Study of Thorium Fuel Cycles for Light Water Reactor VBER-150

The main objective of this paper is to examine the use of thorium-based fuel cycle for the transportable reactors or transportable nuclear power plants (TNPP) VBER-150 concept, in particular the neutronic behavior. The thorium-based fuel cycles included Th+Pu, Th+U, and Th+U and the standard design fuel UOX. Parameters related to the neutronic behavior such as burnup, nuclear fuel breeding, MA stockpile, and Pu isotopes production (among others) were used to compare the fuel cycles.The Pu transmutation rate and accumulation of Pu with MA in the spent fuel were compared mutually and with an UOX open cycle. TheTh+Ufuel cycle proved to be the best cycle for minimizing the production of Pu andMA.The neutronic calculations have been performed with the well-knownMCNPX computational code, which was verified for this type of fuel performing calculation of the IAEA benchmark announced by IAEA-TECDOC-1349.


Introduction
There is a growing interest in small reactors, and specifically TNPP. These reactors could represent a solution for developing countries with energy needs on islands, with remotely located areas without interconnected electricity grids or without the infrastructure required for land-based stationary NPP. Various designs have been developed, but most of the TNPP are based on long-term experience acquired with ship propulsion reactors and with larger land-based stationary reactors.
Depending on the host state requirements, the plant can be operated by the supplier state or by an entity from the receiving country. The host state can acquire a TNPP to own with all the consequences associated with the use of nuclear energy. In other scenario, the supplier state has additional responsibilities that may include delivery and return of the TNPP and its operation if requested by the host state. All operations with the nuclear fuel are done at the zone of responsibility of the supplier state. Anyway, to implement this, it is necessary to design a reactor core with long operation cycles without on-site reloading and shuffling of fuel, which ensures difficult access to the fuel during the entire period of the reactor installation operation including transportation. Some TNPP being considered today for export, but they are innovative concepts and need effort for the demonstration of their viability [1]. The use of thorium based fuel cycle and the exclusion of on-site refueling provides the solution to the nonproliferation problem; therefore, this category of reactors is very attractive for energy supplies in developing countries.
Thorium based fuel cycles are very attractive for producing long-term nuclear energy with low radiotoxicity waste [2,3]. Additionally, thorium fuel cycles could be done through the incineration of weapon grade plutonium (WPu) or civilian plutonium [4,5]. Compared to the U 238 -Pu 239 fuel cycle, Th 232 -U 233 cycle produces less quantity of plutonium and minor actinides (MA: Np, Am, Cm), minimizing   the radiotoxicity associated with spent fuel. Consequently, the production of radioactive waste in a thorium fuel cycle is lower than that in the U-Pu cycle used in traditional light water reactors (LWRs) [6].
The main objective of this paper is to examine the use of thorium based fuel cycle for the TNPP VBER-150 concept, in particular the neutronic behavior. The thorium based  [7]. Unfortunately there is insufficient data of VVER reactors with fuel containing thorium to validate the MCNPX code version 2.6e code. To reach this goal and considering the scope of this paper we performed calculation of the IAEA benchmark announced by IAEA-TECDOC-1349 for a simplified model of a PWR assembly with a plutonium-thorium fuel composition [8]. Similar approach was proposed in [9]. The results obtained from MCNPX version 2.6e code are in good agreement with the results of other participants of this benchmark and the discrepancies can be caused by the use of various methods and nuclear database. The results of benchmark calculations ratified that MCNPX code version 2.6e with available library in XSDIR, ENDF/B VI.2, is adequate for studies of thorium based fuel cycles.
Three different models of VVER-1000 assemblies were designed for the MCNPX calculations with the purpose of selecting the model more suitable for the present study. To reach this goal the neutronic behavior of the reactor during the burnup and cycle duration for the three models was studied.

The Reactor
The TNPP VBER-150 concept is a two-loop modification of the VBER-300 reactor installation [10]. This nuclear reactor International Journal of Nuclear Energy is a small sized loop type pressurized water reactor without on-site refueling for a floating NPP with cogeneration option. The nuclear fuel developed for the VVER reactors will be used for the VBER-150. Refueling, radioactive waste management and repairs could then be provided offsite, in special maintenance centers. Absence of on-site refueling ensures difficult access to the fuel during the entire period of the reactor installation operation including transportation. This TNPP concept is based on a successful multidecade experience in the production and operation of marine propulsion in the Russian Federation. Along with the VVER-type power reactors, the modular shipboard pressurized water reactors represent the most developed reactor technology; it is well examined and proven by successful operation. The total operating life of VVER-type reactors exceeded 1500 reactoryears, and over this period there were no events with the radiation-significant consequences. The operating experience of shipboard reactors exceeds 6000 reactor-years [11,12].

Computer Code Description
The  International Journal of Nuclear Energy    such as depletion/burnup capability based on CINDER90, which works with a 63-energy-group structure. Crosssections for these 63 groups are condensed using a generic spectrum. CINDER90 utilizes decay and energy integrated reaction-rate probabilities along with fission yield information to calculate the temporal nuclide buildup and depletion. The library of data in CINDER90, residing in the CIN-DER.dat library file, includes isotope decay and interaction probability data for 3400 isotopes, including ∼30 fission yield sets and yield data for 1325 fission products. The neutronics analysis performed in this work is the following: (1) Calculate the IAEA benchmark announced by IAEA-TECDOC-1349. Compare the reactor physics parameters: inf and fuel composition as a function of burnup, in a simplified model of a PWR assembly with a Plutonium-Thorium fuel composition.
(2) Calculate parameters related to the neutronic behavior as burnup and cycle duration for the three different models of VVER-1000 assemblies designed for the MCNPX calculations with the purpose of selecting the model more suitable for the present study.

Benchmark
The coordinated research project (CRP) "Potential of Thorium Based Fuel Cycles to Constrain Plutonium and to Reduce Long-term Waste Toxicity" examined different fuel cycle options in which plutonium can be recycled with thorium to incinerate the burner. The potential of the thoriummatrix was examined through computer simulations. Three benchmark tasks for different reactor concepts were performed in order to compare the effect of different methods and databases applied in the countries participating in the CRP. The agreement of the benchmark results generally was very satisfying [8].
The objective in this part of the present work was performing calculation of the IAEA benchmark announced by IAEA-TECDOC-1349 to verify that MCNPX code version 2.6e with available library in XSDIR, ENDF/B VI.2, is adequate for studies of thorium based fuel cycles. We performed calculation of the second benchmark. Five countries participated in this benchmark using its own methods International Journal of Nuclear Energy and computer codes as well as its specific database: India, Israel, Japan, Republic of Korea, and Russian Federation. The benchmark was designed to compare assembly-level calculation methods, by defining a 2D lattice simulating a typical PWR fuel assembly. The benchmark consists of a simplified model of a PWR assembly with a Plutonium-Thorium fuel composition. The model includes a 17 × 17 array of fuel rods, with 25 water hole positions without assembly casing and guide tubes (Figure 1). Burnup calculations were carried out with a constant specific power of 37.7 MW/t (initial heavy metal). Dimensions of the assembly and the material compositions of the fuel (5% PuO 2 + 95% ThO 2 ), cladding (natural zirconium), and moderator (light water with 500 ppm concentration of natural boron) can be found in [8].
We compared the results of criticality and fuel composition as a function of burnup (burnup range from 0 to 60 GWd/t) with those obtained by the participants of the CRP. Table 1 and Figure 2 show the evolution of the multiplication factor during the burnup. Figures 3(a), 3(b), and 3(c) show

Models and Fuel Cycles Studied
In this paper three thorium-based fuel cycles are presented and compared: (i) Th 232 +4.7%U 233 , in this cycle one fissile isotope mainly sustain the criticality of the reactor: U 233 , which represents a certain percent of the fresh fuel, and the U 233 produced by transmutation of fertile Th 232 .
(ii) Th 232 +4.7%Pu 239 , in this cycle two fissile isotopes mainly sustain the criticality of the reactor: Pu 239 , which represents a certain percent of the fresh fuel, and the U 233 produced by transmutation of fertile Th 232 .
(iii) 40%Th 232 +60%U 238 enriched at the 7.83% in U 235 , in this cycle three fissile isotopes mainly sustain the criticality of the reactor: U 235 , which represents a certain percent of the fresh Uranium, U 233 , which is produced by transmutation of fertile Th 232 , and Pu 239 , which is produced by transmutation of fertile U 238 . Table 2 summarizes some of the main parameters of the TNPP VBER-150 concept in whole-core refueling mode used in the MCNPX calculation; detailed information about this reactor model (Figure 4) can be found in [13].
Three models of VVER-1000 assemblies were designed. The first one and second (Figures 5(a) and 5(b)) are an advanced VVER-1000 assembly with two different fuel element types. The thorium oxide fuel is placed into dark pins and the rest are fresh UOX pins. The third one, (Figure 5(c)) is the VVER-1000 assembly with one type of fuel pins; this assembly presents a homogeneous fuel distribution; in the three models the thorium oxide percent, enrichment, and U 235 mass are the same.
The neutronic behavior of the reactor was studied as for burnup and cycle duration for the three models of distribution of the fuel element in the fuel assembly. As is shown in Table 3, the multiplicative properties are higher in the third model (fuel homogeneous distribution in the assembly), as well as the cycle duration in FPD; therefore, the burnup is relatively the highest. For this reason any posterior study presented in this paper will be made with the homogeneous model of fuel distribution.
International Journal of Nuclear Energy

Results
The reactor behavior using the three-thorium-based fuels and the basic UOX cycles was modeled. The fuel burnup in the reactor was calculated using (1). The results of the discharge burnup, fuel cycle duration and eff evolution can be seen in Table 4 and Figure 6: The results obtained for basic cycle of UOX are similar as those reported in [13] for this reactor; results obtained for thorium cycles support the use of thorium as fuel for this reactor. However, it is necessary to go deeper in safety studies regarding the thermomechanical behavior of the nuclear fuel and others.
The mass variation of the fissile isotopes and plutonium isotopes in the reactor as a function of time are shown in  Pu 238 . Figure 8 shows a light depletion of fissile isotope U 233 , product of the new U 233 obtained from transmutation of Th 232 isotope. In Th 232 +U 233 cycle plutonium isotopes are not produced in the reactor. Figure 9(a) shows U 235 decrease with respect to the loaded mass and it can be seen that the mass of U 233 and Pu 239 isotopes presents a quasilinear growth. Figure 9(b) shows that plutonium isotopes grow in all cases except in Pu 238 and Pu 242 ; in these two cases the plutonium masses presents a quasilinear growth. Figure 10(a) shows U 235 decrease with respect to the loaded mass and it can be seen that the mass of Pu 239 isotope tends to reach a constant value. Figure 10(b) shows that plutonium isotopes growth in all cases except in Pu 238 and Pu 242 in these two cases the plutonium masses presents a quasilinear growth. In all cases the fertile isotopes U 238 and Th 232 do not show important relative variation with burnup. Table 5 presented the results of calculation of the fuel breeding coefficient (BC) values for the fuel cycles studied using similar approach proposed in [14]. The greater BC value is obtained for UOX cycle; however, important BC values were obtained for thorium cycles too; in these cases the final mass of fissile fuel left in the spent fuel is greater than UOX cycle. This is due to the quasilinear increase of the U 233 mass in Th 232 -Pu 239 cycle and Th 232 -U cycle (Figures 7(a) and 9(a)) and the quasilinear decrease of the U 233 mass in Th 232 -U 233 (Figure 8), while the mass of Pu 239 is almost depleted in Th 232 -Pu 239 cycle and the mass of U 235 is consumed in a 40% in Th 232 -U cycle. Figure 11 shows the mass variations of the minor actinide isotope Np 237 in the reactor as a function of time for Th 232 -Pu 239 cycle, Th 232 -U cycle, and UOX cycle.
Minor actinide which accumulates more is Np 237 ; it is produced by neutron capture of U 236 and subsequent beta disintegration of U 237 . Figure 11 shows an increase in the accumulation rate of this isotope in Th 232 -U cycle and UOX cycle, in Th 232 -Pu 239 cycle Np 237 mass is negligible.
In the Th 232 -U 233 cycle insignificant masses of minor actinides isotopes are produced in the reactor.

Conclusions
The primary objective of this paper contribute to the neutronics analysis of the TNPP VBER-150 core, innovative concept currently in development.
The main objective of this paper is to examine the use of thorium based fuel cycle for the TNPP VBER-150 concept, in particular the neutronic behavior. The thorium based fuel cycles included Th 232 +Pu 239 , Th 232 +U 233 , Th 232 +U, and the standard design fuel UOX. The Th 232 +U 233 fuel cycle proved to be the best cycle for minimizing the production of Pu and MA in the TNPP VBER-150 concept. Also, it was demonstrated that a relatively low percent of fissile isotopes in the fuel mixture (4.7%) is sufficient to design the reactor core with long operation cycles. Further studies are needed to confirm these results and contribute to development and demonstration of their technical, safety, and economic viability.
The neutronic calculations have been performed with the MCNPX version 2.6e code. Three different models of VVER-1000 assemblies were designed for the calculations with the purpose of selecting the model more suitable for the present study. Unfortunately there is insufficient data of VVER reactors with fuel containing thorium to validate the MCNPX code version 2.6e code. To reach this goal and considering the scope of this paper we performed calculation of the IAEA benchmark announced by IAEA-TECDOC-1349 for a simplified model of a PWR assembly with a plutonium-Thorium fuel composition The results of benchmark calculations ratified that MCNPX code version 2.6e with available library in XSDIR, ENDF/B VI.2, is adequate for studies of thorium based fuel cycles.