Radwaste Management at Operating NPPs
The main sources of the generation of radioactive waste (RAW) are operating NPPs. Radioactive substances are generated during NPP operation at capacity during the process of U235, U233, Pu239 fission in the active zone of the reactor, and in the result of neutron activation of different materials, that are in the active zone, which are the products of nuclear reaction etc. On the average, during the year it is generated from 0,15 to 0,35 cubic meters of liquid radioactive waste and from 0,1 to 0,3 cubic meters of solid radioactive waste per 1 MW, depending upon the capacity and type of reactor facility.
In the process of NPP units operation three main types of RAW are originated, which are gas-aerosol, liquid and solid, and the main term of safety is decontamination of them. Taking into consideration the RAW negative effect into people and environment, it is necessary to carry out the collection and reliable isolation of waste with taking into account the peculiarities of radionuclides and divergences of their physico-chemical and biological peculiarities.
Gaseous radioactive waste management is reduced to their dispersion into environment (waste) after the procedure of remediation. Experience of NPP operation showed that measures taken during the process of NPP design in limit of discharge capacity and compliance with the design regime of operation of equipment for discharge remediation ensured the fulfillment of normative requirements. During all operating time of NPPs in Ukraine in normal mode there were not cases of excess of radioactive discharges capacity over restrictions established by current regulatory documents. Existing cleanup systems at Ukrainian NPPs under conditions of normal operation provides lower levels of discharges than international and national norms.
Control under the levels of discharges of radioactive substances to atmosphere is monitoring by automated system of radiation control installed in all sources of discharges and by laboratory research after the carrying out of sampling.
Liquid RAW management. Generation of liquid radioactive waste (LRW) at NPPs is connected with peculiarities of technological procedure and unforeseen flow of liquid radioactive media; system of LRW management consists mainly of sources of their generation and facilities of storage and reprocessing.
There are produced such types of LRW at NPPs:
- Vat residue as a result of reprocessing of floor drain and laundry drain at evaporator of water cleanup system (active water treatment plant).
- Spent absorbents that flow from filters of active water treatment plant in case of depletion of resources of ion-exchange material including other spent filter materials;
- Sludges and pulps;
- Spent oils and mixed liquids.
Before reprocessing, LRW is collected in the tanks for temporary storage and hold-up. System of LRW storage at NPP consists of gathering facility and its temporary storage.
Vat residue, filter materials along with sludges and lubricants are stored separately. Spent filter materials, absorbents are transmitted by the system of hydrotransport to the reservoirs, where they are stored under the layer of water.
The scheme of initial LRW reprocessing at NPP with reactor of VVER type includes evaporator of water treatment system and unit of reagents. After evaporation of initial liquid radioactive media at water treatment facilities and it is generated vat residue that contains insolible salts of sodium, ferrum, magnesium, calcium etc.
In the process of reprocessing radioactive and other chemical substunces are released from waste and clean water is returned to the technological process.
It is used thermal, sorption and membrance methods for LRW reprocessing at NPPs.
Thermal method that is distillation or evaporation – is the most common and convenient method of LRW reprocessing. It is carried out in special evaporators with heat injection through vapour. Solution of LRW from evaporator is transmitted by pumps to aux evaporator, where it takes place the evaporation to the maximum salt concentration. Vapour of aux evaporator is condensed and returned to the reservoirs of coagulant water, and vat residue is pumped to the tanks for waste storage. Clean water is sent through absorption-trap to clean condensate tanks, from wich it is provided for recycled water use after the examination on radioactivity and salt content.
Sorption method presuposes removal of radionuclides out off liquid waste in the form of solid phase in the result of adsorption, ion exchange, adhesion etc. However because of selectability to some radionuclides method of sorption cannot be considered as the main method of treatment from radionuclides.
Sorption is conducted in special apparatuses in dynamic or static conditions on bulks or precoat filters with special ion-exchange resins.
Membrane method is a method with the help of which removal of radioactive solutions is carried out at molecular level. Among them the most effective is reverse osmosis, electrodialysis and ultrafiltration.
Because of variety of radioactive and nonradioactive pollutants any of the mentioned methods of treatment does not separately provide the remediation of LRW to the necessary level. One of the reasons is the presence of amoniac and oils. That is why the system of treatment at NPPs is a complicated chain of operations, which are performed with special apparatuses that provides different methods of remediation. Therefore, the technology of treatment includes several consecutive operations. In the result of such operations it is released two products: high level concentrated product that is transmitted to the solidification and disposal, and condensate that meets all requirements of water quality for reuse at NPP or for dumping to open water.
Concentrates, received in the result of LRW treatment, are sludges after filtration and chemical treatment, spent ion-exchange resins, vat residues after evaporation. These concentrates are subjected to solidification with the methods of bituminization, cementation, polymerization and others.
Solidification of LRW concentrates is performed by including them into binding materials that can be divided into three main groups: thermoplastic (bitumen and others); thermoset (polyester, carbamide resins etc.); inorganic (cement, gypsum, glass etc.).
Binding materials are to have:
- Low elution that characterizes high insulation properties;
- Compatibility with components of waste concentrate that provides the minimal volume of end product.
- Strength that includes the disruption of solidified product in emergency during transportation;
- Biostability means that solidified products should be resistant to bacteria and microorganisms;
- Radiation resistance that determines gas emission out of solidified products.
Nowadays in many countries it is commonly used the method of bituminization for LRW solidification, during which radioactive waste are mixed with bitumen. Bitumen – distillation product of oil or coal. Water resistance of bitumen provides quite reliable hydro insulation of involved components. Bitumen attracts attention by such positive qualities as proofness, plasticity, sufficient chemical resistance, low cost, resistance to the impact of microorganisms.
Recently, technologies have been developed, where bitumen would be changed into regenerated polymer. Polymerization proceeds without heating, as a rule. Method of polymerization is convenient for fixation of spent ion-exchangers. For this purpose it can be used the same facilities as for the bituminization. In comparison with bitumen polymer products have better properties according to some criteria. Polymers have quite good chemical resistance. Thermosetting resins as kind of binding materials are distinct in simplicity of solidification and some positive properties of solidified product, namely resistance to the effect of mechanical, thermal and radiation load.
Cementation – one of the solidification method of homogeneous (vat residues) as well as heterogeneous (pulps) waste. Process of cementation lies in involving of radioactive substances into Portland cement mark 500 with following generation of solid monolith and it is based on the interaction between viscous substance of cement (calcium oxide, silicates, aluminates etc.) and water contained in waste without temperature rise.
Today one of the most suitable methods of liquid high level radioactive waste is vitrification. The method of vitrification provides the decomposition of significant number of chemical compounds that are components of waste; by this it is eliminated the necessity to take into consideration their harmful effect during the process of disposal and the volume is sufficiently decreased in comparison with bituminization (in 2-4 times) and cementation (in 10 times).
Along with vitrification it is elaborated other solidification methods of waste to receive products that are more thermodynamically resistant than glass, which are able to preserve mechanical and chemical resistance for a long time. Such products include vitroceramics and other kinds of mineral-like ceramics.
Solid radioactive waste management. Technical maintenance and repair of energy supply unit are main sources of solid radioactive waste (SRW) generation in the process of NPP operation.
Parts or details of substituted equipment and pipes, tool that was used during works, electrical and thermal insulation materials, clothes, wiping rags etc. became radioactive during the process of repair works of NPP equipment. System of solid radioactive waste management at NPP includes: collection of waste into initial packing material at sites of their generation; level classification; waste transportation to the centralized sites of collecting and reprocessing; reprocessing of radioactive waste; packing of the initial packing materials with solid radioactive waste into transport containers; transportation of containers with solid radioactive waste to the SRW storage facility by special purpose vehicle; receiving of waste an their unloading to the storage facility section; radioactive waste accounting and reporting.
All types of SRW at NPP are collected into special premises, assorted and while it is necessary they are reprocessed to down-size them.
After reprocessing solid radioactive waste is reserved in storage facilities of SRW that are specially constructed at the territory of NPP. SRW storage facilities are deepened concrete containers that are hydro insulated from underground and atmospheric waters. They are under sever radiation dose monitoring, that is why there are constructed observation wells around the storage facilities, from which it is regularly collected samples of water for radioactive waste content analysis. Storage presupposes the possibility of radioactive waste removal for reprocessing and transportation.
The first step in sequence of stages of SRW management is collection and sorting of them.
Contemporary policy of SRW management presupposes their sorting at the stage of collection by maximum possible number of features and criteria by taking into account the requirements of further stages of radioactive waste management (temporary storage, reprocessing, conditioning and disposal).
Such features are:
- Radioactive peculiarities of radioactive waste;
- Possibility to burn, press them;
- Applicability of decontamination;
- Necessity to fragmentize etc.
Pre-processing operations include decontamination, collection and sorting of radioactive waste, preliminary pressing, fragmentation, drying etc.
The aim of decontamination is volume reduction of radioactive waste, transference of them from higher group to lower one and improvement of personnel, population and environment radioactive protection conditions. Decontamination means the cleanup of equipment surface from radioactive contamination by washing, heating, chemical and electrochemical processes, mechanical and other kinds of cleanup.
SRW reprocessing is understood as any operation that changes their features. The main purposes of reprocessing are: safety improving at further stages of their management, reduction of negative impact on environment, cost cutout for storage and disposal of SRW.
Compliance of reprocessing product with the conditions of further stages of radioactive waste management is the criteria of specific reprocessing method choice, namely conditioning, transportation, temporary storage, and disposal. Requirements of current norms, rules and standards in the sphere of safe radioactive waste management, requirements to the shape, physico-chemical and radiation characteristics etc. have an influence on the choice of radioactive waste reprocessing method.
Mechanical reprocessing of SRW. The purpose of mechanical reprocessing of SRW is to down-size it.
Downsizing of SRW improves the packing of radioactive waste for transportation, storage, disposal or preparing of radioactive waste to the next stage of reprocessing. The main methods of mechanical downsizing include dismantling, sawing, cutting and fragmentation.
Dismantling is used during the process of nuclear power facilities decommissioning with known in the building sphere methods.
Sawing and cutting down-size the overall equipment by using circular, crosscut, chain saws, abrasive discs, plasma cutting, pneumatic and hydraulic impactors etc.
With the help of fragmentation it is reduced the size of SRW (its density is increased) or prepare more homogeneous mixtures of low active SRW. Fragmentation can be used along with pressing or burning.
Pressing of radioactive waste is one of the most productive methods of waste downsizing. Presses are qualified according to the indicators of pressure that it develops. Presses of low pressure (with the force of 10 MN) are aimed for pressing plastic compound, paper, rubber and textile.
Superpressing is pressing with pressure force higher than 10 MN. During the process of superpressing radioactive waste are placed into the drums and then they are pressed. Pressed “briquettes” are packed into other containers that are cylindrical or rectangular.
Thermal reprocessing of radioactive waste. Processes of thermal reprocessing include wide range of oxidating and pyrolytic technologies that are the effective methods of downsizing of burned radioactive waste.
Burning is the most common process of heat treatment. There are a lot of different types of burning facilities for various radioactive waste reprocessing from low level radioactive waste of NPP to high level ones released after nuclear fuel reprocessing. Although the reprocessing of middle level radioactive waste can be more complicated than reprocessing of low level RAW because of the use of protective screens and remote equipment. If there are α-radiators, it is necessary to consider the probability of criticality.
Conditioning. After reprocessing radioactive waste should be in the state that is suitable for transportation, durable storage or disposal. Thus the conditioning is necessary for these purposes. Methods of conditioning can be combined with reprocessing methods (for instance vitrification or melting) or it can be independent (cementing and bituminization).
Very popular is the method of cementing, which is joint grouping into cement blocks of crushed SRW. As a rule, these are small pieces of metal, filters and drums with the waste after superpressing.
With the method of vitrification the glass is melted in fire resistant reactor with auxiliary heating and then it is kept in the molten state (1100-1260 0C) by electrical heating with the help of immersed electrodes. RAW is put into one side of furnace higher than molten glass with the air for burning. Burning occurs due to radiation from molten glass. Initial gas is released from opposite side of furnace. Solid product after burning and non-combustible materials are vitrified and can be continuously removed or composed into matrix that is not leached.
The experience of burning is accepted to the wide range of RAW. The technology of its downsizing by burning is commonly accepted, effective and safe.
The aim of burning of RAW is to reduce the volume of organic and inorganic RAW and give it the compact form in order the end product has high peculiarities on the resistance of leaching.
Methods of chemical reprocessing of RAW are divided into two categories: wet oxidation and chemical oxidation. Wet oxidation is carried out in moist environment that is why it does not release the large quantity of gases as in the usual burning facility. Chemical oxidation can be realized with the use of highly oxidized reagents including permanganates, dichromate, hypochlorite, persulfate, peroxides and nitric or sulfuric acids systems.
Current Situation with Radioactive Waste Management at Ukrainian NPPs
The following facilities are under operation at Zaporizhzhya NPP:
- two deep evaporation facilities (processing of evaporator bottoms);
- solid radwaste incineration facility (processing of solid low-level radioactive waste);
- solid radwaste compaction facility (processing of solid low-level radioactive waste)
Solid radioactive waste sorting facility (processing of solid low-level radioactive waste) is commissioned as part of the complex.
The following facilities for radioactive waste processing are under operation at Rivne NPP:
- centrifuge plant (processing of floor drains);
- two deep evaporation facilities (processing of evaporator bottoms).
The efforts are ongoing on the design of the solid radioactive waste processing complex consisting of:
- retrieval facility;
- fragmentation and sorting facility;
- supercompaction facility;
- system for radiation monitoring in the management of radioactive waste.
Comprehensive acceptance tests (simulator of radioactive waste) is underway at RNPP and ZNPP. Their results will be the basis for making decision on putting the facilities into trial operation.
The following facilities are under operation at Khmelnitsky NPP:
- deep evaporation facility (processing of evaporator bottoms);
- radioactive oil incineration facility;
- centrifuge plant (processing of floor drains).
Only solid low-level radioactive waste facility is under operation at South Ukraine NPP.