Corresponding author: German N. Vlasichev ( vlas@mts-nn.ru ) Academic editor: Georgy Tikhomirov
© 2020 Alina Ye. Pomysukhina, Yury P. Sukharev, German N. Vlasichev.
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:
Pomysukhina AYe, Sukharev YP, Vlasichev GN (2020) Evaluation of the neutronic performance of a fast traveling wave reactor in the Th-U fuel cycle. Nuclear Energy and Technology 6(2): 77-82. https://doi.org/10.3897/nucet.6.54629
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The possibility for all of the uranium or thorium fuel to be used nearly in full is expected in traveling wave reactors. A traveling wave reactor core with a fast neutron spectrum in a thorium-uranium cycle has been numerically simulated. The reactor core is shaped as a rectangular prism with a seed region arranged at one of its ends for the neutron fission wave formation. High-enriched uranium metal is used as the seed region fuel. Calculated power density dependences and concentrations of the nuclides involved with the transformation chain along the core at a number of time points have been obtained. The results were graphically processed for the clear demonstration of the neutron fission wave occurrence and transmission in the reactor. The obtained power density dependence represents a soliton (solitary wave) featuring a distinct time repeatability. Neutron spectra and fission densities are shown at the initial time point, when no wave has yet formed, and at the time of its formation. The wave rate has been calculated based on which the reactor life was estimated. The fuel burn-up has been estimated the ultra-high value of which makes the proposed reactor concept hard to implement. The burn-up of most of both the raw material and the fissile material it produces indicates a high potential efficiency of the developed reactor concept in terms of fuel utilization and nuclear nonproliferation.
Traveling wave reactor, fuel utilization, thorium, nuclear combustion wave, nuclear concentration of nuclides, burn-up depth, reactor life
A great deal of attention is given in scenarios of nuclear power evolution to issues of fuel supply and fuel utilization at nuclear power plants. The specific nature of nuclear fuel combustion in the reactor core affects the plant’s operating conditions, economics, and safety. The potential of nuclear power with only uranium-235 being in use is highly limited and fails to provide for the decisive advantages as compared with power technologies based on other energy sources. Processed natural uranium enriched in the 235U isotope is used in traditional nuclear reactors, both fast and thermal. However, the fissionable isotope is combusted only in part, which reduces the efficiency and cost effectiveness of nuclear fuel utilization. Thus, the design burn-up of fuel in new-generation units with VVER reactors amounts to not more than 5.5% h.a. A higher burn-up (up to 7% h.a.) is reached in BN reactors, and the burn-up has been increased to 12% h.a. in the BN-600 reactor with a potential for bringing the maximum burn-up of fuel in the FAs to 15% h.a. An ultra-high burn-up (tens of percent) is potentially reachable only in high-temperature gas-cooled reactors (VTGR) being under development (
In the first approximation, a traveling wave reactor can be represented as a cylinder (or a parallelepiped) of a pure raw material, such as 238U or 232Th, irradiated by neutrons from its end. In the subsurface region of the cylinder defined by the length of the neutron path, the raw material (e.g., 232Th) transmutes to a material fissionable in accordance with a chain of transformations:
(1)
As the critical concentration of the fissionable material is reached, a self-sustained chain reaction occurs and neutrons enter the adjoining region where the fissionable material starts to accumulate. Therefore, nuclear fission takes place throughout the raw material lengthwise the cylinder. The process of fissions is self-regulating since the fact of the fissile material concentration exceeding to any extent the critical concentration needs to be made up for by its burn-up during the time that is comparable with the neutron lifetime, and the new fissionable material forms for the time comparable with that of the precursor β-decay, and not simultaneously.
The possibility of the neutron fission wave formation in a fast reactor was first shown by S.M. Feynberg and Ye.P. Kunegin in (
The characteristics of burn-up and the velocity of the combustion wave propagation in a small high-temperature gas-cooled reactor were studied in (
A new method for the neutron fission wave arrangement was proposed in (
Two fast reactor concepts are considered in (
A traveling wave reactor in a thorium-uranium fuel cycle was considered in (
The traveling wave reactor core was numerically modeled in a thorium-uranium cycle as part of the study using the WIMS-D4 code. The reactor core is shaped as a rectangular prism of the length 970 cm. A 160 cm seed region with a lateral size of 186 cm is arranged at one of the prism’s ends to form the neutron fission wave. Uranium metal with the 20% enrichment in 235U is used as the seed region fuel. The fertile material region is 800 cm long and consists of a pure raw material, 232Th. The seed region is the external source of neutrons which causes the raw material in the prism’s subsurface layer to transmute to a fissionable material in accordance with nuclear transformation chains. The core is surrounded by the reflector along the edges (see Fig.
The content and key performance of the fuel, the coolant, and the structural materials of the seed region and the fertile region are presented in Table
Volume fractions | Density, g/cm3 | Atomic weight | Concentration (barn·cm)–1 | |
---|---|---|---|---|
Seed region | ||||
Uranium metal | 0.438 | 18.7 | U – 235 | 0.004155 |
U – 238 | 0.01662 | |||
Silicon carbide | 0.162 | 3.2 | Si – 28.09 | 0.007785 |
C – 12 | 0.007785 | |||
Helium | 0.4 | 6.64·10–5 | He – 4 | 1·10–5 |
Fertile region | ||||
Thorium | 0.438 | 11.5 | Th – 232 | 0.01307 |
Silicon carbide | 0.162 | 3.2 | Si – 28.09 | 0.007785 |
C – 12 | 0.007785 | |||
Helium | 0.4 | 6.64·10–5 | He – 4 | 1·10–5 |
The described reactor model burn-up was calculated using the WIMS-D4 code in a plane geometry calculation option. The seed region and fertile region compositions are homogenized. The whole of the reactor core is broken down lengthwise into 31 regions with a step of 32 cm. With the breakdown step reduced from 32 cm to 16 cm, the power density decreases by not more than 4% at its maximum, and changes by about 20% in the energy release (where small) wave tail region. The transport problem was solved at each burn-up step in a two-group energy approximation. The energy of 0.0091 MeV corresponding to the fission spectrum end serves the interface for the groups. It should be noted that the calculation in a 26-group approximation produces practically the same results.
A transport equation in an integral form was solved at the macroscopic cross-section preparation stage by collision probability method for the spectrum calculation in 69 energy groups. At the present time, the WIMS-D4 code uses the base nuclear constant library in 69 groups acquired based on UKNDL and ENDFB6 data.
To demonstrate in a clear-cut manner the existence and nature of the wave process in a fast reactor, the numerical simulation results were graphically processed. Fig.
The neutron fission wave velocity has been found to be equal to 29 cm/year, and the considered reactor life is about 30 years. These results differ substantially from those obtained in (
u = Pt/(ESN0η), (2)
where Pt is the thermal power; E is the energy of one fission event; S is the wave cross-section area; N0 is the concentration of heavy nuclei in the wave region; and η is the fraction of the fissioned nuclei during the wave transmission. In accordance with this formula, the wave velocity in the thorium region, all other parameters being equal, is expected to be nearly twice as high as in the uranium region, this being satisfactorily in line with our numerical estimates.
Fig.
Fig.
One of the essential tasks in studying the possibility for the wave process implementation is to render the reactor critical. The possibility for achieving the reactor criticality at the time the steady-state wave mode is entered in is shown in Fig.
A feature of the modeled system behavior is that the effective multiplication coefficient drops at the initial time point as fuel burns up in the seed region while the energy release wave mode has not yet formed. At the time when keff stops to drop, the reactor enters the energy release wave mode and the multiplication coefficient profile stabilizes. Owing to the initial reactivity margin to the wave mode formation, the existing seed region makes the reactor safety level to decline. This requires the reactivity monitoring and control measures and facilities inherent in traditional plants, including in case of unlikely accidents.
The seed region burning time is observed in Fig.
Figs
It follows from the comparison of the two final diagrams that the neutron spectrum and the fission density spectrum change substantially in the course of the fission wave transmission. The quantity of neutrons in the blanket grows though the spectra form does not change. Interestingly, the fission density in the seed region becomes constant for neutrons with all energies.
The values of the fuel burn-up in the core are calculated using the formula
(3)
where N Th2(0) is the initial concentration of thorium-232; and N Th2(x), N U3(x), N U4(x), N U5(x), N U6(x) are the concentrations of thorium-232, uranium-233, uranium-234, uranium-235, and uranium-236, respectively, in different core portions.
Fig.
Simulation of the traveling wave reactor confirms the possibility for the wave process existence in a fast reactor with thorium fuel. The velocity of the neutron fission wave in a reactor with a seed region, containing 235U-enriched fuel, has been calculated and found to be equal to 29 cm/year. At the same time, the approximate life of the considered reactor will be about 30 years. Most of both the raw material and the fuel it produces (uranium-233) burns up in a substantial core portion which indicates a high potential efficiency of the considered reactor in terms of fuel utilization and nuclear nonproliferation. The burn-up reaches nearly 90% at the life end. The task to provide the engineering support for such an ultra-high burn-up and, accordingly, a prolonged life of 30 years and more should be solved through using coated-particle fuel with a silicon-carbide cladding in a fast reactor.
It should be noted, however, that the considered concept of the traveling wave reactor is not a closed one since a uranium-containing seed region with the 20% uranium-235 enrichment is required to start up each such thorium combusting reactor, for which purpose three extra facilities need to be in operation: those for uranium mining, uranium enrichment, and uranium fuel fabrication. It should be also noted that the use in the seed region of uranium metal having no high radiation stability may turn out improper. Its radiation stability can be increased through alloying additions. Besides, it is required to give a separate consideration to the problem of handling the unloaded fuel which has a high decay heat level and contains much uranium-235 being quite fit for the nuclear weapon manufacturing.