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• Synchrotron X-ray tomographic scans on miniaturised ILW drum – simulant packages. • Grout failure to accommodate internal uranium corrosion – induced volume expansion. • Steel encasement did not exhibit signs of distortion. • Two fold role of the uranium oxide on the behaviour of the waste packages. Within the British nuclear inventory, waste is classified into three main categories, including Intermediate Level Waste (ILW). ILW mainly consists of fuel casing metallic swarf, where parts of uranium metal are also expected to have been agglomerated. ILW treatment route includes the material storage in stainless steel drums and subsequent grout encapsulation. The solidified matrix is believed to maintain the radioactive waste fully confined and immobile. Recent inspections revealed mechanical distortion around the steel liner in a proportion of the inspected drums. The distortion has been ascribed to the corrosion of the encapsulated metallic parts. The generated corrosion products occupy greater volume than the parent metal, resulting in an internal volume expansion which imposes stress/strain on the encapsulants. This study attempts to correlate the level of internal corrosion with the magnitude of package degradation in ILW drum–simulant systems by favouring the use of powerful synchrotron X-rays. [ABSTRACT FROM AUTHOR]

Abstract In the UK, the typical route being followed to handle metallic Intermediate Level Waste (ILW) derived from decladding of irradiated nuclear fuel is to primarily deposit it within stainless steel drums/boxes and subsequently encapsulate it into grout. Thus, a monolithic bond between the waste and the encapsulants can be achieved. However, recent inspections have revealed bulges around the circumference in a considerable proportion of the inspected packages. This distortion has been ascribed to the interior metallic corrosion–induced volume expansion, occurring throughout the waste packages storage period. Throughout this period, the volume expansion taking place in the core of the ILW drums has imposed continuously growing levels of strain on the surrounding encapsulants, i.e. grout and steel confinement. As a result, the encapsulants may exhibit considerable damage, in the form of cracks in the grout or/and steel distortion, threatening the integrity of the packages and, consequently, raising safety concerns within the nuclear community. Herein, experimental work performed on ILW drum–simulant packages is described, attempting to evaluate the performance of the packages under an internal corrosion–induced volume expansion. Graphical abstract Image 1 Highlights • Accelerated corrosion experiments on uranium encapsulated in grout. • Grout degradation due to interior corrosion–induced volume expansion. • Cracks in the grout initiate and propagate during early corrosion stages. • Migration of the corrosion products does not occur at early corrosion stages. [ABSTRACT FROM AUTHOR]

To accurately predict the initiation and evolution of uranium hydride potentially present in nuclear waste containers, studies of simulated conditions are required. Here, for the first time, the uranium-deuterium reaction was examined in-situ, in real time, whilst within grouted media. A deuterium gas control rig and stainless steel-quartz glass reaction cell were configured on a synchrotron beam line to collect X-ray diffraction and X-ray tomography data. It was found that deuteride formation was limited by the uranium and grout thermal conductivities and deuteride initiation only commenced above a threshold temperature. Strong adherence between uranium oxide and grout was also observed. [ABSTRACT FROM AUTHOR]

The encapsulation of metallic waste occurring from the fuel decladding process into cementitious formulations is a typical path within the British nuclear community. The metallic–grout mixture is stored in stainless steel drums which are maintained temporarily in ambient conditions before ultimate disposal. Some of these drums have exhibited bulges ascribed to internal metallic corrosion. The corrosion products, which occupy larger volume than the parent metals, generate stresses on the surrounding material, which inhibit the long term safety of the containment. Herein, finite element modelling was used to address the effects of the internal volume expansion on the grout and the steel integrity. A parametric study is presented to determine how several factors including the type of the grout, the location and the size of encapsulated metallic parts and the friction at the material interfaces may alter the system’s behaviour. [ABSTRACT FROM AUTHOR]

The present work examines the effect of surface geometry on the reaction between hydrogen gas and uranium metal, forming uranium hydride (UH 3 ), a pyrophoric compound of significance to the civil nuclear industry. Hydride formation was initiated on uranium samples that had been patterned with a focused ion beam instrument to form surface arrays of triangular prisms and pillars with differing apex angles. Post reaction analysis indicated preferential hydride formation at the apex of these features. Additionally, once hydride formation had commenced the observed growth rate on the prisms appeared to accelerate in comparison to the rate exhibited on the surrounding surface. [ABSTRACT FROM AUTHOR]

Decommissioning of the damaged Chernobyl nuclear reactor Unit 4 is a top priority for the global community. Before such operations begin, it is crucial to understand the behaviour of the hazardous materials formed during the accident. Since those materials formed under extreme and mostly unquantified conditions, modelling alone is insufficient to accurately predict their physical, chemical and, predominantly, mechanical behaviour. Meanwhile, knowledge of the mechanical characteristics of those materials, such as their strength, is a priority before robotic systems are employed for retrieval and the force expected from them to be exerted is one of the key design questions. In this paper we target to measurement of the standard mechanical properties of the materials formed during the accident by testing small-scale, low radioactivity simulants. A combined methodology using Hertzian indentation, synchrotron X-ray tomography and digital volume correlation (DVC), was adopted to estimate the mechanical properties. Displacement fields around the Hertzian indentation, performed in-situ in a synchrotron, were measured by analysing tomograms with DVC. The load applied during the indentation, combined with full-field displacement measured by DVC was used to estimate the mechanical properties, such as Young's modulus and Poisson's ratio of these hazardous materials. Unlabelled Image • Chernobyl "lava-like" fuel-containing material (LFCM) can be classified as brittle. • Porosity varies depending on the type of the LFCM (brown or black). • The range of Young's modulus for the Chernobyl lavas varies between 5 GPa and 16 GPa. • The stiffness of brown lava is almost 2.5 time greater than that of black lava. [ABSTRACT FROM AUTHOR]

• Time-resolved investigation of uranium corrosion in Magnox sludge • Corrosion initiates within the first 20 days of sample age • Corrosion morphology suggests the formation of uranium hydride (UH 3) • Migration of corrosion products within the sludge has been observed During the early years of nuclear industry within the UK, the typical route followed to handle Magnox (Mg-Al alloy) casing after separation from the nuclear fuel has been immersion in water temporarily in specifically-designed water ponds. Over the decades of storage, Magnox has been corroding in water, resulting in the formation of sludge within the ponds; known as Corroded Magnox Sludge. Uranium, which has been also contained within the ponds as a result of faults occurring during fuel-casing separation, is susceptible to corrosion in water-rich environments. Under certain circumstances, uranium corrosion products can be highly pyrophoric during oxidation in air. This raises serious concerns regarding the safety margins of nuclear pond decommissioning. The present study attempts to shed light on the behaviour of uranium in a Magnox sludge-surrogate environment, using X-ray computed tomography at distinct time intervals over an 18-month period. Focus was given onto the uranium-sludge interface to determine metallic corrosion onset and evolution. [ABSTRACT FROM AUTHOR]

How do you characterise the contents of a sealed nuclear waste package without breaking it open? This question is important when the contained corrosion products are potentially reactive with air and radioactive. Synchrotron X-rays have been used to perform micro-scale in-situ observation and characterisation of uranium encapsulated in grout; a simulation for a typical intermediate level waste storage packet. X-ray tomography and X-ray powder diffraction generated both qualitative and quantitative data from a grout-encapsulated uranium sample before, and after, deliberately constrained H 2 corrosion. Tomographic reconstructions provided a means of assessing the extent, rates and character of the corrosion reactions by comparing the relative densities between the materials and the volume of reaction products. The oxidation of uranium in grout was found to follow the anoxic U + H 2 O oxidation regime, and the pore network within the grout was observed to influence the growth of uranium hydride sites across the metal surface. Powder diffraction analysis identified the corrosion products as UO 2 and UH 3 , and permitted measurement of corrosion-induced strain. Together, X-ray tomography and diffraction provide means of accurately determining the types and extent of uranium corrosion occurring, thereby offering a future tool for isolating and studying the reactions occurring in real full-scale waste package systems. [ABSTRACT FROM AUTHOR]