Parties in a federal lawsuit asking for better protection of Hanford workers from chemical vapors met with a mediator in Seattle on Thursday.
The first round of mediation is expected to continue Friday.
The state of Washington, Hanford Challenge, and Plumbers and Steamfitters Local 598 filed suit against the Department of Energy and its tank farm contractor, Washington River Protection Solutions, in September 2015.
Plaintiffs say workers have developed serious respiratory and neurological illnesses after exposure to chemical vapors associated with waste held in underground tanks at the Hanford nuclear reservation.
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In February, U.S. Judge Thomas Rice agreed to postpone the trial until March 2018. Plaintiffs and defendants asked for the delay to allow them to pursue mediation and see if the case could be settled out of court.