In this paper, the optimization of the assignment of spent fuel assemblies into final disposal canisters is considered. This application is of essential importance as the final disposal canisters are expensive and, on the other hand, there exists a limit for the canister-wise total heat load, which must not be exceeded. The study utilizes mathematical optimization algorithms that have been developed by Ranta in his D.Sc. thesis (Tampere University of Technology, 2012). In the applied formulation, the target of the optimization is to minimize the maximum canister-wise decay heat load at the time of canister formation. The optimization algorithms were utilized for analysing a fictional final disposal scenario for present and expected future spent fuel assemblies of Loviisa NPP. The paper concludes that, despite a huge amount of degrees of freedom, the algorithms are capable of finding practically a global optimum for the considered problem. The implemented software tool can be utilized for further final disposal optimization analyses.
In Loviisa VVER-440-type NPP the coolant outlet temperature of the hot subchannel is constantly monitored during the operation. According to the authority requirement the maximum subchannel outlet temperature must not exceed the saturation temperature. Coolant temperature distribution inside the fuel assembly is affected by the efficiency of the coolant mixing. In order to enhance the coolant mixing the fuel manufacturer is introducing the additional mixing vanes on the fuel bundle spacer grids. In the paper the effect of the different mixing vane modifications is studied with computational fluid dynamics (CFD) simulation. Goal of the modelling is to find vane modifications with which sufficient mixing is reached with acceptable increase in the spacer grid pressure loss. The results of the studies are discussed in the paper.
Recently, an initiative has been made to improve the fuel economy of VVER-440 reactors by implementing a modification on the geometry of the current fuel assembly design. The proposed modification involves reduction of the fuel rod outer diameter from 0.91 cm to 0.89 cm by using 0.01 cm thinner cladding tubes than earlier. The design improvement would shift the neutronics of an under-moderated system slightly towards optimum moderation and, therefore, increase the reactivity of the assembly. In this paper, a neutronics feasibility study on utilization of the proposed new fuel design at Loviisa NPP is carried out. The study involves a comprehensive comparison of two individual equilibrium fuel cycles: one applying current TVEL 2nd generation fuel design and another one where the new fuel design is used. In addition to equilibrium cycle characteristics, also cycle economics as well as back-end effects are considered. The study concludes that the proposed fuel design modification enables to improve the fuel economy of Loviisa NPP.
The CB6 benchmark on VVER-440 final disposal consists of two parts. The decay calculation part is aimed at determination of isotopic composition of irradiated fuel material over the time scale relevant to long-term spent fuel disposal. The second part involves calculation of keff values in a 3D cask configuration at various time points. In this study, the code Serpent is applied in solution of the benchmark. Serpent is a continuous energy Monte Carlo reactor physics code developed at VTT Technical Research Center of Finland. The decay calculation part of the benchmark is, in addition to Serpent, also carried out using point-depletion calculation code ORIGEN. This enables comparison of the results obtained using the two codes exploiting two different methods in solution of the decay equations. The paper demonstrates the decay and keff calculation results as determined in the benchmark specification. The differences in results arising between the two calculation codes in decay calculation, and between the ENDF/B-VII and JEFF-3.1.1 based cross section libraries in keff calculation, are depicted and discussed.
In Loviisa NPP, there are two limiting thermal margins called the enthalpy rise margin and the linear heat rate margin that are monitored during normal operation. Engineering safety factors are applied in determination of both of these factors. The factors take into account the effect of various manufacturing tolerances, impact of the irradiation and simulation uncertainties on the local heat rate and on the enthalpy of the coolant. The engineering factors were re-evaluated during 2015 and the factors were approved by the Finnish radiation and nuclear safety authority in 2016. The re-evaluation was performed by considering all of the identified phenomena that affect the local heat rate or the enthalpy of the coolant. This paper summarizes the work that was performed during the re-evaluation of the engineering safety factors and presents the results for each uncertainty component. The new engineering safety factors are 1.115 for the linear heat rate and 1.100 for the enthalpy rise margin when the old factors were 1.12 and 1.16, respectively. The new factors improve the fuel economy by about 1%.
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