The long-term energy needs of India require utilisation of its large Thorium reserves. Advanced Heavy Water Reactor (AHWR) being developed at BARC envisages large scale utilization of Thorium. A Critical Facility is operational at BARC for validating the reactor physics design and nuclear data for AHWR.

AHWR-CF is a low power research reactor with a nominal power of 100 W. The aluminium reactor tank houses fuel assemblies and moderator. A square box above the reactor tank houses the lattice girders from which the fuel assemblies are suspended. The lattice girders can be moved to vary the pitch. These lattice girders also support the reactor shutdown devices. The top of the square box is closed by a revolving floor, which also permits access to any of the lattice location for handling operations.

Salient features of AHWR-Critical Facility

The major core configurations of AHWR-CF are:

Reference Core

The reference core was initially configured with 55 lattice locations in a lattice pitch of 245 mm, where 49 were occupied by 19 pin natural uranium metal fuel clusters and 6 locations with shut-off rods. Later in 2014, the core was extended to 61 locations with 55 locations for fuel cluster for gaining reactivity in order to perform experiments with thoria based fuel.

AHWR Representative Core

The core for performing detailed experiment with AHWRF will be constituted by replacing the central natural uranium metal fuel clusters with AHWR type of fuel, i.e.54 pin AHWR (Th-Pu Oxide, Th-U233 Oxide) Clusters. Experiments will be performed with AHWR fuel in a typical AHWR neutron spectrum.

AHWR-CF attained its first criticality on 7th April, 2008. The observed critical height for the Reference core configuration was 226.7 cm which agreed well with the estimated value of 226.5 cm.

A large number of reactor physics experiment has been performed in the Reference core of the facility. These experiments ranged from the initial commissioning experiments to various integral and differential measurements. Some of the important experiment carried out at AHWR-CF are as follows:

1. First approach to criticality of AHWR-CF
2. Reactor power calibration by absolute flux measurement by activation method
3. Reactivity worth measurement of six Shut-off Rods and Absorber rod

Differential flux measurement experiments

1. Axial flux distribution measurements on the central fuel cluster
2. Flux measurement at detector location
3. Neutron spectrum measurement on the central cluster by Foil-Activation Technique
4. Measurement of level coefficient of reactivity of Reference Core
5. Fine structure neutron flux measurement in Nat. U Experimental Cluster and Mixed Pin Thoria Cluster placed in central location
6. Gamma scanning of Nat. Uranium and Thorium pin irradiated at central location

Integral measurements

1. Critical height measurement with various types of experimental clusters
1. (NU – ThO₂) Mixed 19-Pin
2. (Th-Pu (1%)) Six Pin Special Cluster
3. (Th-LEU) Six Pin MOX Cluster
4. (NU-ThO₂-NU) 19-pin Sandwich Cluster
5. ((Th-1%Pu) MOX, NU-ThO₂-NU) Mixed 19-pin cluster
2. Measurement of the Moderator Temperature Coefficient of Reactivity
3. Measurement of Westcott neutron spectrum parameters
4. Photoneutron decay studies
5. Subcriticality measurement using neutron noise method
6. Radial and axial flux profile in experimental mixed 19-pin cluster AHWR type fuel with {(Th-1%Pu) MOX, NU-ThO₂-NU}
7. Coolant void worth measurement in experimental Mixed 19-pin cluster AHWR type fuel with {(Th-1%Pu) MOX, NU-ThO₂-NU}