Analysis of the Effect of Vessel Failure and Melt Release on Risk of Containment Failure Due to Ex-Vessel Steam Explosion in Nordic Boiling Water Reactor Using ROAAM+ Framework

Nordic boiling water reactor (BWR) design employs ex-vessel debris coolability in a deep pool of water as a severe accident management (SAM) strategy. Depending on melt release conditions from the vessel and core–melt coolant interactions, containment integrity can be threatened by: (i) formation of noncoolable debris bed or (ii) energetic steam explosion. Melt is released from the vessel affect ex-vessel phenomena and is recognized as the major source of uncertainty. The risk-oriented accident analysis methodology (ROAAM+) is used for quantification of the risk of containment failure in Nordic BWR where melt ejection mode surrogate model (MEM SM) provides initial conditions for the analysis of debris agglomeration and ex-vessel steam explosion which determine the respective loads on the containment. Melt ejection SM is based on the system analysis code methods for estimation of leakages and consequences of releases (computer code) (MELCOR). Modeling of vessel failure and melt release from the vessel in MELCOR is based on parametric models, allowing a user to select different assumptions that effectively control lower head (LH) behavior and melt release. The work addresses the effect of epistemic uncertain parameters and modeling assumptions in MEM SM on the containment loads due to ex-vessel steam explosion in Nordic BWR. Sensitivity and uncertainty analysis performed to identify the most influential parameters and uncertainty in the risk of containment failure due to ex-vessel steam explosion. The results of the analysis provide valuable insights regarding the effect of MELCOR models, modeling parameters, and sensitivity coefficients on melt release conditions and predictions of ex-vessel steam explosion loads on the containment structures.

Rapid and High Aspect Ratio Micro-hole Drilling With Multiple Micro-Second Pulses Using a CW Single-Mode Fiber Laser

Laser drilling is an important industrial process for the production of various sizes of holes. In this paper, we investigate rapid, high aspect ratio microhole drilling using multiple microsecond pulses based on the single pulse drilling technique reported in [17, 18]. It was established that there would be a synergistic effect if a subsequent pulse is irradiated at the target within 100 μs of the previous pulse before the melt solidifies. However, the peak power values of subsequent pulses decrease with higher repetition rates. The results show that the synergistic effect could outweigh the reduction in laser power. Another contributing factor of the synergistic effect is related to the melt ejection efficiency. As the hole deepens, the melt ejection becomes less effective to eject the melt completely out of the hole, resulting in a partially blocked hole. A subsequent laser pulse needs to reopen the hole before the hole can be deepened further. To overcome this hole blocking problem, shooting a subsequent pulse at a higher repetition rate also ensures that the energy absorption is more efficient when a subsequent laser pulse is irradiating at the hole blocking melt which is not yet solidified. This multiple-pulse drilling techniquewas applied for through-hole drilling. It was found that the total drilling times through an 800 μm plate were found to be 634 ms and 21.9 ms at 13 kHz and 20 kHz, respectively. The drilling efficiency at the 20 kHz repetition rate is drastically higher, needing only 428 shots, compared with 8240 shots at the 13 kHz, an improvement of nearly 200 times. It is confirmed that this multiple-pulse drilling technique with microsecond pulses using a 300Wsingle mode fiber laser is a viable technique to produce high aspect ratio through holes with a simple and robust setup for the production environment.