GEN IV Nuclear Reactors

The need for clean and sustainable energy has led scientists and engineers to undertake several projects and experiments for finding new and innovative energy system. Until fusion energy is achieved we have to deal with our existing technologies. Nuclear fission is one such source which has been providing clean energy for industries, transportation and other commercial applications for decades. Apart from certain accidents like Chernobyl and Fukushima it has shown higher prospects among other non-renewables as a sustainable energy source. The popular candidates among renewables are Solar and wind. But the issue with solar or wind is that they are diluted source of energy. And not all parts of the world receive sufficient amount of sunlight or wind. So, these cannot be used as base load electricity supply systems. 

Utilizing the nuclear fission energy along with solar, wind and hydro is going to be vital for modernization. Nuclear is going to play a important role in supplying 24x7 power supply, hence satisfying growing electricity demands in the future. But public acceptance of Nuclear is very meagre, which attributes to the safety and cost of the power plants. 

Looking up for the possible prospects in the sector of nuclear, the Generation IV nuclear reactors are being under development. Scientists and engineers from several countries intend to take up innovative R&D covering all technological aspects related to design, manufacturing, energy conversion and fuel cycles.

GENERATION IV NUCLEAR REACTORS

Fig :- "Generation IV road-map" from Argonne National Laboratory. (Image source)

Six next generation nuclear reactors systems were finalized by the GIF: - Generation IV Forum formed by nine member nations. Technical experts from around the world selected these six nuclear reactor designs for R&D as Generation IV nuclear reactor systems.

• VHTR - Very High Temperature Reactor

• GFR - Gas cooled Fast Reactor

• LFR - Lead cooled Fast Reactor

• MSR - Molten Salt Reactor

• SFR - Sodium cooled Fast Reactor

• SCWR - Super-Critical Water Reactor

These Generation IV systems aim at performance improvement, new application of nuclear systems, sustainable methods for management of nuclear materials and efficient energy conversion systems. These reactors mainly focus on developing high temperature reactor as they have possibilities of process heat application and hydrogen production. The adopted approach also shows reduction in waste production and fuel consumption. 

GOALS FOR GEN IV NUCLEAR REACTORS

1. The Generation IV reactor systems will meet the clean-air objectives,    producing negligible amount of Carbon-dioxide.
2. Long term availability of the energy producing system.
3. Efficient fuel utilization with for worldwide energy production.
4. Advanced waste management systems and improving public and environment protection.
5. These Gen IV systems will have advantage on life cycle cost over other sources.
6. The systems will excel in safety and reliability and minimize reactor core damage or meltdown.
7. Will eliminate the need for offsite emergency operations.
8. They will have restriction against weapon proliferation and will be unattractive for act of terrorism.

GENERATION IV NUCLEAR ENERGY SYSTEMS 

1)Very-high-temperature reactor (VHTR):

It is kind of high temperature but the temperature being higher than the conventional high temperature reactors (as the name suggest). The VHTR is a gas (Helium) cooled, graphite moderated (reflector and in core), and thermal spectrum nuclear reactor design. The core outlet temperatures are >900 degree Celsius. But the goal is to achieve 1000 degree Celsius, which will be appropriate for production of hydrogen fuel by thermochemical processes. The thermal power of the reactor will be such that it will have a heat exchanger for passive decay-heat removal, estimated to be about 600MW. The decay-heat is the heat liberated by the core after shutdown of the reactor operations. This reactor will be used as co-generation plant with electricity generation and hydrogen production, also can be used for other process-heat applications in chemical, oil and iron industries. The production of hydrogen is done from water using electrochemical and hybrid processes with reduced carbon-di-oxide emission. The technical basis of the technology is Tri-structural isotropic coated particle fuel (TRISO). The advantage with such fuels is that they can withstand thermal loads well beyond the limit. Hence, the core will have reduced chances of meltdown in extreme conditions. 

TRISO fuel

The TRISO fuel is made up of Uranium, carbon and oxygen kernel. The kernel is encapsulated by a three layered carbon and ceramic based material that prevent release of radioactive fission products. The particle is small and robust. It is specially made for the use in high temperature and molten salt cooled reactors. They show more resistance to neutron radiations, oxidation and corrosion in high temperature cores. The three layered coating allows these fuel particles to retain fission products under all reactor conditions. 

Generally, the VHTR can be classified as;

1) The Prismatic block type

2) Pebble bed type

Only the shapes of the fuel elements are different, apart from that the overall operations of the VHTR are same. 

For power generation a direct helium cooled cycle can be used in the primary coolant loop, known as direct cycle, or a steam generator can be used for conventional Rankine cycle. For process heat applications such as refineries, petrochemical, metallurgy and hydrogen production an intermediate heat exchanger can be used in a so called, indirect cycle. 

The proposed VHTR has inherent safety, higher efficiency, process-heat application, low operational cost and modular construction.

2)Gas-cooled Fast reactors (GFR):

It is a kind of high temperature, gas (helium)-cooled, fast neutron spectrum and closed fuel cycle (the spent fuel is reprocessed) reactor. The high core outlet temperature > 750°C (typically 800-850°C) is an added value of GFR technology. The fast neutron spectrum assures long-term sustainability of Uranium resource and minimization of waste production through fuel reprocessing. And also this reactor system supports high temperature systems with high thermal efficiency and provides heat for hydrogen production. It requires proper refractory fuel elements and safety designs. The GFR uses the same technological aspects of a VHTR such as materials for different component and energy conversion systems. But demands more attention in it’s reactor core design and safety. The reference thermal output of the proposed design is 2400 MW(th) and with 1200 MW of electrical power. 

The reactor core is contained in a pressure vessel made up of steel.  The core consists of hexagonal fuel elements, with ceramic clad, mixed carbide fuel pins contained in a hexagonal fuel tube. 

For energy conversion the GFR will have a indirect thermodynamic cycle, with helium coolant in the primary loop carrying away heat from the core and a secondary loop with helium-nitrogen mixture. The secondary loop will be incorporated into a closed gas turbine cycle. Proposed power plant designs also consist of a combined cycle which is common practice in natural gas power plants. The waste heat from the gas turbine will be used to generate steam which will drive a steam turbine. Thus, in the advancement of commercial GFR it is essential to establish a type of experimental reactor for qualification of the refractory fuel elements and for full-scale demonstration of the GFR-specific safety systems.

The core structural material for the GFR (cladding, reflector, control rod guides, etc) needs to be developed which can withstand damage caused by fast neutrons and high core operational temperature. The reflector must have specific neutronic properties to avoid neutron leakage and protecting the surrounding in vessel components, an inter-metallic compound of Zr and Si is the favored for the experimental reactor.

z

Further advancement in GFR system depends on development of previously mentioned VHTR, which has same technical aspects as GFR.

3)Sodium-cooled fast Reactors (SFR) :

The SFR is a liquid sodium cooled, closed fuel-cycle and fast neutron spectrum reactor technology. It has a reference reactor outlet temperature of 500-550 degree Celsius and operates at near atmospheric pressure. Closed fuel cycle is considered for better waste management. Using liquid coolant consumes less pumping work as compared to the gas cooled reactors.

A variety of fuel options are being investigated such as oxides and metal alloys. By using liquid sodium as coolant, a high power density can be achieved with low coolant volume fraction and at low pressures. There is a significant thermal inertia in liquid sodium coolant used because of it's advantageous properties, like high thermal conductivity, boiling point, heat of vaporization and heat capacity. For avoiding the sodium to react chemically with air and water a sealed coolant system must be designed. The reactor can be placed in a pool layout or arranged in compact loop layout.

(Thermal inertia : It is a material property that defines the degree of slowness with which it's temperature reaches that of environment.)

Power plant considerations for SFR ranges from 50-300 MW (electrical) in modular reactors to 1500 MW (el.) in large plants. Power plant design include a primary sodium which circulates around the core collecting heat at near atmospheric pressure and a secondary sodium coolant loop between the steam generator and radioactive primary sodium.  Water/steam (Rankine cycle), super-critical carbon dioxide or nitrogen (brayton) cycles can be used as working  fluid in the energy conversion system for thermal efficiency and safety. 

The prime aim of the SFR is the effective management of high grade waste and uranium fuel resource. SFRs are good option for electricity production and is ranked highly for it's prospects for sustainable development. By recycling the spent fuel, decay heat and radio-toxicity is minimized and enables regeneration of fissile elements. 

4)Lead cooled Fast Reactors (LFR):

The LFR is a closed fuel cycle, fast neutron spectrum and high temperature nuclear reactor. The coolant can be either liquid lead or lead-bismuth eutectic. 

(Eutectic system:  It is a homogeneous mixture of substances that melts or solidifies at a single temp. that is lower than the melting point of any of the constituents.)

The lead and LBE coolants are relatively inert liquids with excellent thermodynamic properties. The LFR is envisioned for multiple applications which include electricity generation, hydrogen production and process heat applications. For power plant application three reactor size has been considered : a small 50-150 MW (electricity) modular transportable system with long core life, a medium sized 300-600 MW(el.) and a long term large system with 1200 MW(el.). 

Since it uses fast neutrons and a closed fuel cycle fertile uranium can be transmuted to fissile fuel, LFR assures better fuel management. The LFR is attributed by enhanced safety as a result of considering molten lead as a inert and low-pressure coolant. Fuel sustainability is enhanced by the usage of Lead coolant in conversion cycle. 

LFRs offers other substantial benefits like safety, simplification in design, manufacturing and higher economic performance. 

5) Molten Salt Reactors (MSR) :

MSRs have been investigated since 1950s, but with only one experimental reactor at Oak Ridge National Laboratory in 1960s which was a  thermal-spectrum graphite moderated reactor. The higher economic prospects of PWRs and BWRs have posed tough competition for commercialization of MSRs. In the last decade there has been a renewal of interest in this reactor systems particularly for it's inherent reactor safety and flexibility. 

The MSR has a very special feature of liquid fuel, which attributes to it's inherent safety from accidental reactor meltdown. The Gen IV MSRs will apply closed fuel cycles. This concept can be used as efficient burner of transuranic materials produced from LWRs. MSRs can also be employed as breeder reactors with neutron spectrum ranging from thermal (Thorium fuel cycles) to fast (Uranium-plutonium fuel cycle). These reactor concepts are considered under Generation IV reactors because of the promise for the reduction of radiotoxic wastes and spent fuel usage.

(Transuranic :- Elements with atomic number greater than Uranium. Mainly man-made from nuclear fission activities such as plutonium and americium-241 and are considered high level waste having half-life more than 20 years.)

MSRs are distinguished from other reactors by the fuel used, the fuel is dissolved in molten fluoride salt. R&D on MSRs focus on development on the assets of fast neutron reactors and using molten salt fluorides as liquid fuel and coolant, which excellent physical properties like high boiling temperature and low pressure. 

The core outlet is in the high temperature range of 700-800 degree Celsius and with a proposed electrical output of 1000 MW. The power conversion considered for MSR is a three loop cycles, primary coolant loop which circulates the radioactive fuel-cum-coolant liquid, the heat from the primary loop is exchanged with the coolant salt in the secondary loop and the third thermodynamic loop is a Closed-Brayton power cycle. 

MSRs employ passive safety system, which does not require any external human intervention and takes advantage of natural forces of gravity and convection. Since the fuel is already in liquid phase it eradicates the event of meltdown. In case of an accidental overheating by coolant pumping failure the hot liquid fuel is drained into passively cooled emergency dump tanks through a freeze valve (a plug of salt). The high temperature molten salt fuel/coolant cools inside dump tanks for several hours before secondary cooling can be employed. This formidable safety method is possible because the fuel is in liquid phase. 

There has been significant renewal of interest in liquid fluoride as coolant in nuclear ans non- nuclear applications. With the use of liquid fuel MSR shows unique passive safety characteristics which is not found in solid-fueled reactors. Other advantages include effective fuel utilization and actinide burning. 

6)Super-critical Water Reactors (SCWR):

The SCWR are a kind of high temperature and pressure water-cooled reactors. These systems can operate in a direct or indirect thermodynamics cycles with water above it's critical point (Critical pressure : 22.064 MPa and temperature : 373.95 degree Celsius). The SCWRs combine the technicalities of the hundreds of water-cooled reactors and hundreds of steam power plants which use supercritical water. Both pressure tube and pressure vessel configurations can be used for reactor designs. The reactor outlet temperature is in the range of 510-625 degree Celsius and proposed electrical power plant with an output of 300-700 MW in small scale and large scale power plant with output of 1000-1500 MW. As in the BWRs the supercritical steam will be directly supplied to a high pressure turbine and the feedwater will be cycled back to the reactor. 

The unique features of a SCWR provides several advantages over the conventional water-cooled and moderated reactors:-

1. The thermal efficiency of SCWR can approach upto 44% where as the current water-cooled reactors have efficiency of about 30-36%.
2. Due to use of super-critical water there is a significant rise in core outlet enthalpy which reduces the mass flow rate through the core for a given power output.
3. The higher supercritical steam enthalpy reduces the size of turbine. Hence, making the design less costly than conventional reactors.
4. The containment with emergency cooling and residual heat removal system can be significantly smaller than those of conventional reactors.
5. The steam generators in PWRs and steam separator and dryers in BWRs can be omitted for SCWRs.

The above features offer the potential for development of low cost nuclear power plants with better efficiency and fuel utilization,thus have a clear advantage over the current operating LWRs.

LWR :- Light water reactors

PWR :- Pressurized water Reactors

BWR :- Boiling water Reactors

Conclusion:

Nuclear power is a non-renewable source as fossil fuels but the nuclear fuels are energy dense and can be used longer than some fossil fuels. Currently, Nuclear is the most viable one for electrical generation for the next 50 – 100 years with apparently zero carbon footprint. But they produce radioactive waste which must be stored safely or reprocessed in modern NPPs for trans-mutating it into fissile fuel.

The major driving force for developments in nuclear power plants is thermal efficiency.  

Ranges of gross thermal efficiencies of modern power plants are as the following: 

1) Combined-cycle thermal power plants – up to 62%

2) Supercritical-pressure coal-fired thermal power plants – up to 55% 

3) Carbon dioxide-cooled reactor NPPs – up to 42% 

4) Sodium-cooled fast reactor NPP – up to 40% 

5) Modern watercooled reactors NPP– 30 – 36% 

All current and oncoming Generation III+ NPPs are not that competitive with modern thermal power plants in terms of thermal efficiency, the difference in values of thermal efficiencies between thermal and nuclear power plants can be up to 20– 25%. Therefore, developments of Generation IV NPPs with thermal efficiencies close to or more than those of modern thermal power plants are prime focus for next decade. Also, modern NPPs require better safety features with radioactive waste minimization and efficient fuel utilization. All these advantages in NPPs under Generation IV systems attributes to the cost effective commercialization of  modern NPPs.