Integral water-cooled reactors make efficient use of the primary coolant inventory to prevent core damage under emergency conditions Under LOCA conditions, the steam generators of integral reactors assist decay heat removal for a longer time than in present generation designs since they remain covered by water, cool the primary fluid and reduce the loss of coolant vapor

The following are the basic systems for minimizing coolant inventory loss

• Use of a guard vessel or containment which fits closely around the lower part of the reactor vessel In LOCA this space rapidly fills with coolant ejected from the vessel and the core remains covered In some designs, e g SPWR, this space is connected to the outer water filled pressure suppression environment which gives an adequate supply of water to keep the core covered

• Provision of water by passive means from external tanks The feed may be by gravity at low pressure, gravity from tanks pressurized by automatic connection to the pressunzer in appropriate accidents, or by passive pumping devices such as steam injector pumps

The system using a guard vessel has the advantage of providing for a cheap and simple plant with no need for extra supplies of emergency coolant

The system using gravity feed tanks assisted by steam pressurization has the advantage of operation over the full pressure range The disadvantages are the limited volume of the tanks and the need for isolation valves to isolate pipe breaks outside the containment Besides, steam injectors are not yet proven for the duty required and are undergoing further development