EDF gets approval to raise heat limit in some UK nuclear reactors

(Adds more detail)

By Nina Chestney

LONDON, July 6 (Reuters) - Britain's nuclear regulator said it has approved a request from EDF Energy to increase the maximum operating temperature in the reactors at its Hartlepool and Heysham 1 nuclear plants to avoid reducing reactor power.

In 2014, the two plants were shut down for several months after cracks were found in the reactor boilers. When they restarted, they did so at reduced output to stop high temperatures causing more cracks.

EDF Energy has asked the Office for Nuclear Regulation for permission to raise the operating limit for the upper surface of hot box domes inside the reactors to 390 degrees Celsius from 380 degrees.

Following inspections and assessments, the ONR has agreed to the modification, it said in a report.

Hot box domes are steel structures inside the reactors which form a barrier between different pressures of gas. They have a maximum operating temperature to prevent degradation.

There is a trend of increasing temperature in the hot box dome area as insulation has degraded over time, so EDF Energy has taken steps to reduce the heat, the ONR said.

These measures have not entirely stopped temperatures from rising, so EDF Energy has said it is possible operating temperatures could exceed 380 degrees in the future.

If that happened, EDF Energy could reduce the power of the reactor and/or other steps. However, the firm is trying to avoid this as it can reduce boiler lifetimes, the ONR said.

"The increase in risk associated with operating the domes with a temperature limit of 390°C is acceptably low," the report said.

In addition, the ONR said the future degradation of the hot boxes would be unexpected and likely to appear gradually and the partial failure of the hot box could be detected and is tolerable.

In May, a reactor at EDF Energy's Hunterston B nuclear plant in Scotland was taken offline for additional safety checks after cracks were found in its core.

The development of such cracks is a side effect of ageing but has raised concerns about other old reactors in Britain and Europe. (Reporting by Nina Chestney, editing by David Evans and Alexander Smith)