Because of the higher Coulomb barrier, the temperatures required for + fusion are much higher than those of conventional D-T fusion. Moreover, since both reactants need to be mixed together to fuse, reactions between nuclei of the same reactant will occur, and the D-D reaction ( + ) does produce a neutron. Reaction rates vary with temperature, but the D- reaction rate is never greater than 3.56 times the D-D reaction rate (see graph). Therefore, fusion using D- fuel at the right temperature and a D-lean fuel mixture, can produce a much lower neutron flux than D-T fusion, but is not clean, negating some of its main attraction.