When a valve closes in a steam system, a family of compression waves is generated upstream of the valve. What is meant by “family of waves”? When the valve first starts to close, the first compression wave is generated. As it continues to close, each incremental valve movement generates another incremental wave that travels into the disturbed flow field of the previously generated waves. When the valve reaches full closure a final wave is generated. These waves are referred to here as a “family” of waves. The family has a leading edge (from the initial valve movement) and a trailing edge (from the final valve movement when it comes to full closure).
When this wave family travels through direction changes such as elbows, it causes transient forces acting on the elbows. Engineers designing pipe systems must design for these forces or the pipe and/or pipe supports can fail.
Predicting steam hammer pipe forces is quite complicated. As a result engineers have sought simplified methods believed to be conservative. It was recently shown at the ASME 2022 PVP Conference that the available methods are in fact not conservative. The details are given here in this paper: “A Critique of Steam Hammer Load Analysis Methods” (PVP2022-83715). In short, current methods ignore wave steepening whereby the back of the wave family travels faster than the front. The paper shows how wave steepening can result in transient pipe forces much higher than predicted by current methods. In this situation, the best way forward is to go backwards. More specifically, to go back to basics.
The de facto method of predicting these loads is the Goodling Method from the 1980s. AFT engineers have revealed that the Goodling Method is not always conservative - which means existing fossil and nuclear power stations are not as safe as believed.
Large steam pipe breaks are already analyzed for in existing nuclear power plant safety evaluations. A large steam pipe break will result in increased heat removal from the reactor coolant system, and therefore the effects of such events are already analyzed as postulated accidents.
For instance, chapter 15 of the AP1000 design control document states, in part…
18.104.22.168 Condition IV: Limiting Faults
Condition IV events are faults that are not expected to take place, but are postulated because their consequences include the potential of the release of significant amounts of radioactive material. They are the faults that must be designed against, and they represent limiting design cases. Condition IV faults are not to cause a fission product release to the environment resulting in doses in excess of the guideline values of 10 CFR 50.34. A single Condition IV event is not to cause a consequential loss of required functions of systems needed to cope with the fault, including those of the emergency core cooling system and the containment. The following faults are classified in this category:
Steam system piping failure (major) (see subsection 15.1.5)
Feedwater system pipe break (see subsection 15.2.8)
(Several other postulated accidents are subsequently listed.)
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I see no reason to conclude that US nuclear plants are less safe because of the ASME study referenced in the original post. The analysis of what would happen, given such an event, has already been performed.
The steam hammer experts are saying that the dynamic forces for all steam lines have not been analyzed conservatively. Yes the NRC makes it a nuclear plant licensing condition that they assume large steam line pipe breaks (and all other high energy pipelines).
However, the pipe whipping analysis and the steam jet impingement analysis associated with the assumption of large and small steam line pipe breaks may not have been performed properly. That is why the steam hammer experts say that coal, gas and nuclear plants may not be as safe as previously thought.