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4.3. HEAVY-WATER REACTORS
Y. I. Chang
Argonne National Laboratory
Due to the low neutron absorption cross section of deuterium, reactors utilizing heavy
water as the moderator theoretically can attain higher conversion ratios than reactors using
other moderators. As a practical matter, however, differences in the neutron absorption in
the structural materials and fission products in the different reactor types make the con-
version efficiency more dependent on reactor design than on moderator type. In the study
reported here, a current-generation 1200-MWe CANDU design was chosen as the model for ex-
amining the effects of various fuel cycle options, including the denatured 233U cycle, on
heavy-water-moderated reactors.
The CANDU design differs from the LWR design primarily in three areas: its reference
fuel is natural uranium rather than enriched uranium; its coolant and moderator are separated
by a pressure tube; and its fuel management scheme employs continuous on-line refueling
rather than periodic refueling.
economy was stressed, trying in effect to take maximum advantage of the D20 properties.
on-line refueling scheme was introduced to minimize the excess reactivity requirements.
Unlike in most other reactor systems, in the natural-uranium D,O system the payoff in re-
ducing parasitic absorption and excess reactivity requirements is direct and substantial in
the amount of burnup achievable. These same considerations also make the CANDU an efficient
converter when the natural uranium restriction is removed and/or fueling schemes based on
recycle materials are introduced.
In the development of the CANDU reactor concept, neutron
Penalties associated with the improved neutron economy in the natural-uranium-
fueled CANDU include a large inventory of the moderator (the D20 being a significant por-
tion of the plant capital cost), a large fuel mass flow through the fuel cycle and a loher
thermal efficiency. In enriched fuel cycles, with the reactivity constraint reliioved, the
CANDU design can be reoptimized for the prevailing econonic and resource conditions.
The reoptimization of the current CANCU design involves tradeoffs between economic
considerations and the neutron economy (and hence the fuel utilization). For example,
the D20 inventory can be reduced by a snaller lattice pitch, but this results in a poorer
fuel utilization. Also, the lattice pitch is constrained by the practical limitations
placed on it by the refufling machine operations.
The fuel mass flow rate (and hence the fabrication/reprocessing costs) can be re-
duced by increasing the discharge burnup, but the increased burnup also results in a poorer
fuel utilization. In addition, the burnup has an impact on the fuel irradiation perform-
ance reliability. The fuel failure rate is a strong function of the burnup history, and
The