In 2024, nuclear installations recorded 36 reportable events relevant to safety, compared to 24 in the previous year. The reportable events from the oversight year were distributed across the nuclear installations as follows:

• 6 events at the Beznau 1 NPP
• 7 events at the Beznau 2 NPP
• 1 event at the Beznau NPP
• 10 events at the Gösgen NPP
• 9 events at the Leibstadt NPP
• 1 event at the Mühleberg NPP (decommissioned)
• 2 events at the Central Interim Storage Facility in Würenlingen

The Paul Scherrer Institute and the École Polytechnique Fédérale de Lausanne reported no events in 2024.

Two unexpected reactor shutdowns (SCRAMs) were reported in the 2024 oversight year. The first was an automatic SCRAM, which occurred on 29 April during the shutdown of the Leibstadt NPP (text in German) for the main annual overhaul. The cause was the unexpected opening of the block circuit breaker, which disconnected the plant from the external 380 kV grid. The second automatic SCRAM was triggered on 12 August following a malfunction in the level control system of one of the two steam generators in the non-nuclear section of the Beznau 1 NPP (text in German).

ENSI classified all events as INES Level 0

ENSI classified all reportable events as level 0 on the International Nuclear Event Scale (INES). As such, they were of low significance for nuclear safety.

In 2024, an above-average number of events occurred in the Beznau, Gösgen and Leibstadt NPPs:

• Beznau NPP: Three events were attributed to irregularities in the external power supply, while four were due to age-related leaks. These leaks occurred in components that were unclassified at the time the Beznau NPP was built. As a result, they had been constructed without applying nuclear regulations and were only later assigned a nuclear safety class following regulatory changes. ENSI is closely monitoring the issue of conventionally constructed components that were only assigned a nuclear safety class later in their life cycle.
• Gösgen NPP: Two of the events involved the ingress of foreign material. ENSI addressed this issue as part of an inspection.
• Leibstadt NPP: Among other things, two events related to the solidification of radioactive residues in the cementing box during the filling of waste containers contributed to the higher number of reportable events. The Leibstadt NPP performed an analysis of the events and used the findings to optimise operation of the solidification plant. In addition, in July 2024, the plant launched a project to modernise the solidification control system. With the optimised process conditions in place, the Leibstadt NPP manufactured 91 specification-compliant waste containers without any further anomalies.

ENSI will provide detailed information on the reportable events and findings at the nuclear installations in its annual Oversight Report, to be published in the second quarter of 2025.

As in previous years, emissions of radioactive substances from nuclear installations in 2024 were significantly below the regulatory limits. No unauthorised emissions of radioactive substances (text in German) from Swiss nuclear installations were recorded during the oversight year.

ENSI continues to develop its safety and oversight culture

At the end of November 2024, representatives of various institutions discussed Swiss national culture and examined its influence on the safety culture of the nuclear installations and ENSI’s oversight culture. In addition to ENSI, participants in the Country-Specific Safety Culture Forum in Bern included the operators of the Swiss nuclear installations. The forum was organized by the ENSI, the Nuclear Energy Agency (NEA) and the World Association of Nuclear Operators (WANO). The aim of the forum was to further strengthen safety and security at Switzerland’s nuclear installations. The results will be documented in a report and published.

ENSI checking Nagra’s general licence documents

Also at the end of November, Nagra submitted its application for a general licence for a deep geological repository and an encapsulation plant. The deep geological repository is to be built in the North of Lägern site area, with the encapsulation plant at the site of the Central Interim Storage Facility for Radioactive Waste in Würenlingen. ENSI is responsible for the safety assessment of the applications, with the first step being to check the general licence documents for completeness. This will be followed by a detailed review of the documents, which is expected to be completed by 2027. Commissioning is planned for 2060.