[en] The technique and description of TREC and TREC6 codes for few-group two- and three-dimensional calculations of a channel type reactor performed in terms of the theory of heterogeneous reactor are given. For the solution differential transformation of equations of heterogeneous theory combined with the sine expansion of axial component of neutron flux into the Fourier series and nonlinear iteration procedure for determining the effective multiplication factor are used. The codes permit to attain the highest speed of response when calculating the series of proximate problems (reactor variant calculations; using codes as a modulus in optimization and non stationary problems)