Japan Earthquake: nuclear plants

MadderDoc

But they have kept the reduced rate of nitrogen injection constant now for several weeks with H2 concentration steady at about 0.2 %. It is only during the last few days the H2 level has increased now to about 0.5 %.

SteveElbows

Oops I just noticed that I got this the wrong way round, they had put the RPV nitrogen injection back to previous levels but its the PCV they left at only 5.0N m³/h

r-j

In the US it is after 5 years in a cooling/storage pond. http://en.wikipedia.org/wiki/Dry_cas...#United_States

jim hardy

indeed that's what is described by that attachment.
I am accustomed to valves where the stem lifts the disc directly, but this one may instead lift the spring which would allow the disc to behave as described.

Yes, gage, that bellows keeps discharge pressure away from top of disc. So force up on disc is steam pressure X wetted area and force down is its weight plus pressure inside bellows (atmospheric - that's why bonnet is open type) plus spring force.

I'd like to find the Crosby manual for that specific valve. The link i gave is a bit generic.

Yes, The piston would collapse back down to bottom of cylinder and allow spring to push stem down against disc.

That's how i see it.

old jim

MadderDoc

The reactor would need to be held at a relatively low temperature such as to avoid SRV's opening, and even lower than that, since we also don't want to degrade any part of the PCV. Just to pick a figure, lets say we think we can handle 127^oC or 400 K. A black-body of this temperature will radiate 5.67E-8*400⁴ watt/m². That's about 1.5 kW/m². The surface area of the RPV is in the neighbourhood of 500 m², so that would be 0.7 MW, as the maximum we would like to have in the RPV *if there were no containment around it*. But seeing there is, we would like to have less, and probably much less than 0.7 MW. I know this is not quite the figure you are asking for, but at least it puts a cap to it.

jim hardy

hmm two thoughts while i was eating breakfast.

1. That statement "..valve body is then in a free state.." is written by a technical writer for the report , i'd sure like to find it in the valve documentation. My skepticism is because i am accustomed to a direct plug-stem connection in the smaller safety valves in my experience. So i am allowing the possibility that's an error of translation by a bright fellow who's picked up a manual for pilot operated valve instead of that safety valve. If that's so, then lifting the valve handle would open the valve irrespectine of pressures. Hence my remark about finding the Crosby manual for that valve.
But for now i have to accept what's written.

2. I told you gage pressure. But - is not that valve located inside PCV ? So gage pressure in there is relative not to atmosphere but to PCV pressure.

The sketch you posted shows something called "Eductor" which i haven't figured out yet. I think its purpose is to reduce pressure above disc but outside bellows which helps give that snap-open characteristic you want in a safety valve.

I would like to more completely understand that valve's internals.
As an instrument guy i worked on regulating valves. Safety valves were in mechanical discipline and my small knowledge of them comes from talking with the mechanics who maintained them.
Ahh our regrets in life are mostly about the things we didnt learn. I could have learned more about code safeties.

old jim

MadderDoc

Thanks, that was a great help. Now I think I've got some thinking to do as regards the implications. Up till now I had the SRVs in Fukushima as something that was flipped to stay open, come what may. I can't remember reading about anything done with the SRVs after the time the reactors got depressurised.

jim hardy

Found some more Crosby literature.

The "Balancing Piston" is a backup in case the bellows ruptures.
A person accustomed to control valves might well interpret it as a pilot
and make the mistake i mentioned.

This link takes me to Crosby catalog 310 for their JOS and JBS series valves used on PWR's. Still trying to find exactly what's a 6R10.
http://www.google.com/url?sa=t&rct=j...eKGFteuMsdFGYg
page 59 describes the balancing piston.

And from a generic study of nuclear safety valves, page 11
http://www.osti.gov/bridge/purl.cove...able/41402.pdf
and page 12: page 4 has a photo of BWR safety. The piston that operates the handle is prominent.

Doc, i am ready to believe that "floating state" comment is an innocent error by technical writer.
That stem is i believe directly coupled to the disc, ie raising handle opens valve irrespective of pressure.

So by opening the valve they conneccted RPV to torus . I beieve. So long as their air and batteries held out.

old jim

EDIT sorry to flip on you
but one must go with best info available. I wasn't ready to accuse tech writer nased on my own limited experience. Apologies for the flip-flop.

mheslep

Thanks. This is the approach I'm looking for. Some questions:

Is the temperature of the PCV the critical figure for a walk away state, rather than the temperature of the fuel assembly? I assumed that on the low side no pressure was allowable in walk-away thus 100C was the limit internal to the PCV. Or, if a low steady-state pressure could be sustained indefinitely then something short of a temperature that trips the SRVs? I have no idea.

With regard to heat transfer, convective free air would dominate at about ~10W/m^2/K depending the humidity. If ~100K above ambient is allowable as you suggest, then ~1KW/m^2 is the convective heat transfer, so that a 500m^2 RPV allows .5 MW of decay power. For Daichi 2&3 I gather .5 MW is still many months away.

mheslep

The cask storage stage is the time required to allow deer and rabbits to nuzzle the outdoor casks without cooking them, or to prevent rain/snow from cracking an otherwise super heated cask. The PV (without active cooling) inside containment might sustain a somewhat more elevated power level might for a time without in a radiation release.

MadderDoc

Thanks for the link.

Yes, I think the PCV temperature would be the critical figure. Something just short of a temperature that trips the SRVs would fry the PCV within hours, it only works under normal operation because the PCV is being cooled. With no power no cooling at hand it would be imperative to have the RPV and fuel at a temperature far below what they themselves can withstand. I'd agree that 100C could be the pain threshold for the PCV, rather than the figure I suggested, we also wouldn't like the PCV to loose steam.

MadderDoc

I really don't mind, jim, and I much appreciate your efforts to get it right, not just answered, also as long as I am getting wiser all the time, and I think I am.. :-)

Still most of the stuff about the technical workings of the valve is a bit over my capacity, but I do understand, that it is one crucial point whether that thing called the stem is directly connected to that thing called the disc. Assuming, as you, and everything else I have been able to dig up indicates, they are directly connected. So actuation of the valve would seem to imply that the valve would simply be forced open.

Otoh, the investigation committee who wrote it, and Tepco who I assume read it, have let pass in the interim report the considerations quoted below, which imply contrarily, that the SRV is not unconditionally forced open on actuation, so how to square the conflicting evidence? Could the situation be that the stem/disc action facilitates the opening of the valve, but that a certain steam pressure still would be needed to produce the flow channel?

"In general, the SRVs can be manually opened by remote control, if the RPV pressure is over 0.686MPa in gage. According to the plant parameters released by TEPCO, the Unit 3 RPV pressure at around 2:44 on March 13 was 0.580MPa in gage. Therefore the possibility that RPV pressure was below the required value at the time of the first opening operation at around 2:45 cannot be ruled out. On the contrary, taking into account a shift team operator’s logbook saying that the RPV pressure was 0.8 MPa at around 2:45 on the same day, it can be concluded that the lower pressure was not the real cause of the “fail to open.” To return to TEPCO’s plant parameters, the RPV pressure at Unit 3 around 3:00 on the same day elevated up to 0.770MPa in gage. If so, it is highly possible ...[snip]"

Edit: Jim, I just experienced a potential serendipity, please can I have you take a look page number 68 of the report and ff
http://pbadupws.nrc.gov/docs/ML1111/ML111170549.pdf
What is described there is technically way, way over my head, but just scanning the figures with my eyes
I get much too similar signals from this text as I get from the interim report to ignore.
Perhaps we are looking at Target Rock SRV's not Crosbys.

jim hardy

Thaks m'doc for the encoraging words.

I've found Dresser makes such valves and i found their catalog. It has nice color drawings that definitely show snap-rings . Will post link if i can find it again.

I have a personal dislike for pilot operated valves in this application. So i didnt look into Target Rock.

Crosby's drawings that i've found are lower resolution and dont seem to show a snap ring. In fact some of the drawings look like they'd agree with the TEPCO statement of floating disc.

After making that post i went outside to work on an old engine and thought about this.
I am biased by prior experience so tended to disbelieve the lack of firm stem-to-disc connection. That's my prejudice i realize now.
Given that safety valves evolved from a simple hole covered by a disc with weights stacked on top of it, there's no need for a solid connection to stem.
Indeed there's that historical pecedent.

So i answered before i should have. Realized that upon reflection.
However - the reason i gave answer i did is this:
Assume some reasonable diameter for the disc, say four inches. That gives it area of 4∏ square inches.
When pressure below disc X area of disc equals weight of parts to be lifted by steam , those parts will indeed be lifted by steam. (Assuming spring is held away by operating handle.)
Now 4∏ square inches X 100 psi is 1257 pounds. That's just too much for a disc and retainer to weigh. We're talking about something the size of harmonic damper on a big car engine.
At 2 inches it's still 314 pounds which pushes the credibility limit for me.


I found this line in the 179549 link you gave:
That quote is describing the Target-Rock as you suggested and it sure sounds a lot like what the TEPCO technical writer wrote.


Target-rocks in my plant were maintenance headaches. And a pilot valve caused TMI. So i am biased against pilot valves for 'important to safety' service. I assumed the mechanical designers would stick to the simpler dierct acting design like Crosby.



another serendipity moment for you

Looks like the designers came around .

That's from NUREG/CR-6042 Rev. 2 section 3.7.2.5 , page 7 of 213
http://www.google.com/url?sa=t&rct=j...M8y8zg5f6ovsUw

Fig 3.7-9 on page 33 is a better drawing of the Crosby type valve. Zoomiing in to 300% i still can't tell how it's put together. It's frustrating - when i worked at plant we could go to warehouse and look at the on-hand spare parts.

But i'm coming to my senses now.
We dont know which valve they have.
So i withdraw my accusation against their tech writer, he may well have been describing a different valve than i was looking at. Apology to you, unknown writer.

And i back off my claim in last post. Go ahead with your thinking per TEPCO writer.

M'doc I admire you guys' doggedness and attention to detail. You're doing it right..


Imagine what Microsoft could have been if they had an industrial strength mindset.


old jim


PS - somebody has put pilot valves in main steam safety service. no comment.

jim hardy

From Crosby catalog 310 which claims to represent their Nuclear Main Steam Safety valves, probably JOS type:

so it'll follow stem.

http://www.google.com/url?sa=t&rct=j...ZNJuWyuJ3LE05Q

or CROMC-0297-US.pdf
text from page 4 and see the clip item 29 on page 7.

you're right on with that Target Rock. Wish i knew for sure what valves they have.

old jim

MadderDoc

Yeah. Without that knowledge there may be left unresolved questions, but I think we are now better qualified to express them :-)

I'll assume that the SRVs they have, whatever their brand,
-- in pressure mode, are meant to open automatically at about 1000 psig (about 7-8 MPag), , and reclose at a pressure some 3-10% lower.
-- in actuation mode, are controlled by a differential pressure, which on exceeding 100 psi (0.686 MPa) will make the valve come open from its closed position,
and which on dropping below 50 psi (0.345 MPa) will make the valve come close from its open position.

It is not clear to me which differential pressure we are talking about. Some sources indicate steam inlet differential to PCV , some gauge, ie. steam inlet relative to ambient. Physically it seems the SRVs are inside the PCV, which under normal operation would be at ambient, so it would not matter much with that distinction, but it might under accident conditions. The more 'learned' the description, the more it seems to me indicated there that the PCV pressure factors into the differential pressure -- while the interim report in its considerations judges the available differential pressure only from the RPV gauge pressure. Only once, and only implicitly it says something to the effect that other factors might play a part (my boldface):

"The SRV, functionally, can be opened manually above the RPV pressure of 0.686MPa in gage and remain in an open position down to 0.344MPa in gage after the first actuation. Under this limiting pressure, however, the valve is to be fully closed because valve disk weight exceeds the lifting force. The SRV is, anyhow, less likely to be opened in the lower pressure ranges".

(The explicit specification 'first actuation' of the figures 0.686 MPa/0.344 Mpa would seem to imply that values for the second actuation are (edit: or could be) different)


Edit:
At the time of 'the first actuation' (Unit 3, in the morning of March 13th) the S/C pressure was 0.445 MPa. Assuming a Target Rock SRV with the properties described in the NUREG document this valve would come open when RPV is at 0.345 MPa above S/C pressure, i.e. at 0.790 MPa, or 0.690 MPag. This fits well with the figure 0.686 MPa gauge given by the interim report.

While being actuated, the Target Rock SRV main valve would be kept open by the differential pressure between the RPV and the S/C. IOW, when the RPV pressure would decrease to that of the S/C, 0.445 MPa, or 0.345 MPag, the main valve would come close. Again, this fits well with the figure 0.344 MPa gauge given by the interim report.

jim hardy

Indeed the forces on the valve parts are difference between pressure inside and outside the valve. So if valve is inside PCV , outside of it sees PCV pressure. Or if a pilot valve it may see upstream(RPV) vs downstream(torus) pressures. Can I assume torus is about same as PCV pessure?

Now to RPV pressure:
If the pressure sensor is inside PCV and is a gage pressure sensor, same applies. It'll report difference between RPV and PCV, it knows nothing of atmosphere outside PCV and cannot compensate.
If the pressure sensor is an absolute pressure sensor that's what it will report, absolute pressure.

Gage and absolute pressure sensors are similar but absolute pressure ones are a bit more expensive. That's because they must include a sealed and evacuated chamber for an absolute zero pressure reference. Our Rosemount gage and absolute sensors were identical except that the gage ones leave that reference chamber open to local atmosphere.


Well thanks for the exercise ! Ilearned some things.

If your observed data is fitting with that Target-Rock valve model i'd say that clinches it - they have something similar.
If it really takes 50 psi differential to lift that plug then there's smaller unbalanced areas than i estimated from those Crosby drawings.

Nice work, Doc .
old jim

NUCENG

The pressure difference between the PCV and the suppression chamber in a BWR is limited by vacuum breakers which relieve from the torus to the PCV if the torus pressure exceeds the PCV pressure. Other vacuum breakers relieve from the atmosphere to the torus air space to prevent either the torus or the drywell to become negatively pressurized compared to atmosphere (prevents the crushed beer can syndrome). The drywell pressure may exceed the torus pressure by the submergence head in the torus sowncomers. If the pressure exceeds that the water in the downcomers is displaced and the drywell relieves to the torus.

I know you are an instrument guy, so maybe I should not try to add to your question about pressure instruments. Forgive me if I misunderstood. In PWRs with large dry containments the pressure instumentation transmitters or sensors are usually inside the containment building but outside the shield wall. so they may be measuring gauge pressure to the containment. In BWRs the instrument transmitters or sensors are all located outside the PCV and can be identified as gauge or d/p by the fact that the d/p sensors have two lines and the gage instruments have one which means the difference is to secondary containment pressure. Your discussion of Rosemount pressure sensors is accurate about absolute pressure instrumentation.