This reactor is modeled for depletion calculations. The behavior of special fuel elements that mirror fuel composition as envisioned for large scale transmutation facilities, namely inert-matrix fuels with an increased minor actinide content, are investigated within this reactor environment.
Progress in Nuclear Energy 145 (2022) 104106 Contents lists available at ScienceDirect Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene Inert-matrix fuel for transmutation: Selected mid- and long-term effects on reprocessing, fuel fabrication and inventory sent to final disposal Friederike Frieß ∗, Wolfgang Liebert University of Natural Resources and Life Sciences, Vienna, Department of Water, Atmosphere and Environment, Institute of Safety and Risk Sciences, Peter-Jordan-Straße 76/I, 1190 Vienna, Austria ARTICLE INFO Dataset link: https://github.com/juleylene/AD S-Transmutation-Fuel-Paper.git Keywords: Accelerator-driven-system (ADS) Transmutation Minor actinides (MA) Long-Lived Fission Products (LLFP) Inert-matrix fuel (IMF) ABSTRACT Partitioning and transmutation (P&T) fuel cycles provide a technical approach to ease the problem of radioactive waste disposal Some of the partitioned components of the waste stream are irradiated while others can be used for energy production or are sent to final storage Minor actinides are planned to be irradiated in a fast spectrum nuclear facility to transmute them into stable or short-lived isotopes As minor actinides have negative effects on reactor dynamics, subcritical, accelerator-driven systems are proposed to increase their fraction in the fuel An example is the MYRRHA research reactor to be built in Mol, Belgium This reactor is modeled for depletion calculations The behavior of special fuel elements that mirror fuel composition as envisioned for large scale transmutation facilities, namely inert-matrix fuels with an increased minor actinide content, are investigated within this reactor environment It turns out that gamma dose rates, activity and residual heat from the spent fuel elements present significant challenges for implementing a P&T program Spent inert-matrix fuel element show significantly higher levels than spent fuel elements from fast reactors This requires long cooling periods and poses unprecedented challenges to reprocessing technology The problem is amplified by the fact that it is generally agreed upon that due to low transmutation efficiencies several transmutation steps would be necessary Looking at the radiotoxicity index, the efforts suggested to reduce the minor actinide content in a final repository are justified The long-term safety case of deep geological repositories, however, implies that certain long-lived fission products are more relevant The buildup of some of these radionuclides is investigated for two hypothetical German P&T scenarios Naturally, the amount of fission products increases with continued irradiation But namely the fraction of Cs-135 increases over-proportionally when inert-matrix fuel rich on minor actinides is used Introduction The Belgian government announced its decision to finance roughly one third of the MYRRHA1 (Multi-purpose hYbrid Research Reactor for High-tech Applications) project in 2018 (WNN, 2018) According to the European Strategy Report on Research Infrastructure (ESFRI), MYRRHA is supposed to become operational in 2027 (ESFRI, 2018) The Belgian Nuclear Research Center (SCK⋅CEN), however, states operating the reactor and full power accelerator-driven system (ADS) in 2036 (SCK-CEN, 2020) MYRRHA is supposed to be the first hybrid nuclear research reactor: its design comprises critical and sub-critical core configurations Recently, much work was put into the final adjustments of the cooling system for the MYRRHA reactor core and accelerator technologies (Van Tichelen et al., 2020; Kennedy et al., 2020; Gladinez et al., 2020; Moreau et al., 2019) The focus of the MYRRHA project shifted from research activities to project implementation recently (SCK-CEN, 2019) Besides being introduced as the Experimental Technology Pilot Plant (ETPP) for a lead-cooled fast reactor (LFR) (ESFRI, 2018), MYRRHA is intended to demonstrate the concept of an acceleratordriven system ADS are key components for one possible concept of irradiating high level nuclear waste in a partitioning and transmutation (P&T) fuel cycle Partitioning means the separation of the spent fuel into different waste streams such as uranium, plutonium, minor actinides and fission products It is foreseen that the separated minor actinides are incorporated into a special fuel form and are then irradiated in a (usually fast) neutron spectrum with the objective of ∗ Corresponding author E-mail addresses: friederike.friess@boku.ac.at (F Frieß), liebert@boku.ac.at (W Liebert) ADS: Accelerator-driven system, BWR: Boiling water reactor, EFIT: European Facility for Industrial Sized Transmutation, FE: Fuel element, FP: Fission products, FPD: Full power days, IM-fuel: Inert-matrix fuel, IPS: In-pile test section, LBE: Lead–bismuth eutectic, LLFP: Long-lived fission products, LWR: Light water reactor, MA: Minor Actinides, MOX: Mixed oxide fuel, P&T: Partitioning and Transmutation, PWR: Pressurized water reactor, UOX: Uranium oxide fuel https://doi.org/10.1016/j.pnucene.2021.104106 Received 13 May 2021; Received in revised form November 2021; Accepted 17 December 2021 Available online February 2022 0149-1970/© 2022 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Progress in Nuclear Energy 145 (2022) 104106 F Frieß and W Liebert their transmutation into stable or short-lived isotopes prior to final disposal P&T concepts bring several challenges with them To efficiently transmute minor actinides, it is important to irradiate them in a fast neutron spectrum.2 Additionally, they have undesired safety-relevant effects on reactor dynamics such as the delayed neutron fraction and the Doppler effect This has to be considered in fuel and reactor design The possible amount of minor actinides and plutonium in the fuel for critical core configurations is thus limited (Palmiotti et al., 2011) For a medium-sized sodium-cooled fast reactor, a minor actinide content of approximately 10% seems manageable For a large-size sodiumcooled fast reactor (3000 MWth), this fraction drops to 2.5%–3% (Fazio and Boucher, 2008, 7) In contrast, accelerator-driven systems should allow for minor actinide fractions up to almost 50% (Artioli et al., 2007) Therefore, many P&T concepts rely on accelerator-driven systems that could ensure an efficient throughput of minor actinides Usually, ADS are envisioned for minor actinide transmutation while fast reactors would use the excess plutonium for power generation (Mueller, 2013; Abderrahim et al., 2013; Doligez, 2017; ESNII, 2020) This is called a double-strata fuel cycle In any case, multiple reprocessing and irradiation steps are necessary An ADS consists of a particle accelerator, a spallation target, and a sub-critical reactor core The core never reaches criticality during operation but amplifies the neutrons supplied by an external neutron source, usually a spallation target The number of neutrons in the core is regulated by the variation of the beam current The amplifying nature of the sub-critical core is one key aspect in the safety concept since it prevents exponential criticality excursions in most possible cases (Sarotto, 2017) Consequently, a significantly increased fraction of minor actinides should be possible in the fuel composition The amount of minor actinides in a deep geological repository could be reduced with the implementation of P&T programs But other radionuclides impact radiological safety as well Some of these are fission products with very long half-lives well beyond some 100,000s of years In the 1990s, the transmutation of at least Tc-99 and I-129 was also considered a relevant contribution of P&T to reduce the burden of nuclear waste disposal (NRC, 1996; Jameson et al., 1992; DoE, 1999) This has however been proven to be by far more complicated than anticipated Obstacles are for example that single fission products, sometimes even single isotopes, need to be separated from the spent fuel If they are then placed in a reactor for irradiation, they only consume neutrons and thus effect the neutron economy in the core negatively Consequently, research has ceased (NEA/OECD, 2006a; Doligez, 2017; Frießet al., 2021) The emphasis shifted to the irradiation of minor actinides only If transmutation efficiency for certain systems is evaluated, only the net balance of minor actinides is discussed in most cases (Mueller, 2013; Mansani et al., 2012; Sarotto et al., 2013; Renn, 2014; Liu et al., 2020) The need to transmute long-lived fission products as well is only rarely mentioned, e.g in Shwageraus and Hejzlar (2009) and Chiba et al (2017) In a first step, this article explores the effects a high minor actinide content in inert matrix fuel (IM-fuel) has on the dose rate, the activity and the decay heat of spent fuel elements Simulations are based on a computer model of the planned accelerator-driven system MYRRHA Nuclide compositions in the spent fuel are derived from depletion calculations In a second step, the concentration of selected long-lived fission products in the spent fuel is extracted to estimate the influence of a P&T scenario on the inventory of a deep geological repository The latter is illustrated using two hypothetical scenarios of a potential P&T implementation in Germany The scenarios are based on the highly radioactive waste accumulated by the German nuclear energy program until 2022 With the end of that year, Germany will have shut down all its nuclear power reactors Fig Core layout of the generic ADS core for equilibrium sub-cycle Control rods are not inserted Methods In this chapter the methods used for the analysis are introduced It starts with the description of the reactor model and the computer programs Then the procedure of evaluating the amount of certain isotopes in the spent fuel is explained 2.1 Reactor model In the 7th Framework Program of EURATOM, a FAst Spectrum Transmutation Experimental Facility (FASTEF) was designed (Sarotto et al., 2013; Sarotto, 2017; CORDIS, 2019) The rather detailed FASTEF design is very similar to the MYRRHA reactor The large-scale European Facility for Industrial Sized Transmutation (EFIT) is planned to follow after the proof-of-concept reactor MYRRHA is in operation (Mansani et al., 2012; Artioli et al., 2007; Sarotto, 2017) Since MYRRHA is designed as a hybrid facility, critical and subcritical core configurations have been modeled for validation of the model The critical layout could also be used for transmutation, but within the already mentioned limitations: only a slight increase compared to minor actinide content in current MOX fuel would be possible due to safety reasons Thus the focus of this work is set to the subcritical core The simulations of criticality and neutron flux show good accordance with average values for a typical fast reactor system (Sarotto et al., 2013; Frieß, 2017) The cross section of our generic ADS model based on the MYRRHA core design Sarotto et al (2013) and Sarotto (2012) is depicted in Fig There are six different fuel zones in the core After each sub-cycle of 90 days, the elements in one zone are replaced with fresh fuel elements The other elements are shuffled to the next fuel zone After one full cycle, consisting of six sub-cycles, all fuel elements are replaced by fresh fuel elements Plutonium enrichment and fuel composition are the same for all batches (Sarotto et al., 2013) Geometric dimensions are provided in Table The center assembly hosts the spallation target, which The reason is the relation between fission and absorption cross sections It is significantly more favorable in a fast neutron spectrum Progress in Nuclear Energy 145 (2022) 104106 F Frieß and W Liebert Table Geometric dimensions of the generic ADS model for the MOX fuel elements and the P&T IM-fuel elements The number of fuel assemblies is given at begin of cycle, not at beginning of life The center pin in each assembly is a structure pin Source: The values are taken from Sarotto (2012) Parameter Unit MOX Fuel EFIT/IM-Fuel # of Fuel Elements # of Fuel Rods Radius of Fuel Pin Active Height Cladding Thickness Fuel Assembly Pitch Can Thickness Can Inner Diameter – – cm cm cm cm cm cm 72 126+1 0.271 60.00 0.045 10.45 0.20 9.755 60 0.36 60.00 0.06 10.45 0.20 9.755 The depletion calculation is split into steps of 30 days each After each sub-cycle of 90 days, a 30 day decay step is included This accounts for the reshuffling of the fuel elements Additionally, every third sub-cycle, there is a longer maintenance interval of 90 days (Sarotto et al., 2013) The power of the ADS is 400 MWth The six IM-fuel elements placed in the IPS elements are irradiated 1080 full power days (FPD) This is the irradiation time planned for the EFIT reactor With cladding and structure materials currently available, this irradiation time is not feasible due to high material stress Those burn-ups are considered here nevertheless, since only then efficient transmutation rates can be achieved Only equilibrium sub-cycles where the full number of fuel elements is placed in the core are considered The depletion calculations are started in equilibrium configuration with fresh fuel elements The first cycles of the evaluation were skipped before evaluation to allow 𝑘𝑒𝑓 𝑓 to reach decline During burn-up, 𝑘𝑒𝑓 𝑓 ranges between 0.971 and 0.954 More details on the burn-up calculation, including mass balanced, can be found in Frieß (2017) Table Composition of MOX fuel used in the basic MYRRHA design and IM-fuel under research for transmutation purposes in weight percent Plutonium comprises 45.7% of the transuranium content in IM-fuel Isotope MOX Fuel in wt% EFIT/IM-Fuel in wt% U-235 U-238 Np-237 Pu-238 Pu-239 Pu-240 Pu-241 Pu-242 Am-241 Am-242 m Am-243 Cm-243 Cm-244 Cm-245 Cm-246 Cm-247 0.5 69.5 – 0.5 17.4 6.8 3.6 1.2 – 4.9 – – – – – – – – 2.1 1.7 21.2 15.6 1.8 5.4 41.0 0.14 8.71 0.04 1.63 0.62 0.05