KNOW THY ENEMY'S WEAPONS.

Byline: David Kronke TV Critic

AS RALLYING cries to inspire a nation go, the Department of Homeland Security's new ``Don't be scared, be ready'' ranks somewhere just above ``Uncle!'' As a piece of pragmatic advice, though, it's not bad. Two new documentaries offer viewers opportunities to be both.

Homeland Security's color-coded alarm scheme has thus far proven fairly oblique in terms of how the government decides to ratchet our jittery nerves up or down. Both documentaries suggest that hysteria remains a more viable enemy on our shores than the facts, which nonetheless remain plenty troubling.

``Dirty Bomb,'' a ``Nova'' special report airing tonight, specifically defines its subject and discusses how much of a threat such a bomb poses.

The answer isn't as dire as one might think, but it's still certainly troubling, more in its ramifications than in its reality. A dirty bomb is simply a bomb that, in its simple explosion, might kill a dozen citizens - but when detonated, it might disseminate radiation to a larger area. The question then becomes, how much radiation would fall on the populace - how much more likely is cancer in those affected surroundings - and how large an area would have to be decontaminated?

Alarmist theories are what make dirty bombs so devastating, at least from a terrorist's vantage point. Tonight's ``Nova'' report asks, could terrorists get their hands on enough radioactive material to do us harm? The report suggests that even though the answer isn't all that alarming, the psychological damage would be far graver than the reality.

Tomorrow night, on the Discovery Channel's documentary ``Bioterror,'' hosted by NBC's Tom Brokaw, New York Times reporter Judith Miller confesses that during her research, ``I had trouble sleeping - I never had this problem when I was covering nuclear weapons.'' Before Sept. 11, she adds, ``You could fit everyone worrying about this threat into one small cubicle.'' Biological weapons are powerful due to how vexingly easy they are to conceal, the report notes.

Brokaw knows what he's talking about - his assistant contracted anthrax through a letter addressed to him in 2001. Contaminating victims wouldn't be difficult, the report suggests, but it would take quite a concerted effort, particularly for a widespread attack.

And, as anyone who has read ``Fast Food Nation'' already understands, our food supply may be the most vulnerable. All a terrorist needs to bring foot-and-mouth disease into the United States would fit on a Q-Tip swab. Such attacks would be fairly easily stemmed off, but the economic impact and, again, the psychological damage could be significant.

Of the two documentaries, ``Dirty Bomb'' presents information in a form that discourages hysteria slightly more articulately than Brokaw's ``Bioterror.'' The latter seems to play more like a dance mix of a simple song - an elaborate yet attenuated version on a theme, trying to wring more drama from less material.

NOVA: DIRTY BOMB - Three stars

What: Documentary on our potential next nightmare.

Where: KCET.

When: 8 tonight.

In a nutshell: Informative report explaining both the dangers and limitations of radiological attacks.

BIOTERROR: THE INVISIBLE ENEMY WITH TOM BROKAW - Two and one half stars

What: Documentary on our potential next nightmare.

Where: Discovery Channel.

When: 10 p.m. Wednesday.

In a nutshell: Explains both the dangers and limitations of biological attacks.