The target product of the head-end MUPNSNF pyroprocessing is the oxidized fuel powder, free of cladding segments and with minimal fission products. Suitable powder is required for manufacturing oxide fuel pellets or granules, which are then prepared for metallization through electrolytic reduction of oxides to metal (Zaikov et al. 2024). Volumetric oxidation (voloxidation), or, in a wider context, SNF high-temperature treatment, is the standard process for acquiring a product with the desired characteristics.

A HTT facility mock-up (hereinafter, the mock-up facility), designed for experimental purposes, allows for testing nitriding/denitriding processes, voloxidation of uranium nitride in cladding segments, and subsequent reduction to UO2 with a capacity of tens of kilograms per batch. There are no operational industrial facilities of this kind for spent nuclear fuel available in the world. Primarily, the development of voloxidizers is concentrated on LWR oxide fuel (Uchiyama et al. 1991; Park et al. 2007; Kim et al. 2008; Spencer et al. 2008; Wahlquist et al. 2008; Westphal et al. 2008; Lee et al. 2010; Kim et al. 2017), with limited data available for nitride MNF and SNF oxidation from small laboratory-scale experiments (Dvoeglazov et al. 2016; Dvoeglazov et al. 2018; Momotov et al. 2023). A comprehensive review of spent nuclear fuel HTT technologies is provided in (Nevolin 2020).

By restructuring the crystalline lattice, the mock-up facility can pulverize fuel pellets, separating the fuel from structural parts of the rods, and eliminate volatile fission products such as tritium and radiocarbon from the target product. In this study, the HTT modes proposed by Potapov et al. (2023) were tested.

Rotating retort furnaces are generally employed in the heat treatment of bulk materials, like in uranium oxide production, to ensure thorough particle mixing for consistent temperature and reaction gas exposure. The HTT facility for MUPNSNF utilizes this principle, but with notable distinctions due to its operational characteristics:

This paper addresses results of the development of the mock-up facility for the Reprocessing Module of the Experimental and Demonstration Energy Complex and its testing at the manufacturer’s plant and in the shielded cell with an inert (nitrogen) atmosphere.

Fig. 1 shows the HTT facility mock-up, which is a batch electric resistance heating furnace. It features a rotating retort, with its main units mounted on a support frame. A single charge of feedstock (segments of fuel claddings with nitride MNF) weighs a maximum of 15 kg. This represents the highest batch the HTT facility mock-up can handle, given the size constraints for the inert cell.

The product enters the retort from the container through the loading chamber on the left side of the mock-up facility. A gas-tight container docking system ensures tightness of the container-chamber interface.

By changing the inclination of the rotating retort and the “right” auger spiral in it, the product is advanced into the heated reaction zone. The “left” auger spiral prevents the product from shifting from the retort’s working area on the right side. After heat treatment, the product is discharged by altering the angle of the rotating retort and reversing its rotation, using the appropriate auger spiral. A dedicated electric drive on the mock-up frame allows for adjustments of the retort angle.

In the reaction zone, the retort is surrounded by a heating chamber that manages thermal treatment of the product. The heating chamber is equipped with thermal insulation, a hearth, a lid, and heating panels that include thermocouples for process control and monitoring. Remote installation and removal of the heating panels are possible inside a shielded inert cell with a manipulator.

A rotary seal system serves to seal the retort’s internal volume, establish a vacuum (or excess pressure) inside, and manage radial runout as the retort spins. In the rotary seal system, bellows fix misalignment between the retort and the loading chamber and cover the retort thermal expansion.

The mock-up facility includes a gas supply system with required flow rates, an off-gas treatment system, and a control system.

The experiments were aimed at testing the design efficiency of the mockup facility for MUPNSNF high-temperature treatment and establishing its actual performance under specified operation conditions.

The test objectives were:

The surrogates was chosen conservatively after reviewing literature on the properties of UN, UN_1.5, U₃O₈, and UO₂ powders after the HTT. This selection ensured the surrogate powder had no less particle terminal velocity, whereas its flowability, measured by the angle of repose, was comparable to these materials.

The simulated cladding were made of stainless steel tube segments (10 mm diameter, 0.5 mm wall thickness, and 20, 30, or 40 mm lengths). Al₂O₃ powder (1–5 μm particle size) served as a surrogate SNF powder. The average angle of repose for the Al₂O₃ powder was 38°.

For oxidation resistance tests of the 10CrNi45Al alloy retort material (GOST 5632-2014 (2019)), the specimens were set on zirconium oxide supports and placed in a stagnant air environment inside the high-temperature furnace. The specimens were held in a steady-state mode at a temperature of 1150 °C for 12, 24, 48, 100, and 864 hours. Fig. 2 shows the test time dependence of the corrosion rate. The corrosion rate of the 10CrNi45Al alloy decreased abruptly as oxidation time extended up to 100 hours, presumably due to a continuous protective oxide layer formed. After 860 hours of testing, the corrosion rate did not exceed0.04 mm/year.

The simulated products – 20 mm tube segments, 30 mm rod segments, fine aluminum oxide powder (Al₂O₃), and garnet sand (GS) for waterjet cutting – were used to perform tests for completeness of the product discharge without gas purging.

All of the rod segments were discharged completely. The undischarged coarse garnet sand made up no more than 4% of the charge, with most instances showing less than 1%. The finely-pulverized Al₂O₃ powder exhibited the worst discharge efficiency. Only 75% of the charge was released after the initial Al₂O₃ powder loading. The other 25% deposited on the retort surface. When 1.2 kg of Al₂O₃ powder was loaded and unloaded in follow-on tests, a decrease in the mass of the undischarged product was observed. The final two tests even resulted in a negative difference between the charged and undischarged powder masses, implying that the powder accumulated after previous runs had discharged from the retort. It could indicate that a stable amount of undischarged residue, ranging between 0.68 and 0.58 kg, was accumulating in the mock-up facility.

The loading and subsequent unloading of the Al₂O₃ powder +claddings mixture demonstrated that all claddings were discharged, leaving 39.7% of the powder in the retort. When viewed with a video endoscope, the retort inner volume displayed a thick compacted layer of Al₂O₃ powder on its surface and mixing blades.

Another batch of Al₂O₃ powder, free of cladding segments, was subjected to a new loading/unloading test that involved heating, 5-hour annealing at 1000 °C, and complete cooling. In the absence of claddings, the powder was unloaded fully, and the results indicated no compaction.

The issue of forming a dense layer on the retort surface, caused by the cladding segments and incomplete powder product discharge, was definitively confirmed by further tests with a powder-cladding mixture. So, it was decided to use a mechanical hammer knocking device in the mock-up facility.

In the final test of the upgraded mockup facility, the powder was discharged in full. It was found in dense compacted lumps that resisted crumbling in pouring out, but collapsed under considerable force. Thus, the issue of compacting the powdered product against the retort walls in the mock-up facility was resolved.

The temperature field in the empty retort was measured under isothermal conditions at 700 °C, 900 °C, and 1240 °C. As Fig. 3 shows, the temperature difference in the product area did not exceed 35 °C.

The experiment to map temperature fields inside the retort under different thermal conditions revealed the hottest surface areas of the HTT facility mockup. The hottest surfaces with temperatures of up to 400 °C were found on both sides of the retort at the heating chamber, specifically the loading chamber and the retort rotation drive.

Leak tests were performed while the retort rotated at 2 rpm in mixing mode under steady-state thermal conditions at 20 °C; 500 °C; 700 °C; 900 °C and 1000 °C. In the process, a vacuum pump of the local off-gas treatment system generated a vacuum of minus 50 kPa in the retort. Then, the pump was turned off, the shut-off valve on the vacuum line was closed, and the pressure increase (due to the gas leakage) in the retort was measured with a pressure transducer. The rate of gas leakage into the retort through the seals was determined by analyzing the time-dependent vacuum curves (Fig. 4). At a vacuum of 50 kPa, the rate of gas leakage into the retort was at most 0.06 m³/h. The gas leakage rate at a vacuum of 10 kPa was less than 0.015 m³/h that makes 6% of nitrogen supplied to the retort during nitriding and 0.33% of the gas supplied to the retort during the MNF oxidation and reduction. Thus, it was shown that under the operating parameters of the mock-up facility (vacuum and temperature) the additional gas leakage into the retort through the seals did not affect the HTT processes significantly.

In addition, the dust carryover with off-gases was evaluated using a simulated pulverized MNF (fine Al₂O₃ powder) at operating temperatures of 500 °C, 850 °C, and 1150 °C, with exhaust gas flowing at 5 m³/h (under normal conditions). A filter installed at the off-gas outlet of the retort was used to capture the dust. The filter element was made of mesh No. C 685-03Kh18N9T-VI. Based on the results of measuring powder mass collected in the filter after purging in different modes the dependence of the Al₂O₃ dust mass flow rate from the retort was determined as it shown in Fig. 5. There was a nonlinear increase in the dust flow with temperature, and it was almost double in argon compared to air.

The test results predicted that the maximum dust would occur during the denitriding operation in argon at a temperature of 1150 °C.

The tests were performed in an inert shielded cell at JSC “SCC” (Kuzmin et al. 2024) in a nitrogen atmosphere.

For the experiments, the MNF comprised a mixture of depleted uranium mononitride pellets and uranium nitride pellets enclosed in EP823 alloy steel tubes, simulating the chemical composition of the UN nuclear fuel rod segments. The MNF total mass was 8489.19 g, comprising 7418.74 g of uranium nitride pellets and 1070.45 g of cladding segments (324 pieces, 2 pellets per piece). The estimated mass of uranium in the charged UN was 7006.6 g.

The tests were conducted in two steps for one MNF batchto verify the following HTT modes:

The entire process of treatment, including heating and cooling, took about 100 hours.

Upon completion of the nitriding and denitriding, 6417.8 g of the MNF in cladding segments were discharged and sieved. The complete pulverization of the MNF pellets did not take place after nitriding and denitriding, as expected; the MNF in the claddings made up 25% of the entire fuel loaded into the mock-up facility. The particle size distribution in the MNF powder is given in Fig. 6. The bulk density of the powder was determined using a measuring cup and yielded a value of (6.5 ± 0.5) g/cm³.

Some cladding segments were dumbbell-shaped with a diameter of their widest parts increased by 0.5 mm and the one of their narrowest parts increased by 0.2 mm.

Conversion to mononitride was incomplete, either. X-ray diffraction analysis of the denitriding product indicated that it was a mixture of uranium mononitride and sesquinitride in near-equal proportions.

After the analysis, the MNF, with all its claddings, was reloaded into the HTT facility mockup and then underwent successive oxidation and reduction operations.

Oxidation was conducted in an Ar–10% O₂ gas mixture at 1 m³/h, followed by reduction in an Ar–4% H₂ gas mixture at the same flow rate. The oxygen concentration in the off-gas dropped to 2% during oxidation and then raised back to 10% over six hours. As reduction occurred, the hydrogen concentration in the off-gas saw a swift decline to 1.1%, followed by a recovery to 4% after 9 hours.

After the oxidation and reduction processes (step 2), uranium dioxide was a coarse powder with particles less than 100 µm making up just 12.5% (most particles were expected to fall between 50 and 100 µm). The sieve analysis results are shown in Fig. 7. When comparing the particle fractions, the bulk density for particles between 500 and 2000 µm was 1.99 g/cm³, whereas for particles between 100 and 500 µm, it measured 2.29 g/cm³.The mass of the discharged product was 5677.7 g.

The angle of repose of the powder ranged from 41° to 42°. The stainless-steel plate surface with roughness Ra 0.8 allowed for a sliding angle of about 30°, while the surface with roughness Ra 3.2 resulted in an sliding angle of about 45°. No visible residues remained on the plates after the powder was poured out.

The contents of 317 out of 324 claddings poured out as a powder. All claddings were intact, except for a single one that exhibited a longitudinal crack covering one-third of the segment length.

Compacted contents were found in seven segments of the simulated fuel rod claddings, which collectively weighed 46.24 g and showed no evidence of spilled-out powder. The powder filled up some of them, while in other segments, the MNF pellet pieces or whole pellets moved freely within claddings. One segment retained its original shape and showed no visible signs of chemical reaction. Obviously, it remained outside the reaction zone of the mock-up facility throughout the four high-temperature processing stages.

X-ray diffraction analysis of the product sample after oxidation and reduction showed that the powder was primarily UO₂ with minor amounts of U₂N₃; the U₃O₈ phase was not found.

The HTT facility mock-up was partially taken apart to find out how much uranium-containing material had deposited in its internals and where it was located. This involved using an endoscope to examine the retort and the loading chamber from the inside followed by cleaning the inner surfaces from the HTT deposits. The woven wire mesh filter of the local off-gas treatment system was opened to remove dusty uranium compound products after the HTT process. A material balance of uranium-containing product distributed over the units of the mock-up facility was drawn, as shown in Table 1.

The material balance gave the following quantitative evaluation of uranium deposits in the internals of the facility mock-up:

Presumably, the undischarged residue resulted from the particles breakthrough the woven wire mesh filter into the local off-gas treatment system pipes. Thus, the losses arising from the dust carryover from the mock-up facility were calculated to be 29%.

According to the MNF test results, the nitriding and denitriding processes were incomplete. These operations can be abandoned due to their low efficiency for the MNF pulverization, confining the MUPNSNF high-temperature treatment to oxidation and reduction only.

The loading and unloading modes require modification. Compacting the powdered material on the retort surface by fuel rod cladding segments is a probable reason for UN and UO₂ depositing in the mock-up facility. To ensure the complete product discharge, the operating modes of the mock-up facility require adjustment, specifically by increasing the amplitude and force of the mechanical hammer knocking device.

The tests revealed that 29% of the uranium in the loaded model product batch was carried over as dust, taking into account the amounts both retained by and passed through the filter.

The design of the HTT facility mock-up requires upgrades to minimize deposits in the internals and the dust carryover, and prevent a break of the dust particles through the woven wire mesh filter. These upgrades will also enable remote removal of the deposits from the gas cleaning filter. In addition, life tests of the mock-up facility are required to determine its lifetime.