MD11 CREW DIVERTED WHEN INTL RELIEF CREW WAS UNABLE TO OBTAIN PROPER REST.

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I WAS DISPATCHER AT ACR AB ATL-NRT. 13 HRS 17 MINS TIME ENRTE VIA NORTHERN CANADA; ALASKA; RUSSIAN; FAR EAST. 2 FULL CREW (2 CAPT; 2 FO). AT XX16Z (ABOUT 4 1/2 HRS INTO FLT) I WAS ADVISED BY ACARS (DATA LINK) THAT RELIEF CREW GOT NO REST ON BREAK; THAT CREW WAS UNFIT; AND REQUESTED FLT PLAN CALHO (NEXT FIX) TO PDX. I RESPONDED VIA ACARS; REQUESTING DETAILS AND FOR VOICE CONTACT EITHER VIA SATCOM OR HF. CREW CALLED AT XX43Z ADVISING THAT THEY WERE PROCEEDING TO PDX FOR CREW CHANGE AND THAT IT WAS UNSAFE TO CONTINUE AND THEY HAD ALREADY OBTAINED CLRNC TO DO SO. I ADVISED CAPT THAT BASED ON HIS DECLARATION OF UNSAFE TO CONTINUE AND HIS DECISION TO DIVERT WITHOUT CONSULTATION WITH DISPATCHER; CONSIDERING THAT I HAD NO OPPORTUNITY TO EVALUATE PROPOSED RTE; DEST; AND ALTERNATE; INCLUDING APPROPRIATE NOTAMS; THAT I CONSIDERED THAT HE WAS PROCEEDING UNDER HIS EMER AUTH. DURING THE PERIOD BTWN XX16Z AND XX43Z; I ALSO CONSIDERED THE POSSIBILITY OF A SECURITY THREAT DUE TO LACK OF FURTHER INFO. I FELT THAT THE CREW MAY BE ACTING UNDER DURESS. I MADE THE APPROPRIATE CODED INQUIRY AND RECEIVED AN APPROPRIATE RESPONSE INDICATING NO THREAT EXISTED.