Economics of nuclear power plants to the extent of 3.4 billion pounds. In general, coal and nuclear plants have the same types of operating costs (operations and maintenance plus fuel costs). However, nuclear has lower fuel costs but higher operating and maintenance costs. Nuclear plants require fissile fuel. Generally, the fuel used is uranium, although other materials may be used (See MOX fuel). In 2005, prices on the world market for uranium averaged US$20/lb (US$44.09/kg). On 2007-04-19, prices reached US$113/lb (US$249.12/kg). On 2008-07-02, the price had dropped to $59/lb. Fuel costs account for about 28% of a nuclear plant's operating expenses. As of 2013, half
Evolutionary history of plants billion years ago, and that communities of complex, multicellular photosynthesizing organisms existed on land in the late Precambrian, around . Evidence of the emergence of embryophyte land plants first occurs in the mid-Ordovician (~), and by the middle of the Devonian (~), many of the features recognised in land plants today were present, including roots and leaves. Late Devonian (~) free-sporing plants such as "Archaeopteris" had secondary vascular tissue that produced wood and had formed forests of tall trees. Also by late Devonian, "Elkinsia", an early seed fern, had evolved seeds. Evolutionary innovation continued into the Carboniferous and still continues