To change moderator purity:

! COMPUTE DENSITY AND WGT% FOR COOLANT          
INFO: ::              TMP: <<Tcool>> K 
                      PUR: <<Pcool>> ATM% 
                      CALC DENS WATER >>Dcool<<
                      LIB: WIMSD4 FIL: IAEALB
                      ISO: 3 <<nam1H>> <<nam2D>> <<O16>> 
                      CALC WGT% D2O >>wgt1Hc<< >>wgt2Dc<< >>wgtOXc<< ;

[Add the following lines:]
! Void
EVALUATE Dcool := Dcool 0.001 * ;