Meteorological conditions

Various weather conditions have been analysed including rain, seiches, tsunamis, floods, earthquakes, wind, tornados, lightning, snow, hail, extreme temperatures, cyclones, sandstorms and waterspouts.^[12]

Earthquakes

Since the design of Doel 3 and 4 and the first TJH of Doel 1 and 2^[12]: 50 the reactors were designed to be subject to earthquakes such as the Richter scale 5.6 Zulzeke-Nukerke quake in 1938. At a distance of 75 km to the epicentre, this is the most significant historical earthquake for Doel. The earthquake resulted in horizontal ground accelerations of up to 0.058 g ^[12]: 51 and formed the design base for Doel 1 and 2. Doel 3 and 4 were designed for peak ground accelerations of 0.1 g.^[12]: 50 After the Fukushima Daiichi nuclear disaster probabilistic safety studies performed by the Royal Observatory of Belgium predicted an earthquake with peak ground acceleration of up to 0.081 g every 10,000 years.^[12]: 52 The design was subsequently analysed for earthquakes up to 0.17 g,^[12]: 70 which is equivalent to a 1 in 100,000 year earthquake.^[12]: 48

Floods

The Doel plant was originally designed for a 10,000 year flood of 9.13 m TAW (Tweede Algemene Waterpassing).^[12]: 90 The highest measured level was 8.10 m TAW during the North Sea flood of 1953.^[12]: 78 The site was filled to 8.86 m TAW during construction and fitted with a seadyke of 12.08 m TAW. The other dykes around the site have a height of 11 m TAW.^[12]: 20 Probabilistic studies performed after Fukushima showed the height of a 10,000 year flood had slightly increased to 9.35 m TAW, 22 cm higher than the studies performed in the 1960s.^[12]: 91 The highest possible tsunami is lower than 0.5 m.^[12]: 77 During post-Fukushima stress tests a dyke failure with a water level of 10.2 m was simulated. Due to the presence of bulkheads and pedestals in the buildings no safety functions were endangered.^{[12]: 98–108}

Safety systems

Besides regular primary level safety systems, in common with most nuclear power plants in the world, Doel has secondary level safety systems that can autonomously keep the power plant safe during large external accidents such as the crash of an aircraft, external explosions or loss of the primary level.^{[12]: 30 [12]: 14} The primary level systems have a redundancy of three or four times.^: 26–29 The secondary level systems are 2x100% or 3x50%.^{[12]: 30–33} and have their own heatsink separate from the primary heatsink, the Scheldt river. Doel 1 and 2 have aircoolers ^[12]: 32 while Doel 3 and 4 have three separate artificial cooling ponds.^[12]: 33

Double containment

Nuclear plants are designed with multiple physical barriers to prevent fission products escaping into the environment. In the case of a pressurized water reactor there are three barriers: the fuel cladding which surround the fuel pallets, the primary circuit which houses the fuel rods and finally the containment building in which the primary circuit is built. In Belgium it was decided to add an extra double containment barrier.^[13] The primary containment, a steel sphere in Doel 1 and 2 and a pre-stressed concrete cylinder with steel liner in Doel 3 and 4, is surrounded by a secondary containment made of 1.2 to 1.3 m thick reinforced concrete. The space between both containments is kept at sub-atmospheric pressure and filters are used to filter potential leaks of the primary containment.^[12]: 14

Filtered containment venting system

In answer to a question of die Grünen in the Bundestag, the German parliament, the German government replied that the Belgian nuclear power plants do not have filtered containment venting systems installed. In German nuclear reactors these were already built in after the Chernobyl disaster in 1986. Other countries followed this example, latest after the Fukushima nuclear disaster. This kind of system allows for the containment pressure to be relieved in the event of a severe accident. The non-condensible gases that cause pressure within containment to rise are released through a stack (or chimney) via a filtration system that removes large quantities of fission products from the effluent.^[14][15]

As part of the stress tests following the Fukushima incident, this issue had already been identified to be included in the stress-test action plan (BEST). Construction of containment filter venting systems at Doel 3 and 4 was completed in 2017, with Doel 1 and 2 scheduled to be operational in 2020.^[16][17]

Turbo feed pump

Every reactor has at least one steam driven feedwater pump which can supply the steam generators with water to cool down the reactor. These turbine driven pumps can cool down the plant even when no electrical power is available to power the motor driven feedwater pumps during a station blackout like the Fukushima Daiichi nuclear disaster.^[12]: 147 In a boiling water reactor, like those in Fukushima, the heat removal capacity of the pumps is limited as the steam that drives the turbines is radioactive and thus has to be stored.^[18] This is not the case with a PWR due to the use of steam generators. Steam can simply be removed via a chimney. Water supplies on site are sufficient to keep the plant safe for dozens of days.^[12]: 147

Cooling towers

While Unit 1 and 2 have no cooling towers, Unit 3 has one 169.48 metres tall and Unit 4 has one 172.61 metres tall. The latter is the tallest cooling tower in Belgium.

2011 Doel 4 INES-2 incident

There has been one INES-2 incident on the International Nuclear Event Scale. After the replacement of an auxiliary feedwater turbopump, during the 2009-outage, one of the three feedwater pumps of the first level was adjusted to a lower level of rotational speed, delivering insufficient flow rates during certain circumstances.^[25][26] The two other first level feedwater pumps and three second level feedwater pumps were still available. The incident was rated 2 on the scale rather than 1 because the "Federal Agency for Nuclear Control felt that Electrabel waited a little too long to report the incident".^[27]

2012 Doel 3 pressure vessel cracks

In June 2012 Doel 3 was shut down for its third scheduled 10-year in-service inspection. Ultrasound inspections revealed a number of cracks in the lowest forged ring of the reactor pressure vessel. The cracks were determined to be an original manufacturing defect, not related to tension, and theoretically of no risk.^{[28][29][30][31]}

The reactor remained offline for further inspections and assessment for a year. Eventually the nuclear regulator judged short-term safety concerns initially raised had been solved in a satisfactory manner and that the reactor could operate safely, and it was restarted in June 2013.^[32][33]

2014 Doel 4 turbine incident

In August 2014, there was a major incident in the non-nuclear part of the plant. The main turbine overheated while operating without oil. A valve had been deliberately opened, which rapidly evacuated 65,000 litres of oil to an underground storage tank. The procedure was normally used in case of fire. The valve was normally secured by a padlock.^[34] Authorities and the plant operator suspected an act of deliberate sabotage. The unit was eventually back on grid by 19 December 2014. Combined with the outage of Doel 3 and Tihange 2, blackouts were not ruled out for the winter period of 2014–2015.^[35]

In December 2016, Electrabel requested that the justice department in Brussels prolong the search for the perpetrator of the turbine sabotage. Repairs cost more than 100 million euros.^[36]

Transmission

The power plant has two electrical substations. Two transmission lines run to Zandvliet and Kallo from the 150 kV station. The 380 kV station has three lines to Avelgem, Mercator in Kruibeke and Zandvliet. Elia is the transmission system operator.^[37]

The line to Zandvliet crosses the Scheldt River using one of the highest transmission towers in Europe. The tower is 170 metres (560 ft) high and built on a caisson in the middle of the river. The line is part of the interconnection between the Dutch and Belgian grids.

Cooling towers

With a height of 169 metres(554 feet),^[38] the two cooling towers are the most visible structures in the Port of Antwerp. Due to their proximity to the Dutch-Belgian border, the towers and the accompanying wet steam can be seen over large areas of the Dutch provinces of Zeeland and western North Brabant. Since the spring of 1996, one of the cooling towers has hosted a nest of peregrine falcons.^[39]

Security

On 15 March 2016, the federal government of Belgium decided 140 soldiers would guard the nuclear sites and that Electrabel should cover the costs.^[40] In late 2015 it had already been decided that a specially trained department of the federal police would guard the nuclear sites.^[41] After the 2016 Brussels bombings, on 22 March the nuclear powerplants of Doel and Tihange were preventively evacuated, the standard procedure when the threat level in Belgium reaches Level 4. The powerplants continued running with minimal staff.^[42]