In this study, a preliminary neutronic design of an Accelerator-Driven Subcritical System is bestowed. The design of the reactor core with HEU Thorium Oxide fuel is coupling with proton accelerator and spallation target. The feasibility of ADS system as a neutron source for HEU burning, and isotopes production is evaluated. The multiplication factor Keff, the production of 233U and depletion of 235U can be evaluated by MCNPX. Introducing thorium fuel into the core with HEU offers an effective way to produce more 233U isotopes and to burn 235U isotopes more efficiently.
Additionally, production of less minor actinides (MA) and generation of energy can be achieved from this process.
In varied countries, the accelerator-driven subcritical system, popularly called the accelerator-driven system (ADS), is recently being studied with specific objectives. In USA, Europe, and Japan, the most goals of ADS are incorporating inherent safety in nuclear energy systems and providing long-term solutions to nuclear waste disposal by burning plutonium and minor actinides and transmutation of long-lived fission products.
The main goal in India is to develop ADS for safe and efficient breeding of 233U from the abundant resources of thorium and provide sustainable nuclear energy security .
Among the various ADS concepts being studied, the principal ones are:
In addition, the ADS has heat removal and electricity generation equipment.
The claims of increased safety characteristics are supported the very fact that the ADS operates in a non-self-sustained chain-reaction mode that reduces criticality issues. The ADS is operated during a sub-critical state and remains sub-critical, irrespective of the accelerator being on or off. Moreover, the accelerator might offer a convenient control mechanism for sub-critical systems that might scale back the necessity of control rods. The sub-criticality itself adds an extra level of operational safety with relation to criticality insertion accidents .
The fast energy amplifier is appropriate for the next core loadings containing thorium-bearing fuels :
Accelerator-driven sub-critical assembly system (ADS), with a family of energy amplifiers (EA), and thorium as the breeding fuel gives probably vital blessings in thorium fuel cycle (once-through or closed) in terms of minimizing radiotoxicity of nuclear waste and making certain proliferation-resistance at the same time.
Denaturing the thorium by the addition of 238U may mitigate the risks of proliferation because of the 238U reduce the effective enrichment in 233U of the reprocessed uranium (as the chemical reprocessing cannot separate the 238U from the 233U). Dilution (or denaturing) of fissile materials by their isotopes will complicate the utilization of fissile materials in nuclear explosive devices (NED) .
In the second stage of the India’s three-stage programme, Fast Breeder Reactors (FBRs) are fuelled by fuels supported Pu mixed with reprocessed Uranium recovered by reprocessing of the first stage spent fuel and 232Th will be introduced as a blanket material where 233U can be produced .
In this study, a conceptual design of ADS for burning high enriched uranium (HEU) in addition to power generation utilizing thorium for 233U breeding will be applied. This can be considered as an advantageous option for countries that do not have spent nuclear fuel but have a huge amount of thorium.
The ADS model that employed in this simulation depends on the typical fast neutron spectrum, lead-bismuth accelerator-driven transmutation system in Trellue analysis . By bombarding 1GeV proton beam into a cylindrical LBE target, neutrons are produced from the spallation reaction. The cylindrical core of 140 cm radius is loaded with 180 fuel assemblies.
However, during this work, rather than loading the ADS core with reprocessed fuel as in the original design, 96 seed assemblies of HEU and 84 blanket assemblies of thorium fuel were inserted into the individual regions. This heterogeneous approach can facilitate to modify the fuel assembly fabrication and in-core fuel management.
The separating of the fertile and fissile materials, ideally, there would be no competition for neutron absorption between them; therefore, the capture rate in fertile fuel would be optimized, and 233U conversion ratio within the blanket may well be increased. The breeding 233U from thorium fuel will compensate the burnt EU within the HEU fuel, therefore decreasing the swing of core reactivity.
Figure (1) shows the vertical and horizontal sectional views of the seed-blanket ADS. In order to improve the neutron economy, most of the thorium blankets assemblies are placed at the periphery of the core.The rest of the 232Th assemblies are distributed inside the core along with the HEU seed assemblies around the ADS target.
Sodium is employed as a coolant in the system. In Table (1), the main design parameters of the ADS reactor are presented.
The initial content of the core (thorium and HEU fuels) was determined in order to achieve approximately initial Keff of 0.96. This was achieved by attempting different 235U enrichments and calculating the effective multiplication factor Keff in each case. The isotopic composition of the core is shown in Table (2).
MCNPX is a general-purpose Monte Carlo radiation transport code designed to track particle types over broad ranges of energies. MCNPX program began in 1994 as an extension of MCNP4B and LAHET 2.8 in support of the Accelerator Production of Tritium Project (APT). The work envisioned a formal extension of MCNP to all particles and all energies; improvement of physics simulation models; extension of neutron, proton, and photonuclear libraries to 150 MeV .
The growth of computing power caused a sustainable increase of the share of Monte Carlo codes in nuclear reactor and nuclear criticality research and development. These Monte Carlo codes can provide the most accurate locally dependent neutronic characteristics in realistic 3D geometries of any complexity. Among them, only the general-purpose radiation transport code MCNPX [9, 10] is fully capable to treat ADS-related problems, since it tracks almost all particle types of nearly all energies.
In the upper energy region (above 20 MeV) it relies on the model calculations using different intra-nuclear cascade, pre-equilibrium and equilibrium model combinations. However, recent progress in nuclear data library extensions to higher energies (up to 200 MeV) allows implementing evaluated nuclear data in neutronic analysis of ADS systems .
The calculation of neutron multiplication factor for ADS was performed by MCNPX code using KCODE option. In an ADS, Keff must be between 0.95 and 0.98 indicating that ADS is a subcritical system . ADS reactors, as a matter of fact, are sub-critical in all conditions and power levels.
The system was simulated with Th-HEU fuel assemblies loaded and the multiplication factor was calculated. The loaded core is shown in Figure (1) and the multiplication factor was adjusted to be 0.9604 at the beginning of cycle (BOC) where the chain reaction could sustain. The variation of Keff with EFPD is shown in figure (2).
The mass evolution is an important parameter to prove the HEU burning and 233U breeding in the seed and blanket ADS fuel. Figure (3) shows the element inventory in the 840 MWth ADS using thorium – HEU fuel. At the end of the cycle (EOC), a significant amount of EU is burnt. This can be seen from the figure as the amount of 235U was about 1.841E+03 kg at the BOC while it was about 9.638E+02 kg at the EOC, meaning about 52.4% of the initial content of 235U was burnt.
Mean while a great amount of 233U was bred. From figure (3), it can be noticed that 5.700E+03 kg of thorium existed at the BOC; at the EOC, about 5.281E+03 kg of thorium, 2.638E+02 kg of 233U in addition to small fractions of other actinides such as 233Pa were present.This gives a ratio of about 4.63% of thorium converted into 233U. Important contribution accounted for 239Pu (mainly due to the conversion of 238U).
In this case, enriched 235U starter core is surrounded by a blanket of fertile fuel (thorium). Enriched fuel produces neutrons that generate power and convert fertile fuel to fissionable one. Figure (4) shows the depletion of 235U content due to the external neutrons from the ADS system and fission process.
The exposure to neutron flux gradually decreases 235U concentration. Each neutron absorbed in 235U decreases the fuel concentration and thus the macroscopic fission cross section, which causes the reactivity decrease.
On the other hand, the neutron capture by 232Th forms 233U. The percentage change in 232Th concentration is not significant due to its high concentration in the ADS core. However, production of the small amount of 233U is extremely important for the core performance. Figure (5) shows the breeding rate of 233U from the BOC to the EOC in the ADS core.
ADS is capable of burning any type of fuel and the choice of thorium potentially provides low radiotoxicity, fuel diversity, and proliferation resistance.
An important target for using thorium fuel is to breed as much fissile isotope 233U as possible in addition to decrease the amount of 239Pu produced for safeguards concerns. The use of ADS with HEU and Thorium blanket fuel achieves both goals.
The ADS used in this work resulted in the conversion of about 4.63% of initial content of thorium isotopes into 233U. Moreover, the amount of 239Pu produced in the system (about 328kgafter one cycle) is much lower than that produced from the other reactors like the PWR (about 1700kg after one cycle ). ADS also shows a high capability for 235U isotope burning (about 52.4% of the 235U initial content was burnt during the first cycle).
Therefore, energy generation can be achievable by the use of ADS fueled with HEU and Thorium that also results in a significant fertile-to-fissile conversion as well as burning of HEU.