ICONE15-10492 The INET's progress of silica gel adsorption for zirconium

Abstract

A High Level Liquid Waste (HLLW) partition process, trialkyl phosphine oxides (TRPO) process has been developed by INET (Institute of Nuclear and new Energy Technology) of Tsinghua University in China to recover the Minor Actinides (MA) and Long-Lived Fission Products (LLFP) since 1980's. The feasibility of TRPO process was proved by hot tests and Pilot-plant scale experiment. HLLW liquid waste arising from spent nuclear fuel reprocessing process contains zirconium, which will deteriorate MAs and LLFPs' recovery from HLLW. Therefore, as first step 0f the TRPO process, removal of Zr is studied. In INET, studies on the adsorption of silica gel for Zr have been carried out. In this paper the progress of silica gel adsorption is reviewed and a technical process is introduced. Purchased silica gel was used to study the adsorption behavior of Zr and other nuclides in HLLW. The result of static adsorption shows that the static capacity of Zr on silica gel is about 20 mg Zr/g in 1.0〜4.0 mol/L HNO_3 solution; the adsorption distribution coefficient of Pu(IV) is about 0.7〜1.4 ml/g; U and Np are not adsorbed on silica gel; among all fission products, besides Zr only a part Mo is adsorbed on silica gel, while other fission products (Fe, Mn, Sr etc.) are not adsorbed. The result of dynamic adsorption shows that the effective capacity of silica gel column is 3.5 times of bed volume; a little Pu is adsorbed on silica gel together with Zr; the adsorbed Zr and Pu in column can be partly eluted by HNO_3; 0.2 mol/L H_2C_2O_4 can be used to elute Zr and Pu completely. In order to reduce the volume of waste silica gel, the regeneration and reuse of silica gel was studied. The 0.2 mol/L H_2C_2O_4 eluted silica gel column can be reused at least six times and the dynamic adsorption property of Zr is almost the same each time. We also studied the measurement of Pu adsorbed on silica gel and found that the radioactivity specific activity of Pu remained on the silica gel after H_2C_2O_4 elution is about 1×10^4 Bq/kg, which means the waste silica gel belongs to non-α-emitting waste. After the removal of Zr by the technical process introduced in this paper, HLLW can be treated with TRPO process. The final eluted Zr/Pu/H_2C_2O_4 mixed solution can be solidified as medium radioactivity solid waste by cement solidification method.