Bolstering Borssele17 August 2017
Dutch flood management expertise has been applied to radiation monitoring systems at the Borssele nuclear plant.
The Fukushima accident in March 2011 put the environmental risks related to extreme weather conditions in focus. Shortly afterwards, the first reviews of the safety measures applied at the Fukushima Daiichi site revealed a misjudgement of safety margins. Consequently, European national and supranational authorities asked for risk and safety assessments (“stress tests”) to be performed for the 143 nuclear power plant units in the European Union. The goal of the stress tests was to double-check “the ability of nuclear facilities to withstand damage from hazards such as earthquakes, flooding, terrorist attacks or aircraft collision”.
At the Borssele nuclear power plant operated by Elektriciteits Produktiemaatschappij Zuid-Nederland (EPZ) in the Netherlands, safety has been a priority since the unit, a 515MW pressurised water reactor (PWR), started operations in 1973.
The main extreme weather scenarios for external incidents at the Borssele site are related to storms and flooding. Borssele had already taken precautions to prevent water damage including sea wall construction, installation of diesel generators with tanks in sealed rooms and investment in additional pumps. Although the initial results of the stress tests gave “absolutely no reason to question the safety of the Borssele Plant” according to the Dutch Minister of Economic Affairs, Agriculture and Innovation, EPZ decided on an plan to further increase robustness.
In a post-accident scenario (like Fukushima), a major task along side accident mitigation is to inform the population living in the vicinity of the plant about the radiation levels. To ensure the radiation monitoring system will continue to operate if the equipment installed on the ground is not available due to raised water level, EPZ has enhanced its existing gamma monitoring system.
The new system consists of seven additional gamma dose rate measurement probes (GammaTRACER XL2), one new radionuclide identification and dose rate measurement probe (SpectroTRACER) and a radio transmission instrument (ShortLINK). Bertin Instruments designed all the devices, drawing on the expertise of Saphymo. Two factors were fundamental in the design of the radiation monitoring system: ensuring its resistance to extreme coastal weather conditions, and ensuring it could work autonomously from electrical and public communication networks.
What does resistance to extreme weather conditions close to the North Sea mean for gamma monitoring probes? Firstly, probes had to be installed above the water level, even in the case of extreme storm surges and flooding. All monitoring probes at Borssele are installed on 2m high masts. The masts are 17cm in diameter with robust concrete foundations to withstand extreme wind conditions (up to 202km/h for severe storms with high winds and 450 km/h for tornados).
The gamma-monitoring probes have also been designed to last several decades in robust sealed aluminium housings, despite the corrosive salt-water environment.
Another challenge concerned the availability of the probes in case of external power supply or mobile data transmission network failure. For this, the monitoring system has to be designed to work independently from the electrical network, i.e. on batteries. The low power consumption technology for the GammaTRACER and SpectroTRACER probes as well as for
the ShortLINK radio data receiver allows autonomous operation for up to ten years (depending on the measurement cycle). Additionally, the monitoring system should be able to communicate all measurement data in case of mobile network overload or blackout. With the ShortLINK radio transmission system, the dose rate measurement data can be transmitted up to 5km to a central server for several days, using only battery power.
With this new system, the Borssele nuclear power plant aims to fulfil requirements from nuclear regulators by showing that external hazards have been considered. The objective is to be ready in case of a severe incident, especially flooding or storm.
In addition to the new system, Borssele is still using the old monitoring system based on the former GammaTRACER basic probes, which are still working 22 years after installation.