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Management of Commercially Generated Radioactive Waste - U.S. ...

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Draft p. 3.6.4<br />

Issue<br />

360<br />

ALTERNATIVE DISPOSAL CONCEPTS<br />

A statement is made regarding waste disposal in trenches: ". . a plate being sub-<br />

ducted would have moved only tens <strong>of</strong> kilometers during that time (250 to 500 thousand years)<br />

and would not be subducted fast enough for waste diposal purposes." This conclusion does<br />

not follow from the discussion proceeding it in the same paragraph.<br />

a. How far would the waste have to move during that time to be subducted fast enough<br />

for waste disposal purposes? Reference?<br />

b. What might the impact be <strong>of</strong> the waste not being subducted fast enough? (208-NRC)<br />

Response<br />

In general terms the first several hundred years are the most hazardous. During that<br />

period, the plate would have moved less than 200 m, which for practical purposes means it<br />

has not moved at all. The water column in a trench is dynamic and unpredictable, and con-<br />

tains many complex processes such as turbidity currents and landslides in addition to those<br />

we know <strong>of</strong> in the.open ocean. In other words, the most unpredictable regimes in the oceans<br />

are the trenches, and for that reason have been removed from consideration for the purpose<br />

<strong>of</strong> the Statement. Responding to the specific comments:<br />

a. The waste would have to be moved on the order <strong>of</strong> 100's <strong>of</strong> kilometers in order to<br />

be subducted.<br />

b. Should the waste not be subducted fast enough, it could become part <strong>of</strong> the over-<br />

riding plate and end up on the continental slope.<br />

Draft p. 3.6.4<br />

Issue<br />

"The time needed to contain the waste (250 to 500 thousand years)," shows again a lack<br />

<strong>of</strong> understanding <strong>of</strong> the role <strong>of</strong> containment and isolation." (154)<br />

Response<br />

To establish a "target" time <strong>of</strong> containment and isolation, the longest half-life <strong>of</strong><br />

the waste constituents is chosen--that <strong>of</strong> Pu (25,000 yrs)--and multiplied by 10, which<br />

yields 250,000 to 500,000 years. The waste will contain many kinds <strong>of</strong> nuclides with widely<br />

varying half-lives, and ultimately DOE intends to separately address each nuclide,.its<br />

half-life, and its necessary containment time to meet the pertinent criteria.

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