Just as it is in LWRs, the main objective of reactor core design is to improve economy while ensuring safety. The requirements for safety are ensuring shutdown capability, limiting reactivity insertion rate, getting self controllabil­ity, preventing fuel failure, limiting the power distribution, and ensuring sta­bility. The main requirements for economy are getting adequate burnup and breeding performance. The basic design principles are summarized in Table 4.1.

(1) Reactor shutdown

The design conditions of the reactor shutdown system are determined in order to shut down the reactor safely and surely at abnormal incidences. For that purpose, the following criteria are adopted.

• Provide two independent shutdown systems.

• Design at least one system able to shut down the reactor at low temper­ature with the required shutdown margin even if the control rod with the highest reactivity worth is fully withdrawn and stuck.

pper end plug

Handling head

Tag gas capsule

Upper spacer pad

Cladding

spacer pad

Upper blanket fuel pellet

W rapper tube

A-Across section

Fig. 4.3 Driver fuel assembly and core configuration of japanese prototype reactor monju [6]

• Design the shutdown systems so that, even if one shutdown system completely fails, another system can keep the reactor subcritical at low temperature.