Accident Loading Conditions Research Articles

Experimental simulation of asymmetric heat up of coolant channel under small break LOCA condition for PHWR

During postulated small break loss of coolant accident (SBLOCA) for Pressurised Heavy Water Reactors (PHWRs) as well as for postulated SBLOCA coincident with loss of ECCS, a stratified flow condition can arise in the coolant channels as the gravitational force dominates over the low inertial flow arising from small break flow. A Station Blackout condition without operator intervention can also lead to stratified flow condition during a slow channel boil-off condition. For all these conditions the pressure remains high and under stratified flow condition, the horizontal fuel bundles experience different heat transfer environments with respect to the stratified flow level. This causes the bundle upper portion to get heated up higher as compared to the submerged portion. This kind of asymmetrical heating of the bundle is having a direct bearing on the circumferential temperature gradient of pressure tube (PT) component of the coolant channel. The integrity of the PT is important under normal conditions as well as at different accident loading conditions as this component houses the fuel bundles and serves as a coolant pressure boundary of the reactors. An assessment of PT is required with respect to different accident loading conditions. The present investigation aims to study thermo-mechanical behaviour of PT (Zr, 2.5wt% Nb) under a stratified flow condition under different internal pressures. The component is subjected to an asymmetrical heat-up conditions as expected during the said situation under different pressure conditions which varies from 2.0MPa and 4MPa. In order to simulate partially voided conditions inside PT, asymmetric heating has been carried out by injecting power to selected heater pins of the upper section of the 19 element fuel bundle simulator housed in a PT. This simulates nearly a stratification level of a half filled reactor channel. Through this technique an expected maximum circumferential temperature gradient of around 440°C, has been attended from top to bottom periphery of PT. Tests also cover a power range of 8–11kW which simulates different decay power levels. An asymmetric ballooning over eighty percent of PT length is observed for all the experiments and the deformation is mostly located to the upper part of the PT. The PT integrity is observed for lower internal pressure tests however a local failure has been observed for the test at 4.0MPa. This is found to be due to excessive local strain prior to establishment of contact with Calandria Tube.

The PROMETRA Program: Fuel Cladding Mechanical Behavior under High Strain Rate

An assessment of the mechanical properties of the highly irradiated fuel claddings under high strain rate has been carried out in the framework of the PROMETRA program undertaken by the French Institut de Radioprotection et de Sûreté Nucléaire in collaboration with Electricité de France and Commissariat à l’Energie Atomique (CEA). Three types of tests, including burst tests, hoop and axial tensile tests, have been performed at CEA-Saclay hot laboratories to determine the cladding tensile properties to use in the SCANAIR code. The prototypicality of each test with regard to the reactivity-initiated accident loading conditions can be addressed and analyzed in terms of strain or stress ratio. The high-strain-rate ductile mechanical properties of irradiated ZIRLO and M5 alloys derived from the PROMETRA program and their comparison to the stress-relieved irradiated Zircaloy-4 are reported. Then, the clad brittle behavior, in particular for highly corroded or spalled Zircaloy-4 cladding, is investigated.

Leakage via a through-wall circumferential crack in a piping or tubing system under accident loading conditions

The paper derives an expression which gives the crack opening area associated with a through-wall circumferential crack in a piping or tubing system that is fabricated from a ductile material when there is extensive plastic deformation at the cracked section. An estimate of such an area is important if there is a concern with regard to the amount of leakage for accident condition loadings.

The flexibility of a two-dimensional piping system that is subjected to out-of-plane deformation

In the context of the stability of a circumferential through-wall crack in a Type 304 stainless steel piping system that is subjected to accident loading conditions, the paper analyses a simple model that simulates the out-of-plane deformation of a two-dimensional model piping system. The instability criterion is expressed in terms of the material tearing modulus T MAT and the applied tearing modulus T APP , the latter being correlated with an effective pipe length parameter L EFF , which is a measure of the system's elastic flexibility. It is shown that out-of-plane deformation can be associated with higher L EFF values than in-plane deformation, and this has an adverse effect on crack stability. The higher values of L EFF are due to a pipe's higher torsional flexibility, as compared with its bending flexibility.

The instability criterion for a circumferential through-wall crack in a bend in a stainless steel piping system: The effects of bend profile and bend angle

Motivated by the problem of intergranular stress corrosion cracking of Type 304 stainless steel in Boiling Water Nuclear Reactor piping systems, the paper examines the stability of a circumferential through-wall crack in a bend in a piping system that is subjected to simulated accident loading conditions. The instability criterion is expressed in terms of the material tearing modulus T MAT and the applied tearing modulus T APP (or the effective pipe length L EFF ). The main thrust of the paper is the modelling of the bend profile and an assessment of the effect of bend angle on the instability criterion.

LMFBR subassembly response to simulated local pressure loadings

The structural response of liquid metal fast breeder reactor (LMFBR) subassemblies to local accidental events is of interest in assessing the safety of such systems. Problems to be resolved include failure propagation modes from pin to pin and from subassembly to subassembly. Factors which must be considered include: (a) the geometry of the structure, (b) uncertainty of the pressure-energy source, (c) uncertainty of materials properties under reactor operating conditions, and (d) the difficulty in performing in-pile or out-of-pile experiments which would simulate the above conditions. The main effort in evaluating the subassembly response has been centered around the development of appropriate analyses based on the finite element technique. Analysis has been extended to include not only the subassembly duct structure itself, but also the fluid environment, both within subassemblies and between them. These models and codes have been devised to cover a wide range of accident loading conditions, and can treat various materials as their properties become known. The effort described here is centered mainly around an experimental effort aimed at verifying, modifying or extending the models used in treating subassembly damage propagation. To verify the finite element codes under development, a series of out-of-pile room temperature experiments has been performed on LMFBR-type subassembly ducts under various loading conditions. The duct sections were instrumented to measure internal pressure, duct midflat strains and deflection of the mid-flat and corners. Since moderate deflections were expected, and effect influence on the radial deformation would occur over a relatively short length. Preliminary calculations and subsequent static and dynamic tests demonstrated that for the range of deformations expected in single subassembly prior to failure, a shortened duct section of only 30.48 cm in length was sufficient to provide a central section over which axially uniform conditions prevailed. As a result, with axial motion of the end plates constrained, the deformation over the uniform deflection range corresponds to two-dimensional, plane-strain conditions and a two-dimensional, finite element computer code could be applied. Tests were subsequently performed on several ducts made of type 316 stainless steel which were either annealed or 50% cold-worked. Material properties of the ducts used in the experiments were determined by testing samples obtained from each duct. Also, diamond point hardness measurements were obtained across the subassembly duct flats in order to establish that the material properties were uniform. Comparisons were then made between the code calculations and experimental results which demonstrated remarkable agreement, thus lending confidence to the code's ability to predict duct response, at least under quasi-static loading. Further preliminary work was performed on the dynamic response of hexcans to a pressure pulse designed to duplicate a postulated local event.