The operation modes of the GT-MHR categorized as abnormal operation occurrences or pre-accidental conditions are listed below.

(1) Modes with reactivity and power distribution variations:

1.1. Inadvertent removal of one or several of the most effective control rods from the reactor core;

1.2. Inadvertent insertion of one or several of the most effective control rods into the reactor core;

1.3. Inadvertent insertion of absorbing elements from the reactor shutdown system hoppers into the reactor core;

1.4. Incorrect loading of a fuel assembly into the reactor core and the operation of such a fuel assembly.

(2) Modes with a decrease in heat removal from the primary circuit:

2.1. Complete stop of water circulation through PCU heat exchangers;

2.2. Ruptures of PCU cooling water system pipelines within and beyond the containment;

2.3. SCS failures in standby mode (ceasing of water circulation and ruptures of SCS cooling water system pipelines within the containment).

(3) Modes with a decrease in coolant flow rate through the reactor core:

3.1. Failure of a turbomachine or failure of individual turbomachine components which require the emergency shutdown of a turbomachine;

3.2. Inadvertent opening of the bypass shut-off and control valves of the control and protection system of the turbomachine;

3.3. Increase of bypass flows in the primary coolant circulation system due to inadvertent opening of valves or due to depressurization of in-vessel components.

(4) Modes with inter-circuit depressurization:

4.1. Inter-circuit depressurization involving the primary circuit and circuits of the PCU and SCS cooling water systems.

(5) Modes with loss of power supply:

5.1. NPP blackout — loss of normal (main and backup) power supply for the system’s own needs with a loss of the external load of the generator.

(6) Modes with abnormal refuelling and nuclear fuel handling:

6.1. Inadvertent withdrawal of a control rod during refuelling;

6.2. Failure of heat removal from the reactor core during refuelling;

6.3. Failure of the drum of spent fuel assemblies to cool;

6.4. Drop of a fuel assembly during refuelling (into the reactor or into the drum of spent fuel assemblies);

6.5. Drop of a fuel assembly transportation container during refuelling.

(7) Modes with external impacts:

7.1. Design basis or maximum design basis earthquake;

7.2. Impact of air shock wave;

7.3. Aircraft crash.

The initiating events of design basis accidents for the GT-MHR are categorized in brief below.

(1) Accidents with primary circuit depressurization:

1.1. Primary circuit depressurization due to a loss of leaktightness or the guillotine break of a primary circuit pipeline with a coolant leak into the containment and further air ingress to the primary circuit: —Rupture of small lines (with equivalent outer diameter of less than or equal to 30 mm);

—Rupture of a bypass pipeline in the control and protection system of the turbomachine (the

equivalent outer diameter is 250 mm);

—Depressurization of a standpipe of the reactor control and protection system.

1.2. Rupture of the pipelines of the helium transportation and storage system beyond the containment.

(2) Accidents with abnormal fuel assembly cooling conditions:

2.1. Partial clogging of the fuel assembly flow area by a fuel assembly fragment.

(3) Accidents with disruption of normal refuelling and nuclear fuel handling modes:

3.1. Dropping of heavy objects and damage of fuel assemblies during refuelling;

3.2. Depressurization of fuel assembly handling equipment;

3.3. Fuel assembly damage during refuelling.