Actinide mixed oxide conversion by advanced thermal denitration route

Abstract

In the framework of generation IV development for nuclear reactors, actinide mixed oxides are considered for multirecycling plutonium fuels and for transmutation targets of minor actinides. In this context, new processes are being developed for either the synthesis of mixed uranium-plutonium oxide compounds for MOx fuel or uranium-americium target fabrications. The main purposes are to simplify and step up industrial processes as well as to decrease actinide dust dispersion, and liquid effluent and gaseous releases. Among options for conversion route, a novel and patented advanced thermal denitration in presence of organic additives was established successfully to synthesize UO_2+δ, U_0.55Pu_0.45O_2±δ, and U_0.9Am_0.1O_2-δ oxides. Here, we describe the different intermediate steps of this process together with the characterization of the oxides obtained. The data highlight several advantages of this new route for actinide conversion.

Introduction

This study is part of the recycling of recoverable materials (U, Pu actinides) from spent nuclear fuels. Conversion is a key step at the interface between the separation and purification processes and fabrication steps of uranium-plutonium oxide fuels called MOx (Mixed Oxides). Briefly, it consists in converting quantitatively a solution containing the products obtained from purification processes. Generation IV (GEN IV) reactors should allow a multi-recycling of Pu as well as the transmutation of minor actinides, but conversion routes must suit additional needs compared to the current process:

• Continuous process with higher throughput will be necessary to increase conversion rates as the amount of matter to convert;

• Conversion must integrate fluxes with the minimum of possible additional operations, avoiding redox adjustments or concentration steps of feeding actinide solutions still for a better efficiency;

• Conversion process must be able to synthesize oxide powders with different Pu contents for both LWR and SFR MOx fuel fabrication;

• MOx synthesis has to be favored to prevent the proliferation risk associated to Pu multi-recycling;

• MOx or mixture of oxides synthesized must be adapted for fuel pellet shaping directly, without any grinding and sieving steps to reduce dust generation;

• Management of effluents and gas release should be simple and efficient processes.

Meeting these requirements explains the large number of studies conducted so far to synthesize mixed oxides containing both uranium and plutonium.

In the literature, actinide co-conversion routes can be categorized into three major families: precipitation routes, sol-gel routes and denitration routes. Although precipitation processes offer a complementary partitioning factor and allow the synthesis of a single phase U_1-xPu_xO_2±δ powder after a single calcination, these processes require a redox adjustment (U(IV)/Pu(III) oxalic co-conversion), bring filterability difficulties and produce effluents hardly manageable (carbonate conversion route) [[1], [2], [3], [4], [5], [6], [7]]. Sol-gel routes come in two forms: internal or external gelations [[8], [9], [10]]. They have the advantage of not forming dust with the synthesis of oxide microspheres, but whatever internal or external gelation routes, they involve the managing of a large amount of hazardous effluents, including ammonium nitrate solutions, and they cannot achieve the targeted cadences [7,10]. Denitration routes are more direct conversion pathways. Among them, microwave-assisted denitration of actinide salts was developed and industrialized by Japan Nuclear Fuel Industry Limited [11,12]. The dielectric properties of uranyl nitrates and plutonium nitrates, initially hydrated, make it possible to carry out microwave thermal co-denitration in four steps: heating, concentration, denitration, followed by a last complementary step for maintaining the denitrated product heating. The recovered product is a U₃O₈/PuO₂ mixture followed by calcination under a reducing atmosphere to form UO₂/PuO₂ mixture [13,14]. In Nitrox process [15], a concentrated solution has to be prepared first and cold crystallization follows. The powder obtained is then dehydrated and denitrated, with the objective of staying at a temperature below the melting temperature corresponding to the instantaneous decomposition of the salt [16]. MDD (modified direct denitration) process was developed at Oak Ridge National Laboratory [17,18]. This process involves the addition of ammonium nitrate to the initial solution of uranyl nitrate. This leads to the formation of double nitrate salts of uranyl and ammonium, which decompose without going through a melting state. A last step of calcination under reducing atmosphere is required to form a UO₂/PuO₂ mixture [7,19,20]. Advantages of all these thermal denitration processes are the rapidity of the reaction and the absence of filtration step. However, two major drawbacks can be noticed on these routes: a NOx treatment has to be added to avoid release and/or corrosion problems, and the production of actinide oxide dusts either directly or due to the requirement of a milling step prior to fuel pelletization [7].

As a result, none of the conversion routes studied so far suits all the additional specifications exposed below and associated to GEN IV fuel cycle. For these reasons, it was necessary to develop a new way of conversion with simultaneous management of uranium and plutonium. A present proposed chemical route is inspired by NPG (nitrate polyacrylamide gel) synthesis method [[21], [22], [23], [24]]. It permits a mono [25] or poly cationic [21] oxide synthesis and comprises four distinct steps. An aqueous solution containing cations and additives is prepared and then gelified in a crosslinked polymer by adding an initiator. The gelation makes it possible to trap all the cations present in solution in a homogeneous manner. This gel is then dehydrated to synthetize a xerogel which is then calcined to form the final oxide [21,25]. So far, this chemical route had never been applied to actinide oxides. Such a synthesis method is flexible, coping with high versatility in terms of acidity, cation concentration, use of chelating agents and choice in starting monomer [[26], [27], [28], [29], [30], [31]]. Based on these results, this work explores the so called “Advanced thermal Denitration in presence of Organic Additives (ADOA) process” conversion route, to synthetize mixed actinide oxides. It will also describe all the developments and adjustments performed to suit the requirements linked to the synthesis of nuclear fuel precursors (e.g. sulphur and chloride free using safe reagents).

This paper presents the synthesis by advanced thermal denitration in presence of organic additives of UO_2+δ, U_0.55Pu_0.45O_2±δ and U_0.9Am_0.1O_2-δ enabling to demonstrate the feasibility of such conversion route applied to single or mixed actinide solutions.