Fabrication, chemical and thermal stability studies of crystalline ceramic wasteform based on oxyapatite phosphate host LaSr₄(PO₄)₃O for high level nuclear waste immobilization

Highlights

• First time report on simulated phosphate oxyapatite wasteform.

• La_0.6Pr_0.1Nd_0.1Sm_0.1Gd_0.1Sr₄(PO₄)₃O (WF1) contains 17.95 wt% loading of rare earths.

• WF1 exhibits average thermal expansion co-efficient (α_avg) of 10.7 ± 1.2 × 10⁻⁶ K^-1 (298–973 K).

• WF1 shows negligible leaching of RE³⁺ and P⁵⁺ ions.

Abstract

Reprocessed high-level nuclear waste (HLW) contains range of radioactive components. Crystalline oxyphosphate apatite ceramic of the formula LaSr₄(PO₄)₃O [LSS] was investigated as a host for HLW immobilisation. The systematic study of solid solubility limit of individual rare earth ion substitution leads to the formulation of simulated wasteform of the formula La_0.6Pr_0.1Nd_0.1Sm_0.1Gd_0.1Sr₄(PO₄)₃O (WF1) with the waste loading of 17.95 wt% of rare-earth ions. Both parent and WF1 were synthesized by precipitation method. The thermal stress and groundwater inventory at the repository site can severely affect the wasteform performance, in addition to radiation and mechanical effects. Hence, the fabricated composition with high-level nuclear waste loading must be screened basically for chemical, thermal and radiation resistance. The present study investigated the thermal stability (by TGA), thermal expansion behaviour (by HT-XRD) and chemical durability (MCC-5) of the composition (WF1). The weight loss of WF1 being 2.2% and the average thermal expansion co-efficient (α_avg) of 10.7 ± 1.2 × 10⁻⁶ K^-1 in the temperature range (298–973 K) were comparatively lower than the parent phase, LaSr₄(PO₄)₃O. The WF1 showed resistance to leaching of RE³⁺ and P⁵⁺ with only the leaching of Sr²⁺ ion whose leach rate was of the order 10^-3-10^-4 gm^-2d^-1.

Introduction

High-level nuclear waste (HLW) contributes to just 3% volume of all the disposed radioactive materials. However, the matter of concern lies in the fact that it accounts for 95% of the radioactivity since it contains vast variety of radionuclides in the waste stream having long half-life (Oelkers and Montel, 2008). HLW of spent nuclear fuel differs in their composition and concentration of the radionuclides present (Djokic, 2013). Subsequently, immobilisation of these wastes into appropriate chemically, thermally and radiation resistive host matrix to formulate wasteform is paramount important prior to ultimate disposal. Wasteforms can be tailored either considering the geological conditions at the repositories or based on the composition of the waste stream (Ewing et al., 2016). If the later one is chosen, having a panoramic perspective in perceiving the critical importance of various challenges it may come across safety analysis assessment must be the priority, because, however, remote the chosen disposal site is, there is always a risk involved owing to unexpected hydrogeological factors such as tectonic events, thermal stress (Olszewska et al., 2015). Because of the presence of concentrated beta and gamma emitters heat gets generated, and as a result, the properties of the wasteform may change (Okamoto et al., 1991). It is mainly the thermal pressurisation from the decay heat, which is one of the factors that impact the effectiveness or integrity of the wasteform (Okamoto et al., 1991). The heat produced not only depends on the constituents of radionuclides, but also the volume of waste that is loaded in the wasteform (Okamoto et al., 1991). Understanding the thermal properties of the wasteform is pivotal because, during the period of heat generation within the wasteform, a variable temperature distribution to surrounding materials develops both in space and time (Roxburgh, 1987). As a result of geothermal effect, the temperature increase of 20 to 30 °C per kilometre depth of the earth and rapid temperature increase in the disposal region after 50–100 years may cause the increase in temperature of the site (Olszewska et al., 2015; Roxburgh, 1987). Initially, the decay process is rapid (Roxburgh, 1987), and the heat released may bring about major alterations in the properties of wasteform and can eventually cause the possibility of water entry and wasteform dissolution (Bradley, 1980).

As the consequence of wasteform alteration, radionuclide leaching and the subsequent migration into the biosphere are of serious peril. This is the most feasible way, in which radioactive waste find its way to the biosphere than other proposed mode of spread (stress condition is owing to mechanical interference via physical/mine excavation) (Okamoto et al., 1991; Roxburgh, 1987). Hence, the details of thermal and chemical durability of a wasteform serve as the vital prerequisite.

To avoid the state of the unfathomable fate of wasteform at the disposal site, chemical and thermal stability studies of simulated wasteform are a couple of fundamental features that are required to comprehend, while contemplating the performance of real wasteform for disposal purpose. During reprocessing itself, up to 15 wt% of phosphate is generated as a waste, because of the usage of bismuth phosphate or tributyl phosphate as phosphate sources (Ewing and Wang, 2002). Hence an effort to make use of a phosphate-based matrix to accommodate the complex waste stream constituents (alkali, alkaline earth, transition, rare-earth ions) and radioactive phosphorus content, solves the issue. It is important to note that phosphates of radionuclides are mostly insoluble in groundwater, which is worth considering here, as the possibility of waterborne leakage of phosphate immobilised radionuclides at the repository site is less, even if it encounters high thermal stress. In the past, a series of phosphate-based glasses such as sodium phosphate glass, alumino phosphate glass, zinc phosphate glass and lead phosphate glass were explored to check the performance in each of the compositions (Sengupta, 2012). But most of these phases exhibited relatively low thermal stability and are highly corrosive in nature (Oelkers and Montel, 2008). Further, iron phosphate-based glasses were developed, which showed extreme chemical durability and solved the issue of corrosiveness. Although it was identified as one of the improved hosts in the glass matrices, once crystallisation occurs it would markedly decrease the durability (Sengupta, 2012). An alternate approach involves the development of single-phase phosphate-based ceramic hosts to immobilise the waste constituents. Phosphates such as monazite [LnPO₄](Ln = La- Gd) (Dacheux et al., 2013), xenotime [LnPO₄] (Ln= Ho- Lu, Y) (Vance et al., 2011), Rhabdophane [LnPO₄.nH₂O] (Ln = La-Dy) (Qin et al., 2017), brabantite [Ln³⁺_1–2xAn⁴⁺_xM_x²⁺(PO₄)] (Dacheux et al., 2013; Popa et al., 2016), sodium zirconium phosphate [NaZr₂(PO₄)₃] (NZP) (Shrivastava and Chourasia, 2008) and apatite [Ca₁₀(PO₄)₆X₂] (Trocellier, 2001) were explored. Recent reports indicate studies on few other phosphate-based phases such as florencite [LnAl₂(PO₄)₂(OH)₆] (Janeczek and Ewing, 1996), β-TPD [β-Th₄(PO₄)₄P₂O₇] and β- TUPD [β-Th_4-xU_x(PO₄)₄P₂O₇] (Brandel et al., 2001).

Rare earth phosphates, be it monazite or xenotime, they are the type of material that possess high melting temperature, large thermal expansion, low thermal conductivity (Feng et al., 2013) and high chemical durability (Ewing and Lutze, 1991). Rhabdophane rare-earth phosphate structure type is stable up to 650 °C (Anfimova et al., 2014), which on further heating subsequently gets converted to monazite or xenotime structure (Rafiuddin and Grosvenor, 2016). Another analogue of monazite host called cheralite/ brabantite (2REE³⁺ ⬄3Ca²⁺+An⁴⁺) found to exhibit good chemical durability (Veilly et al., 2008). NZP, on the other hand, is one of the low thermal expanding materials (Buvaneswari et al., 2004) possessing good leach resistance (Sugantha et al., 1998). In this series, phosphate-based apatites are the class of materials identified as the most abundant ore of phosphate mineral that accounts for > 95% of total phosphorus content found on earth crust (Jacobson et al., 2000), which is highly pliable and making it possible to accommodate a variety of ions into its structure. Various classes of phosphate-based apatites such as haloapatites, hydroxyapatite and britholites are identified to accommodate Cs⁺ (Kumar and Buvaneswari, 2013), Sr²⁺ (Kumar and Buvaneswari, 2013), Cl⁻ (Kim et al., 2005), I⁻ (Guy et al., 2002; Coulon et al., 2016), RE³⁺ (Ardanova et al., 2010) and An⁴⁺ (Terra et al., 2004) [RE³⁺= La, Nd, Sm and Eu and An = U, Pu and Th]. Phosphate-based apatites possess reasonably good thermal stability (Tõnsuaadu et al., 2012) and are leach resistive ceramics (Coulon et al., 2016; Zhang et al., 2019).

In this series, current work focussed on the wasteform host of the formula REA₄(PO₄)₃. Effort has been made to fabricate simulated wasteforms hosting rare earth and alkaline earth elements and study its thermal and chemical durability.