Think of it as a Pandora's Box of plutonium.

That's sort of how Hanford officials feel as they prepare to peek inside an underground tank half-full of plutonium-laced, radioactive sludge that has sat untouched for 23 years.

Could flaming hydrogen gases spew out?

Or could some strange chemical reaction gurgle up something dangerous?

Or will putting a bunch of people and equipment on the ground above the tank cave it in?

A cautious first peek inside the Plutonium Finishing Plant's underground Tank 241-Z-361 is planned in about a month.

The tank popped up on Hanford's radar screen more than a year ago as a significant unsolved question - and as a radioactive baby brother to Hanford's 177 more famous, monster-sized, underground waste tanks.

It's expected to take up to a year to sample and analyze the estimated 20,000 gallons of sludge laced with an estimated 66 pounds of plutonium in Tank 241-Z-361. It probably will take several years and many millions of dollars to get it out.

"We need to move very slowly because we could be putting people at risk," said Duane Bogen, B&W Hanford Co.'s project manager for the tank.

The chained-off site of the buried tank looks rather ordinary.

Five yellow pipes stick 3 or 4 feet out of a gravel patch along the PFP complex's south fence. It is through these pipes that scientists plan to peek inside the tank.

Grids are chalked out in green on the ground around the gravel patch where a weighted robot tested the soil to see if it might cave in. So far, it hasn't.

But a 1975 photo - from a camera poked through one of the pipes - shows what may be lurking underneath.

Orangish sludge. Steel inner walls largely eaten away by acidic goop. Rough-looking concrete of uncertain strength now that another 23 years have passed.

Tank 241-Z-361 is 26 feet long, 13 feet wide and roughly 17 feet deep - about the size of two backyard swimming pools stacked atop each other.

When the PFP refined plutonium during the Cold War, its wastes went through a series of holding and sampling tanks before disposal. In Tank 241-Z-361, the solids and sludges containing plutonium settled out before the liquids were pumped to the next disposal stage.

The tank was used from 1949 to 1973. Most of its remaining liquids were pumped out in 1975 - leaving slightly less than 8 feet of sludge behind. Hanford sampled and analyzed that sludge in 1975 and 1977.

Then, the tank sat largely forgotten for 20 years. But in May 1997, a PFP chemical tank exploded on the fourth story of a building a couple hundred feet from Tank 241-Z-361.

In the explosion's aftermath, PFP workers hunted for other semiforgotten potential hazards. And they zeroed in on this tank, untouched since the mid-1970s.

No one had sufficient understanding of what was in the tank to risk opening one of the "risers" - the pipes leading from the tank to the surface - to take a look inside.

All sorts of records were hunted down and studied. About 30 former PFP workers were interviewed. Their combined memories produced a fragmented, incomplete picture, Bogen said.

Bogen and Pete Knollmeyer, the Department of Energy's assistant manager for facilities transition, said several potential hazards were identified and studied, including:

Could the plutonium in the tank go "critical" - unleashing bursts of radiation? This is considered unlikely.

Could opening a pipe leading into the tank create some dangerous chemical reaction? Again, Hanford scientists concluded this is unlikely.

Is enough hydrogen being generated by the sludge to produce potentially flammable gases? This question is unresolved.

How solid is the tank after decades of being eaten away by acidic wastes? Are wastes leaking? Will the tank collapse when people and heavy equipment are put on top of it? These questions remain largely unresolved.

The possibility exists that a bridge might be built over the site to keep the weight of any heavy equipment off the ground over the tank.

Hanford expects to open one riser in about a month to ventilate the tank and collect vapor samples.

Video cameras will be inserted through the pipes to see the tank's innards. By spring, Hanford hopes to start collecting samples of the several layers of soupy to peanut-butter-like sludges to analyze.

Sampling is expected to be done by late summer, and the analyses done by fall, at a cost of about $4.5 million.

After that, Hanford officials aren't sure how they will attack the problem; they need the analyses before they can devise a plan.

The sludge could be pumped out. Electrodes could be inserted into the tank to zap the sludge into a glass. Or the sludge could be mixed with cement.

The estimated overall price tag is uncertain. However, Hanford officials believe it will be significantly less than a $40 million worst-case scenario suggested in the spring.