Safety of tunnels at Yucca studied

Suzanne Struglinski

Thursday, Oct. 23, 2003 | 9:47 a.m.

ROCKVILLE, Md. -- The effects of rocks and dust falling inside the tunnels onto engineered barriers at the proposed Yucca Mountain repository are still being studied by Nuclear Regulatory Commission researchers, scientists told a commission panel Wednesday.

Researchers explained to the Advisory Committee on Nuclear Waste, an advisory panel for the commission, how they measure and analyze safety systems and components of the tunnel that will hold containers of high-level nuclear waste that is proposed to be stored at Yucca, 90 miles northwest of Las Vegas.

The tunnel is expected to degrade over time, scientists said, but by how much and exactly what the effects will be on the engineered barrier -- planned now to be titanium drip shields -- covering the waste containers and the containers themselves is still being calculated under various conditions. These are part of the remaining "key technical issue agreements" the Energy Department is working on submitting to the NRC.

Raj Nataraja, progam element manager, said Wednesday some parts of the repository design issue have been closed, but 23 unanswered questions on the stability of the tunnel and rockfall effects are still being worked on.

NRC researchers used examples of how they plug in different accident scenarios, such as a canister breach or failure of a crane, into formulas on a computer program to estimate consequences.

Researcher Doug Gute said the waste package response to earthquake activity over time has not yet been assessed, but noted that rock should not affect the packages because of the drip shield. But he added the department is re-evaluating the drip shield and other designs for the repository.

Researcher Goodluck Ofoegbu pointed out that in abandoned mines tunnels have collapsed underground and sinkholes developed at the surface.

The NRC researchers also pointed out that the commission staff and the department have different views on the overall magnitude of rock pieces accumulating on the drip shield and other issues in this remaining unsolved questions.

Mark Board of Bechtel SAIC, the project's main contractor, said researchers need to be careful when comparing the project to mining studies. He said rock in mines is "pushed to heavy stress levels" due to the "high extraction ratio."