The Kraken computational framework is a new modular calculation system designed for coupled core physics calculations. The development started at VTT Technical Research Centre of Finland in 2017, with the aim to replace VTT’s outdated legacy codes used for the deterministic safety analyses of Finnish power reactors. In addition to conventional large PWRs and BWRs, Kraken is intended to be used for the modeling of SMRs and emerging non-LWR technologies. The main computational modules include the Serpent Monte Carlo neutron and photon transport code, the Ants nodal neutronics solver, the FINIX fuel behavior module and the Kharon thermal hydraulics code, all developed at VTT. The core physics solution can be further coupled to system-scale simulations. In addition to development, significant effort has been devoted to verification and validation of the implemented methodologies. The reduced-order Ants code has been successfully used for steady-state, transient and burnup simulations of PWRs with rectangular and hexagonal core geometry. The Ants–Kharon–FINIX code sequence is actively used for the core design tasks in VTT’s district heating reactor project. This paper is a general overview on the background, functional description, current status and future plans for the Kraken framework. Due to the short history of development, Kraken has not yet been comprehensively validated or applied to full-scale core physics calculations. A review of previous studies is instead provided to exemplify the practical use.

In 2019 the government of Finland made a decision to phase out of coal in energy production in a period of just ten years. The Finnish energy sector is currently looking for alternative technologies to replace coal-fired power plants, used especially in large cities for producing electricity and low-temperature heat for the local district heating network. The production of low-carbon electricity is expected to grow within the near future, along with the commissioning of the Olkiluoto 3 nuclear power plant and increasing share of wind power. The lost district heating capacity, however, is more difficult to replace. To anticipate the transition, municipal energy companies have turned their attention to clean alternatives, including nuclear energy. In an effort to meet the government climate goals, VTT Technical Research Centre of Finland has launched a project to design a small, simplified and passively safe PWR for district heating applications. The heating plant consists of one or multiple 50 MW reactor modules, operating on natural circulation at around 120°C temperature. The design combines conventional LWR technology with an innovative containment function, capable of decay heat removal without any mechanical moving parts. The reactors can be constructed partially or fully underground, or retro-fitted into an existing boiler plant. This paper presents an overview of the pre-conceptual reactor design, together with some general background on district heating reactor technology. More detailed design and safety analyses are provided in two separate papers at this ICONE-28 conference.

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Development of a small district heating reactor was started at VTT Technical Research Centre of Finland. The concept features a 50 MW reactor that operates at low temperature and pressure. Traditional LWR technology, passive safety functions and natural circulation are combined in the integrated design. This paper presents the thermal-hydraulic design and transient analyses done with Apros simulation software. The studied cases include station blackout with reactor trip and as an ATWS scenario, and small break LOCA in the lower downcomer. During the station blackout transients, both temperature and pressure remained at safe levels. The innovative containment design functioned as planned and was capable of efficient decay heat removal. The small break LOCA ceased the natural circulation, but the core was not uncovered at any point and the core temperatures remained low. The results from the thermal-hydraulic analyses are promising and show that the reactor design is capable of producing low temperature heat to the district heating network. The analyzed transients posed no risk to reactor safety, and the passive containment function was capable of removing decay heat efficiently. These preliminary analyses give valuable insight to the design work in the future.