ASN to monitor corrosion in La Hague evaporators

A series of measurements of the thickness of the steel walls of some of the evaporators at Areva's La Hague reprocessing plant in northern France has indicated faster than expected corrosion. The country's nuclear regulator, the Autorité de Sûreté Nucléaire (ASN), said it will monitor Areva's work to minimize future corrosion of these evaporators.

A series of measurements of the thickness of the steel walls of some of the evaporators at Areva's La Hague reprocessing plant in northern France has indicated faster than expected corrosion. The country's nuclear regulator, the Autorité de Sûreté Nucléaire (ASN), said it will monitor Areva's work to minimize future corrosion of these evaporators.

A cutaway showing the structure of the La Hague evaporators (Image: ASN)

The UP3-A and UP2-800 facilities at the La Hague plant each contain 15 workshops where one particular phase of reprocessing operations are carried out. In these workshops, used nuclear fuel is dissolved in acid and the uranium and plutonium then extracted through a chemical process. The remaining solution contains dissolved fission products, which account for most of the radioactivity of the used fuel.

In the R2/T2 workshops of the UP3-A and UP2-800 facilities, the fission product solutions are concentrated using evaporators (three per plant) that heat the solution to evaporate the acid, which is then recycled. These evaporators are heated by water superheated to a temperature of about 145°C and at a pressure of about 10 bar circulating in half-tubular welded circuits formed on the outer surfaces of the boilers. The concentrated solutions are then vitrified to form glass packages that are then stored at the La Hague site awaiting final disposal in a deep geological repository.

The evaporators were designed in the 1980s using a steel selected by Areva for its corrosion resistance. At that time, Areva ensured the thickness of their steel walls would provide good resistance to earthquakes and the pressure of the heating coils, even after corrosion. The evaporators - designed for an operating life of 30 years - were commissioned between 1989 and 1994.

Due to the pressure of the water circulating in their heating coils, the evaporators are classified as "equipment under nuclear pressure" and are therefore subject to monitoring programs and periodic inspections set by the regulator.

Since 2006, French nuclear operators have been required to carry out comprehensive safety reviews of their installations every ten years. As part of a review of the UP3-A facility started in 2010, ASN requested that Areva perform thickness measurements on the evaporators to monitor aging.

The evaporators are encased in reinforced concrete and are inaccessible to staff due to the ambient level of radioactivity. Areva therefore took measurements using a probe. Thickness measurements were performed in 2012. Additional measurements were taken in 2014 and 2015. The latest results - submitted in December 2015 - indicated a faster rate of corrosion than expected when the evaporators were designed.

On 11 February, the ASN commission questioned Areva chairman Philippe Varin and CEO Philippe Knoche on the corrosion of the evaporators.

In a 24 February statement, ASN said: "These results are likely to call into question the medium-term safety of the facility. In fact, the resistance of such equipment to the pressure of their heating circuits or earthquakes could be challenged in the coming years and potentially from 2018 for the most degraded evaporator."

ASN said it has decided to statutorily regulate the continued operation of the evaporators. Areva is required to strengthen supervision of the evaporators and install additional means to limit the consequences of a leak or rupture, such as isolation facilities and advanced detection systems.

The regulator also said it would monitor the development of corrosion of the evaporators and will check that Areva complies with the conditions set for their continued operation. It noted, "ASN could enforce the shutdown of the plant in case of excessive deterioration."

Researched and written
by World Nuclear News