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Nuclear characterization of the spent nuclear fuel in a reactor core is essential, especially in case of severe accidents. The radionuclide inventory and its activity can assist in the management of spent fuel handling, transport or reprocessing.In this paper, the core of Fukushima Daiichi Unit-1(FD-U1) accident was modeled using the Monte Carlo code (MCNPX 2.7) linked to the depletion calculation code CINDER'90 and ENDF/B-VII.0 cross section data library. The isotopic inventory and the activity of the radionuclides for the burned fuel were calculated. The input to the code depends on the previous evolution of the reactor core configurations, dimensions and material of the fuel assemblies, initial uranium enrichment, fuel burn-up and reactor core operational history.The calculations were validated with experimental measurements which were carried out by the Japan Nuclear Energy Safety Organization (JNES) and verified with published results using ORIGEN2-code by Japan Atomic Energy Agency (JAEA). The validation and verification results were in good agreement.The masses, activities, specific activities, half-lives and decay schemes for the actinides and fission products were calculated at the time of the accident and after 50 years cooling time. The calculations showed that, total activity of the burned fuel in the core at the time of the accident was 9.86E+19Bq and after 50 years was 1.89E+17Bq and the higher inventory concentration in the fuel was dominated by the trans-uranic elements. Also, the specific activity in the core at the time of the accident and after 50 years cooling time was found to be 1.84E+15Bq/g and 5.86E+12Bq/g, respectively. These calculations are required for nuclear characterization of the corium and in the estimation of the radiological consequences of the source term in the environment. Also, the results can support the recovery program for Fukushima Daiichi-Unit-1.

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Democratic Republic of Congo (DRC) has a TRIGA mark II research reactor called TRICO II, its design power is 1.00 MW. The reactor was in extended shutdown state since November 2004. The DRC government has decided to resume its operation using the last uploaded core. One of the safety features to be determined before putting the spent fuel into the reactor core is the calculation of its excess reactivity, radionuclide inventories as well as its discharge burn-up. The spent fuel was modeled and simulated by using Monte Carlo software, MCNPX code. The input data and the horizontal and vertical modeling for the fuel pins, control rods and moderator were done. The model results were validated by calculating the effective delayed neutron fraction (β eff) and the worth of the control rods. The results of the criticality and fuel burn-up were compared with the reference design parameters and with the experimental measurements and it were found in good agreement. The calculations showed that the last uploaded core has 47.00 g of 235U which represent only 2% of fissile materials. The depletion analysis results showed that the highest radio-activities come from 151Sm, 137Cs, 90Y, 90Sr and 85Kr.

High fuel burn-up and long cycle length are the main objectives for economic and reliable operation of Advanced Nuclear Power Reactors. The selected cladding material to stand for high burn-up and long cycle length is HANA-4 alloy. AP1000 core was developed through increasing the fuel enrichment to higher values than the initial values and replacing the ZIRLO cladding by HANA-4 cladding to achieve higher burn-up and longer cycle length. The initial core and the developed core were simulated using Monte Carlo N-Particle Transport Code MCNPX. The criticality control parameter, core cycle length and spent fuel radionuclides inventory were calculated. The results showed that the developed reactor core can reach a cycle length up to 22 months at fuel discharge burn-up 75GWD/MTU safely using HANA-4 cladding compared to the initial design core which can reach to 18 months cycle length at fuel discharge burn-up of 60 GWD/MTU.