Temperature Reactivity Effects in Pebbles of a High-Temperature Reactor Fueled with Reactor-Grade Plutonium

This work presents the temperature reactivity effects occurring in pebbles of a high-temperature reactor fueled with reactor-grade plutonium, without any additional resonance absorbers. Burnup calculations are performed for pebbles loaded with various amounts of plutonium per pebble. During burnup, branching calculations are carried out to calculate k as a function of the uniform temperature. For a high plutonium mass per pebble and low burnup values, k decreases with uniform temperature, which indicates a negative uniform temperature coefficient of reactivity (UTC). However, for a low plutonium mass per pebble, as well as for a high plutonium mass per pebble in combination with high burnup, k is maximal at a particular uniform temperature. Below this temperature, the UTC is positive, while above this temperature it is negative. Branching calculations with only a varying moderator temperature show almost the same behavior, which indicates that the contribution of the fuel temperature plays a minor role for the mentioned effect. To understand the reactivity effect, the moderator temperature coefficient (MTC) is investigated by two methods. In the first method, changes in reaction rates of individual nuclides and their corresponding contributions to the MTC are calculated, whereas in the second method the four-factor formula has been used. For the fresh fuel cases, the positive coefficient at low temperature is due to a positive coefficient for the thermal utilization factor. For high moderator temperatures, the coefficient for the resonance escape probability renders the MTC negative. This trend is basically caused by the shift of the high-energy tail of the Maxwell-spectrum toward the 1-eV resonance of ²⁴⁰Pu, and toward the resonances of ²⁴¹Pu, which leads to an increase of the capture-to-fission ratio.