THORIUM FUEL CYCLE UNDER VVER AND PWR CONDITIONS
Juraj Breza1,2, Petr Dařílek1, Vladimír Nečas2
, ,
1 VUJE Inc., Okružná 5, 91864 Trnava, Slovakia
2 Slovak University of Technology, Faculty of Electrical Engineering and Information Technology,
Ilkovičova 3, 81219 Bratislava, Slovakia
ABSTRACT
The study of thorium fuel cycle with plutonium as a supporting fissile material was performed under VVER-440 and PWR conditions. Our analysis was focused on the plutonium transmutation potential, amount of separated plutonium in equilibrium cycle and total amount of finally disposed plutonium. Calculations were performed by the HELIOS spectral code.
The ability of plutonium transmutation was pointed out in both reactor cases.
1. INTRODUCTION
One alternative for the management of plutonium accumulated in the world from different nuclear power programs is to incinerate it in pressurized water reactors. There are several alternatives of advanced fuel which can be used for this purpose. Well known is mixed oxide fuel (MOX), but irradiation of MOX fuel causes second generation of plutonium. Another option is inert matrix fuel and thorium fuel on which several research projects are focused nowadays. Inert matrix and also thorium fuel do not produce secondary plutonium under irradiation, they can be used as plutonium and minor actinides (MA) bearer and their irradiation will lead to reducing the amount of plutonium and MA’s cumulated in the spent fuel.
This paper is focused on the thorium fuel cycle in light water reactor. The potential of the thorium-matrix for two different light water reactor types has been examined and their transmutation rates compared through computer simulations. All calculations were performed by HELIOS 1.9 spectral code. To perform calculations with a new type of fuel, benchmark calculations are needed to verify the obtained results. This part was referred for this code
in [1] and HELIOS 1.9 [2] is suitable for thorium fuel calculations.
2. THORIUM FUEL CYCLE
The thorium fuel cycle generally starts with UOX spent fuel reprocessing, where plutonium isotopes are separated. These separated PuOX isotopes are mixed with ThOX and burned in a light water reactor again. A simplified scheme of the VVER-440 thorium cycle with plutonium content in fresh PuThOX fuel is in Fig 1, the scheme is similar for the PWR cycle. The difference is in the Pu content in the UOX spent fuel and the content of Pu in fresh PuThOX fuel, which is listed in Table 1. This model of thorium cycle is based on [3].
Fig. 1. VVER-440 thorium fuel cycle
Table 1. Pu content in different reactor types
VVER-440 / PWRPu content in UOX spent fuel / 1.24 wt% / 1.11 wt%
Pu content in fresh PuThOX fuel / 9.74 wt% / 8.00 wt%
Pu-239 content in fresh PuThOX fuel / 5.40 wt% / 4.08 wt%
Due to a low content of plutonium in spent UOX fuel for direct reprocessing of one spent fuel assembly to one fresh fuel assembly with thorium-plutonium fuel, while reachingasimilar multiplication factor as with UOX fuel, the plutonium content in the thorium fuel was estimated. Estimation of plutonium content for both cases, VVER-440 and PWR reactor types, is shown in Fig. 2 and Fig. 3. Fuel with a low content of fissionable elements does not reach operational multiplication ability.
3. MODELS OF FUEL ASSEMBLIES
Two models of fuel assemblies were prepared. The first one, Fig. 4a, is the VVER-440 assembly, the second one, Fig. 4b is the PWR fuel assembly. Figures are cross sections of the assemblies.
Fig. 2. Plutonium content estimation for VVER-440 reactor
Fig. 3. Plutonium content estimation for PWR reactor
The assemblies were computed in an infinite lattice – neutrons which escape from one surface of the assembly enter the assembly from the other surface. The models were prepared and fuel cycles were calculated by HELIOS 1.9 spectral code.
Target burn-up is the same for all types of fuels, 50 GWd/tHM in 5 cycles of 320 days. Basic data of the assemblies are listed in Table 2.
Table 2. Basic parameters of assembly models
VVER-440 / PWRPin (cell) pitch (cm) / 1.22000 / 1.26492
Pellet outer diameter (cm) / 0.76000 / 0.82532
Pellet hole diameter (cm) / 0.12000 / -
Cladding thickness (cm) / 0.07500 / 0.061705
Fuel operating temperature (K) / 873.15 / 873.15
Cladding operating temperature (K) / 599.15 / 599.15
Coolant temperature (K) / 579.15 / 579.15
Natural boron in light water (ppm) / 456 / 456
Fig. 4a. VVER-440 model of assemblyFig. 4b. PWR model of assembly
4. RESULTS
Calculation properties and variables were set in both cases to fulfil the main goal – to reach target burn-up of 50 GWd/tHM in 5 years of 320 days core cycle.
The results are prepared for the equilibrium cycle, which means that the amount
of plutonium in the thorium fuel is obtained by separation of plutonium from UOX spent fuel which still produces electricity rather than from the stored spent fuel assemblies.
Table 3 summarizes the initial plutonium and minor actinides content and production during the cycle. The transmutation rate is shown in different units, in %, as well as in g/tHM and in kg/TWhe for VVER reactor type.
Table 3. Main results
VVER-440 / PWRUOX / PuThOX / UOX / PuThOX
Pu initial (kg/tHM) / 0 / 97.38 / 0 / 80.00
Pu in spent fuel after 5y cooling (kg/tHM) / 12.37 / 51.39 / 11.06 / 33.77
MA in spent fuel after 5y cooling (kg/tHM) / 1.48 / 6.15 / 1.36 / 5.15
Pu transmutation rate (%) / 0 / 47.22 / 0 / 57.80
Pu transmutation rate (kg/tHM) / 0 / 45.98 / 0 / 46.23
Rate of Pu and MA in
the fuel entering reactors (%) / 0 / 9.74 / 0 / 8.00
Rate of Pu and MA in the spent
fuel, after 5-years cooling (%) / 1.38 / 5.75 / 1.24 / 3.89
Amount of separated Pu (kg/TWhe) / 0 / 24.57 / 0 / -
Pu transmutation rate (kg/TWhe) / 0 / 11.60 / 0 / -
Total amount of disposed Pu (kg/TWhe) / 26.02 / 12.97 / - / -
Results for PWR reactor were not been studied in more detail, plutonium transmutation potential was shown in similar values as in VVER-440.
5. CONCLUSIONS
Both investigated reactors fully loaded with thorium-plutonium fuel can transmute about 46 kg of Pu/tHM but PWR has a better efficiency (initial amount of Pu is only 80 kg/tHM).
As it can be seen, introduction of an advanced fuel cycle in light water reactors does not solve the problem of plutonium burning. It only allows producing less of plutonium than
it is produced today with classical UOX fuel.
The early introduction of this novel type of fuel cycle into operation will lead to reducing the plutonium production in the nuclear power plant. In the case of non-equilibrium cycles, where plutonium will be obtained from the stock of spent fuel, this cycle will burn plutonium into approx. 50% of today store.
ACKNOWLEDGEMENT
This project has been partially supported by the SlovakGrantAgency for Science through grant No. 1/3160/06. We would like to thank RED-IMPACT project participants for valuable advice.
LITERATURE
[1]J. BREZA et al., “PWR and VVER Thorium Cycle Calculation”, in Proceedings of 16th Atomic Energy Research Symposium, September 25-29, 2006, Bratislava, Slovakia, KFKI Budapest (2006), pp. 685-691.
[2]Studsvik Scandpower, HELIOS Documentation, Studsvik Scandpower, April 2000, Norway.
[3]L. BOUCHER, at al.: DRAFT - Definition of Technical Data and Detailed Hypotheses, RED-IMPACT, November 2005.