Operator error was to blame for the unauthorised release of contaminated water into the Clyde from Hunterston B Power Station in November.
Radioactive waste was discharged for half an hour longer than scheduled during the incident, which was immediately reported to the Scottish Environment Protection Agency.
Hunterston B, owned by eDF Energy, is authorised by SEPA to release aqueous radioactive waste into the Firth of Clyde.
In a report presented to the Hunterston site stakeholders group, Station director Colin Weir said: "This is a normal part of our activities and the authorisation from SEPA contains many conditions.
"One of these conditions specifies when the waste can be discharged. Waste should only be released within the period between one hour after high tide and one hour before low tide. This makes sure the waste is being drawn away from the shore. 
"In November, operator error meant the the discharge pump was left running for around half an hour longer than scheduled. This meant that it as stopped around half an hour before low tide.
"This represented a non-compliance with our permit conditions. Hunterston B immediately informed SEPA and conducted a thorough investigation and we have identified some improvements to our procedures which will stop it from happening again."
SEPA had sent a warning letter to the station but has recognised that the discharge was stopped whilst the tide was still ebbing, and that there does not appear to be any environmental impact or harm resulting from the extended discharge. The matter was discussed at last Thursday's site stakeholder group meeting.
Andy Taylor of Hunterston B said: "It was a simple human error - it happened over a shift handover - as it was being handed over to oncoming operative and it wasn't an effective handover conducted and it was only later in the shift some time after the discharge should have stopped that it was noticed. We have strengthened our shift handling arrangements, which were proceduralised before, and have elevated the nature of the tasks including our log-in activities. It came down to a human error between shifts."
Mr Taylor explained samples of the water were regularly taken of the discharges, and said: "It is contaminated as it has either been cooling a piece of plant with activated steel in it, or it is from a pump which has been pumping out liquid. These systems are very varied, but it is very low levels. We spend a lot of time with our technicians out in the environment and collect farm and marine samples, and we look for any trace levels, and we find that the level of radioactivity is negligible." 
SEPA's Keith Hammond stated that Hunterston had alerted SEPA immediately to the issue, as they are required to do, and Hunterston B had carried out an investigation into the matter within 48 hours, and a further investigation took place to establish whether any contraventions to compliance issues had taken place.
Mr Weir added: "There were no other compliance issues surrounding this release; the station did not go over its consented volume of release and there is no evidence that any of the material was deposited on the beach.
"Both gaseous and aqueous discharges arising from the normal plant operations remain at levels well below those authorised by SEPA."
Mr Hammond added: "In terms of compliance scores in 2015, Hunterston B have six licences from SEPA, four of which were excellent, and two were good, and that is satisfactory on all the licences, and is quite an achievement."