Assessment of 137Cs decontamination of concrete by the reagent method

Abstract

The particulars of radioactive contamination of concrete wastes by 137 Cs were studied. x-Ray phase analysis and chemical analysis show that clayey materials, including Al 2 O 3 , Fe 2 O 3 , K 2 O, and MgO, on which 137 Cs sorption is possible, were present in the concrete wastes. The content and form in which 137 Cs was found in radioactive concrete wastes from nuclear power facilities as well as in model samples were determined. When the wastes were treated with nitric acid the binder dissolved and a polydisperse suspension formed. The suspension consisted of three phases: solution, fine suspension, and rapidly settling precipitate of heavy filler particles. x-Ray phase analysis was performed and the 137 Cs mass ratio and distribution in the phases were determined. The possibility of decontaminating the concrete by a reagent method was evaluated.Large amounts of radwastes, a considerable fraction of which consist of concrete structures, are inevitably produced when nuclear power facilities are decommissioned. There are many known methods for decontaminating concrete. Usually, concrete structures are decontaminated by mechanical removal of the surface layer using different equipment, as a result of which secondary wastes in the form of fragmented concrete are formed [1]. Ordinarily, such wastes are conditioned by incorporation into cement prepared using liquid radwastes [2] or by saturation with a highly penetrating cement solution [3]. As a result, the volume of the cemented radwastes and correspondingly the storage costs increase.As a rule, concrete wastes are classified as medium-and low-level. 137 Cs is one of the main contaminating γ-and β-emitting radionuclides. Its half-life is about 30 years, so that cool-down of the wastes to lower the activity to a safe level is of little use and in many cases removing this isotope from concrete will make it possible to remove such wastes from the radwaste category.The objective of the present work is to assess the possibility of volume 137 Cs decontamination of fragmented concrete wastes. Concrete is a nonuniform composite material, so that to develop a method of decontamination it is necessary to study the form in which the radionuclides, specifically, 137 Cs, are found in the wastes.The objects of study were concrete wastes from the rehabilitation of radiologically hazardous facilities: sample 1 -a 25-kg concrete block with γ-ray dose rate 0.1 m away from the surface to 500 μSv/h and sample 2 -fragmented wastes with γ-ray dose rate 0.1 m from the surface not exceeding the background level. In addition, model samples obtained by contaminating clean concrete with a radioactive solution as well as by cementing a radioactive solution were studied.For γ-spectrometric analysis, the waste fragments were comminuted and average samples were taken. The samples were subjected to radiometric analysis performed using an ORTEC-GEM 35P (Finland) automated four-channel gamma spectrometer with a semiconductor detector in the central laboratory of the Moscow Scientific and Industria...

Editors' Choice—Review—Electro-Kinetic Decontamination of Radioactive Concrete Waste from Nuclear Power Plants

“…In respect of cesium in concrete, Nikolayev et al suggested that clayey minerals and cement binder is the main source of cesium in concrete. 64 In addition, they suggested that 30-50% of the 137 Cs in concrete is in a fixed form, and only 5% of the total amount of 137 Cs is highly fixed in concrete. Cesium is one of the elements that contaminates concrete by direct penetration, thus most of the cesium isotopes might bond to the concrete microstructure through adsorption.…”

“…85 However, the trend observed by Faiz et al and Nikolayev et al differed from that of Rahman et al for the mobility of cesium. 31,77,85 Fig . 21 shows the cumulative leaching rate of cesium in Portland cement mortar for different time intervals (0-7 days, 7-16 days, 16-23 days, 23-34 days).…”

“…The effect of the pH on the leaching rate of contaminants during decontamination was estimated from the literature. The leaching rate of each nuclide with regard to pH in the solution used for decontamination is estimated based on the test data reported in the previous studies, 31,[74][75][76][77][78][79] as follows. The specimens of series A and C of Castellote et al, 36 Frizon et al, 37 and Popov et al 40 consisted of cement mortar; thus, the leaching test results of Van Gerven et al 76 and Faiz et al 77 that employed mortar specimens were used for the estimation of leaching rate.…”

“…The specimens of series A and C of Castellote et al, 36 Frizon et al, 37 and Popov et al 40 consisted of cement mortar; thus, the leaching test results of Van Gerven et al 76 and Faiz et al 77 that employed mortar specimens were used for the estimation of leaching rate. Especially, Van Gerven et al reported test data for the effect of carbonation on the leaching rate of cobalt, so this was applied for the specimens of Castellote et al 36,76 The specimens of series B of Castellote et al, 36 Popov et al, 41 and Kim et al 39 involved concrete; hence, the leaching test results of Hohberg et al 78 and Nikolayev et al 31 that employed concrete specimens were used for the estimation. This leaching rate estimation is not concerned with the manufacturing process of concrete samples, which is investigated in a later section.…”

“…25 Chemical techniques use chemical reactions with water, chemical gel, oxidants/reductants, chelating agents, or acids/bases. 19,[26][27][28][29][30][31] z E-mail: msshin@unist.ac.kr; sungyeolchoi@kaist.ac.kr…”