The Saint-Laurent-des-Eaux nuclear accident of 1980 13 March 1980, two fuel elements melted A sudden rise in the radioactivity in the reactor pressure vessel led to a reactor trip. The alarms sounded, reactor A2 suffered a partial core meltdown. This meltdown was triggered by the detachment of a piece of sheet metal in the cooling system, blocking a section of it and causing a local rise in the fuel temperature. 20 kg of uranium melted after the reactor trip. Professor Pierre Pellerin, head of the SCPRI, explained to the NPP surveillance committee that “the pressure inside the reactor was equivalent to thirty times atmospheric pressure and a few discharges had to be carried out in order to depressurise the reactor pressure vessel”. The cumulative discharges of radioactive effluents remained low because a waiting period was observed before depressurising the vessel, knowing that the fuel was irradiated. The small volumes discharged remained below the limits authorised at that time, governed by decree. Damage and return to service The quantity of melted fuel was smaller than in 1969 (20 kg as opposed to 50 kg), but the fuel was more radioactive because it had accumulated the fission products and minor actinides during its two years of utilisation in the reactor. The reactor clean-up and repair operations lasted 29 months and involved five hundred EDF employees and subcontractors. The uranium dust dispersed in the reactor building during the accident represented a contamination risk for a long time. Several tonnes of lead were brought into the reactor building to provide radiological protection. The clean-up and repair work lasted until 1982. The facility was restarted in October 1983. The two GCRs A1 and A2 were definitively shut down in April 1990 and May 1992 respectively. Much later, in 2015, a controversy broke out concerning discharges of plutonium into the river Loire following the accident (see next page). The Saint-Laurent-des-Eaux NPP is located on the municipality of Saint-Laurent-Nouan in the Loir-et-Cher département(*) on the banks of the river Loire, between Orléans (30 km upstream) and Blois (28 km downstream). The accidents concern only the two old gas-cooled nuclear reactors A1 and A2, which are currently being decommissioned, and the two associated waste (graphite sleeves) storage silos. These two reactors were commissioned in 1969 and 1971 and shut down in April 1990 and May 1992 respectively. This NPP also comprises two PWRs, B1 and B2, which have been operating since 1983. They each have a unit power of 915 megawatts. A fact-finding mission was undertaken in 2015 The two events were subsequently rated level 4 (accident) on the INES scale (see p. 6), adopted by the IAEA in 1994 in the wake of the Chernobyl accident. A fact-finding mission carried out at the request of the Minister for Ecology concluded that there had been low-level discharges which did not exceed the standards in effect at the time of the events. The year 1980 was also marked by two noteworthy incidents at the Saint-Laurent-des-Eaux NPP. • 13 February 1980 Further to a very rapid increase in power linked to shortcomings in the operating instructions, the cladding of several fuel elements melted, without the uranium suffering the same fate. • 21 April 1980 A container exploded in a pool storing spent fuel bars removed from the reactor and whose cladding was damaged (pending their transfer off the site). Fission products were released into the pool water. * Administrative region headed by a Prefect. Nuclear accidents and developments in nuclear safety and radiation protection • 9
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