Thursday, March 15, 2018

Compact High Temperature Reactor - CHTR - Towards Meeting India's Process Heat Requirement

New sustainable Technologies to meet the country's burgeoning energy need responsibly.

As per the Ministry of Power's report, "Growth of Electricity Sector in India from 1947-2017", it's current annual, per capita consumption of electricity is around 1122 kWh. For it to reach a stage of moderately high human development, it has to register annual per capita consumption upwards of 5000 kWh. For a Hydrocarbon deficient India, that is a tall order by any estimate. It has already had to nearly double its Crude Oil import over a decade, 2006-2007 [111.15 MT] to 2015-2016 [202 MT], while its Coal import has registered almost 500% increase over the same period [43.08 MT Vs. 199.88 MT] [source]. Thus, any attempt to  reach the upper-middle-income country group by 2047, via the current approach is quite unsustainable.

Compact High Temperaure Reactor CHTR 04 Schematic Diagram

The National Hydrogen Energy Board, convened in 2004, proposed for India to adopt the use of Hydrogen as a fuel, introducing it into its energy mix, to diversify its energy basket. As per the recommendations of the task force, listed in its 'National Hydrogen Energy Roadmap', a greater focus must be made towards transitioning transport vehicles towards using hydrogen as fuel, so as to have the greatest positive impact.

Hydrogen, as a fuel, offers some unique & significant advantages. It has the highest energy density amongst all fuel [120.7 MJ/kg], while also being clean, emitting no Greenhouse gases.  A hydrogen economy can, thus, prove greatly beneficial, both, since it is a technically superior fuel, as also helping India achieve energy security. Some of the issues that need to be addressed before it can be widely adopted include sourcing sufficient quantity of hydrogen, safe storage & handling, infrastructure development etc. Hydrogen can be generated by various means, such as coal gasification, steam reforming, electrolysis of water, high temperature thermochemical splitting of water, from biological sources etc..

Hydrogen System - National Hydrogen Energy Roadmap - India


Of these, the thermo chemical splitting process of water holds the greatest promise, as it has the potential to generate large quantities of hydrogen, with a high degree of efficiency [40-57%]. Temperatures of ~1000 degrees Celsius is needed to carry out the process. For any transition towards a hydrogen-based economy, it is essential that large-scale generation of hydrogen be an economical proposition, which primarily boils down to [no puns intended] being able to achieve the high temperatures needed economically & on a commensurate scale.

With this end in view, the Department of Atomic Energy [DAE], in India, is working towards development of High Temperature Reactor [HTR] technologies. Classified as a Generation IV reactor, the Compact High Temperature Reactor [CHTR] programme, being spearheaded by it's Bhabha Atomic Research Centre [BARC], is proposed to be a Technology Demonstrator platform, to validate concepts & prove technologies necessary to build full-scale HTR.

Operating on the Brayton Cycle, BARC's design is expected to generate 100 KWth of Thermal Power. It's primary function would be to produce heat at  around 1100 degree Celsius temperature, needed to split the water molecule, for liberation-generation of Hydrogen, via Thermo-chemical process. The Reject Heat could, then, be used to generate electricity, while Waste Heat could desalinate water.

Compact High Temperaure Reactor CHTR 01 Reactor Core


The CHTR consists of a Core made of 19 Nos. of hexagonally formed hollow Beryllium Oxide [BeO] Moderator rods, within which the fuel would be housed. A mixture of Uranium-233 & Thorium-232, weighing 2.4 kg & 5.6 kg respectively, would fuel the CHTR's Core, that would require refueling every 15 years.

To facilitate high fuel burnup, needed for generating the high temperature [compared to the ~300o C, generated in Power Reactors], the fuel particles would receive a Tri Isotropic [TRISO] coating. Each particle is a mix of fissile, fertile & burnable poison, covered in 4 layers of protection. Each Core would house 14 Million such TRISO coated microsphere fuel particles. These fuel particles would then be pelletized into Fuel Compacts of 10 mm diameter & 35 mm long each, placed inside hollow Graphite Rods, to be used in the CHTR's Core. Each Core would contain around 6840 such pellets.

Compact High Temperaure Reactor CHTR 08 TRISO Fuel Core

During operation, a Eutectics alloy of Lead [44.5%] & Bismuth [55.5%] would serve as coolant, absorbing the Nuclear heat generated. An appropriate coolant, given that, among other reasons, it has a Melting Point of 123 degree Celsius & Boiling Point of 1670 degrees Celsius, thus allowing flow to occur in a non-pressurised atmosphere, via natural convective circulation. Liquid Sodium, flowing through the Heat Exchanger,  will absorb this heat from the coolant, for end-use utilization.

Compact High Temperaure Reactor CHTR 02 Graphite Fuel Tube


BARC has also zeroed in on the use of Prismatic Rods of Beryllium Oxide as Reflector [6 movable, 12 movable], in addition to Graphite, to confine the Neutrons within the Core, for perpetuating Chain Reaction.

It is, pretty much, engineering Safety into the CHTR's system. This is exemplified no better than the use of Lead-Bismuth Eutectic alloy, with its 1670 degrees Celsius boiling point, as coolant, to transfer heat energy at around 1100 degrees of operating temperature. Similarly, the TRISO-coated fuel microspheres that maintain it's leak integrity till 1600 degrees can, therefore, safely function under the designed operating temperatures of the CHTR.

The negative operating characteristics of the fuel [Doppler Coefficient], the Moderator [Temperature Coefficient] & Coolant [various Reactivity effects], adds to it's safe characteristics. Proliferation-proof is the underlying USP of such Reactors, since they do not produce any usable fissile material.

As keeping with the current focus of implementing passive systems of safety measures, the CHTR too will be equipped with multiple levels of redundancies of passive safety systems.

Not surprisingly, the Technological-Engineering challenges that need to be addressed to Compact High Temperaure Reactor CHTR 09 Schematic Diagramrealise this prototype High Temperature Reactor, is quite a list. This is due to no small part owing to the extreme environmental condition in the CHTR's Core, because of  its high operating temperature - nearly 266% higher than conventional Power Reactors. The corrosive properties of Pb-Bi coolant, for example, gets even more prominent at those high temperature, requiring use of materials that can withstand it, as well as techniques to mitigate it.  Selecting materials & choosing manufacturing processes to fabricate the component becomes a challenge in itself. In many cases, it becomes imperative to develop a suitable manufacturing/fabrication process before component production can actually commence - COTS not always of much help.

For manufacturing the TRISO-coated fuel particles, for example, BARC has had to adapt the Internal Gelation Process [IGP], to fabricate fuel kernels made of Carbide & Oxides of Uranium & Thorium. For applying the TRISO coating, it has, similarly, had to develop suitable Chemical Vapour Deposition [CVD] process to apply the multiple layers of Pyrolytic Carbon & Silicon Carbide coating, deriving exact parameters of temperature, volume flow rate, pressure needed.

The table, below, gives an overview of the technologies that needs to be validated, prior to integration into the CHTR. It must be noted that the current status of development is likely to be further ahead, give that this list has been sourced from a reference, published a while ago.

Compact High Temperaure Reactor CHTR 03 New Technologies

That said, the CHTR proposes to use Uranium-233 in its fuel mix, a scarce resource in the country, that India plans to breed in larger quantities its Stage-II Nuclear Power Plant Reactors, to be built as part of its unique 3-Stage Nuclear Power Programme. High gestation period of technologies, listed in the table above, require, therefore, for developmental efforts to be carried out in the present time, to be ready to harness the material, upon availability.

The Compact High Temperature Reactor is also answer to the question, posed a few years ago. Thank you to the anonymous person, who recently enquired about it.

For a detailed, technical overview of the various aspects of the CHTR, BARC Highlights [Chapter 3], provides the most comprehensive information.

Along with BARC's pursuit of developing the CHTR, it has also already initiated preliminary studies to undertake development of its follow-on, a practical HTR, capable of generating Hydrogen on industrial scale. The Innovative High Temperature Reactor [IHTR] is proposed to be a 600 MWth setup, that would incorporate systems & technologies validated by the CHTR. Preliminary studies, based on Thermal Hydraulics & Temperature Distribution analysis, suggest adopting a Pebble Bed Reactor Core design, using  Molten Salt as Coolant.

Innovative High Temperaure Reactor IHTR 05


Some additional technology thrusts, specific to the IHTR include, manufacturing pebble-type fuel compacts, reprocessing the pebble fuel, loading & unloading system for the fuel, manufacturing process for fabricating large-size components from the brittle Nuclear-grade Graphite, among others.

DAE/BARC has been quite quiet about progresses made developments in it's High Temperature Reactor development efforts. One can only hope that work on this front has been going on apace, absence of public updates, notwithstanding. A technology of National importance needed to achieve energy independence & security.


Also Read: How To Design A Nuclear Power Reactor, the A, B, C, Ds