A key element in the nuclear energy production system is nuclear fuel. The same, expensive high-tech product, over the creation of which hundreds of engineers work for the past 50 years. However, severe accidents, for example, at the Three Mile Island and Fukushima Daiichi nuclear power plants, have shown that under extreme conditions nuclear fuel is likely to fail and the accident will lead to significant consequences. Hayman Ricoover in June 194? for the program of transport reactors of the US Navy. This material has the necessary properties and has proved itself for a long time.
 
 
As the material of tablets, heavy metal oxide is commonly used. Usually it is uranium dioxide, much less often - a mixture of oxides of uranium and plutonium. Used in modern power reactors, uranium with 235U isotope content not exceeding 5%, including uranium of natural isotopic composition (~ ???%) or slightly enriched uranium.
 
 
With the preservation of these common features over the past decades, a gradual change in the "secondary" design features of fuel assemblies has occurred. This improved the consumer fuel properties, its reliability and safety, providing a tangible competitive advantage with early versions of fuel assemblies.
 
 
Here are some of them:
 
 
 
Increase in the level of enrichment: in the 1970s it hardly exceeded 3%, whereas today the maximum enrichment for light water reactors is approaching 5%. Along with the increase in the level of enrichment, it is profiled in the core of the reactor - up to the differences between the parts of the tablet in perspective fuel.
 
Increase the load of uranium by mass. This change was primarily due to a change in the geometry of the fuel elements and the structural parts of the fuel assemblies. For example, for WWER reactors, the length of the "fuel" part of the assembly increased by approximately 15 cm. For Western design reactors, the amount of fuel assemblies in the assembly of the square profile changed over time: it was 15x1? it became 17x17.
 
Significant changes in zirconium alloys. A vivid example of the modernization of shell materials is the widespread introduction of niobium as one of the main alloying elements. In contrast to past alloys, in which niobium was absent or contained in small quantities (Zircaloy ? Zircaloy 2), materials containing about 1% niobium began to dominate. This applies, for example, branded alloys Westinghouse (Zirlo, improved Zirlo, AXIOM), Framatome /Areva (alloys M? Q) and Rosatom (alloys E11? E635). In this case, such components as tin, nickel and chromium were reduced or excluded in a number of zirconium alloys. Technologies were improved to minimize the content of hafnium in a zirconium alloy.
 
Complex improvement of fuel assembly designs. During the development, some components of the assembly structure (shrouds and TVS covers) were excluded. Solutions have emerged that increase the strength of the fuel assembly, its resistance to deformations, solutions that provide additional integrity of TVEL (introduction of antidebrasive filters) and satisfy new regulatory requirements, for example, seismic resistance. The construction of the fuel assemblies was made collapsible, thereby permitting the replacement of individual fuel elements and continued operation.
 
 
Above are listed not all, but rather the most basic changes in the design of fuel, which occurred from the moment of the fabrication of the first fuel assemblies.
 
 

The stumbling block is

 
From the first point you can guess that the current fuel assemblies for a long time of development have already managed to reach the limits of efficiency and safety, but at least two factors now oblige designers to continue improving nuclear fuel further.
 
 
Considering the enormous specific power of the energy release of the light water reactor core ~ ​​150 W /cm? combined with the possibility of introducing positive reactivity or loss of cooling in this complex system, engineers designing reactors from the very beginning understood the importance of designing safety systems.
 
 
In order to develop a mitigation strategy, in the event of accidents, two types of events were taken as a basis for the design of safety systems: events based on positive reactivity input (3R3r3283 reactivity insertion accident (RIA) [/i] ) And events based on loss of coolant ( loss-of-coolant accident (LOCA) ). The basic security systems were specifically designed to respond to these design events.
 
 
But the experience of such accidents at both the Three Mile Island and Fukushima Daiichi NPP proved that with multiple failures and imposition of initial events, active security systems are not able to cope with the functions assigned to them, especially the removal of residual heat from fuel assemblies located in the core .
 
 
The equation of heat transfer in its simple form explains well what is happening in the nuclear reactor during the development of an accident with loss of heat sink:
 
 

 
 
The left-hand side of the equation describes the temperature change (
? T [/i] ) In time (
? t [/i] ); This change is also determined by the heat capacity of materials in the core ( pCr ). The first term on the right-hand side in the general case presents in a simplified form heat exchange processes (conduction, convection and radiation) for removing heat from the core. The second term is the amount of heat generated in the core (
? Q [/i] ).
 
 
During the course of the aforementioned events, the core cooling mode is violated, the first term on the right side becomes numerically smaller and the heat Q gradually causes an increase in temperature. Since the active zone of the reactor becomes partially or completely stripped (the level of water falls, the water changes to steam), the effectiveness of the heat removal from the core decreases sharply, TVEL temperatures continue to grow, which serves as the beginning of chemical and physical degradation of fuel elements. Physical degradation of the fuel element membrane begins at temperatures (700-1000 ̊С) and causes blistering and rupture of the shells.
 
 
Chemical degradation is expressed, mainly, by steam oxidation of zirconium. The key factor is the exothermicity of this reaction. And of course, the product of this reaction is explosive hydrogen. For example, ~125 kg of zirconium in each fuel assembly of a pressurized reactor produces about 820 MJ of heat and more than ?700 moles of gaseous hydrogen upon reaction with steam.
 
 
Depending on the design of the light water reactor, about 25-40 tons of zirconium is present in the core, with full oxidation of which a huge amount of heat will be produced, in addition to the residual (at best) energy release of the fuel itself.
 
 

 
The value of the thermal capacity of the system as a function of the reactor shutdown time, taking into account the exothermic reaction of zirconium oxidation
 
 
The generated hydrogen, in turn, will not peacefully accumulate, and without properly triggering systems for its disposal, will lead to a large-scale explosion or fire, while the active zone can melt the hull and take up the concrete of the reactor compartment.
 
 

The concept of fuel resistant to accidents
 

 
The apocalyptic scenario described above basically repeats the events at the Fukushima nuclear power plant in 2011. This event led to the revision of a number of nuclear safety standards, especially concerning serious design and beyond design basis accidents (with complete de-energization of the reactor installation and loss of the coolant). Incidentally, thanks to this accident, in many regions of the world, the competition of nuclear power plants with other energy sources has become aggravated, which considerably increases the requirements for the economy of nuclear power plants and their safety (with equal or sometimes losing economic performance, investors may prefer non-nuclear energy sources).
 
 
This factor significantly increases the requirements for all elements of electricity generation at nuclear power plants, especially nuclear fuel. In the current decade, work has been intensified to create a fundamentally new fuel that can withstand the conditions of severe accidents while maintaining or improving economic performance and safety in normal operation. A lot of developments of this kind were collectively called Accident Tolerant Fuel (ATF) - fuel with increased resistance to accidents.
 
 
At the heart of the philosophy of changes in the design of fuel assemblies is the replacement of materials of the main components of nuclear fuel, mainly fuel tube envelopes and fuel pellets for materials that will be more resistant to processes occurring at the time of the accident.
 
 
Sheath of TVEL
 
 
The main approach in the choice of materials for the fuel element for fuel ATF is the need to eliminate or reduce the degree of steam zirconium reaction and, as a consequence, the release of additional heat and hydrogen. A quick and obvious solution is the application of a protective coating on the surface of a zirconium sheath. Thin coatings on the zirconium shell should have minimal effect on the thermal and neutron-physical characteristics of the fuel. Scientists have found that good resistance to vapor oxidation at high temperatures is chromium, aluminum, silicon. These impurities demonstrate stability in a high-temperature vapor environment, despite the fact that they can react somewhat with steam.
 
 

 
The rate of parabolic oxidation for various materials in a vapor, depending on the temperature
 
 
As can be seen from the graph, the oxidation rate of these materials, which form and are accordingly protected by their oxide films, is two orders of magnitude lower than the rate of zirconium oxidation. Reduction of the rate of steam oxidation directly affects the rate of heat and hydrogen evolution in the presence of heavy LOCA in the core of the reactor.
 
 
The coating of TVEL envelopes with metallic chromium is now considered the most promising technology for further development. Also, TVEL shells without zirconium are considered promising, for example, FeCrAl and SiC /SiC materials.
 
 

 
Testing of the shell of the silicon shell of EnCore fuel (Westinghouse) at a temperature above 1300ºC
 
 
Fuel cells
 
 
The second most important direction in the development of ATF fuel is the choice and justification of the material of the fuel matrix, which will have better thermal conductivity than classical ceramics. This, in turn, requires solving a number of emerging problems: preventing chemical reactions of the shell and fuel, swelling and damage to the shell of fuel, localization of fission products, etc.
 
 
The Institute of Atomic Energy of South Korea (KAERI) is working on the creation of microcells (microcells) to increase the retention capacity of fission products and the best thermal conductivity compared to conventional uranium dioxide pellets.
 
 

 
Concept of a microcell fuel pellet
 
 
The figure shows a conceptual illustration where it can be seen that the grains or granules of UO2 are surrounded by a thin wall. The main task of creating such tablets is to reduce the yield of fission products from the tablet. The improved ability to retain fission products reduces stress-corrosion cracking from the inside of the fuel element caused by iodine and cesium.
 
 
It is expected that this can positively influence the strength of the fuel rods. Also, the microelement structure will prevent massive fragmentation of the tablet in the event of an accident, thereby ensuring additional retention of radioactive fission products.
 
 
The thermal conductivity of such tablets can be increased by the addition of materials with a high thermal conductivity, for example, using metals in the form of a wall of one element.
 
 

 
Typical structure of microcellular tablets with metal
 
 
This change will reduce the temperature in the center of the tablet under normal and emergency conditions of TVEL operation.
 
 
In order to understand how the above innovations are implemented in practice, I will give the following example. Westinghouse creates a tolerant fuel under the brand name EnCore, which is a U3Si2 uranium silicide tablet, enclosed first (in the first stage of this program) in a shell of chrome-plated zirconium alloy Zirlo.
 
 
It is expected that fuel from uranium silicide will exceed the traditional dioxide by more than 5 times in thermal conductivity and by ~ 1/5 in density, and the absorption of neutrons by the carbide-silicon shell should be ~ 1/4 less than for zirconium alloys.
 
 
Due to the last two parameters, the company expects to keep EnCore enrichment within 5%, which will facilitate its promotion on the market. In 201? Westinghouse plans to establish an experimental production of fuel elements in a chrome zirconium sheath, in 2019 - to begin testing them as part of the regular fuel assemblies at Byron NPP in the United States.
 
 

Results of

 
The changes described above areare part of the actual projects now. Perhaps in the future we will witness the introduction of more than exotic solutions.
 
 
For now, it is only worth noting that the appearance of economically profitable and new projects of nuclear fuel from the point of view of security will help strengthen the position of nuclear energy in the world.
 
 
Author: Yatsenko Mikhail.
 
 
Sources:
 
 
 
Kurt A. Terrani, "Accident tolerant fuel cladding development: Promise, status, and
 
challenges ";
 
IAEA-TECDOC-1797 «Accident Tolerant Fuel Concepts
 
for Light Water Reactors »;
 
Materials of the journal "Atomic Expert" № ? May 2018.