Advanced Heavy water Reactor features

ADVANCED HEAVY WATER REACTOR (AHWR)

India is developing the Advanced Heavy Water reactor (AHWR) as the third stage in its plan to utilise thorium to fuel its overall nuclear power program. It is a 300MW vertical pressure tube type reactor using heavy water as moderator and boiling water as coolant in natural circulation mode at low pressure (~ 70 bar).

The calandria has 500 vertical pressure tubes and the coolant is boiling light water circulated by convection. Each fuel assembly has 30 Th-U-233 oxide pins and 24 Pu-Th oxide pins around a central rod with burnable absorber. Burn-up of 24 GWd/t is envisaged. It is designed to be self-sustaining in relation to U-233 bred from Th-232 and have a low Pu inventory and consumption, with slightly negative void coefficient of reactivity.

The major changes from the PHWR design is that AHWR is a thorium fuel based, Light water replaces the high pressure heavy water coolant circulated using a pump .
During its designed plant life of 100 years AHWR will generate 65% of the power from ThO2 based fuel. AHWR is the first of its kind in the world not only because of its most attractive feature of heat removal from the reactor core by natural circulation under all conditions but also due to the fact that it incorporates a host of other passive safety features that are in line with the approach being pursued world over for development of inherently safe reactor system by incorporating safety features that do not call for any human intervention or any active control devices for reactor safety.

The AHWR Fuel

The initial core will be made up of entirely (Th, Pu-239) MOX fuel assemblies.

• The U233 bred in the 54 pin (Th, Pu-239) MOX fuel pin will be progressively recovered and recycled as (Th, U233) MOX.
• At equilibrium, the core of AHWR will consist of composite fuel assemblies each having 24 nos. of (Th, Pu239) MOX pins and 30 nos. of (Th, U233) MOX pins arranged in three consecutive rings having fissile material compositions as shown hereunder:
  (i) 12 (Th-U233)O2 pins with U233 enrichment of 3.0% in inner most or 1st ring.
  (ii) 18 (Th-U233)O2 pins with U233 enrichment of 3.75% in intermediate or 2nd ring.
  (iii) 24 pins consists of (Th-Pu)O2 pellets with plutonium enrichment of 3.25% in outermost or 3rd ring.
  Fissile isotopes content of plutonium will go down from initial 75% to 25% level at equilibrium discharge burn-up level (which would not be possible to recycle in AHWR but can be recycled to FBR or ADSS with fast neutron spectrum).

To reduce the overall inventory of waste, it is envisaged that Th and U233 will be recycled in AHWR. Even though U234 produced (along with U235 and U236) by neutron capture in U233 has negative influence on reactivity, it might be possible to recycle U233 in AHWR with only a marginal penalty of less than 1000 MWd/Te on discharge burn-up for each recycling.
The initial core characteristics and equilibrium characteristics on AHWR are shown in Table 2 and Table 3 respectively. Plutonium in AHWR burns faster due to large absorption cross section

that leads to loss in reactivity. An option is kept available in AHWR to reconstitute the fuel cluster after an averaged discharge burn-up of 24,000 MWd/Te. In reconstitution, only plutonium pins in outer rings are replaced by fresh fuel. Rest of the fuel cluster remains as it is. It is possible to obtain an additional burn-up of upto 20,000 MWd/Te from the reconstituted cluster. The cluster reconstitution improves U233 production and reduces the reprocessing load due to increase in average cluster burn-up. Reconstitution of fuel cluster involves multiple enrichments for the (Th-Pu)O2 pins, which will affect the fuel fabrication. However, reconstitution improves fuel conversion and hence economics of fuel cycle.
The fuel cycle time of AHWR is 8 years : 4 years for residence in reactor residence and two years for cooling (to allow for >99.9% conversion of Pa-233 to U233), 1 year of reprocessing and 1 year for refabrication. For the initial few years, annual reload would consist of (Th-Pu)O2 clusters only.