Boring
Explain the function of the moderator in a fission reactor. Compare heavy and light water as moderators.
What are the three major components of a fuel channel, and from what materials are they made?
Describe the CEG (climb-enabled glide) theory of irradiation creep.
The CEG theory explains irradiation creep as dislocation glide that is enabled by dislocation climb. Neutron irradiation produces excess point defects that allow dislocations to climb past obstacles.
What is an important restriction on temperature for hot rolling of calandria tubes that must be imposed during processing? Why?
During hot rolling of calandria tubes the temperature must be kept below the B-transus (~865 °C for zircaloy). This is because entering the B-phase causes loss of a-phase texture and leads to coarse grain growth which increases corrosion resistance.
Which structural component controls the gross sag of the CANDU fuel channel? Why?
The component that controls the gross sag of the CANDU fuel channel is the pressure tube. The pressure tube is the only component under high temperature and pressure, and under neutron irradiation.
An important application of deformation equations has been SLAR. What does SLAR stand for? Describe SLAR operation.
Which structural components control the gross sag of the CANDU fuel channel? Why?
The pressure tube controls the gross sag of the CANDU fuel channel because it operates at high temperature, high internal pressure, and high neutron flux causing thermal irradiation creep.
What are the typical effects of displacement damage on the properties of structural reactor materials?
Displacement damage from neutron irradiation creates point defects and defect clusters, which leads to irradiation hardening and embrittlement, reduced ductility, and irradiation creep growth.
What factors have been found to correlate with the diametral strain rate of pressure tubes during service?
The diametral strain rate of pressure tubes during service correlates strongly with neutron flux, temperature, and applied stress.
What manufacturing variable is principally responsible for the crystallographic texture of fuel cladding? What is a desirable texture for fuel cladding and why is it a desirable texture?
Describe the manufacturing route for a calandria tube from the ingot stage onwards. What is an important restriction on temperature that has to be imposed during the processing? Why?
Calandria tubes are made from Zircaloy ingots by vacuum melting, β-forging, extrusion, then multiple cold reduction with intermediate vacuum anneals, and finally a stress-relief anneal. Processing temperatures must be below the β-transus, 865 °C, to prevent a loss of α-phase texture and coarse grain growth. This could degrade the mechanical stability and corrosion resistance.
Describe the effect of temperature during extrusion on the crystallographic texture of Zr-2.5Nb tubes. What are some of the deformation mechanisms believed to be involved in the development of texture during extrusion?
What features of the microstructure of Zr-2.5Nb pressure tubes are affected by irradiation? What features are not?
What features of the micro-structure of Zr-2.5Nb pressure tubes are affected by irradiation? What features are not?
The fine-scale features of the microstructure of Zr-2.5Nb pressure tubes are affected by irradiation. Β-Nb precipitates coarsen and dissolve, dislocation density increases, irradiation defect loops form, and Nb redistributes between phases. The α-Zr crystallographic texture is unaffected by irradiation. The texture of this structure is established during manufacturing.
What characteristics of the pressure tube are monitored during inspection? What characteristics are monitored by channel removal?
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.
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.
Improved manufacturing and heat treatment:
Describe the anisotropy of irradiation growth of pressure tubes (DAD model) and explain in terms of the crystallographic texture and growth strains in a single crystal.
Explain what happened for the ex-service spacer D3Q13, what microstructures were involved in their brittle fracture, and why the fracture of a pinched/unpinched section of the same spacer look different?
D3Q13 is made of Ni alloy so cavities or helium bubbles caused embrittlement of spacers basically. The pinched/unpinched zones served at the different temperatures, causing the difference of cavity size/distribution and gamma prime phase dissolution, dislocation density as well. As a result, the unpinched zone indicated worse degradation than the pinched zone!
Explain the large variability observed in the fracture toughness of earlier pressure tubes, and how the manufacturing route has changed to obtain consistent high toughness.
Describe the stages of development of microstructure in annealed Zircaloy-2 during irradiation at about 300 °C and their relationship to microchemistry.
Annealed Zircaloy-2 irradiated at 300 °C first develops point defects that cluster into small dislocation loops. With increasing fluence these loops grow and increase in density which causes irradiation hardening. At highest fluence radiation enhanced diffusion leads to solute redistribution which contributes to irradiation growth, creep, and reduced ductility.
Describe the manufacturing route for a pressure tube from the ingot stage onwards. Which two micro-structural features contribute most to the strength of the finished tube, and in what stages of manufacturing are these micro-structural features produced?
Pressure tubes are made from Zr-2.5Nb by vacuum-melting ingots, β-forging, hot extrusion, intensive cold working, quenching from the β-phase field, and finally annealing in the α+β region. The two microstructural features that contribute most to the strength of the finished tube are the: (1) Elongated and textured α-Zr grains which are produced during cold working. (2) And fine β-Nb precipitates produced during quenching and annealing.
In 1983, a pressure tube in a CANDU reactor ruptured during reactor operation explain what steps have been done to prevent a similar failure from happening again (tube design, spacers, and monitoring).
To prevent it from happening again several measures were implemented. The pressure tube design was improved to have a more consistent microstructure and higher fracture toughness.
Spacer design designs and materials were upgraded and the SLAR (Spacer Location and Repositioning) was introduced.
Enhanced monitoring now tracks creep, sag, and hydrogen concentration.
Why is the hydrogen isotope concentration of a pressure tube important? Name an important fracture mechanism related to the hydrogen isotope concentration and describe its key features.
The hydrogen isotope concentration in a pressure tube is important because hydrogen precipitates as brittle zirconium hydrides. This process severely reduces ductility and fracture toughness. Delayed Hydride Cracking, or DHC, is a fracture mechanism where hydrides repeatedly precipitate and fracture crack tips causing stepwise crack growth. DHC is caused by hydride buildup in stressed or cooled regions.
In 1983, a pressure tube in a CANDU reactor ruptured during reactor operation. Explain the factors that led to this disaster
In 1983 a CANDU pressure tube ruptured due to hydrogen pickup and delayed hydride cracking (DHC). Garter spring spacers shifted which caused contact between the calandria and pressure tubes. This caused fretting wear which created a surface flaw that promoted hydrogen accumulation and hydride precipitation. Under stress, DHC initiated the flaw and grew slowly until unstable rupture happened.