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

Authors: Daniel Evelio Milian Lorenzo; Daniel Milian Pérez; Lorena Pilar Rodríguez García; Jesús Salomón Llanes; Carlos Alberto Brayner de Oliveira Lira; Manuel Cadavid Rodríguez; Carlos Rafael García Hernández
Journal: International Journal of Nuclear Energy
 (2013)

---

## Abstract

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 Th232+Pu239, Th232+U233, and Th232+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. The Th232+U233fuel cycle proved to be the best cycle for minimizing the production of Pu and MA. The neutronic calculations have been performed with the well-known MCNPX computational code, which was verified for this type of fuel performing calculation of the IAEA benchmark announced by IAEA-TECDOC-1349.


---

## Body

## 1. 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 U238-Pu239 fuel cycle, Th232-U233 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 fuel cycles included Th232+Pu239, Th232+U233, Th232+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 thorium based fuel cycle was examined through computer simulations.The neutronic calculations have been performed with the MCNPX version 2.6e code [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.


## 2. The Reactor

The TNPP VBER-150 concept is a two-loop modification of the VBER-300 reactor installation [10]. This nuclear reactor 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 reactor-years, 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].


## 3. Computer Code Description

The neutronic calculations have been performed with the MCNPX version 2.6e code with the available library in XSDIR, ENDF/B VI.2, on a standard personal computer with Intel(R) Core(TM) i3-2328 M CPU of 2.20 GHz and 4 GB RAM. MCNPX code is based on the Monte Carlo method and incorporates new possibilities used in the present study, such as depletion/burnup capability based on CINDER90, which works with a 63-energy-group structure. Cross-sections 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 CINDER.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:Kinf 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.(3)
Calculate parameters related to the neutronic behavior such as burnup, nuclear fuel breeding, MA stockpile, and Pu isotopes production (among others), for the TNPP VBER-150 concept.The control parameters in the MCNPX calculations are the following:Source size per cycle: 2000Initial guess forK-infinite: 1.0Number of settle cycles: 50Total cycles to run: 1000Total histories: 2,000,000.


## 4. 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 thorium-matrix 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 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 (Figure1). 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% PuO2 + 95% ThO2), cladding (natural zirconium), and moderator (light water with 500 ppm concentration of natural boron) can be found in [8].Figure 1
PWR fuel assembly model in MCNPX.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. Table1 and Figure 2 show the evolution of the multiplication factor during the burnup. Figures 3(a), 3(b), and 3(c) show the relative concentration of Th232, Pu239, and Pu241 during the burnup. 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 found correspond to those reported in [8]. These discrepancies can be caused by the use of various methods and nuclear database. Benchmark calculations performed contribute to the verification of MCNPX code for studies of thorium cycles in PWR and at the same time demonstrates the ability of the team in it use for burnup calculations.Table 1
K
inf as function of burnup.
Burnup, GWd/T
Russian Federation
Japan
Republic of Korea
India
Israel
MCNPX ver. 2.6e
0
1.1890
1.1987
1.1864
1.2076
1.1956
1.19775 ± 0.00047
0.5
1.1569
1.1670
1.1551
1.1736
1.1643
1.16734 ± 0.00048
20
1.0298
1.0521
1.0303
1.0372
1.0290
1.05898 ± 0.00047
40
0.9147
0.9527
0.9167
0.9104
0.9119
0.95610 ± 0.00041
60
0.8315
0.8657
0.8310
0.8294
0.8314
0.86561 ± 0.00038Figure 2
Comparison ofKinf as function of burnup.(a) Comparison of relative concentrations of Th232. (b) Comparison of relative concentrations of Pu239. (c) Comparison of relative concentrations of Pu241.
(a)
(b)
(c)


## 5. Models and Fuel Cycles Studied

In this paper three thorium-based fuel cycles are presented and compared:(i)
Th232+4.7%U233, in this cycle one fissile isotope mainly sustain the criticality of the reactor: U233, which represents a certain percent of the fresh fuel, and the U233 produced by transmutation of fertile Th232.(ii)
Th232+4.7%Pu239, in this cycle two fissile isotopes mainly sustain the criticality of the reactor: Pu239, which represents a certain percent of the fresh fuel, and the U233 produced by transmutation of fertile Th232.(iii)
40%Th232+60%U238 enriched at the 7.83% in U235, in this cycle three fissile isotopes mainly sustain the criticality of the reactor: U235, which represents a certain percent of the fresh Uranium, U233, which is produced by transmutation of fertile Th232, and Pu239, which is produced by transmutation of fertile U238.Table2 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].Table 2
Main parameters of the TNPP VBER-150 concept.
Parameter
Value
Thermal power (MWt)
350
Electrical power (MWe)
110
Core height (m)
2.2
Core diameter (m)
2.42
Core volume (m3)
9.02
Coolant and moderator
H2O
Number of fuel assembly
85
Type of fuel assembly
Same as VVER-1000
Power density (MW/m3)
39
Fuel mass (tons)
22.3Figure 4
TNPP VBER-150 concept model in MCNPX.Three models of VVER-1000 assemblies were designed. The first one and second (Figures5(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 U235 mass are the same.(a) and (b) TNPP VBER-150 concept advance fuel assembly model in MCNPX. (c) TNPP VBER-150 concept fuel assembly model in MCNPX.
(a)
(b)
(c)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 Table3, 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.Table 3
Main results for the TNPP VBER-150 concept assembly models.
Parameter
Model Figure5(a)
Model Figure5(b)
Model Figure5(c)
InitialKeff
1.18813 ± 0.00022
1.21255 ± 0.00021
1.22083 ± 0.00022
Full power days (FPD)
1160
1178
1200
Discharge burnup (GWd/T)
17.27
17.46
18.03


## 6. 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 Keff evolution can be seen in Table 4 and Figure 6:
(1)Fuelburnup(GWd/ton)=Power(GW)·Time(days)Fuelmass(ton).Table 4
Results for the considered fuel cycles.
Parameter
Fuel cycles
Th232-Pu239
Th232-U233
Th232-U
UOX
Discharge burnup (GWd/T)
26.29
39.06
18.03
31.30
Full power days (FPD)
1750
2600
1200
2083Figure 6
K
eff evolution for the fuel cycles studied.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 Figures7(a) and 7(b) for Th232-Pu239 cycle, Figure 8 for Th232-U233 cycle, Figures 9(a) and 9(b) for Th232-U, cycle and Figures 10(a) and 10(b) for UOX cycle.(a) Mass depletion of the fissile isotopes (Pu239 and U233). (b) Plutonium isotopes variation in the reactor (Pu238, Pu240, and Pu241).
(a)
(b)Figure 8
Mass depletion of the fissile isotope (U233).(a) Mass depletion of the fissile isotopes (U235, U233, and Pu239). (b) Plutonium isotopes variation in the reactor (Pu238, Pu240, Pu241, and Pu242).
(a)
(b)(a) Mass depletion of the fissile isotopes (U235 and Pu239). (b) Plutonium isotopes variation in the reactor (Pu238, Pu241, Pu241 and Pu242).
(a)
(b)Figure7(a) shows an important Pu239 decrease with respect to the loaded mass and it can be seen that the mass of U233 isotope presents a quasilinear growth. Figure 7(b) shows that plutonium isotopes grow in all cases except in Pu238. Figure 8 shows a light depletion of fissile isotope U233, product of the new U233 obtained from transmutation of Th232 isotope. In Th232+U233 cycle plutonium isotopes are not produced in the reactor. Figure 9(a) shows U235 decrease with respect to the loaded mass and it can be seen that the mass of U233 and Pu239 isotopes presents a quasilinear growth. Figure 9(b) shows that plutonium isotopes grow in all cases except in Pu238 and Pu242; in these two cases the plutonium masses presents a quasilinear growth. Figure 10(a) shows U235 decrease with respect to the loaded mass and it can be seen that the mass of Pu239 isotope tends to reach a constant value. Figure 10(b) shows that plutonium isotopes growth in all cases except in Pu238 and Pu242 in these two cases the plutonium masses presents a quasilinear growth. In all cases the fertile isotopes U238 and Th232 do not show important relative variation with burnup.Table5 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 U233 mass in Th232-Pu239 cycle and Th232-U cycle (Figures 7(a) and 9(a)) and the quasilinear decrease of the U233 mass in Th232-U233 (Figure 8), while the mass of Pu239 is almost depleted in Th232-Pu239 cycle and the mass of U235 is consumed in a 40% in Th232-U cycle.Table 5
Reactor's BC for the fuel cycles studied.
Cycle
BC
Th232+Pu239
0.50
Th232+U233
0.51
Th232+U
0.55
UOX
0.70Figure11 shows the mass variations of the minor actinide isotope Np237 in the reactor as a function of time for Th232-Pu239 cycle, Th232-U cycle, and UOX cycle.Figure 11
Np237 mass variation for the Th232-Pu239, Th232-U, and UOX cycles.Minor actinide which accumulates more is Np237; it is produced by neutron capture of U236 and subsequent beta disintegration of U237. Figure 11 shows an increase in the accumulation rate of this isotope in Th232-U cycle and UOX cycle, in Th232-Pu239 cycle Np237 mass is negligible. In the Th232-U233 cycle insignificant masses of minor actinides isotopes are produced in the reactor.


## 7. 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 Th232+Pu239, Th232+U233, Th232+U, and the standard design fuel UOX. The Th232+U233 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.



---

*Source: 491898-2013-12-23.xml*