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

Thorium as an Energy Source - Opportunities for Norway
The HTGR fuel element manufacture process was established in Germany, KFA Jülich and
NUKEM, to make the reactor “pebbles” of about 60 mm diameter for the ATR and THTR reactors.
About 650 000 such pebbles were manufactured. The coatings on the HTGR fuel particles operate
at very nearly the same temperature as the fuel, 800 to 1300 ºC. These high temperatures
intensify effects from radiation-induced dimensional changes, fission gas pressure build-up, and
temperature-controlled diffusion processes. The effectiveness by which the fission products are
retained in the different fuels is governed by a complex interrelation between many factors
including radiation damage, swelling, chemical corrosion and composition changes at elevated
temperatures. It can be deduced that, while HTGR carbide fuel can withstand very high
temperatures for a long time and high burnup compared with LWR oxide fuel, it is much more
complex and expensive to operate.
4.3 Fuel Properties and Performance
Considerable experience has accumulated in thorium fuel fabrication. For the Fort St. Vrain
reactor 2448 hexagonal graphite fuel elements, 7.1 million fuel compacts and 26 000 kg of fissile
and fertile material in TRISO-coated fuel particles were produced. This included almost 25 000 kg
of thorium. It was irradiated at temperatures greater than 1300ºC to a maximum burnup in the
fissile particles of 16 per cent fissions in initial metal atoms (FIMA) to a maximum fast neutron
fluence of 4.5·10 25 n/m 2 with no evidence of significant coating failure. Over 50 tonnes of thorium
fuel in ceramic form, clad in Zircaloy, was also manufactured for the Shippingport LWBR core.
4.3.1 LWR Fuel Performance
A number of irradiation tests was performed in water-moderated reactors to evaluate the in-pile
behaviour of various (Th,U)O2 containing fuel rods fabricated by either pelletizing, vipac or
sphere-pac.
Fuel performance was extensively studied within the Oak Ridge National Laboratory irradiation
program which started in 1961. The initial results on the performance characteristics were found
to be favourable [38], [39]:
• In general, all thoria based fuels performed better than urania based fuels;
• Sol-gel derived (Th,U)O2 fuels meet the basic performance requirements of a nuclear fuel;
• Vibration compacted (Th,U)O2 fuel rods perform as well as those containing pressed and
sintered pellets at moderate heat ratings up to 300 W/cm and burnup levels of 40 000
MWd/tHM (Mega Watt days per tonne Heavy Metal).
The work was continued by investigating the performance of (Th,U)O2 fuel under different test
conditions such as:
• High burnups and moderate heat ratings;
• High burnups and intermediate heat ratings;
• Moderate burnups and high heat ratings;
• Processing variables affecting the irradiation performance.
Additionally, special irradiation test programs had been performed under the Light Water
Breeder Reactor Program and in the Halden Boiling Heavy Water Reactor [40] and in power
reactors [41]. The behaviour of Th-based fuel in PWRs at heat loads up to 680 W/cm and to
burnups of 80 000 MWd/tHM was studied. The Th-based fuel performed excellently compared to
standard LWR fuel.
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