Vitrification of HLW Produced by Uranium/Molybdenum Fuel Reprocessing in COGEMA’s Cold Crucible Melter

Abstract

The performance of the vitrification process currently used in the La Hague commercial reprocessing plants has been continuously improved during more than ten years of operation. In parallel COGEMA (industrial Operator), the French Atomic Energy Commission (CEA) and SGN (respectively COGEMA's R&D provider and Engineering) have developed the cold crucible melter vitrification technology to obtain greater operating flexibility, increased plant availability and further reduction of secondary waste generated during operations.The cold crucible is a compact water-cooled melter in which the radioactive waste and the glass additives are melted by direct high frequency induction. The cooling of the melter produces a solidified glass layer that protects the melter's inner wall from corrosion. Because the heat is transferred directly to the melt, high operating temperatures can be achieved with no impact on the melter itself.COGEMA plans to implement the cold crucible technology to vitrify high level liquid waste from reprocessed spent U-Mo-Sn-Al fuel (used in gas cooled reactor). The cold crucible was selected for the vitrification of this particularly hard-to-process waste stream because it could not be reasonably processed in the standard hot induction melters currently used at the La Hague vitrification facilities : the waste has a high molybdenum content which makes it very corrosive and also requires a special high temperature glass formulation to obtain sufficiently high waste loading factors (12 % in molybdenum).A special glass formulation has been developed by the CEA and has been qualified through lab and pilot testing to meet standard waste acceptance criteria for final disposal of the U-Mo waste. The process and the associated technologies have been also being qualified on a full-scale prototype at the CEA pilot facility in Marcoule. Engineering study has been integrated in parallel in order to take into account that the Cold Crucible should be installed remotely in one of the R7 vitrification cell. This paper will present the results obtained in the framework of these qualification programs.

Phase separation and crystallization of borosilicate glass enriched in MoO3, P2O5, ZrO2, CaO

“…Molybdenum and phosphorus are poorly soluble in silicate [1] and sodium borosilicate [2], [3] glasses (less than 1 mol%). Increasing their concentrations can lead to the separation of stable or metastable phases and to crystallization [4,5].…”

MoO3 incorporation in magnesium aluminosilicate glasses

“…GCMs wasteform can be a useful compromise between glasses and ceramics, being easier and less expensive to prepare than conventional ceramics, but offering higher durability than glasses [40][41]. This is the approach adopted to date in France for waste with high MoO3 content [2,14]. Nevertheless, the glasses developed here represent an alternative approach in that promote high MoO3 solubility in a fully vitreous wasteform.…”

“…However, post-operational clean-out wastes from Sellafield in the UK and some French wastes (e.g. [2]) contain high levels of MoO3 hence identifying glass composition with high MoO3 compatibility is desirable.…”

“…MoO4 2-tends to form alkali or alkaline earth molybdate crystals on separating out from the glass network and therefore the molybdate solubility can be related to the cations with which the MoO4 2-tetrahedra are associated. Tailoring glass composition so as to improve molybdate solubility in borosilicate glasses has been undertaken by a number of workers [2,[14][15][16][17]; however, the solubility is still low. For example, Caurant and co-workers [14,17] have found that increasing the B2O3 level in a Ca-Na borosilicate glass, which is supposed to consume more Na + cations thereby enabling more MoO4 2-units to be connected with Ca 2+ , does not enhance molybdate capacity in glass in reality.…”

“…Some of the high level nuclear waste (HLW) produced in the UK and France contains a high concentration of MoO3 [1][2], one of the most challenging oxides occurring in the vitrification of radioactive wastestreams on account of its low solubility (≤ 1 wt% ) in the conventionally used borosilicate glasses [3]. MoO3 at higher levels in nuclear waste glasses can cause the formation of "yellow phase" (a mixture of alkali and alkaline earth molybdates with chromates and sulphates) [4][5], which is detrimental to HLW vitrification process.…”