In the construction of a Nuclear Power Plant [NPP], attainment of Criticality is an occasion, where the developer & operator collectively concur that, "the kid has come of age". A moment to rejoice where, otherwise, grave, sombre individuals betray their usual persona, displaying unrestrained glee, hugging one another, feeding, eating the customary लड्डू/पेड़ा/बर्फी. Subsequent measures primarily involve sustaining what has been achieved. Reactor going Critical is the last major milestone in the Commissioning of a NPP. The sense of satisfaction & happiness among stakeholders is, therefore, understandable.
Work is underway, in India, to put a stop to this.
Deep within the facilities of the Bhabha Atomic Research Centre [BARC], India's foremost Nuclear establishment, one whose Airspace is secured by emplaced SAM batteries, whose premises are guarded by personnel of the elite CISF & where access control is second nature, Engineers & Scientists are working on an all-new Reactor Technology. One that no other country in the world has sustainably demonstrated, so far.
Criticality is the state of a Power-generating Reactor, where it generates more power than what it consumes. Thus, now, it becomes a net Power producer. The Reactor is in Neutron equilibrium. The number of Neutrons released is equal to the sum of the Neutrons participating in Fission plus those that escape. The Power output is a function of the Neutrons at play. By maintaining constant Neutron count, power stays constant.
![Radio Frequency Quadrupole [RFQ] - Indian Accelerator Driven Subcritical Reactor - 01](https://1.bp.blogspot.com/-ztvf0c6fV1s/X3MT6Z9-XmI/AAAAAAAAPPg/-b0i-Am2kesDQ2ttpUqvQBRIndBPo6UygCLcBGAsYHQ/s949/Radio%2BFrequency%2BQuadrupole%2B-%2BRFQ%2B-%2B01.jpg "Radio Frequency Quadrupole [RFQ] - Indian Accelerator Driven Subcritical Reactor - 01")
An Accelerator Driven System, or Accelerator Driven Subcritical [ADS] Reactor, is one that generates Power, but never goes Critical. Essentially, the Subcritical Core releases net deficit Neutrons &, left to it's own devices, will encounter a Self-terminating Chain Reaction. To keep it going, the balance is made-up with the help of a Neutron Accelerator. Employing Spallation reaction, a high-energy stream of Protons impinge on a heavy metal atom, such as Lead or Bismuth, leading to Neutron release. This is, then, accelerated & fed into the Core. A practical Spallation target is one that releases 30-40 Neutrons per Proton.
Inherent safety is a defining characteristic of such a facility. The Reactor can not have an uncontrolled run of Chain Reaction, of the type the 3 Mile Island encountered. Switching off the Accelerator would eventually terminate the Fission, bringing system to a safe state. Passive cooling systems would remove the heat generated in the intervening period.
That apart, ADS Reactors hold special significance for India's own, unique 3-Stage Nuclear Programme. The higher, externally-sourced availability of Neutrons can scale-up the irradiation process of it's abundantly available, fertile Thorium 232 [Th-232], turning them into fissile Uranium 233 [U-233]. As per the 2011 USGS estimate, India alone possesses more than 50% of the global Thorium reserve. Indian ADS design envisages a Hybrid Reactor, that can use Th-232, as the main fuel, performing in-situ Transmutation. Successfully realised, this will help leapfrog the need to build-up a sufficient stockpile of U-233, as planned in the 3-stage programme, before commencing commercial operations. This parallel effort can make-up for the delays being encountered in India's 3-Stage programme, towards Thorium utilisation.
More efficient fuel utilisation can be achieved. The externally-sourced Neutron would facilitate a higher fuel burnup. Thus, one can extract more energy from a given mass of Uranium fuel, than possible in Critical Reactors.
The vexing issue of the safe disposal of Radioactive waste is another challenge that stands to benefit from Subcritical Rectors. Current practise involves encasement & storage in deep geological repositories, passing the buck into the future. Fission by-products such as Highly Radiotoxic Transuranic Americium, Neptunium, Curium & Fission Products like Iodine-129, Caesium-135, Technetium-99 have half-life of Million years.
Present-day Indian Power Reactors generate these Radioactive by-products in significant amounts. Except for Plutonium, that finds use as a Neutron trigger for Fast Reactors & Military applications, the remaining elements are kept in storage, unaddressed. It, therefore, is vital to grab the bull by it's horns & address the elephant in the room. Conventional Thermal Reactors are inadequate in burning these Radioactive wastes, primarily due to factors of Delayed Neutron Fraction & Negative Temperature Feedback conditions, under which they function.
![Accelerator Driven Sub-critical System [ADS] Reactor - 01](https://1.bp.blogspot.com/-EHxfMOx-CtA/X3MeroGBCHI/AAAAAAAAPQM/J6mxqkKx-T4JSQaM0c7yd9fFfbLUYiarwCLcBGAsYHQ/s1139/Accelerator%2BDriven%2BSub-critical%2BSystem%2B%255BADS%255D%2BReactor%2B-%2B01.jpg "Accelerator Driven Sub-critical System [ADS] Reactor - 01")
In ADS, on the other hand, 1 can maintain conditions to use them as fuel. Thus, not only would one generate energy from it, during this process, they would be converted into, either, stable elements, or Radio Nuclides with much shorter half-life.
With all the virtues ADS promises to deliver, one would not be faulted for asking, why, then, Critical Reactors? For one, the concept of ADS, in it's current form, is relatively recent that, in 1995, Dr. Carlo Rubbia & his team first proposed while Researching at CERN. Thus, the many decades of Scientific & Engineering rigour needed to realise a Nuclear Concept, as have been initiated earlier for Fast Reactor & Fusion Reactor technologies, start from that date.
India's own interest in the ADS concept gained speed in the early part of this Millennium. It has adapted it's PURNIMA Research Reactor, located within BARC, to carry out associated studies. It, today, forms the nucleus of India's Subcritical drive. Besides synthesising the Science of ADS, one has to address major Engineering challenges in order to realise a commercially-viable system. Some of them include:
- Developing a rugged high current, high energy Proton accelerator, that would perform reliably all year-round.
- An effective Cooling system, that carries away the extreme heat generated during Spallation.
- A high power Spallation target, having good Neutron economy, while also withstanding the physical degradation due to the irradiation effects.
- A method to monitor the sub-criticality of the Core, that would determine Reactor performance.
An understanding of the Reactor's behaviour is first gained via conceptualisation computer simulation models. Running the code, based on the Multigroup Neutron-Transport Theory, recursively iterating it to refine performance, before zeroing in on the most acceptable model. This would be verified out on table-top experiments, be it a table weighing a few tonnes. Feedback from these experiments would be incorporated into the model, refining it further. A cycle that repeats till physical experiments produce designed outcomes. A successful table-top experiment would allow for progressively scaling up of the endeavour, culminating into a commercial scale ADS. An amalgamated series of concurrent & sequential, multi-pronged activities, spanning 3-4 decades of sustained, well-funded efforts.
BARC has concluded that the Accelerator needed to operate a full-scale Subcritical Reactor would consume around 15% of the power generated, with the remaining available for distribution. A sustainable proposition. To further improve performance, it has proposed a design, called One-way Coupled Fast Thermal ADS Reactor.
It combines operations of, both, Fast & Thermal Reactors. The Neutron is, first, accelerated in a smaller Lead-cooled Fast region having fissile Plutonium. Here, the Lead also acts as the Spallation target. It, then, enters the main Thermal region of the Core, where it does it's business. This approach would accrue up to 80% reduction in Power consumption. Blanketing the Fast region with Radioactive wastes, would burn them into benignity [ref: above]. This is also the answer to the question asked earlier.
BARC is, currently, in the process of studying the Thermal Region of such a Reactor, at a scale, in it's specially setup Beryllium Oxide Reflected & HDPE Moderated Multiplying Assembly [BRAHMMA] facility. The aim is to, first, understand it's behaviour at low power levels. Thus, a Neutron Generator source, capable of operating at levels up to 14 MeV, powers the system. The Rig is modular in design, that allows Researchers to vary the sub-criticality, by operating it with different fuel compositions - Natural Uranium, Slightly Enriched Uranium [SEU], Low Enriched Uranium [LEU], or a mix, as per requirement.
Subsequent plans with the Rig also include studying the Fast Reactor component of the Indian design, introducing a Thorium-Plutonium Mixed Oxide [MOX] fuel. This stage is additionally vital due to the role it would play in Radioactive waste disposal.
Indian interests in Accelerator-based Science & Engineering took roots very soon after Independence. In 1953, the Tata Institute of Fundamental Research [TIFR] setup India's first facility - a 1 MeV Cockroft-Walton Accelerator, also known as a Cascade Generator. The Kolkata-based Variable Energy Cyclotron Centre [VECC] developed India's first indigenous system in 1978. The 224 cm Variable Energy Cyclotron facility continues to function to this day, having received intervening upgrades. Superconducting LINAC got setup in the country, as an upgrade to TIFR's Pelletron, yet another type of Accelerator, in the early 90s. The biggest Accelerators India operates are housed within the facilities of the Raja Ramana Centre for Advanced Technology [RRCAT], in Indore - the Synchrotron Radiation Source, Indus-1 & Indus-2. The latter imparts particles with 2.5 GeV energy level. Around 100 Accelerators, of various types & sizes, are located in India. An extremely modest number, compared to neighbouring & continental countries.
A RRCAT collaboration with the Fermi Labs, in play, would facilitate knowledge sharing. This also enables India to provide sub-systems for it's future Accelerators.
As one would have concluded, Indian ADS efforts are still predominantly in the Study phase, gaining an understanding of Subcritical Physics, building up the knowledge database. A lot of ground to be covered.
For India, commercial utilisation of Thorium is the end-goal, to end the chronic energy deficiency holding it back. It promises an assurance of an equitable, green & sustainable attainment of per capita Energy consumption, ranking alongside developed Nations. Energy independence, unencumbered by Strategic imperatives, can be achieved. The country stands to be a global provider of this clean energy solution.
Towards this, India has ongoing, multiple programmes to harness the fuel - the 3-Stage Programme, Advanced Heavy Water Reactor [AHWR], High Temperature Reactor [HTR], Molten Salt Reactor [MSR], Metallic Fuel Fast Breeder Reactor. The quiver holds many arrows. ADS is one more addition to it.
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References:
- 8.1 ACCELERATOR DRIVEN REACTOR SYSTEMS - http://www.barc.gov.in/publications/eb/golden/reactor/toc/chapter8/8.pdf
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Accelerators for Science and Society, S KAILAS and R CHIDAMBARAM - https://insa.nic.in/writereaddata/UpLoadedFiles/PINSA/Vol85_4Cumulative.pdf
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Accelerator-driven sub-critical reactor system (ADS) for nuclear energy generation, S S KAPOOR - https://www.ias.ac.in/article/fulltext/pram/059/06/0941-0950
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ADS Programme and Associated Developments in India-Roadmap and Status of Activities., P.K. Nema, P. Singh, P. Satyamurthy, S.B. Degweker, V.K. Handu - https://www-pub.iaea.org/MTCD/Publications/PDF/P1433_CD/datasets/papers/ads_int-03.pdf
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ADS Programme and Associated Developments in India- Roadmap and Status of Activities. - https://www-pub.iaea.org/MTCD/publications/PDF/P1433_CD/datasets/presentations/ADS-INT-03.pdf
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Overview of Activities on Accelerator Driven Subcritical System in India, Amar Sinha - http://accapp20.org/wp-content/2017/data/pdfs/134-22919.pdf
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Application of Accelerators for Nuclear Systems: Accelerator Driven System (ADS), Pramod Kumar Nema - https://www.researchgate.net/publication/251712612_Application_of_Accelerators_for_Nuclear_Systems_Accelerator_Driven_System_ADS
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BEAM DYNAMICS FOR THE 1 GeV PROTON LINAC FOR ADS, Rajni Pande, Shweta Roy, T. Basak, S.V.L.S. Rao and P. Singh - http://www.barc.gov.in/publications/nl/2007/200710-22.pdf
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Developmental Studies of High Current Linac for Indian ADS program, P. Singh - https://www.rrcat.gov.in/jas08/talk/ps_RRCAT.pdf
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Existing and upcoming particle accelerators in India, B. Lalremruata, Swaminathan Kailas, Vasant Bhoraskar, S. Ganesan - https://www.researchgate.net/publication/320948083_Existing_and_upcoming_particle_accelerators_in_India
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OVERVIEW OF ACTIVITIES ON ACCELERATOR DRIVEN SUBCRITICAL SYSTEM IN INDIA, Amar Sinha, Tushar Roy, Rajeev Kumar - http://accapp20.org/wp-content/2017/data/pdfs/134-22919.pdf
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Thermal hydraulic studies of spallation target for one-way coupled Indian accelerator driven systems with low energy proton beam, V MANTHA, A K MOHANTY and P SATYAMURTHY - https://www.researchgate.net/publication/225100969_Thermal_hydraulic_studies_of_spallation_target_for_one-way_coupled_Indian_accelerator_driven_systems_with_low_energy_proton_beam
Indian ADS Programme, P. Singh - https://indico.cern.ch/event/564485/contributions/2379331/attachments/1403459/2149741/psingh_ADSIndia.pdf
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