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ACR-1000 to ensure minimum damage
of the fuel during normal operation, and
during accident scenarios
Reference fuel for the ACR-1000
is the 43-element CANFLEX-ACR
(CANDU FLEXible) bundle, which
incorporates 42 elements with 11.5 mm
OD, 2.4% enriched LEU and one 20-mmdiameter
central element with burnable
neutron absorbers (BNA). Sheath material
is Zircaloy-4.
ACR-1000 fuel acceptance criteria
for normal operation were used to
systematically evaluate any potential
damage mechanisms that could affect
fuel robustness. This ensures that fuel
cannot be damaged in fulfilling design
requirements for normal operation.
Design changes, listed below, help to
minimize fuel damage during normal
operation and accidents:
• More highly subdivided 43-element
CANFLEX-ACR fuel bundle,
lowering fuel element ratings and
reducing the power-related damage
mechanisms
• Fuel pellet geometry optimized to
minimize sheath strains and fission
gas pressure
• CANLUB interlayer thickness
increased to improve resistance to
damage due to power ramp failures
• Fuel sheath thickness defined to
maintain its intrinsic collapsibility
• Fuel bundle endplate geometry
modified to improve irradiated fuel
bundle strength during refuelling
operations
• Use of CANFLEX-ACR fuel
bundle with AECL’s patented flowenhancing
sheath appendages,
providing increased margin to dryout
in postulated accident conditions
• Central fuel element containing
BNAs to control the coolant void
reactivity, thus minimizing potential
for fuel damage in the case of a
postulated large-break loss-ofcoolant
accident (LOCA)
5. How does ACR-1000 minimize
damage to the fuel in case of a loss-ofcoolant
accident
The ACR-1000 design has
incorporated some new features to
minimize fuel damage that might occur
during a postulated large-break Loss-of-
Coolant Accident:
• Reduced core lattice pitch (distance
between the fuel channels), reducing
the coolant void reactivity (CVR)
during a postulated large-break
LOCA
• Increased calandria-tube diameter,
resulting in reduced moderatorto-fuel
ratio, which reduces the
moderator volume and, hence,
reduces the CVR
• Enhanced fuel design, with the centre
element containing zirconia with
BNAs, further reducing the CVR
All of the above features combine
to give a small negative CVR value for
nominal end-of-life conditions, such that
the power transient during a large-break
LOCA is benign.
Changes to the fuel design make the
fuel less susceptible to failure during a
LOCA. As above (Question 4), the more
subdivided CANFLEX-ACR fuel bundle
lowers fuel element ratings and reduces
the power-related damage mechanisms
while fuel pellet geometry minimizes
sheath strains and fission-gas pressure,
ACR-1000 Four Unit Layout
reducing the likelihood of fuel failures
during power transients.
Finally, the ACR-1000 design has
retained the two independent fast-acting
reactor shutdown systems, which are the
well-established means of limiting the
reactivity transient during a postulated
large-break LOCA in traditional CANDU
reactors. As a result of all of these
enhancements, calculations show that
during a postulated large-break LOCA,
there will be no fuel failures in the ACR-
1000 reactor design.
6. What innovative fuel cycles have
been used in ACR-1000 to maximize fuel
effi ciency and to minimize concerns of
proliferation
The reference fuel for the ACR-
1000 has a uniform 2.4% enrichment.
The ACR-1000 uses the advanced
CANFLEX ® fuel bundle, developed
as the optimal carrier for CANDU
advanced fuel cycles. Development is
underway to increase enrichment and
burnup, to further improve economics.
In addition, Recovered Uranium (RU)
from conventional reprocessing can be
burned efficiently in the ACR-1000, with
the addition of fissile LEU or plutonium
(Pu). The reactor can operate with a
full core of 2.4% LEU, or with RU plus
fissile to 2.4% Heavy Element (HE). The
on-power refuelling capability permits
switching back and forth between the two
fuel types, without any hardware changes
to the safety/control systems.
Additionally, spent ACR-1000 fuel
with a residual fissile content of about
1%, opens the possibility of its re-use
in existing CANDU reactors. The ACR-
1000 is also amenable to thorium fuel
cycles. The simplest case, feasible in the
short term, is the Once-Through Cycle
(OTT). This is easy to implement, with
no reprocessing required, to achieve a
burnup of about 21,000 MWd/TeHE.
This cycle also creates a “reservoir” of
Uranium-233 (233U) for future use. In
the longer term, a closed-cycle option
offers burnups to 40,000 MWd/TeHE.
Spent fuel is reprocessed to recycle 233U,
and burnup can be tailored by adding Pu
to fresh bundles.
Proliferation-resistance results from
a combination of technical design features,
operational modalities, institutional
arrangements and safeguards measures.
In CANDU technology, these features are
strongly linked and self-enforced, with
the result that their combination is greater
than the sum of the parts. CANDU technology
has always incorporated intrinsic
proliferation-resistance features—derived
from the fundamental physics of naturaluranium
or LEU-fuelled reactors.
While these inherent barriers
minimize the attractiveness of CANDU
technology as a target for proliferation,
external measures provide verification
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