Regulatory oversight of nuclear safety in Finland. Annual ... - STUK

4.2 OlkiluOtO NPP uNits 1 aNd 2
Annual maintenance at Olkiluoto 1
Olkiluoto 1 had a refuelling outage from 1 to 10
May 2011. The outage lasted two days longer than
planned due to the time taken to repair the faults
discovered in the diesel generator and in the outer
isolation valve of one of the main steam pipes.
Twenty-five per cent of the fuel in the reactor
was replaced with fresh fuel. In other respects, the
work carried out mainly consisted of inspections,
maintenance, repairs and tests of systems, equipment
and structures, such as fuel inspections and
leak tests on the two hundred or so containment
isolation valves.
During the outage, a fuel rack in the reactor
hall fuel pool tilted slightly when spent fuel was
moved to one side of the rack. The structure of the
rack is such that a lop-sided load like this changes
the centre of gravity, and the rack may tilt. The
matter had not been taken into account when producing
the transfer plan and when carrying out
the actual transfers. TVO will develop its procedures
and instructions so that a similar event can
be prevented. The event did not put the integrity of
fuel at risk.
Annual maintenance at Olkiluoto 2
The annual maintenance outage at Olkiluoto 2 took
place between 10 May and 8 June 2011 and lasted
over three days longer than planned. The delay was
caused in particular by investigations and repairs
of the damage observed in the pistons and control
cylinders of the valves in the system required for overpressure
protection of and residual heat removal from
the primary circuit.
Almost 25 per cent of the nuclear fuel in the reactor
was replaced with fresh fuel during the annual
maintenance outage. A fuel leak was detected at the
plant unit soon after the start-up following the 2010
annual maintenance outage. TVO monitored the magnitude
of the leak and its development by regular
measurements during the operating cycle. During this
annual maintenance outage, the leaking fuel assembly
was identified and removed from the reactor.
Some major modifications were carried out during
the annual maintenance outage. They included
the replacement of the inner isolation valves of the
main steam pipes (see the description in Section
STUK-B 147
inspecting the outage plans and modification work
documentations and by carrying out site inspections
during the annual maintenance.
STUK has found that TVO is capable of planning
and implementing annual maintenance outages
in a safe manner. However, STUK identified
scope for development in spare parts management,
planning and resource allocation of modifications
implemented during the annual maintenance as
well as in decision-making related to analysing
events. Following its inspection performed before
the annual maintenance, STUK required TVO
to produce a report of spare parts maintenance
regarding systems and components important to
safety and submit it to STUK by the end of
November 2011. The shortage of spare parts encountered
during the annual maintenance outages
further emphasised the importance and topicality
of the issue. The observations made during the
2010 annual maintenance indicated that there
is scope for improvement in the way TVO plans
modifications and allocates resources for them.
Following the 2010 annual maintenance, STUK required
TVO to investigate the underlying problems
that have led to delays in planning and to the deficiencies
in the contents of documents submitted to
4.2.5), the replacement of a low-voltage mechanism in
one subsystem (see the description in Section 4.2.5),
the modernisation of the main seawater pumps, the
replacement of pipes in the seawater system, the replacement
of low-pressure turbines, the replacement
of the generator and the replacement of the generator’s
cooling system. In addition to the modifications, numerous
inspections, maintenance operations, repairs
and tests of systems, equipment and structures were
carried out.
TVO tested the operation of the new generator and
its voltage regulator on 11 and 13 June after the annual
maintenance. An operational transient occurred
during both tests resulting in the plant dropping off
the national grid. The first transient was caused by
an incorrect setting of the voltage regulator. The latter
transient was due to an error in planning the tests.
The voltage regulator setting was corrected before
ramping up the reactor power. The events did not put
the plant or its surrounding environment at risk. The
events did not cause any disturbance to power supply
to the plant’s systems.
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