Power Dip Damages ESS Moderator; Replacement Moderator 2 Targets 2026 Neutron Test

A power dip at the end of 2025 damaged a critical moderator-reflector plug at the European Spallation Source in Lund, prompting ESS management and in-kind partner Forschungszentrum Jülich to prioritise the second moderator as the recovery route and target installation and testing in the last quarter of 2026. The incident slowed progress toward producing the facility’s first neutrons and shifted earlier plans that had aimed for beam-on-target milestones in 2025 and user operation in 2026.

Kevin Jones, ESS Director for Machine and Operations, said the second moderator was always planned as a replacement and that, after the setback, “we will now make every effort to ensure that moderator 2 will be ready to be installed and tested in the last quarter of 2026.” Moderator 2, which was already in production at the time of the incident, uses a different geometry and is expected to deliver stronger neutron flux to instruments, increasing discovery potential.

Project leadership has set clear near-term planning steps. Andrew Kimber, ESS Project Director, said the facility is focused on minimising the incident’s impact and “to produce neutrons and welcome scientists to do experiments by the end of 2027.” Kimber added that ESS would explore all acceleration options and aimed to establish a revised plan by March 2026. ESS messaging also states teams are working “to establish a new timeline for achieving production of first neutrons within a year.”

Commissioning work on the accelerator and target system continues alongside moderator recovery. ESS has logged a Beam on Dump milestone last year and is preparing for a second Beam on Dump commissioning period, while an earlier milestone—first beam through the first DTL tank—has been passed. The accelerator’s design targets remain 2.0 GeV energy and an ultimate 5 MW average beam power delivered by a LINAC running long pulses at 14 Hz with 2.86 ms pulses and a 62.5 mA peak current.

Target station hardware and scale underline the challenge of replacing a damaged moderator. The monolith stands roughly 6 m in diameter and 8 m high with about 6,000 tons of steel shielding, while the rotating tungsten target is 2.5 m in diameter and built from thousands of tungsten bricks. ESS continues integrated testing of target-system components, the organisation says, to be ready to receive protons once the new moderator is installed.

Instrument and operations targets remain ambitious: JACoW and technical briefings describe a long-term goal of serving 22 instruments, with 16 under construction and a nearer-term ESS target of about 15 operational instruments as the facility ramps. If Moderator 2 is installed and validated in Q4 2026, ESS intends to sustain a revised pathway toward welcoming scientists by the end of 2027.