Pressure tube in a horizontal channel-type PHWR

The following Sections 3.2.4.1 to 3.2.4.3 are based on practices from Canada and other States that operate horizontal channel-type PHWRs. In China, the integrity of the pressure tube under an interaction with fuel bundles is considered out of the scope for fuel design qualifciation.

3.2.4.1. Local plastic strain due to contact with molten material Reported criteria

No information on this fuel failure criterion was specifically requested from the PHWR States.

Local plastic strain due to contact with molten material is not considered as a safety criterion in some States. This criterion is also not applicable to vertical channel-type PHWRs, e.g.

Atucha-1 and Atucha-2.

Discussion

A concern during the early phase of events involving power increases is the potential for the power pulse to increase fuel temperatures sufficiently high that molten UO2±x/Zircaloy could form. In the unlikely event of this melt relocating outside the fuel bundle contacting the pressure tube, the local thermal interaction may cause failure of the pressure tube.

An important factor influencing the extent of fuel channel deformation due to melt contact is the internal pressure in the pressure tube at the time of melt contact [120]. Tests performed with high internal pressure induced greater pressure tube deformations than tests performed at low pressure with similar masses of melt relocating on them. The mass of melt that contacted the pressure tube also played a role in fuel channel integrity. Test programmes demonstrated the ability of an as-received ballooned pressure tube in contact with the calandria tube to withstand a very intense, relatively short lived hot spot induced by molten Zircaloy-4, provided the calandria tube was well cooled.

Local melt heat transfer to the pressure tube is a major phenomenon for large break LOCAs and flow blockage in single channel events. Molten fuel sheaths probably would not relocate onto

the pressure tube as Zircaloy with dissolved oxygen wets uranium dioxide decreasing the driving force for relocation. However, for fast heat-up transients, there is a potential for some melt from end plates and end caps.

The most important factor in determining fuel channel deformation caused by melts is the cooling condition on the outside of the calandria tube. As mentioned above, another important factor is the internal pressure tube pressure at the time of melt contact [120]. The key phenomena occurring after a channel flow blockage would be:

 Coolant boil-off;

 Cladding heat-up and melting;

 Dripping of molten Zircaloy from the fuel elements;

 Thermal interaction between the molten Zircaloy and the pressure tube;

 Localized deformation of the pressure tube or calandria tube;

 Potential channel failure.

If the coolant surrounding the fuel is in nucleate boiling, the channel will remain cool, limiting the area over which fuel channel deformation occurs, providing a good heat conduction path from the melt to the surrounding water. In addition, failure may be inhibited if a rewet front moves from the cooler calandria tube as heat from the melt dissipates. Extensive experimental data was acquired on this phenomenon [120] and very conservative criteria were derived from these experiments. The pressure tube failure mechanism by contact with molten material in horizontal channel-type PHWRs is studied with limits of the amount of melt the pressure tube can sustain. This is done for cases where melt occurs before or after the pressure tube contact with the calandria tube, and in relation to pressure tube internal pressure and calandria tube outside surface heat transfer.

As in the previous section, this mechanism was not included in the IAEA questionnaire and, thus, no information was provided by the participating States.

3.2.4.2. Local plastic strain due to fuel element contact Reported criteria

No information on this fuel failure criterion was specifically requested from the PHWR States.

Local plastic strain due to fuel element contact is not considered as a safety criterion in some States. This criterion is also not applicable to vertical channel-type PHWRs, e.g. Atucha-1 and Atucha-2.

Discussion

Channel integrity could be threatened by the potential formation of local hot spots on the pressure tube by forced fuel element contact before pressure tube ballooning ensure a hard contact with the calandria tube. If such a hot spot was to occur, there is a potential for sufficient local strain to rupture the pressure tube. Failure occurs when a local region of the pressure tube necks down to a knife-edge. The Canadian industry is aware of this potential pressure tube failure mechanism for large break LOCA and is conducting joint research and development work to investigate the criteria that, if met, would prevent pressure tube failure.

For a ballooned pressure tube, the hot spot is expected to quench with minimum or no local deformation due to the enhanced heat transfer to the moderator.

Fuel elements can contact the pressure tube from several types of bundle deformation: (a) fuel element bowing; (b) bundle sagging/slumping, and (c) bundle settling.

Fuel element bowing can be due to differential axial thermal expansion across the fuel bundle cross-section, hydraulic drag, gravity or mechanical loading bending moments (e.g. bundle impact, constrained axial expansion). Due to the design features of the fuel elements for horizontal channel-type PHWRs, there are two possible types of contact between the pressure tube and the fuel element. The first type is contact with the fuel elements bearing pad and the other is contact with the fuel element sheath.

Experiments that study the first type of contact have shown that this process is self-limiting, where the heat transfer to the pressure tube is arrested by the local deformation of the pressure tube under the bearing pads once a hot spot has developed. Due to the plasticity of the fuel elements, the second type of contact could be maintained during the pressure tube local deformation. In this case, the integrity of the pressure tube depends on the contact load, the internal pressure inside the pressure tube, and the extent of the hot spot. The pellet-to-sheath contact forces and their effects on fuel element rigidity are not easy to quantify in most experiments [121]. The most thoroughly characterized tests were conducted without pellets inside the sheath, so the stiffness of the fuel elements may have been lower than expected under actual conditions [121].

Bundle sagging and slumping are deformation due to gravitational loading for temperatures above 973 K. In particular, sagging is a rapid deformation in the α/β transformation temperature range (1081 K – 1281 K) [122]. A few experiments have been done that involve fuel bundle slumping, but many of the tests in these experiments had high uncertainties [121].

Bundle settling refers to changes in the end plate geometry due to applied loads and temperature. While theoretical aspects of the deformation are known, the complexity of the bundle end plate geometries is high.

The information available on fuel bundle sagging, slumping, and settling is inadequate to characterize the onset of bundle deformation. Also, the large temperature differences in the tests between the top and bottom of the bundle, and axially along the bundle, may have caused some non-characteristic deformations. However, due to the large temperature sensitivity of the creep rate at elevated temperatures, the time of failure, during heating transients, are expected to be predicted accurately [123].

As in the previous section, this mechanism was not included in the IAEA questionnaire and, thus, no information was provided by the participating States.

3.2.4.3. Rolled joint failure due to fuel string relocation Reported criteria

No information on this fuel failure criterion was specifically requested from the PHWR States.

Rolled joint failure due to fuel string relocation is not considered as a safety criterion in some States. This criterion is also not applicable to vertical channel-type PHWRs, e.g. Atucha-1 and Atucha-2.

Discussion

As described in Section 3.2.3.5, during events like a large break LOCA, the coolant flow can change direction (flow reversal) and accelerate the fuel strings (or bundles) in the channels. If the bundles acquire sufficient kinetic energy during the acceleration time, the impact of the bundles against the channel-end structures could damage the channels. In Canada, the industry is aware of this potential fuel channel failure mechanism for large break LOCA and is conducting joint research and development work to investigate the criteria which, if met, would prevent fuel channel failure.

It is argued that this mechanism is physically impossible for the estimated conditions during accidents of horizontal channel-type PHWRs. However, it is considered prudent to establish a fuel acceptance criterion until this proof is obtained.

The pressure tube and end fittings in the fuel channel of a horizontal channel-type PHWR are joined together by roll-expanded joints. The pressure tube is rolled into the end fitting to a specified reduction in the wall thickness of the pressure tube. During roll expansion, the pressure tube material is forced into three circumferential grooves in end fitting to provide a strong and leak-tight joint. During the flow reversal during a large break LOCA, the rolled joint is expected to experience high strain-rate produced by the fuel string impact loads. At sufficient load, the pressure tube will fail near the corner of the inboard groove before the joint slips with a fracture surface at about 45º to the tube axis [103].

To assess the fuel string kinetic energy necessary for this failure mechanism, a set of experiments were performed for various kinetic energies. In these impact tests, the maximum value of the kinetic energy to prevent rolled joint failure was assessed. In tests with a 12-bundle fuel string, the maximum value was determined to be 14.6 kJ at a velocity of 6.3 m/s, with a standard error of 0.24 m/s in the velocity measurement [103].

Because it is unclear if this failure mechanism could be active during certain accident sequences, it is considered prudent to have a safety criterion for this pressure tube failure mechanism.