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Case Study - Fire Damaged Reactors
 In
this section we will be looking at a case where
a nuclear reactor core was damaged by fire. We
will be taking a brief look at the cause of the
fire and then investigating the various difficulties
encountered, as well as methods and options being
implemented during the decommissioning process
in this difficult case. This has been divided
into the following categories:
Background
- 1946 - Britain still had considerable worldwide
defense responsibilities & commitments,
thus they needed an independent deterrent.
- 1947 - Work began on 2 reactors at Windscale
Cumbria UK (Pile 1 and Pile 2). The purpose
of these was for the production of weapons-grade
plutonium.
- 1950 - Pile 1 went critical in October.
- 1951 - Pile 2 went critical in June.
- 1952 - The first plutonium produced by midyear.
- Figure 1 shows the pile from above at this
time.
Problem
- Wigner growth in the graphite moderator was
known of at the time (and design taking that
into account), however the phenomenon of Wigner
energy was not known.
- What is Wigner energy? It is an increase in
potential energy due to the displacement of
atoms in the lattice when bombarded by neutrons.
- How to avoid this problem: The stored energy
in the graphite moderator can be released by
annealing.
- What if annealing not carried out? A spontaneous
and potentially dangerous release of heat can
occur. 1952 - Such a release of energy was first
seen in Pile 2 in May when an unexpected temperature
rise was seen in the core. Soon after a similar
event occurred in Pile 1 while the reactor was
shut down.
- The Wigner energy phenomenon was finally recognized
and deliberate annealing was periodically conducted
on each reactor from this time till October
1957.
- 1957 - The period between anneals was extended
from 30,000 to 40,000 MWdays.
Disaster
- 1957 - October 7, an anneal had begun on Pile
1 but a fire started in the heart of the core
which was not extinguished until October 11,
four days later.
- The fire showed that there were very serious
flaws in the design of air-cooled reactors,
both from a technical and safety point of view.
- Both Piles have been shut down since the accident
and no reactors of this design have ever been
built again.
POCO
- 1958-1961 - The Post Operational Clean Out
(POCO).
- The POCO included:
- Insertion of control & shutoff rods,
none of which pass through the fire-affected
zone (FAZ)
- Dismantling & removal of operating
gear
- Sealing of the outlet air ducts connecting
the core space to the chimney
- Provision of ventilation plant to provide
containment via depression
- Removal of the fuel cartridges from the
air outlet and outlet ducts; Isolation of
the water duct from the cooling pond; Clean
out, draining and sealing of the water duct
The In Between Years
- 1961-1990 - There was not too much that could
be done on the core due to the current technology
of the time.
- Since the current critical state of the core
was unknown periodic core surveys were carried
out.
- These were to determine if any neutron poisons
needed to be injected into the core prior to
dismantling operations.
- There is also the problem of possible voids
within the FAZ, which could cause a collapse
during dismantling.
Phase 1 - Remote Control
- Another pass-time during the "in-between
years" included planning, design and construction.
- Planning - Obviously involves figuring out
- "How the hell are we gonna pull apart
this wreak?" - and without killing anyone?
- As one would expect, the high background radiation
levels vastly restricted significant human access
to the worksite.
- Design & Construction - The solution to
this problem was to build a dedicated remote
operated vehicle (ROV), in fact they made a
few of them!
- Introducing:
- Below Left - The Norman submersible ROV.
- Below Right - The Cyclops ROV (used for
inlet air duct fuel and debris collection).
- Both these ROVs were instrumental in phase
1 of decommissioning Pile 1. The ROV's were
used in various ways. For example:
- Repairing and creating watertight seals
in the ducts
- Removing radioactive sludge and lose contaminated
particles
- Removing obstacles for other ROV's
- And also for freeing ROV's that got stuck!
- Basically the ROV's did all that was not accessible
by humans in some way or another.
- A further in-depth report on how they were
used can be found at this website:
- http://www.ukaea.org.uk/windscale/nearticle.htm
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Phase 2 - Core Dismantling
- UKAEA carried out optioneering studies to
examine a number of commercial proposals to
decide how to carry out the work of dismantling
the core.
- Some of these options included carrying out
the work:
- In air;
- Under water;
- In an Argon atmosphere.
- An Argon inerting solution was eventually
decided upon as it was determined the there
should be no chance of fire in the core during
dismantling.
- At the time the article used was written preparation
work was being carried out to allow core dismantling.
- This can be split into 3 categories:
- Material and structures peripheral to
the core;
- Undamaged graphite blocks and fuel from
the core;
- Fire damaged zone potentially fused and
disordered.
- It has been decided that they intend to dismantle
the core from both the inlet air and water duct.
- A transfer tunnel is being constructed to
allow waste to be moved from the water duct
to the inlet air duct.
- The use of manipulators deployed on vertical
masts in the inlet air and water ducts to dismantle
the core was closely examined.
- This was finally rejected on operability and
ALARA grounds.
- Thus as with phase 1, all processes will have
to be carried out by ROV with CCTV feedback.
- Core dismantling will proceed from top to
bottom using the ROV's equipping a variety of
tools and placing the waste in skips for transfer
to the WPF.
Predicted Waste
- All materials within the core are assumed
to be waste.
- Predicted waste includes:
- Graphite 2 x 106 kg;
- Stainless steel 11 x 103 kg
- Isotope cartridges 2 x 103 kg;
- Aluminium 7 x 103 kg
- Uranium 15 x 103 kg;
- Cadmium 0.6 x 103 kg
- Mild steel 140 x 103 kg
- Cast iron 4 x 104 kg
- The exception to this of course is the
undamaged fuel cartridges, as these are
reprocessible.
Currently And Beyond
- At this point in time phase 2 of the Pile
1 decommissioning process is underway.
- Final decommissioning and dismantling is expected
to be completed sometime during the year 2015.
- Below is an artist's depiction of Pile 1 now
(Top) compared to the year 2050 (Bottom).
References
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