The key objective to be achieved as part of implementing a closed fuel cycle for two-component nuclear power system is to switch to using mixed uranium-plutonium fuel.

The BN-800 reactor is Russia’s first reactor based on mixed oxide uranium-plutonium (MOX) fuel that provides the ability to achieve a breeding ratio of over 1. Compared to uranium fuel, specific to uranium-plutonium fuel is in increased radioactivity, which requires dedicated fabrication facilities and respective onsite handling procedures.

The capability of MOX fuel to operate in conditions of the BN-800 reactor core was justified as a result of irradiating experimental FAs (EFA) in the BOR-60, BN-350 and BN-600 reactors. It was found, among other things, that the behavior of MOX fuel under irradiation is similar to that of oxide uranium fuel, and the key factor that limits the burnup of fuel is radiation resistance of the fuel cladding structural material (Kinev 2011; Vasilev et al. 2011; Vasiliev et al. 2013).

Commercial production of MOX fuel for the BN-800 reactor has been created at FSUE MCC. Due to a delay in the launch of this production against the BN-800 commissioning deadline, the start-up core, formed in 2014, consisted predominantly of fuel assemblies with uranium fuel manufactured by MSZ JSC. The MOXFAs fabricated at pilot production facilities of FSUE PO Mayak and RIAR JSC accounted for 16% of the total number of FAs (Belov et al. 2022). In the period of the hybrid core operation, the share of MOXFAs was changing depending on the available fabrication capabilities (Belov et al. 2022, 2025).

The transition to full MOX fuel core began with the 8-th reactor refueling in 2021and was practically completed by the beginning of operation cycle 11 (2022) when the share of MOXFAs in the core amounted to 93%. Uranium FAs were finally discharged from the core in the course of 12-th refueling. Thus, starting with operation cycle 13 (2023) the reactor core was composed fully of MOXFAs (Belov et al. 2022).

Due to differences in the nuclear properties of MOX and uranium fuel, the core’s neutronic performance and the FA irradiation conditions were changing through the reactor operation period under review (Belov et al. 2025).

The paper presents information on the experimental justification for the operating capability of MOX fuel and its use in the BN-800 reactor, including the change in irradiation conditions.

Essential in building the country’s first BN reactors was to produce a robust design of a fast neutron reactor. This was easier to do due to using the most proven fuel based on uranium dioxide which did not require a new fuel fabrication facility and was possible thanks to the well-developed uranium enrichment infrastructure available domestically. The development of MOX fuel began with fabrication, irradiation and post-irradiation examination of experimental fuel assemblies (EFA).

The first MOX EFAs were irradiated in the BOR-60 experimental reactor. In 1970–1992, pelletized MOX fuel was tested as part of EFAs composed of 19 fuel pins of Ø 6.9 × 0.4 mm in size. The content of plutonium in fuel was from 15% to 30%. The irradiation parameters achieved are presented in Table 1.

The fuel pins tested in the BOR-60 reactor retained serviceability reserve. The fuel post-irradiation examination results made it possible to justify the requirements for MOX fuel for the BN-350 and BN-600 reactors and to develop fuel fabrication methods to provide less chemical interaction between fuel and the cladding.

In 1988, four EFAs were manufactured for testing in the BN-350 reactor with MOX fuel pins designed for a burnup of 10% of heavy atoms (h.a.) and more. In these EFAs, the content of plutonium in fuel to provide the required irradiation parameters was 27.5%. The achieved irradiation parameters are presented in Table 2.

The results of the BN-350 EFA fuel examination have shown that all fuel pins had an operating capability margin.

The most representative tests were those in the BN-600 reactor where 42 EFAs with pelletized MOX fuel were tested. All fuel pins had a cladding of austenitic steel (ChS68-ID CW grade). Of these, 39 EFAs with fuel pins of the BN-600 design (with an upper axial blanket) and three EFAs of the BN-800 type with an upper sodium plenum and a separate upper absorbing shield in the form of a bundle of boron carbide containing absorber elements. Accordingly, the BN-800 EFAs used shortened fuel pins with an increased upper weld damaging dose. In total, there were about 5 thousand fuel pins tested successfully. All EFAs operated with no deviations found or their integrity lost throughout the irradiation time. The maximum irradiation parameters achieved in the BN-600 reactor for pelletized MOX EFAs are presented in Table 3.

The BN-800 EFA testing has confirmed the operation capability of FAs with a sodium plenum between fuel and the upper absorbing shield.

As a result of the post-irradiation EFA examination:

As noted above, before commercial fabrication of MOX fuel was started, the BN-800 reactor operated with a core equipped predominantly with uranium FAs. The share of MOXFAs during this period did not exceed 22%. Taking into account its specific configuration, the core used in the initial period of the reactor operation was called the hybrid core.

To flatten the core power distribution, the hybrid core had three types of uranium FAs used depending on fuel enrichment: the core’s central part is formed of uranium FAs with an enrichment of 18.5% (low enrichment zone or LEZ), the intermediate part is formed of FAs with an enrichment of 21% (medium enrichment zone or MEZ), and the peripheral part is formed of FAs with an enrichment of 24% (high enrichment zone or HEZ). MOXFAs with a plutonium (low-activity) enrichment of 18.7% and 19.5% for pelletized fuel and vibrocompacked fuel are installed respectively in the core’s peripheral part based on optimization regarding power distribution and sodium void reactivity effect (Kuznetsov et al. 2017a).

The FA array is surrounded by one row of the radial blanket (RB) assemblies containing depleted uranium dioxide. Further, there are steel shielding assemblies (SSA) and boron shielding assemblies (BSA), based on natural boron carbide, followed by the in-vessel storage (IVS) for spent FAs. In the first operation cycle, the in-vessel storage was filled with dummy assemblies (steel FA simulators).

For compensation of excessive reactivity, regulation and protection of the reactor, there are 30 CPS rods in the BN-800 core, including two control rods (CR) with natural boron carbide, 16 shim rods (SR) with boron carbide (60% boron-10 enrichment), nine emergency protection (EP) rods with boron carbide (92% boron-10 enrichment), and three hydraulically suspended passive protection (PP) rods also with boron carbide (92% boron-10 enrichment).

Hot-pressed boron carbide is used in CPS rods and in boron shielding assemblies.

There was a neutron source in the hybrid core center contained in a steel assembly (two ampoules with californium) with an intensity of 10⁹ n/cm²·s.

In the first operation cycle, there were six permanent reactivity compensator assemblies (PRCA), structurally similar to boron shielding assemblies, in the reactor core. The PRCAs compensate for the excessive reactivity caused by the absence of fission products in the core (no FAs with partial fuel burnup the presence of which is typical of steady-state refueling mode).

The layout of the hybrid core for the first operation cycle is shown in Fig. 1.

The design of pelletized MOXFAs used in the hybrid core is the same as that of FAs for the full MOX fuel target core having an upper sodium plenum and a boron carbide absorbing shield above it. Uranium and vibrocompacted MOXFAs had a conventional, as used in BN-600 reactor, design with an upper axial blanket. The height of the fuel columns was equal in all hybrid core FAs (900 mm) and the same as in the full MOX fuel core (Fig. 2).

The BN-800 reactor was operated with the hybrid core in the course of the reactor’s operation cycles 1 through 8. Its composition was changing during this period relative to the startup configuration depending on the MOX FA delivery. MEZFAs were installed into the core instead of spent pelletized MOXFAs after operation cycles 4 and 5. All MOXFAs were installed within the HEZ boundaries. The enrichment zone boundaries of the hybrid core during operation cycle 8 and those in the full MOX fuel core are nearly the same, and the difference consists in a slightly larger MEZ (36 FAs more).

The main design-basis mode of the reactor operation is steady-state refueling characterized by the same number of FAs reloaded during each refueling and the same duration of operation cycle. With the adopted scattered batch FA refueling pattern, there are simultaneously assemblies with different irradiation times in the core, being uniformly distributed through the core. There are three such batches formed in the main part of the core (480 FAs), and four batches in the peripheral part (84 FAs). This is done in such a way as to avoid significant azimuthal and radial redistribution of the content of fissile fuel elements in the course of FA reloading.

At the initial stage of the BN-800 operation, a special refueling mode was used to achieve a steady state for the core, in which FAs that had not fully exhausted their life were discharged from the core. To minimize losses for lack of energy generation by fuel assemblies, where possible, these FAs unloaded to the in-vessel storage were returned to the core for further irradiation.

In each operation cycle, the core was composed such that to provide a reactivity margin to be enough for it to operate for 155 equivalent full power days (efpd) taking into account compliance with the relevant regulatory requirements for reactivity balance. As a result, by the beginning of operation cycle 4, the distribution of FAs into batches depending on the number of operated intervals was practically the same as for the design-basis refueling steady-state mode.

Data on the number of FAs of different types in the hybrid core in the course of its operation are presented in Table 4.

The design lifetime of the hybrid core FAs was assumed to be the same as that of FAs for the full MOX fuel core:

Depending on the reactor operating conditions and the existing possibilities for the operation cycle and FA lifetime extension, the duration of operation cycle 5 was increased by 19 efpd.

The transition from the hybrid core to a full MOX fuel core was begun in the course of the 8-th refueling (before operation cycle 9 was started) in January 2021 and was carried out by sequential replacement of spent uranium FAs for fresh pelletized MOXFAs (Belov et al. 2022). Starting with operation cycle 13 (as from the autumn of 2023), the BN-800 reactor core comprises only MOXFAs.

The change in the core composition in the process of transition to the full MOX fuel core is shown in Table 5. The layout of the full MOX fuel core is shown in Fig. 3.

The peculiarity of the FA irradiation parameters for the full MOX fuel core, compared to the hybrid core, is a 15% higher neutron flux level. This is explained by plutonium’s neutronic peculiarities, as compared with uranium-235, which define the former’s better neutron multiplication properties and, accordingly, less content of fissile isotopes in fuel.

In the period of the hybrid core operation, MOXFAs were installed within the HEZ boundaries to optimize power distribution in the core: pelletized MOXFAs were installed in the HEZ row 1, and vibrocompacted MOXFAs, with allowance made for operating parameters, were installed in the peripheral row cells. In the course of the hybrid core operation, with regard for the above difference in the neutron flux level for uranium and uranium-plutonium fueled cores, MOXFAs were characterized by relatively low irradiation parameters, which were below the values reached for experimental assemblies and the values corresponding to those for irradiation in the full MOX fuel core. To justify the FA capability to operate in conditions of the same irradiation parameters as in case of the full MOX fuel core, the operating time of 34 pelletized MOXFAs was increased by one fuel cycle. As a result, a damaging dose of ~ 90 dpa and a burnup of 10% h.a. were achieved in the most heated FA. All fuel pins in the given assembly remained leak-tight after the irradiation.

The period of transition to the full MOX fuel core was characterized by a phased growth, as the share of MOXFAs in the fuel load increased, in the neutron flux density, and, accordingly, in the key operating parameters of the fuel pins, which defined their capability to operate (linear thermal power, cladding damaging dose and fuel burnup).

The values of the operating parameters for MOX fuel pins in the initial period of the BN-800 operation (from the hybrid core to the end of the transition to the full MOX fuel core) are given in Table 6.

Work is been currently under way to switch to a core using FAs with fuel cladding of more radiation resistant austenitic steel (EK164 grade) predicted to be capable to operate with the fuel cladding damaging dose values of ~ 115 dpa (Nikitina et al. 2016). Switching to this steel grade allows one to expect a maximum burnup of 12% h.a. to be achieved and the FA lifetime to be increased from 465 to 620 efpd.

The initial materials for the MOX fuel fabrication are plutonium and depleted uranium dioxides. The existing specifications permit various contents of plutonium and uranium isotopes in initial materials.

The detailed design of the full MOX fuel core was developed assuming that plutonium with a fixed (base) isotope composition (the same as for plutonium extracted from VVER SNF with a ²³⁹Pu content of 67%) would be used for the fuel fabrication. To allow raw materials with different isotope compositions to be used in the MOX fuel fabrication, a procedure is employed to adjust the mass fraction of plutonium in the uranium-plutonium mixture (Belov et al. 2025). This procedure is based on ensuring reactor criticality parameters at the end of the operation cycle. Such approach provides the reactivity margin required for the reactor operation in the course of the design-basis operation cycle. Calculations have shown that adjusting so the mass fraction of plutonium in fuel provides retention of the core performance and the FA operating conditions within the specified design limits with any combination of FAs differing in the isotope composition.

The procedure for adjusting the mass fraction of plutonium was introduced both at pilot production facilities of FSUE PO Mayak and RIAR JSC and at production facilities of FSUE MCC. Possibility was achieved so for using plutonium with any isotopic composition for the MOX fuel fabrication and for using in the core combinations of FAs with different contents of isotopes in fuel.

Low-activity plutonium (with a mass fraction of the ²³⁹Pu isotope in plutonium amounting to not less than 92%) was used to fabricate the hybrid core fuel at pilot production facilities of FSUE PO Mayak and RIAR JSC, as well as in the process of fine-tuning commercial production of fuel at the newly built production facility of FSUE MCC (Belov et al. 2025). Such material is characterized by relatively low radiation impact on personnel. High-activity plutonium dioxide began to be used for the MOX fuel production at FSUE MCC. The initial batch of MOXFAs based on high-activity plutonium extracted from VVER SNF (with the mass fraction of the ²³⁹Pu isotope in plutonium being in a range of 68% to 86%) was loaded into the reactor in the 10-th refueling (the summer of 2022) (Belov et al. 2025).

It is planned to fabricate MOX fuel further only based on high-activity plutonium. Using such plutonium warehoused at FSUE PO Mayak requires it to be repurified of americium accumulated in the process of plutonium storage. The currently achieved capacity of FSU MCC’s plutonium repurification unit (PRU) is not enough to supply raw materials for two programs: to provide the BN-800 reactor with MOX fuel and to generate MNUP fuel for the BREST-OD-300 reactor. In this connection, it is planned to apply freshly obtained in SNF reprocessing high-activity plutonium dioxide for the MOX fuel production without being repurified in the PRU, i.e. without removing the accumulated americium. The initial batch of FAs with such fuel was loaded into the core before operation cycle 14 started in July 2024.

Currently, as part of the Integrated Program for Justifying the Burning of Minor Actinides, the possibility is under consideration, as applied to the BN-800 reactor, for using fuel with an addition of minor actinides (americium and neptunium). To confirm experimentally the operating capability of such fuel, three FAs have been in the process of in-pile irradiation starting with operation cycle 14, which contain four fuel pins with americium and neptunium added in the amount of respectively 0.9% and 0.6% of the heavy metal weight.

MOX fuel began to be developed for being used in BN reactors with irradiation of EFAs in the BOR-60 experimental reactor and in early BN-350 and BN-600 power reactors. Based on the EFA irradiation experience, information was obtained on the behavior of fuel pins in conditions of irradiation, criteria of their working capacity were defined, and the burnup limiting factor (fuel cladding damaging dose) was determined.

The BN-800 became the country’s first BN reactor in which MOXFAs were used routinely. Due to the delayed commissioning of the MOX fuel production facilities, a hybrid core, formed predominantly of uranium FAs, was used in the initial period of the reactor operation in 2015–2021. The share of MOXFAs in this period was from 3% to 22%.

The transition to full MOX fuel core began in the summer of 2021 starting with the 8-th reactor refueling when the first major batch of MOXFAs was loaded into the core instead of spent FAs

By the beginning of operation cycle 11 (the summer of 2022), the share of MOXFAs amounted to 93%. Uranium fuel assemblies were fully discharged from the BN-800 reactor core after operation cycle 12.

Due to the difference in the neutronic properties of uranium and mixed uranium-plutonium fuel, the conditions for irradiation of MOX fuel pins during the transition to the full MOX fuel core changed noticeably but were nevertheless within the design limits in conditions of the reactor rated power.

Plutonium of different grades, in a range from low-activity plutonium (with the ²³⁹Pu isotope content of not less than 92%) to high-activity plutonium with a high content of the highest number plutonium isotopes, is used for the BN-800 reactor fuel. To allow plutonium with various isotope compositions to be used in production, a procedure was introduced to adjust the mass fraction of plutonium depending on the actual content of isotopes in raw material. This procedure is based on ensuring the reactor reactivity margin required for the reactor operation during an operation cycle of the rated duration (155 efpd).

Activities are been currently under way to replace sequentially FAs with fuel pin cladding of ChS68 steel (cladding damaging dose of up to 87 dpa) for FAs with fuel pin clad from more radiation resistant austenitic steel of the EK164 grade (damaging dose of up to 116 dpa), this will allow the fuel lifetime to be increased from 465 to 620 efpd and a maximum fuel burnup of 12% h.a. to be achieved.

Work is also continued to find out if it is possible to use the BN-800 reactor for burning minor actinides (neptunium and americium) at the expense of adding these long-lived nuclides homogeneously to MOX fuel. In 2024, irradiation of three combined FAs was started which, along with standard pins, contain four fuel pins with an addition of americium and neptunium. Based on the results of testing these fuel pins, their operating capability is expected to be confirmed experimentally which is required to burn minor actinides.