MLG ON SLOW TAXI AFTER LNDG HAD LOSS OF ACFT CTL ON ICE AND SNOW COVERED TXWY. ENTERS CLOSED RWY.

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I WAS THE FO AND THE PF. LANDED ON RWY 6R AT ANC FROM THE ILS APCH. BRAKING ACTION WAS RPTED ON ATIS AS MU 33; 43; 42 FOR TOUCHDOWN; MID; AND ROLLOUT (FAIR; GOOD; FAIR TO GOOD BRAKING). RWY 6L WAS CLOSED. TOUCHDOWN AND LNDG ROLLOUT WERE NORMAL AND UNEVENTFUL. CTL OF THE ACFT WAS TRANSFERRED TO THE CAPT WHEN THE ACFT HAD SLOWED TO TAXI SPD; AT WHICH POINT I COMMENTED TO HIM THE ANTISKID WAS RELEASING SLIGHTLY DURING BRAKING. TWR INSTRUCTED US TO EXIT 6R AT D; CROSS 6L; AND CONTACT GND. EXITING RWY 6R AT D WAS UNEVENTFUL UNTIL WE BEGAN TO CROSS RWY 6L. AT THAT POINT THE L TURN FOR TXWY D TIGHTENS UP TO EXIT RWY 6L. AT THIS POINT; THE CAPT STATED HE COULDN'T TURN THE ACFT TO THE L; NOR WAS HE ABLE TO STOP STRAIGHT AHEAD. HE WAS ABLE; HOWEVER; TO MAKE A R TURN AND CONTINUE DOWN RWY 6L (HEADED NE). I CONTACTED TWR IMMEDIATELY AND ATC (TWR CTLR) RESPONDED BY STATING THERE WAS NO PROBLEM AND TO EXIT AT 6L AT TXWY C; WHICH WE DID. I DID RPT THAT RWY 6L WAS VERY SLICK AND THE CTLR STATED THAT THE GND PERSONNEL WERE 'WORKING ON IT' AT THIS TIME. A SLOWER TAXI SPD (OURS WAS VERY SLOW) AND A BETTER SURFACE WOULD HAVE ASSISTED IN THIS OCCURRENCE.