22.08.2015   DESIGN FEATURES TO ACHIEVE. DEFENCE IN DEPTH IN SMALL AND. MEDIUM SIZED REACTORS

PASSIVE SAFETY DESIGN FEATURES OF AHWR The main inherent safety features of AHWR are

• Negative void coefficient of reactivity;

• Negative fuel temperature coefficient of reactivity;

• Negative power coefficient of reactivity;

• Double containment system;

• Absence of main circulating pumps;

• High pressure and low pressure independent emergency core cooling system (ECCS) trains;

• Direct injection of ECCS water into the fuel cluster.

The important passive safety features and systems in AHWR are:

TABLE VI-1. MAJOR DESIGN CHARACTERISTICS OF AHWR [VI-1]

Attributes

Design particulars

Major design specifications

Core configuration

Vertical, pressure tube type

Fuel

Pu-ThO2 MOX, and 233UO2-ThO2 MOX

Moderator

Heavy water

Coolant

Boiling light water

Number of coolant channels

452

Pressure tube inner diameter

120 mm

Pressure tube material

20% Cold worked Zr-2.5% Nb alloy

Lattice pitch

245 mm

Active fuel length

3.5 m

Calandria diameter

7.4 m

Calandria material

Stainless steel grade 304L

Steam pressure

7 MPa

Mode of core heat removal

Natural circulation

MHT loop height

39 m

Shutdown system-1 (SDS-1)

40 mechanical shut off rods

Shutdown system-2 (SDS-2)

Liquid poison injection in moderator

Thermal hydraulic characteristics

Circulation Type

Natural for normal operating as well as hot shutdown conditions

Coolant Conditions

Core inlet: 532 K, 2237 kg/s; Core outlet: 558 K, average exit quality 18.2%

Steam and feed water conditions

Steam at outlet from steam drum: 7 MPa, 558 K, 407.6 kg/s Feed water at inlet to steam drum: 403 K

Fuel temperatures during normal operation

For maximum rated channel:

Fuel centre line: 1213 K, Clad surface: 572 K

The maximum permissible clad temperature is 673 K.

Reactivity feedbacks

Condition

Temperature and void effects

Reactivity change (mk)

Channel temperature (300 K at cold critical to 558 K at hot standby)

+2.5

Moderator temperature (300 K to 353 K)

+3.0

Reactivity feedbacks (continued)

Fuel temperature (558 K at hot standby to 898 K at full power)

-6.5

Coolant void (density from 0.74 at hot standby to 0.55 g/cc at full power)

-2.0

LOCA at full power (density change from 0.55 to 0.0 g/cc) Xenon load

-4.0

Equilibrium load

-21.0

Transient load 30 min. after shutdown from full power

< -1.0

Peak load 300 min. after shutdown from full power Other neutron physical parameters

-7.0

Delayed neutron fraction, b (without photon neutrons)

0.003

Prompt neutron lifetime, l, sec.

0.00022

• Core heat removal by natural convection of the coolant during normal operation and in shutdown conditions;

• Decay heat removal by isolation condensers (ICs) immersed in a large pool of water in a gravity driven water pool (GDWP);

• Direct injection of ECCS water into the fuel cluster in a passive mode during postulated accident conditions, such as loss of coolant accidents (LOCAs), initially from the accumulators and later from the GDWP;

• Containment cooling by passive containment coolers during LOCA;

• Passive containment isolation via formation of a water seal in the ventilation ducts, following a large break LOCA;

• Passive shutdown through injection of poison to the moderator, using high pressure steam, in case of the low probability event of failure of the wired (sensors, signal carriers and actuators) mechanical shutdown system (SDS-1) and the liquid poison injection system (SDS-2);

• Passive concrete cooling system to protect the concrete structure in a high temperature zone.

The availability of a large inventory of water in the GDWP at higher elevation inside the containment facilitates sustainable core decay heat removal, ECCS injection, and containment cooling for at least 72 hours without invoking any active systems or operator actions.

Passive safety features/systems of the AHWR are described in brief below.

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