GENIUS: Fabrication of Gen-IV nitride fuels

Dr. Mikael Jolkkonen, Dept. of Reactor Physics, KTH, Stockholm

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Nitride fuels in the pastThe advantages of nitride fuel have long been recognised. Investigations of their suitability for practical use started fifty years ago, and later the BR-10 reactor operated for 18 years on uranium nitride fuel.

The nuclear fuel industry is presently based on uranium oxide and MOX. Perhaps for this reason, most attempts to produce nitride fuels have used oxides as starting material.

The oxide is in that case converted to nitride by reduction with carbon dust and simultaneous nitriding with N2 (carbothermic nitriding). That method has some disadvantages:

• The process requires very high temperatures ( > 1700 °C )

• Some oxygen and carbon remains dissolved in the produced nitride

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Properties of nitride fuels (1)

Compilation by courtesy of Jérémy Caruso, 2010.

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Properties of nitride fuels (2)

Graphics from Minato et al., “Thermochemical and thermophysical properties of minor actinide compounds,” Journal of Nuclear Materials 389 pp. 23-28, 2009.

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• Only one type of solid actinide nitride (cubic mononitride)

- exception: Th2N3, U2N3 at high PN2

• No changes in crystal structure with increasing temperature

• Actinide mononitrides are mutually soluble (within limits)

The actinide mononitrides have

- high fissile density

- high melting point ( >2700 °C )

- acceptable thermal stability (UN, PuN)

- excellent thermal conductivity

- good compatibility with most materials including steel and liquid lead/LBE

- proven reliability and reprocessability (UN, PuN)

Properties of nitride fuels (3)

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However,

1. Manufacture is more complicated than for oxides.

2. Sintering to high density is difficult.

3. Pure AmN has limited thermal stability (safety limit ~ 1800 K).

4. 14N is partially converted to radiotoxic 14C during burn-up.

5. Nitride powders oxidise in air, and nitride pellets decompose in hot water.

We will discuss these issues one by one.

Properties of nitride fuels (4)

Production of uranium mononitride pellets, LLNL.

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Carbothermic nitriding leaves traces of oxygen and carbon. Cause: The product is essentially a nitrogen-rich monophasic carboxynitride that can never be rendered 100% pure. Suggestion: Metal, not oxide, as starting material. This is a low-temperature process ( 400 - 1200 °C ).

Materials are hard to mill, sinter poorly, produce low-density pellets.Cause: Materal (nitride) properties and particle size of powders.Suggestion: Nitriding of uranium hydride (spontaneousy formed as very fine dust), very high temperature sintering.Or: Hot-pressing (Spark-Plasma Sintering, SPS) of the nitride powder.

Powders become degraded by air and moisture, also risk of ignition.Cause: Handling, transfer and storage during preparation.Suggestion: Metal → hydride → nitride conversion in one single run. Nearly all work and handling performed in dry nitrogen atmospere.

1. Difficulties with manufacture

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Joseph E Burke and Cyril Stanley Smith (1947): The Formation of Uranium Hydride. Journal of the American Chemical Society 69, pp. 2500-2502.

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Synthesis, analytical run (by TGA)

UH3 - UN

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In our research programme, we have had reason to study methods for producing mixed nitrides with inert additives. This work also has value for producing mixed actinide nitrides. However, we are currently working with U-Zr solid solutions. Initially, we produced ZrN separately.

Inert additives: Zirconium

ZrH2 (intermediate product) ZrN (final product, incomplete reaction)

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TGA (thermogravimetric analysis) is useful for establishing reaction temperatures. Yet an exothermic reaction may proceed differently in analytical scale (rapid heat loss) and preparative scale (self-heating sample). And TGA technology is not feasible on preparative scale.

Our problem was that we in fact did not know exactly how the reaction proceeded in our preparative furnace.

Our diploma thesis worker T. Hollmer developed a system for both regulating the reaction by electronic control of the gas supply (earlier done manually), but more importantly also for monitoring and quantifying the reaction on-line by measuring and integrating the gas consumed and produced in the reactions

U + 1.5 H2 → UH3 (1.5 mol gas consumed / mol UH3)UH3 + 0.75 N2 → UN1.5 + 1.5 H2 (0.75 mol gas produced / mol UN1.5)UN1.5 → UN + 0.25 N2 (0.25 mol gas produced / mol UN)

Monitoring technology advances

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Synthesis, preparative run, hydriding only (by gas monitoring)

(This graph shows hydriding of Zr, which is performed analogously to U)

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Quality of the productsUN and ZrN powders of good quality are produced. Capacity is approx. 50 g UN per 2 days or 100 g ZrN per 2 days.

UN is produced as very fine, free-flowing powder. ZrN requires additional milling before mixing with UN and pressing.

XRD analysis reveals very low amounts of foreign phases in the powders. Lattice parameters are in close agreement with the literature values.

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2. Sintering to high densityThe density achieved depends on material purity, nitrogen content, particle size, temperature, sintering atmosphere et c. Additives ("sintering aids") may be used to improve the results.An alternative to traditional sintering (by baking at extreme heat) has been tried at KTH. We have produced pellets of 90-94 %TD by SPS, "Spark Plasma Sintering". Sintering times are reduced to 5 - 15 min.

We have by this method produced pellets of pure UN and ZrN as well as mixed nitride pellets. The method is very well suited for less thermostable nitrides, such as AmN.

← UN

ZrN →

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Solid-solution mixed nitridesA crucial step is the formation of solid solution between UN and PuN, AmN et c. which in traditional methods are initially present as separate grains in the pressed pellet.

Since the formation of solid-solution is a slow, diffusion-controlled process, it requires many hours of sintering near 2000 °C. It will thus not occur during a short SPS sintering run.

A solution would be to use the corresponding U-An alloy as raw material in the nitride synthesis. We hope that this would result in a mixed nitride powder that will permit the use of rapid sintering methods such as SPS.

We are presently working with U-Zr alloys produced by arc melting, in order to test this concept.

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Alloying experimentsThe arc furnaces presently available to us have small capacity (sample size ≈ 10 g). We are working with such samples of U-Zr alloy to find out whether a true solid-solution mixed nitride powder can be directly produced in the hydriding-nitriding process.

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Choice of model systemThe U-Zr alloy may not be ideal for predicting behaviour of MA-containing fuel materials. Apart from the chemical dissimilarity of Zr, these alloys are known to not have the atttracive property of forming fine powders spontaneously upon hydriding.

An alloy of U and rare earth metals (e.g. neodymium or cerium) might be more suitable as a U-Pu-MA model system.

Shuai et al.: Hydrogen absorption–desorption properties of UZr0.29 alloy. Journal of Nuclear Materials 301 (2002) pp 203–209.

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Preliminary results - alloy reactivityUnlike the pure metals, the U-Zr alloy reacted sluggishly with hydrogen. Some nitride powder was produced, but some metallic residues were also present after the reaction.

There may be several reasons for this:

- Insufficient reaction time due to long diffusion path- Lowered thermodynamic activity of metals in solid solution (alloy)- Formation of dense solid nitride layer with metallic appearance

Twice or thrice repeated synthesis typically reduces all of the alloy to nitride powder.

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Preliminary results - alloy nitrides (1)Synthesis from U-Zr alloy does not necessarily produce solid-solution nitride. At such (low) temperatures where the reaction equilibrium is shifted towards hydrides, U and Zr have a solubility gap. Thus we can expect UN and ZrN to form separately.

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Preliminary results - alloy nitrides (2)Conversely, at such (high, 700 °C) temperatures where U and Zr are mutually soluble, the reaction equilibrium is shifted towards metallic uranium. UH3 is not stable unless excessive H2 pressures are applied.

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Preliminary results - monophasic alloyA solid-solution (U,Zr)N powder would be valuable as it would allow direct SPS pressing of mixed-nitride pellets. One option could be to perform low-temperature hydriding of a quenced (forced-solution) U-Zr alloy. Preliminary analysis indicates that rapid cooling indeed traps the majority of Zr in the U phase. Care must then be taken that phase separation does not occur during hydriding, which should be performed at very modest temperatures. It is still unclear what microscopic structure the produced (UH3,ZrH2) product would have.

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Ongoing work - monophasic alloyA second option is to indeed carry out the synthesis at such temperature where U and Zr are mutually soluble. We must then dispense with the separate hydriding step and perform direct nitriding either with N2, N2 + H2 mixture, or NH3. We expect the first results within short.

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Application to plutoniumWhile the mentioned problems complicate synthesis of nitrides from U-Zr alloy, no such difficulty is expected with Pu. In fact, while PuH3 decomposes already at 260 °C, PuH2 is stable well above the melting point of metallic Pu. At the same time, Pu and Zr are mutually soluble down to below 400 °C. As PuH2 is not isostructural with ZrH2, extended mutual solubility of the hydrides seems not to be the case, but interesting mixed hydride compounds exist in the relevant temperature region and can be promising precursors for nitriding.

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The actinide nitrides tend to dissociate upon melting or even before melting. This is more pronounced in the nitrides of the minor actinides.

Evaporation of americium in the production of MA-bearing fuels is a major concern and has been observed in the carbothermic nitriding process as well as during sintering. These manufacturing steps require temperatures near or above the AmN dissociation temperature.

Using the low-temperature method of synthesis from metal alloy, and the fast method of SPS sintering, would considerably alleviate these problems. However, neither of these unconventional methods are likely be adopted by the nuclear fuel industry any time soon.

The in-core stability of MA-bearing nitride fuels has been studied by modelling at KTH. As e.g. AmN will be in solid solution with other nitrides, its chemical activity will be lowered and the tendency to dissociation decreased. Temperatures exceeding 1500 °C will be required to evaporate any macroscopic amounts of material.

3. Thermal stability

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For small test batches, the production of 14C is of minor concern. However, if nitride fuels would be widely used for a century, the produced 14C is sufficient to elevate atmospheric amounts considerably above natural levels.

14C can in principle be captured at reprocessing and turned into concrete-like blocks, but this would add a new category of radioactive waste and to some extent defeat the purpose of GenIV reactors.

The generally accepted conclusion is that isotopically purified 15N must be used in the production of nitride fuel. This adds a tolerable cost.

However, any unconsumed 15N must be recycled in the production process. The conventional method of carbothermic nitriding is very wasteful with N2 (a large flow is needed to remove produced CO) and recovering such volumes may be difficult.

Fortunately, synthesis from metal is more economical in this respect.

4. 14C production

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5. Sensitivity to waterPure UN is incompatible with hot water or steam. This is of course nearly irrelevant for fast reactor applications, as long as fuel production is performed in dry atmosphere. However, there is some interest in nitride fuels also for LWR applications.

ZrN shows much higher (but not complete) resistance. This is presumed to be due to formation of a dense zirconia surface layer.

We are investigating whether this mechanism may protect a mixed (U,Zr) nitride, and at which compositions the effect is seen.

Our first results indicate that an unexpected variable is of great importance for the resistance: the oxygen content. In fact, commercial ZrN of poor quality (oxynitride) is far superior to our own high-purity product.

Thank you for your attention