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.

Jules Horowitz Reactor (JHR) irradiation test devices (so called ADELINE and MADISON) must undergo a periodic inspection every 40 months (French ESP(N) Regulation). The first step of inspection proposal was performed from October 2019 to March 2020 and concerns application of non-destructive methods for electron beam welds, while examining the possible methods and locations that could be utilized. This study has been performed in collaboration between VTT Technical Research Centre of Finland Ltd and French CEA with VTT employee secondment at CEA Cadarache. The initial conclusions of this work are the following: • Two inspection methods are likely needed. • Eddy current is likely suitable for surface examination while volumetric ultrasonic inspection can be used for tube bodies. • Inspection under-water in a storage pool (EPI) is likely the best option for location in the JHR. • Some automation and remote controls will be needed during the inspection process. The inspection area is quite large as the whole tube bodies will be inspected. • Internal surfaces located between the nested tubes likely cannot be inspected for corrosion and therefore, sealing the space from oxygen and water will be likely be the best option. • Experimental reference is a critical next step of the work to assess and qualify the inspection methods. As the next step, CEA will continue discussions on requirements of the mock-ups: for eddy current inspection, a Zircaloy-4 planar specimen with artificial reflectors is needed. For ultrasonic reference, a representative mock-up of the cylinder with a weld and artificial defect(s) is needed. The mock-up testing will provide an experimental reference to CIVA calculations as simulations of this work have been performed with CIVA software, which is extensively used in the industry and research organizations in simulation of non-destructive control. This study will allow further steps in the future, up to the qualification of inspection methods for JHR test devices.