For the proposed novel procedure of immobilizing HLW with magnesium potassium phosphate cement (MKPC), Fe2O3 was added as a modifying agent to verify its effect on the solidification form and the immobilization of the radioactive nuclide. The results show that Fe2O3 is inert during the hydration reaction. It slows down the hydration reaction and lowers the heat release rate of the MKPC system, leading to a 3°C-5°C drop in the mixture temperature during hydration. Early comprehensive strength of Fe2O3 containing samples decreased slightly while the long-term strength remained unchanged. For the sintering process, Fe2O3 played a positive role, lowering the melting point and aiding the formation of ceramic structure. CsFe(PO4)2, or CsFePO4, was generated by sintering at 900°C. These products together with the ceramic structure and absorption benefit the immobilization of Cs+. The optimal sintering temperature for heat treatment is 900°C; it makes the solidification form a fired ceramic-like structure.
The utilization of nuclear energy resulted in the accumulation of large amounts of liquid high-level radioactive waste (HLW) which contains environmentally hazardous elements like plutonium and other actinides in addition to fission and corrosion products [
Magnesium phosphate cements (MPCs) are cementitious materials that are formed through a solution acid-based reaction between dead burnt magnesia and phosphate. Retarder and mineral admixtures may be added during hydration reaction to achieve proper workability or specific properties [
Earlier MPCs featured a “two-part” system, consisting of dead burnt magnesia and a soluble orthophosphate, i.e., NH4H2PO4 (ADP) or KH2PO4 (KDP) [
Since the reaction between MgO and ADP releases ammonia, in consideration of the secondary pollution control, we prefer to adopt the magnesium potassium phosphate system as the raw material. The hydration reaction is governed by [
Nuclear reactors periodically unload spent fuel containing unburnt nuclear fuel, abundant fission fragments, and their decay products [
As shown in the reaction equations, aqueous HLW could be directly immobilized by the introduction of binding agents. The cement-like solidification process is characterized by its low energy inputs, the simplicity of realization, as well as the minimization of secondary radioactive waste, and the low mobility of the nuclide ion. Compared to conventional cementation, MKPC has perceptible advantages: possibility of solidifying liquid wastes within a wide range of pH, high loading capacity toward HLW, etc. Interest in the use of binding phosphate materials for radioactive waste immobilization has risen during last few years. Singh et al. investigated immobilizing 99Tc with MKPC Ceramicrete. The solidified form was achieved and had a comprehensive strength no less than 30 MPa. The solidification mechanism of 99Tc was proven to be the combination of mechanical enclosing and chemosetting [
Scientists of ANL (USA) offered the use of phosphate materials (Ceramicrete) for immobilization of low-level and technetium-containing simulant waste solutions as well as for incorporation of Pu-containing ash. At the Khlopin Radium Institute (Russia) and INEEL (USA), the possibility of incorporating a simulant of low-level ash remainder of combustible radioactive waste into iron phosphate matrices was also studied [
In our previous study, a novel procedure of HLW immobilization was developed in which the liquid HLW was added as a substitute for the mixing water of the MKPC system. This procedure makes the processing very simple and direct. The solidified form has advantages over glass solidification or synroc solidification in terms of chemical stability and heat resistance, etc. Further study showed that the MKPC solidification form has good thermal resistance; it keeps intact even after sintering at 1400°C for hours, and the sintering makes MKPC form into real fire ceramics. In consideration of the heat releasing of the HLW, the MKPC form needs to withstand quite high temperatures after the HLW disposal. So, we believe that the presintering is beneficial for the durability and stability of the solidified forms as well as for the immobilization of the nuclide.
Iron phosphate has been proven to be appropriate for hazardous material immobilization. Sales, B. C. et al. reported that the lead-iron phosphates glass performs well as stable storage of high-level nuclear waste [
Iron oxide is one of the most abundant metal oxides on earth. Its abundant and inexpensive characteristics make it a promising candidate to partially replace the magnesium for forming phosphate cements. It is meaningful to investigate the effect of fe2o3 on the immobilization of aqueous high-level waste with magnesium potassium phosphate ceramic. An experimental study was carried out in this paper to verify the function of the iron oxide in both the hydration reaction and the sintering process of the MKPC matrix.
The magnesium potassium phosphate cement paste was prepared through a mixture of dead burnt magnesium oxide (MgO), acidic phosphate (KDP), and borax in specific proportions. The chemical characteristics of the raw materials are listed in Table
Characteristics of raw materials.
Component | Characteristics | Supplier |
---|---|---|
Dead burnt magnesium oxide powder (MgO) | Industrial grade, >95% MgO | Liaoning Xinrong Mining Group Co. Ltd., China |
KDP (KH2PO4) | Industrial grade, >98% KDP | Qingzhou Guanghui Chemical Plant Co. Ltd, China |
Borax (Na2B4O7·10H2O) | Industrial grade, >95% borax | Tibet Pengdu Boron Industry Co. Ltd., China |
The aqueous HLW considered in this study is the effluent of spent fuel postprocessing. During the postprocessing procedure, the spent fuel from the nuclear power reactor is mechanically cut and then dissolved in nitric acid. The solution is then filtered and clarified, and residual U and Pu are extracted for recycling by extraction agents, e.g., TBP. The aqueous HLW is discharged by spent fuel recycling and posttreatment processes. Generally, it is a mixture of concentrated nitric acid and various kinds of nitrate solution that contain the majority of the radiation and toxicity of the spent fuel. Due to the safety considerations and experimental conditions, a simulated aqueous HLW was used to represent the nuclear power reactor aqueous HLW. Cations were introduced into nitric acid by corresponding nitrate. Nuclides were represented by their nonradioactive isotopes. We believe this is appropriate in chemical view. The simulated aqueous HLW was prepared according to the composition listed in Table
Composition of typical power nuclear reactor aqueous HLW (g/L) [
| | | | | | | | | | | |
---|---|---|---|---|---|---|---|---|---|---|---|
15.9 | 0.074 | 2 | 17.4 | 0.45 | 51.2 | 2 | 8.2 | 0.61 | 0.82 | 0.78 | 2.05 |
K3PO4·7H2O was introduced into aqueous HLW to adjust the pH value in advance. Fe2O3 powder was mixed with MgO homogeneously as an additive. All above-mentioned chemicals are analytically graded products.
Solidified blocks were prepared through the following steps. (1) Add K3PO4·7H2O into aqueous HLW to adjust pH to the specific value. (2) Add KDP and Borax into the liquid HLW at specific proportion and stir to make the liquid (solution and sediment) a homogeneous mixture. (3) Add mixture of MgO and Fe2O3 into the liquid and stir the hydration reaction. Pour the paste into cubic mold when the flowability is appropriate. (4) Vibrate by hand, use glass rod to densify paste, and avoid the opening. (5) Cure at ambient room temperature for hours. (6) Remove mold and retrieve blocks for further processing and study. (7) After ambiently curing for 7 days, the solidification forms were sintered for 2 hours at specific temperature in a muffle furnace. Heating rate of the muffle furnace was set to 5°C/min.
The basic formula of MKPC was listed in Table
Basic formula of MKPC.
Component | MgO | KDP | Borax | Water |
---|---|---|---|---|
Ratio | 100 | 25 | 12 | 16 |
Hydration heat release was tested by an eight-channel micro calorimeter (Thermonetrics TAMair). The compressive strength of the hardened blocks was measured according to the standard of
Fe2O3 was added at a dosage series of 0%, 3%, 6%, 9%, and 12% (wt% of MgO to replace MgO) to investigate the effect on MKPC hydration.
The exothermic curve of the mixture during hydration can be seen in Figure
Exothermic curve of the mixture during hydration.
Fe2O3 was added at a dosage series of 0%, 6%, 9%, and 12% (wt% of MgO to replace MgO) to investigate the effect on mechanical property. Samples of the original MKPC and the sintered MKPC were tested for comprehensive strength (Figure
Comprehensive strength.
Aqueous HLW was used to substitute mixing water completely in the proposed immobilization procedure. Since liquid HLW is a very concentrated acid liquor (pH<1), it may degrade the cement system and lead to failure of the immobilization. So the liquid HLW was treated in advance to adjust its pH value. K3PO4·7H2O was used as buffer agent since its solution is alkalic and the introduction of K3PO4·7H2O does not bring in extra elements. The neutralization reaction produces the sediment nuclide phosphate, which is more stable than hydroxide. Thus, the pretreatment benefits immobilization and eliminates the negative effect of nitric acid.
HLW was pretreated to specific pH (3, 5, 7) and then incorporated into the MKPC system mix water to form solidification blocks. Original samples and samples containing 9% Fe2O3 were tested to find the effect of Fe2O3 on the immobilization form.
As can be seen in Figure
Comprehensive strength of solidification forms. ①HLW pH=3, ②HLW pH=5, ③HLW pH=7, ④HLW pH=5, sintered samples.
The adiabatic temperature curve of the hydration reaction was plotted. As seen in Figure
Adiabatic temperature curve (Fe2O3 dosage=9%).
The unsintered solidification form and the solidification form sintering at 900°C were prepared and analyzed by XRD. For the XRD spectrum, see Figure
XRD spectrum of solidification forms (①unsintered, ②sintered at 900°C).
In order to investigate the effect on the microstructure of the solidification form, HLW was pretreated to specific pH levels and incorporated in the MKPC matrix. The original form and Fe2O3 containing samples were prepared. All sample series were sintered at 900°C and compared with unsintered samples. SEM photos were taken and compared.
When HLW pH=3, the microstructure is mainly a small particle, and the crystal is underdeveloped because of the rapid hydration reaction (①② in Figure
SEM photo of unsintered samples (HLW pH=3, ①②Fe2O3 free, ③④9% Fe2O3 addition).
When HLW pH=5, the crystal developed into a bigger formation. The reason is that free H+ gets less, slowing down the reaction. The prolonged setting time makes better developed crystal (①② in Figure
SEM photo of unsintered samples (HLW pH=5, ①②Fe2O3 free, ③④9% Fe2O3 addition).
When HLW pH=7, the crystal agglomeration region did not show on the vision field. The solidification form developed into a bigger compact plate structure. The bigger compact plate structure is a result of enough setting time for the production of K-struct struvite. The pH=7 environmentally lessens the free H+ and benefits the crystal growth (①② in Figure
SEM photo of unsintered samples (HLW pH=7, ①②Fe2O3 free, ③④9% Fe2O3 addition).
As to all samples that sintered at 900°C, the microstructures present for the fire ceramic-like structure are all very compacted without visible pores (Figures
SEM photo of 900°C sintered samples (HLW pH=3, ①②③Fe2O3 free, ④⑤⑥9% Fe2O3 addition).
SEM photo of 900°C sintered samples (HLW pH=5, ①②③Fe2O3 free, ④⑤⑥9% Fe2O3 addition).
SEM photo of 900°C sintered samples (HLW pH=7, ①②③Fe2O3 free, ④⑤⑥9% Fe2O3 addition).
According to mechanism research, immobilization of Cs+ with MKPC system mainly depends on the following reactions. (a)Cs+ replaces the metal ion with minor radius thus entering the crystal lattice. (b)Cs+ is absorbed by metal phosphate. Cs+ leaching behavior of samples containing free Fe2O3 and Fe2O3 as well as sintered and unsintered samples was tested according to the MCC-1 method. HLW was pretreated to a specific pH (3,5,7) first before being incorporated in. For the leaching rate of Cs+, see Figure
Leaching rate of Cs+ (①unsintered samples, ② 900°C sintered samples, ③1000°C sintered samples).
For unsintered samples, the addition of Fe2O3 lowered the leaching rate slightly while the variation tendency stayed the same with the Fe2O3 free samples, which is why the leaching rate of Cs+ is lower in higher HLW pH value. In an acidic environment, free H+ makes the MgO dissolve faster and thus sped up the hydration reaction. This makes Fe2O3 have fewer chances for entering the crystal lattice or for being absorbed. Thus, harming the immobilization of Cs+ and making it escape easily from the solidification form. Addition of Fe2O3 makes the form more compacted because of the accumulation of its particles. Denser structure prevents the flushing out of the Cs+.
For sintered samples, sintering at 900°C makes the solidification form a fire ceramic structure. Cs+ replaces K+ and forms MgCsPO4 and CsFePO4 both of which have better water resistance. The addition of Fe2O3 lowered the leaching rate significantly since it lowered the melting point and helped the formation of ceramic.
Sintering at 1000°C cut the Cs+ leaching rate of Fe2O3 free samples but improved the Cs+ leaching rate of Fe2O3 containing samples dramatically. This again proved that the addition of Fe2O3 lowered the melting point, destroying Fe2O3 containing samples in the 1000°C sintering, and Cs+ was leached out easily because of the failure of encapsulation and absorption.
As an additive for the novel procedure of HLW immobilization, in which the liquid HLW was added to substitute mixing water of the magnesium potassium phosphate cement system, the effect of Fe2O3 can be concluded as follows.
Addition of Fe2O3 lowers the heat release rate of the hydration reaction significantly, thus lowering the temperature of the mixture during hydration, while not harming the comprehensive strength of the test blocks.
Fe2O3 increases the compactness of the samples due to the accumulation of particles. For sintered samples, the existence of Fe2O3 lowers the melting point and generates more liquid phase in the sintering; this lowers the firing temperature effectively.
Fe2O3 is an inert matter for the hydration reaction, but it plays a positive role in sintering process. It lowers the melting point and helps the formation of ceramic structure. The sintering process produces CsFe(PO4)2 and immobile Cs+ more effectively.
When taking its cheap price and remarkable availability into consideration, Fe2O3 may play an important role as an additive in the application of HLW immobilization with MKPC.
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
The authors declare that all received funding in the “Acknowledgments” section did not lead to any conflicts of interest, and there are no other possible conflicts of interest in the article.
The authors would like to acknowledge the financial support listed: International Cooperation Project of National Nature Science Fund of China, Grant No. 0211002321124; Chongqing Municipal Education Commission Science and Technology Planning Project, Grant No. KJ1600636; Student Science and Technology Innovation Fund of Chongqing Technology and Business University, Grant No. 173018; Environmental Pollution Control Team Fund of Environment and Resources College, CTBU.