Fukushima's Sea-Salt Problem, Continued

In previous blogs I fretted about the use of seawater to cool Fukushima, because these are boiling water reactors. As hundreds of tons of seawater boiled away, a heavy load of minerals remained around the core and below. While most experts agree seawater injection via the feedwater lines was the right thing to do given the alternative (no cooling at all, and prompt meltdown), there is good reason to believe that salt buildup is blocking effective cooling of the fuel assembly in Unit 1 and perhaps the others. 

Unit 1 is what I've been following, because it's an old model with fewer protections, because the upper containment walls and roof blew off, and because the main trend of pressure readings in the reactor pressure vessel (RPV) has been steadily upward for three weeks, to nearly one megapascal. That's still well below the manufacturer's pressure rating, but does make one wonder what it means, and why the rising trend hasn't leveled off.

Late in March NRC staff considered the scant evidence, ran models, and advised the Japanese that salt deposits in Unit 1 most likely were jammed into the lowest part of the reactor pressure vessel (RPV) up to the bottom of the fuel assembly. That's a lot of salt.

There is some hope that the tonnage of salt could be less now, given that Tepco began injecting freshwater on March 25. And it makes sense that fresh water could dissolve salt and carry it out in solution.

Tepco could estimate the salt-removal rate by sampling the heated water for mineral content as it leaves the RPV. Are they? Who knows! Very little operational data is on line.

The pressure rise in Unit 1's RPV could be a good thing, or a bad thing. Maybe it hints that Tepco is more confident of its improvised plumbing now. But it would be worrisome if it means that a chunk of mineral is blocking water flow at an outlet and the only way Tepco has been able to maintain a critical flow rate (called the minimum debris retention injection rate, or MDRIR) has been to dial up the pressure. 

MDRIR sounds like just another metric in a long list but it means "enough emergency cooling water to keep the wrecked fuel -- the debris -- from melting through the reactor pressure vessel." So maintaining flow at or above the MDRIR does matter. The latest official guesstimate is that crumbled fuel has gathered at the bottom of Unit 1's RPV, but has not melted the steel wall and escaped into the primary containment. Pressure readings appear to support that conclusion.

Getting the salt out from the bottom of the RPV will depend on jet pumps and recirculation pumps that can force water to the bottom of the vessel.

The emergency cooling water has been entering the middle part of the vessel via the feedwater lines. Therefore the bottom section, where wrecked portions of fuel assembly have piled up, isn't being cooled well. ( But there's less decay heat now, so that's one bit of good news.)

Restoration of proper cooling will depend on successful completion of some very difficult tasks in and around the bottom of the semi-wrecked and highly radioactive building: replacing seals, repairing motors, calibrating instruments and transducers, checking for cracked pipes and hangers, and freeing up stuck valves. 

Notice how congested it is in the lower reaches, below compartments of reinforced concrete that prevent work by overhead cranes.

No workers can carry out such heavy-duty repairs until highly radioactive water is removed and surfaces are cleaned. So until flow is restored to the bottom part of the RPV through the recirculation circuit, Unit 1 is going to continue suffering from blocked arteries.