26 March 2019 http://www.controlengeurope.com Control Engineering Europe

SEISMIC MONITORING

AND PROTECTION

T

he various regulatory bodies

around the world have gone

back to basics to consider

not only the design basis for

operational sites but also

revisit the risk analysis, the accident

management strategy and the periodic

safety review policy. Considering

that many nuclear power stations

are located on the coast, the risk of

flooding from both seismic events

and severe weather has been put

into particular focus. Of significant

importance has been the review of

methodology used to derive the seismic

hazard and how that hazard has been

mitigated through the design process

and present day operational systems.

No two nuclear plants are the same

in terms of their approach to seismic

monitoring and protection. Sensonics

has identified that some sites utilise

data from the national network of

geophysical instruments, while others

implement independent monitoring

and shutdown on each critical plant

item. In each case, the derivation of

the required seismic monitoring and

protection strategy must meet with the

safety case and provide appropriate risk

mitigation.

The structural effects to be expected

at a site from an earthquake result

from the vibration induced by the

event, classified in terms of seismic

response spectra. This defines the

ground acceleration magnitude versus

frequency, typically over a range of

0.1Hz to 100Hz.

Two such types of spectra are

specified, the Operational Basis

Following the Fukushima incident the nuclear industry has a renewed focus on risk

mitigation from extraordinary events caused by nature. Control Engineering Europe

looks at the trends in seismic monitoring and protection systems, technologies adopted

and current best practice driven by these enhanced risk management demands.

MONITORING

Earthquake (OBE) and the Design Basis

Earthquake (DBE), based on a predicted

worst case seismic event within a

specified period of time (for example

OBE may be specified within 100 years).

Secondary response spectra are

derived from the ground accelerations

through modelling to predict the

response of each structure and each

level within that structure. A nuclear

plant will allocate several seismic

categories for specifying the design

requirements and assess according

to the safety class. For example the

highest or most stringent category will

demand the equipment or process be

tested to the DBE level plus a margin

(+40% is recommended in IEEE- 344,

Standard for Seismic Qualification

of Equipment for Nuclear power

generating Stations), since the process

must still remain operable to the design

basis even if other less critical plant

processes may have failed above the

OBE level. Any earthquake above the

OBE level may result in the plant being

shutdown and to remain shutdown

until post analysis / inspection has

determined the plant is safe to continue

operations.

The challenge is to design and

construct in a cost effective manner to

meet with the seismic categorisation

and to provide sufficient design margin.

It may not be possible for all equipment

or processes through either analysis or

testing to meet with its categorisation

fully and this is where independent

seismic monitoring systems can be

utilised to provide detection of the OBE

event and to bring the process to a safe

state. Not only must these monitoring

systems be robust to seismic events

but they also need to exhibit high

levels of availability beyond the DBE

magnitude event to maintain a valid

alarm function. In combination with

the seismic requirements various safety

standards are applied to obtain a stated

availability, with EN IEC 61508 being the

most common approach. Adherence

to such a standard provides a stated

system reliability and availability

while at the same time providing an

understanding of the systematic failures

and ensuring compliance with the

EN IEC 61508 life cycle model.

Where to start?

The starting point with any seismic

monitoring design is the sensor. There

is a clear technical difference between

the types of sensors that are used for

seismic protection and those used for

geophysical earthquake monitoring.

Geophysical seismic monitoring utilise

broadband magnet & moving coil

(electrodynamic) sensor arrangements

capable of measuring micro g

acceleration events with sinusoidal

periods of over 100 seconds.

Strong motion sensors for seismic

protection applications only need to

provide a resolution down to 1mg and a

response to 10 seconds; while historically

electrodynamic sensors have been

used, nowadays for these applications

piezoelectric-based accelerometers are

preferred as they match the technical

requirement closely and provide higher

reliability as they have no moving parts.

A trend in vibration monitoring is