The state temporarily stopped a Hanford 300 Area project Thursday after it accidentally released a bigger-than-planned amount of tritium into the air this week.

Tuesday's release was the second time in 3 1/2 months the Pacific Northwest National Laboratory project in Building 325 has released an unplanned burst of radioactive tritium. The radiation exposures to the public were tiny and posed no health threat, said Al Conklin, the state health department's manager for radioactive emissions.

The project's accumulated tritium releases in 1998 still are within the limits of its state health department permit - but barely.

The project's state permit allows releases of 379 curies of tritium radiation a year. The permit's intent is for the project to release small amounts at a time without coming close to its total limit.

The project has released 330 curies so far in 1998 - of which slightly more than 200 curies came in the two unexpected bursts.

PNNL spokesman Greg Koller said the lab still is investigating the cause of of this week's release of roughly 90 curies - much more than the five to 10 curies that were originally planned.

The 325 Building - called the Radiochemical Processing Laboratory - contains a project in which PNNL scientists extract tritium from reactor fuel rods that were irradiated at the Department of Energy's Idaho Falls site.

This is test data for DOE to analyze the fea sibility of modifying Tennessee Valley Authority reactors to produce tritium to boost the explosive power of atomic bombs.

The tritium is a gas that is caught in a special trap. The tritium releases into the air are supposed to be in the 5- to 10-curie range.

But on Aug. 26, a broken seal on the equipment sent 118 curies into the air. PNNL notified the state, then fixed the problem.

But Tuesday, another 80 curies were released with another 10 escaping Wednesday before everything was sealed off, Koller said.

PNNL and the state health department don't known yet if the cause this time was an equipment failure or human error, Conklin and Koller said.

However, the state issued a stop work order on the tritium project until the problem is tracked down and fixed, they said.

PNNL "has been very cooperative," Conklin said.

In measuring the potential human impact, lab officials calculated the potential radiation dose someone standing at the nearest public spot to the lab could have received.

That figure came out to 0.1 millirem, Koller said. The average Tri-Citian receives about 560 millirem annually from natural background radiation, according to PNNL.

Tritium is one of the less dangerous radioactive substances, Conklin said.

PNNL is required to send a report to the state by Dec. 18 on the incident's cause and proposed corrective actions.