High-temperature reactors core cross section

Figure 15.5 High-temperature reactor core cross section. S/D, Shut-down.

A proper analysis of the tine dependent behavior of.a reactor operating on thermal neutrons must take into account the important effects on its criticality, reactivity, and stability which arise from such factors as fission i products of high thermal-neutron capture cross-section, depletion, temperature, average neutron lifetime in the reactor, flux level, and reactor period. As has been seen in the requirements placed on the.reactor, considerable excess reactivity must be built into the active core before start-up. The control rods must keep the reactivity below the critical value before and during start-up. 

C (n,y) C Due to the large graphite inventories in the cores of high-temperature reactors, this reaction is the main source of C production there. In LWR fuels, however, this nuclear reaction is of little significance because of the low carbon content in the fuel (<10 ppm), the low isotopic abundance of C of 1.1%, and its small thermal neutron capture cross section of about 1 10 cm. ... 

This ensures that while the volumes of components in the core are changing, the mass of each material remains constant. Finally, the cross sections of the materials being used in the core were corrected to those at operating temperature The vast majority of the materials in the core do not have readily available cross sections at the desired temperature. The NJoy code was used to generate cross sections for the different materials in the reactor [MacFarlane, 1994]. The temperatures used were the peak values from the FEPSIM model. These temperatures are probably somewhat high, but are a much better approximation than using room temperature cross sections. This run resulted in a k-effective of 1.022 0.001. The net reactivity swing of the system was 2. With a total excess reactivity of 5.7 this leaves another 3.7 for bumup and other losses. 

---