DERIVATION OF THE DESIGN BASIS

1.27. The design basis for containment systems should be derived primarily from the results of the analysis of relevant postulated initiating events, which are defined in Appendix I of Ref. [1]. The postulated initiating events that should be considered include those of internal and external origin that could necessitate the performance by the containment of its intended functions and those that could jeopardize the capability of the containment to perform its intended safety functions.

1.28. Relevant elements of the design basis for normal operation (power operation, refuelling and shutdown) should be derived from the following requirements:

— To confine the radioactive substances produced by neutron or gamma radiation,

— To remove the heat generated,

— To provide for the necessary access and egress of personnel and materials,

— To perform containment pressure tests and leak tests,

— To contribute to biological shielding.

Internal events

1.29. Internal events that should be considered in the design of the containment systems are those events that result from faults occurring within the plant and that may necessitate the performance by the containment of its functions or that may jeopardize the performance of its safety functions. They fall essentially into five categories:

(1) Breaks in high energy systems located in the containment: The containment should be able to withstand high pressures and tempera­tures, as well as pipe whips and fluid jet impacts.

(2) Breaks in systems or components containing radioactive material located in the containment: The containment should be able to confine the radioactive material.

(3) System transients causing representative limiting loads (e. g. pressure, temperature and dynamic loads) on the containment systems: The containment should be able to withstand these loads.

(4) Containment bypass events such as loss of coolant accidents (LOCAs) in interfacing systems or steam generator tube ruptures: Appropriate provisions for isolation should be in place.

(5) Internal hazards: It should be verified that internal hazards will not impair the containment functions.

1.30. Typical internal events that should be considered in the design of containment systems are as follows:

— LOCAs;

— Various failures in the steam system piping;

— Breaks in the feedwater piping;

— Steam generator tube ruptures in a pressurized water reactor;

— Inadvertent opening of a pressurizer safety valve or relief valve in a pressurized water reactor, or of a safety relief valve in a boiling water reactor;

— Condensation oscillations and ‘chugging’ of liquid-gas mixtures during blowdown in a boiling water reactor;

— Breaks in lines connected to the reactor coolant pressure boundary, inside or outside the containment;

— Leakage or failure of a system carrying radioactive liquid or gas within the containment;

— Fuel handling accidents in the containment;

— Internal missiles;

— Internal fires;

— Internal flooding.