376 / 536
2.4.4 Monitoring the resistance of reactor vessels
The reactor vessel is one of the essential components of
a PWR. For a 900 MWe reactor, it is 14 m high, 4 m in
diameter and20 cmthick. Itweighs 300 tonnes. It contains
the reactor core and its instrumentation. Innormal operating
conditions, the vessel is entirely filled with water, at a
pressure of 155 bar and a temperature of 300°C.
Regular monitoring of the state of the reactor vessel is
essential for the following two reasons:
•
The vessel is a component for which replacement is not
envisaged, owing to both technical feasibility and cost.
•
Rupture of this item is not considered in the safety
assessments. This is one of the reasons for which all
steps must be taken as of the design stage, in order to
guarantee its resistance for the operating lifetime of the
reactor, including in the event of an accident.
In normal operation, the vessel’s metal slowly becomes
brittle under the effect of the neutrons from the fission
reaction in the core. This embrittlement makes the vessel
particularly sensitive to pressurised thermal shocks or to
sudden pressure surges when cold. This susceptibility is
also aggravatedwhen defects are present, which is the case
for some of the reactor vessels that have manufacturing
defects under their stainless steel liner.
ASN regularly examines the files related to the vessels
transmitted by EDF in order to ensure that the in-service
behaviour demonstration for the vessels is sufficiently
conservative and complies with the regulations.
The AdvisoryCommittee for Nuclear Pressure Equipment
was consulted at the end of 2015 concerning the file
transmitted by EDF to substantiate the in-service resistance
of the 1,300 MWe reactor vessels.
2.4.5 Monitoring steam generator maintenance
and replacement
SteamGenerators (SG) comprise two parts, one of which
is a part of the primary system and the other a part of
the secondary system. The integrity of the main steam
generator components is monitored, more specifically
the tube bundle, which is particularly important for
the safety of the facility. This is because any damage
to the tube bundle (corrosion, wear, cracking, etc.)
can lead to a primary system leak to the secondary
system. Furthermore, a SteamGenerator Tube Rupture
(SGTR) would lead to the bypassing of the reactor
containment, which is the third containment barrier.
Steam generators are the subject of a special in-service
monitoring programme, established by EDF, reviewed
periodically and examined by ASN. After inspection,
tubes that are too badly damaged are plugged to remove
them from service.
Mechanical and chemical cleaning
of steam generators
Over time, the SGs tend to become cloggedwith corrosion
products from the secondary system exchangers. This
takes the form of a build-up of soft or hard sludge on
the tubesheet, fouling of the walls of the SG tubes and
clogging of the foliate channels of the tube support plates.
The corrosion products form a layer of magnetite on
the surface of the internals. On the tubes, the layer of
deposits (fouling) reduces the heat exchange capacity.
In the foliate channels, the deposits prevent the free
circulation of the water-steammixture (clogging), which
creates a risk of damage to the tubes and the internal
structures and which can degrade the overall operation
of the steam generator.
Toprevent ormitigate sucheffects, various solutions areused
tominimisemetal deposits: preventive chemical cleaning
or mechanical cleaning (using hydraulic jets), material
replacement (brass by stainless steel or titanium alloy,
which aremore corrosion-resistant) in certain secondary
system exchanger tube bundles, along with an increase
in the pH of the secondary system.
Replacement of steam generators
Since the 1990s, EDFhas been running a SteamGenerator
Replacement programme (SGR) for those SGswith themost
heavily degraded tube bundles, with priority being given
to those made from Inconel 600 without heat treatment
(600 MA) and then those made from Inconel 600 with
heat treatment (600 TT).
The replacement campaign for SGs with a tube bundle
made of 600MA (26 reactors) was completed in 2015 on
reactor 3 at the Le Blayais NPP. It is being pursued with
the replacement of SGswithheat treated Inconel (600TT)
tube bundles. The operations to replace those of reactor 2
at the Paluel NPP will take place in 2016.
The principles of demonstrating
the in-service resistance of reactor vessels
The regulations in force require in particular that the licensee:
• identify the situations that would result in an impact on the
equipment;
• take measures to understand the effect of ageing on the
properties of the materials;
• take steps to ensure sufficiently early detection of defects
prejudicial to the integrity of the structure;
• eliminate all cracks detected or, if this is impossible, provide
appropriate specific justification for retaining such a type of
defect as-is.
UNDERSTAND
376
CHAPTER 12:
EDF NUCLEAR POWER PLANTS (NPPs)
ASN report on the state of nuclear safety and radiation protection in France in 2015