Abstract: With its passive characteristics, anti-single point failure characteristics, and simple system, the heat pipe reactor has become a hot research topic in advanced nuclear power. The loading criticality is an essential and important link before the heat pipe reactor is put into operation. Different from the traditional modular design of heat pipe fuel element, the new heat pipe reactor uses bulk fuel, and the traditional extrapolation fuel element/assembly and other extrapolation criticality experimental techniques have poor adaptability. Different from the KRUSTY reactor, the new heat pipe reactor has four side reflectors, and simultaneously lifting four side reflectors does not meet the regulations, which makes the new heat pipe reactor face safety challenges in the process of loading and extrapolation criticality. Therefore, combined with the characteristics of the reactor, the scheme of assembling bulk fuel was firstly determined, and gradually lifting the side reflectors to reach the criticality. Because of the large reactivity of bulk fuel, the core assembly process may face critical safety risks. To solve this problem, the critical safety in the core assembly process was calculated and analyzed by using Monte Carlo calculation program, and the safe and controllable scheme was determined. In the extrapolation process, the neutron fluence rate calculated by the detector is very important, and the size of the new heat pipe reactor is small, and the space available for the arrangement of the neutron source and the detector is limited. At the same time, the new heat pipe reactor is a fast reactor, and the neutron calculated by the detector is a thermal neutron. In order to solve the statistical influence of the neutron source and the detector on the neutron fluence rate in the extrapolation process, the Monte Carlo calculation program was used to determine the arrangement of the neutron source and the detector as well as the thickness of the detector cladding. In order to meet the principle of half addition, the new heat pipe reactor was proposed to use the extrapolation of the side reflector volume to reach the critical point. However, the direct extrapolation of the side reflector volume caused the convex problem on the extrapolation curve. The extrapolation criticality experimental technology was studied, and a modified method was proposed to weight the volume by taking the reactivity value of the side reflector as the weight factor. The extrapolation process was simulated by using this technology, and the feasibility was verified, which provides an important guidance for the loading and extrapolation criticality of the new heat pipe reactor.