High breeding with light water cooling is not easy to be achieved. The main obstacle is the moderating effect of light water, which softens the neutron spectrum. Decreasing the volume ratio of coolant to fuel is normally introduced as a way to harden the neutron spectrum and achieve breeding with light water cooling. Therefore, the tight-lattice assembly was proposed to design reactors cooled by light water with hard neutron spectrum. However, most of them were High Conversion LWRs and none achieved high breeding to meet the growth rate of energy demand in advanced countries.
Tightly packed fuel assembly is designed for the purpose of high breeding. The number ratio of hydrogen atoms to heavy metal atoms (H/HM) in this assembly is significantly reduced to less than 0.1 which is about 1/6 of that of Reduced-Moderation Water Reactor (RMWR). Super Fast Breeding Reactor (Super FBR) is one kind of Supercritical Light Water Cooled Reactors (Super LWRs), which adopts these assemblies, obtaining high breeding of CSDT (less than 50 years).
The high breeding performance of Super FBR indicates that, application of the tightly packed fuel assembly on conventional LWR-type reactors, such as BWR-type or PWR-type reactor, may also be effective in achieving high breeding. Compared with Super FBR, the conventional LWR-type reactors with technologies which are currently in use are expected to be easier to implement. When comparing the two main LWR types, BWR-type and PWR-type, BWR-type gains more advantages on breeding, since the coolant is boiling water that generates larger amount of void in the reactor core, leading to a harder neutron spectrum. Meanwhile, from the viewpoint of safety, the negative void reactivity should be satisfied, which is consistent with conventional LWRs. From the viewpoint of neutron economy, high enrichment should be avoided as well.
This study aims to design the BWR-type reactor with the tightly packed fuel assemblies, which attains both high breeding and negative reactivity.
