applied sciences Article System Studies on the Fusion-Fission Hybrid Systems and Its Fuel Cycle Mikhail Shlenskii 1,* and Boris Kuteev 2,* 1 Plasma Physics Department, National Research Nuclear University Moscow Engineering Physics Institute, Kashirskoe Hwy 31, 115409 Moscow, Russia 2 Tokamak Department, National Research Center “Kurchatov Institute”, Akademika Kurchatova pl. 1, 123098 Moscow, Russia * Correspondence: [email protected] (M.S.); [email protected] (B.K.) Received: 8 November 2020; Accepted: 24 November 2020; Published: 26 November 2020 Abstract: This paper is devoted to applications of fusion-fission hybrid systems (FFHS) as a powerful neutron source implementing transmutation of minor actinides (MA: Np, Am, Cm) extracted from the spent nuclear fuel (SNF) of nuclear reactors. Calculations which simulated nuclide kinetics for the metallic fuel containing MA and neutron transport were performed for particular facilities. Three FFHS with fusion power equal to 40 MW are considered in this study: demo, pilot-industrial and industrial reactors. In addition, needs for a fleet of such reactors are assessed as well as future FFHSs’ impact on Russian Nuclear Power System. A system analysis of nuclear energy development in Russia was also performed with the participation of the FFHSs, with the help of the model created at AO “Proryv”. The quantity of MA that would be produced and transmuted in this scenario is estimated. This research shows that by the means of only one hybrid facility it is possible to reduce by 2130 the mass of MA in the Russian power system by about 28%. In the case of the absence of partitioning and transmutation of MA from SNF, 287 t of MA will accumulate in the Russian power system by 2130. Keywords: fusion-fission hybrid system (FFHS); fusion neutron source (FNS); minor actinides; partitioning and transmutation (P&T); closed nuclear fuel cycle 1. Introduction 1.1. Prerequisites for Minor Actinides Transmutation One of the most crucial issues for nuclear engineering is to create a closed nuclear fuel cycle that would provide the use of nuclear energy for a long time (more than 1000 years) and improve the safety, ecology and economics of nuclear technology. This issue includes the problem of spent nuclear fuel (SNF) and radioactive waste (RW) management. Minor actinides (MA—Np, Am, Cm) comprise about 0.1–0.15% of SNF mass, have high radiotoxicity and a relatively long half-life [1,2]. One of the ways to manage MA is to dispose of them in geologic repositories. However, there is another approach where MA are transmuted into lighter nuclides. This paper describes the important problems of MA management and a way to solve those using FFHSs. The value of radiotoxicity shows the real harmful potential of a particular nuclide—thus it is used by a majority of authors. Radiotoxicity is a result of the multiplication of nuclide radioactivity and effective dose coefficient (or DPUI—Dose Per Unit Intake, Sv/Bq) [2]: Radiotoxicity = A e(50) (1) · Appl. Sci. 2020, 10, 8417; doi:10.3390/app10238417 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 8417 2 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 16 wherewhere A isis the nuclide’s activity activity (Bq) (Bq) for for a a particular particular moment, moment, (50)e(50) isis thethe eeffectiveffective dosedose coecoefficientfficient (Sv(Sv/Bq)/Bq) thatthat isis thethe dosedose (Sv)(Sv) resultingresulting fromfrom thethe intakeintake ofof 11 BqBq fromfrom aa specificspecific nuclidenuclide over over 50 50 years. years. As reported in [[1,2]1,2] the typical SNF of a light wa waterter reactor (LWR) consists of more than 95% U, about 1% Pu, only 0.1% MA and about 3–4% fissionfission products (FP). Plutonium and MA (despite their small percentage)percentage) provideprovide the the main main contribution contribution to to the the long-term long-term radiotoxicity radiotoxicity of radioactiveof radioactive waste waste [2], (Figure[2], (Figure1). The 1). radiotoxicity The radiotoxicity of fission of fission products produc declinests declines much faster much compared faster compared to that of to actinides. that of Itactinides. reaches theIt reaches radioactive the radioactive equilibrium equilibrium with respect wi toth the respect uranium to the ore uranium in about 300ore yearsin about [2]. 300 If SNF years is not[2]. reprocessedIf SNF is not and reprocessed transmuted, and it reachestransmuted, the natural it reaches radiotoxicity the natural level radiotoxicity only after 100,000 level only years after [2]. At100,000 the same years time, [2]. At in the same European time, Union in the alone,European nuclear Union power alone, plants nuclear produce power annually plants produce around 2500annually tons around of SNF [25001], almost tons of the SNF same [1], amount almost the is produced same amount in the is USA produced [3]. in the USA [3]. Figure 1. Radiotoxicity ofof SNFSNF (spent(spent nuclearnuclear fuel)fuel) and and its its components components (for (for PWR, PWR, UOX, UOX, 60 60 GW GW·d/t)d/t) [1 ]. · [1]. The concept of radiation equivalent treatment of radioactive nuclides in nuclear fuel cycle [4] is quiteThe attractive concept because of radiation it could equivalent provide a treatment great reduction of radioactive of safety nuclides requirements in nuclear and costs fuel forcycle the [4] RW is repositoryquite attractive ([5], p. because 516, [6]). it Thiscould concept provide implies a great disposing reduction of of radioactive safety requirements waste that has and a radiotoxicitycosts for the levelRW repository equal to the ([5], natural p. 516, U or [6]). Th oreThis level concept (or at implies least reduced disposing radiotoxicity of radioactive level). waste that has a radiotoxicityHowever, level nowadays, equal to manythe natural EU countries U or Th (andore level not just(or at EU least countries) reduced have radiotoxicity chosen a level). phase-out scenarioHowever, for nuclear nowadays, energy many (like Sweden EU countries [7] and (and Germany) not just and EU/or countries) have decided have not chosen to reprocess a phase-out SNF andscenario simply for tonuclear dispose energy of it (like in geologic Sweden repositories [7] and Germany) (like the and/or USA have [1,7]). decided There arenot severeto reprocess problems SNF withand simply this approach. to dispose It requiresof it in geologic controlling repositories SNF for an (like impractical the USA length[1,7]). There of time, are meaning severe problems it is very expensivewith this approach. (due to the It requires time and controlling storage space SNF needed), for an impractical probably unsafe,length of not time, to mentionmeaning the it is direct very disposalexpensive of (due SNF willto the deprive time and humanity storage of space a great needed), source ofprobably power. unsafe, not to mention the direct disposalBecause of SNF of nuclearwill deprive power humani RW problem,ty of a great “pure” source thermonuclear of power. fusion reactors seem like a good alternativeBecause as of they nuclear produce power much RW less problem, RW during “pure” their th operationermonuclear if the fusion correct reactors materials seem are like chosen a good [8]. However,alternative the as fusionthey produce reactor ismuch a very less complex RW during facility their that operation is in fact ine if ffithecient correct in utilizing materials very are valuable chosen thermonuclear[8]. However, the neutrons. fusion Atreactor the same is a timevery fusion-fission complex facility hybrid that systems is in fact have inefficient relatively in easily utilizing achieved very plasmavaluable parameters thermonuclear compared neutrons. to “pure” At the thermonuclear same time fusion-fission fusion reactors hybrid [9]. systems have relatively easilyThus achieved reprocessing plasma parameters the spent nuclear compared fuel to via “pure” separating thermonuclear minor actinides, fusion reactors Pu and [9]. U seems a promisingThus reprocessing way forward the in developmentspent nuclear of fuel the via nuclear separating power. minor Pu and actini U coulddes, Pu be and used U in seems fission a reactorspromising again. way forward Fission productsin development could beof usedthe nuclear as sources power. of radiation,Pu and U reducingcould be theused amount in fission of radioactivereactors again. waste. Fission As mentioned products above, could FPbe lose used their as radiotoxicitysources of radiation, much faster reducing than actinides. the amount The last of radioactive waste. As mentioned above, FP lose their radiotoxicity much faster than actinides. The last problem, concerning MA, could be solved via the transmutation of MA in fission reactions which produce fission products. This strategy is called partitioning & transmutation (P&T) [1]. Appl. Sci. 2020, 10, 8417 3 of 15 problem, concerning MA, could be solved via the transmutation of MA in fission reactions which Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 16 produce fission products. This strategy is called partitioning & transmutation (P&T) [1]. AA very very hard hard spectrum spectrum is requiredis required for for the the fission fission of most of most MA dueMA todue the to threshold the threshold character character of their of fissiontheir fission cross-sections cross-sections (Figure 2(Figure). In Figure 2). 2In, twoFigure types 2, oftwo neutron types cross-sectionof neutron cross-section are shown: fission are shown: and capture.fission and The capture. ratio of theirThe ratio average of their values average for a particularvalues for neutrona particular spectrum neutron (α )spectrum is crucial ( toα) eisff ectivecrucial transmutation.to effective transmutation. Table1 shows Table that the 1 shows capture-to-fission that the capture-to-fission ratio for the spectrum ratio for of the a light spectrum water of reactor a light iswater bigger reactor than 1is forbigger most than actinides 1 for most while actinides for the fastwhile reactor for the spectrum fast reactor this spectrum ratio less this than ratio 1 for less most than actinides.1 for most At actinides.