Reactor Physics

Atomic and Nuclear Physics

Notation of nuclei
Notation of nuclei

A knowledge of atomic and nuclear physics is essential to nuclear engineers, who deal with nuclear reactors. It should be noted that atomic and nuclear physics is very extensive branch of science. Nuclear reactor physics belongs to an applied physics as a particle physics or nuclear chemistry. These branches have common fundamentals. Atomic and nuclear physics describes fundamental particles (i.e. electrons, protons, neutrons), their structure, properties and behavior. These physical fundamentals consist of:

See also: Atomic and Nuclear Physics

Interaction of Radiation with Matter

Interaction of Radiation with Matter
Interaction of Radiation with Matter

The design of all nuclear reactors and other nuclear systems depends fundamentally on the way in which nuclear radiation interacts with matter. In nuclear reactor we can typically encounter with one of these types of radiation:

See also: Interaction of Radiation with Matter

Neutron Diffusion Theory

Neutron Diffusion Theory is the branch of science that deals with the study and application of neutrons and its behavior within the nuclear core or in various environments. The Diffusion Theory provides theoretical basis for a neutron-physical computing of nuclear cores. The term theoretical has to be emphasized.

In order to design a nuclear reactor properly, it is necessary to be able to predict how the neutrons will be distributed throughout the system. This, in general, is a difficult problem, for the neutrons move in a reactor in complicated paths as a result of repeated collisions. To a first approximation the overall effect of these collisions is that the neutrons undergo a kind of diffusion in the reactor medium, much like the diffusion of one gas in another. This approximation allows solving such problem using the diffusion equation.

Nuclear Reactor Theory

The nuclear reactor theory is based on diffusion theory. The key term of the reactor theory is “criticality” of the reactor. Using the term “criticality” may seem counter-intuitive as a way to describe normalcy. The word often describes situations with potential for disaster. Nevertheless, in the context of nuclear power, “criticality” indicates that a reactor is operating safely and the neutron flux is stable at a given power level.  In a critical reactor, there is a balance between the number of neutrons produced in fission and the number lost, either by absorbtion in the reactor or by leaking from its surface. The nuclear reactor theory solves the central problem in the design of a reactor, what is the calculation of the size and composition (nuclear fuel) of the system required to mantain this balance.

Reactor Kinetics – Zero Power Physics

Reactor criticality
Reactor criticality. A – a supercritical state; B – a critical state; C – a subcritical state

The nuclear  reactor theory is concerned with critical nuclear reactors operating at constant power. But the reactor must not be always in the critical state. The reactor must be  noncritical (supercritical or subcritical) in order to change the reactor power. The study of the behavior of neutron populations in a noncritical ractor is called reactor kinetics. This theory introduces a new unit – reactivity – of a nuclear reactor. The reactivity characterizes the deflection of reactor from the critical state.

It should be noted this theory can be used for a reactor at low power levels, hence  “zero power criticality” (i.e. a reactor without reactivity feedbacks, 10E-8% – 1% of rated power). First periods of a reactor operation (reactor startup and power maneuvering at low power levels) are well described by this theory. Once the reactor core heats up (thermal power outweighs a decay power), reactivity feedbacks change the dynamics of the reactor.

Reactor operation – Full Power Physics

Iodine pit - Xenon poisoning
Iodine pit – Xenon poisoning

This branch of nuclear engineering deals is very specific and dependent on a reactor type. This article focuses on the most common type of reactors – Pressurized Water Reactors. PWRs have much in common with Boiling Water Reactors, but the operation of the reactors is not the same. On the other hand for example fast neutron reactors have completely different physics than PWRs and BWRs.

The degree of criticality at high power levels (~1% – 100% of rated power) is influenced by many variables. The most important are summarized below.

  • Nuclear Fuel and Fuel Loading Pattern
  • Fuel Depletion
  • Neutron Leakage
  • Control Rods and Chemical Shim
  • Reactivity Feedbacks
  • Burnable Absorbers
  • Xenon and Samarium Poisoning

Basic reactor operation physics includes these operation states:

  • Reactor Startup (“going critical”)
  • Control of the Reactor and Power Maneuvering.
  • Axial Offset Control.
  • Xenon Oscillations
  • Reactor Shutdown.
  • Emergency Shutdown – SCRAM

Thermohydraulics of Nuclear Core

critical heat flux and DNB (Departure from Nucleate Boiling)
If the heat flux of a boiling system is higher than the critical heat flux then DNB (Departure from Nucleate Boiling) may occur.

Heat removal from nuclear reactors is also key issue of reactor physics. For a reactor to operate in a steady state, with an internal power distribution that is independent of time, all of the heat released in the system must be removed as fast as it is produced. This is accomplished by passing a coolant (water in case of PWRs or BWRs) through the core and through other regions where heat is generated.

The thermohydraulics deals with thermal behavior (heat generation, heat removal) and hydraulic behavior (Delta P, cross-flow issue, flow pattern) of the nuclear core. The nature and operation of this coolant system is one of the most important considerations on the design of a nuclear reactor. In the PWR reactors, there are phenomena which must be avoided. Namely DNB (Departure from Nucleate Boiling) and coolant saturation are very dangerous phenomena.

Behavior of Fuel Rod

Thermal and mechanical behavior of nuclear fuel constitute one of three key core design disciplines. Nuclear fuel is operated under very inhospitable conditions (thermal, radiation, mechanical) and must withstand more than normal conditions operation. For example temperatures in the centre of fuel pellets reach more than 1000°C (1832°F) accompanied by fission-gas releases. Behavior of nuclear fuel is associated with many phenomena. The most important are summarized below.

  • Fuel swelling
  • Fuel densification
  • Thermal expansion
  • Fuel relocation
  • High Burnup Structure
  • Pellet-Cladding Interaction
  • Pellet-Coolant Interaction
  • Fission-Gas Releases

Applied Core Physics

Core physics is the branch of science that deals with the study of behavior of specific nuclear core. The core physics provides methodological basis for a core design. Each nuclear core is due to a fuel reloading

Full-scale physical computing of reactor core includes these types of computations:

  • Core Design computing
    • Fuel cross-section computing. Neutron cross-sections  must be calculated for for fresh fuel assemblies usually in two-Dimensional models of the fuel lattice.
    • Loading Pattern search. The best arrangement of new fuel assemblies and used fuel assemblies is searched for in the core design from the viewpoints of safety and economy.
    • Fuel Cycle depletion. Behavior and key safety parameters of each proposed fuel cycle (power distribution, reactor kinetics parameters, control rods efficiency, etc.) must be analyzed for entire duration of the proposed cycle.
  • Reload Safety Assessment
    • Neutron-physical computing
    • Thermal-hydraulic computing
    • Fuel rod behaviour computing

 Safety of Nuclear Reactors

Meaning of ‘(nuclear) safety’ 

Definition by IAEA:

‘Safety’ is the achievement of proper operating conditions, prevention of accidents and mitigation of accident consequences, resulting in protection of workers, the public and the environment from undue radiation hazards.

Nuclear safety is very broad branch of nuclear engineering. The three primary objectives of nuclear reactor safety systems as defined by the U.S. Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition and prevent the release of radioactive material. Appropriate redundant and diverse safety systems are installed to ensure safety of nuclear reactor.

Most of nuclear power plants introduce a ‘defence-in-depth‘ approach to achieve maximum safety, this approach is constituted of multiple safety systems supplementing the natural features of the reactor core.