quarterly report on npp events - europa · 3.1 manual reactor scram due to pressure and power...

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

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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.

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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.



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).



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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.



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.

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Fig

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GETTING IN TOUCH WITH THE EU

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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

quarterly report on npp events - europa · 3.1 manual reactor scram due to pressure and power...

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