WO2021024004A2 - Handling of solid radioactive waste with low and intermediate activity - Google Patents

Abstract

The invention relates to the handling of low and intermediate level solid radioactive waste. According to the handling one performs the steps of collecting low and intermedi- ate level solid radioactive waste to be treated, mixing the collected solid waste with glass-forming additives, comminuted and then homogenizing said mixture, and feeding the thus obtained comminuted and homogenized mixture into a molten and swirling material (M) with a melt level (ML) arranged in a reaction space (R), wherein said feeding is performed under pressure and below the melt level (ML) of the molten material (M).

Description

HANDLING OF SOLID RADIOACTIVE WASTE WITH LOW AND INTERMEDIATE ACTIVITY

The present invention relates to the handling, in particular to the processing, of solid radioactive waste with low and intermediate activity level.

Radioactive waste is any waste that has become radioactive in nuclear power plants, but also waste generated in the fields of medicine and other sciences, in dustry and agriculture that shows some degree of radioactivity.

In literature, a distinction is made between low level, intermediate level and high level radioactive waste. These must all be stored and handled / processed differ ently. In the management of nuclear waste, the activity of the radiating material and its amount are of primary importance, however, in many cases radioactive waste is also classified on the basis of the half-life of the radiating material in the waste. Here and from now on, low and intermediate level radioactive waste refers to radioactive waste with a half-life of up to at most 30 years.

Nuclear power plants generate significant amounts of low and intermediate level solid radioactive waste every year. For example, the Paks Nuclear Power Plant (Hungary) has accumulated about 1,300 m³ of low and intermediate level solid ra dioactive waste in the last ten years. This amount is made up of outerwear con taminated with radioactive material (such as textile working dresses, work protec tive gloves, work safety boots, etc.). Most of the solid radioactive waste can be compressed, during compression the compressible waste is compacted into 200 I canisters. The above annual volume is already the compacted volume, i.e. the volume compressed to half, 40% of the original volume. Accordingly, the quantity currently stored is about 6,500 canisters; the average weight of 1 m³ of com pressed solid waste is about 400 kg.

Considering the above example, the total amount accumulated in Hungary (over ten years) is approx. 520,000 kg, that is, about 52,000 kg/year of low and interme diate level solid waste is generated regularly. It is treated / processed at a rate of 1425 kg / day and 60 kg / hour. Until treatment / processing has taken place, this radioactive waste must be stored under appropriate conditions. A landfill carved into granite was opened in Bataapati (Hungary) ten years ago, with an investment cost of about HUF 70 billion. Designating the location of the required repositories is a very lengthy and costly task that must be done with extreme care, and the lo cation of the landfill itself must meet extremely strict safety considerations.

Various compression / compaction solutions are used to reduce the volume of ra dioactive waste intended for long-term disposal, usually by placing the waste in metal canisters and arranging said canisters in shafts deepened into a rock layer.

Another possible solution to reduce the volume of low and intermediate level solid radioactive waste generated is to reduce the volume by incineration, but in such a case the further treatment of the ash-like material and the "retention" of the gase ous radiating effluents pose further problems, see e.g. the pieces of equipment sold by HTT (Pitt Meadows, BC, Canada; www.httcanada.com).

In other solutions, falling e.g. into the field of plasma technology, the treatment of low- and intermediate-level, radioactively contaminated materials compacted in iron canisters of 200 litres in volume performed by feeding said canisters one by one into a plasma takes a considerable amount of time before the heat effect of the plasma treatment reaches from the outer surface of the canister fed into the plasma to the inner core of said canister, as it takes place for the plasma treatment plants of Zwilag AG (WOrenlingen, Switzerland). During this significant amount of time, and due to the simultaneous feeding of large mass, evaporation will also be significant, which is attempted to be compensated by making use of complex gas filtration; this, however, results in additional costs and wastage of energy.

Evaporation is a significant problem with all treatment / processing methods to date. When fed into or through the gas space, a significant amount of material to be treated inevitably avoids actual / immediate entry into the melt. Mainly the more volatile substances and due to the high external heat effect even metal vapours will leave the reaction space directly through the gas space, thereby carrying with them some of the radioactive contamination that should remain in the melt.

In light of the above, our aim is to improve and, where appropriate, eliminate the technical disadvantages / deficiencies associated with the above-referred man agement of low and intermediate level solid radioactive waste. In particular, our aim is to provide a solution for the treatment and / or processing of low and inter mediate level solid radioactive wastes wherein evaporation can be significantly re duced or even eliminated. Our further aim is to implement a process and an appa ratus by means of which low and intermediate level solid radioactive wastes can be processed in an efficient and energy-saving way, along with significantly reduc ing the risk to the environment.

In our investigations it has been found that if low and intermediate level solid ra dioactive waste is introduced directly into a molten and swirling material located in the specific reaction space of a plasma reactor, below the melt level and under pressure, and together with glass-forming additives, in accordance with the method of claim 1, surprisingly, the above aims are achieved. Preferred further embodiments of the invention are set forth in claims 2 to 14.

Accordingly, the low and intermediate level solid radioactive waste materials to be treated are collected instead of iron canisters, preferably in polyethylene (PE) bags, in a plant located not far from the place of origin of said waste materials, i.e. without temporary storage and external transport, and the collected daily volume is preferably processed immediately. As a result of the use of glass-forming addi tives, radioactive contaminants from the processed solid waste will leave the closed area of the processing plant only in the form of a small amount of glass bricks ensuring safe encasement for the contaminants.

The material to be treated / processed in the PE bags, together with the additives suitable for glass-formation, is fed directly to a melt of about 1800 to 2000 degrees Celsius, after grinding, without the need for additional material and energy for compacting into iron canisters, thereby the amount of melt gets increased. The upper region of the melt is occasionally tapped from the reaction space into mould(s), and the moulds are cooled to form glass bricks from the melt, which can then be transported to the landfill.

The direct introduction of the material to be treated, under pressure, into a melt ar ranged in a substantially closed reaction space, said melt being kept in swirling by one or more transferred arc thermal plasma torches, is carried out by screw feed- ing, i.e. by feeding the waste to be treated continuously in small batches and at a controlled rate; evaporation, thus, gets essentially completely eliminated. The di rect introduction of the waste to be treated into the melt in this way also eliminates the risk of backflow, and prolongs the life of the screw by keeping the moving metal structure away from the reaction space.

The difference between the heat transfer coefficient of the hot arc gas mixture (e.g., nitrogen gas, argon gas, carbon dioxide gas, optionally a mixture thereof, etc.) present between the plasma torches and the melt and the heat transfer coef ficient of the (partly glass) melt kept in swirling is three orders of magnitude. Due to the intensive heat transfer conditions of the plasma torches, the waste added to the high temperature melt reaches the temperature prevailing in said melt very quickly, its organic matter content decomposes and becomes gaseous within about 1 to 2 seconds, that is, practically immediately, while its inorganic matter content with the radioactive contaminants melts and gets mixed with the melt.

Unlike at incineration, the organic matter content evaporates as a gas, with no air and nitrogen content therein, thus no NO_x formation takes place and, hence, NO_x filtration is practically unnecessary.

By adding water vapour to it, the gasified organic matter content can be converted to CO/H2 fuel, which is simply incinerated after dry / wet gas cleaning. The CO/H₂ fuel can also be used for heat production if burnt under controlled conditions.

The filter elements used in the wet / dry gas cleaning and the resulting sludge are recycled to the process inlet, wherein they, together with the previously filtered ra dioactive contaminant content accumulated in the filter elements / sludge, are added to the waste to be treated / processed.

By this complex treatment procedure, only multiply purified gaseous combustion products and radioactive material encased in glass bricks to be still stored / rested are released to the environment, the latter being strictly isolated from the environ ment.

By the procedure shown here, the volume of waste to be treated / processed is re duced by its incinerated organic matter content and increased by the addition of glass-forming additives. Overall, a volume reduction of about 80% and a weight reduction of about 60% are achieved, along with significant protection of the envi ronment.

The solution for the treatment of low and intermediate level solid radioactive waste according to the invention will now be described with reference to the accompany ing drawings, wherein

- Figure 1 is a schematic block diagram of a plasma reactor system accord ing to the invention with a molten lower space reaction chamber for the treatment of low and intermediate level solid radioactive waste, as well as a system of ancillary units connected thereto; and

- Figure 2 is a schematic illustration of an apparatus for accomplishing a pre ferred embodiment of the inventive process for the treatment of low and in termediate level solid radioactive waste, wherein introduction of a homoge nized substance, prepared for treatment, into the apparatus takes place by continuous screw feeding at a controlled rate and the substance com pressed into a truncated cone into the melt space of the reaction space un der pressure, just below the melt level of the melt in the reaction space.

Figure 1 shows a system according to the invention for the treatment and / or processing of low and intermediate level solid radioactive waste, comprising a plasma reactor 100 having an internal, substantially closed reaction space R which includes an upper section R-3, a middle section R-2 located below the upper sec tion R-3 and a lower section R-1 located below the middle section R-2, said sec tions are interconnected and in communication with each another. The upper sec tion R-3 of the plasma reactor 100 comprises one or more gas outlets 0-1 formed preferably at its highest point, i.e. at the top thereof. Sections R-3 and R-2 pass into each other continuously, sections R-3 and R-2 can only be separated from each other on the basis of their functions to be discussed later. In the upper sec tion R-3, one or more second type plasma torches P-2 are arranged. In the middle section R-2, one or more first type plasma torches P-1 are arranged, the first and second type plasma torches being plasma torches with different arc forming me dia. Sections R-2 and R-1 also pass into each other continuously; however, in the operating state of the plasma reactor 100, a melt level ML of a molten material M in the lower section R-1 also defines physically a boundary between sections R-2 and R-1. It will be apparent to those skilled in the art that the melt level ML is lo cated in a band of predetermined height at the boundary of sections R-2 and R-1, i.e. its location can be well defined. The lower section R-1 is preferably provided with at least one tapping opening 0-2. In the operating state of the plasma reactor 100, the tapping opening 0-2 is located below the melt level ML, directly below the band of given height defined for the melt level ML. The lower section R-1 is pref erably provided with at least one screw feeder I-3 for introducing the low and in termediate level solid radioactive waste to be treated into the lower section R-1 of the plasma reactor 100. An inlet B of the screw feeder I-3 in communication with the lower section R-1 is located below the melt level ML. The inlet B is preferably located within the middle third of the lower section R-1. In addition, the lower sec tion R-1 is provided with a tapping opening 0-3 preferably at its lowest point, i.e. at the bottom of the plasma reactor 100. Ancillary units associated with the plasma reactor 100 for a complex management of low and intermediate level solid radio active waste will be described in more detail below.

As shown in Figs. 1 and 2, after a short (e.g. only a daily) storage, the incoming material formed by the solid radioactive waste with low and medium activity to be processed (or treated) is fed into a grinding unit 1-1 together with glass-forming additives (e.g. quartz sand, soda, limestone grind, as known to those skilled in the art) required for encasement into glass, where the mixture is comminuted / ground. The substances thus fed are then homogenized in a mixing section I-2. By the screw feeder I-3, the mixture thus prepared for treatment gets compacted in a cy lindrical tube (made, preferably, of ceramic) of reduced diameter and then fed into the lower section R-1 through the inlet B as a compressed mixture. The rate of feeding said compressed mixture consisting of the waste and the additive, under pressure, is changed automatically, if necessary, by means of suitable actuator means (not shown), depending on CO concentration measured by sensors (not shown) arranged in the reaction space R. As is known to those skilled in the art, the value of said pressure can be controlled by the degree of compaction achieved by the screw feeder I-3, said degree of compaction is adjusted by a controllable electric motor (see Fig. 2) for driving the screw feeder I-3.

The mixture of the waste material to be processed and the additive is in the molten state in the lower section R-1 of the reaction space R. The material in the molten state is kept in constant swirling by the one or more first type plasma torches P-1. The first type plasma torches P-1 used here are transferred arc plasma torches with a power of up to at most 80 kW. Preferably, at least three plasma torches P-1 are used. The plasma torches P-1 are arranged at the circumference of the reac tion space R, preferably at equal angular distances from each other, and are di rected in such a way that their operation gets the molten material M in section R-1 into swirling or keeps it in swirling when the plasma reactor 100 operates. Upon addition of the material to be treated, it enters into a melt of large mass, kept in swirling at a temperature of 1800 to 2000 degrees Celsius, wherein its state of matter changes in 1 to 2 seconds. The largely carbonaceous gasifiable content rises above the melt level ML of the molten material M, while the substances that can be encased into glass (i.e. the vitrifiable substances) remain in the melt. It is of particular importance that the radioactive contaminants, due to their higher specific gravity, remain in the melt and gradually get deeper and deeper in it until they reach the bottom of the reaction space R. The level of molten material M in the re action space R is kept between given limits. When the melt level ML reaches an upper limit, the molten material M is tapped until said melt level ML reaches a lower limit. Thereby, a scheduled tapping of the upper part of the molten material M is performed through the tapping opening 0-2, whereby, in successive steps, a given amount of molten material M also containing a maximum amount of radioac tive contaminants, preferably prescribed by law(s), is withdrawn from the reaction space R. Tapping is preferably carried out into ceramic vessels 10 of nearly rec tangular shape, and the thus obtained vitrified substance, after cooling, is removed from said vessels 10 and transported to the final landfill for long-term storage in the smallest volume. Tapping is also possible through the tapping opening 0-3 lo cated at the lowest point of the reaction space R, where the radioactive contami nants are present in a higher concentration. This molten phase is tapped sepa- rately, along with keeping the relevant safety regulations, and, after solidification, is deposited in harmony with the regulations, depending on the level of activity. In principle, it is possible that, if sufficiently long swirling time is applied, the glass discharging through the 0-2 tapping opening and solidifying no longer requires special and expensive storage, as its radioactive contaminant content is negligible, or at least below the prescribed limit.

In the middle section R-2 of the reaction space R, the gaseous substance exiting the melt separates from the entrained liquid droplets in a slow vertical flow at a speed of 2 to 3 m/s; the droplets fall back into the lower section R-1 , so that a pure gaseous substance enters the upper section R-3 of the reaction space R.

In the upper section R-3 of the reaction space R, one or more second type plasma torches P-2 are arranged. The second type plasma torches P-2 used here are tangentially blowing, water vapour blown arc plasma torches using water vapour as the arc forming medium and having a power of up to at most 100 kW. The number of plasma torches P-2 used is preferably at least three. The plasma torches P-2 in the section R-3 are arranged in such a way that their operation gets the gaseous medium performing a slow vertical flow upwards into rotation. To this end, the plasma torches P-2 are preferably arranged at the circumference of the upper section R-3, preferably at equal angular distance from each other. The car bon (C) content of the rotating, swirling gaseous material is converted to carbon monoxide (CO) by the oxygen (0₂) content of the water vapour blown as the arc forming material of the plasma torches, while the hydrogen content of the water vapour continues to flow upwards essentially unchanged as combustible gas. The radioactively contaminated particles / grains remaining in the rotating gas drift to the mantle of the plasma reactor 100 and flow back to the lower section R-1 of the reaction space R, i.e. the high specific gravity solid radioactive particles return to the melt from the upper section R-3 as well.

Before open-air incineration, the combustible gas generated in the upper section R-3 of the reaction space R as described above is discharged from the plasma re actor 100 through the one or more gas outlets 0-1 of the upper section R-3 and then directed through a filter unit comprising a coarse ceramic filter F-1 , a fine ce- ramic filter F-2 and a water jet washer F-3 arranged one after the other in the given order in the flow direction of the combustible gas. The applied filter unit also traps any iodine contamination. Preferably, the coarse ceramic filter F-1 has a pore size of 0.5 to 5.0 mm. Preferably, the fine ceramic filter F-2 has a pore size of 5 to 50 pm. When the clogging level of said ceramic filters reaches approx. 40%, the ceramic filters are replaced and fed into the grinding unit 1-1 , which is the input to the material feed to the plasma reactor 100, and then ground together with the waste to be treated. Condensed sludge from the water jet washer F-3 is recycled to the reaction space R and thus to the system / treatment procedure through the water vapour plasma torches P-2. Flence, only (i) the gas purified in several stages and incinerated, and (ii) the radioactive contaminants with the prescribed maxi mum activity level incorporated into a glass matrix after solidification of the melt tapped through the tapping opening 0-2 with a volume decreased by about 80% compared to the original volume of the solid radioactive waste will be released into the environment. If now the glass bricks obtained in the presented procedure are arranged in a suitable landfill, the storing capacity of said landfill increases at least fivefold.

Thus, based on Figures 1 and 2, low and intermediate level solid radioactive wastes are processed through the following steps / units.

Pre-treatment of incoming material: receiving, cutting, and grinding the incoming material in the grinding unit 1-1; homogenizing the incoming material in the mixing section I-2 with appropriate glass-forming additives; supplying the mixture of said incoming material and the additives after compaction at a given pressure below the melt level ML of the molten material M by the screw feeder I-3 driven by a con trollable electric motor.

Block of first type plasma torches: complete melting and / or gasification of the substance fed under pressure by heat transfer trough one or more transferred arc plasma torches P-1 at the temperature of 1800 to 2000 degrees Celsius in the molten lower section R-1 of the reaction space R, simultaneously keeping the melt in swirling by the torch(es). Block of second type plasma torches: depending on the carbon content, convert ing to fuel gas (CO/H₂) by one or more water vapour plasma torches P-2 in the upper section R-3 of the reaction space R, with a residence time of at least 3 sec onds, simultaneously keeping the gaseous substance in rotation by the tangential blowing of the torch(es).

Exits for materials leaving the reaction space: gas outlet(s) 0-1 for gaseous prod ucts; tapping opening 0-2 for tapping low activity melt into frequently used, shaped cooling-receiving vessels 10, after cooling, the tapped melt converts to a glass product; and, optionally, tapping opening 0-3 for tapping intermediate activity melt requiring special care, after cooling, the tapped melt also converts to a glass prod uct.

Filtering / washing block: coarse ceramic filter F-1 , fine ceramic filter F-2, water jet washer F-3, and optionally, drip separator and return channel F-4 arranged one af ter the other in flow direction. Outputs recycled to open-air or treatment procedure: flaring (K-1) the doubly fil tered and washed gas; recycling (K-2) the saturated ceramic filters to the inlet of the system / procedure; recycling (K-3) the wash water sludge to the plasma torches P-2.

Summary In comparison with conventional compression and storing in deep shafts in canis ters or making use of conventional incineration, the volume of low and intermedi ate level solid radioactive wastes is reduced to at least about one-tenth of the original volume (or about one-fifth of the compressed volume) by the complex treatment procedure according to the invention. The comminuted incoming material which is supplemented with glass-forming ad ditives (e.g., quartz sand, soda, limestone grain) is introduced under pressure, in a controlled way, below the surface of the glassy melt at a temperature of 1800 to 2000 degrees Celsius arranged in the reaction space. Continuous swirling, mixing and heat transfer of the melt is ensured by one or more transferred arc plasma torches.

The organic matter content of said incoming material evaporates as a gas, bursts to the surface of the melt and then converts to gaseous fuel CO/H₂ by the addition of water vapour. After a dry / wet gas cleaning, it is released to open-air through incineration; optionally, heat recovery thereof may be considered and implemented in a known manner.

The filter elements used in and the sludge obtained from said wet / dry gas clean ing step are recycled to the processing plasma reactor, where they are melted to- gether with their radioactive material content and then tapped from the middle sec tion of the plasma reactor with the inorganic materials in order to form glass bricks. Water vapour is introduced into the process by one or more water vapour plasma torches.

The obtained glass bricks can be transported to the landfill in use. Due to the size of said glass bricks, the landfill's storage capacity increases at least fivefold. In ad dition, the encasement of radioactive contaminants into a glass matrix is safer than e.g. the storage in iron canisters of 200 litres in volume (of. storage, transport - loading - being damaged - accidents), thus the risk of accidents possibly involving radiation contamination significantly decreases.

Claims

1. A process for the treatment of low and intermediate level solid radioactive waste, characterized by the steps of collecting low and intermediate level solid radioactive waste to be treated, mixing the collected solid waste with glass-forming additives, comminuting and then homogenizing said mixture, and feeding the thus obtained comminuted and homogenized mixture into a molten and swirling material (M) with a melt level (ML) arranged in a reaction space (R), wherein said feeding is performed under pressure and below the melt level (ML) of the mol ten material (M).

2. The process according to claim 1 , characterized in that said feeding into the molten material (M) is carried out by means of a controlled rate screw feeder (I-3).

3. The process according to claim 1 or 2, characterized in that swirling of the molten material (M) is induced and maintained by at least one transferred arc plasma torch (P-1) arranged at the circumference of the reaction space (R).

4. The process according to any one of claims 1 to 3, characterized in that the temperature of the molten material (M) is maintained between 1800 and 2000 degrees Celsius.

5. The process according to any one of claims 1 to 4, characterized by fur ther steps of gasifying organic matter content of the waste to be treated by con tacting said waste with the molten material (M), and mixing the thus obtained gaseous phase substance with water vapour in the reaction space (R), thereby converting said gaseous phase substance at least partially to a CO/H₂ mixture be fore said gaseous phase substance leaves the reaction space (R).

6. The process according to claim 5, characterized in that water vapour is introduced into the reaction space (R) by at least one water vapour blown arc plasma torch (P-2) arranged at the circumference of the upper section (R-3) of the reaction space (R).

7. The process according to claim 5 or 6, characterized by using the at least one plasma torch (P-2) to blow water vapour into the reaction space (R) in a direc- tion tangential to the reaction space (R), thereby bringing said CO/H₂ mixture into swirling and separating solid particles from said mixture by making the solid parti cles to collide into a mantle delimiting the reaction space (R).

8. The process according to any one of claims 5 to 7, characterized by filter ing and washing the CO/H₂ mixture leaving the reaction space (R), thereby pro- ducing a purified CO/H₂ mixture substantially free of solid particles suitable for fur ther use.

9. The process according to any one of claims 5 to 8, characterized in that filtering and washing of the CO/H₂ mixture is carried out by passing the CO/H₂ mixture through a filter unit comprising a coarse ceramic filter (F-1), a fine ceramic filter (F-2) and an water jet washer (F-3) arranged one after the other in the given order in a flow direction of said CO/FI₂ mixture, wherein upon reaching a predeter mined clogging level, said ceramic filters are replaced, the spoilt ceramic filters are added to the collected solid waste to be treated and processed together with the waste, and wherein a condensed sludge from the water jet washer (F-3) is recy- cled to the reaction space (R) through the at least one water vapour plasma torch (P-2).

10. The process according to any one of claims 1 to 9, characterized by fur ther steps of contacting the waste to be treated with the molten material (M), thereby bringing inorganic matter content and radioactive contaminant content of said waste into melt phase, tapping molten material (M) with a predefined amount of radioactive contaminant from the upper part of the molten material (M) in the re action space (R), pouring the tapped molten material into at least one mould, so lidifying the tapped molten material and, optionally, transporting the obtained so lidified material to a landfill.

11. The process according to any one of claims 1 to 10, further comprising the step of removing at least a part of the radioactive contaminants from the reac tion space (R) from time to time by tapping molten material rich in radioactive con taminants from the lowest part of the molten material (M).

12. The process according to any one of claims 1 to 11, further comprising collecting said low and intermediate level solid radioactive waste to be treated in polyethylene bags.

13. The process according to any one of claims 3 to 12, characterized in that at least three transferred arc plasma torches (P-1) are used.

14. The process according to any one of claims 6 to 13, characterized in that at least three water vapour blown arc plasma torches (P-2) are used.