Variabla inloppsstrypningar i kokvattenreaktorer

Sammanfattning: In a nuclear reactor core there are fuel assemblies with different age (i.e. how long the assembly has been in the core) and burnup (i.e. how much energy that has been extracted from the assembly). The fresh assemblies have higher reactivity, which means they generate more thermal power, and therefore need a higher coolant flow rate than the older assemblies. In order to redistribute the coolant flow from the older assemblies to the fresher assemblies, the core can be divided into different throttling zones. The older assemblies are placed in the periphery of the core which has higher throttling, while the fresher assemblies are placed in the central parts of the core which has lower throttling. This type of throttling is done in the core inlet. However, there is also an opportunity to throttle the flow into each separate fuel assembly, which is done in the bottom nozzle of the assembly. This type of throttling is usually constant during the whole time the assembly is in the core and is specific for the assembly type, which means that it does not contribute to the redistribution of coolant flow from older to fresher assemblies. In this project, so called variable throttling was studied. This means that the throttling of each assembly is increased after the assembly has been in the reactor core for a specific time (e.g. one or two years) and the need of coolant flow hence has decreased. By doing this, while also decreasing the total coolant flow rate through the core, the amount of void (steam) in the core increases but enough coolant flow is supplied to the fresher assemblies due to the increased redistribution of the flow. An increased amount of void leads to a shift of the neutron spectrum to higher energies (“hardening” of the spectrum), which in turn leads to better breeding. Better breeding means that more fissile material, mainly Pu-239, is produced in the core during operation. A lower enrichment can therefore possibly be used, which would reduce the fuel costs. The results show that, by implementing variable throttling, the breeding is improved and the relevant safety requirements are also fulfilled. According to the calculations, the enrichment in the fresh fuel could possibly be lowered with up to 0.068 weight percent, which means that the fuel costs would be reduced with up to approximately 8 MSEK per fuel cycle.