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CHME 476 Nuclear Fuel Cycles

Published : 27-Sep,2021  |  Views : 10


1.If the core must have excess reactivity in order to increase power, then how does this affect load following with SMRs? 
2.Select one ATF fuel concept and one cladding concept. Conduct the risk assessment matrix given in Table ES-1 of the (Light Water Reactor Accident Tolerant Fuel Performance Metrics.pdf) report and discuss the results. Select the fuel and cladding from the (Overview of International Activities in Accident Tolerant Fuel Development for Light Water Reactors.pdf) presentation. Feel free to use other resources though


1.If the core must have excess reactivity in order to increase power, then how does this affect load following with SMRs

In many nuclear power plants there is a need to operate in the load following mode. The small modular reactors are utilized in these reactors for the purpose of co-generation. The load following idea defines the potential for a power plant to adjust its power output. The adjustment is highly dependent on the demand and price for electricity fluctuations from the state national grid during the day operations. The load following is achieved in the nuclear power plants by immersing the control rods into the pressure vessels in the reactor.  The core power density is given as

Where Fq is the power peaking factor and the P’’’ is the maximum power density in the core.

Combining the two equations we are able to obtain the total power at the core as,


The reactor designers and physicists are able to maximize this ratio using the control rods, new designs, and varying enrichment. The temperature is increased to create a feedback mechanism in the reactor through Doppler broadening, thermal expansion, and density changes which will induce spectral shifts. These changes will impact the reactivity, thus causing transients (MIT , 2009).  It is noted that when power increases to its operating level, additional negative reactivity is introduced by an increase in temperature. Excess reactivity occurs as the value of rho, if all the control poisons or additives are removed from the core. These additive refer rods and power adjustment components. The large excess reactivity is avoided as they need a lot of poison to compensate at the beginning of the cycle and they tend to require extra care.

The increase in the excess reactivity by inserting control rods as power is reduced helps in improving the load following with the SMRs. The control rod position adds a very large amount of negative reactivity that increases to shutdown margin (SDM). The instantaneous amount of reactivity by which a nuclear reactor core is subcritical, or can be made subcritical from its present condition, with the most reactive control rod fully withdrawn from the core at any time during the core cycle denotes the SDM.  The same thing on a reactor shutdown occurs though in this case the shutdown rods may not be inserted. This as well increases the SDM slightly. During the power operation, control rods must be above a certain minimum height to ensure that there is adequate SDM on a trip (Industry Learning, 2014).

2.Select one ATF fuel concept and one cladding concept. Conduct the risk assessment matrix and discuss the results.

The SiC sandwich concept. This is an ATF fuel concept that has been adopted in Europe especially in France. In the first phase, oxidation tests are carried out in LWR nominal conditions for 3500h. the mechanism used to perform the oxidation are clearly understood. The pyrocarbon interphase is not affected: no reduction of mechanical strength is observed in the test. The test does a recession of the alloy in the nominal oxidation conditions. The numerical simulation of the rod behavior is developed in the normal behavior. The Fuel-SiC interface issues are handled in the second phase of the tests. It is observed that there is corrosion during the nominal conditions and the high temperature oxidation and irradiation tests proves quite promising. The SiC sandwich is a candidate for experimental comparison and numerical approach evaluations.

Figure 1 SiC AFT Concept (Anon., 2013)

 There are three key categories of cladding materials that can be considered in the nuclear fuel cycle,

  • Fully metallic cladding
  • Fully ceramic cladding
  • Hybrid cladding

The coatings or wraps involved in cladding are either on the inner or outer part of the tubes. There is a proposed prioritization that assumes the primary material meets basic requirements for use in a fuel rod. A test matrix for the candidate fuel and cladding includes all the licensing criteria and it identifies the existing test standards or characterization tests for which standards must be developed for future qualifications. In this paper, the fully ceramic cladding concept is adopted and there are several tests performed on it to determine if it is risky or if it is feasible enough. It is important to note that when cooling cannot be restored during the course of a severe accident, the accident is bound to accelerate or proceed in the same magnitude.

The ATF designs are developed to meet the LWE operations, safety, and fuel cycle constraints. They are evaluated over all potential performance regimes such as fabrication or ability to manufacture, the normal operations and anticipated operational occurrences, postulated accidents as well as the severe accidents, and the use, storage, and transportation of fuel including the potential for future reprocessing. Reduced oxidation and hydrogen generation is the key benefit of alternative cladding and materials. The ATF cladding development efforts focus on materials with more benign steam reactions. The advanced steels, refractory metals, ceramic cladding, innovative alloys with dopants, and Zircaloy with coating or sleeve. The ATF fuel concept taken into consideration in this case is high density fuels, oxide fuels with additives, and micro-encapsulated fuels. The micro-encapsulated fuel concept is much more suitable as it is adopted in Asia and Europe. When designing or developing the concepts in the strategic plan, the first thing is to perform a feasibility study on the advanced fuel and clad concepts. This is done by bench-scale fabrication, irradiation tests, steam reactions, furnace tests, and modelling of the mechanical properties. Development and qualification follows and the product is commercialized.

Risk assessment table

Performance Regime

Performance Attributes

(For large-scale deployment)

Expert Opinion

Recommended Actions



Fabrication/ manufacturability


Millions ft. of clad/year.

~ 300 million pellets/year

Economic-cost of raw materials and fabrication process

Current fabrication plant enrichment limits.

Manageable fissile material content

Compatible with large scale production needs (material availability, fabrication techniques, waste etc.)

Compatible with quality and uniformity standards





Control rod compatibility

Reprocessing potential

Proliferation potential

Access to raw materials and corrosion of plants due to water reactivity.

Highly ability to manufacture as the plants and extraction sites for raw materials are already established.

Normal operation and AOOs


Overall neutronics

Linear Heat Generation Rate (LHGR) to centerline melt

Power ramp, ~ 100W/m/min

Reduced flow (departure from nucleate boiling, DNB)

Flow induced vibrations

Surface roughness effects

Safe shutdown-earthquake

External pressure

Axial growth (less than upper nozzle gap)

Utilization or burnup (12,18, or 24 month/cycle)

Thermal hydraulic interaction

Reactivity control systems interaction

Mechanical strength, ductility (beginning of life and after irradiation)

Thermal behavior (conductivity, specific heat, melting)

Chemical compatibility/ stability

Chemical compatibility with and impact on coolant chemistry

Fission product behavior

The melting point is important under RIA and LOCA conditions

Thermal conductivity many impact DNB under loss of flow conditions


The higher melting point is preferred.

Higher thermal conductivity is preferred.

Higher diffusivity is preferred

Lower coefficient of thermal expansion is preferred

Postulated Accidents (Design Bias)


Prompt reactivity insertion

Post-DNB behavior

Loss of coolant conditions

Thermal shock

Steam reactions

Thermal behavior (conductivity, specific heat, melting)

Chemical compatibility/ stability

Chemical compatibility with and impact on coolant chemistry

Fission product behavior

Fissile density

Cross sections

Reactivity feedback coefficients

Low parasitic absorption. New concepts should retain fission products as well as UO2

Higher fissile density is preferred.

Desire high fission cross sections

Severe Accidents (Beyond Design Bias)


Thermal shock

Chemical reactions

Combustible gas release

Long term stability in degraded state

Mechanical strength and ductility

Thermal behavior

Chemical compatibility/stability

Fission product behavior

Combustible gas production

Yield strength.


Creep rate

Modulus of elasticity


Lower yield strength, higher toughness, rapid creep rate during normal operations, and structural rigidity is required during normal operation.

Used Fuel storage/Transport/Disposition


Handling, placement, and drying loads, future reprocessing potential

Mechanical strength, ductility

Thermal behavior

Chemical stability

Fission product behavior

Water reactivity

Clad compatibility

Phase stability

Fission product chemistry


There are no adverse reactions between fuel and cladding under normal conditions.

Lower water reactivity is desired


Anon., 2013. Advanced Fuel Campaign. s.l.:Oak Ridge National Laboratory.

Industry Learning, 2014. Operator Generic Fundamentals Reactor Theory- Reactivity Coefficients. s.l.:s.n.

MIT , 2009. Neutron Science and Reactor Physics. Fall ed. Carlifonia: MIT OpenCourseWare.

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