The paper deals with the influence of the strategy for using separated plutonium from spent fuel of thermal and fast reactors, as well as homogeneous burning of americium in the BN reactor core on the balance of americium in the Russian nuclear fuel cycle throughout the 21^st century. The assessment is carried out with the use of mathematical modeling of nuclear materials movement including nuclide transformations throughout the nuclear energy system based on the CYCLE code. Scenario modeling of the americium and Pu-241 accumulation was carried out in Russia’s two-component nuclear energy system model with thermal (VVER) and fast (BN) reactors. In so doing, spent nuclear fuel (SNF) reprocessing was simulated in 2 options: as priority reprocessing of SNF of VVER reactors (1) or SNF of BN reactors (2). In addition to americium accumulation in the system without burning, the accumulation of this actinide was studied taking into account its homogeneous burning in the MOX-fuel of fast reactors at the level of its equilibrium content of ~1%. It has been shown that the priority reprocessing of VVER spent fuel makes it possible to reduce the americium accumulation by the end of the century by ~8 tons, and the effect is achieved by using freshly separated plutonium with short-term cooling, thus, by priority the americium source is eliminated, without directly handling it. Homogeneous addition of americium to the fuel of fast reactors of the BN-1200 type at a level of ~1% makes it possible to stop accumulation of americium in a two-component energy system by 2070, stabilizing it at a level of ~40 tons in the scenario with priority reprocessing of VVER SNF and ~50 tons in the scenario with priority reprocessing of BN SNF.

two-component nuclear energy system, modeling, fast reactor, spent nuclear fuel, plutonium, americium, burning

One of unsolved problems that accompany the present-day nuclear power development is accumulation of long-lived minor actinides and, primarily, Am-241 (Dekusar et al. 2019). Approaches to utilization of this isotope have long been discussed in the scientific and technical literature (Rabotnov 1996; Actinide and Fission Product Partitioning and Transmutation, Status and Assessment Report 1999; Status of Minor Actinide Fuel Development 2009; Camarcat et al. 2011; Homogeneous versus Heterogeneous Recycling of Transuranics in Fast Nuclear Reactors 2012; Gulevich et al. 2020), but, despite an abundance of ideas and proposals, they are still far from real technical and technological implementation.

Meanwhile, it is well known that americium in a fast reactor core can both be produced under the action of nuclear reactions, and fission or transmute into other minor actinides (Gulevich et al. 2020). In this case, it will accumulate if its content in the fuel is below the “equilibrium” level, and will decrease otherwise. The “equilibrium” content of americium in the in-core fuel and in the ex-core fuel cycle is determined primarily by the content of the Pu-241 isotope in the isotope vector of plutonium, which, with a half-life of ~ 14.3 years, gives birth to Am-241.

Since the Pu-241 fission importance is almost one and a half times higher than that of Pu-239, it “burns” very effectively in a fast spectrum, thus, reducing the potential production of americium in the nuclear power fuel cycle and significantly reducing direct handling in future. And the higher is the Pu-241 content in the fuel, the more of it “burns” in a run through the core. With the initial content in the fuel of more than 10% (such plutonium is referred to as freshly separated high-burnup plutonium), the reduction can be more than twofold.

Therefore, the content of the Pu-241 isotope in the fast reactor fuel can have major effect on the amount of americium and the scope of its handling in a nuclear energy system formed by thermal and fast reactors.

This paper looks into the effect that the strategy of using plutonium separated from spent nuclear fuel of thermal and fast reactors, as well as homogeneous burning of americium in the BN reactor core, has on the balance of americium in the fuel cycle of nuclear power throughout the 21^st century. The assessment is based on mathematical simulation of the nuclear material movement processes taking into account the change in the nuclear material nuclide composition in the nuclear energy system (NES) using the CYCLE code (Kalashnikov et al. 2016). At the same time, a two-component NES with thermal and fast reactors is considered, which reflects the key characteristics of the advanced nuclear energy system in Russia.

Switching Russia’s nuclear power to a two-component regime with thermal and fast reactors and a single closed nuclear fuel cycle (NFC) is a strategic goal of the Rosatom State Corporation for the coming decades (The White Paper of Nuclear Energy 2020). However, the options of plutonium fuel cycle closure and plutonium management strategies as part of two-component nuclear power may be different (Alekseev et al. 2016; The White Paper of Nuclear Energy 2020; Tuzov et al. 2022). Here two approaches can be distinguished.

The first approach suggests that plutonium from thermal reactor SNF after a long-term cooling (up to 30 years or more) is used for the fast reactor startup. The content of the Pu-241 isotope in such plutonium is ~ 2 to 3%, and its isotope vector is much closer to the “equilibrium” composition of plutonium produced in a fast neutron reactor than that for freshly separated high-burnup plutonium. This makes a reactor operating with such plutonium much safer in terms of minimizing the lifetime reactivity margin and reducing the probability of reactivity accidents (The White Paper of Nuclear Energy 2020). On the other hand, the reserves of such plutonium are very limited and clearly insufficient to start up many fast reactors. At the same time, this plutonium requires repurification from americium, which further must be either stored or transmuted in a nuclear reactor (Tuzov et al. 2023). There are about 15 to 20 tons of such americium accumulated in the thermal reactor SNF in Russia (Dekusar et al. 2019).

The second approach, which has recently been initiated in connection with the conversion of the BN-800 reactor to full MOX fuel loading, suggests that plutonium is involved in the fuel cycle, which allows for a small content of americium. Moreover, the Pu-241 content in the plutonium vector is 10 to 12%, and the Am-241 content in the fuel is up to 1%. Such plutonium does not need repurification, which will significantly improve the technical and economic indicators of the fuel cycle.

Since the Pu-241 isotope in a fast reactor core “burns up” much faster than other plutonium isotopes, its content is more than halved in a run going down to ~ 5%. Thus, the acuteness of the problems of direct handling of americium is significantly reduced. However, due to rapid “burning” of the Pu-241 isotope, the use of freshly separated high-burnup plutonium does not contribute to minimization of the reactivity margin and extension of the reactor refueling interval, which may lead to a number of technical and economic constraints.

Both of these approaches are closely associated with the strategy of reprocessing spent nuclear fuel of fast and thermal reactors. Ultimately, the strategy’s objective comes down to the following: which, fast reactor or thermal reactor SNF, is the preferred choice for reprocessing in the specific conditions of the selected nuclear power development scenario. These two options are discussed below.

Based on the scenario modeling of two-component nuclear power with thermal (VVER-type) and fast (BN-type) reactors, a quantitative assessment of Pu-241 and americium production until 2100 is made. Model scenarios are based on a condition that the integrated installed capacity of Russian NPPs will grow to ~ of 31 GW (el) in 2035, to ~ 45 of GW (el) by 2050, and to ~ 91 GW (el) by 2100.

The boundary conditions to construct the scenarios were the Russian reserve of available natural uranium (512 thousand tons) (The White Paper of Nuclear Energy 2020) and the duration of the external fuel cycle for the reactors under consideration, which is equal to seven years for the SNF of VVER-1000/1200/TOI reactors, six years for the BN-800 and BN-1200M reactor core fuel, and three years for the fertile material of the radial blanket. The capacity structure in all the scenarios of development of a two-component NES with thermal and fast reactors is presented in Fig. 1 (Troyanov et al. 2024).

Figure 1. 

SNF reprocessing capacity for scenarios 2 and 3 (a) and scenarios 4 and 5 (b).

For fuel reprocessing and fabrication, all the scenarios give priority to the batches with the shortest storage time.

In the simulation of a two-component NES, consideration was given to the reference scenario of an open NFC with thermal reactors only (scenario 1) and two groups of a closed NFC scenarios with thermal and fast reactors, which correspond to the two above SNF reprocessing options:

• in scenarios 2 and 3, priority is given to large-scale reprocessing of thermal reactor SNF (VVER-1000 and VVER-1200/TOI) to be started in 2042 with the launch of the BN-1200M SNF reprocessing delayed until 2059. The SNF reprocessing capacities are determined based on the balance of separated plutonium – its integral supply at any given time should not be less than its loss without an excessive quantity accumulated;
• in scenarios 4 and 5, in contrast, priority is given to the BN-1200M SNF reprocessing, starting in 2038, with the launch of the thermal reactor SNF reprocessing delayed until 2054, also based on the plutonium balance.

In all the scenarios, americium received at its warehouse comes both from SNF reprocessing and from repurification of earlier separated plutonium. Scenarios 2 and 4 assume complete repurification of the warehouse separated plutonium from americium in the course of MOX fuel fabrication, and scenarios 3 and 5 are allowed to contain 1% of americium by weight of heavy metal, which roughly corresponds to its “equilibrium” content in the BN-1200M reactor. In scenarios 2 and 4, therefore, “clean” fuel is always loaded into the fast reactor core, and in scenarios 3 and 5, americium is always homogeneously present in the fuel at the level of its equilibrium content (Tuzov et al. 2023).

All the scenarios consider the technologies tested in Russia and do not take into account innovative thermal reactors (VVER-S and VVER-SKD), and no RBMK SNF reprocessing is envisaged until the end of the century. The possibility to use plutonium in thermal reactors is not considered either. This approach reflects the current state of the Russian nuclear power industry both in near and medium term.

Fig. 1 shows the volume of SNF reprocessing to provide fast reactors with plutonium, and Fig. 2 shows the amount of unreprocessed SNF in storages until 2100. The figures do not show RBMK SNF.

Figure 2. 

Amount of SNF in storage facilities. a. For scenarios 2 and 3 b. For scenarios 4 and 5.

As one can see in Fig. 2, with priority given to the VVER SNF reprocessing, as early as by 2040 its accumulation will stop at a level not exceeding 10 thousand tons, and after 2050, the storage facilities to be vacated after the required retrofitting can be used to store SNF of BN reactors, the amount of which is 5 to 10 times less. In the scenarios with priority given to the BN SNF reprocessing, the VVER SNF accumulation issue will be resolved throughout the century with an escalation to 14 thousand tons by 2055.

Fig. 3 shows the total amount of Pu-241 contained in the external fuel cycle for the scenarios under consideration. The areas limited by the curves are nearly directly proportional to the amount of americium produced by the decay of plutonium by 2100. The comparison of the figures shows that the amount of such americium is obviously less in the case of the scenario with priority reprocessing of thermal reactor SNF. No additional costs are required. There is only a need for a number of organizational measures.

Figure 3. 

Pu-241 in the external fuel cycle. a. For scenarios 2 and 3 b. And scenarios 4 and 5.

At the same time, in addition to americium produced by the Pu-241 decay in the course of SNF storage and reprocessing, as well as storage of separated plutonium, this actinide is also produced during in-pile fuel irradiation. Fig. 4 shows the total accumulation of americium in the fuel cycle for the considered scenarios, including scenarios with homogeneous burning of americium at the level of its equilibrium content in the BN-1200M fast reactor core.

Figure 4. 

Production of americium in the NES fuel cycle for different scenarios.

In addition, this figure shows, by way of comparison, the amount of americium produced in NES with the same installed capacity, but consisting only of natural uranium thermal reactors, i.e. without plutonium recycling (reference scenario 1).

The calculations using the CYCLE code have shown the following results. As expected, the largest amount of americium (122 tons by 2100) accumulates in the event of a NES with a natural uranium-based open fuel cycle with thermal reactors (scenario 1). And in this case, the accumulation of americium is directly proportional to the energy generation of the nuclear energy system. The only, to some extend technically feasible way to solve the problem of americium would be its final immobilization in geological formations.

NFC closure with the use of fast BN-1200M reactors (~ 62 GW in 2100) without purposeful burning of americium results in a decrease in the americium accumulation to 109 tons in the case with priority reprocessing of BN SNF (scenario 4) and to 101 tons in the case with priority reprocessing of VVER SNF (scenario 2). In this case, however, the accumulation of americium grows as energy generation increases. Note that a decrease in the americium production occurs without any manipulations with it, but only due to plutonium being involved in the fuel cycle, including the Pu-241 isotope as the major source of americium production.

Commissioning of fast reactors makes it possible to implement their fundamental property, that is an excessive amount of secondary neutrons for minor actinides burning, specifically, of americium. It is assumed in the studies that implementation of the simplest option, homogeneous burning in the core fuel at the level of equilibrium content of americium (~ 1% of the heavy metal load), will start since 2035 at all BN-1200M reactors of the Russian nuclear power industry. The calculations have shown that the accumulation of americium will decrease to ~ 49 t by 2100 in the event of priority reprocessing of BN SNF (scenario 5) and to ~ 41 t in the event of priority reprocessing of VVER SNF (scenario 3).

The curves that describe americium accumulation flatten in this case and reach a nearly stationary level in the second half of the 21^st century (in fact, fast reactors become storages of americium). The value of this level is defined not only and not so much by the system’s energy generation, but by the ratio of the installed capacities of the thermal and fast reactors under consideration. In the event of conversion to a NES consisting of 100% fast reactors, the content of americium in the fuel cycle will be determined only by the performance of these reactors and by the power level of such an energy system.

Fig. 5 shows the distribution of americium production by reactor types and the locations of plutonium in a two-component NES for the considered scenarios with a closed NFC.

Figure 5. 

Production of americium in the fuel cycle of a two-component NES. a. For scenario 2; b. For scenario 3; c. For scenario 4; d. For scenario 5.

Implementation of the option with homogeneous burning of americium at the level of equilibrium content undoubtedly requires a feasibility study and is likely to affect the economic characteristics of the fast reactor fuel cycle.

It should also be noted that the figures concerning the decrease in americium production presented in the paper, appear to be the maximum ones for the considered set of conditions. Further decrease in the production is possible with an increase in the share of fast reactors in the NES. The latter is possible in case of increase in their breeding ratio or if an additional source of plutonium is found. The decrease in the accumulation of americium, if technologically feasible, can be also caused by a shorter time of the external fuel cycle of fast reactors, as well as by a shorter VVER SNF cooling time. In the scenarios considered, the amount of plutonium by 2100 proves to be insufficient for further increase in the share of fast reactors. The additional source of plutonium may be RBMK SNF or SNF from Russian-designed foreign NPPs, in the event of its return to the Russian territory. However, any SNF also contains additional americium. In particular, RBMK SNF will contain about 12 tons of americium by 2100. Therefore, such opportunities require an additional study.

Scenario modeling of accumulation of americium and plutonium-241 in Russia’s two-component nuclear energy system has been performed in the study taking into account priority reprocessing of either VVER reactor SNF or BN reactor SNF, as well as taking into account homogeneous burning of americium in fast reactor fuel at the level of its equilibrium content (~ 1%).

It has been shown that the priority reprocessing of VVER SNF allows accumulation of americium to be reduced by ~ 8 tons by the end of the century. At the same time, the effect is achieved by the use of freshly separated plutonium after short-term cooling, thus, the source of americium is eliminated as a priority in a fast reactor, without directly handling it.

To avoid accumulation of americium, an option of its homogeneous addition to BN-1200 fast reactor fuel at the level of its equilibrium content was also considered. This makes it possible to stop accumulation of americium in the two-component system by 2070, stabilizing it at a level of ~ 40 tons in the scenario with priority reprocessing of VVER SNF and ~ 50 tons for the scenario with priority reprocessing of BN SNF.