The initiating events/combinations of events for beyond design basis accidents regarding the GT-MHR are

categorized in brief below.

(1) Beyond design basis accidents with loss of power supply sources:

1.1. Blackout;

1.2. Blackout with a complete RCCS failure;

1.3. Blackout with a failure of the actuation of the reactor emergency protection (shutdown) system (anticipated transient without scram — ATWS).

(2) Beyond design basis accidents with reactivity variation (taking into account additional failures):

2.1. Inadvertent withdrawal of several of the most effective control rods from the reactor core with an actuation failure of the reactor emergency protection system (ATWS).

(3) Beyond design basis accidents with a decrease of the coolant flow rate through the reactor core (taking

into account additional failures):

3.1. Failure of the turbomachine or failure of individual turbomachine components, requiring an emergency shutdown of the turbomachine, accompanied by an actuation failure of the reactor emergency protection system (ATWS).

(4) Beyond design basis accidents with primary circuit leakage (taking into account additional failures):

4.1. Primary circuit depressurization with a blackout and an ingress of a considerable amount of air into the primary circuit (guillotine break of a standpipe of the control and protection system);

4.2. Primary circuit depressurization with failure of the reactor protection system to actuate (ATWS), a blackout, and the ingress of a considerable amount of air into the primary circuit (guillotine break of a standpipe of the control and protection system);

4.3. Rupture of the transportation and storage system helium pipelines beyond the containment, followed by a failure of the system of activity localization within the primary circuit and by a blackout;

4.4. Inter-circuit depressurization between the primary circuit and the PCU or the SCS cooling water circuits, followed by a failure of the isolation systems and a blackout, and by ingress of a considerable amount of water into the primary circuit.

VII — 4.2. Acceptance criteria

The acceptance criteria used for NPP designs with modular high temperature gas cooled reactors (HTGR) are as follows:

— Radiation safety criteria, which specify allowable radiation doses for personnel and population during normal plant operation and in the case of accidents;

— Probabilistic safety criteria, which establish the allowable overall probability of severe beyond design basis accidents and the probability of maximum reactivity releases during such accidents.