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U–Pu fuel cycles in an EM2 reactor
Progress in Nuclear Energy 85 (2015) 764e770
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Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Startup and burnup strategy for TheU/UePu fuel cycles in an EM2 reactor Y.W. Ma a, b, c, X.X. Li a, b, X.Z. Cai a, b, C.G. Yu a, b, C.Y. Zou a, b, J.L. Han a, b, J.G. Chen a, b, * a
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China CAS Center for Excellence in TMSR Energy System, Chinese Academy of Sciences, Shanghai 201800, China c University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2015 Received in revised form 7 September 2015 Accepted 14 September 2015 Available online xxx
General Atomics (GA) is developing the Energy Multiplier Module (EM2) which is a compact gas-cooled fast reactor as one of candidates of the Generation-IV nuclear energy systems. In the EM2 core, low enriched uranium is used as igniting fuel and depleted uranium is used for converting and burning. It indicates that EM2 can maintain critical operation for more than 30 years without refueling. To further study the TheU fuel cycle performance in the EM2, two kinds of start-up strategies with TheU (Th þ 233U) and semi TheU (Th þ enriched 235U) are evaluated. Neutronics characteristics, such as the effective multiplicity factor (keff) and conversion ratio (CR) are analyzed from neutron usage point of view. The simulated results for the two kinds of fuels are compared with the UePu fuel from the design of GA. The analysis gives an insight into the pros and cons of UePu and TheU fuel cycles in terms of the breeding capability and the discharged radio-toxicity. The breeding performance of the second generation EM2 is also presented and compared with that of the first generation EM2. It indicates that the multi-generation EM2 can deepen the burnup and reduce the waste management pressure for each kind of fuel loading strategy. © 2015 Elsevier Ltd. All rights reserved.
Keywords: EM2 Fuel cycle Breeding Radio-toxicity
1. Introduction Current commercial nuclear power reactors, mostly Light Water Reactors (LWRs), use about 5% enriched uranium with oncethrough fuel cycle. The fuel utilization is less than one percent of the natural uranium, which will consume up the whole world's estimated uranium in a few decades along with the growing demand of energy (David, 2005). Meanwhile, it also inevitably leads to release of radioactive waste with a considerable amount of transuranium (TRU) (Alajo and Tsvetkov, 2011; Salvatores, 2009). Six advanced reactor concepts selected for Generation-IV nuclear energy systems are under development worldwide to meet the goals of effective resource utilization and waste minimization, improved safety, enhanced proliferation resistance, and reduced system cost (Locatelli et al., 2013; Gyorgy and Czifrus, 2015). Three of them are fast reactors: Gas-cooled Fast Reactor, Lead-cooled Fast Reactor and Sodium-Cooled Fast Reactor. Fast reactor systems have
* Corresponding author. Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail address: [email protected] (J.G. Chen). http://dx.doi.org/10.1016/j.pnucene.2015.09.010 0149-1970/© 2015 Elsevier Ltd. All rights reserved.
effective capability to breed fissile fuel and transmute nuclear waste and eventually realize the ultimate utilization of nuclear fission energy. Therefore, fast reactors are considered to be potential to solve the essential issues for long-term development of nuclear energy, such as fuel supply and back-end spent fuel processing (Artisyuk et al., 2005; Tucek, 2004). The recently emerged concept of Small Modular Reactor (SMR) also addresses the above these issues for nuclear energy utilization (Liu and Fan, 2014; Hidayatullah et al., 2015; Shin et al., 2015; ElGenk and Palomino, 2015; Rowinski et al., 2015). It provides an energy option with low carbon emission, enhanced safety conviction, convenient construction and operation, therefore becoming more attractive especially in the Post-Fukushima era (Kessides and Kuznetsov, 2012). One kind of SMRs named Energy Multiplier Module (EM2) was proposed by General Atomics (GA) based on the Gen-IV reactor concept of gas-cooled fast reactor (Schleicher and Back, 2012). The EM2 was designed to burn the depleted uranium (DU) or used nuclear fuel (UNF) in convert and burn mode which converts fertile to fissile and then burning it in situ (Parmentola and Rawls, 2012). The EM2 ignited with low enriched uranium and loaded with DU as
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fertile fuel can operate for more than 30 years without refueling. And a burnup of 140 GWd/t which is more than double that of any traditional light water reactor, could be achieved (Joe et al., 2010). Furthermore, the discharged fuel from EM2 can be recycled to ignite a new EM2 by only removing a fraction of fission products (FPs) (Parmentola and Rawls, 2012). The multi-generation mode of fuel utilization ensures the excellent sustainability of EM2. Besides UePu fuel cycle, TheU fuel cycle could also be feasible to be employed in EM2. TheU fuel cycle has been widely studied in the past as a possible alternative to UePu fuel cycle (IAEA, 2005; Lung and Gremm, 1998; Furukawa et al., 2008; Fortini et al., 2015; Brown et al., 2015). Furthermore, it can be in principle utilized sustainably in both thermal and fast reactors (Kuegler et al., 2007; MacDonald et al., 2004; Fiorina et al., 2013a; Fiorina et al., 2013b; NEA-OECD, 2002). Use of Th in a burner design for a closed nuclear fuel cycle is appealing because of the potential to achieve high fuel utilization and low build-up of hazardous isotopes (Ashley et al., 2014; Csom et al., 2012; Srivastava et al., 2011). Therefore, the neutronic performance in EM2 based on TheU fuel cycle (abbreviated as U3eTh) is evaluated in this work. Another thorium based fuel option ignited by enriched 235U (abbreviated as U5eTh) and the conventional fuel proposed by GA (abbreviated as U5eDU) are also analyzed for comparison. Then the neutronics analysis is extended to the second generation EM2 (Gen-II EM2) fueled with the reprocessed fuel that was discharged from the first generation EM2 (Gen-I EM2). The methodology for core modeling and the neutronic tool are introduced in Section 2. The calculated results and the detailed discussion are presented in Section 3. The conclusions are given in Section 4. 2. Methodology 2.1. Reactor description and modeling The schematic EM2 core exhibited in Fig. 1 includes the starter zone, the fertile zone, the inner Be2C and the outer graphite reflectors. Fissile fuel and fertile fuel (uranium carbide and thorium carbide in the form of porous pellets) in the core could maintain the balance of reactivity increase from the production of new fissile fuel and reactivity decrease owing to fuel depletion and FPs accumulation. Therefore EM2 can operate for several decades without refueling but just depending on the initial fuel loading. The core contains 91 fuel assemblies arranged in a right hexagonal prism, and each fuel assembly contains 91 fuel rods. The coolant flow channels (triangle) are arranged between the fuel rods. The important parameters referred to the GA's design are shown in Table 1 (Schleicher and Back, 2012; Parmentola and Rawls, 2012). In addition, some other parameters like material
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Table 1 EM2 design parameters. Parameter
Value
Thermal power (MWth) Active core diameter/Height (cm) Core diameter (cm) Fuel lattice Lattice diameter (mm) Lattice height (cm) SiC thickness (mm) Starter zone HM Inventory (t) Volume (m3) Fertile zone HM Inventory (t) Volume (m3) Total HM inventory (t) Other materials Inner reflector Outer reflector Coolant Cladding
500 196/273 280 Hexagon 18.5 30.3 1 21.2 2.6 19.5 2.4 40.7 Be2C Graphite Helium SiC
density are determined according to empirical data. The structural material used in the core including the fuel cladding, is SiCeSiC composite (Schleicher and Back, 2012). Helium is used as coolant and the core outlet temperature is assumed to be 850 C, corresponding to 48% thermal conversion efficiency. Be2C and graphite are used as reflector to ensure a flat power profile for relatively uniform irradiation rates in the core (Parmentola and Rawls, 2012). The numerical analysis is based on the TRITON control module in SCALE6.1 which can perform reliable depletion and decay analysis for reactor physics applications using problem-dependent cross-section processing and rigorous treatment of neutron transport (Scale 2011). Three-dimensional Monte Carlo transport code KENO-VI coupled with point-depletion and decay analysis module ORIGEN-S are used within the TRITON sequence based on the cross section library V7-238 (238-group ENDF/B-VII). A fast reactor library is selected during the TRITON execution, which contains fast fission product yields and cross sections are weighted based on a typical fast reactor spectrum (Sheu et al., 2013). In order to preserve a better accuracy during the calculation, the option PARM is selected for 388 isotopes in the transport updating for depletion. Depletion step is chosen by considering both computation efficiency and accuracy (Allen and Knight, 2010), which is relatively compressive (20e50 days) at the early operation stage and has a larger span (200e500 days) after 200 days.
Fig. 1. Schematic core of EM2 from vertical view (a) and side view (b).
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2.2. Fuel loading consideration
Table 2 Loading portion for the three kinds of fuels.
The EM2 system is featured with the characteristics of fuel breeding and burning. Three fuel loading strategies U3eTh, U5eTh and U5eDU are analyzed to compare the neutronic properties. To evaluate the breeding capability, conversion ratio (CR) for each fuel type is first calculated considering various possibly involved fissile and fertile materials (Li et al., 2015):
Fuel type
Starter zone Loading 233
U3eTh U5eTh U5eDU (GA)
235 235
Uþ Uþ Uþ
232
Th U U
238 238
Fertile zone Mass ratio
Loading
Mass ratio
11% þ 89% 12% þ 88% 12% þ 88%
232
100% 100% 0.2% þ 99.8%
232 235
Th Th Uþ
238
U
3. Results and discussions
CR ¼
ð232 Th
234 U
238 U
240 Pu
233 PaÞ
Rcapture þ þ þ Rabsorption ð233 U þ 235 U þ 239 Pu þ 241 PuÞ
;
(1)
where Rcapture denotes the neutron capture of fertile isotopes (232Th, 234U, 238U, 240Pu and 233Pa) while Rabsorption stands for the neutron absorption of fissile isotopes (233U, 235U, 239Pu and 241Pu). From the neutron balance point of view, keff and CR are two competitive parameters. In order to keep critical and achieve breeding, both keff and CR should be higher than 1, which can be used to determine a relatively reasonable ratio of fissile to fertile in the EM2 core. Fig. 2 shows the respective CR and keff with various mass fractions of 235U (Mass235 U =Mass235 Uþ238 U ) for U5eTh and U5eDU, and 233 U (Mass233 U =Mass233 Uþ232 Th ) for U3eTh. The enrichment of 235U is limited to under 20% because it is the preferred enrichment level for civil reactor to minimize overall proliferation risks (Glaser, 2005). The 235U enrichment of 12% adopted by GA allows both critical and CR higher than 1 (Schleicher and Back, 2012). Under such a similar consideration, the 233U mass ratio is set to be 11% for U3eTh and the 235U mass ratio is 12% for U5eTh. Th fuel is loaded in the fertile zone for the two scenarios. The related fuel loading parameters based on the analyzing results are shown in Table 2. There are plenty of available 238U and TRU elements in the discharged fuel of the Gen-I EM2. If FPs can be partitioned effectively from the end-of-cycle EM2 fuel, the heavy metal fuel can be reloaded as driver fuel to ignite a new EM2. For simplicity, it is assumed that all FPs are removed and the reprocessed fuel is reused in the Gen-II EM2. The neutronics of startup fuel with some MAs for the Gen-II EM2 may differ somewhat from that of a “pure” fuel (for example U5 þ Th). But this minor difference would not bring out a significant influence on the evolution of the Gen-II. The total fraction of fissile fuels and minor actinides (MAs) is therefore still kept at about 12% for simplicity. Under this condition, only fertile fuel is needed to start a new EM2, which is of considerable importance to reduce fissile demand and improve fuel utilization efficiency.
Fig. 2. Initial CR and keff for different mass ratios of
235
U or
233
U in the starter zone.
According to the fuel loading parameters in Table 2, the neutronics simulations are performed for the EM2 from Gen-I to Gen-II. 3.1. Neutron spectrum and reactivity evolution Neutron spectrum and keff are analyzed to compare the breeding performance of each kind of fuel loading. The neutron spectra of the three types of fuels for the Gen-I EM2 at BOL (beginning of life) are displayed in Fig. 3. The resonance dips between 1 eV and 1 keV result from the resonance absorption of 238U and/or 232Th. The differences between the spectra appear at around the range of 2 eVe100 eV where the resonance peak of 238U is broader but smaller than the lowest resonance of 232Th. The keff of the three types of fuels for Gen-I as a function of operation time is shown in Fig. 4 (top panel). It can be seen that the keff for U3eTh decreases monotonically with the operation time. However, the keff evolutions of U5eTh and U5eDU fuel are different from the U3eTh case, which first increase and then decrease. It also indicates that U5eTh has the lowest initial excess reactivity and the deepest burnup. To explore its variation sources, keff can be broken up into separate k and kþ , which stand for the reactivity decrement due eff eff to both fission and absorption of initial loaded heavy nuclides (HN) and the reactivity increment from the newly-produced HN by absorption of fertile fuel, respectively. The corresponding variants can be written as
k eff ðtÞ ¼ ðtÞ ¼ kþ eff
HNðinitialÞ X i HNðnewÞ X
keff ði; tÞ
HNðinitialÞ X
keff ði; 0Þ
i
(2)
keff ði; tÞ;
i
where HN(initial) and HN(new) denote the initial loaded and the
Fig. 3. Neutron spectra of three kinds of fuels for the Gen-I EM2.
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the operation time for U3eTh. As for U5eTh and U5eDU, k is first eff smaller and then greater than kþ , which leads to first increasing eff and later decreasing of keff with time. The maximum difference between reactivity decrement and increment corresponds to the peak value of keff, which is around 14 years for U5eTh and 9 years for U-DU. While the point of intersection of the two lines means keff reaches its initial value at the startup time. In short, U3eTh has the greatest reactivity increment in the three kinds of fuels; however due to its even higher reactivity decrement during the whole operation time, its keff manifests a monotonous decline. It also indicates that the deepest fuel discharge burnup of U5eTh stems from the largest gap between its reactivity increment and decrement. The reactivity variation is determined by the contributions of important actinides in the core. Therefore, it is interesting to extract the detailed evolution of reactivity contributions from various important nuclides including fissile and fertile nuclides. For U3eTh, the variation of the initial loaded 233U can be evaluated by
N233 U ðtÞ ¼ N233 U ðt ¼ 0Þ$eF$ðsn;g þsn;f Þt ;
Fig. 4. Evolution of keff: Gen-I (top) and Gen-II (bottom).
newly-produced HN. HN(initial) includes 233U and Th for the U3eTh loading, 235U, 238U and Th for the U5eTh loading, 235U, 238U and DU for the U5eDU loading, while HN(new) includes all the possible newly-produced HN from the initial fuels. The contribution of HN i at time t to keff can be calculated by (Yu et al., 2015).
keff ði; tÞ ¼
Rp ði; tÞ R ði; tÞ$nði; tÞ ¼Pf ; Ra ðj; tÞ þ LðtÞ Rd ðtÞ
(3)
j
where Rp and Rd refer to the neutron productive rate and the neutron disappear rate, respectively; Rf(i) and nðiÞ represent the neutron fission rate and the average neutron number per fission for HN i, respectively; Ra(j) and L represent the neutron absorption rate for nuclide j (including fuels and various possible absorption materials in the core) and the neutron leakage rate, respectively. With k and kþ , one can know the explicit evolution of the eff eff reactivity decrement from the original fuels (mostly from fissile fuel) and the reactivity increment from the newly produced fuels. Fig. 5 gives the variations of k (absolute value) and kþ with time eff eff for the three fuel loadings for the Gen-I EM2. For the U3eTh case, the variant speed of k is always greater than that of kþ during the eff eff þ whole operation, which results in a greater k than k at any eff eff operation time. Therefore, the keff decreases monotonously with
Fig. 5. Variation of reactivity.
(4)
where F denotes the neutron flux; s refers to the one group average capture or fission cross section. Because the total amount of 233U in the core can be calculated by SCALE, one can thus know the net amount of 233U bred from 232Th using Eq. (4). For U5eTh and U5eDU, since the bred fissile (239Pu and/or 233U) can be distinguished from the initial loaded fissile (235U), their respective contribution evolution can be extracted directly by SCALE. For the starter zone, it can be seen from Fig. 6a that 235U, 239Pu and 238U dominate the reactivity contribution (more than 94%) for U5eTh and U5eDU. Despite the continuous consumption of 235U, 239 Pu bred from 238U compensates for the reactivity loss from 235U. After 11 years, the reactivity contribution of 239Pu exceeds that of 235 U, and then keeps dominant during the rest of operation time. As for U3eTh, one can see that the initial loaded and bred 233U also provides the majority of reactivity contribution during the whole operation time. Meanwhile the reactivity contribution from 232Th for U3eTh is significantly lower than that of 238U for U5eTh and U5eDU, which leads to the faster decline of reactivity of fissile for U3eTh and consequently the much less operation time despite its greater reactivity from the initial loaded and bred 233U. For the fertile zone, the difference between the reactivity variations of U5eTh and U5eDU mainly results from the contribution of fertile isotopes, since they have identical loading composition in the starter zone. As shown in Fig. 6b, for the U5eDU loading, 238U and bred 239Pu contribute the majority of reactivity in the fertile zone. 239Pu has a larger increment in the first 5 years and then tends to be gradual compared to 233U, which explains a larger initial keff and a faster decrease for U5eDU compared to U5eTh. As for U3eTh, it has the same fuel loading in the fertile zone with U5eTh and their main reactivity contribution comes from bred 233U. Bred 233 U in the fertile zone gets more than 239Pu after 5 years, which brings a higher reactivity contribution. The keff of the Gen-II EM2 is shown in Fig. 4 (bottom panel). It can be seen that Gen-II has a deeper burnup compared to Gen-I because of the existence of much more transuranic (TRU) which have a considerably better fission property in fast neutron energy region. Most TRUs like 237Np, 241Pu, 243Pu and 244Cm are capable to occur fission reaction, therefore it is helpful to deepen the burnup of the EM2. Furthermore, the discharged fissile fuel from Gen-I is sufficient to initiate Gen-II. The Gen-I with U3eTh and U5eDU can produce about 2.5 ton fissile nuclides while only 2.1 ton and 1.6 ton are required for Gen-II, respectively. As for U5eTh, 2.6 ton is produced in Gen-I but only 1.9 ton is needed to restart Gen-II.
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Fig. 6. Contribution to keff from important actinides. a: starter zone; b: fertile zone.
3.2. CR for the fuel loadings CR is an important parameter for EM2, which should be equal to or larger than 1.0 during a long enough operation time in order to achieve a sufficient breed-and-burn. Fig. 7 presents the CR evolution of the three fuel loadings for Gen-I. For all the three scenarios, CR shows a decline trend due to the decrease of capture reaction rate of fertile nuclide. Additionally, various reactivity contributions of fissile nuclides (233U, 235U and 239Pu) result in different absorption reaction rate evolutions. It also plays an important role in the CR evolution for U3eTh, U5eTh and U5eDU. The reaction rate evolution of a nuclide is determined by the variants of its atomic density, exposed neutron flux and mean microscopic cross section. The variation of mean microscopic cross section is relatively small considering the slight shift of neutron spectrum from BOL to EOL (end of life). Take U3eTh for example, Eq. (1) can be simplified as the ratio of 232Th capture reaction rate and 233U absorption reaction rate. As seen from Fig. 8, both the absorption reaction rate of 233U (Ra) and the capture reaction rate of 232 Th (Rc) decline with operation time while the latter has a much larger decline speed. As mentioned in Section 3.1, 233U is produced and consumed in the starter zone, yet the net amount of 233U is decreased; meanwhile 233U bred from 232Th in the fertile zone is continuously accumulated. Hence the neutron flux tends to slightly decline in the starter zone but increase in the fertile zone with time.
Fig. 7. Evolution of CR for Gen-I.
As noted previously, the variation of mean microscopic absorption cross section for 233U is negligible. Thus the total absorption reaction rate of 233U in both zones resulting from the comprehensive effect of neutron flux and atomic density variation is slightly decreased during the operation time. As for 232Th, the atomic density declines in both zones. Its effect on the reaction rate evolution is far above the variation brought by neutron spectra shift as well. Therefore, the decrease of 232Th atomic density dominates its declining capture reaction rate. Based on the analysis above, the reaction rate evolutions of 233U and 232Th cause the monotonic decline of CR for U3eTh. With the similar analysis to the above, one can understand the CR evolution for U5eTh and U5eDU in Fig. 7. The initial CR value is determined according to the mass ratio (see Fig. 2) considering both criticality and breeding. The CR evolution of U5eDU decreases quickly during the first 3 years and then tends to be gradual. It is because the neutron absorption reaction rate of newly produced 239 Pu is larger than that of consumed 235U during the first 3 years. Afterwards, U5eDU shows a slower decrease of neutron capture reaction rate (mainly from 238U) compared to U5eTh (mainly from 238 U and 232Th), which results in the flattest decline trend of CR evolution of U5eDU after 3 years. In a word, CR evolution in the EM2 mainly relies on atomic density and neutron flux variations. The three kinds of fuel loadings can achieve acceptable breeding capability. And a reasonable CR is favorable to deepening burnup.
Fig. 8. Reaction rate analysis of U3eTh.
Y.W. Ma et al. / Progress in Nuclear Energy 85 (2015) 764e770
3.3. Radio-toxicity for the fuel loadings Nuclear waste management has become more important, radiotoxicity is a commonly used international benchmark to describe the hazards in used nuclear fuel. EM2 has a potential in reducing radio-toxicity when thorium is considered as a fuel option. The radio-toxicity R of nuclide i with inventory Ni and decay constant li can be calculated using
RðtÞ ¼
X
Ri ðtÞ ¼
i
X
ri li Ni ðtÞ;
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Table 3 Radio-toxicity and its proportion for three kinds of fuel options after 10 years cooling. The value of zero denotes that the radio-toxicity is smaller than 0.1%. Fuel type Th Gen-I
U3eTh U5eTh U5eDU Gen-II U3eTh U5eTh U5eDU
0.7% 0.2% 0.0% 0.2% 0.2% 0.0%
Pa
U
Pu
Am
Cm
Total (Sv/GWth$y)
1.3% 0.1% 0.0% 0.2% 0.1% 0.0%
97.7% 19.4% 0.0% 99.3% 27.6% 0.0%
0.2% 75.9% 87.3% 0.2% 68.0% 83.1%
0.0% 3.6% 6.9% 0.0% 3.3% 8.7%
0.0% 0.8% 5.7% 0.0% 0.8% 8.2%
2.74 5.44 7.62 1.49 1.96 6.26
108 108 108 108 108 108
(5)
i
where dose coefficients ri is given by the International Commission on Radiological Protection (Nuttin et al., 2005). Calculation of waste radio-toxicity R(t) are done using the code ORIGEN-S. The radio-toxicities of the Gen-I and Gen-II EM2 with the three discharged fuels after a cooling time of 10 years are compared in Fig. 9. The total radio-toxicity and the proportion of each nuclide are shown in Table 3. The long term fuel activity is predominantly determined by transuranic nuclides like U, Pu, Am and Cm. U5eDU has the highest radio-toxicity level since more plutonium and americium are produced during the operation compared to U5eTh and U3eTh. 239Pu and 240Pu dominate the radio-toxicity of U5eDU. U3eTh displays a lower radio-toxicity level compared to other two kinds of fuel loadings in the first 10,000 years. It confirms that TheU fuel cycle has a significantly lower radio-toxicity due to the less bulid-up of TRUs compared to UePu fuel cycle. The midterm and long-term radio-toxicity of the 233U and/or Th loading fuels is dominated by 233U and 234U, and the latter is responsible for the radio-toxicity growth around 10,000 years. The radio-toxicity of the Gen-II EM2 has the similar trend to the Gen-I EM2 but with a lower level due to the deeper burnup with effective incineration of TRU. The ratio of the total radio-toxicity (see Table 3) of Gen-II to that of Gen-I is about 0.54 for U3eTh, 0.36 for U5eTh, 0.82 for U5eDU at 10 years, and about 0.55 for U3eTh, 0.36 for U5eTh, 0.61 for U5eDU at 5 107 years.
produce 2.5 ton 233U and only 2.1 ton is needed to initiate the Gen-II EM2. The U5eTh fuel loading is better than the other two cases from the viewpoint of burnup and initial reactivity control. 238U in the starter zone has a bigger reactivity contribution than 232 Th, which can significantly slow down the consumption of 235 U as well as the decrease of keff for U5eTh compared to U3eTh, while 233U bred from 232Th in the fertile zone has a larger reactivity contribution than bred 239Pu. These cause the U5eTh loading has the longest operation time. Spent fuel from TheU fuel cycle has a radio-toxicity almost one order of magnitude lower than that of UePu fuel cycle in the first 10,000 years due to much less neutron capture for 233U and thereby less minor actinides production. Moreover, introduction of multi-generation EM2 can significantly reduce the waste management pressure due to the deeper burnup of TRU. With further optimization such as adjustment of fuel loading and core geometry, the EM2 with U3eTh is expected to achieve a better breed and burn performance. Acknowledgments This work is supported by the Chinese TMSR Strategic Pioneer Science and Technology Project under Grant No. XDA02010000 and the National Natural Science Foundation of China under Grant No. 91326201.
4. Conclusions References The neutronic properties of an EM2 based on three kinds of fuel loadings are analyzed. Conclusions drawn from this work are as follows: The U3eTh fuel loading can achieve sustainable operation in multi-generation EM2. The Gen-I EM2 with the U3eTh fuel can
Fig. 9. Radio-toxicity of EM2 Gen-I and II.
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Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Startup and burnup strategy for TheU/UePu fuel cycles in an EM2 reactor Y.W. Ma a, b, c, X.X. Li a, b, X.Z. Cai a, b, C.G. Yu a, b, C.Y. Zou a, b, J.L. Han a, b, J.G. Chen a, b, * a
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China CAS Center for Excellence in TMSR Energy System, Chinese Academy of Sciences, Shanghai 201800, China c University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2015 Received in revised form 7 September 2015 Accepted 14 September 2015 Available online xxx
General Atomics (GA) is developing the Energy Multiplier Module (EM2) which is a compact gas-cooled fast reactor as one of candidates of the Generation-IV nuclear energy systems. In the EM2 core, low enriched uranium is used as igniting fuel and depleted uranium is used for converting and burning. It indicates that EM2 can maintain critical operation for more than 30 years without refueling. To further study the TheU fuel cycle performance in the EM2, two kinds of start-up strategies with TheU (Th þ 233U) and semi TheU (Th þ enriched 235U) are evaluated. Neutronics characteristics, such as the effective multiplicity factor (keff) and conversion ratio (CR) are analyzed from neutron usage point of view. The simulated results for the two kinds of fuels are compared with the UePu fuel from the design of GA. The analysis gives an insight into the pros and cons of UePu and TheU fuel cycles in terms of the breeding capability and the discharged radio-toxicity. The breeding performance of the second generation EM2 is also presented and compared with that of the first generation EM2. It indicates that the multi-generation EM2 can deepen the burnup and reduce the waste management pressure for each kind of fuel loading strategy. © 2015 Elsevier Ltd. All rights reserved.
Keywords: EM2 Fuel cycle Breeding Radio-toxicity
1. Introduction Current commercial nuclear power reactors, mostly Light Water Reactors (LWRs), use about 5% enriched uranium with oncethrough fuel cycle. The fuel utilization is less than one percent of the natural uranium, which will consume up the whole world's estimated uranium in a few decades along with the growing demand of energy (David, 2005). Meanwhile, it also inevitably leads to release of radioactive waste with a considerable amount of transuranium (TRU) (Alajo and Tsvetkov, 2011; Salvatores, 2009). Six advanced reactor concepts selected for Generation-IV nuclear energy systems are under development worldwide to meet the goals of effective resource utilization and waste minimization, improved safety, enhanced proliferation resistance, and reduced system cost (Locatelli et al., 2013; Gyorgy and Czifrus, 2015). Three of them are fast reactors: Gas-cooled Fast Reactor, Lead-cooled Fast Reactor and Sodium-Cooled Fast Reactor. Fast reactor systems have
* Corresponding author. Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail address: [email protected] (J.G. Chen). http://dx.doi.org/10.1016/j.pnucene.2015.09.010 0149-1970/© 2015 Elsevier Ltd. All rights reserved.
effective capability to breed fissile fuel and transmute nuclear waste and eventually realize the ultimate utilization of nuclear fission energy. Therefore, fast reactors are considered to be potential to solve the essential issues for long-term development of nuclear energy, such as fuel supply and back-end spent fuel processing (Artisyuk et al., 2005; Tucek, 2004). The recently emerged concept of Small Modular Reactor (SMR) also addresses the above these issues for nuclear energy utilization (Liu and Fan, 2014; Hidayatullah et al., 2015; Shin et al., 2015; ElGenk and Palomino, 2015; Rowinski et al., 2015). It provides an energy option with low carbon emission, enhanced safety conviction, convenient construction and operation, therefore becoming more attractive especially in the Post-Fukushima era (Kessides and Kuznetsov, 2012). One kind of SMRs named Energy Multiplier Module (EM2) was proposed by General Atomics (GA) based on the Gen-IV reactor concept of gas-cooled fast reactor (Schleicher and Back, 2012). The EM2 was designed to burn the depleted uranium (DU) or used nuclear fuel (UNF) in convert and burn mode which converts fertile to fissile and then burning it in situ (Parmentola and Rawls, 2012). The EM2 ignited with low enriched uranium and loaded with DU as
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fertile fuel can operate for more than 30 years without refueling. And a burnup of 140 GWd/t which is more than double that of any traditional light water reactor, could be achieved (Joe et al., 2010). Furthermore, the discharged fuel from EM2 can be recycled to ignite a new EM2 by only removing a fraction of fission products (FPs) (Parmentola and Rawls, 2012). The multi-generation mode of fuel utilization ensures the excellent sustainability of EM2. Besides UePu fuel cycle, TheU fuel cycle could also be feasible to be employed in EM2. TheU fuel cycle has been widely studied in the past as a possible alternative to UePu fuel cycle (IAEA, 2005; Lung and Gremm, 1998; Furukawa et al., 2008; Fortini et al., 2015; Brown et al., 2015). Furthermore, it can be in principle utilized sustainably in both thermal and fast reactors (Kuegler et al., 2007; MacDonald et al., 2004; Fiorina et al., 2013a; Fiorina et al., 2013b; NEA-OECD, 2002). Use of Th in a burner design for a closed nuclear fuel cycle is appealing because of the potential to achieve high fuel utilization and low build-up of hazardous isotopes (Ashley et al., 2014; Csom et al., 2012; Srivastava et al., 2011). Therefore, the neutronic performance in EM2 based on TheU fuel cycle (abbreviated as U3eTh) is evaluated in this work. Another thorium based fuel option ignited by enriched 235U (abbreviated as U5eTh) and the conventional fuel proposed by GA (abbreviated as U5eDU) are also analyzed for comparison. Then the neutronics analysis is extended to the second generation EM2 (Gen-II EM2) fueled with the reprocessed fuel that was discharged from the first generation EM2 (Gen-I EM2). The methodology for core modeling and the neutronic tool are introduced in Section 2. The calculated results and the detailed discussion are presented in Section 3. The conclusions are given in Section 4. 2. Methodology 2.1. Reactor description and modeling The schematic EM2 core exhibited in Fig. 1 includes the starter zone, the fertile zone, the inner Be2C and the outer graphite reflectors. Fissile fuel and fertile fuel (uranium carbide and thorium carbide in the form of porous pellets) in the core could maintain the balance of reactivity increase from the production of new fissile fuel and reactivity decrease owing to fuel depletion and FPs accumulation. Therefore EM2 can operate for several decades without refueling but just depending on the initial fuel loading. The core contains 91 fuel assemblies arranged in a right hexagonal prism, and each fuel assembly contains 91 fuel rods. The coolant flow channels (triangle) are arranged between the fuel rods. The important parameters referred to the GA's design are shown in Table 1 (Schleicher and Back, 2012; Parmentola and Rawls, 2012). In addition, some other parameters like material
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Table 1 EM2 design parameters. Parameter
Value
Thermal power (MWth) Active core diameter/Height (cm) Core diameter (cm) Fuel lattice Lattice diameter (mm) Lattice height (cm) SiC thickness (mm) Starter zone HM Inventory (t) Volume (m3) Fertile zone HM Inventory (t) Volume (m3) Total HM inventory (t) Other materials Inner reflector Outer reflector Coolant Cladding
500 196/273 280 Hexagon 18.5 30.3 1 21.2 2.6 19.5 2.4 40.7 Be2C Graphite Helium SiC
density are determined according to empirical data. The structural material used in the core including the fuel cladding, is SiCeSiC composite (Schleicher and Back, 2012). Helium is used as coolant and the core outlet temperature is assumed to be 850 C, corresponding to 48% thermal conversion efficiency. Be2C and graphite are used as reflector to ensure a flat power profile for relatively uniform irradiation rates in the core (Parmentola and Rawls, 2012). The numerical analysis is based on the TRITON control module in SCALE6.1 which can perform reliable depletion and decay analysis for reactor physics applications using problem-dependent cross-section processing and rigorous treatment of neutron transport (Scale 2011). Three-dimensional Monte Carlo transport code KENO-VI coupled with point-depletion and decay analysis module ORIGEN-S are used within the TRITON sequence based on the cross section library V7-238 (238-group ENDF/B-VII). A fast reactor library is selected during the TRITON execution, which contains fast fission product yields and cross sections are weighted based on a typical fast reactor spectrum (Sheu et al., 2013). In order to preserve a better accuracy during the calculation, the option PARM is selected for 388 isotopes in the transport updating for depletion. Depletion step is chosen by considering both computation efficiency and accuracy (Allen and Knight, 2010), which is relatively compressive (20e50 days) at the early operation stage and has a larger span (200e500 days) after 200 days.
Fig. 1. Schematic core of EM2 from vertical view (a) and side view (b).
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2.2. Fuel loading consideration
Table 2 Loading portion for the three kinds of fuels.
The EM2 system is featured with the characteristics of fuel breeding and burning. Three fuel loading strategies U3eTh, U5eTh and U5eDU are analyzed to compare the neutronic properties. To evaluate the breeding capability, conversion ratio (CR) for each fuel type is first calculated considering various possibly involved fissile and fertile materials (Li et al., 2015):
Fuel type
Starter zone Loading 233
U3eTh U5eTh U5eDU (GA)
235 235
Uþ Uþ Uþ
232
Th U U
238 238
Fertile zone Mass ratio
Loading
Mass ratio
11% þ 89% 12% þ 88% 12% þ 88%
232
100% 100% 0.2% þ 99.8%
232 235
Th Th Uþ
238
U
3. Results and discussions
CR ¼
ð232 Th
234 U
238 U
240 Pu
233 PaÞ
Rcapture þ þ þ Rabsorption ð233 U þ 235 U þ 239 Pu þ 241 PuÞ
;
(1)
where Rcapture denotes the neutron capture of fertile isotopes (232Th, 234U, 238U, 240Pu and 233Pa) while Rabsorption stands for the neutron absorption of fissile isotopes (233U, 235U, 239Pu and 241Pu). From the neutron balance point of view, keff and CR are two competitive parameters. In order to keep critical and achieve breeding, both keff and CR should be higher than 1, which can be used to determine a relatively reasonable ratio of fissile to fertile in the EM2 core. Fig. 2 shows the respective CR and keff with various mass fractions of 235U (Mass235 U =Mass235 Uþ238 U ) for U5eTh and U5eDU, and 233 U (Mass233 U =Mass233 Uþ232 Th ) for U3eTh. The enrichment of 235U is limited to under 20% because it is the preferred enrichment level for civil reactor to minimize overall proliferation risks (Glaser, 2005). The 235U enrichment of 12% adopted by GA allows both critical and CR higher than 1 (Schleicher and Back, 2012). Under such a similar consideration, the 233U mass ratio is set to be 11% for U3eTh and the 235U mass ratio is 12% for U5eTh. Th fuel is loaded in the fertile zone for the two scenarios. The related fuel loading parameters based on the analyzing results are shown in Table 2. There are plenty of available 238U and TRU elements in the discharged fuel of the Gen-I EM2. If FPs can be partitioned effectively from the end-of-cycle EM2 fuel, the heavy metal fuel can be reloaded as driver fuel to ignite a new EM2. For simplicity, it is assumed that all FPs are removed and the reprocessed fuel is reused in the Gen-II EM2. The neutronics of startup fuel with some MAs for the Gen-II EM2 may differ somewhat from that of a “pure” fuel (for example U5 þ Th). But this minor difference would not bring out a significant influence on the evolution of the Gen-II. The total fraction of fissile fuels and minor actinides (MAs) is therefore still kept at about 12% for simplicity. Under this condition, only fertile fuel is needed to start a new EM2, which is of considerable importance to reduce fissile demand and improve fuel utilization efficiency.
Fig. 2. Initial CR and keff for different mass ratios of
235
U or
233
U in the starter zone.
According to the fuel loading parameters in Table 2, the neutronics simulations are performed for the EM2 from Gen-I to Gen-II. 3.1. Neutron spectrum and reactivity evolution Neutron spectrum and keff are analyzed to compare the breeding performance of each kind of fuel loading. The neutron spectra of the three types of fuels for the Gen-I EM2 at BOL (beginning of life) are displayed in Fig. 3. The resonance dips between 1 eV and 1 keV result from the resonance absorption of 238U and/or 232Th. The differences between the spectra appear at around the range of 2 eVe100 eV where the resonance peak of 238U is broader but smaller than the lowest resonance of 232Th. The keff of the three types of fuels for Gen-I as a function of operation time is shown in Fig. 4 (top panel). It can be seen that the keff for U3eTh decreases monotonically with the operation time. However, the keff evolutions of U5eTh and U5eDU fuel are different from the U3eTh case, which first increase and then decrease. It also indicates that U5eTh has the lowest initial excess reactivity and the deepest burnup. To explore its variation sources, keff can be broken up into separate k and kþ , which stand for the reactivity decrement due eff eff to both fission and absorption of initial loaded heavy nuclides (HN) and the reactivity increment from the newly-produced HN by absorption of fertile fuel, respectively. The corresponding variants can be written as
k eff ðtÞ ¼ ðtÞ ¼ kþ eff
HNðinitialÞ X i HNðnewÞ X
keff ði; tÞ
HNðinitialÞ X
keff ði; 0Þ
i
(2)
keff ði; tÞ;
i
where HN(initial) and HN(new) denote the initial loaded and the
Fig. 3. Neutron spectra of three kinds of fuels for the Gen-I EM2.
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the operation time for U3eTh. As for U5eTh and U5eDU, k is first eff smaller and then greater than kþ , which leads to first increasing eff and later decreasing of keff with time. The maximum difference between reactivity decrement and increment corresponds to the peak value of keff, which is around 14 years for U5eTh and 9 years for U-DU. While the point of intersection of the two lines means keff reaches its initial value at the startup time. In short, U3eTh has the greatest reactivity increment in the three kinds of fuels; however due to its even higher reactivity decrement during the whole operation time, its keff manifests a monotonous decline. It also indicates that the deepest fuel discharge burnup of U5eTh stems from the largest gap between its reactivity increment and decrement. The reactivity variation is determined by the contributions of important actinides in the core. Therefore, it is interesting to extract the detailed evolution of reactivity contributions from various important nuclides including fissile and fertile nuclides. For U3eTh, the variation of the initial loaded 233U can be evaluated by
N233 U ðtÞ ¼ N233 U ðt ¼ 0Þ$eF$ðsn;g þsn;f Þt ;
Fig. 4. Evolution of keff: Gen-I (top) and Gen-II (bottom).
newly-produced HN. HN(initial) includes 233U and Th for the U3eTh loading, 235U, 238U and Th for the U5eTh loading, 235U, 238U and DU for the U5eDU loading, while HN(new) includes all the possible newly-produced HN from the initial fuels. The contribution of HN i at time t to keff can be calculated by (Yu et al., 2015).
keff ði; tÞ ¼
Rp ði; tÞ R ði; tÞ$nði; tÞ ¼Pf ; Ra ðj; tÞ þ LðtÞ Rd ðtÞ
(3)
j
where Rp and Rd refer to the neutron productive rate and the neutron disappear rate, respectively; Rf(i) and nðiÞ represent the neutron fission rate and the average neutron number per fission for HN i, respectively; Ra(j) and L represent the neutron absorption rate for nuclide j (including fuels and various possible absorption materials in the core) and the neutron leakage rate, respectively. With k and kþ , one can know the explicit evolution of the eff eff reactivity decrement from the original fuels (mostly from fissile fuel) and the reactivity increment from the newly produced fuels. Fig. 5 gives the variations of k (absolute value) and kþ with time eff eff for the three fuel loadings for the Gen-I EM2. For the U3eTh case, the variant speed of k is always greater than that of kþ during the eff eff þ whole operation, which results in a greater k than k at any eff eff operation time. Therefore, the keff decreases monotonously with
Fig. 5. Variation of reactivity.
(4)
where F denotes the neutron flux; s refers to the one group average capture or fission cross section. Because the total amount of 233U in the core can be calculated by SCALE, one can thus know the net amount of 233U bred from 232Th using Eq. (4). For U5eTh and U5eDU, since the bred fissile (239Pu and/or 233U) can be distinguished from the initial loaded fissile (235U), their respective contribution evolution can be extracted directly by SCALE. For the starter zone, it can be seen from Fig. 6a that 235U, 239Pu and 238U dominate the reactivity contribution (more than 94%) for U5eTh and U5eDU. Despite the continuous consumption of 235U, 239 Pu bred from 238U compensates for the reactivity loss from 235U. After 11 years, the reactivity contribution of 239Pu exceeds that of 235 U, and then keeps dominant during the rest of operation time. As for U3eTh, one can see that the initial loaded and bred 233U also provides the majority of reactivity contribution during the whole operation time. Meanwhile the reactivity contribution from 232Th for U3eTh is significantly lower than that of 238U for U5eTh and U5eDU, which leads to the faster decline of reactivity of fissile for U3eTh and consequently the much less operation time despite its greater reactivity from the initial loaded and bred 233U. For the fertile zone, the difference between the reactivity variations of U5eTh and U5eDU mainly results from the contribution of fertile isotopes, since they have identical loading composition in the starter zone. As shown in Fig. 6b, for the U5eDU loading, 238U and bred 239Pu contribute the majority of reactivity in the fertile zone. 239Pu has a larger increment in the first 5 years and then tends to be gradual compared to 233U, which explains a larger initial keff and a faster decrease for U5eDU compared to U5eTh. As for U3eTh, it has the same fuel loading in the fertile zone with U5eTh and their main reactivity contribution comes from bred 233U. Bred 233 U in the fertile zone gets more than 239Pu after 5 years, which brings a higher reactivity contribution. The keff of the Gen-II EM2 is shown in Fig. 4 (bottom panel). It can be seen that Gen-II has a deeper burnup compared to Gen-I because of the existence of much more transuranic (TRU) which have a considerably better fission property in fast neutron energy region. Most TRUs like 237Np, 241Pu, 243Pu and 244Cm are capable to occur fission reaction, therefore it is helpful to deepen the burnup of the EM2. Furthermore, the discharged fissile fuel from Gen-I is sufficient to initiate Gen-II. The Gen-I with U3eTh and U5eDU can produce about 2.5 ton fissile nuclides while only 2.1 ton and 1.6 ton are required for Gen-II, respectively. As for U5eTh, 2.6 ton is produced in Gen-I but only 1.9 ton is needed to restart Gen-II.
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Fig. 6. Contribution to keff from important actinides. a: starter zone; b: fertile zone.
3.2. CR for the fuel loadings CR is an important parameter for EM2, which should be equal to or larger than 1.0 during a long enough operation time in order to achieve a sufficient breed-and-burn. Fig. 7 presents the CR evolution of the three fuel loadings for Gen-I. For all the three scenarios, CR shows a decline trend due to the decrease of capture reaction rate of fertile nuclide. Additionally, various reactivity contributions of fissile nuclides (233U, 235U and 239Pu) result in different absorption reaction rate evolutions. It also plays an important role in the CR evolution for U3eTh, U5eTh and U5eDU. The reaction rate evolution of a nuclide is determined by the variants of its atomic density, exposed neutron flux and mean microscopic cross section. The variation of mean microscopic cross section is relatively small considering the slight shift of neutron spectrum from BOL to EOL (end of life). Take U3eTh for example, Eq. (1) can be simplified as the ratio of 232Th capture reaction rate and 233U absorption reaction rate. As seen from Fig. 8, both the absorption reaction rate of 233U (Ra) and the capture reaction rate of 232 Th (Rc) decline with operation time while the latter has a much larger decline speed. As mentioned in Section 3.1, 233U is produced and consumed in the starter zone, yet the net amount of 233U is decreased; meanwhile 233U bred from 232Th in the fertile zone is continuously accumulated. Hence the neutron flux tends to slightly decline in the starter zone but increase in the fertile zone with time.
Fig. 7. Evolution of CR for Gen-I.
As noted previously, the variation of mean microscopic absorption cross section for 233U is negligible. Thus the total absorption reaction rate of 233U in both zones resulting from the comprehensive effect of neutron flux and atomic density variation is slightly decreased during the operation time. As for 232Th, the atomic density declines in both zones. Its effect on the reaction rate evolution is far above the variation brought by neutron spectra shift as well. Therefore, the decrease of 232Th atomic density dominates its declining capture reaction rate. Based on the analysis above, the reaction rate evolutions of 233U and 232Th cause the monotonic decline of CR for U3eTh. With the similar analysis to the above, one can understand the CR evolution for U5eTh and U5eDU in Fig. 7. The initial CR value is determined according to the mass ratio (see Fig. 2) considering both criticality and breeding. The CR evolution of U5eDU decreases quickly during the first 3 years and then tends to be gradual. It is because the neutron absorption reaction rate of newly produced 239 Pu is larger than that of consumed 235U during the first 3 years. Afterwards, U5eDU shows a slower decrease of neutron capture reaction rate (mainly from 238U) compared to U5eTh (mainly from 238 U and 232Th), which results in the flattest decline trend of CR evolution of U5eDU after 3 years. In a word, CR evolution in the EM2 mainly relies on atomic density and neutron flux variations. The three kinds of fuel loadings can achieve acceptable breeding capability. And a reasonable CR is favorable to deepening burnup.
Fig. 8. Reaction rate analysis of U3eTh.
Y.W. Ma et al. / Progress in Nuclear Energy 85 (2015) 764e770
3.3. Radio-toxicity for the fuel loadings Nuclear waste management has become more important, radiotoxicity is a commonly used international benchmark to describe the hazards in used nuclear fuel. EM2 has a potential in reducing radio-toxicity when thorium is considered as a fuel option. The radio-toxicity R of nuclide i with inventory Ni and decay constant li can be calculated using
RðtÞ ¼
X
Ri ðtÞ ¼
i
X
ri li Ni ðtÞ;
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Table 3 Radio-toxicity and its proportion for three kinds of fuel options after 10 years cooling. The value of zero denotes that the radio-toxicity is smaller than 0.1%. Fuel type Th Gen-I
U3eTh U5eTh U5eDU Gen-II U3eTh U5eTh U5eDU
0.7% 0.2% 0.0% 0.2% 0.2% 0.0%
Pa
U
Pu
Am
Cm
Total (Sv/GWth$y)
1.3% 0.1% 0.0% 0.2% 0.1% 0.0%
97.7% 19.4% 0.0% 99.3% 27.6% 0.0%
0.2% 75.9% 87.3% 0.2% 68.0% 83.1%
0.0% 3.6% 6.9% 0.0% 3.3% 8.7%
0.0% 0.8% 5.7% 0.0% 0.8% 8.2%
2.74 5.44 7.62 1.49 1.96 6.26
108 108 108 108 108 108
(5)
i
where dose coefficients ri is given by the International Commission on Radiological Protection (Nuttin et al., 2005). Calculation of waste radio-toxicity R(t) are done using the code ORIGEN-S. The radio-toxicities of the Gen-I and Gen-II EM2 with the three discharged fuels after a cooling time of 10 years are compared in Fig. 9. The total radio-toxicity and the proportion of each nuclide are shown in Table 3. The long term fuel activity is predominantly determined by transuranic nuclides like U, Pu, Am and Cm. U5eDU has the highest radio-toxicity level since more plutonium and americium are produced during the operation compared to U5eTh and U3eTh. 239Pu and 240Pu dominate the radio-toxicity of U5eDU. U3eTh displays a lower radio-toxicity level compared to other two kinds of fuel loadings in the first 10,000 years. It confirms that TheU fuel cycle has a significantly lower radio-toxicity due to the less bulid-up of TRUs compared to UePu fuel cycle. The midterm and long-term radio-toxicity of the 233U and/or Th loading fuels is dominated by 233U and 234U, and the latter is responsible for the radio-toxicity growth around 10,000 years. The radio-toxicity of the Gen-II EM2 has the similar trend to the Gen-I EM2 but with a lower level due to the deeper burnup with effective incineration of TRU. The ratio of the total radio-toxicity (see Table 3) of Gen-II to that of Gen-I is about 0.54 for U3eTh, 0.36 for U5eTh, 0.82 for U5eDU at 10 years, and about 0.55 for U3eTh, 0.36 for U5eTh, 0.61 for U5eDU at 5 107 years.
produce 2.5 ton 233U and only 2.1 ton is needed to initiate the Gen-II EM2. The U5eTh fuel loading is better than the other two cases from the viewpoint of burnup and initial reactivity control. 238U in the starter zone has a bigger reactivity contribution than 232 Th, which can significantly slow down the consumption of 235 U as well as the decrease of keff for U5eTh compared to U3eTh, while 233U bred from 232Th in the fertile zone has a larger reactivity contribution than bred 239Pu. These cause the U5eTh loading has the longest operation time. Spent fuel from TheU fuel cycle has a radio-toxicity almost one order of magnitude lower than that of UePu fuel cycle in the first 10,000 years due to much less neutron capture for 233U and thereby less minor actinides production. Moreover, introduction of multi-generation EM2 can significantly reduce the waste management pressure due to the deeper burnup of TRU. With further optimization such as adjustment of fuel loading and core geometry, the EM2 with U3eTh is expected to achieve a better breed and burn performance. Acknowledgments This work is supported by the Chinese TMSR Strategic Pioneer Science and Technology Project under Grant No. XDA02010000 and the National Natural Science Foundation of China under Grant No. 91326201.
4. Conclusions References The neutronic properties of an EM2 based on three kinds of fuel loadings are analyzed. Conclusions drawn from this work are as follows: The U3eTh fuel loading can achieve sustainable operation in multi-generation EM2. The Gen-I EM2 with the U3eTh fuel can
Fig. 9. Radio-toxicity of EM2 Gen-I and II.
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