Quarterly report on NPP events
July – September 2019
A. Ballesteros
G. Manna
M. Peinador
P. Pla
Z. Simic
M. Strucic
J. Vegh
2019
Ref. Ares(2019)6646706 - 28/10/2019
i
Contents
Foreword .............................................................................................................. 2
Abstract ............................................................................................................... 3
1 Introduction ...................................................................................................... 4
2 Events short-listed ............................................................................................. 5
3 Events selected ................................................................................................. 6
3.1 Manual reactor scram due to pressure and power oscillations ............................ 6
3.2 Reactor trip due to a reactor coolant pump motor failure .................................. 8
3.3 Wall thickness weaknesses on differential pressure lines on main steam and feedwater systems ....................................................................................... 8
3.4 Manufacturing anomaly in steam generators ................................................... 9
2
Foreword
In the European Union, a regional network, the European Clearinghouse on Operating
Experience Feedback for Nuclear Power Plants, has been established to enhance nuclear
safety through improvement of the use of lessons learned from operating experience.
The European Clearinghouse is composed mainly of European nuclear safety regulatory
authorities and their technical support organisations. It is operated by dedicated staff
from the European Commission´s Joint Research Centre.
3
Abstract
This newsletter provides Feedback on Operating Experience (OEF) from significant safety
related events at nuclear power plants (NPPs) worldwide, compiling the NPP events that
were reported publicly in July - September 2019.
4
1 Introduction
This newsletter provides Feedback on Operating Experience (OEF) from significant safety
related events at nuclear power plants (NPPs) worldwide, every three months. It is
intended to provide timely information to the Clearinghouse members about recent
significant events, with a real or potential impact on nuclear or radiation safety. The
report is intended to be complementary to other international reporting systems such as
the International Atomic Energy Agency (IAEA) IRS, rather than duplicate the information
provided by them. Usually the information used to prepare the report is publicly available
and the information is notified promptly, in advance of other reporting systems. Only
events that are considered to be likely to have lessons applicable to EU NPPs are
selected.
Event selection for reporting in this newsletter is a two stage process. All the information
found on relevant web sites is initially screened and the events that match at least one of
the following criteria are short-listed for further consideration:
— Unplanned or unexpected automatic or manual reactor trips;
— Events rated at INES Level 2 or above;
— Significant radiological events;
— Real or potential challenges to nuclear safety or defence in depth; including recurrent
events and actuation of systems;
— Events with common cause failure aspects;
— Events with lessons learned worth being disseminated;
— Events requiring the entry into emergency operating procedures
Furthermore staff may occasionally short-list other events based on other criteria.
The final selection of the events is made by the JRC Clearinghouse Selection Committee.
The following criterion is adopted to guide the Committee’s final selection:
— Level of actual or potential effect on safety;
— Events rated at INES Level 2 or above; and
— Significance of lessons learned for EU NPPs.
Clearly the criteria above are open to a degree of interpretation and judgment and the
selection committee is comprised of suitably qualified and experienced personnel who by
applying engineering judgment and through consensus, arrive at the final selection.
Finally, no comparison should be made among countries with regards to the number and
significance of events, as the number of nuclear power plants, the reporting criteria and,
most significantly, the information made available to the public, varies widely among
countries.
5
2 Events short-listed
Gathering event information for short-listing involves searching potential sources of
operating experience information including relevant world-wide websites. When NPP
related event reports are identified as potential candidates for the shortlist the
information is translated into English, wherever necessary, for the purpose of screening
and possible inclusion in this newsletter. The sources of the event information are
referred to in an event list compiled for the purposes of screening which then results in
the initial short-list.
The short-list of events considered for inclusion in this quarterly report are drawn from
NPPs world-wide and can be found in the database on our website, accessible to
Clearinghouse members. The following information is collected: title of the event; date of
event or date of reporting if date of incident not available; event description; INES level
(if available) and name of the NPP.
6
3 Events selected
Four events were selected from the short-list for this Newsletter:
1. 29/04/2019: Manual reactor scram due to pressure and power oscillations (US /
NINE MILE POINT 1).
2. 26/05/2019: Reactor trip due to a reactor coolant pump motor failure (US /
ARKANSAS ONE 2)
3. 13/06/2019: Wall thickness weaknesses on differential pressure lines on main
steam and feedwater systems (Germany / BROKDORF).
4. 09/2019: Manufacturing anomaly in steam generators (France / FRAMATOME).
The information provided is extracted from publicly available and other authorised
sources. More detailed information on these events may become available in due course,
either from the original source or through international operating experience sharing
systems.
3.1 Manual reactor scram due to pressure and power oscillations
NINE MILE POINT 1 – 29/04/2019
While the unit was operating at 84.6% reactor power, with power ascension in progress,
the operator inserted a manual reactor scram due to pressure and power oscillations. The
scram resulted in shrink of the water level in the reactor vessel and the low level setpoint
was reached. This resulted in a High Pressure Coolant Injection initiation as designed.
The non-optimal tuning of the turbine control resulted in pressure control issues in the
80-90% power region during reactor startup that produced reactor pressure, level and
power oscillations greater than those allowed without a backup pressure regulator
available. A manual reactor scram was inserted per procedure.
The organisation did not effectively incorporate appropriate learnings from previous plant
events and operating history that would have provided adequate flexibility in the
operating strategy to mitigate the risks of increased oscillations. A similar event had
occurred in 2017 with the reactor at 4% power, also producing pressure oscillations. The
cause of this event had been a partial blockage on a pressure sensing line from the
turbine control system.
The scope of the corrective actions is to incorporate operational history and events into
Special Operating Procedures.
Editor´s comment – This event has been highlighted because of the potential
significance of its lessons to be learned.
Power oscillations in BWRs are a well-known issue, and they are normally avoided by
keeping the reactor operation point sufficiently far from unstable regions. However, the
evolution of new fuel designs or new core configurations demand a constant feedback on
the topic, particularly, if power oscillations present local variations (not the case in this
event). Operating experience should be used to provide operators with adequate margins
to avoid unstable regions during power variations and with flexible strategies to exit
safely from the oscillations region.
7
8
3.2 Reactor trip due to a reactor coolant pump motor failure
ARKANSAS ONE 2 – 26/05/2019
The reactor automatically tripped due to a fault and subsequent trip of one of the reactor
coolant pumps (RCP). After the reactor trip, the unit was stabilized in Mode 3 with
Emergency Feedwater initially being used to maintain plant temperature under automatic
control. At the time of the event, the plant was operating at 100% rated thermal power
in Mode 1.
After investigation, it was found that the motor of the pump failed with a phase-to-
ground fault. Examination of the failed stator identified that one of the compression
fingers had come loose and migrated into contact with the rotor, damaging the stator coil
insulation.
Subsequent research identified that a similar failure mechanism had occurred in 1979 on
another RCP at the same plant. The original equipment supplier (GE) subsequently
modified all four RCP motors with mitigating strategies that included welding at several
locations and the addition of epoxy impregnated felt around the fingers to prevent
vibration. However, these mitigating actions and related operating experience were not
replicated in subsequent motor specifications or operating experience documents. For
this reason, the epoxy felt was not employed in future motor refurbishments due to
misunderstanding of its purpose.
Editor´s comment – This event has been highlighted because of the potential
significance of its lessons to be learned.
A lesson to be learned is the importance of appropriate management of knowledge
stemming from operational experience, even if related to events occurred long time ago.
The original event back in 1979 was reported and a corrective action was put in place
that allowed for more than 30 years of successful RCP operation. However, operating
experience was not used to update the relevant documentation, in this case the motor
specifications, which led, over time, to reoccurrence of the event.
3.3 Wall thickness weaknesses on differential pressure lines on
main steam and feedwater systems
BROKDORF – 13/06/2019
After findings in another reactor (Grohnde NPP), the plant operator carried out
inspections on differential pressure tubing used by pressure instrumentation in the main
steam and feedwater systems. From a total of 30 lines inspected, the operator found that
the wall thickness was lower than expected in 12 of them.
The weaknesses detected were due to corrosion, located in areas where condensation of
the medium occurs due to transition from insulated to not insulated sections of the test
leads.
The minimum wall thickness required was not exceeded in any of the differential
pressure lines, and their integrity was still guaranteed.
All lines where the wall thickness was found to be lower than 2 mm as well as all non-
tested lines were replaced. Further measurements will be carried out later.
The operator has rated this event as INES 0 (no or very low safety or no radiological
significance).
Editor´s comment – This event has been highlighted because of the potential
significance of its lessons to be learned.
9
Regulators may want to draw attention on the potential corrosion in differential pressure
lines or other instrumentation lines where condensation of the medium can be expected
due to transitions from insulated to not insulated sections.
3.4 Manufacturing anomaly in steam generators
FRAMATOME - 09/2019
Framatome has identified a deviation in a manufacturing process applicable to different
major nuclear equipment. The process affected is the thermal treatment applied to
certain welded joints on some major components such as steam generators or
pressurisers.
Steam generator shells are essentially composed of several ring forgings, hemispheric
bottom and SG head welded together. Once the welded material cools down after the
welding, the residual stresses must be relieved by a thermal treatment. In the case of
the steel used for the steam generator ring forgings, the thermal treatment applied to
these welds must be within the range 595 °C – 620 °C. Different methods and tools are
available to complete the thermal treatment. Since 2008, Framatome has been using a
specific type of electrical heaters on insulating blankets ("moufles chauffants"). These are
laid around the outer and inner walls of the steam generator shell, with eight or nine
modules being required to cover the entire weld length, and arranged in four bands to
cover the entire width of the ring welding and base material around the weld (see Figure
1).
Recently, Framatome has conducted simulations showing that the temperatures obtained
by this process may be significantly above the maximum prescribed limit in some areas
of the weld joint and significantly below the limit in other areas. A temperature too low
may result in the residual stresses not sufficiently relieved, while a too high temperature
may cause a reduction in the elasticity limit, the resistance to traction and the toughness
of the material.
EdF has identified 16 steam generators in operation concerned by the anomaly, in
addition to seven steam generators already manufactured but not yet in service
(planned for the EPR at Flamanville and for Gravelines NPP). Also the pressuriser
intended for the EPR is concerned.
Framatome and EdF have submitted to the safety authority a justification showing that
the integrity of the equipment is not compromised. The safety authority, after review and
analysis, has authorised the operation of the concerned reactors and has asked the
operator to conduct non destructive testing and measurements during the next outages
in order to confirm the assumptions made. Furthermore, the operator has been
requested to set up a dedicated work programme to fully address the causes of this
event.
Editor´s comment – This event has been highlighted because of the potential
significance of its lessons to be learned.
Regulatory bodies in other countries may have an interest to verify the similarity of
thermal treatment processes for ring forgings stress relieving in the case of the steam
generators in operation in their respective areas.
10
Fig
ure
1:
Manufa
ctu
ring a
nom
aly
in s
team
genera
tors
(Sourc
e:
IRSN
)
GETTING IN TOUCH WITH THE EU
In person
All over the European Union there are hundreds of Europe Direct information centres. You can find the address of the centre nearest you at: http://europea.eu/contact
On the phone or by email
Europe Direct is a service that answers your questions about the European Union. You can contact this service:
- by freephone: 00 800 6 7 8 9 10 11 (certain operators may charge for these calls),
- at the following standard number: +32 22999696, or
- by electronic mail via: http://europa.eu/contact
FINDING INFORMATION ABOUT THE EU
Online
Information about the European Union in all the official languages of the EU is available on the Europa website at: http://europa.eu
EU publications You can download or order free and priced EU publications from EU Bookshop at:
http://bookshop.europa.eu. Multiple copies of free publications may be obtained by contacting Europe
Direct or your local information centre (see http://europa.eu/contact).
http://europea.eu/contacthttp://europa.eu/contacthttp://europa.eu/http://bookshop.europa.eu/http://europa.eu/contact
Electronically signed on 28/10/2019 10:34 (UTC+01) in accordance with article 4.2 (Validity of electronic documents) of Commission Decision 2004/563
1 Introduction2 Events short-listed3 Events selected3.1 Manual reactor scram due to pressure and power oscillations3.2 Reactor trip due to a reactor coolant pump motor failure3.3 Wall thickness weaknesses on differential pressure lines on main steam and feedwater systems3.4 Manufacturing anomaly in steam generators
2019-10-28T09:51:06+0000
Top Related