June 15 (Reuters) – CLP Holdings Ltd (0002.HK), Hong Kong’s largest power supplier, said it had recorded a small rise in radioactivity in reactor cooling water at a nuclear plant in the southern Chinese city of Shenzhen last month.

“The reactor cooling water is sealed in completely and isolated from the external environment, thus causing no impact to the public,” CLP said on Tuesday.

CLP’s statement followed a report by Washington-based Radio Free Asia that the Daya Bay nuclear plant had suffered a large radiation leak that was threatening public safety.

Preliminary assessments indicated there was a very small leakage at a fuel rod in Unit 2 of the power station, the company said in a statement obtained by Reuters on Tuesday.

The level of radioactivity had since remained stable over the last two weeks, it said.

Operations at the Daya Bay nuclear power plant had not been affected, said the company, adding that the situation did not fall within the International Nuclear Event Scale, which measures the significance of nuclear accidents.

Daya Bay nuclear power plant, 25 percent owned by CLP, is located about 50 km (30 miles) from densely-populated Hong Kong and supplies a quarter of the city’s power.

The power station, which comprises two nuclear reactors with a generating capacity of 984 megawatts each, has been controversial in Hong Kong, where activists have questioned the safety of the facility.

Activists have raised questions about the time it took for CLP to issue a statement about the leak.

“We are concerned about the plant’s slow reaction (to the incident) and the long duration it took them to act on the situation,” said Prentice Koo, a Greenpeace campaigner in Hong Kong.

Hong Kong’s Security Bureau said the Hong Kong Observatory’s radiation monitoring stations had not registered any unusual levels since May 23. The bureau said it would study the incident further and follow up with CLP. (Reporting by Alison Leung and Darren Chen; Editing by Don Durfee and Chris Lewis)