The Office for Nuclear Regulation has highlighted several areas of concern in the Generical Design Assessment of Hitachi-GE's ABWR nuclear reactor design.
The design is planned for use at Horizon's Wylfa site in Wales.
The ABWR design is somewhat different to those previously used in the UK with a different configuration of reactor core and turbine. ONR said work on "reactor chemistry" in this area - an issue that has cross-cutting implications for the fuel in the reactor as well as other hardware, and the operating regime - "is proving to be a very challenging topic".