Upgrading Apsara

‘APSARA’, ASIA’S OLDEST RESEARCH REACTOR, located at the Bhabha Atomic Research Centre (BARC), Trombay, India, was resurrected as ‘Apsara-Upgraded’ or ‘Apsara-U’ in September 2018. It had been shut down in 2009 for a comprehensive upgrade and life-extension programme. The ‘new’ 62-year old Apsara-U, a swimming-pool-type research reactor, has double the rated power of its older version, and advanced safety features and more proliferation-resistant fuel.

India’s Department of Atomic Energy (DAE) expects Apsara-U, whose systems and components have been ‘designed, manufactured and installed according to the requirements of the latest safety standards’, to remain viable as a neutron source for research and development (R&D) purposes well into the future. It will be used for beam tube research, production of radioisotopes, neutron activation analysis, neutron radiography, shielding studies, material irradiation and development and testing of neutron detectors.

Higher flux

A key feature of the refurbishment has been the uprating of Apsara-U to 2MWt from 1MWt. This has enabled the maximum thermal neutron flux generated at the in-core irradiation position to rise to 6.1 × 10¹³ n/cm²/s from 1.0 × 10¹³ n/cm²/s.

With the higher thermal neutron flux generated at rated power, radioisotopes that were previously produced in the decommissioned Cirus can now be produced in Apsara-U. In fact, Apsara-U is expected to increase indigenous production of radioisotopes for medical applications by about 50%.

The maximum thermal neutron flux in the reflector region is now 4.4 × 10¹³ n/cm²/s and seven experimental and irradiation positions have been provided in this region in the new grid plate.

The reactor pool block, inside the reactor building, is a concrete structure whose inner face is lined with stainless steel plates to house a reactor pool filled with demineralised water. The reactor pool is 8.5m × 3.4m × 9.6m. Provision has been made for a thermal column and shielding experiment facility in the pool block itself. The reactor pool also accommodates process piping, instrumentation and spent fuel storage cages.

The reactor pool block provides structural support for a movable trolley located at the pool top, from which the core is suspended. Apsara-U’s core has three specific locations inside the reactor pool, two of which site beam tubes. In all, eight beam tubes are provided, six of them 100mm in diameter and two of 150mm diameter. The beam holes themselves have been provided with motorised inner and outer gates.

The maximum fast neutron flux in the in-core water hole of the Apsara-U is 1.4 × 10¹³ n/cm ²/s, ten times that Cirus (now decommissioned) or Dhruva (still operating) could manage. The old Apsara proved rather useful in ratifying the design adequacy of India’s 500MWe Prototype Fast Breeder Reactor (PFBR) in Kalpakkam, which is expected to reach first criticality in the coming months. The Apsara-U is expected to carry on this tradition and further contribute to the ‘second stage’ of India’s three-stage nuclear programme, which is focused on large-scale FBR based power generation capacity.

Better neutron economy

The higher rated power has been attained with a new core design that achieves better neutron economy. According to BARC, Apsara-U’s reactor core is mounted on a 140mm thick aluminium grid plate, with 64 lattice positions arranged in an 8x8 square array with a lattice pitch of 79.7mm. The central 4x4 lattice positions of the core are loaded with fuel assemblies. Two layers of beryllium oxide (BeO) reflector assemblies clad in aluminium surround the core, in order to provide the desired level of core reactivity while sustaining high thermal neutron flux levels over a large radial distance around the reactor core. This arrangement is useful for material studies and isotope production.

The central 4x4 lattice positions of the core consist of eleven standard fuel assemblies comprising 17 fuel bearing plates, two control fuel assemblies, similar except they can accommodate hafnium absorber plates, two shut-off-rod fuel assemblies and one hollow beryllium oxide plug. The BeO plug will be used for high neutron flux irradiations in addition to the irradiation positions provided in the reflector assembly.

The use of four fast-acting hafnium shut-off rods, two of which double-up as the reactor’s control rods (and are supplemented by a hafnium fine control rod) has enhanced safety and maintained the availability of spots in the core matrix for irradiation purposes.

Despite the uprating, the new reactor core design ensures a negative reactivity coefficient from zero to full power levels. The earlier ‘single-channel’ on/off-type reactor power regulating system has been replaced by a triple-channel proportional control system using neutron power and reactor period signals. The reactor has also been equipped with emergency power supply for safety-related equipment.

The turn to LEU

In keeping with global non-proliferation trends, the Apsara-U uses low- enriched uranium (LEU) fuel instead of the highly-enriched uranium (HEU) that was used in the original. LEU (17%) uranium silicide dispersed in aluminium (U₃Si₂-Al) plate type fuel has been chosen. Its favourable features include high uranium loading density in the fuel, good thermal conductivity, an excellent blister resistance threshold, stable swelling behaviour under irradiation, high fission-gas-retention capability and easy fabricability. U3Si2 is synthesised by the DAE using the powder processing route with uranium metal powder and silicon powder as the starting materials. Aluminium-6061, which is an alloy of nuclear grade, is the chosen cladding material, with high thermal conductivity, a small cross section for neutron absorption and good compatibility with the U₃Si₂-Al dispersion matrix.

Enhanced safety

Construction of the refurbished Apsara reactor pool, annex building, pump house and dump was complete in 2017, and DAE is satisfied that it has met seismic and shielding adequacy standards. Apsara-U also has a new reactor hall and a new electrical substation.

Prior to refurbishment, a safety evaluation of the Apsara’s reactor building, which dates from 1955, had been carried out with reference to the guidelines outlined in IAEA-TECDOC-1347. A detailed seismic analysis found that the footings of the old reactor building had to be strengthened. Since that was not possible, a strengthening scheme with elasto-plastic dampeners in the frames and corner truss bracings of ISMC 200 box sections, which would reduce moments in the corner columns, was envisaged.

The renovated reactor building also has a closed loop ventilation system that maintains a temperature of 24±1°C and a relative humidity of 50±5%. The system caters to the optimal functioning of electronic equipment, personnel comfort and ensures that there is more than one air change every hour in the reactor building. A cleanup system that uses iodine and HEPA filters has been incorporated.

Apsara-U’s extensive refurbishment includes new control systems, shielding and core cooling structures and components that meet current safety codes. The radiation field at the pool top is maintained at acceptably low levels via the primary coolant flow from the top to the bottom of the core, with a hot water layer at the top of the pool water. The delay tanks provided at the core outlets are sized to permit N-16 activity to decay, reducing the radiation field in the process equipment room to permissible levels.

Overall, Apsara-U is a move by the DAE to rekindle its old assets with a balance of utility and safety. The learnings from this project are likely to be ported by the DAE to other refurbishment programmes.  

Author information: Saurav Jha, Author and commentator on energy and security, based in New Delhi