Radioactive Waste: Can Membranes Reduce The Fear Factor?

It’s no wonder that the word "radioactive" scares people. I remember Chernobyl. I was raised on The Simpsons, with its bumbling nuclear plant operators and three-eyed fish. I've followed the Fukushima tragedy closely for Water Online. But, scary or not, nuclear power isn't going anywhere soon — a fact that points to the need for enhanced safety and environmental responsibility.

At the recently wrapped International Atomic Energy Agency (IAEA) conference, IAEA director-general Yukiya Amano announced the "continued growth in the use of nuclear power," noting that there are 437 nuclear power reactors currently operating in 30 countries, and 70 more reactors under construction. Thirty-three countries, he said, are considering, planning, or starting nuclear programs. Such activity produces a lot of liquid radioactive waste (LRW), but nuclear plants aren't the only source.

Beyond what is produced for fuel supply — from the purification and enrichment of uranium, the operation of power reactors, within spent fuel reprocessing, or even during the decommissioning of nuclear facilities — there is also institutional waste to deal with. Institutional LRW arises from the production and use of radioisotopes in medicine, research, industry, and agriculture.

How best to treat liquid radioactive waste

According to an expert in the field, Dr. Grażyna Zakrzewska-Trznadel of the Institute of Nuclear Chemistry and Technology in Warszawa, Poland, advances made in membrane treatment can "supersede many non-effective, out-of-date methods," such as ion exchange/sorption, chemical precipitation, and/or evaporation. Membranes have some inherent drawbacks, but they also have more potential, and the drawbacks can be managed.

Membranes applied for the treatment of radioactive solutions are subject to scaling and fouling that reduce the permeate flux. The membranes need to be periodically cleaned — as with other applications — but they should also be designed and operated in a way that minimizes detrimental phenomena. That’s because the consequences of these disadvantages are more severe than in conventional installations; the sustained fouling in the systems used for radioactive waste processing can result in a huge rise of radioactivity and may cause a hazard for operating staff, Zakrzewska-Trznadel explains. Membranes exposed to ionizing radiation may change their separation and permeation characteristics, especially membranes made from polymers, which can undergo various structural changes.

To combat these potential issues, membrane resistivity and stability should be tested before selecting the membrane for a specific application. The resistivity problems are avoided by the use of inorganic membranes, such as ceramic or metallic barriers. They’re not only resistant to an aggressive chemical environment, but they can withstand “all kinds of radiation,” says Zakrzewska-Trznadel. Commercial membranes can be modified in the laboratory to change their permeation behavior and separation characteristics. The modifications may include combining different materials, introducing functional groups, or incorporating additives into the membrane matrix.

Solid, liquid, and gaseous wastes are classified as low-level, intermediate-level, or high-level radioactive waste. Low- and intermediate-level LRW produced in nuclear facilities, laboratories, and hospitals create a particular problem because of large volumes of effluent with low levels of radioactive materials. Membranes, meanwhile, offer large-volume reduction and high-decontamination factors as key benefits. The third major benefit offered by membranes is low energy consumption (relative to competing technologies), which is always an important consideration. Zakrzewska-Trznadel cites additional advantages: Membranes can operate under moderate temperature and pressure conditions, they are flexible and easily scaled-up, and they can be simply combined with other treatment methods.

A deeper dive into the various membrane technologies indicates why Zakrzewska-Trznadel sees membranes as the future of LRW treatment. She gives the following analysis of research conducted for each membrane type, as prepared for her paper “Advances in membrane technologies for the treatment of liquid radioactive waste,” supported by performance results from example applications.

Reverse osmosis

Laboratory and pilot plant experiments carried out at the Institute of Nuclear Chemistry and Technology (INCT) showed that reverse osmosis (RO) can be useful for the treatment of liquid low-level radioactive institutional wastes. The application of membranes at the beginning of the cleaning cycle reduced significantly the energy consumption of the whole system.

Example: The RO method developed by INCT has been implemented at the Radioactive Waste Management Plant (RWMP) in Swierk, Poland.  It is a three-stage RO system: The first two stages are used for decontamination to produce the effluent that, after radiometric control, can be safely released into the sewage system. The third stage is applied for the volume reduction by concentrating retentate obtained from the first two stages. Spiral-wound RO modules were used in the installation.

Nanofiltration

The nanofiltration (NF) process allows for the separating of monovalent ions from multivalent ions, which are retained by the membrane with pores in the 0.001 to 0.01 μm (micrometers) size range (microns [μ] are a familiar equivalent to micrometers). The process can be used for separation of organic compounds of moderate molecular weight from monovalent ions present in the solution. The well-known application of NF in the nuclear industry is boric acid recovery from contaminated cooling water in nuclear reactor.

Example: Australian Nuclear Sciences and Technology Organisation (ANSTO) tested different commercial nanofiltration membranes in a cross-flow membrane cell for the treatment of uranium mill effluents. The rejection of uranium was greater than 75 percent. Some of the tested membranes showed potential for separation of radium, sulphate, and manganese.

Ultrafiltration

Ultrafiltration (UF) uses membranes of pore sizes between 0.001 and 0.1 μm. In such a case, dissolved compounds pass through the membrane, while colloid and suspended matters are rejected. In the nuclear industry, UF can be used for removal of all substances that are present in radioactive waste in colloidal or suspended form. UF can be also applied as a pretreatment stage before reverse osmosis. Very often UF is combined with sorption, precipitation, or complexation in one hybrid process of enhanced ultrafiltration. Small ions bound by macromolecular agents form complexes, which can be retained by UF membranes. Radioactive cations can be also separated in the precipitation process, forming less soluble particles (carbonates, phosphates, and oxalates or hydroxides), which can be later retained by the UF membrane. The hybrid complexation-UF or precipitation-UF methods are effectively applied in several plants processing alpha[α]-bearing radioactive waste streams.

Example: Waste originating from floor drains and equipment drain tanks, as well as from the reactor coolant system, was processed with UF at the Callaway Nuclear Generating Station in the U.S. (near Fulton, MO). Four UF modules were used to treat reactor coolant water; 70 percent of radioactivity and suspended solids were removed with UF, and permeate was additionally polished with ion exchange to the minimum detectable level.

Microfiltration

Microfiltration (MF) membranes reject bigger particles and macromolecules with size of 0.1 to 1 μm. In nuclear technology, the process is used either for pretreatment purposes or for concentration of coarse particles after precipitation process. For high-level radioactive wastes, ceramic filters are applied to handle the high decontamination and concentration factors for certain effluents.

Example: At the Idaho Nuclear Technology and Engineering Center (INTEC) that belongs to the Idaho National Engineering and Environmental Laboratory (INEEL), cross-flow microfiltration for the remediation of radioactive waste was developed. Solids formed from precipitation and absorption of radioactive substances during reprocessing of nuclear fuel required the separation from the liquid before solvent extraction and ion exchange. Cross-flow filtration with the sintered Hastelloy filter (from Mott Corporation) was tested. The filter specifications included a 0.480-in. inner diameter, 6-in. length, and 0.5-μm pore diameter. Its performance was tested for different solids loading from 0.19 to 7.94 weight percent (wt%). Filtrate flux rates for each solid loading displayed a high dependence on transmembrane pressure and negative dependency on axial velocity. Cross-flow filtration seemed to be a viable method for removal of undissolved solids from INEEL radioactive slurries.

Electric membrane processes

The main electric membrane processes, in which the electric potential is the driving force, are electroosmosis, electrodialysis, and membrane electrolysis. They are carried on with the use of ion-exchange membranes, and have been tested in the context of application for nuclear technologies. Electroosmosis was used for dewatering sludge after filtration or gravitational sedimentation [26]. With this method, high retention factors with minimal membrane fouling were achieved. Electroosmosis was accompanied by electrophoretic transport that reduced fouling and prevented small charged particles from passing through the membrane. The result was a 99.99 percent retention factor, while the volume reduction factors were lower due to the reduction in transport velocity with increasing conductivity of the liquid.

Example: Sodium super ion conducting ceramic membranes — NaSICON — were studied for separation of sodium from radioactive wastes produced by the U.S. Department of Energy [52]. Comparative studies of Nafion and NaSICON membranes used for the electrochemical separation of sodium ions from radioactive waste [53] were conducted. Experiments demonstrated the advantages of ceramic membranes, such as resistance to radiation, negligible electroosmotic transport of water, and lack of cesium-137 (¹³⁷Cs) transport. In the case of Nafion membranes, about 60 percent of ¹³⁷Cs passed from the anolyte to the catholyte. AFN (organic, fouling-resistant) ion-exchange membranes were tested in Korea for the recovery of boric acid from wastewater [54]. Their use allowed for the 87 percent reduction of the volume of the concentrate. Ion-exchange membranes obtained by radiation grafting of polyacrylic acid on a matrix of polyethylene and Teflon were tested in Egypt in terms of removal of zirconium from solutions containing uranium.

Liquid membranes

Liquid membranes employ membrane contactors to form supported liquid membranes. Liquid membrane processes are in a relatively early stage of development, although they have already found several potential applications in the mineral mining industry and have become an interesting field of exploration for nuclear technologies because of their good separation ability toward specific radionuclides. Liquid membrane processes include techniques such as bulk liquid membranes (BLMs), emulsion liquid membranes (ELMs), and supported liquid membranes (SLMs). High selectivity is achieved in facilitated transport by introducing the carrier of high affinity to one of separated components.

Example: Supported liquid membranes, also called immobilized liquid membranes, have been used in a number of applications, including uranium mining and the removal of uranium from seawater. In SLM, treated and stripping phases are separated by a porous barrier, with an organic phase inside the pores. In order to increase the mass transport rate between the phases, the carrier, which complexes one of separated species, is introduced to the organic phase. Such a facilitated transport produces not only fast transport rates, but also results in higher selectivity of the process. SLMs were studied in the nuclear industry for radioactive waste treatment and the removal of actinides and fission products from the waste after reprocessing of nuclear fuel. Radionuclides such as cesium, strontium, cerium and europium were separated from radioactive solutions to reduce the radiotoxicity of the waste. Uranium, plutonium, and americium produced in the back-end of fuel cycle were removed from the wastes.