The Pressurized Heavy Water Reactor (PHWR-220) is a 235 MWe pressure-tube type reactor derived from the Canadian CANDU design. It uses heavy water (D2O) both as a primary coolant and as a neutron moderator. Pressure tubes made of Zr — 2.5%Nb contain the 0.5 m long cluster-type fuel bundles containing 19 Zircaloy-4 cladded pins of natural UO2. Each pressure tube contains 12 fuel bundles. A total of 306 horizontal pressure tubes and surrounding moderator make up the core, which is surrounded by a light-water calandria vessel. Unlike most reactors that use batch refueling, the PHWR is refueled on a continuous basis using two refueling machines — one on either end of the core. Refueling is accomplished by inserting a fresh fuel bundle into one end of the pressure tube and collecting the spent fuel bundle that is forced out on the other end, which is then transported to a spent fuel area. Refueling is bi-directional with eight bundles being refueled at a time. Four reactor coolant pumps are used to circulate the primary coolant through two independent primary heat transport circuits, each providing coolant to 153 pressure tubes. Two recirculating U-tube steam generators on each primary heat transport circuit transfer heat to the secondary side for power conversion.

A double containment system is used for each reactor with an interior pre-stressed concrete structure surrounded by a reinforced concrete structure. The negative pressure gap between the two structures facilitates leak detection.

India currently operates 16 PHWR-220 units at five different locations. The reactors are typically deployed as twin units with some shared systems. Key parameters for the PHWR-220 design are given in Figure 2.14. [12]

Key parameters Electrical capacity: 235 MWe Thermal capacity: 755 MWt

Configuration: Pressure tube

Primary coolant: Heavy water

Moderator: Heavy water

Primary circulation: Forced Outlet temperature: 293 °C RV diameter/height: 6.0 m/4.2m Steam generator: Recirculating

U-tube (4)

Power conversion: Indirect Rankine

Fuel (enrichment): UO2 (< 5%)

Reactivity control: Rods

Refueling cycle: Continuous

Design life: 40 years

Status: 16 units in operation

Figure 2.14 PHWR-220 (India) — Nuclear Power Corporation of India Ltd (NPCIL).

2.3.1 India: AHWR-300-LEU design

The Advanced Heavy Water Reactor (AHWR-300-LEU) being developed by the BhaBha Atomic Research Center (BARC) is an extension of the PHWR-220 with

Gravity driven
water pool

Steam drum Refueling machine

Vertical calandria

Figure 2.15 AHWR-300-LEU (India) — BhaBha Atomic Research Center (BARC).

several significant differences. Although it retains use of heavy water for neutron moderation, it uses light water as the primary coolant. Fuel bundles consist of 54 pins of cladded mixed-oxide fuel, including 30 thorium-uranium oxide pins and 24 plutonium-thorium oxide pins. The fuel bundles are contained in vertical pressure tube channels. The U and Pu content in the mixed-oxide fuel is maintained below 5%. Natural circulation of the primary coolant is used with a direct Rankine cycle for power conversion. The steam/water mixture that exits the core is circulated to an external steam separator drum where the steam is directed to the turbine and the water condensate is returned to the feedwater header.

Currently it is intended that the AHWR-300-LEU will be used in co-generation mode and produce 2400 m3/day of potable water by extracting a portion of the steam from the low-pressure turbine. Key parameters and a representative graphic for the AHWR-300-LEU design are given in Figure 2.15. [13]