Solidification Of High-level Wastes Research Articles

Optimization of the Solidification Method of High-Level Waste for Increasing the Thermal Stability of the Magnesium Potassium Phosphate Compound

The key task in the solidification of high-level waste (HLW) into a magnesium potassium phosphate (MPP) compound is the immobilization of mobile cesium isotopes, the activity of which provides the main contribution to the total HLW activity. In addition, the obtained compound containing heat-generating radionuclides can be significantly heated, which increases the necessity of its thermal stability. The current work is aimed at assessing the impact of various methodological approaches to HLW solidification on the thermal stability of the MPP compound, which is evaluated by the mechanical strength of the compound and its resistance to cesium leaching. High-salt surrogate HLW solution (S-HLW) used in the investigation was prepared for solidification by adding sorbents of various types binding at least 93% of 137Cs: ferrocyanide K-Ni (FKN), natural zeolite (NZ), synthetic zeolite Na-mordenite (MOR), and silicotungstic acid (STA). Prepared S-HLW was solidified into the MPP compound. Wollastonite (W) and NZ as fillers were added to the compound composition in the case of using FKN and STA, respectively. It was found that heat treatment up to 450 °C of the compound containing FKN and W (MPP-FKN-W) almost did not affect its compressive strength (about 12–19 МPa), and it led to a decrease of high compressive strength (40–50 MPa) of the compounds containing NZ, MOR, and STA (MPP-NZ, MPP-MOR, and MPP-STA-NZ, respectively) by an average of 2–3 times. It was shown that the differential leaching rate of 137Cs on the 28th day from MPP-FKN-W after heating to 250 °C was 5.3 × 10−6 g/(cm2∙day), however, at a higher temperature, it increased by 20 and more times. The differential leaching rate of 137Cs from MPP-NZ, MPP-MOR, and MPP-STA-NZ had values of (2.9–11) × 10−5 g/(cm2∙day), while the dependence on the heat treatment temperature of the compound was negligible.

Solidification of high level waste using magnesium potassium phosphate compound

Abstract Compound samples based on the mineral-like magnesium potassium phosphate matrix MgKPO4 × 6H2O were synthesized by solidification of high level waste surrogate. Phase composition and structure of synthesized samples were studied by XRD and SEM methods. Compressive strength of the compounds is 12 ± 3 MPa. Coefficient of thermal expansion of the samples in the range 250–550 °C is (11.6 ± 0.3) × 10−6 1/°C, and coefficient of thermal conductivity in the range 20–500°С is 0.5 W/(m × K). Differential leaching rate of elements from the compound, g/(cm2 × day): Mg - 6.7 × 10−6, K - 3.0 × 10−4, P - 1.2 × 10−4, 137Cs - 4.6 × 10- 7; 90Sr - 9.6 × 10−7; 239Pu - 3.7 × 10−9, 241Am - 9.6 × 10−10. Leaching mechanism of radionuclides from the samples at the first 1–2 weeks of the leaching test is determined by dissolution (137Cs), wash off (90Sr) or diffusion (239Pu and 241Am) from the compound surface, and when the tests continue to 90–91 days - by surface layer depletion of compound. Since the composition and physico-chemical properties of the compound after irradiation with an electron beam (absorbed dose of 1 MGy) are constant the radiation resistance of compound was established.

Hydrolytic and thermal stability of magnesium potassium phosphate compound for immobilization of high level waste

The samples of the magnesium potassium phosphate (MPP) compound have been synthesized during solidification of high level waste (HLW) surrogate. The compound consists of a main phosphate phases Mg1.1Na0.35K0.45PO4 × (4–6)H2O and MgCs0.5Na0.2K0.3PO4 × (5–6)H2O. Differential leaching rates of 239Pu, 152Eu and 90Sr from the MPP compound after heat treatment (450 °C) are 7.8 × 10−9; 1.7 × 10−7 and 8.8 × 10−6 g cm−2 day−1, respectively. The coefficient of thermal expansion of MPP compound—(11.6 ± 0.3) × 10−6 °C−1; coefficient of thermal conductivity averages 0.5 W m−1 K−1. The properties of MPP compound meet the regulatory requirements for solidified HLW.

Application of acoustic emission for monitoring the quality of glass granules and ceramic samples

In technological processes for solidification of high-level wastes it is necessary to establish remote control of the apparatus and mechanisms, to monitor the technological parameters, and to monitor the quality of both the intermediate product and the final parts. In many cases, the properties of the intermediate products can be judged from the parameters of the technological process being monitored, while tests of the final parts which must be put into long-term storage or buried in geological formations, require an independent technical solution. Nondestructive methods of monitoring, for example, the acoustic-emission method, based on the detection of signals emitted from solids to which a force is applied [1], are being increasingly used. In standard methods of ultrasonic flaw detection the amplitude of the acoustic signal with the crack length increasing by 0.01-0.1 mm is hundreds of times greater than the change in amplitude of an ultrasonic signal reflected by the same growing crack. The maximum sensitivity achieved under laboratory conditions in investigations of the growth of a fatigue crack reached 0.025 ~m/cycle [1]. It should also be noted that ceramic piezoelectric transducers, employed for recording acoustic signals, also have high radiation and thermal resistance [2]. The laboratory apparatus for recording the acoustic signals consisted of separate units and devices, connected with one another according to the scheme shown in Fig. 1. The heating element (not shown in the figure) consisted of a vertical resistance furnace, which made it possible to perform experiments with the temperature of the heating zone reaching 1050~ Depending on the experimental conditions, we installed in the heating zone either a melter-crucible with a die at the bottom to obtain glass granules or the top (hot) part of a ceramic sound duct with a platform on which the experimental ceramic sample was placed. The initial product for preparing the glass granules consisted of presynthesized (prefound) glasses with different composition. The ceramic samples consisted of cylindrical tablets, which were pressed out of ground clay and mixed with additives simulating real high-level wastes. Phosphate and borosilicate glasses with different founding temperature were used in the experiments with glass granules. The finished glass samples were placed in a crucible, heated up to a definite temperature, and granulated by the method of free drop efflux. The temperature in the drop-formation zone was measured. The fused-glass drops formed either fell on a receiving area of the sound duct, where they were formed into granules and cooled to a certain temperature, or they were directed by a mechanical system into a collector container. After cooling, the granules were held for an additional time on the receiving area and then were removed. This process was repeated many times. During the experiment the spectra of the amplitudes of the envelopes of the acoustic signals were recorded continuously. Time was measured from the signal recorded at the moment a glass drop fell on the receiving area of the sound duct. Figure 2 displays typical spectra of granules which had defects in the form of cracks of different size. The defects vary over a wide range: from microcracks, visible only under high magnification, to chips and large cracks, observed by the unaided eye. For example, granules whose spectra are presented in Fig. 2a consisted of sectioned spheres, whose bases and volumes were penetrated by cracks of length up to several millimeters and chips. Figure 3 displays the spectra of glass granules obtained by cooling of phosphate glass drops on a heated area of a sound duct. The granule whose acoustic emission spectrum is displayed in Fig. 3a contained microcracks on its base and several microcracks in the volume; the spectrum of the amplitudes, which is shown in Fig. 3b, corresponds to a visually "ideal" granule. All granules of the same composition and the same size, whose acoustic emission spectra correspond to the range between the positions a and b in Fig. 3, had the same (within the limits of accuracy of the measurements) mechanical strength under a static load. The objective of the experiments with ceramic samples was to determine from the acoustic signals the possibility of observing defects, formed during the heat-treatment process. The samples were placed on the working area of the ceramic sound duct, which was placed in the heating zone of the resistance furnace, and heated or cooled with different

Contributions to the Risk Evaluation of a High-Level Waste Solidification Plant

An analysis has been made dealing with the evaluation of risk of a high-level waste solidification plant. Fault-tree methodology was used to identify all possible accidents and to evaluate their probability of occurrence. The analysis shows that with regard to risk, the most important accidents occur during off-gas cleaning. Furthermore, the routine risk is of the same order of magnitude or greater than the most important accident risks.

A usable method for the management of high-level waste from nuclear fuel reprocessing

A process for the solidification of high level waste (HLW)-solutions is described. Final product of this process is not, as usual, a glass-block but a block of glass-metal compound called VITROMET. The main features of the process, the properties of the product and the possibilities for engineered storage and disposal are: denitrition of the waste stream, concentration, formation of phosphates or borosilicates, continuous melting, formation of glass beads, incorporation in a corrosion resistant lead-alloy matrix and sealing in a canister that may be placed first in engineered storages then in stable geological formation.

High-Level Waste Solidification: Applicability of Fluidized-Bed Calcination to Commercial Wastes

Candidate processes for commercial high-level waste solidification are radiant-heat spray calcination, rotary-kiln calcination, and fluidized-bed calcination. Radiant-heat spray and rotary-kiln calcination have been studied only on a pilot-plant scale; plant-scale fluidized-bed solidification of U.S. Atomic Energy Commission high-level wastes has been operating for more than 10 years. Cold pilot-plant studies using the radiant-heat spray and fluidized-bed processes are currently underway on simulated commercial wastes. Encouraging results to date show that the existing fluid-bed solidification process pioneered at the Idaho Chemical Processing Plant is applicable to commercial waste processing with some process and equipment modifications. These modifications are mainly in areas connected with uncontrolled heating during postulated collapse of the fluidized bed, off-gas cleanup, and equipment design for compatibility with total remote maintenance.