Nitrogen under cladding of fuel pins with mixed uranium-plutonium nitride

Nitrogen under cladding of fuel pins with mixed uranium-plutonium nitride* Aleksey F. Grachev1, Liudmila M. Zabudko1, Evgeny E. Marinenko2, Sergey I. Porollo2, Anna V. Belyaeva3, Feodor N. Kryukov3, Vadim G. Teplov3, Mikhail V. Skupov4 1 Proryv JSC, 1 Ramenskiy Blvd, 119607 Moscow, Russia 2 IPPE JSC, 1 Bondarenko Sq., 249033 Obninsk, Kaluga Reg., Russia 3 NIIAR JSC, 9 Zapadnoye Sh., 433510 Dimitrovgrad, Ulyanovsk Reg., Russia 4 VNIINM JSC, 5a Rogov St., 123098 Moscow, Russia Corresponding author: Sergey I. Porollo ([email protected]) Academic editor: Georgy Tikhomirov ♦ Received 25 June 2025 ♦ Accepted 24 September 2025 ♦ Published 9 October 2025 Citation: Grachev AF, Zabudko LM, Marinenko EE, Porollo SI, Belyaeva AV, Kryukov FN, Teplov VG, Skupov MV (2025) Nitrogen under cladding of fuel pins with mixed uranium-plutonium nitride. Nuclear Energy and Technology 11(3): 225–229. https:// doi.org/10.3897/nucet.11.173059 Abstract In gas-bonded fuel pins the interior space is filled with helium in order to provide heat removal from the fuel. Under irradiation, as a result of gaseous fission products release, the initial gas composition under the fuel pin cladding changes, which leads to a change in its thermophysical characteristics. Post-irradiation examinations have shown that the partial pressure of nitrogen under the fuel pin cladding increases with fuel burn-up increase, since nitrogen atoms also release from uranium-plutonium nitride fuel under the cladding, in addition to the krypton, xenon and helium inert gases. The mechanisms of release of inert gases and nitrogen from nitride are different: the diffusion mechanism for inert gases and the knocking out of fission fragments for nitrogen. The difference in the gas release mechanisms leads to a significant quantitative difference in the gases release under the cladding. The specific nitrogen yield under the fuel pin cladding is significantly lower than the specific yield of other gases, but its presence is an important factor, since the dissociation temperature of the mixed nitride, as well as the nitriding of the inner surface of the cladding, depends on the nitrogen pressure under the fuel pin cladding. A generalization and analysis of the results of post-irradiation examinations of the nitrogen content in the gas mixture under the claddings of 87 investigated fuel pins after irradiation in the BN-600 reactor as part of 17 experimental fuel assemblies with mixed uranium-plutonium nitride fuel, irradiated to maximum fuel burn-up from 3 at. % to 9 at. % is carried out in the paper. Keywords mixed uranium-plutonium nitride fuel, experimental fuel pins, experimental fuel assembly (EFA), combined experimental fuel assemble (CEFA) post-irradiation examination (PIE), nitrogen partial pressure, fission gas products, cladding nitriding, inert gases Copyright Grachev AF et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Nuclear Energy and Technology 11(3): 225–229 DOI 10.3897/nucet.11.173059 Research Article * Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2025, no. 2, pp. 40–49. Grachev AF et al.: Nitrogen under cladding of fuel pins226 Introduction In order to provide heat removal from the fuel in gas-bonded fuel pins, the pin internal space is filled with helium. Depending on the fuel pin design and its fabrication technology, the quantity and pressure of helium under the cladding may vary. The release of gas fission products at the irradiation results in the initial gas composition change under the fuel cladding which changes the fuel pin thermophysical parameters. Therefore, the study of the kinetics and mechanisms of gas release from the fuel as a result of irradiation is one of the most important PIE task. In the Porollo et al. 2017; Grachev et al. 2020 papers the PIE results on the release of gas fission products (Xe and Kr) and helium under the claddings of uranium and uranium-plutonium nitride pins after the irradiation in the BR-10 and BN-600 reactors are presented. It has been found that xenon, and then helium and krypton are released from nitride fuel in the largest quantities. Systemic post-irradiation examinations at NIIAR JSC have shown that the gas mixture under the irradiated nitride fuel cladding includes nitrogen as well. Despite the fact that, in percentage terms, the amount of nitrogen under the cladding is not too large compared to other gases, it is an important factor, since the dissociation temperature of the mixed nitride, as well as the nitriding of the inner surface of the cladding, depend on the nitrogen pressure under the pin cladding. This paper summarizes and analyzes the results of post-irradiation examinations of the nitrogen content in the gas mixture under the claddings of 87 examined fuel pins after irradiation in the BN-600 reactor as part of 17 experimental fuel assemblies with mixed uranium-plutonium nitride fuel, irradiated to maximum fuel burn-up from 3 at. % to 9 at. %. Results of measurements of nitrogen content under the nitride fuel pin cladding of BN-600 reactor As defined by specifications, at the fabrication the internal space of the BN-600 reactor fuel pins with nitride uranium-plutonium fuel is filled by helium with a volume content of not less than 94% under atmospheric pressure. Assuming that the rest of the gas mixture is air (78% of nitrogen), the maximum partial pressure of nitrogen under cladding in the initial state is not expected to exceed 4.7·10-3 MPa. The puncturing, prior to irradiation, the cladding of two “witness” fuel pins (No. 0050, No. 0420) from the CEFA-6 combined experimental fuel assembly showed that the partial pressure of nitrogen in these pins was 2.8·10-3 and 3.12·10-3 MPa, that is, the fuel pins complied with the specifications (Fig. 1). At the cladding puncturing of the BN-600 nitride pins carried out in NIIAR JSC, the gas composition was also analyzed along with measuring of the total gas amount under the cladding, including measurement of the nitrogen percentage. Fig. 1 shows the average values of the nitrogen partial pressure under CEFAs pin claddings together with the partial pressure of nitrogen in the CEFA-6 “witness” pins, and presents the design nitrogen pressure in the non-irradiated fuel pin filled by helium to 94%. Fig. 2 shows the average values of the partial nitrogen pressure under the examined EFAs fuel pin cladding. Results of experimental data analysis Fig. 3 shows the values of the nitrogen partial pressure under the cladding as a function of fuel burn-up. As it can be seen, the partial pressure of nitrogen under the cladding increases as fuel burn-up grows. Fig. 4 shows the change in the partial pressure of nitrogen under the cladding of examined pins as a function of the nitrogen mass fraction in fuel (specification data). Figure 1. Averaged values of the nitrogen partial pressure under the cladding of all nitride fuel pins (CEFAs-1, 2, 3, 6, 7) after irradiation in the BN-600 reactor, and of CEFA-6 “witness” fuel pins (No. 0050, No. 0420); * - design value of nitrogen pressure in the 94% helium-filled fuel pin. Figure 2. Averaged values of the nitrogen partial pressure under the cladding of all EFAs nitride pins after irradiation in the BN600 reactor. Nuclear Energy and Technology 11(3): 225–229 227 It follows from the presented data that • in all cases, with the exception of fuel pins No. 99 of CEFA-7, No. 68 of EFA-17, and No. 107 of EFA18, the partial pressure of nitrogen under the cladding of irradiated fuel pins is less than the initial value (nitrogen pressure in “witness” fuel pins); for CEFA-6, CEFA -2 and CEFA -3, the post-irradiation nitrogen pressure decreased more than tenfold as compared with the initial pressure; • the partial pressure of nitrogen under the examined claddings pins increases as fuel burn-up increases; • the partial pressure of nitrogen under the examined claddings pins does not depend on the mass fraction of nitrogen in mixed nitride fuel. Measurements of the nitrogen content under the fuel cladding has shown that, in most cases, the post-irradiation partial pressure is lower than the initial design pressure or the pressure in the CEFA-6 “witness” fuel pins (see Figs 1, 2). Specifically, this is apparent for fuel pins with the lowest fuel burn-up values (CEFA - 6, CEFA - 2, CEFA - 3). The post-irradiation partial pressure of nitrogen in these fuel pins is an order of magnitude smaller than the pressure of nitrogen at the fuel pin fabrication. This means that the nitrogen contained in fuel pin in the gas phase interacts at the very irradiation beginning practically fully with the fuel and/or with cladding. In principle, both of these processes are possible. The nitriding of inner cladding surface was observed for most of the examined mixed nitride pins (Porollo et al. 2022). At increased temperatures gaseous nitrogen can react with pre-stoichiometric uranium nitride (Nekrasov 1957). An analysis of specification data for the mixed nitride composition in examined EFAs pins shows that the nitrogen content in most fuel pins is lower than the exact stoichiometric composition of 5.55 wt. %. At the same time, it follows from Fig. 3 that the mixed nitride burn-up increase leads to an increased amount of nitrogen under the cladding. The only source of nitrogen in the pin is mixed nitride, in which half of the total atoms numbers are nitrogen atoms. One of the possible mechanisms for the nitrogen atoms release is through these being knocked out by fission fragments formed in fission of uranium or plutonium atoms (Olander 1976; Lustman 1985). Unlike inert gas atoms (Kr, Xe, He), which can move freely in uranium nitride at a sufficiently high temperature, nitrogen atoms are located in the crystal lattice nodes, and their movement by diffusion is limited. Nitrogen atoms, which are located near the free surface, can be however knocked out by fission fragments and pass into gas under the fuel cladding. This process is shown schematically on Fig. 5, from which it can be seen that elastic collisions with fission products can result in the lattice atoms (uranium, plutonium or nitrogen) being knocked out the nitride surface. The intensity of this process is defined by many factors, the most important of these being the fission fragment generation rate (fission rate), the path length of primarily knocked out atoms and the nitride open surface area. And the greatest uncertainty in the calculations of the nitrogen atom knocked out rate from irradiated mixed nitride is associated with estimation of the fuel pellet open surface area in the fuel pin. In the initial state, there are more than one hundred fuel pellets in a fuel pin with the geometrical surface totaling some 250 to 300 cm2. Besides, sintered mixed nitride pellets have open porosity, which needs to be taken into account as well. At the beginning of irradiation at the reactor power increase, fuel pellets are fragmented which results in their open surface increased several-fold. Further irradiation leads to gas-filled pores forming within the fuel grains and at the grains boundary. After a certain burn-up value is achieved, Figure 3. Partial pressure of nitrogen under nitride pin cladding as a function of fuel burn-up: Blue square – CEFA fuel pins; Red circle –EFA fuel pins. Figure 4. Partial pressure of nitrogen under nitride pin cladding as a function of the nitrogen mass fraction in fuel: Blue square – CEFAs fuel pins; Red circle – EFAs fuel pins. Grachev AF et al.: Nitrogen under cladding of fuel pins228 the pores at the grain boundaries integrate into channels extending through the fuel pellet surface. This expands greatly the surface through which nitrogen atoms can be knocked out into the pin gas space. The estimated amount of nitrogen released under pin cladding after the irradiation in the BN-600 reactor, according to the dependencies presented in Olander 1976; Lustman 1985, with uncertainties taken into account, fits fairly well the measured values of the nitrogen pressure in irradiated fuel pins. The specific yield of nitrogen from mixed nitride at similar conditions is many times less than the specific yield of gas fission products (xenon and krypton) or helium (Fig. 6), which indicates once again that the yield of nitrogen atoms is only from rather thin surface layer of fuel pellets. Nevertheless, having nitrogen under the fuel cladding is important in terms of the mixed nitride dissociation at high temperatures. It is known that uranium nitride melts congruently at a temperature of 3100 K and a nitrogen pressure of 2.5·10-1 MPa and higher, the melting temperature of uranium-plutonium nitride being somewhat lower (~3045 K) (Carvajal-Nunez, Prieur, Manara 2014). At a lower nitrogen pressure, heating nitride to a high temperature causes it to dissociate. The maximum temperature of mixed nitride at normal reactor operation does not exceed 1500 - 1600 °С, this fitting the equilibrium nitrogen pressure above uranium nitride equal to ~6·10-9 MPa (Hayes et al. 1990). Adding nitrogen to a closed system, such as fuel pin, up to pressure of ~2·10-3 MPa will shift the uranium nitride decomposition reaction towards higher temperatures, and equilibrium will be reached in the system at a much higher temperature of ~2400 °C (Hayes, Thomas, Peddicord 1990). Scarce data show that the nitrogen pressure above mixed nitride at low temperatures is higher than that above uranium nitride, while this difference is smaller at a higher temperature (Rogozkin et al. 2003). One can therefore state that for intact fuel pins with a fuel burn-up of ≥ 8 at % the release of nitrogen from mixed uranium-plutonium nitride is an additional factor that prevents its dissociation. Conclusion The measurements presented in this paper for the nitrogen content under the cladding of experimental nitride fuel pins irradiated in the BN-600 reactor have shown that the post-irradiation partial pressure of nitrogen in irradiated fuel pins is in most cases lower than the initial design pressure value. This means that the nitrogen gas contained in fuel interacts practically fully at the very beginning of irradiation with the fuel and/or the cladding. At the same time, an analysis of the post-irradiation examination results show that the partial pressure of nitrogen under the cladding of examined fuel pins is higher at higher fuel burn-up, since the irradiation results in nitrogen atoms release from mixed nitride under the cladding in addition to inert gases (krypton, xenon and helium). The release mechanisms for inert gases and nitrogen differ greatly: it is a diffusion mechanism for inert gases and knocking out by fission fragments for nitrogen. The difference in the gas release mechanisms leads to a greater quantitative difference of gas release under the cladding. The specific yield of nitrogen under the pin cladding is much lower than that for other gases. However, for intact fuel pins with a burn-up of ≥ 8 at.% the release of nitrogen from mixed uranium-plutonium nitride is an additional factor that prevents its dissociation. Figure 5. Diagram of the atom knocking out from the crystal surface layer by fission fragments (Lustman 1985). Figure 6. Specific yield of krypton (Red square) and nitrogen (Blue circle) from mixed nitride fuel after irradiation in the BN-600 reactor. Nuclear Energy and Technology 11(3): 225–229 229 References Carvajal-Nunez U, Prieur D, Manara D (2014) Melting point determination of uranium nitride and uranium plutonium nitride: a laser heating study. 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