Radioactive sea water from Fukushima Daichi has hit the west coast of the United States
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Nuclear power generation process converts Uranium into many different radioactive elements (Argonne National Laboratory).

The Fukushima Daiichi plant has about 1744 tons of spent nuclear fuel and under 100 tons of fuel in the reactors. The irradiated nuclear fuel contains the following approximate quantities of radioactive chemicals that represent a health risk when disbursed into the environment.

Molybdenum 99

1 months danger (400 pounds)
yellow oxide slightly soluble in water

Iodine 131

2 months danger (200 pounds) potential for a few thyroid cancer deaths
90% thyroid affinity - iodized salt provides some protection
yellow readily vaporized/dissolved

Cesium 137

500 year danger (70,000 pounds) potential for a large number of deaths
liquid near room temperature; readily vaporized/dissolved
electrolyte activity - salt substitute provides some protection (potassium & sodium)

Europium 155

40 year mild risk (300 pounds)
white/pink insoluble 99% affinity for soil
0.02% affinity for liver and bone

Strontium 90

500 year danger (70,000 pounds)
red/yellow/white
10% affinity for bone - calcium supplements provide some protection

Samarium 151

1000 year mild risk (5,000 pounds)
grayish yellow 99% affinity for soil
slight affinity for liver and bone - mineral supplements provide some protection

Plutonium 239

200,000 year danger (300,000 pounds)
yellow 99.9% affinity for soil
potential for 700,000 fatalities

Krypton 85

100 year mild risk (3,000 pounds)
colorless gas - no affinity for the body outside

Nuclear materials are incredibly hazardous. Nuclear fuel is replaced in rounds of 1/3 of the total about every 1000 days.

Radioactive chemicals produce considerable heat in spent/irradiated fuel after power production is shut down. Nuclear waste cannot be buried because of heat produced by ongoing radioactive decay.

Spent fuel rods must remain submerged in water for decades.

Molybdenum - lasts a few months - does not vaporize or dissolve
Iodine - lasts half a year - carried easily by water and air - vaporizes at oven temperatures
Cesium - lasts several hundred years - dissolves in water
Plutonium - lasts millions of years - particles carried by water