Nooclear
What factors have been found to correlate with the diametral strain rate of pressure tubes during service? Describe the correlations.
1. Increased Neutron Flux: During service there is increased exposure to neutron flux from the core. this leads to an increase in creep and thus an increase in diametral strain
2. Elevated Operating Temperature: The increase in temperature will lead to an increase in creep and thus an increase in diametral strain.
Why is the hydrogen isotope concentration of a pressure tube important? Name an important slow cracking mechanism related to the hydrogen isotope concentration and describe its key features.
1. The formation of the hydride is dependent on the hydrogen isotope
2. A mechanism related to this is delayed hydride cracking [DHC]
3. some key features of this are: Time dependent crack growth due to precipitation and hydride expansion at stress points
Describe the stages of development of microstructure in annealed Zircoloy-2 during irradiation at about 300°C and their relationship to microchemistry.
1) Unfaulted Loops Form:
structure becomes saturated after 1-2 dpa
denuded zones appear at grain boundaries
2) Faulted loops begin to form:
loops multiply after 3-5 dpa due to microstructure
formation is affected by the purity of the alloy
What is an important restriction on temperature for hot-rolling of calandria tubes that has to be imposed during the processing? Why.
Hot rolling below 800 degrees C to drop below the alpha + beta phase (must be avoided) to ensure a fine particle distribution for corrosion resistance
Explain the function of the moderator in a fission reactor? Compare heavy and light water as moderators.
The moderator's job is to slow down fast neutrons without absorbing them. This makes it easier for uranium to capture the neutrons and sustain the chain reaction.
Heavy: uses natural uranium as fuel allowing more neutrons to be available for fission.
Light: require fuel that is enriched in ²³⁵U to maintain a sustained chain reaction.
What are the typical effects of displacement damage on the properties of structural reactor materials?
The following can be observed in reactor materials after experiencing displacement damage:
-Increase in yield strength
-Reduced ductility
-Reduction in work hardening rate
Explain the large variability observed in the fracture toughness of earlier pressure tubes, and how the manufacturing route has been changed to obtain consistent high toughness.
Due to improper and inconsistent manufacturing methods
We have addressed this by adding quadruple melting process to limit factors such as chlorine concentration
What features of the microstructure of Zr-2.5Nb pressure tubes are affected by irradiation? What features are not?
The following features ARE affected by irradiation:
1. Dislocation structure
2. State of Beta-phase
3. Fe distribution
4. Nb distribution/precipitation
The following features ARE NOT affected by irradiation:
1. Grain structure
2. Texture
Describe the anisotropy of irradiation growth of pressure tubes and explain it in terms of the crystallographic texture and growth strains in a single crystal.
Zirconium pressure tubes experience anisotropic behavior under irradiation due to their textured microstructure
The crystallographic orientations of grains influence the accumulation of defects, causing uneven growth strains within single crystals and leading to varying dimensional changes
What are the three major components of a fuel channel, and from what materials are they made?
Tubes
Calandria tube: Zr-2
Pressure tube: Zr 2.5 Nb
End fittings
Stainless steel
An important application of deformation equations has been SLAR. What does SLAR stand for? Briefly describe the SLAR operation.
Spacer Location and Repositioning (SLAR)
Tool able to locate and reposition "as found" spacers within a pressure tube
Configuration using deformation model/ FE code
Post-SLAR "time until contact" can be calculated
Linear induction motor (LIM) used to reposition spacers
In 1983, a pressure tube in a CANDU reactor ruptured during reactor operation. Explain the factors that led to this event, and explain what has been done to prevent a similar failure from happening again.(tube design, spacers and monitoring)
The rupture of a CANDU reactor pressure tube was caused by displaced spacers, which caused the hot pressure tube to come into contact with the cold calandria tube. This contact caused the creation of a blister on the outer surface of the pressure tube. This blister then propagated and caused the rupture.
To prevent this from happening in the future the Spacer Location and Repositioning (SLAR) method was introduced to ensure that the spacers remained in their designated positions
What characteristics of the pressure tube are monitored during service, by inspection? What characteristics are monitored by channel removal?
The following Characteristics and properties are monitored during service, either by inspection or by channel removal
Inspection
1. Elongation
2. diameter'
3. Sag
4. Deuterium ingress
5. In-service wear
6. Spacer location
By channel removal
1. Fracture properties
2. End-of-life irradiations in test reactors
What manufacturing variable is principally responsible for the crystallographic texture of fuel cladding? What is a desirable texture for fuel cladding and why is this texture desirable?
The plastic deformation of fuel cladding is primarily responsible for the crystallographic texture.
Radially oriented is the most desirable texture for fuel cladding because it because it allows for ductility in biaxial loading and increases fracture toughness
Which structural component controls the gross sag of the CANDU fuel channel? Why?
Calandria tubes are used to control the gross sag because:
1. They have increased deformation resistance
2. they have 5x the creep resistance of the pressure tubes
What two developments have been or are being introduced to improve the performance of calandria tubes under hypothetical accident conditions? Give a brief description of each.
Enhanced Alloy Composition: alter the composition of Zr-2.5 Nb tubes to optimize material characteristics in extreme conditions (modifying alloy constituents/refining distribution in tube)
Advanced Manufacturing Techniques: manufacturing methods that precisely control the microstructure/tube properties. Processes such as forging or heat treating produce more resilient material.
Describe CEG (climb enabled glide) theory of irradiation creep
The CEG theory describes how dislocations in a material move under shear stress when exposed to radiation. Dislocations can glide (move along their slip planes) and climb (move perpendicular to their slip planes) due to the presence of vacancies and self-interstitial atoms (SIAs) generated during irradiation.
Explain the concept of grouping and separation in designing a safe reactor system.
Grouping refers to each safety related system being assigned to one group. Because of this, each group should be able to independently perform most if not all required safety functions.
Separation refers to the distancing, both through barriers and space of these safety groups so that they are able to function if an isolated part of the plant is impaired.
Explain the concept of leak-before-break for a pressure tube, with reference to its fracture and delayed hydride cracking characteristics?
- "Leak-before-break" suggest cracks in pressure tubes propagate slowly, allowing detection before reaching critical size
- Requires monitoring to catch cracks early, considering both slow crack growth characteristics and potential delayed hydride cracking in zirconium alloys, preventing failures
What factors have been found to correlate with the diametral strain rate of pressure tubes during service? Describe the correlations.
1. Increased Neutron Flux: During service there is increased exposure to neutron flux from the core. this leads to an increase in creep and thus an increase in diametral strain
2. Elevated Operating Temperature: The increase in temperature will lead to an increase in creep and thus an increase in diametral strain.