The determination of the crack resistance of the protective shell of a nuclear reactor under thermal fourced load has been made in the article. The reactor design under consideration is used on several NPPs in Ukraine. The case of the presence of an initial semi-elliptic cracks in the reactor's protective shell is considered. The thermal fourced load is comprised of internal pressure and temperature load during emergency cooling. In order to assess the crack resistance of the reactor, in accordance with the recommendations of the IAEA, the stress intensity factor (SIF) is used. To substantiate the reliability, the definition of the CIF is carried out by energy and direct methods. The CIF determination by the energy method is performed on the basis of the calculation of the invariant integral J*. The CIF determination by a direct method is based on a known distribution of displacements and stresses in the vicinity of the crack top. The solution of the problem is accomplished by finite element method (FEM). For the calculation of invariant J * integrals in FEM discrete models, a reaction method is used which showed high efficiency for a wide range of problems. Definition of the CIF by a direct method is carried out in the special neighborhood of crzck top. When solving the problem, a discrete model of a fragment of a reactor with a semi-elliptic crack with a relation to the elliptic semiaxes of 0.67 and 0.33 is considered. The configuration and size of the crack are taken in accordance with the existing IAEA recommendations for reactors. The graphs of the distribution of stresses, displacements and CIF along the front of the semi-elliptic crack are presented. CIF acquires it’s the highest values at the point most remote from the inner surface of the reactor's protective shell. It is shown that taking into account temperature loads significantly increases the value of CIF compared with the calculation only from the effect of internal pressure. It was found that a crack with a half-bonded ellipse of 0.33 is more dangerous than 0.67.