Question 17.7: The average temperature of the fuel in a TRIGA reactor immed......
The average temperature of the fuel in a TRIGA reactor immediately increases by 300°C after a control rod is withdrawn from the core. If the reactor is exactly critical at the time the rod is withdrawn (ρ = 0), how much negative reactivity is
introduced into the core through the process of temperature feedback? Assume the UZrH fuel contains enriched uranium (i.e., U-235). How long do you expect the power peak to last?
Earlier in the chapter, we mentioned that the fuel temperature coefficient in a TRIGA reactor is about minus one dollar per every 100°C. Hence, if the fuel temperature is immediately raised by 300°C, the reactivity must fall by 3 dollars. Since a TRIGA reactor is fueled with U-235, the value of β required to achieve a prompt critical condition is 0.0065. This translates to a total negative reactivity insertion of −3 × 0.0065 = −0.0195 (−3.0β). According to our previous discussion, the power pulse should last about 20 milliseconds (0.020 seconds).