Major modifications and upgrades
As a result of the Three Mile Island accident in 1979, all Swedish reactors were required to install FCVS - Filtered Containment Venting System - and ICSS - Independent Containment Spray System. In case of an accident with core degradation and loss of all cooling systems the ICSS can still limit the containment pressure, and should this fail the FCVS can relieve the containment pressure with limited releases of radioactivity. The systems went into operation at Barsebäck in 1985 and for the other plants in 1989.
As a result of the Barsebäck strainer incident in 1992[1] the recirculation strainer capacity was significantly increased for R1 in 1992 and R2 in 1994. Units R3 and R4 made similar upgrades in 2005.
During the 2005-2015 period, significant improvements were made regarding fire separation, redundancy and diversification of various safety systems, particularly for the older plants R1 and R2.
The original steam generators for the Ringhals PWRs had tubes of Inconel-600 and details in the construction that made them prone to cracks and corrosion. This triggered huge efforts for inspection, maintenance and repair. Although designed for 40 years of operation, both unit R2 and R3 replaced their steam generators after 14 years, i.e. 1989 and 1995. Unit R4, starting later, could take advantage of being aware of the problems, and due to high attention to water chemistry and maintenance efforts the R4 steam generators were replaced in 2011 after 28 years of operation. All the new steam generators have tubes of Inconel-690 and improved design, and have had very few problems and high availability. This is especially true for R2 which, as of 2017, had been operating 28 years with the new steam generators.
As a result of analysis and observations from the Three Mile Island accident, the Ringhals PWR reactors installed PAR - Passive Autocatalytic hydrogen Recombiners in 2007. Each reactor contains a number of PAR units with catalytic plates, which after a severe core degradation can process the hydrogen emanating from the fuel cladding oxidation within a few hours and hence reduce the risk for violent hydrogen combustion.
All European nuclear power plants including Ringhals had to perform stress-tests after the Fukushima accident 2011, where consequences of certain beyond-design-events should be assessed.[2] The availability of FCVS - Filtered Containment Venting System, ICSS - Independent Containment Spray System and PAR - Passive Autocatalytic hydrogen Recombiners, turned out to be valuable capabilities in these beyond-design-events and no immediate plant modifications were proposed as a post-Fukushima-response. On the contrary Ringhals made significant upgrades in the emergency preparedness organization in the size, education and training of the staffing, and in the independence and durability (power supply, communications, etc.) for certain systems and buildings.
In December 2014, the Swedish regulator required that all Swedish reactors should be equipped with an Independent Core Cooling System - ICCS - before 31 December 2020.[3] The system should be able to cool the reactor during 72 hours without any supply of water, power, fuel or other consumables, and should survive external events (seismic events, hard weather etc.) with an estimated probability of exceedance of 10−6/yr. The requirements are influenced by the so-called "Forsmark-event" 25 July 2006 and the Fukushima accident. Design of and preparations for ICCS is (2017) already underway for R3 and R4, but will not be made for R1 and R2 since they were planned to be shut down before 2021.[4] However, the Swedish Radiation Safety Authority stated that in case that the reactors were to be shut down when they were originally planned to be closed, i.e. around 2025, they could be operated without additional upgrades after 2020.[5]
