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

Thorium as an Energy Source - Opportunities for Norway
turbine and, in turn, a generator to produce electricity. The steam cycle is typically 33 % efficient.
A principal layout of a PWR is shown in Figure 14.2.
Figure 14.2: Pressurized Water Reactor (PWR).
The uranium used in PWR fuel is usually enriched several percent in U-235. After enrichment
the uranium dioxide (UO2) powder is sintered to create hard, ceramic pellets. The cylindrical
pellets are then put into tubes of a corrosion-resistant zirconium metal alloy (Zircaloy). The fuel
rods are grouped in fuel assemblies, called fuel bundles, which are positioned in the reactor core.
A typical PWR has fuel assemblies consisting of 14x14 to 17x17 fuel rods, with a length of about 4
meters. A large reactor would have about 150 - 250 such assemblies with a total of 80 - 100 tonnes
of uranium.
Re-fuelling for most commercial PWRs is after 18 - 24 months where approximately one third of
the core is replaced.
Another design belonging to the light water reactor type is the Russian VVER. The reactor is of
the PWR design. The fuel is low enriched (ca. 2.4 – 4.4wt% U-235) uranium dioxide (UO2) pressed
into pellets and assembled into fuel rods.
14.2 Appendix B2: Boiling Water Reactor (BWR)
The Boiling water reactor (BWR) also belongs to the light water reactor type and is the second
most common nuclear reactor in commercial operation. Today, there are 94 BWR units generating
electric power accounting for about 21 % of nuclear reactors installed. BWRs use ordinary light
water (H2O) both as coolant and moderator. In a BWR, water is constantly fed into the bottom of
the primary vessel and then boils in the upper part of the reactor core. The steam generated, at a
pressure of 70 bars and temperature around 290˚C, is routed directly to the turbine.
A modern BWR fuel assembly comprises 74 to 100 fuel rods, and there are up to approximately
800 assemblies in a reactor core, holding up to about 140 tonnes of uranium. Fuel load and
efficiency are similar to the PWR. A schematic drawing of a BWR is shown in Figure 14.3.
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