以冷媒R134a萃取鈾金屬螯合物

Abstract

核能發電所產生的高放射性核廢料僅約核燃料總量的3%，剩餘的97%為可回收再利用的新能源-鈾及鈽。現今最普遍的核燃料後處理流程需將核燃料溶解於濃硝酸，並以含螯合劑之有機溶劑萃取，因而會產生大量廢液。利用超臨界二氧化碳流體(SF-CO2)來取代傳統有機溶劑進行萃取，在收集過程中洩壓便可氣化而與萃取物分離，達到濃縮的效果，大幅地減少樣品處理體積，是目前主要的發展。本實驗嘗試以液態冷媒1,1,1,2-tetrafluoroethane（R134a）取代SF-CO2，操作壓力比SF-CO2降低約100個大氣壓以上，螯合劑除了使用有機磷試劑類，也選用不含氟的β-diketone類螯合劑，實驗証明此兩類螯合劑所形成之螯合物於R134a中皆有很高的溶解度，且使用TBPO (Tributylphosphine oxide)萃取鈾，回收率最高可達96.3%。另外，為了減少大量濃硝酸的使用，先將螯合劑結合硝酸形成錯合物，溶解於R134a直接進行鈾萃取，實驗發現萃取效率最高能達90.9 %，証明以此方法可避免濃硝酸之直接使用，所以又進一步的達到R134a的應用價值，在萃取安全性上得到改進。

Around 97% of the spent fuel generated in nuclear power plants can be recovered and reused. The spent fuel is usually dissolved in concentrated nitric acid and extracted using chelate – containing organic solvent. Uranium and plutonium can be recovered efficiently in this process; however, large amount of waste organic solvent must be reprocessed. Supercritical fluid carbon dioxide (SF-CO2) has been introduced recently to replace the organic solvent. SF-CO2 evaporates after extraction and separates from the extracts; thus to save the endeavor to reprocess the extraction solvent. We, in this study, attempted using liquid 1,1,1,2-tetrafluoroethane (R134a) in substituting SF-CO2 as the extraction solvent due to the much lower operational pressure of R134a. Several organophosphorus and non-fluorine b-diketone chelate agents were examined in the extraction. The solubilitis of uranium complexes with these agents in R134a were found very high. Extractions of uranium using these chelates were also investigated. The uranium recovery was as high as 96.3% using chelate tributylphosphine oxide. In addition, complex compounds of nitric acid and the chalate agents were synthesized. These compounds – containing R134a also showed great extraction ability to uranium. It took the advantage of using no nitric acid directly to the spent fuel and thus improved the safety of the process.