Nuclear power is believed to be a key to the energy security for a developing country like Vietnam where the power demanding increases rapidly every year. Nevertheless, spent nuclear fuel from nuclear power plants is the source of radiotoxic and proliferation risk. A conceptual design of ADS utilizing thorium fuel as a based fuel and reprocessed fuel as a seed for nuclear waste transmutation and energy production is proposed as one of the clean, safe, and economical solutions for the problem. In the design, 96 seed assemblies and 84 blanket assemblies were inserted into the core to make a heterogeneous subcritical core configuration. Introducing thorium fuel into the core offers an effective way to transmute plutonium and minor actinide (MA) and gain energy from this process. Transmutation rate as a function of burnup is estimated using MCNPX 2.7.0 code. Results show that by using the seed-blanket designed ADS, at 40 GWd/t burnup, 192 kg of plutonium and 156 kg of MA can be eliminated. Equivalently, 1
Demand for power is rising as Vietnam’s economy expands. Electricity demand growth has been 14% per annum and is expected to be 15% per annum by 2015 and then slowing by 2020, though other figures suggest 10% per annum. In order to improve the energy security, the government has approved the nuclear power development plan in Vietnam. Due to the plan, 2000 MWe nuclear power plant at Phuoc Dinh in the Ninh Thuan province should be online by 2020. A further 2000 MWe was planned at Vinh Hai nearby, followed by a further 6000 MWe by 2030. A high demand scenario would give 8000 MWe in 2025 and 15,000 MWe (10% of total) in 2030 at up to eight sites in five provinces [
Thorium with its abundance is gaining a considerable attention as the fuel candidate to replace uranium fuel. When choosing the based fuel for the system, taking into account the ability of eliminating large amount of TRU waste since the production of these elements in thorium cycle is significantly reduced compared with the uranium cycle and energy gaining from this process, thorium is an appropriate candidate to be the based fuel in the ADS system. To start the fission reaction, reprocessed fuel (Pu + MA) is loaded into the core as seed and thus transmuted.
In this paper, conceptual designs of ADS for transuranic waste transmutation and power generation utilizing thorium blanket and reprocessed fuel as a seed are proposed. The ADS configuration and the calculation code are described in Section
The model of ADS used in this simulation was conducted from the typical fast neutron spectrum, lead-bismuth accelerator-driven transmutation system in Trellue research [
Vertical and horizontal sectional views of the seed-blanket ADS design (scale is given in cm).
In the calculations, Pu and MA are recovered from the reprocessing scheme assumed in Tsujimoto research [
Fuel composition of thorium and reprocessed fuel.
Nuclides | Number density (atoms/b-cm) |
---|---|
Reprocessed fuel | |
235U |
|
236U |
|
237Np |
|
238Pu |
|
239Pu |
|
240Pu |
|
241Pu |
|
242Pu |
|
241Am |
|
242Am |
|
243Am |
|
243Cm |
|
244Cm |
|
245Cm |
|
246Cm |
|
16O |
|
Thorium fuel | |
232Th |
|
Gd |
|
16O |
|
MCNPX is an extension of MCNP-4B and LAHET with the improvement of physics simulation models; extension of neutron, proton, and photonuclear libraries to 150 MeV; and the formulation of additional variance-reduction and data-analysis techniques [
In order to investigate the neutronics characteristics and transmutation potential of the seed and blanket thorium-reprocessed fuel ADS, the MCNPX code was employed for the calculation using the core configuration illustrated in Figure
Performance characteristics of the ADS.
Core diameter (cm) | 280 |
Core length (cm) | 300 |
Fuel pin radius (cm) | 0.315 |
Pin pitch (cm) | 0.89 |
Cladding thickness (cm) | 0.031 |
Thorium assemblies/reprocessed fuel assemblies | 84/96 |
Thorium weight/reprocessed fuel weight | 2.45/1 |
LBE target radius (cm) | 15.0 |
Accelerator current (mA) | 13–30 |
Spallation yield (n/s) | 30 |
Power output (MWth) | 840 |
Cycle length (days) | 430 |
Radial power peaking factor at BOC | 2.50 |
Axial power peaking factor at BOC | 1.21 |
Radial power peaking factor at EOC | 2.09 |
Axial power peaking factor at EOC | 1.20 |
Void coefficient at BOC (×10−2 |
−7.53 |
Burnup (GWd/MT) | 40 |
|
|
BOC |
|
EOC |
|
Burnup reactivity swing | 0.0634 |
Neutron energy spectra for different regions in ADS core are demonstrated in Figure
Neutron energy spectra for different regions in ADS core.
The mass evolution is the key parameter to prove the TRU waste transmuting potential of the seed and blanket thorium-reprocessed fuel ADS. Figure
Isotopic inventory for 40 GWd/t fuel burnup.
In order to compare the TRU transmutation capability of different systems, the actinide balances per TWthh in PWR [
Comparison of the actinide balance per ton of TWthh for different systems.
Actinides (kg/TWthh) | PWR (UOX) 41 GWd/tHM | ARR (spent UO2) 150 GWd/tHM | Seed and blanket ADS 40 GWd/ton of spent fuel |
---|---|---|---|
Pu | +11.3 | −16.3 | −22.6 |
Np | +1.18 | −2.3 | −11.6 |
Am | +1.04 | −13.1 | −10.7 |
Cm | +0.03 | +4.0 | +3.9 |
The development plan of nuclear power expresses the quick expansion of nuclear power in Vietnam in the near future. By 2027, eight 1000 MWe class reactors are planned to come online. The large amount of nuclear waste from those nuclear power plants will bring serious radiotoxic and proliferation risks. Massive research and development (R&D) works on how to deal with these hazardous materials are needed to be done and should start from now on.
Regarding the radiotoxicity and decay heat, HLWs which include TRUs and LLFFs are the most hazardous materials. TRUs and LLFFs, respectively, occupy 1.1% and 0.6% of the conventional spent nuclear fuel from a LWR. The element masses in the waste for UOX with an initial enrichment of 4.2%, an average thermal burnup of ~50 GWd/t of heavy metal and after 5 years of cooling time, are demonstrated in Table
Composition of nuclear waste after 50 GWd/t of heavy metal burnup.
Nuclide | Element (kg/GWe/yr) at 50 GWd/tHM | Element (kg/8 GWe/yr) in 2025 |
---|---|---|
U | 20765 | 166120 |
Np | 16.0 | 128 |
Pu | 255.2 | 2041.6 |
Am | 16.4 | 131.2 |
Cm | 1.7 | 13.6 |
LLFFs | 127.2 | 1017.6 |
Considering the fact that Vietnam is one of the most crowded countries with a small total area (approximately 331.210 km2), a permanent geological repository to dispose the HLW is hardly an option. By taking the advantages as discussed previously, thorium-reprocessed fuel ADS is considered as a safe, clean, and economical solution of TRU waste management problem. From the consumption of Pu and MA of ADS system in Table
Although great amount of validation and verification are needed to be done to finalize the design of the thorium-reprocessed fuel ADS, this study offers a potential solution to solve the TRU waste management in Vietnam. However, not only TRU waste but also LLFFs in the spent fuel of LWRs should be transmuted into stable or short-lived nuclides in order to eliminate the permanent geological repository. The LLFFs could be separated and transmuted in the dedicated facility. One of the options is that it can be loaded in the core reflector of ADS where the thermal neutron spectrum is utilized to transmute those nuclides. This approach is now under estimation and will not be discussed in the scope of this paper.
The proposed fuel cycle with the ADS utilizing thorium and reprocessed fuel is illustrated in Figure
Fuel cycle with the ADS utilizing thorium and reprocessed fuel.
The seed and blanket thorium-reprocessed fuel ADS core concept was investigated in this study. As confirmed previously, besides producing energy, a significant amount of TRUs was transmuted using the seed and blanket thorium-reprocessed fuel ADS. At 40 GWd/t burnup, 192 kg of plutonium and 156 kg of MA can be eliminated. It implies that 1 GWth ADS can be able to transmute the TRU waste from 2 GWth LWRs. 14 units of ADS would be required to deal with the TRUs from the future reactors to be constructed in Vietnam and add about 4 GWe (40% thermal efficiency) to the grid. It is noteworthy that even though the results are approximated, it is reasonable to discuss the transmutation potential of the ADS. This result proposes a promising solution to solve the TRU waste management in Vietnam in the next few decades.