Level measurement in GE BWRs

rmattila

I've been looking at the level measurement data from the Fukushima plants during the accident, and wondering if the somewhat suspicious behaviour is due to clogging of the impulse tubes ore some other malfunction, or if the measurement is arranged in such a way that it does not measure the core level in this kind of situation.

Being only familiar with the ASEA BWRs with the internal recirculation pumps, I've been looking at the GE documentation on the jet pump type BWR:s ( http://www4.ncsu.edu/~doster/NE405/M...escription.pdf ) trying to comprehend how the level measurement system works in these reactors. However, no information on that particular detail is given in that (otherwise wonderful) documentation.

Is there someone who could help an ignoramus by sharing the following information:

• Is the level measurement based on impulse pipe-type pressure transmitters and a pressure sensor?
• If so, is the location of the lower impulse pipe above or below the core shroud support shelf?

If my thinking is correct, the location of the lower impulse pipe (above or below the core shroud shelf) determines whether the level to be measured is the actual core level (=the level inside the shroud) or the level in the annular region between the diffuser pipes. This should make no difference as long as the level is above the top of the diffuser pipes (which is the state the plant is supposed to be in), but if the level falls below the top of the diffuser pipe, then it is important to know which level we are actually seeing in the measurement results.

NUCENG

BWR reactor level measures water level outside the shroud. It uses two taps into the vessel, The top tap has a condensing pot to keep the reference leg full. At least some US plants have keepfill systems to keep the reference leg full under flashing conditions or shutdown. The lower tap is outside the shroud just above the core support plate. The pressire difference between these two points is related to level The maximum pressure difference would be a vessel dry or full of steam down to the bottom tap (max dp = minimum measureable level). The minimum pressure would be when level of water rises above the upper tap. (Zero dp = maximum measureable level). If the reference leg is not full the level indication is erroneously high.

Some plants may have installed hydrosonic level detection, but I have no knowledge what they have at Fukushima.

THe release of hydrogen and radiation clearly indicates water level dropped below the Top of Active Fuel (TAF) If there is a leak in piping or systems attached above the core support plate are broken and leaking, and makeuo water is being added through the core spray or LPCI systems the level of water outside the shroud would drop until the tops of the jet pumps are uncovered. This keeps level inside the shroud up to a level sufficient for core cooling with continuiing makeup. The only penetrations below the support plate are the vessel drain and control rod drives and a few instrument connections.

rmattila

Thanks for the info. So, whenever the level in the core drops below the tops of the jet pump diffuser pipes (-1 m or so), the level measurement gives information on the water level between the shroud and pressure vessel, not in the core area. All we know (if the level measurement works properly) is that the core level is below -1 m. The core could even be completely drained, and there could still be a couple of meters of water to be measured in the downcomer.

This is due to the special design of the GE BWRs with the bottom of the downcomer separated from the core at low water levels in order to prevent core from draining in case of a rupture in the jet pump pipes. This is different from the internal recirculation pump reactors (or the old ASEA external pump reactors), where the level in the downcomer is always equal to that in the core. I doubt that many Europeans have realized this difference.

rmattila

I finally found a better description of the GE BWR instrumentations, and it appears that there are two pressure difference sensors in addition to those designated as "level measurements": the pressure difference meters used for measuring the flow through a jet pump diffusor pipe and through the core bottom plate.

These measurements have their taps in the same volume as the core, and the one in the diffuser pipe - if functional - could probably be utilized to measure the level in the tank even after the level falls to below the top of the diffusor pipe. Otherwise, I really don't see how levels lower than -1200 mm could be measured, if the lower taps indeed are above the shroud support plate.

Not that it matters any more, but if the flow meter readings have been plotted in the early phases of the accident, this data could possibly give some additional information regarding the behaviour of the level and uncovering of the core.

jim hardy

Here's an article by some other guys who had similar questions.
it's like an impressionism painting. - not quite enough detail but conveys the idea.

http://www.pbnc2010.org.mx/pdfs/tech...umentation.pdf

old jim

rmattila

The diagrams shown in the video: http://www.youtube.com/watch?v=CsuKsS-Gl5k

hint that the lower tap they use for reactor water level measurement would indeed be in the diffusor pipe and NOT in the downcomer region above the shroud support plate. If this indeed is the case, there finally seems to be some sense in giving out the level measurement data.