Sotan said:
Hello everybody.
This is my first post on these forums.
But I have been following this thread for a while as I am interested in the discussion of Fukushima plant developments.
I am in no way a specialist in this field or in any physics field therefore I doubt I could contribute to these discussions. But I do happen to know Japanese at a fair level, though, and I thought… perhaps I could help with the Japanese translations, sometimes. Unless there's somebody better at this too, that is.
This is a test posting; please tell me if I am useful or I am in the way; no offense will be taken if it's the latter.
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This is an attempt to translate & summarize some of the things related to ADS from the TEPCO report of 20131213, link given in the posts above.
- ADS is mentioned first on page 32, in a chapter dedicated to the issue "The cause of the sudden/fast decrease of pressure in reactor #3 (and the possibility of it being due to some hole that appeared in the main installations of the reactor)".
The situation or level of knowledge before this study:
It was believed that the sudden decrease in reactor 3 pressure that occurred on March 13 around 19:00 hours was the result of operator action - that is, the opening of SRV (Safety Reliev Valves?).
The results of this study:
It was established that the decrease of reactor pressure occurred while the operators were making preparations to manually decrease the pressure. There is the possibility that the pressure decreased as specific conditions were met for the activation of ADS.
(jumping to page 33 - graph of reactor pressure vs time; on the time axis time increases from right to left)
(moving to page 34)
Investigation of the conditions needed for the activation of ADS
The sudden, quick decrease in pressure could be explained by the activation of ADS, but we used to believe that the conditions for the activation of ADS had not been met on reactor 3.
*one of the conditions for the activation of ADS is making sure that the low pressure water system is ready for operation.
The diagram on page 34 indicates that while 3 factors needed for ADS activation were indeed cleared, the output of pumps for the system of removing residual heat and the system for spraying the inside of the reactor was insufficient (these pumps couldn't be operated due to loss of electric power).
The conclusion is that, logically speaking, the ADS system was not supposed to operate.
We investigated the possibility of ADS ending up operating, in spite of the fact that the logical procedure for its operation did not appear as having being achieved.
(moving on to page 35)
We thought, what if the conditions for the operation of ADS were in fact met. What about this possibility.
Due to the rise in pressure in the S/C (suppression chamber?), even though the pump(s) in the residual heat removal system were not functioning, the fact that a certain (significant) value of pressure on the output of this pump could be read might indicate that the conditions for ADS operation were in fact met.
(the diagram indicates that) S/C pressure reaches 0.455 MPa (abs) -> the pressure is transmitted -> pressure gauge measures a value that exceeds the 0.344 MPa needed for ADS activation
(moving on to page 36)
The actually measured data as well as analysis data were considered in relation with the decrease in reactor pressure.
The graph on this page shows that various actually measured parameters (the SRVs and the water level) are consistent with the hypothesis that the ADS had in fact been activated around 08:56 hours.
(moving on to page 37)
Considering the possibility that the cooling by water might have been insufficient, they are modifying the proposed graphic depiction of reactor 3 damage as shown in these drawings. Left is what they used to believe - right is what they think now.
Alright, this is where I come in.
ADS (Automatic Depressurization System) is a part of each plant's ECCS (Emergency Core Cooling System) package. ADS does not directly cool the fuel in itself, but it is designed to automatically reduce reactor pressure to a level low enough that a low pressure ECCS system could inject water to the reactor. ADS works by sensing certain plant parameters, and, if those parameters are met, automatically lifting a select number of SRV (Safety Relief Valves) in their power operated relief mode to depressurize the reactor to the suppression pool.
The logic for an ADS initiation is roughly as follows:
Reactor water level < Level 1 (about 20 feet below normal, and a few feet above the fuel, this also starts the low pressure ECCS pumps)
AND
reactor water level < Level 3 (This is the low water level scram signal, about 2-3 feet below normal)
AND
any low pressure ECCS discharge piping has sufficient pressure for injection (signifies a low pressure ECCS pump is running)
AND
high drywell pressure (> 1.68 PSIG, this also auto starts all ECCS systems and puts the plant into LOCA mode)
Once all of the above are met, an alarm goes off in the control room for 105 seconds. If the operator does not manually inhibit the ADS system before the 105 seconds is up, ADS activates. ADS will blow down the reactor until the conditions are clear and the operator manually resets ADS.
If high drywell pressure is not present, but all the other conditions are present, the system will automatically activate after several (~6) minutes. High drywell pressure means a LOCA is in progress, so the ADS blowdown needs to happen as soon as possible. Without high drywell pressure, the 6 minute timer is long enough to give the operators time to restore cooling systems, and short enough that the fuel remains safe.
It looks like the torus/wetwell pressure was high enough to make the system think the low pressure ECCS pumps were running. The low pressure ECCS pumps take pressure from the suppression pool, so it makes sense that the logic was made up.
