The Diet’s NAIIC report’s executive summary seems largely spot-on, drawing only a few questionable conclusions (e.g. keeping the earthquake-as-cause notion open). However, one serious mistake cries out to be exposed. On page 31, the report says the unit #1 staff’s stoppage of the Isolation Condenser emergency cooling system, 11 minutes after the earthquake, was a questionable action. They further judge Tepco’s explanation to be “irrational”. Actually, the Tepco staff did was precisely what they should have done. Further, the report apparently makes this egregious claim based on misunderstood testimony, which suggests the panel has limited understanding of Boiling Water Reactor (BWR) operations.
While producing steam, the BWR fuel core is engulfed in a mixture of water and steam. The steam “bubbles” are typically called “voids”. When cooling water flow is increased, enough of the voids collapse to cause the level of the steam/water mixture to decrease. This is called “shrink”. If the increase in cooling water flow is great enough, the rate of shrink will be considerable. In parallel, accelerated flow can cause reactor vessel pressure to literally plummet. In order to insure that “shrink” does not cause the upper portion of the core to become uncovered, and further to insure against core structural damage due to thermal shock, a firm limit on how fast core temperature is allowed to drop. With the F. Daiichi BWR’s, this is 55oC per hour. This is a technical specification that any trained reactor operator will adhere to.
The source of cooling water for the Isolation Condenser is condensate from either the plant storage tank or the turbine condenser itself…as long as there is power to run the supply pumps. Before the tsunami knocked out all electrical supplies, the IC system was so efficient that the rate of cooling, pressure drop, and shrinkage was considerable. Why? Because the steam from the reactor was more than 300oC and the cooling water from the condensate system was no more than 100oC. The great differential in temperature certainly condensed the reactor steam flowing into the IC very rapidly, and the water being returned to the RPV must have been in the order of 100oC cooler than the RPV temperature itself. The rate of void collapse in the core and resulting water/steam level shrink was unquestionably significant. I suspect it was greater than the staff had anticipated. Some staff may have said they also wished to see if there might be a leak from the RPV because the rate of decrease surprised them, but this was not the reason they shut the IC down. The technical specifications were followed…period!
Thus, the F. Daiichi unit #1staff’s actions and Tepco’s explanations were entirely correct. On the other hand, questioning the actions of the staff and calling Tepco’s explanations irrational is an exercise in ignorance.