30 Containment systems Core overheating

Consequently, they blocked the emergency water injection systems which had been automatically actuated. The core overheated and partially melted. The releases were negligible from the health protection point of view because of the presence of an effective containment. 

Transfer system. The principal system used to transfer fuel and other components to and from the reactor vessel is the closed loop ex-vessel machine (CLEM) shown in Fig. A.2. The CLEM loads all components into the reactor vessel and removes all components from the reactor vessel and operates only when the reactor is shutdown. Under most conditions, CLEM moves a core component pot (CCP) that contains the fresh or SNF fuel to and from the reactor vessel. In a fast reactor, the core power density is very high thus, there is significant decay heat in each SNF assembly immediately after reactor shutdown. To prevent fuel failure from overheating, the SNF is kept in sodium at all times to ensure effective cooling. This is accomplished by transferring each fuel assembly in its own pot of sodium—the CCP. CLEM is also used to transfer a variety of other components within the reactor containment. 

The release of fission products and structural materials from the overheated and, finally, molten reactor core represents the initial and, therefore, most important component of the source term. It provides vapor and aerosol sources for radionuclide transport in the reactor primary system and, further on, to the containment. 

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