Development and evaluation of synthesized adsorbents for removal and immobilization of PWR primary corrosion products

Abstract

Radioactive corrosion products such as $^{60}Co$, $^{58}Co$, $^{64}Cu$, $^{51}Cr$, $^{54}Mn$, $^{59}Ni$, $^{59}Fe$, $^{65}Zn$ are released into primary coolant due to corrosion of steam generator and other structural materials of the nuclear power reactors and get activated under neutron flux. The radioactive corrosion products deposit on the primary coolant systems and on the fuel rod surfaces. They provide an intense occupational radiation exposure to the workers, affects the heat transfer efficiency and can also lead to the fuel failure.

The purpose of this study is to investigate the removal of activated corrosion products by organic (EDTA-functionalized mesoporous silica) and inorganic (hydroxyapatite) solid based adsorbents and their subsequent conversion into a dense monolith by pressure-less sintering without sintering aids. Such a process could be a convenient method for the treatment of radioactively contaminated aqueous solutions coming from the primary coolant systems, decommissioning activities, and research installations.

Ethylenediaminetetraacetic acid functionalized mesoporous silica (EDTA-SBA-15) was synthesized by co-precipitation method. It was then used to efficiently remove corrosion products generated in nuclear reactor coolants. The un-functionalized SBA-15 and EDTA-functionalized SBA-15 showed the equilibrium sorption capacity for Co(II), Cu(II) and Ni(II) up to 0.1 mmol/g and 1.4 mmol/g, respectively. The study further investigated the removal of metal ions (Co(II), Cr(III), Cu(II), Mn(II), Ni(II), Fe(II), and Zn(II)) from their mixture. EDTA-SBA-15 had a good stability over a wide pH range from 1 to 7, in concentrated ionic media and under gamma irradiations (1 kGy).

To evaluate the final fate of the captured corrosions and to reduce the secondary waste volume, calcium hydroxyapatite (HA) was introduced due to its unique properties of acting like an adsorbent and binding matrix for final disposal. HA was synthesized and applied to remove individual and mixtures of corrosion products from aqueous solutions. Results showed more than 95 % removal of corrosion products in both cases. The direct solidification of adsorbed corrosion products was achieved by pressure-less conventional sintering. The sintered matrix had a good hardness (3.62 ± 0.32 GPa) and a relative sintered density > 97% after heat treatment at 1150 $^\circ$C. The measured compressive strength (207.3 ± 9.5 MPa) was significantly higher than the established waste immobilization criteria of the US (3.5 MPa) and Russia (4.9 MPa). Standard product consistency test and the standard ANS/ANSI leaching test were performed for final matrix’s durability testing. The CRUD consolidated matrix had a normalized leaching rate ranging from 3.4⨯$10^{-2}$ to 5.3⨯$10^{-6} g/m^2/day$ and the leaching index was greater than 7 for all the metal ions (which is higher than usual acceptable value of 7). Physico-mechanical properties were measured after 90 days leaching test and results were very close to pre leaching test values showing no swelling and physical degradation. Moreover, the sorption and immobilization process involved no additional chemical treatments, additives (gypsum, slaked lime, sodium silicate, etc.), and sophisticated equipment. Therefore, the proposed waste management route has no adverse effects on the ecosystem and can be highly efficient to immobilize adsorbed waste and reduce the secondary waste volume.

Finally, the CRUD captured EDTA and CRUD captured HA adsorbents were mixed together and optimized to transform them into a solid monolith. The results showed good agreement of physico-mechanical properties, chemical stability, and the leaching resistance with CRUD-HA matrix.