Reactor 4 and the Spent Fuel Pools. Same thing, Reactors cannot self power their own systems


American Nuclear Society Fukushima Diiachi ANS Report

II.B.4. Fukushima Daiichi Unit 4
The total AC power supply for Unit 4 also was lost because of the earthquake/tsunami; therefore, the functions of cooling and supplying water to the SFP were lost. The SFP temperature increased to 84°C (183°F) by 4:00 a.m. on March 14. At ~6:00 a.m. on March 15, an explosion that was thought to be a hydrogen explosion occurred in the reactor building, severely damaging part of the building. At first, this was thought to be from fuel uncovery, heatup, and hydrogen production. Therefore, over the next several days, several different schemes were used to add water—via helicopter, fire truck, and concrete pump truck. Both freshwater and seawater were used. Later, photographs indicated that there was no overheat damage to fuel in the SFP, and the source of hydrogen was traced to backflow through the standby gas treatment system ducting that shared a common piping at the NPP stack with Unit 3, whose containment was being vented.
II.C. Spent-Fuel Situation at Fukushima
Daiichi NPS
Damage to stored used fuel resulting in the release of radioactive material can result from several mechanisms:
• a sustained loss or degradation of effective active cooling of the SFP water
• loss of SFP water inventory
• physical impact of a dropped heavy object
• a combination of the above mechanisms.
Loss of cooling could lead to boiling of the SFP water. The time before the SFP water level drops sufficiently to result in fuel overheating depends on the amount of water in the SFP as well as the heat load of the spent fuel. In the absence of a leak in the SFP, this time could range from several days to a couple of weeks depending on the details of the SFP design and the decay heat.
Conditions at the NPS during the accident suggested that these mechanisms may have existed. However, the evidence is that no damage occurred to the fuel in the Unit 5 SFP, the Unit 6 SFP, or the common SFP. The September 2011 supplemental report by the Japanese government to the International Atomic Energy Agency (IAEA) concluded that it is most likely that water levels in the Unit 1 through Unit 4 SFPs were recovered before any spent fuel was exposed and damaged [1]. No subsequent evidence has emerged to counter these conclusions.
When the off-site power and all but one of the EDGs were lost at the NPS because of the earthquake/tsunami, normal cooling of the SFPs was lost. The available EDG allowed cooling to be restored to the Unit 5 and Unit 6 SFPs before the temperature of these SFPs increased significantly. Power was also restored to the common SFP cooling system before its temperature increased significantly.
On March 12, a hydrogen explosion damaged the upper portion of the structure surrounding the refueling bay on Unit 1. While this explosion may have resulted in material falling into the SFP, there is no evidence that damage to the fuel occurred. Beginning on March 31, a concrete pumping truck was used to provide makeup inventory to the Unit 1 SFP. An alternative cooling water system has since been put in service for Unit 1. As of September, the SFP water in Unit 1 has been maintained at <35°C (95°F).
Water addition using existing Unit 2 SFP piping began on March 20 and was intermittent. A sample of the water in the Unit 2 skimmer surge tank was taken on April 16. Analysis of this sample suggests that the spent fuel was not damaged. By May 31, a dedicated system incorporating a heat exchanger was in service. An alternative cooling system is in operation, and as of September, the SFP water in Unit 2 has been maintained at <35°C (95°F).
On March 14, a hydrogen explosion damaged the structure housing the Unit 3 refueling pool. Water spray by water cannon and water drops by helicopter started March 17. By March 27, water addition to the pool was accomplished by use of a concrete pump. Use of existing SFP piping to restore SFP inventory began in late April. A video recording made in the Unit 3 SFP was released on June 16 that showed debris from the containment structure that had fallen into the SFP. It was not possible to confirm the structural integrity of the fuel racks using the video recording. It is likely that no damage has occurred to the spent fuel. As of September, the SFP water in Unit 3 has been maintained at <35°C (95°F).
Because of the relatively high decay heat associated with the fuel in the Unit 4 SFP (all fuel had been removed from the Unit 4 RPV in December 2010), special concern was focused on this SFP. When the refueling floor containment structure was severely damaged because of an apparent hydrogen explosion early in the morning of March 15, this concern was intensified. Initially, since the Unit 4 RPV was defueled, the source of the hydrogen was thought to be the stored used fuel, implying that SFP inventory had been lost early in the accident. Later, the source of the hydrogen was determined to likely be from Unit 3, via a pathway to the Unit 4 refueling floor, leaking through a shared pipe to the stack.
Unit 4 SFP temperatures were reported to be 84°C (183°F) on March 14 and 15. Water was intermittently sprayed from trucks beginning March 20. Nevertheless, the reported SFP temperature on March 24 was 100°C (212°F). Water was introduced to the SFP using concrete pumps starting March 25, which offered a more reliable method of delivering water to the SFP.
Additional evidence of the condition of the used fuel in the Unit 4 SFP was inferred from a series of assessments of specific radionuclides from samples taken of the SFP water. Evaluation of the radiochemical assessments supported the proposition that the source of the hydrogen that led to the destruction of the Unit 4 reactor bay superstructure was Unit 3. A video recording of the Unit 4 SFP was released on May 9. This video recording did not show evidence of extensive damage. In fact, the fuel racks appeared to be intact with little debris visible in the SFP.
In April, a concern developed centered around the strength of the structure supporting the Unit 4 SFP. Between May 31 and June 20, steel support pillars were installed to provide protection against damage that might result from additional seismic events.
In late September, the temperature in the Unit 4 SFP was <40°C (104°F), and a new system to provide active cooling was in operation. This is a typical SFP temperature.
end quote.

Same deal at the system concept level. Reactors are not rigged to power their own essential system valves, although the actual pumps have integral steam turbines driving them. The spent fuel pools became vulnerable and then became emitters. The destruction of the outer reactor buildings resulted in direct emission of radionuclides from the spent pool fuel fire in No 4 to vent into the atmosphere.

Google Bob Alvarez. eg


One Response to “Reactor 4 and the Spent Fuel Pools. Same thing, Reactors cannot self power their own systems”

  1. CaptD Says:

    I think it is important to double check the “fault tree” in every reactor because it will not only tell us much about how they were affected by the earthquake but point out problems where the logs don’t match the data, something TEPCO is World Famous for doing.

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