Fuel assembly

A nuclear reactor comprising:

• a pressurized heavy water reactor;
• and
• a fuel assembly disposed in the pressurized heavy water reactor, the fuel assembly comprising:
• a plurality of elongated fuel elements mounted to each other, each of the plurality of fuel elements having a spirally twisted, multi-lobed profile that defines a plurality of spiral ribs, each of the plurality of fuel elements comprising:
• a fuel kernel comprising fuel material disposed in a matrix of metal non-fuel material, the fuel material comprising fissile material, and
• a cladding surrounding the fuel kernel,
• wherein a moderator:
• fuel ratio in a region of the fuel elements is 2.4 or less, the moderator:
• fuel ratio being an area ratio within a cross-section that is perpendicular to longitudinal axes of the plurality of fuel elements and extends through the plurality of fuel elements, the area ratio is a ratio of:
• (1) a total area available for moderator flow for the plurality of fuel elements to (2) a total area of the fuel kernels of the plurality of fuel elements.