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Depletion calculation and accurate inventory of fission products in a nuclear system are required for criticality, safety and spent fuel management. Actual trend is to use Monte Carlo methods. It is well know that the fission process produces a large number of nuclides, some of which have a significant impact on the nuclear properties of the core and its behavior. In this study, we propose to determine the influence of fission products on the behavior of the IAEA 10 MW benchmark reactor. Even if nowadays we have powerful computing capability and we can solve the full system of fission products, such calculations are cumbersome and not needed because most of fission products have low absorption rates and therefore their precise concentrations calculation are not required. The practice is to identify and use only the nuclides which can have a significant absorption cross section. From the entire fission products of the available fissionable actinides, 214 nuclides have been considered. Their selection was essentially based on their absorption rates. To carry out the calculation, 81 were treated explicitly and 133 were lumped into pseudo fission products. A computational method has been developed for burnup and criticality calculations using MCNP5-ORIGEN coupling scheme. The MIXE_ACE program was developed and incorporated within this coupling scheme in order to mix and rewrite in ACE format the selected cross sections of the pseudo fission products for each burnup step. The mass weight of the constituent nuclides was used. The initial one group cross sections library for ORIGEN was generated using average flux spectrum in the core. Using the above methodology, an estimation of k eff and cross sections during depletion calculations has been carried out for the IAEA 10 MW reactor based on UZrH1.6 fuel. The results are compared to those of ANL (Argonne National Laboratory), MCNP6 and other calculations by using selected fission products from WIMS library. Generally, the results are satisfactory but some discrepancies exist. The differences can be explained mainly by the nature of the fission products considered in the calculation and especially their cross sections.

In the nuclear safety field, the thermomechanical phenomena that occur during an accident in a nuclear power plant (NPP) are of particular importance. During some reactor severe accidents, the decay energy of the core is transferred via the reactor cooling system (RCS) to other parts of this circuit. The associated heatup of RCS structures can lead to pressure boundary failures; with notable vulnerabilities, i.e., the lower head vessel, the pressurizer surge line, the hot leg nozzles, etc. The potential for a primary circuit component rupture is of particular concern because fission products could be released into the environment by such a failure. The risk is higher in-vessel; the relocated debris toward the lower head vessel, following a potential melting of the core, leads to the rupture of the most important protective barrier. Primary circuit structures are subject to the effects of high pressure and high temperature throughout their service life. Such loadings generally lead to creep rupture. This risk is more likely when these structures experience transients or severe accidents. One of the most studied accidents is the station blackout (SBO). SBO without operator actions accident (TMLB' sequence) is considered as one of the most likely scenarios that may threaten the integrity of vulnerable RCS pressure boundaries. In this manuscript, we have evaluated, using the system code RELAP5/SCDAPSIM 3.4, the damage caused by rupture of some structures of the primary circuit of the Surry NPP. The structures analyzed are the surge line, the hot leg nozzles and the lower head vessel. RELAP5/SCDAPSIM 3.4 results indicate that surge line (hot leg) failures will be the first failures in the RCS pressure boundary.