As radiation fears from the ongoing nuclear drama escalate in Japan – and even on the West Coast of the United States – a reality check is in order.

Broadly speaking, there are two sources of radiation from the Fukushima Daiichi nuclear power plant: radioactive uranium fuel from the reactor cores and used fuel pools at the site, and radioactive particles blown into the air.

The uranium on-site poses a danger only to people within a short distance, namely the work crews. The laws of physics and geometry explain why.

Uranium fuel generates three types of radiation — alpha, beta and gamma.

Alpha radiation consists of the nucleus of helium atoms and is too slow and heavy to penetrate a sheet of paper, let alone the suits worn by the workers.

Beta radiation, composed of electrons (or, rarely, the positively charged twins of electrons, called positrons), is still too weak to bore past the skin. But beta radiation can cause severe burns, as happened to some first responders at Chernobyl. Again, radiation suits protect against it.

Gamma radiation — very similar to X-rays — is the real worry at the facility. Any exposed uranium fuel acts like “an X-ray machine that never turns off,” said John Boice, director of the International Epidemiology Institute in Rockville. Gamma rays can be stopped by enough lead and concrete.

Despite the grave peril gamma radiation poses to workers at Daiichi, that radiation decreases rapidly as one moves away. The reason is a simple rule of geometry called the inverse square law. That law means that someone standing one kilometer from a gamma radiation source will receive just one one-millionth the dose absorbed by anyone sitting on the reactor.

As for the steam and smoke seen rising from the site — the “plume” that has caused so much worry — it, too, can carry particles that emit alpha, beta and gamma radiation.

Alpha and beta radiation is only a risk if people inhale or ingest large amounts of any traveling particles. That’s why the Japanese government told residents between the 12.5-mile evacuation zone and 19 miles from the facility to stay indoors.

Gamma radiation from traveling particles could pose a health risk but only if a large enough concentration of those particles accumulate. And, as various mathematical models show, smoke and steam from the site disperses — not concentrates — as it travels.

So far, radiation readings in Tokyo and elsewhere in Japan — although increasing above normal — suggest that very little radiation has escaped the 12.5-mile evacuation zone. Not enough to pose a health risk, anyway, according to a slew of radiological health experts.

Of course, the situation is still developing, and larger releases could occur. The radioactive elements from the site stay radioactive for decades and might accumulate in the environment. Ultimately, that accumulation — first in grass, then in cow’s milk — is what caused the greatest human toll from the Chernobyl accident 25 years ago.