Energetics of metastudtite and implications for nuclear waste alteration

Abstract

Metastudtite, (UO 2 )O 2 (H 2 O) 2 , is one of two known natural peroxide minerals, but little is established about its thermodynamic stability. In this work, its standard enthalpy of formation, −1,779.6 ± 1.9 kJ/mol, was obtained by high temperature oxide melt drop solution calorimetry. Decomposition of synthetic metastudtite was characterized by thermogravimetry and differential scanning calorimetry (DSC) with ex situ X-ray diffraction analysis. Four decomposition steps were observed in oxygen atmosphere: water loss around 220°C associated with an endothermic heat effect accompanied by amorphization; another water loss from 400°C to 530°C; oxygen loss from amorphous UO 3 to crystallize orthorhombic α-UO 2.9 ; and reduction to crystalline U 3 O 8 . This detailed characterization allowed calculation of formation enthalpy from heat effects on decomposition measured by DSC and by transposed temperature drop calorimetry, and both these values agree with that from drop solution calorimetry. The data explain the irreversible transformation from studtite to metastudtite, the conditions under which metastudtite may form, and its significant role in the oxidation, corrosion, and dissolution of nuclear fuel in contact with water. Metastudtite is the main natural peroxide mineral (1) due to the irreversible dehydration of studtite (2). Both these phases may be found in spent nuclear fuel exposed to water (3-5). Burns et al. (6), by single crystal diffraction of a natural sample, determined the crystal structure of studtite to be monoclinic with space group C2/c. The structure consists of edge-sharing UO 8 -polyhedra chains. The water molecules are located on two different positions between these chains. Although there are different, yet very similar, models for metastudtite, thus far no structure has been determined (7,8). It is highly probable that metastudtite, like studtite, consists of chains of edge-sharing UO 8 polyhedra with the water molecules located between the chains. To better depict the bonding situation and the different oxygen atoms, it is proposed to write the sum formulas as (UO 2 )(O 2 )(H 2 O) 2 ·2H 2 O for studtite and (UO 2 )(O 2 )(H 2 O) 2 for metastudtite [in analogy to Burns et al. (6)].Studtite and other polyoxouranylates have a potentially important role as alteration phases in geological repositories for nuclear waste, specifically regarding the interactions of nuclear waste with the aqueous environment (9, 10). They have also been proposed as corrosion products in sea water after the FukushimaDaiichi nuclear plant accident (10-12).In a nuclear waste repository, the high alpha dosage will be the dominant factor after the first thousand years of storage (13). In combination with groundwater, the alpha radiation will lead to formation of H 2 O 2 (3, 4, 9, 14). These very localized oxidative conditions could trigger the genesis of studtite or metastudtite even if the overall conditions are reducing (14). The formation of metastudtite on UO 2 samples as a direct effect of alpha radiolysis of water w...

Thermodynamic studies of studtite thermal decomposition pathways via amorphous intermediates UO3, U2O7, and UO4

“…However, as shown by thermodynamic investigations by Kubatko et al [4] and Guo et al [11], both studtite and metastudtite have positive enthalpies of formation, 22.3 ± 3.9 and 15.8 ± 1.7 kJ/mol, from -UO 3 , H 2 O and O 2 , respectively. This suggests that once studtite or metastudtite form, eventual decomposition may release soluble U(VI) [4,11]. Hence, understanding the decomposition pathway of studtite may help to clarify aspects of UO 2 fuel degradation involving exposure to water.…”

“…Studtite, (UO 2 )O 2 (H 2 O) 2 ·2H 2 O, and metastudtite, (UO 2 )O 2 (H 2 O) 2 , form as alteration products on spent nuclear fuel (SNF) in aqueous environments [1][2][3]. They are important where spent fuel is altered during surface or geological storage, in nuclear accidents, and in the processing of uranium ores [1][2][3][4][5][6][7][8][9][10][11][12]. The formation of studtite in a nuclear waste repository is favorable where H 2 O 2 is present under locally oxidative conditions [1,2,4,5], with the H 2 O 2 generated and replenished by water radiolysis caused by high radiation dosage [13].…”

“…The formation of studtite in a nuclear waste repository is favorable where H 2 O 2 is present under locally oxidative conditions [1,2,4,5], with the H 2 O 2 generated and replenished by water radiolysis caused by high radiation dosage [13]. However, as shown by thermodynamic investigations by Kubatko et al [4] and Guo et al [11], both studtite and metastudtite have positive enthalpies of formation, 22.3 ± 3.9 and 15.8 ± 1.7 kJ/mol, from -UO 3 , H 2 O and O 2 , respectively. This suggests that once studtite or metastudtite form, eventual decomposition may release soluble U(VI) [4,11].…”

“…The decomposition of studtite upon heating under oxygen or argon atmospheres occurs in several steps [11,[15][16][17][18][19][20][21]. Irreversible dehydration to metastudtite begins around 60 °C.…”

Thermodynamics of formation of coffinite, USiO 4

“…) species (47). Organic matter in the natural system (6,8,12,18,28) could then play an important reducing role to precipitate coffinite as summarized by Deditius et al (8).…”