THORIUM AS AN ENERGY SOURCE - Opportunities for Norway ...

Thorium as an Energy Source - Opportunities for Norway
The AHWR fuel cycle will be implemented in stages – starting with the fuel cycle of “AHWR
Critical Facility” which is being built at BARC followed by a closed fuel cycle of AHWR which will
ultimately be self-sustaining with respect to U-233. The initial core will be made up entirely of
(Th, Pu-239) MOX fuel assemblies, each being made of 54 fuel pins. Successively, the U-233 bred
in the (Th, Pu-239) MOX fuel will be recovered and recycled as (Th, U-233) MOX. At equilibrium,
the core of the AHWR will consist of composite fuel assemblies each having 24 (Th, Pu-239) MOX
pins and 30 (Th, U-233) MOX pins arranged in three consecutive rings having fissile material
compositions:
1st ring (inner ring): 12 pins consisting of (Th,U-233)O2 with U-233 enrichment of 3.0wt%.
2nd ring (intermediate): 18 pins consisting of (Th,U-233)O2 with U-233 enrichment of 3.75wt%.
3rd ring (outer ring): 24 pins consisting of (Th,Pu)O2 with Pu enrichment of 3.25wt%.
The content of fissile plutonium isotopes will go down from an initial 75 % to 25 % at the
equilibrium discharge burn-up level. However, to reduce the overall inventory of the waste, it is
envisaged that thorium and U-233 will be recycled in the AHWR. Even though U-234 produced
(along with U-235 and U-236) by neutron capture in U-233 has a negative influence on reactivity,
it might be possible to recycle U-233 in AHWR with only a marginal penalty of less than 1000
MWd/tHM on discharge burn-up for each recycling.
The objective was to achieve a negative void coefficient of reactivity with light water as a coolant
[62]. The core was optimized with respect to the fissile content, the maximum attainable
reactivity, and low power peaking factors. Plutonium in the AHWR burns faster due to its large
absorption cross-section that leads to a loss in reactivity. An option is available in the AHWR to
reconstitute the fuel cluster after an averaged discharge burn-up of 24 000 MWd/tHM. Only
plutonium pins in the outer rings are replaced by fresh fuel. The rest of the fuel cluster remains
as it is. It is possible to obtain an additional burn-up of up to 20 000 MWd/tHM from the
reconstituted cluster which improves U-233 production and reduces the reprocessing load.
Reconstitution of the fuel cluster involves multiple enrichments for the (Th,Pu)O2 pins, which will
affect the fuel fabrication. However, the reconstitution improves the fuel conversion and hence
the economy of the fuel cycle.
The plutonium composition has a strong bearing on the void coefficient. Although it is possible to
achieve a negative void coefficient at the beginning of cycle, the void coefficient becomes positive
as plutonium burns. With both plutonium and U-233 as fuel, the delayed neutron fraction (βeff) is
low. The βeff for AHWR has been calculated as 2.8 mk 5 . Since a void collapse would introduce a
positive reactivity, it will be one of the design basis accidents.
The fuel cycle time of the AHWR is 8 years of which 4 years are in-reactor residence, 2 years of
cooling (to allow for >99.9 % conversion of Pa-233 to U-233), 1 year of reprocessing and 1 year for
refabrication. For the initial few years, annual reload would consist of (Th,Pu)O2 clusters only.
Part of the recovered thoria from the reprocessing plant will be recycled into the reactor
immediately by using it for the fabrication of (Th,U-233) MOX pins. The rest will be stored for 17
- 20 years (~ 10 half-life of Th-228). Then it would be similar to fresh thorium and could be used
to fabricate (Th,Pu)O2 pins.
5 The reactivity unit mk is equal to 10 −3 , where k refers to the value 1 of the multiplication factor in critical
condition.
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