Real‐time monitoring of crystal accumulation in the high‐level waste glass melters using an electrical conductivity method

Abstract

During the vitrification of high‐level radioactive waste (HLW) in HLW melters in the Waste Treatment and Immobilization Plant located in Washington State, spinel crystals [Fe, Ni, Mn, Zn]2+[Fe, Cr]23+normalO4 may precipitate from glass and accumulate in the melter riser, preventing the discharge of molten glass into canisters. Therefore, an effort is being made to develop an electrical conductivity method to monitor crystal buildup in the melter riser. A vertically configured electrical conductivity (EC) probe with an alumina shaft and Pt‐10%Rh‐electrodes was designed and tested in standard conductivity solutions and glass melts both with and without spinel crystals. The EC probe measured conductivity in conductivity solutions within 10% of their certified values and showed a linear relationship with increased spinel layer thicknesses. Testing in silicate glass containing spinel crystals allowed for the determination of spinel conductivity as a function of temperature. The conductivities of spinel crystals of 20‐24 S/m at 800°C were in excellent agreement with the conductivity of trevorite (NiFe2O4) crystals at 800°C reported in the literature. The conductivities of spinel crystals and measured changes in the conductivity across the accumulated layer allowed for a successful measurement of spinel crystal accumulation in simulated HLW glass.

Impact of non‐framework cation mixing on the structure and crystallization behavior of model high‐level waste glasses

“…[2][3][4] A similar problem is that of the impact of non-framework cations on the crystallization of (Fe, Ni, Mn, Zn) II (Fe, Cr, Mn) III 2 O 4 mixed stoichiometry spinel crystals in the glass discharge riser of the melter during idling (where the melter temperature is lowered to 850−900 • C when not in operation). [5][6][7] Here, it should be noted that the problem of spinel crystallization in nuclear waste melts has been well investigated and reported in the literature, but only at an empirical level, where the effort has been focused on developing constraints to limit the crystallization of spinels from borosilicate glass in the HLW melter. 1,[8][9][10] However, the validity of these empirical constraints is now being challenged.…”

“…The problem pertaining to the crystallization of alkali aluminosilicate‐based phases during the cooling of the melts (in the steel canisters) and its impact on their chemical durability has been widely reported 2–4 . A similar problem is that of the impact of non‐framework cations on the crystallization of (Fe, Ni, Mn, Zn) II (Fe, Cr, Mn) III 2 O 4 mixed stoichiometry spinel crystals in the glass discharge riser of the melter during idling (where the melter temperature is lowered to 850−900°C when not in operation) 5–7 . Here, it should be noted that the problem of spinel crystallization in nuclear waste melts has been well investigated and reported in the literature, but only at an empirical level, where the effort has been focused on developing constraints to limit the crystallization of spinels from borosilicate glass in the HLW melter 1,8–10 .…”

Crystallization behavior of iron‐ and boron‐containing nepheline (Na2O·Al2O3·2SiO2) based model high‐level nuclear waste glasses

“…In the first case, iron interacts with other transition‐metal cations (for example, Ni 2+ , Mn 2+ , Cr 3+ ) in the glass melter to form spinels (eg, NiFe 2 O 4 ). The formation of spinel crystals in the glass melter is problematic, because large insoluble crystals can settle on the floor of the melter and partially or completely block the discharge throat and riser . In the second scenario, the as‐formed spinel crystals tend to act as nucleation sites for the crystallization of nepheline during cooling of HLW glass in canisters, which results in a waste form with poor chemical durability .…”