WO2019220001A1

WO2019220001A1 - Method of treating liquid radioactive waste containing boron

Info

Publication number: WO2019220001A1
Application number: PCT/FI2018/050374
Authority: WO
Inventors: Jussi-Matti MÄKI; Heikki Leinonen
Original assignee: Fortum Power And Heat Oy
Priority date: 2018-05-18
Filing date: 2018-05-18
Publication date: 2019-11-21

Abstract

A method of recovering radiochemically clean boron from aqueous liquid radioactive waste which contains radionuclides and boron. The method comprises contacting the liquid waste with at least one ion exchange material (5a, 5b, 6a, 6b, 7a, 7b) in order selectively to remove radioactive nuclides to produce essentially nuclide-free aqueous waste; recovering the waste water thus obtained; removing water from the aqueous waste at a temperature of at least 30 °C to provide an aqueous solution in which boron is primarily present in the form of boric acid dissolved in water; and lowering the temperature of the water to 25 °C or lower so as to precipitate boric acid to form a dispersion of solid boric acid in water. The dispersion thus obtained is subjected to a separation operation and the solid phase comprising radiochemically clean boron is recovered in the form of a solid. A liquid phase comprising radiochemically clean water is also recovered..

Description

Method of treating liquid radioactive waste containing boron

Field of Invention

The present invention relates to purification of liquid radioactive waste. In particular, the present invention concerns a method of recovering radiochemically clean boron from aqueous liquid radioactive waste which contains radionuclides (hereafter nuclides) and boron.

Background

Pressurized water reactors (PWR), including Russian-type VVER-reactors, use boron to control reactivity. Some nuclear power plants (NPPs) have strict release limits for boron releases to waterways, especially rivers. European Chemicals Agency’s (ECHA)

Registeration, Evauluation, Authorisation and Restriction of Chemicals -regulation (REACH) has included boric acid as a candidate in the“substance of very high concern” -list. ECHA is discussing to set boron limits in REACH to PNEC“probable no effect concentration” (PNEC) limits of less than 10 Bq/kg. Thus, REACH is expected to increase the pressure to minimise the boron releases to waterways even further.

Fortum’s NURES® process for treatment of liquid radioactive waste, and the inorganic selective ion exchangers (IX) CsTreat®, SrTreat®, and CoTreat® have been successfully used at several nuclear sites for the past 25 years. Use cases have been demanding, either due to high salt concentration (evaporator concentrates, reprocessing liquids, and reverse osmosis (RO) rejects) and/or due to high decontamination factor (DF) requirement.

A combined cesium and boron removal system for treating evaporator concentrates, is disclosed by E. Tusa et al. The boron recovery process consists of following steps: pH adjustment of effluent with NaOH or other caustic substance to a value in the pH range for high solubility of boron (boric acid or sodium borate); evaporation to increase boron concentration; addition of acid to achieve crystallization of boron in a crystallizer tank; and finally filtration with a pressure filter. As reported, the combined cesium and boron removal system is capable of removing at least 70 % of boron as sodium borate from the evaporator concentrates. The recovered boron is a radiochemically clean and dry alkaline borate.

However, the treated evaporator concentrate, still contains a fraction of the original boron - upto 30% percentage of the process in toto, filtrate from the pressure filter needs to be retreated or recycled back to evaporation. Furthermore, the amounts of alkaline agents used are large and extensive processing and additional cations may influence the operation of the cesium ion exchanger. The use of alkaline agents increases the liquid volumes that need to be treated.

Summary of the Invention

There is a need for eliminating at least a part of the problems relating to the art and to provide an improved process for treating liquid radioactive waste so as to produce radiochemically clean boron in solid form by filtration and radiochemically.

Accordingly, it is an aim of the present invention to achieve a combined radionuclide and boron removal process is used to recover boric acid or alkaline borate as radiochemically clean crystalline boron (boric acid or alkaline borate cake).

The present method comprises the steps of

- contacting the liquid waste with at least one ion exchange material in order

selectively to remove radioactive radionuclides to produce essentially radionuclide- free (non-radioactive) aqueous waste;

- recovering the waste water thus obtained;

- removing water from the aqueous waste at a temperature of at least 30 °C to

provide an aqueous solution in which boron is primarily present in the form of boric acid dissolved in water, said boric acid having a concentration greater than its maximum solubility at 25 °C;

- lowering the temperature of the water, under agitation, to 25 °C or lower so as to precipitate boric acid to form a dispersion of solid boric acid in water;

- subjecting the dispersion thus obtained to a separation operation; and

- recovering a solid phase comprising radiochemically clean boron in the form of solid particles and a liquid phase comprising radiochemically clean water. More specifically, the present invention is characterized by what is stated in the characterizing part of claim 1.

The invention provides considerable advantages.

The present method represents an efficient way of removing boric acid as radiochemically clean, in particular crystalline boron particles, for example in the form of a filtration cake. The method can be used to remove boron from both demineralized system waters and also from evaporator concentrates. Boron can be removed as part of NPP’s normal operation or during decommissioning, when the whole boron inventory (amounting to tens of tons) has to be released in a short time period.

It is expected that many plants will have difficulties in complying with the future EU’s REACH regulation if direct boron release to waterways is sought. As demonstrated by the examples below, very high decontamination factors for boron can be achieved using a cost efficient, simple, and robust process. In particular, by the invention, it becomes possible to recover PWR’s boron as radiochemically clean crystalline filtration cake that can be disposed of in conventional facilities with minimal secondary waste.

In summary, by means of the invention, boron removal from liquid waste can be increased to at least 90 %, in particular up to 99 % or even more. The volumes of treated water can be efficiently reduced. Boron can be retrieved in the form of a pure, crystalline substance which has many uses.

Next, embodiments will be examined in more detail with reference to the attached drawing which, by means of a process scheme shows the principal stages of one configuration according to the present technology. Embodiments

A basic flow diagram of an embodiment of the present process is shown in the figure.

The main components the presented embodiment are the following:

- an ion exchange system of sequentially arranged inorganic ion exchangers for selective absorption of radionuclides (reference numerals 1, 2, 3, 4, 5a, 5b, 6a, 6b, 7a, 7b and 8)

- an evaporator section (reference numerals 11 and 12),

- a crystallizer 15, and

- a pressure filter 18 with a filtrate tank 19.

In the drawings, a number of pumps are also depicted. They have been given the reference numerals 9, 10, 13, 14, 16 and 17.

The treatment method comprises typically the following steps:

1. Nuclide removal pre-treatment step. Selective nuclide removal is conducted in order to purify the boric acid solution. Liquid waste is fed, typically from a storage tank, into a feed tank 1 which is provided with a mixer. From the mixed tank 1 , the liquid waste is conducted by pumping 9 through at least one, typically two sequentially arranged prefilters 3, 4 for removing suspended solids and similar components which may influence the absorption in the ion exchangers. The effluent of the prefilters is then conducted to a first ion exchange section or“zone”, for example comprising two column s 5a, 5b, which can be operated in parallel or typically and preferably one at the time, one column allowing for regeneration or replacement of ion exchange proper while the other is being run with waste liquid.

There can be several ion exchange zones, the drawing shows three sets of ion exchange columns, viz. 5a, 5b; 6a, 6b; and 7a, 7b, making up for three different ion exchange zones.

The ion exchange resin system can be, for example, of the NURES® type, applicable for removal of radioactivity to a very low level so that the end product (crystalline boric acid) can be released from radiological control. Thus, in one embodiment, NURES® uses selective ion exchangers are used to achieve high decontamination factor and to minimize secondary waste volume. In one preferred embodiment, at least some, preferably all of the ion exchangers (5a, 5b;

6a, 6b; and 7a, 7b) are used in once-through column operation.

Typical radionuclides that have to be removed include Cs-l37, Sb-l24 & 125, and Co-60. Other gamma-emitting corrosion and fission products, such as Mn-54 and Ag-l lOm, are also expected to be encountered, albeit at lower activities.

When fuel pool and system waters, such as waters from primary or secondary circulations, are treated, Co and other transition metal fission/corrosion products are typically in ionic form. Co and other transition metal radionuclides are expected to be bound to complexing agents in evaporator concentrates.

In one embodiment, oxidation of complexing agents, such as EDTA and oxalic acid, is used for improving efficiency of IX to be effective when treating evaporator concentrates. It is the aim that after the NURES® treatment radioactivity is below the detection limit for all the radionuclides.

As can be seen from the drawing, the effluent from the first ion exchange zone (5 a, 5b) can be recirculated through a second feed tank 2 in which components influencing ion exchange can be added.

After the section of 1, 2, 3 or more ion exchange zones, the treated, non-radioactive waste liquid can be gathered in an intermediate storage tank 8, from which it can be fed using a pump 17 further to the next section. 2. The next step is the concentrating step. This is a step wherein water is removed by a physical operation, such as evaporation or membrane separation. In one embodiment, the evaporation is carried out at above room temperature, in particular at ambient pressure. An evaporator 11 can be used to increase boric acid concentration close to the solubility limit of boric acid in water, i.e. to 200 g/l at 100 °C or more. The boron acid is obtained as a bottoms product of the evaporator 11 and pumped to crystallization.

In another embodiment, reverse osmosis (RO) is used to increase boric acid concentration above the solubility at room temperature.

High temperature-high pressure reverse osmosis (RO) or membrane distillation methods (for example osmotic distillation) can also be used to increase the boric acid concentration.

In one embodiment, when reverse osmosis is used, the temperature of the feed has to be raised above 25 °C, in particular to at least 30 °C or preferably to at least 40 °C elevated in order to keep the boric acid soluble.

Distillate 12 from an evaporator will contain some ppms of boric acid. In case of reverse osmosis, the permeate might contain even grams of boric acid if treatment is conducted at low pH.

Thus the distillate/permeate has to be processed further if very low boric acid

concentration (< 10 Bq/kg) is required. Possible further treatment steps can comprise e.g. additional RO steps or additional evaporation or combinations thereof.

The pH can be elevated before the concentrating step to further increase boric acid solubility with alkaline agents, such as NaOH or KOH. Thus, in one embodiment, the pH of the nuclide-free aqueous waste obtained from removal of radioactive nuclides is increased to at least 7, for example to at least 9, in particular to at least 12, by the addition of an alkaline agents, in particular an alkaline agent selected from the group of alkali metal and alkaline earth metal hydroxides and carbonates and combinations thereof.

However, in one embodiment, the nuclide-free aqueous waste obtained from removal of radioactive nuclides is subjected to removal of water without adjustment of pH, for example the aqueous waste is subjected to removal of water as such, even without any modification. 3. Crystallization step. After evaporation the boric acid is cooled down in controlled manner in a crystallizer 15, resulting in boric acid slurry. The crystallizer tank 15 preferably uses a scraper to remove crystallized boric acid from crystallizer walls.

In one embodiment, boric acid solution is cooled to room temperature or below room temperature in a crystallization vessel. After cooling pH is lowered if solution pH was in the soluble region of pH=7 or rH>12 during the concentrating step.

In a preferred embodiment, agitation is used for avoiding boron deposition on surfaces.

4. In the last step, the slurry comprising the bottoms product of the crystallizer 15 is pumped 16 to a separation operation or a combination of separation operations for separating the crystallized, precipitated matter formed by the boric precipitate from the liquid.

In one embodiment, the boric acid slurry is filtered using e.g. pressure filter 18. Boric acid or alkaline borate is washed, compressed and dewatered and optionally dried during the filtration. In another embodiment the solid matter is separated by centrifugation and then further dewater or dried or both. Combinations of filtration and centrifugations can be employed as well.

Once the crystals are removed from the supernatant, they do not have a tendency to reattach onto the walls.. The resulting boric acid cake can be re-used or disposed of as conventional, non-radioactive waste.

In one embodiment, filtrate gathered in filtrate tank 19 from the pressure filter 18 or centrifugation (containing ~40 g/L of boric acid) is recycled for use in the treatment or recovered or discarded. In a preferred embodiment, the boric acid containing water is recycled. In one embodiment, the ratio of recycled aqueous liquid to fresh feed is about 1 :100 to 100:1, for example 1 :10 to 10:1.

The end result of the above described process is crystalline radiochemically clean boric acid or sodium borate that can be released from radiological control. The following non-limiting examples illustrate the present technology.

The process was demonstrated in a pilot in Germany using actual PWR boron water. The plant was recently shut down and is currently being decommissioned. The aim of the pilot was to recover a batch of solid boric acid and to demonstrate that the obtained boric acid is radiochemically clean.

The pilot contained all the major components of the full system: NURES® ion exchanger system including cobalt and cesium selective ion exchangers CsTreat® and CoTreat®, evaporator, crystallizator, filtration. The pilot was successful and crystalline boron cake was nearly free of gamma-emitting radionuclides and the activity was below exemption limit. Additional ion exchanger tests were conducted later in laboratory in Helsinki in order to further optimize the ion exchange (“IX”) process and to obtain additional Sb removal.

NPP’s boric acid concentrate was used as feed liquid in the tests. The concentrate contained liquid from primary circuit, thus it is the most active system liquid at the plants.

The radiochemical composition of the feed liquid is shown in Table 1. As will appear, Sb- 125 was the most prevalent radionuclide in this solution. In some other systems in this plant Co-60 activity was slightly higher and Sb-l25 activity much lower.

Chemically the solution was a typical PWR system water, containing 40 roughly g/L boric acid and some ppms of Li (from LiOH addition) and silica. Other impurities are present in trace concentrations. pH of the solution was elevated to 7 using NaOH. pH adjustment is a case of optimization, as pH close neutral pH is required for optimal Co abatement.

However adding NaOH will introduce competing ions which will lower IX performance and will produce more secondary waste. Test showed that optimal pH is below 7.

Activity after the IX process is also shown in Table 1.

Most of the radionuclides are below detection limit after IX. In the second sample (out of three) there was 1.9E+03 Bq/m3 of Co-60 present. However, it is possible that this measurement was an artefact as the two other samples contained no Co-60 activity. There was only minimal Sb-l24 and Sb-l25 abatement. Sb-l24 and -125 are typically in oxyanionic species and their ion exchange is most effective at low pH. Sb species do not typically show affinity for the neutral/anionic boric acid/sodium borate. Therefore Sb abatement is not as critical as removal of cationic Co-60, for example. Despite this it was agreed that more efficient Sb abatement shall be demonstrated in laboratory later on as removing all the gamma-emitting radionuclides simplifies actual treatment from

radioprotection and licensing point of view.

Table 1. Radiochemical properties of the feed liquid

After the IX treatment, 25 L of liquid was evaporated to 5 L and the solution was allowed to crystallize overnight. The resulting slurry was filtered next day using a laboratory pressure filter unit. The filtering cycle is following: slurry is fed into the filter chamber and excess water is removed by pressing with filter’s diaphragm with > 10 bar pressure. After this cake is washed by feeding clean water inside the chamber and the cake is pressed again. Finally, the cake is dried using compressed air.

Two filtrations were conducted and total 500 g of boric acid was recovered. In the second filtration too much wash water was fed into the chamber and channelling occurred, leaving the cake slightly damp. This was due to the fact that it was not possible to do proper process control with the non-automated lab pressure filter. Despite this the results were good, as shown in Table 2 and the recovered boric acid could be released from radiological control. In the second cake there was some Co-60 activity present (4 Bq/kg), whereas Sb- 124 and Sb-l25 activity was slightly higher in the first cake. H-3 activity was the biggest contributor to the total activity (requirement is that åi Ai/FRi < 1 , where Ai is the specific activity for a nuclide and FRi is the corresponding free release limit).

There may be some H-3 present in the boric acid due to the reversible ion exchange taking place between water molecules and boric acid. Additionally, sodium borate will frequently contain some crystal water (i.e. also tritiated water) and is formed already at pH=7.

Conducting the crystallization at slightly lower pH will reduce H-3 activity in the recovered boric acid.

Table 2. Radiochemical properties of the recovered crystalline boric acid and the German free release limits for corresponding nuclides

Additional laboratory tests

An additional long-term laboratory test was also conducted later on at the plant to verify that IX could be conducted at lower pH and also to test the treatment capacity of CsTreat® and SrTreat®.

The test was conducted at pH=6 in order to minimize sodium borate formation. Co-60 activity was slightly higher in the feed (close to MBq/m3) compared to original tests. All the gamma-emitting radionuclides expect for Sb-l24 and Sb-l25 (Co-60, Cs-l37, Ag- 1 lOm, Te-l23m) were below detection limit for the duration of tests, 4000 ion exchanger bed volumes.

Additional laboratory tests were conducted in Finland in order to demonstrate the achieve Sb abatement. Two different Sb selective ion exchangers were tested: Fortum’s own SbTreat and another commercially available ion exchanger. Both IX materials are inorganic and oxide-based. Sb-l24 and -125 activity in the test solution was close to MBq/m³. Batch tests were used to find the optimal pH for the treatment. Best results were achieved at low pH. Both SbTreat and the Japanese material removed all Sb from the solution, indicating that the treatment capacity is very high. Column test using the Japanese material was conducted to verify that column performance was as good as the batch test indicted. Sb-l24 and -125 activity were below detection limit for the duration of the tests, 1000 ion exchanger bed volumes. Thus, it was demonstrated that all the gamma-emitting

radionuclides could be removed from the test solution.

Reference numerals

1, 2, 8 Feed tank

3, 4 Prefilter

5a, 5b Ion exchange column

6a, 6b Ion exchange column

7a, 7b Ion exchange column

11 Evaporator

12 Overhead tank

15 Crystallizer

18 Pressure filter

19 Filtrate tank

9, 10 Pump

13, 14 Pump

16, 17 Pump

List of references

Non-Patent Literature

E. Tusa, E. Mattila and B. Szabo,“New System Installed for Nuclide Removal and Boron Recovery at the Paks NPP in Hungary,” in Waste Management 2004 proceedings, Tucson, 2004.

Claims

1. A method of recovering radiochemically clean boron from aqueous liquid radioactive waste which contains nuclides and boron, comprising the steps of

- contacting the liquid waste with at least one ion exchange material in order

selectively to remove radioactive nuclides to produce essentially nuclide-free aqueous waste;

- recovering the waste water thus obtained;

- removing water from the aqueous waste at a temperature of at least 30 °C to

- subjecting the dispersion thus obtained to a separation operation; and

- recovering a solid phase comprising radiochemically clean boron in the form of solid particles and a liquid phase comprising radiochemically clean water.

2. The method according to claim 1, wherein the step of removing water from the aqueous waste comprises a method selected from the group of evaporation and membrane separation, such as reverse osmosis, or a combination thereof

3. The method according to claim 1 or 2, wherein removing water from the aqueous waste comprises evaporating water at ambient pressure and a temperature of 30 to 100 °C, in particular 40 to 100 °C, for example 50 to 100 °C.

4. The method according to any of the preceding claims, wherein 10 to 90 %, in particular 20 to 85 %, preferably at least 50 %, for example at least 75 % by weight of the water of the aqueous waste is removed.

5. The method according to any of the preceding claims, comprising removing water from waste water having a pH in the range of 4 to 6.

6. The method according to any of the preceding claims, wherein the nuclide-free aqueous waste obtained from removal of radioactive nuclides is subjected to removal of water without adjustment of pH.

7. The method according to any of claims 1 to 5, wherein the pH of the nuclide-free aqueous waste obtained from removal of radioactive nuclides is increased to at least 7, for example to at least 9, in particular to at least 12, by the addition of an alkaline agents, in particular an alkaline agent selected from the group of alkali metal and alkaline earth metal hydroxides and carbonates and combinations thereof.

8. The method according to any of the preceding claims, wherein the dispersion is subjected to filtration so as to recover the boron as a filter cake.

9. The method according to claim 8, wherein filtration is carried out using a pressure filter, in particular using a rotary filter drum.

10. The method according to any of the preceding claims, wherein the boron is recovered as crystalline, radiochemically clean boric acid or sodium borate.

11. The method according to any of the preceding claims, wherein the boron recovered has a maximum content of radioactive nuclides of less than 10 Bq/kg.

12. The method according to any of the preceding claims, wherein the liquid phase recovered after filtration has a maximum content of radioactive nuclides of less than 10 Bq/kg.

13. The method according to claim 11 or 12, wherein the boron and the liquid phase recovered has a maximum content of radioactive nuclides of less than 10 Bq/kg, said maximum content being the total content of radioactive nuclides selected from the group of Cs-l37, Sr-90, Sb-l24, Sb-l25, Co-60, Mn-54 and Ag-l lOm and mixtures thereof.

14. The method according to any of the preceding claims, wherein the aqueous liquid radioactive waste being contacted sequentially with at least two inorganic ion exchangers, each ion exchanger being selective for one of Cs-l37, Sr-90, Sb-l24, Sb-l25 and Co-60.

15. The method according to claim 14, wherein the aqueous liquid radioactive waste being contacted sequentially with at least two, preferably three inorganic ion exchangers, at least one, preferably two, in particular three, being used in once-through column operation.

16. The method according to any of the preceding claims, wherein the total content of at least one, preferably two or several, in particular all of the radionuclides selected from the group of Cs-l37, Sr-90, Sb-l24, Sb-l25, Co-60, Mn-54 and Ag-l lOm is below detection limit.

17. The method according to any of the preceding claims, wherein the boron content of the liquid waste is at least 10 g/l, in particular about 15 to 60 g/l.

18. The method according to any of the preceding claims, comprising recycling at least part of the radiochemically clean water to the aqueous liquid radioactive waste which contains nuclides and boron.