March 27, 2013
One question has lurked behind the scenes at Fukushima Daiichi for more than two years. We know cooling water is being pumped into the three Reactor Pressure Vessels (RPVs) that contain damaged or fully-melted nuclear fuel. We know the water is being fed in at a rate of ~ 5 tons per hour for units 1&2 (21 gallons per minute), and 7 tons per hour for unit #3 (29 gallons per minute). We know there are two pathways of flow into the RPVs; one through the feed-water system piping and the other through the core spray ring inside the three RPVs. We know that the water being pumped into the RPVs is not staying in there because the levels appear to be constant. We know the water eventually finds its way into the basements of the outer reactor building and turbine building basements. What we don’t know is how the water gets from the RPVs and into the basements. Where is the water going?
There is little doubt that the cooling water is leaking out of the three reactor systems and into the outer reactor building basements, and from there to the attached turbine basements. There is one main theory posed by Tokyo Electric Company and the Japanese government as to how the water is getting into the outer reactor building basements. The problem is that the theory itself doesn’t seem to hold water! The evidence to date indicates that the leaks are located in places where Tepco is either not yet looking, possibly due to the high radiation levels involved. Let’s look at the theory and find out why it appears to be questionable.
The official notion is that the RPVs are leaking into the large, donut-shaped suppression pools (toruses) that surround the bottom of the Primary Containment Vessels (PCVs). The PCVs are massive steel-reinforced concrete structures that completely enclose the RPVs. The outer walls of the PCVs are several feet thick, top-to-bottom, with high-density concrete surrounding a spider-web-like network of several-inch-thick steel reinforcement bars. The PCVs are shaped like an incandescent light bulb, with the spherical portion on the bottom. The bottom houses the 600,000 gallon torus tank. Massive piping connects the torus to the inner part of the PCV outside of the reactor pedestal. The pedestal is robust cylindrical structure, also made of steel-reinforced concrete, with walls several feet thick and has the several-hundred-ton RPV sitting on top. There is an opening in the base of the pedestal for the hydraulic tubing attached to the control rod drive mechanisms (CRDMs) that are attached to the bottom head of the RPV. The hydraulic tubes that pass through the opening are packed solid with special material to provide integrity similar to the steel-and-concrete walls.
Now, here’s the problematic part. Any water flow out of the vessel must first make its way through the pedestal wall, most-probably through the CRDM opening (generally acknowledged as the pedestal’s weakest section). The drywell area around the pedestal is open to the pipes that connect to the torus, outside the drywell but still inside the PCV. To get from there, the water would have to leak from the torus structure, into the “torus room” with walls made of (again) several-feet-thick steel-reinforced concrete. The torus room walls are believed to be leaking into the outer reactor building basement. However, arthroscopic and robotic inspections of the torus rooms reveal…nothing! The toruses show no signs of an integrity compromise…no cracks and no apparent leaks. Unit #2’s torus room is dry, so the probability that the leak out of the unit #2 RPV going through the torus is about nil. Units #1&3 have water in their torus rooms about half-way up the curved walls of the tanks. However, there is no indication of a flow of water through the rooms. Flow rates of between 20 and 30 gallons per minute (the gas station flow into your car’s tank is about 2 gallons per minute) would certainly create surface disturbances – tiny ripples at the very least – but the waters appear to have a mirror-like calmness. In fact, the waters in the bottom of the unit #1&3 torus rooms look stagnant. Thus, it appears unlikely that the unit #1&3 leaks follow the official theoretical path out of the PCVs and into the outer building basements. Then there’s the problem of getting through the thick torus room walls, which is in itself quite unlikely because all the observed water levels are below penetrations through the walls.
Despite the evidence to the contrary, Tepco/Tokyo continues to hold fast to their theory. In fact, they are planning to pump a grout-like material into the three torus tanks, fill them with the stuff, and when the grout solidifies they believe it will stop the leaks out of the system and in to the outer building basements. I may be wrong, but trying to stop the leaks by filling the toruses with grout will have little effect on the outflows.
In my honest opinion, the leaks may very well be located outside the PCV walls. Is all the water being pumped into the RPVs actually getting there? Or, is some of it leaking out of the feed-water piping and inner core spray piping that was severely over-heated and over-pressurized the first five days of the accident before going through the PCV wall? Then there’s the possibility of leaks from piping attached to the core recirculation system. The recirculation system itself is entirely inside the PCV, but some auxiliary system piping comes from outside the PCV and connects into the recirculation piping. One such tie-in, Reactor Water Cleanup, is designed to hold full system pressure. Another, Residual Heat Removal (RHR), is not built to contain high operating pressures. RHR is used during shutdown periods for refueling and maintenance when everything is depressurized. RHR removes the heat of radioactive decay while fuel is being removed from the RPV and eventually replaced for the next power run. Most of the low pressure RHR piping is outside the PCV and inside the outer reactor building. If I was a betting man, I’d lay my wager on the leaks being out of one or more of the systems attached to the RPV, but outside the PCV. I think it likely the outflow is coming from system piping not built to handle the pressures experienced the first five days of the accident.