atw 2018-10

atw Vol. 63 (2018) | Issue 10 ı October
DECOMMISSIONING AND WASTE MANAGEMENT 522
| | Fig. 2.
Range of Hazards Considered for Robot Controlled Laser Cutting Facility.
The range of UK regulatory
requirements addressed during the
installation and operation of NNL’s
Robot Controlled Laser Cutting
Facility included the following: Fire
Precautions Act [4]; Electricity at
Work Regulations [5]; Control of
Substances Hazardous to Health
Regulations (COSHH) [6]; Pressure
Systems Regulations; Provision and
Use of Work Equipment Regulations
(PUWER) 1992 [7]) and Environmental
Permitting Regulations [8]. The
requirements of relevant standards
including; the Safe Use of Lasers
(BE EN 60825-1 [9] and BS EN ISO
11553-1 [10]), and the use of Robots
(BS EN ISO 10218-1:2011 [11] and BS
EN ISO 10218-2:2011 [12]) were also
considered.
Safety case
Figure 2 provides an overview of the
hazards considered for NNL’s Robot
Controlled Laser Cutting Facility.
The primary focus of this paper
is on the integrated Safety Case
approach to the management of the
combination of the key radiological,
chemotoxic, laser, and robotic/
Programmable Logic Controller (PLC)
hazards.
The approach for developing
the NNL’s Robot Controlled Laser
Cutting Facility safety case is
summarised as:
• Identification of hazards
• Assessment of hazards and identification
of suitable safety measures
• Substantiation of safety measures
• Implementation of safety measures
A structured and systematic examination
of NNL’s Robot Controlled
Laser Cutting Facility has been undertaken
using HAZard and OPerability
(HAZOP) studies to identify potential
problems that may represent risks to
personnel or equipment, or prevent
efficient operation.
Hazards are then assessed and
safety measures are identified in the
safety case.
The foundation of NNL’s Robot
Controlled Laser Cutting Facility HMS
is based upon a hierarchical approach
to safety measures (i.e. ERICPD)
which is used to minimise or eliminate
the exposure to hazards:
• Elimination of the hazards wherever
possible,
• Reduction of the hazard by substitution
with a less hazardous form
if possible,
• Isolation and Control of the hazard
with Passive / Engineering controls
to prevent/mitigate the
hazard where appropriate,
• Reliance upon Personal protective
control to mitigate the hazard, and
Discipline with procedural controls
is the ‘last line’ of defence.
The HMS developed for the facility
has been used to identify safety
measures which are proportionate to
hazard severity and demonstrate
there is sufficient strength in depth
and the risk is ALARP.
The key radiological, chemo toxic,
laser and robotic/PLC hazards are
discussed in further detail below
together with the development of a
method to be harmonised and
managed taking into consideration
the competing needs of different
hazard disciplines.
Radiological assessment
Laser cutting for nuclear decommissioning
has the potential to disturb
contamination present on the surface
of the substrate, or contained inside
vessels being cut resulting in the
volatilisation of material leading to
the formation of submicron particles.
Trials using NNL’s Robot Controlled
Laser Cutting Facility have
shown that all of the loose contamination
present within the cut path
could be made airborne and further
experimental work is currently being
undertaken to provide clarity. The
impact upon any loose material and
volatile radionuclides in the vicinity of
the cut path is also considered.
The radiological safety assessment
follows a rigorous process and is
required as part of Nuclear Installations
Site Licence Con ditions.
The individual hazards identified
by HAZOP studies for NNL’s Robot
Controlled Laser Cutting Facility are
presented in the form of a number of
fault sequences. Each fault sequence
starts with an initiating event that
could lead to unwanted consequences
and places a demand on a set of safety
measures. The assessment of the fault
sequence includes failure of some or
all of these safety measures.
The radiological safety assessment
specifies the Engineering and/or
Operational Safety Measures that
need to be in place to minimise the
risks to acceptable levels, ie ALARP
and ensure the adequacy of safety.
The concept of defence in depth is
fundamental to radiological safety to
prevent accidents and if pre vention
fails, to limit potential consequences.
For significant faults (>20mSv to
worker and >0.1mSv public) Design
Basis Analysis (DBA) requires the
designation of a passive safety
measure, (such as the laser cutting
facility enclosure wall), or two key
independent safety measures, (such
as high integrity door interlock
arrangements and other Control,
Electrical and Instrumentation Equipment
(CE&I) with predefined action
on failure and substitution arrangements).
Alternatively, it is possible in
some instances for Operational Safety
Measures to be claimed, which must
be carried out to prevent possible
harm /dose uptake.
For lesser significant faults, DBA
requires the designation of one safety
measure, which can either be passive,
or an item of CE&I equipment that
does not need to have any predefined
action on outage or substitution
arrangements. Alternatively, it is
possible to in some instances for
Operational Safety Measures, about
operator actions, or plant conditions
to be claimed which support the safety
case.
There is no requirement to iden tify
any safety measures in the safety case
for radiological hazards which fall
below the site low consequence
thresholds.
The various engineering safety
measures in the safety case are
Decommissioning and Waste Management
Laser Cutting for Nuclear Decommissioning: An Integrated Safety Approach ı Howard Chapman, Stephen Lawton and Joshua Fitzpatrick