The Jules Horowitz Reactor, a new material testing reactor, is currently under construction on the CEA Cadarache site (in the South of France). A nuclear heating rate unequaled in Europe will be provided: 20 W.g −1 for a nominal thermal power of 100 MW. There is no differential calorimeter in the state of the art that measures such a nuclear heating rate. This article presents a new differential calorimeter configuration dedicated to this very high nuclear heating rate. On the one hand, the chosen configuration is validated and calibrated experimentally under laboratory conditions by using a new heating system, allowing the application of a wide range of electrical power never tested in the literature and ten times higher compared to that commonly used. Then the calorimetric cell response is estimated under real conditions with an analytical model. Finally, 3-D thermal numerical simulations of the whole calorimeter are performed under real conditions. Some parameters, such as the size of the cell, the intercell space, and the nature of the sample, are tuned to propose a very compact differential calorimeter.