Interest in small modular nuclear reactors, which are planned to have short construction times, standard designs and simplified operation, as well as a much higher fuel utilisation, is showing a resurgence. The travelling wave nuclear reactor (TWNR), based on the breed and burn principle, is one of the options being considered.
Any optimal energy technology for the future must meet stricter standards than in the past; in addition to being economically attractive, it now must also be environmentally benign, sustainable and scalable to global use. For stationary energy, only one existing resource comes close to meeting all the technical and societal requirements for an optimal energy source: nuclear energy. Despite the obvious and unique effectiveness of nuclear energy of new generation Reactors, there are difficulties in achieving the nontrivial properties of an ideal nuclear reactor of the future.
First, nuclear fuel should be natural, that is, non-enriched uranium or thorium. Second, traditional control rods should be absolutely absent in reactor active zone control system. Third, despite the absence of the control rods, the reactor must exhibit inherent safety. This means that under any circumstances the reactor active zone must stay at a critical state, that is, sustain a normal operation mode automatically, with no operator actions, through physical causes and laws that naturally prevent an uncontrolled chain reaction.
Generation V reactors are being developed to comply with the above requirements. One of the most promising designs is a “burn and breed” type of reactor where the fission occurs in a wave that moves through the fuel. This type of reactor is known by several names: the travelling wave nuclear reactor (TWNR) and the “Candle” reactor (constant axial shape of neutron flux, nuclides densities and power shape during life of energy production).
A travelling wave reactor is a device where a fission or “burning” zone moves in a wavelike fashion through a column of nuclear fuel. The fuel consists of fertile material such as natural uranium and thorium, or spent fuel uranium from a conventional reactor, which is converted to fissile material as the burn zone moves through the column. This process is similar to that of a breeder reactor. The progress of the burn zone can be very slow, of the order of a few cm per year, and the TWR can operate for several decades on a single batch of fuel. The limiting factor appears to be the lifetime of fuel cladding and other materials used in the reactor. One of the foreseen advantages is that it can be constructed safely in a range of sizes, from 100 to 1000 MWe and could be constructed in modular form.
There are two ways in which this principle is being applied:
Fuel utilisation efficiency
TWRs are characterised by a high fuel burnup. Fuel burnup is the amount of fuel that is converted to energy in the fission process. This is most easily expressed as a percentage of the fertile fuel that is used. TWRs are capable of a 40% or higher burnup ratio, compared to approximately 1% for conventional reactors. TWRs are thus estimated to be capable of offering a 40-fold gain in fuel utilisation efficiency compared to conventional light-water reactors burning enriched fuel. The high burnup ratio also reduces the need for refuelling, as more energy is obtained from the same amount of fuel.
The spent fuel does not require reprocessing. The waste treatment for the system is much simpler than a system with reprocessing. The technologies and facilities of uranium enrichment and spent fuel reprocessing are the most important items for nuclear proliferation. This system does not require these technologies and facilities. Since the reactor has a long life and refueling is not required, the reactor vessel can be sealed during reactor life, which helps physical protection. All these characteristics enhance the sustainability of nuclear energy.
The concept of TWR technology was developed from traditional fast breeder reactors The TWNR is not a new concept, and the initial proposal of a fast reactor design that could sustain a breed-and-burn condition using only natural uranium or depleted uranium as fuel was made in 1958 by Savelli Feinberg . Feinberg imagined what we now call a breed-and-burn reactor. Early proposals featured a slowly advancing wave of nuclear fission through a fuel source, like a cigar that takes decades to burn, creating and consuming its fuel as the reaction travels through the core. But Feinberg’s design couldn’t compete during the bustling heyday of atomic energy. Uranium was plentiful, other reactors were cheaper and easier to build, and the difficult task of radioactive-waste disposal was still decades away .
Breeder (Breed and burn (BB)) reactor principles
The Breeder reactor is one which consumes fissionable fuel from its core and breeds fissile fuel from fertile blanket fuel. The capability of a given breeder reactor is decided by its breeding ratio, which is the ratio of fissile material created to fissile material consumed. Natural uranium, as it is mined, consists of 0,7% U235 and 99.3% U238. In conventional reactors, uranium fuel is enriched to contain 3 to 5% of U235 . Spent fuel from a conventional reactor contains uranium with a content of 0,8% of U235, the balance being U238 . In a breeder reactor, stable fertile isotopes of uranium (U238) are converted by fast neutron bombardment to fissile material, usually plutonium. BBR can thus use both natural uranium and spent fuel as fuel. The conversion process is illustrated in Fig. 3.
Absorption of a neutron in the U238 nucleus yields U239. The half-life of U239 is approximately 23,5 min. U239 decays (negative beta decay) to Np239 (neptunium), whose half-life is 2,36 days. Np239 decays (negative beta decay) to Pu239.which then produces heat through fission,and also results in fission products and free neutrons, which sustain the conversion process of more U238.
The process has to be “ignited” with fissile material, usually enriched uranium, but once started continues as long as there is U238 available. The process can also be ignited by “spent” fuel from a BBR, which contains neutron emitting plutonium. A series of BBR could be “spawned” from a single batch of enriched uranium over decades. The principal has been applied in ways which have resulted in a number of variations. The major differences are configuration, either radial or axial wave, and the method of heat removal or cooling.
Axial travelling wave reactors
Several designs have been developed, the best known being the “Candle” reactor
The Candle reactor
In this concept, developed by Hiroshi Sekimoto, a burning region moves through the column of fuel, leaving spent fuel behind (Fig. 4). In the top position of the burning region, the produced neutrons leak to the fresh fuel region, and in the bottom position of the burning region the fission products (FPs) are accumulated by fission reactions of the fissile materials .
The Candle burnup strategy has the following general merits:
The Candle could be applied effectively several designs such as reactors with block fuel arrangement, as the progressing wave allows removal of spent fuel blocks and addition of fresh fertile blocks, allowing operation to be unlimited by the height of the core. This is shown in fig 4. As applied to a high temperature gas reactor (HTGR).
The principle can also be applied effectively to small reactor design . The speed of the burning region of a typical small design is 4 cm/y. 20 years operation requires only 80 cm of the core fuel height, and it is possible to design a reactor, which does not require refueling. The maintenance of this reactor becomes simple, safe and easy. The reactor is barely critical and redistribution of fuel usually reduces its criticality performance. Therefore, it is almost free from CDA accident. All these features make our reactor very safe and reliable, and its operation and maintenance become very simple and do not require any highly trained specialists. 
The stationary wave nuclear reactor
In a SWNR the burn zone or wave is stationary and fuel is moved relative to the wave zone. The most common design is a radial wave as illustrated in Fig. 2. The core has four zones. The localised fission zone contains the initial fissile material which may contain the U235 that decays producing fast neutrons. These are captured in the surrounding breeding zone, converting a fertile isotope like U238 into a fissile isotope like Pu239, which itself decays producing between two or three fast neutrons. The fresh zone contains unreacted fertile material, and the depleted zone contains mostly fission products and leftover fuel .
The Terrapower SWNR
TWR-P is a 1475 MWth/600 MWe gross liquid sodium cooled, fast neutron spectrum reactor that uses U-10%Zr metallic fuel with HT-9 ferritic-martensitic stainless steel clad. The 4 m diameter, 5,5m tall cylindrical core sits near the bottom of a 13,3 m diameter, 17,65 m tall reactor vessel which is enclosed within a guard vessel.
The reactor, which is in design phase, is an axial wave reactor that uses liquid sodium as a coolant, and operates at a temperature of around 550°C. The breed-and-burn wave of the TWR does not itself move. Instead, the fuel in the core is moved in and out of the breed-burn region which remains stationary as a “standing” wave. Fuel shuffling will be automatic, and won’t need the reactor to be opened. The reactor has a once-through (open) nuclear cycle. It operates at around 550°C, with the heat being removed by liquid sodium to drive steam turbines.
Previous large-scale fast breeder power stations have also used liquid sodium cooling, for example, and the TWR is a pool type reactor (primary heat exchangers and pumps are immersed in the reactor tank) which is also well known. The major difference lies in the core. This is approximately cylindrical and composed of hexagonally shaped fuel bundles. The fuel bundles contain a combination of enriched and depleted uranium metal alloy fuel pins clad in steel tubes. Fig x shows the proposed reactor design.
Core geometry is a major and important part in the SWR. The core is cylindrical and has hexagonal fuel bundles. Core has two types of fuel that are fissile and fertile. The fissile fuel is located in central zone called active control zone and producing most of the generated power.
Fertile fuel is placed in core periphery called fixed control zone. The core has very special design feature i.e. fuel shuffling. In fuel shuffling, after predetermined time, high burn-up fuel assemblies in central core moved to the core periphery by means of instrumentation in the reactor vessel in shutdown condition.
The TWR is projected to have a levelised cost of electricity that is lower than that of LWRs being built today. Capital cost estimates for the TWR yield overnight costs that are similar to equivalent estimates for modern LWRs. Meanwhile, the TWR holds large advantages in its operating costs due to lower fueling and disposal requirements. Over its 60-year life, a 1,15 GW TWR refueled with unenriched uranium would cost between $4-billion and $5-billion less to operate than an equivalent LWR or traditional SFR. Eliminating the need for reprocessing plants and reducing the need for enrichment saves additional hundreds of billions of dollars in infrastructure development costs. In scenarios where TWRs represent a part of the future projected growth of nuclear energy, beginning in the 2030s (~450 GW of TWR capacity by 2100), these cost reductions would total more than $1-trillion dollars .
High fuel efficiency, combined with an ability to use uranium recovered from river water or sea-water (which has been recently demonstrated to be technically and economically feasible) suggests that enough fuel is readily available for TWRs to generate electricity for 10 billion people at United States per capita levels for million-year time-scales . It is estimated that the Earth’s rivers carry into the ocean a flux of uranium several times greater than that required to replace the implied rate-of-consumption, so that the Earth’s slowly-eroding crust may provide a readily-accessible flow of uranium sufficient for all of mankind’s anticipated energy needs for as long as the sun shines and the rain falls .
TWRs are claimed to naturally retain their efficiently-expended fuel for century length time-scales, so that they intrinsically pose minimal safety and security transportation hazards in addition to being full-scale carbon-free energy sources .
The energy value of the depleted uranium stockpiles (“waste”) accumulated in the US is equivalent to, when used in B&B reactors, up to 20 centuries of the total 2010 USA supply of electricity. Therefore, a successful development of B&B reactors could provide a great measure of energy sustainability and cost stability .
 SM Feinberg: “Discussion Comment,” Record of Proceedings of Session B-10, ICPUAE, United Nations, Geneva, 1958.
 E Greenspan: “A Phased Development of Breed-and-Burn Reactors for Enhanced Nuclear Energy Sustainability”, Sustainability, 2012.
 T Ellis, et al: “Traveling-Wave Reactors: A Truly Sustainable and Full-Scale Resource for Global Energy Needs”, Proceedings of ICAPP ‘10, San Diego, 13 – 17 June 2010.
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 Y Ohaka, et al: “Neutronic Characteristics of Candle Burnup Applied to Block-Type High Temperature Gas Cooled Reactor”, ICAPP ’05 Seoul, 15 – 19 May 2005.
 H Sekimoto: “Application of “candle” burnup to small fast reactor”, 5th International Conference on Nuclear Option in Countries with Small and Medium Electricity Grids, Dubrovnik, 16 – 20 May 2004.
 M Koziol: “TerraPower’s Nuclear Reactor Could Power the 21st Century”, IEEE spectrum, 1 June 2018.
 J Gilleland: “The Traveling Wave Reactor: Design and Development”, Engineering 2, 2016.
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