was auch den Tatsachen entspricht. NP schrieb, der Reaktor sei "ausgeschaltet" worden.

Einen Reaktor kann man nicht einfach "ausschalten".

Hier ist die genaue Sequenz der Abläufe:

https://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/...

Der wesentlich Punkt war jedoch, daß NP behauptet hat, es solle getestet werden, ob mit der thermischen Energie (also der Nachwärme nach "Ausschalten" des Reaktors) noch Energie gewonnen werden könnte, bis die Notstromversorgung anspringt. Meine wesentliche Antwort war, daß das inkorrekt war, denn man wollte die Rotationsenergie der nachlaufenden Welle und Turbine nutzen.

Was dann die Aussage von NP sollte, "der Reaktor sollte ausgeschaltet werden" und was das mit meinem Post zu tun hat, erschließt sich mir nicht. Zudem ich gepostet hatte, daß der Reaktor für den Versuch heruntergefahren würde.

Timeline

The timeline which follows has been compiled following a review of a large number of reports and it represents what is considered the most likely sequence of events, but there remain some uncertainties.
April 25
01:06 The scheduled shutdown of the reactor started. Gradual lowering of the power level began.
03:47 Lowering of reactor power halted at 1600 MW (thermal).
14:00 The emergency core cooling system (ECCS) was isolated (part of the test procedure) to prevent it from interrupting the test later. The fact that the ECCS was isolated did not contribute to the accident; however, had it been available it might have reduced the impact slightly.
The power was due to be lowered further; however, the controller of the electricity grid in Kiev requested the reactor operator to keep supplying electricity to enable demand to be met. Consequently, the reactor power level was maintained at 1600 MWt and the experiment was delayed. Without this delay, the test would have been conducted during the day shift.
23:10 Power reduction recommenced.
24:00 Shift change.
April 26
00:05 Power level had been decreased to 720 MWt and continued to be reduced. Although INSAG-1 stated that operation below 700 MWt was forbidden, sustained operation of the reactor below this level was not proscribed.
00:28 With the power level at about 500 MWt, control was transferred from the local to the automatic regulating system. The operator might have failed to give the 'hold power at required level' signal or the regulating system failed to respond to this signal. This led to an unexpected fall in power, which rapidly dropped to 30 MWt.
00:43:27 Turbogenerator trip signal blocked in accordance with operational and test procedures. INSAG-1 incorrectly reported this event occurring at 01:23:04 and stated: "This trip would have saved the reactor." However, it is more likely that disabling this trip only delayed the onset of the accident by 39 seconds.
01:00 The reactor power had risen to 200 MWt and stabilised. Although the operators may not have known it, the required operating reactivity margin (ORM) of 15 rods had been violated. The decision was made to carry out the turbogenerator rundown tests at a power level of about 200 MWt.

Der Reaktor wurde also nicht "abgeschaltet", sondern wie ich vorher geschrieben hatte, "heruntergefahren".

01:03 A standby main circulation pump was switched into the left hand cooling circuit in order to increase the water flow to the core (part of the test procedure).
01:07 An additional cooling pump was switched into the right hand cooling circuit (part of the test procedure). Operation of additional pumps removed heat from the core more quickly leading to decreased reactivity, necessitating further absorber rod removal to prevent power levels falling. The pumps delivered excessive flow to the point where they exceeded their allowed limits. Increased core flow led to problems with the level in the steam drum.
01:19 (approx.) The steam drum level was still near the emergency level. To compensate, the operator increased feedwater flow. This raised the drum level, but further reduced reactivity to the system. The automatic control rods went up to the upper tie plate to compensate but further withdrawal of manual rods was required to maintain the reactivity balance. System pressure began to fall and, to stabilise pressure, the steam turbine bypass valve was shut off. Since the operators were having trouble with the pressure and level control, they deactivated the automatic trip systems to the steam drum around this time.
01:22:30 Calculations performed after the accident found that the ORM at this point proved to be equal to eight control rods. Operating policy required that a minimum ORM of 15 control rods be inserted in the reactor at all times.
01:23 (approx.) Reactor parameters stabilised. The unit shift supervisors considered that preparations for the tests had been completed and, having switched on the oscilloscope, gave the order to close the emergency stop valves.
April 26: the test
01:23:04 Turbine feed valves closed to start turbine coasting. This was the beginning of the actual test. According to Annex I of INSAG-7, for the following approximately 30 seconds of rundown of the four coolant pumps, "the parameters of the unit were controlled, remained within the limits expected for the operating conditions concerned, and did not require any intervention on the part of the personnel."
01:23:40 The emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core, increasing the reactivity at the bottom of the core.
01:23:43 Power excursion rate emergency protection system signals on; power exceeded 530 MWt.
01:23:46 Disconnection of the first pair of main circulating pumps (MCPs) being 'run down', followed immediately by disconnection of the second pair.
01:23:47 Sharp reduction in the flow rates of the MCPs not involved in the rundown test and unreliable readings in the MCPs involved in the test; sharp increase of pressure in the steam separator drums; sharp increase in the water level in the steam separator drums.
01:23:48 Restoration of flow rates of MCPs not involved in the rundown test to values close to the initial ones; restoration of flow rates to 15% below the initial rate for the MCPs on the left side which were being run down; restoration of flow rates to 10% below the initial rate for one of the other MCPs involved in the test and unreliable readings for the other one; further increase of pressure in the steam separator drums and of water level in the steam separator drums; triggering of fast acting systems for dumping of steam to condensers.
01:23:49 Emergency protection signal 'Pressure increase in reactor space (rupture of a fuel channel)'; 'No voltage - 48 V' signal (no power supply to the servodrive mechanisms of the EPS); 'Failure of the actuators of automatic power controllers Nos 1 and 2' signals.
01:24 From a note in the chief reactor control engineer's operating log: "01:24: Severe shocks; the RCPS rods stopped moving before they reached the lower limit stop switches; power switch of clutch mechanisms is off."

Xenon poisoning was a significant contributor to the Chernobyl accident. Xenon-135 is produced in the reactor by the decay of the fission product iodine-135 (I-135). As I-135 has a half-life of 6.7 hours, Xe-135 will continue to build up after a reactor has been shut down. (Xe-135 itself has a half-life of 9.2 hours, so will eventually decay.) Xe-135 is a very strong neutron absorber and is 'burned' in the process of absorbing neutrons. During normal operation, the production of Xe-135 is balanced by the reaction rate. When the power of the Chernobyl 4 reactor dropped at 00:28 on 26 April, Xe-135 would have built up making it difficult to raise the reactor power. Attempts to raise the reactor power at this point led to so many control rods being withdrawn that the emergency protection system was brought to a state where termination of the nuclear reaction could not be guaranteed. [Back]

DT