogging and other problems. Spending more time in the zone of radioactive exposure for the sake of regular or emergency maintenance increases the dose load on service personnel.

[0006] The goal of the claimed invention is to remove the abovementioned shortcomings.

[0007] The technical result of the claimed invention consists in decreasing the dose load on service personnel during the process of cleaning liquid radioactive waste, simplifying the liquid radioactive waste cleaning process, and enhancing the reliability and safety of the liquid radioactive waste conditioning process.

[0008] In order to achieve said technical result, a method is proposed for cleaning liquid radioactive waste, which includes feeding liquid radioactive waste into an storage tank, adding sorbents into the stated storage tank, mixing the liquid radioactive waste and sorbents in the tank, separating the used-up sorbent from the solution, wherein the sorbent is fed into the storage tank in a package made of materials which are soluble in an aqueous medium.

[0009] Package can be made of water-soluble polymer film, specifically from polyvinyl alcohol. The separation of the used-up sorbent from the solution can be carried out by filtrating the suspension, composed of the sorbent and solution, through the tank which is equipped with at least one filter element at its exit. Moreover, the obtained suspension can be sent back to additional cleaning, while the used-up sorbent will be dumped in the storage tank, placed in the protected concrete block and sent off to be cemented. These blocks are the final result of LRW processing and can be sent directly for burial or can be used as construction materials for building storage facilities. Therefore, implementation of this method excludes the usage of complex in operation and requiring specialized maintenance equipment, which allows to significantly improve the level of radiological protection of the service personnel in the process of conditioning liquid radioactive waste.

[0010] Implementation Examples:

Example 1

[0011] The proposed method was used to process LRW containing:

total salt content - 228 g/l; pH=10,9;

specific activity of strontium-90, 4.2*10⁴ Bq/l;

specific activity of cobalt-60: 1,5*10⁴ Bq/l.

[0012] The tank was pumped with 12M³ of LRW of the aforementioned composition and then 30kg of selective sorbent in a form of dry powder based on manganese dioxide, placed in a package made of polyvinyl alcohol, were added, and 30kg of powder sorbent based on copper sulfide, also placed in a package made of polyvinyl alcohol, were added after that. The particle size of the powder sorbents did not exceed 0.5mm. After the package dissolution and a three-hour mixing, LRW was directed to storage tank, featuring two filter elements with a pore size of 0,4 and 0.1mm. Afterwards, a filtrate separated from the powder sorbent was passed through a storage tank, comprising fifty kilograms of granular selective sorbent based on manganese dioxide. The total specific activity of isotopes, remaining in LRW accounted for not more than 10 Bq/l.

Example 2

[0013] The suggested method was used to process LRW containing:
Boric acid g/L, pH = 4;

Cs -137	5,2* 106 Bq/l;	Co-60	3,1* 104 Bq/l;
Ag-110	8,1*103 Bq/l;	Sr-90	1,9*105 Bq/l.

[0014] The tank containing 10M³ of LRW was consecutively pumped with 20kg of selective sorbents in a form of dry powder (with a particle size of less than 0.3mm), placed in a package made of polyvinyl alcohol, of the following composition: copper ferrocyanide, magnesium phosphate and zirconium hydroxide.

[0015] After the package dissolution and a five-hour mixing, LRW was pumped through two storage tanks with a pore size of 0,2mm in the first filter and 0.1mm in the second one. Afterwards, a filtrate was passed through three consecutively connected tanks, comprising 60L of mechanical mixture of selective sorbents with granular size of 3mm.

[0016] Mechanical mixture consisted of homogeneously mixed sorbents:

20L of copper ferrocyanide,

20L of magnesium phosphate,

20L of zirconium hydroxide.

[0017] The total specific activity of isotopes, remaining in LRW made up not more than 10 Bq/1.

[0018] The claimed invention allows to decrease the dose load on service personnel during the cleaning process of liquid radioactive waste and to significantly simplify the cleaning process of liquid radioactive waste.

Claims

1. A method is proposed for cleaning liquid radioactive waste, which includes feeding liquid radioactive waste into an storage tank, adding sorbents into the said tank, mixing liquid radioactive waste and sorbents in the tank, separating the used-up sorbent from the solution, wherein the sorbent is fed into the storage tank in a package made of materials which are soluble in an aqueous medium.

2. Liquid radioactive waste cleaning method of Claim 1, wherein a package is made of polyvinyl alcohol.