lessons from windscale's nuclear legacy - Ingenia

LESSONS FROM WINDSCALE’S NUCLEAR LEGACY
WEALTH CREATION
LESSONS FROM
WINDSCALE’S
NUCLEAR LEGACY
the industry can manage the
legacy issues surrounding
nuclear power. Indeed, the
WAGR decommissioning was a
European demonstration project,
managed by the OECD Nuclear
Energy Agency, for safe and
cost-effective decommissioning
of nuclear reactors. More
specifically, the decommissioning
has provided the industry with
valuable experience in areas
such as remote handling, and
has underlined the importance
of establishing ways to get waste
out of the facility and what would
happen to it after its removal.
ALWAYS INNOVATING
Piles 1 and 2 at Windscale were
an extraordinary technological
achievement for their time.
Removing the reactors has also
driven significant engineering
innovations in nuclear clean-up.
Constructed in the late 1940s,
the Piles comprise two graphitemoderated,
air-cooled reactors
and their associated facilities. Each
reactor consisted of 2,000 tonnes
of graphite blocks in an octagonal
stack, 15 m in diameter and 7.5 m
long, surrounded by a biological
shield of reinforced concrete
2 m thick.
The owner of any new nuclear power station built in the UK will
have to produce detailed plans for its eventual closure before
construction can begin. Lessons learnt from decommissioning
reactors and waste disposal at the current plants will provide
valuable information for future developments. Peter Mann,
formerly head of site for Babcock at Windscale in Cumbria,
describes the work undertaken on the Windscale Piles and the
Windscale Advanced Gas-Cooled Reactor.
Part of the charge face of the Pile 1 reactor – the worker is removing the sealant from the locking device on the charge plug to inspect the fuel channels
The decommissioning
at Windscale has led to
the development of new
techniques that are directly
applicable to the UK
decommissioning programme.
As one of the first countries
to operate nuclear power
stations, the UK can draw on
50 years’ experience in dealing
with reactors at the end of
their commercial life. Thanks
to this history, the UK’s nuclear
industry is now one of the
leaders in decommissioning
redundant nuclear facilities.
The industry has already had
to develop techniques to deal
with three nuclear reactors
at Windscale – the Windscale
Advanced Gas-Cooled Reactor
(WAGR) and Piles 1 and 2.
While some of the challenges
at Windscale are specific to
those reactors, many of the
techniques developed to
decommission these reactors
are relevant to other nuclear
power stations.
The expertise gained will
support the decommissioning
of the Magnox breed of
reactors, most of which have
now closed. The last four
Magnox reactors, at Oldbury
and Wylfa, are scheduled for
closure by 2014 having been
given lifetime extensions.
There are a number of similarities
between Magnox and the
WAGR reactors. They all have a
graphite core, or moderator, and
a steel pressure vessel, along
with CO 2 cooling with similar
heat exchangers. These reactors
were also built in the same era,
using similar materials such as
asbestos. Babcock managed
the site (as UKAEA) until 2008. It
was then contracted to provide
senior personnel within the site
management team at Windscale,
working with the Nuclear
Decommissioning Authority
(NDA) and Sellafield. It is
applying this expertise in working
with Japan on an ongoing
project decommissioning the
Tokai gas cooled reactor.
The devastation at Fukushima
power station throughout the
Spring of 2011 focused the
world’s attention on nuclear
safety. The problems posed by
a burnt out reactor have already
been faced in the UK owing to
the fire at Windscale in the 1950s
and it is too early to say whether
there are relevant parallels that
can be used for Fukushima.
In the longer term, the
experience from Windscale
will help in decommissioning
new nuclear power stations.
In general terms, this includes
reassuring the UK public that
The Pile 1 reactor charge face where fuel is loaded into the reactor
34 INGENIA INGENIA ISSUE 48 SEPTEMBER 2011 35

LESSONS FROM WINDSCALE’S NUCLEAR LEGACY
WEALTH CREATION
The use of epoxy resin to encapsulate fuels
and radioactive waste has been trialled
and attracted significant industry interest
which could pave the way for its use in future
decommissioning projects
POLYMERS TO HOLD NUCLEAR WASTES
As part of the decommissioning of the Windscale Piles, Babcock, on behalf of Sellafield Ltd, has
conducted trials on the use of epoxy resin to immobilise fuel and radioactive waste. While the civil
nuclear industry makes widespread use of cement to contain higher activity wastes, this material
cannot easily encapsulate some wastes, such as reactive metal or ion-exchange resins. The work on
polymers to encapsulate nuclear wastes has reached an advanced stage and has attracted significant
interest in the UK, where the use of polymers has been limited to small volume waste streams at
Trawsfynydd and Harwell.
The high-level waste from the Windscale Piles includes a significant proportion of metallic uranium
in finned aluminium cartridges, graphite boats, aluminium isotope cartridges containing a variety
of known compounds and fuel debris. Work was undertaken to develop a polymeric encapsulation
system, including trials with inactive and active wastes, to assess the ability of the polymers to
produce an acceptable wasteform. The aim of the work was to support the process leading up to the
Radioactive Waste Management Directorate’s (RWMD) issuance of a Letter of Compliance (LoC) and to
gain the directorate’s endorsement for encapsulating the waste.
The work included selection of suitable polymers and studies of how they would interact with
other materials in the wastes. The project also looked into the performance of the materials during
processing and their behaviour when exposed to radiation and to heat generated by radioactive
decay. Further trials demonstrated that the materials could be handled safely and could comply
with the standards set out
in the LoC for geological
disposal. These trials have
shown that the polymers
can meet the requirements
for RWMD acceptance of
the wasteform for geological
disposal.
Research so far has
concentrated on the use of
polymers to withstand high
doses of radiation. The NDA
Direct Research Portfolio
has funded further work at
lower dose rates of radiation,
similar to those experienced
as a waste encapsulant, and
under the kind of conditions
they would experience in a
repository. These tests will
also investigate the long term
performance of the polymers.
If successful, the polymer will
be applicable to the widest
Section of Windscale liner containing simulated waste from Pile 1, including
graphite, metal fines, fuel cartridges and isotope cartridges encapsulated in
epoxy resin
possible range of wastes
within the UK’s inventory of
nuclear wastes.
Following a serious fire in 1957,
both Windscale reactors were
shut down. All of the fuel was
removed from Pile 2 while only
a very small proportion of fuel
remains in the fire-damaged
parts of Pile 1. In the 1990s,
Babcock, then UKAEA, began to
decommission, disassemble and
clean up both reactors.
Phase 1 of decommissioning
ended in 1999. The work
included putting dams in the
original water ducts, previously
used to transfer fuel from the
reactor to the cooling pond, and
in the inlet and exhaust air ducts
to seal the massive concrete
biological shield surrounding
the core. This allowed the
installation of a ventilation and
monitoring system to replace
the existing natural circulation
which had been difficult to
monitor. The clean-up team also
installed monitoring systems in
and around the core to measure
temperatures, levels of radiation
and the amount of radioactive
material in the core and airflow.
Outside the core, the clean-up
team removed accumulations of
old fuel and isotope cartridges,
used to irradiate materials in the
reactor, in the water and air ducts.
Clearing the water ducts
was particularly challenging: it
involved removing fuel debris
from the fire as well as sludge
submerged in 750 m 3 of water.
To achieve this, Babcock and its
contractors adapted technologies
developed for the North Sea oil
industry. They deployed remotely
operated vehicles on existing rails
in the ducts, manoeuvring them
from a control console. In this
way, the decommissioning team
could explore the underwater
areas where there might have
been radioactive material. The
operation provided the tools
needed to handle and clear the
solids and sludge. This approach
not only ensured safe operations
but was also highly cost-effective.
Completion of Phase 1 of
the clean up left the plant in
a safe condition until further
decommissioning could begin.
Subsequent work has focused on
developing a remote retrieval and
handling system that can remove
the main hazardous materials,
remains of fuel and isotope
cartridges, from Pile 1’s damaged
core, while work on Pile 2 has
included removing the reactor’s
last remaining isotope cartridges.
An important achievement
in the work on Pile 1 has
involved gaining approval from
the nuclear regulators for the
safety case to allow the team
to look inside the reactor’s fireaffected
zone for the first time
since the fire. The team used an
endoscope to inspect this area
and take pictures. The surveys
gave the clean-up team a better
understanding of the condition
of the area. They used the
results to design and develop
techniques to remove fuel and
other radioactive isotopes, and
to devise a strategy for waste
treatment, storage and disposal.
RETRIEVING
THE WASTE
The decommissioning engineers
developed specialist retrieval
equipment to remove the fuel and
isotopes from Pile 1. The so-called
Fuel Channel Retrieval Tool (FCRT)
can remove fuel elements and
isotope cartridges remotely and
transfer them to a processing
facility next to Pile 1, where the
debris will be segregated, put
into waste liners and the radiation
measured for the records.
To achieve this, an access
slot will be cut in the top of the
reactor, the concrete pile cap,
within a containment area. An
overhead crane and manipulator
gantry will deploy the FCRT.
Cutting the access slot – using
a combination of standard
industrial techniques and remote
operations involving core drills
and diamond wire equipment –
will allow the decommissioning
team to dismantle and remove
redundant steel structures and
components within the void,
followed by remotely operated
deployment of the FCRT.
A carbon fibre telescopic mast
– carbon fibre minimises weight
and avoids compromising the
strength of structural components
– is installed through the access
slot so that the FCRT can be
positioned anywhere in the
discharge void to retrieve fuel and
debris from channels within the
graphite core. A combination of
‘end effecters’ (grabs, scoops and
loosening tools) operated using
camera signals and controls sent
through the telescopic tubes, will
unblock channels, retrieve fissile
material and collect the cartridges
and debris in containers. A
slightly smaller Isotope Channel
Retrieval Tool (ICRT) will remove
isotopes in the same way.
The retrieved waste will go
to the waste separation cell for
encapsulation in an epoxy-based
polymer to contain the waste
for safe disposal. The use of
epoxy resin to encapsulate fuels
and radioactive waste has been
trialled and attracted significant
industry interest which could
pave the way for its use in future
decommissioning projects (see
panel Polymeric Encapsulation).
Once the fuel and isotopes
have been removed, subsequent
stages will include using
remotely operated hydraulic
grippers to remove the control
and shutdown rods, followed
by construction of a modular
containment structure, gaining
access to the core and removal
of the graphite blocks. This
will be done using a variety of
industry-proven mechanical and
thermal cutting tools, including
hydraulically or electrically
driven devices such as shears,
diamond wire band saws
and grinders, thermal cutting
technologies such as plasma
arc and oxy-propane torches,
and gripping and lifting tools.
Ultimately, this will be followed
by demolition of the bioshield,
once the remaining material
and internal surfaces of the
reactor have been removed or
decontaminated.
A schematic representation of Windscale Pile reactors. This shows the reactor
building and the chimney with its filter gallery, from which the air was
discharged having passed through the reactor. It also shows the air ducts
through which air was drawn into the reactor and the water ducts which
provided the export route to the adjacent cooling pond for the discharged fuel
36 INGENIA INGENIA ISSUE 48 SEPTEMBER 2011 37

LESSONS FROM WINDSCALE’S NUCLEAR LEGACY
WEALTH CREATION
Removal of one of four 190 tonne heat exchangers from the Windscale
Advanced Gas-Cooled Reactor
Pressure vessel
and insulation
Hot box
Loop tubes
Neutron shield
Fuel channels
Graphite core and steel
restraint structure
Thermal shield
Diagrid, Ring Beam etc
A cross-section through the WAGR bio-shield and reactor vessel showing
the graphite core and other structural and operational components
WAGR
The Windscale Advanced
Gas-Cooled Reactor (WAGR)
was a prototype for the UK’s
second generation of nuclear
power stations, the AGRs,
which followed on from
Magnox stations. WAGR started
operations in 1962, shutting
down in 1981. A forerunner
of a family of 14 reactors on
seven sites in the UK, the WAGR
was CO 2 -cooled, graphitemoderated
and fuelled with
uranium dioxide in stainless
steel cans. The reactor consisted
of a graphite moderator, the
structural core, 4.6 m diameter
and 4.3 m high, housed in a
cylindrical reactor vessel with
hemispherical ends. The reactor
and its heat exchangers sat in a
steel containment building that
was 40.8 m high and 41.1 m in
diameter, the easily recognisable
‘golf ball’ that came to symbolise
nuclear power in the UK.
The WAGR decommissioning
programme had several
objectives. It set out to
demonstrate the feasibility of
dismantling a nuclear reactor
safely and at acceptable cost.
The programme was also
designed to establish a route
and appropriate authorisation
procedures for disposing of
the radioactive waste. Another
important task was to acquire
and record the information,
data and expertise that would
support the design and
subsequent decommissioning
of nuclear power plants,
especially gas-cooled
reactors (see Information
for Posterity).
The programme has been
managed through a series of
one- to two-year campaigns
that commenced in the 1990s.
The campaigns followed a
‘top down’ approach, dealing
with the various structures and
materials that made up the
reactor’s core.
These were principally:
• the hot box – located above
the reactor, this structure
received the gas coolant and
channelled it to four
heat exchangers
• the neutron shield –
constructed of graphite and
steel, this absorbed radiation
to protect workers on the
operating floor
• the graphite core and
reflector – 230 tonnes of
interlocking blocks of
graphite that formed a matrix
to position the fuel elements
• the diagrid and tundish –
support structures for the
graphite core, made up of
90 tonnes of steel
• pressure vessel – a cylindrical
vessel, weighing 118 tonnes,
with hemispherical ends.
These components sat within
the reactor’s pressure vessel
which was itself supported
inside a concrete bioshield (a
massive structure providing
structural integrity and
personnel protection from
radiation). The sequence of
campaigns was:
1 install remote dismantling
machine
2 remove operational waste
from the 253 fuel channels
3 dismantle the hot box
4 remove the six loop tubes
5 dismantle the neutron
shield
6 remove the graphite core
and steel restraint structure
7 dismantle the thermal shield
8 reduce the size of the lower
structures and remove them
from within the reactor’s
pressure vessel
9 remove the steel pressure
vessel which housed
the reactor core, and its
surrounding insulation
10 remove the two thermal
columns and the outer
ventilation membrane from
within the reactor vault.
Working from the control room, the
operators can use the hoist to deploy
various tools, such as grabs, oxy-propane
torches, shears and grinders.
Hoist maintenance room
8 tonne waste hoist
Retractable shield door
Sentencing cell
slew beam
Operator
viewing
position
Carousel floor
racks and baskets
Existing heat exchanger
raised by appox 12 m
SENTENCING
CELL
WASTE
ROUTE OUT
Mast
Reeling drums
REACTOR
VESSEL
This final part of the
decommissioning of WAGR
was completed in the summer
of 2011.
The Remote Dismantling
Machine (RDM) constructed
for the project has played a
vital role in dismantling WAGR.
Operated from a purpose-built
control room in a building next
to WAGR, the RDM consists of an
extendable mast supporting a
remotely controlled manipulator.
Suspended crane rails enable
a three-tonne hoist to travel
across the reactor vault into the
adjacent cells. Working from
Upper containment
Platform lift winch
Transfer hoist cable
feed system
(accumulator)
Lower containment
and entrylock
Rotating floor shield
Rotating floor
shield slew bearing
Transfer hoist slew
beam bearing
To hoist and grab
maintenance cell
Transfer hoist
slew beam
3 tonne transfer
hoist and grab
Manipulator platform
and service feeds
Manipulator
Concrete reactor
bio-shield
Diagram showing the WAGR buildings following adaptation for decommissioning. It shows the rotating Remote
Dismantling Machine mounted centrally above the reactor vessel and the manipulators and hoists used to remove the
components during dismantling. The route through which the waste is exported is also shown
the control room, the operators
can use the hoist to deploy
various tools, such as grabs,
oxy-propane torches, shears and
grinders. A hydraulic manipulator
allows these tools to move in
all directions and to reach the
full depth of the reactor vessel.
During the programme, the
decommissioning team designed
specialist tools so that the RDM
could perform specific tasks
during the various campaigns.
LESSONS LEARNT
While new nuclear power stations
will have decommissioning
‘designed-in’, the work on WAGR
has highlighted that it will still be
important to take an adaptable
approach to decommissioning.
Technology, legislation and
policy will inevitably change
over the operational life of a
nuclear reactor. For example, we
can expect to see considerable
advances in remotely operated
vehicles, mobile cranes, modular
and mobile plant and equipment,
and the ability to decontaminate
equipment. After all, there has
been plenty of progress in these
areas since WAGR was built,
allowing the decommissioning
teams to work in ways that were
not possible when the engineers
designed the reactors.
It is also important to consider
the costs of the technologies
used in decommissioning. On
this front, off-the-shelf equipment
can often prove economically
advantageous. For example, the
decommissioning operators used
commercial CCTV cameras on
WAGR. Unlike radiation-hardened
cameras. commercial cameras
may not last as long, but they are
considerably cheaper and the
total cost is lower.
Work on the WAGR also
demonstrated the value of
being able to adapt existing
systems and structures to
avoid the need to build new
structures for decommissioning.
38 INGENIA INGENIA ISSUE 48 SEPTEMBER 2011 39

LESSONS FROM WINDSCALE’S NUCLEAR LEGACY
For example, the waste was
removed from the reactor
through two of the bioshields
for the heat exchangers so that
the decommissioning team
could benefit from the shielding
concrete. To achieve this, the
team had to raise the heat
exchangers 12 m to make space
available. The operators used
diamond drilling to create the
openings into the reactor vault
to provide access for the RDM
hoist’s transport system.
A further lesson learnt has
been in significantly reducing
the number of waste packages.
Cutting the waste to appropriate
sizes and careful packing ensured
efficient use of the space in
the waste box. It also proved
beneficial to take advantage
of radioactive decay to allow
disposal as low-level waste rather
than intermediate-level waste.
The self-shielded waste package
also hugely simplified handling
and on-site storage.
LOOKING AHEAD
Having demonstrated that
it is possible to manage the
decommissioning of the Piles,
completing their clean up is now
a lower priority for the NDA than
other legacy issues at Sellafield.
The Piles can now move into
a period of surveillance and
maintenance, based on the
extensive work done on the
safety case, allowing the NDA and
Sellafield to allocate resources to
projects that have higher priority.
The decommissioning of
WAGR has successfully met
significant challenges. This work
puts the UK’s engineering sector
in a strong position to contribute
to the growing demand for
decommissioning the many
nuclear power stations that will
reach the ends of their working
lives over the coming years.
BIOGRAPHY
Peter Mann is Director PBO Support with Babcock International
Group’s nuclear business. He is a chartered engineer with
over 30 years’ experience in the nuclear industry, the last 15
of which have been directly involved with management of
decommissioning programmes, notably at Windscale where
he was head of site for some 10 years, and previously at
Hunterston A where he was Magnox site director.
The author would like to thank Michael Kenward OBE for his help in the
writing of this article.
Removal of the WAGR reactor pressure vessel top dome
INFORMATION FOR POSTERITY
An important aspect of the work on the Windscale Piles has
been in collecting and managing information. This is particularly
important given the protracted timescales involved, which can
run into decades. Babcock’s state-of-the-art computer-based
interactive tool will enable the project to retain knowledge for the
future, while aiding daily use of the documentation.
The first ‘document’ to use this approach was the Interactive
Safety and Environment Overview Report. This internal
management and communication tool sets out the strategy, or
roadmap, for the safety case and its justification. The document
makes extensive use of drawings, video and animation to illustrate
decommissioning concepts and schemes. In some 15,000 pages,
it presents information in layers, so that users can navigate the
documentation at an appropriate level of detail.
The decommissioning team used similar interactive
electronic techniques to develop the Decommissioning Safety
Case. The user can navigate the document quickly and easily,
using interactive screens. They can search the entire suite of
documents for keywords, and can view videos and animations.
The technique offers significant advantages in document
management that could have wider future application.
The UK now has a thriving nuclear decommissioning market
– the Nuclear Decommissioning Authority (NDA) has successfully
fostered an environment where many companies from the UK
and USA compete for decommissioning work. Much of the work
completed to date has intellectual proprty owned by UK and
USA government agencies and is available to all, with the private
sector organisations bringing further innovation to complete the
hazard reduction faster and at lower cost.
40 INGENIA