Probabilistic safety criteria specify the basic safety indices of an NPP in probabilistic terms as the following:

(a) To avoid the need for population evacuation beyond plant boundaries established by the regulatory requirements regarding the location of NPPs, it is necessary to target a probable maximum release of no more than 10-7 per reactor per year; the value of this maximum release, established by the same regulatory documents, corresponds to radiation dose limits for the population in the case of beyond design basis accidents;

(b) The overall probability of severe beyond design basis accidents (evaluated on the basis of probabilistic safety analysis) should be targeted not to exceed 10-5 per reactor per year.

The GT-MHR NPP project establishes operation limits and conditions, safe operation limits and conditions, and design limits for abnormal operation conditions, including design basis accidents. Maximum fuel temperature, which shall not exceed 1600°C, is considered one of the most important design limits for pre­accidental situations and design basis accidents.

The operation limits for process parameters and characteristics of reactor plant equipment are specified based upon:

— Analytical results for reactor plant parameters and equipment operating conditions during normal operation, taking into account measurement errors;

— The evaluation of a control range of reactor plant process parameters during normal operation with evaluation of the accuracy of keeping these parameters within the control range, taking into account errors of the measurement and automation means.

Presently, the operation limits and the safe operation limits for fuel elements of the GT-MHR have not been established.

Safe operation limits for the basic process parameters are established to protect physical barriers against damages during abnormal operation. Barriers are protected by the safety systems, which have actuation set points assigned with some margin relative to safe operation limits or equal to them.

The range of safe operation limits corresponds to the list of plant process parameters according to which protection of the plant is provided. For the GT-MHR, this list includes:

— Reactor neutron (thermal) power;

— Helium pressure in the reactor;

— Containment pressure;

— PCU cooling water system pressure;

— Turbomachine rotor speed;

— Coolant temperature at the reactor outlet;

— Coolant temperature at the low pressure compressor inlet;

— Coolant temperature at the high pressure compressor inlet;

— Activity of the primary coolant.

The operation limits and safe operation limits for process parameters and reactor plant equipment characteristics, established as indicated above, are given in Tables VII-2 and VII-3. Design limits adopted for the analysis of design basis accidents are given in Table VII-4.

TABLE VII-2. OPERATION LIMITS AND SAFE OPERATION LIMITS FOR PROCESS PARAMETERS Process parameter Value Operation limit Safe operation limit Reactor power, MW(th) 620 660 Primary coolant temperature, °C: — At the reactor inlet 500 Not established — At the reactor outlet 870 890 Helium pressure in the reactor, MPa 7.5 7 5 +0‘5 7 -1.4 Primary coolant activity, Bq/l 1.5 x 107 3.0 x 107 Turbomachine rotor speed, rpm 3180 3300 Containment fluid pressure, MPa Vacuum not less than 50 kPa 0.15 relative to the environment PCU cooling water pressure, MPa 1.1 1 0 -0‘5 +0.2

TABLE VII-3. OPERATION LIMITS FOR THE EQUIPMENT Equipment Operation limits Temperature, °C Pressure, MPa Reactor vessel 440 7.5 Lower support plate 500 Not established Reactor core barrel 500 Not established Upper restricting device 550 Not established Fuel assembly 1300 Not established Units of: — Replaceable side reflector 800 Not established — Permanent side reflector 500 Not established — Central reflector 1200 Not established — Upper reflector 500 Not established Control and protection system (CPS) rods 700 Not established CPS standpipe casing Not established 7.5 Shutdown cooling system (SCS) unit casing Not established 7.5 Tube system of the SCS heat exchanger Not established 7.5 Power conversion unit (PCU) vessel 140 7.5 Connecting vessel: — Cold gas duct 500 7.5 — Hot gas duct 870 7.5

TABLE VII-4. DESIGN LIMITS ADOPTED FOR THE ANALYSIS OF DESIGN BASIS ACCIDENTS Barrier Safety criteria Note Fuel Primary circuit Maximum temperature of coated fuel particles shall not exceed 1600°C Primary circuit pressure shall not exceed 8.6 MPa Design limit Containment -Containment pressure shall not exceed 0.5 MPa Design limit -Fluid leak from the containment shall not exceed 1% of the volume per day at a pressure of 0.5 MPa Design limit

Acceptance criteria for operating modes

The operating modes (regimes) are rated as acceptable based on the following:

— Normal operation modes — non-excess of the operation limits;

— Modes with abnormal operation occurrences, including pre-accidental situations — non-excess of the safe operation limits;

— Design basis accidents — non-excess of the safe operation limits and the design limits for design basis accidents;

— Beyond design basis accidents — non-excess of the specified radiation criteria.

Summary of approaches to the provision of radiation safety

Radiation safety of personnel, the population and the environment is provided according to the following basic concepts:

• Radiation impact on personnel, the population and the environment during normal operation and accidents does not exceed limits established in the GT-MHR project, which are in full compliance with regulatory documents;

• The reactor plant structures and means of radiation protection and radioactive product localization (isolation) are designed taking into account technical and administrative measures aimed at a reduction of radiation levels and air radioactivity in the NPP rooms, at a reduction of emissions of radionuclides to the environment, and at a reduction of radiation doses to personnel and the population, as well as at maintaining these radiation parameters at a reasonably achievable low level.

(1) Physical barriers

Provision of radiation safety is based on the use of physical barriers intended to prevent releases of radioactive products into the environment.

(2) Biological shielding

Biological shielding is one of the barriers to the propagation of ionizing radiation from the reactor plant. According to regulatory requirements, biological shielding is designed with a margin factor of two for the radiation dose rate.

(3) Technical and administrative measures

Several administrative and technical measures are provided for in the project to maintain radiation doses to personnel and the population at a minimum possible level:

— Establishment of a buffer area and a restricted access area around the NPP;

— Execution of the radiation, dosimetric, and process control;

— Establishment of a restricted access area and a ‘free’ area at the NPP;

— Use of closed circuits with radioactive fluids;

— Filtering of radioactive substances emitted into the environment;

— Use of the containment to retain radioactive products.

Fuel handling operations are performed using protective containers to avoid fuel assembly damage and radioactive product release. Appropriately shielded containers are provided to protect personnel against radiation impacts during dismantling of reactor unit components.

The effective annual radiation dose for the population beyond the buffer area during normal operation of the GT-MHR is much lower than the quota of 20 pSv/year established in regulatory documents. Under abnormal operation conditions, the release of radioactive substances and/or ionizing irradiation does not exceed safe operation limits adopted in the design for normal operation.