Corresponding author: Nikita V. Kovalev ( kovalev@khlopin.ru ) Academic editor: Yury Kazansky
© 2020 Nikita V. Kovalev, Boris Ya. Zilberman, Nikolay D. Goletsky, Andrey B. Sinyukhin.
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.
Citation:
Kovalev NV, Zilberman BYa, Goletsky ND, Sinyukhin AB (2020) A new approach to the recycling of spent nuclear fuel in thermal reactors within the REMIX concept. Nuclear Energy and Technology 6(2): 93-98. https://doi.org/10.3897/nucet.6.54624
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A review of simulated nuclear fuel cycles with mixed uranium-plutonium fuel (REMIX) was carried out. The concept of REMIX fuel is one of the options for closing the nuclear fuel cycle (NFC), which makes it possible to recycle uranium and plutonium in VVER-1000/1200 thermal reactors at a 100% core loading. The authors propose a new approach to the recycling of spent nuclear fuel (SNF) in thermal reactors. The approach implies a simplified fabrication of mixed fuel when plutonium is used in high concentration together with enriched natural uranium, while reprocessed uranium is supposed to be enriched and used separately. The share of standard enriched natural uranium fuel in this nuclear fuel cycle is more than 50%, the share of mixed natU+Pu fuel is 25%, the rest is fuel obtained from enriched reprocessed uranium. It is emphasized that the new approach has the maximum economic prospect and makes it possible to organize the fabrication of this fuel and nuclear material cross-cycling at the facilities available in the Russian Federation in the short term. This NFC option eliminates the accumulation of SNF in the form of spent fuel assemblies (SFA). SNF is always reprocessed with the aim of further using the primary reprocessed uranium and plutonium. Non-recyclable in thermal reactors, burnt, reprocessed uranium, the energy potential of which is comparable to natural uranium, as well as secondary plutonium intended for further use in fast reactors, are sent as reprocessing by-products to the storage area.
REMIX, spent nuclear fuel, nuclear fuel cycle, mixed fuel, uranium-plutonium fuel, plutonium, reprocessed uranium, recycling, cross-cycling
The reprocessing of spent nuclear fuel is considered with the aim of using the energy potential of the remaining uranium and plutonium produced, eliminating the SNF storage and previously accumulated reprocessed products, and saving natural uranium (
There are about 24 thousand tons of SNF accumulated in the Russian Federation. Approximately 650–700 tons of SNF are unloaded from the reactors of Russian NPPs annually, while no more than 15% of this volume is reprocessed (
In Russia, the concept of a two-component nuclear energy system is adopted, which includes both types of reactors (with a thermal neutron spectrum – VVER and a fast neutron spectrum – BN). Within this nuclear energy system, it is supposed to use a partial core loading with MOX fuel (~ 9% Pu mixed with depleted uranium) in thermal reactors. The transition period before the start of reactors with a fast neutron spectrum may consist in recycling reprocessed nuclear materials as a mixed fuel for thermal reactors, possibly more efficient than MOX fuel, with a partial core loading.
Since 2005, the V.G. Khlopin Radium Institute in collaboration with the Kurchatov Institute has been developing REMIX fuel (REMIX – REegenerated MIXture of U-Pu oxides), which involves recycling both reprocessed uranium and plutonium to load the entire core of existing VVER-1000/1200 reactors (
The authors of the article carried out numerous calculations of nuclear fuel cycles using software that simulates nuclear fuel burnup based on the Monte Carlo method. The main tool in the calculations was SCALE 6.2 (
Various NFC schemes were simulated using mixed uranium-plutonium fuel. The analysis considered a group of Russian-designed thermal reactors of the VVER-1000/1200 type throughout the entire period of their operation. After the fuel burnup and cooling, the SNF isotopic composition was evaluated for the possibility and expediency of its further use. With a residual energy potential of the SNF higher than that of natural uranium, the nuclear materials of this fuel are used in simulating the next recycle.
The authors investigated the basic NFC options and developed a new one based on mixed REMIX uranium-plutonium fuel.
The basic option is REMIX-A (
The REMIX-A recycling scheme is shown in Fig.
REMIX-C is a development of REMIX-A that implies directing reprocessed uranium (repUEn) from SNF reprocessing to re-enrichment to about 4–5% 235U, followed by its mixing with the calcined plutonium-uranium master mixture left at reprocessing plant. A natural enriched uranium makeup is used. Excess nuclear materials are excluded, but reprocessed uranium should be transported to the enrichment plant and backward.
In terms of fuel supply for reactors being built abroad under Russian projects, the REMIX-A/C concept looks optimal for the entire life cycle, since Russia intends to provide services for reprocessing SNF during recycling and returning nuclear materials to customers, but already in the form of mixed uranium-plutonium fuel and radioactive waste (Nuclear Fuel Reprocessing 2020). The decrease in the accumulation of SNF is at the same level as in the case of REMIX-A with complete reprocessing.
The REMIX-C recycling scheme is shown in Fig.
In the REMIX-B option, it is not intended to use a makeup during recycling; accordingly, the amount of mixed fuel and its final SNF are reduced. The Pu content in this fuel can exceed 4%. After the fuel burnup of ~ 47 GW day/tHM, approximately 1.25% of plutonium is formed from enriched natural uranium. Accordingly, for the production of mixed fuel with a plutonium content of 4%, it is necessary to reprocess three to four times more SNF. To achieve the necessary effective fuel enrichment, the reprocessed uranium enrichment method is used. The fabrication of REMIX-B fuel requires great precision in mixing the components; however, the amount of secondary fuel is several times less than the original SNF from natural uranium.
The REMIX-B recycling scheme is shown in Fig.
An estimated economic analysis of NFCs based on prices from open sources (
The authors propose a new approach – REMIX-E – that implies a simplified fabrication of mixed fuel when plutonium is used in high concentration together with enriched natural uranium, while reprocessed uranium is supposed to be enriched and used separately. This NFC option seems to be optimal in terms of quick start and makes it possible to reduce the accumulation of SNF at the level of other options. This fuel composition was patented in Russia (
In the REMIX-E NFC, three types of fuel are used: enriched natural uranium (natUEn): 56%, enriched reprocessed uranium (repUEn): 18% and mixed fuel (natUEn-Pu): 25%. The low share of mixed fuel results from the increased initial plutonium concentration of 5%; the fuel is “compressed”. It is important to note that this natUEn-Pu fuel uses enriched natural rather than reprocessed uranium. Such production can now be organized at existing MOX production facilities. Once-reprocessed uranium is supposed to be used, as it is done now, at existing facilities in the form of ERU (enriched reprocessed uranium) fuel. Most of the fuel in this NFC is standard enriched natural uranium. Natural uranium is saved at the level of 25–30%. On average, 2.2 kg of MA is formed from one tHM of the initial fuel.
The REMIX-E recycling scheme is shown in Fig.
In this NFC, there is no restriction on the cross-cycling duration. By way of illustration (see Fig.
The REMIX-E approach makes it possible not to accumulate SNF at all. A distinctive feature of this cycling option is that it generates not SNF but burnt reprocessed uranium, non-recyclable in thermal reactors, which energy potential is comparable with that of natural uranium, and secondary plutonium.
Figures
At present, VVER-1000 reactors operate on an 18-month fuel cycle. Every 18 months, 67 fuel assemblies out of 163 are loaded and unloaded (for VVER-1200 – 72 fuel assemblies). Some fuel assemblies have two 18-month fuel cycles in the reactor, and some have three. Each fuel assembly contains 0.465 tHM; therefore, over a period of 3×18 months, 93.5 tHM are used. For calculating material balances, the period of 3×18 months (taking into account overloads and maintenance of about 4.5 years) is taken for a full cycle of a fuel batch, for which all fuel reaches an average burnup of 47 MW×day/kg. While this fuel batch is cooled and reprocessed, and new fuel is fabricated from it, it is necessary for the reactor to operate on exactly the same fuel batch. As a result, the concept of pair cycles of fuel batches emerges. Over the period of the reactor operation for about 63 years, approximately seven pair cycles are obtained. Since batches of fresh fuel assemblies are constantly uploaded, the cycles do not go sequentially, but layering on top of each other. The result is an excess of the seven pair cycles. In the concept of reprocessed fuel, enlarged cycles are composed of a zero (uranium) cycle and cycles with fuel recovered from reprocessed nuclear materials. For the REMIX-E concept, the number of recycles does not matter, i.e., it can be indefinite. Even in the second or third cycle, the composition of the REMIX-E mixed fuel stabilizes and does not change further. Note that for REMIX-A/C low-plutonium fuel, which implies multi-recycling, the recycling duration is limited to six or seven recycles due to the plutonium isotopic composition degradation during recycling.
Regarding REMIX-A, it can be said that only 14% of this fleet (i.e., six reactors) should run on natural fuel, while even now the corresponding Russian capacities are supplying such fuel for 30–35 reactors and efforts are under way to expand this production. Suppose that fabricating mixed oxide fuel, we can rely on its supply only for 10–15 reactors, i.e., about 30% of the total capacity or 250 tons/year with a calculated increase in the reactor fleet up to 50–55. Then, it makes sense to consider low-plutonium fuel mixed with reprocessed uranium (REMIX) only for a limited group of 3–5 reactors, including the experimental group for the phased development of the closed nuclear fuel cycle or commercially for foreign small groups of reactors provided that this nuclear fuel cycle is economically justified.
If the REMIX-E scenario is used, natUEn-Pu fuel accounts for only 15% of the capacities in one Pu-cycle and also 6% in the second Pu-cycle; the main share falls on standard natUEn-fuel and partly on enriched reprocessed uranium (repUEn).
The authors propose a new approach to recycling SNF in thermal reactors within the REMIX concept. The REMIX-E NFC, as compared to other REMIX concepts, according to our estimates, seems to be optimal in terms of quick start and can completely eliminate the accumulation of SNF in the form of SFA.
A specific feature of the REMIX-E approach is the endless cycling of the same fuel, natUEn-Pu and repUEn, produced from standard components, i.e., primary reprocessed uranium and primary energy-grade plutonium from standard enriched natural uranium fuel. As a result, the production of such fuel eliminates the problems of mixing SNF from another fuel in one reprocessing chain, as in the case of multi-recycling low-plutonium REMIX fuel; repUEn transporting to the facility for reprocessing SNF and fabricating uranium-plutonium fuel is also excluded.