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.

A diluted water plug can form inside the primary coolant circuit if the coolant flow has stopped at least temporarily. The source of the clean water can be external or the fresh water can build up internally during boiling/condensing heat transfer mode, which can occur if the primary coolant inventory has decreased enough during an accident. If the flow restarts in the stagnant primary loop, the diluted water plug can enter the reactor core. During outages after the fresh fuel has been loaded and the temperature of the coolant is low, the dilution potential is the highest because the critical boron concentration is at the maximum. This paper examines the behaviour of the core as clean or diluted water plugs of different sizes enter the core during outages. The analysis were performed with the APROS 3D nodal core model of Loviisa VVER-440, which contains an own flow channel and 10 axial nodes for each fuel assembly. The wide-range cross section data was calculated with CASMO-4E. According to the results, the core can withstand even large pure water plugs without fuel failures on natural circulation. The analyses emphasize the importance of the simulation of the backflows inside the core when the reactor is on natural circulation.

An automatic loading pattern optimization tool called ALPOT has been developed for Loviisa VVER-440 reactors. The ALPOT code utilizes combination of three different optimization methods. The first method is the imitation of the equilibrium pattern that is the optimized pattern in case the cycle length and the operation conditions are constant and the same shuffling pattern is repeated from cycle to cycle. In practice, the algorithm imitates assemblies’ operation year distribution of the equilibrium pattern stochastically. The function of the imitation algorithm is to provide initial patterns quickly for the next optimization phase, which is performed either with the stochastic guided binary search algorithm or the deterministic burnup kernel method depending on the choice of the user. The former is a modified version of the standard binary search. The standard version goes through all possible swaps of the assemblies and chooses the best swap at each iteration round. The guided version chooses one assembly, tries to swap it with every other possible assembly and performs the best swap at each iteration round. The search is guided so that the algorithm chooses the assemblies at or near the most restrictive fuel assembly first. The kernel method creates burnup kernel functions to estimate burnup variations that are required to achieve desired changes in the power distribution of the reactor. The idea of the kernel method is first determine the optimal burnup distribution that minimizes the maximum relative assembly power using the created kernel functions and a common solver routine. Then, the burnups of the available fuel assemblies are matched with the obtained burnup distribution.

We use the Serpent Monte Carlo code to produce total and partial albedo boundary conditions that can be used to model the Loviisa NPP VVER-440 core with the nodal neutronics tools of Fortum. The albedo generation process is described in detail. The dependence of the generated albedos on boron content and water density is investigated and a clear distinction is noted in water density dependence between regions containing mostly water and those containing mostly structural materials. The Serpent generated albedos are currently used in production calculations for modeling the Loviisa reactors at Fortum.

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.