Measuring of High Neutron Fluence Rate by Reversible Specific Heat Capacity Method

China Advanced Research Reactor (CARR) is a high-performance, multipurpose, safe and reliable nuclear reactor device developed by China Institute of Atomic Energy. Its highest undisturbed thermal neutron fluence rate reaches 8X1014 cm-2s-1 and main technical indicators are among the top in the world. In this paper, the reversible specific heat capacity method was used for the measurement of neutron fluence rate of CARR. It is always difficult to measure high neutron fluence rate directly. At present, the basic methods for measuring neutron fluence rate in world are fission track method and 4^(3-7 coincidence absorption method. The two methods for measuring neutron fluence rate have their own advantages and disadvantages, and measuring ranges are also different. However, the upper limit of measurement for the two methods generally does not exceed 108 cm-2 s-1. Therefore, they are not suitable to measuring high neutron fluence rate directly. The linear extrapolation method is generally used to measure the high neutron fluence rate now. Firstly, the absolute value of neutron fluence rate corresponding to low power is measured by neutron activation method. And then the high neutron fluence rate under high power is extrapolated according to the linear relationship between neutron fluence rate and power. With the rapid development of thermal analysis instruments and measurement technology, it is possible to measure the high neutron fluence rate by measuring the thermal properties of neutron irradiated materials. Since the measuring range of the thermal analysis instrument has no upper limit, the more radiation the sample is exposed to, the more obvious the change in thermal properties of the material is, and the more accurate the measurement results arc. Therefore, thermal analysis method has great advantages in measuring high neutron fluence rate. In the previous research, based on differential scanning calorimetry (DSC) method, the authors of this paper obtained the neutron fluence rate of the reactor by measuring the annealing heat of boron-doped materials after neutron irradiation. In this paper, the DSC method was improved. Firstly, modulated differential scanning calorimetry (MDSC) method was used for measure the change of specific heat capacity of irradiated samples. Secondly, the neutron fluence in the samples were obtained based on the linear relationship between the neutron fluence and the change of specific heat capacity of the sample. Finally, the neutron fluence rate of the reactor was calculated in combination with other parameters such as irradiation time and sample mass. The measured results are about 3% consistent with the reference value, which prove that the reversible specific heat capacity method can directly measure the neutron fluence rate. This paper makes a beneficial attempt to improve the measurement of high neutron fluence rate. © 2023 Atomic Energy Press. All rights reserved.