POWER April 2014 - 36

NUCLEAR POWER
1. Map of the UK legacy sites. Courtesy: Magnox Ltd./Nuclear Decommissioning
Authority
of the Magnox Optimized Decommissioning
Program (MODP). Burnett confirmed that
the NDA's goal is to decommission existing
facilities and restore the sites to delicensed
conditions suitable for alternative uses, albeit
influenced by the existing level of environmental
or health risks and resources needed
to achieve a passively safe and secure state.
In the 1990s, " Safestore " was determined
to be the preferred UK strategy for decommissioning
Magnox power stations. This
entails activities carried out in a number of
steps, separated by periods of C&M.
Dose decay is initially dominated by cobalt60,
decreasing by orders of magnitude until
subjugated by longer-lived radionuclides such
as niobium-94 and silver-108m. After 135
years following reactor shutdown, there is no
further significant reduction in dose rate over
time. The physical, chemical, and mechanical
properties of the graphite core within the
vessel are not expected to change either during
storage or dismantling (see sidebar " The
Three Stages of Decommissioning " ).
Burnett said that in order to accomplish
its mission and stimulate competition, the
NDA is a single contracting organization for
implementing a national integrated strategy
consisting of an incentivized, three-tiered
contractor and supply chain management
program for conducting the cleanup:
from outside the nuclear industry are being
introduced, where feasible. Lessons learned
at Bradwell and Trawsfynydd will eventually
be applied across the entire Magnox fleet,
achieving C&M status many years earlier
than expected for the eight remaining Magnox
reactor sites: Berkeley, Chapelcross,
Dungeness A, Hinkley Point A, Hunterston
A, Oldbury, Sizewell A, and Wylfa.
Magnox Reactors
The complexity of the proposed cleanup was
amplified because most facilities, such as the
fleet of Magnox reactors, were " one-offs. "
Ongoing design modifications and construction
methods meant that each new station
was slightly more refined, and different from
the previous model.
Magnox is short for " magnesium non-oxidizing, "
from the magnesium and aluminum
alloy-clad reactor fuel rods. Magnox reactors
are pressurized, carbon dioxide-cooled,
graphite-moderated reactors that use natural
uranium as fuel and a Magnox alloy as fuel
cladding. The spent fuel could not be stored
indefinitely because of its chemical reactivity,
so it had to be reprocessed.
36
The Magnox reactor was the first of eight
small prototype units built at Calder Hall and
Chapelcross. Although Magnox reactors were
initially dual-purposed, combining power
generation with plutonium production for military
purposes, the latter function was eventually
conducted only at Sellafield, which also
handled fuel reprocessing. Subsequent units
were scaled up for electricity production.
Magnox steel reactor pressure vessels
are typically spherical, 20 meters (m) in diameter,
weigh 5,000 metric tons (mt), have
a nonremovable top, and contain 30,000 to
40,000 individual 60-centimeter fuel elements.
Fueling and defueling is conducted
through core penetrations using specialized
machines. By comparison, U.S. light-water
reactors are compact because the reactor vessels
are smaller and are designed so that the
top can be removed, giving direct access to
the reactor core, enabling all of the fuel to be
removed in a relatively short period.
Magnox Optimized
Decommissioning Program
POWER interviewed Dr. Brian Burnett, head
of programs at the NDA, for an inside account
www.powermag.com
■ Tier 1: The sites are operated by Site
Licensed Companies (SLCs, such as
Magnox Ltd.) and their Parent Body Organizations
(PBOs, for example, EnergySolutions
Inc.) under contract to the NDA.
The SLCs hold the nuclear site licenses
and are responsible for day-to-day operations
and delivery of site programs.
■ Tier 2: Tier 2 suppliers work directly with
the Tier 1 SLC and subcontract to Tier 3
and below. Services include designing and
manufacturing specialized equipment and
construction.
■ Tier 3: Lower-tier companies fill certain
niche requirements. Certain Tier 3 suppliers
have worked across multiple SLCs,
including the Sellafield, Magnox, and
Dounreay sites. Some 2,000 suppliers support
Magnox Ltd.
To maintain transparency, the NDA is accountable
to the public and the UK government.
Milestones are established to monitor progress
against clearly defined endpoints. Political, socioeconomic,
and timescale options are communicated
to a wide range of stakeholders that
can affect decision-making outcomes.
Strategic Programs
A significant benefit of the MODP was
grouping common strategic programs to efPOWER
| April 2014
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POWER April 2014

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