A week ago, Tepco detected radiation exceeding 10,000 mSv/h (i.e. 10 Sv/h) at an outdoor location near unit 1. This is the highest dose so far recorded outside a reactor core at the site. The previous record was 4,000 mSv/h in unit 2. To put this in perspective, about 7 Sv is already a fatal dose. Therefore 40 minutes at that location would mean certain death from radiation sickness. The local hourly dose of 10 Sv is 100 times as much as the 100 mSv a nuclear worker may normally be exposed to over a total of five years. It is 40 as much as the 250 mSv the government has permitted workers to be exposed to during the Fukushima disaster efforts.
While it’s alarming to hear about record radiation levels being found months after three of the reactor buildings exploded, most likely this radiation is not something new but has been there since the early days of the disaster. It was detected at two locations, an outdoor stack serving units 1 and 2 for venting purposes, and an indoor filter room on the second floor of the turbine hall of unit 1. Both locations are connected using a high pressure steel pipe used for emergency venting. Tepco recently started some construction work near the stack, erecting a steel and plastic cover around unit 1 and perhaps that increased human activity around there led to the discovery.
When Fukushima-I was hit by a station blackout (total electric power failure) after being flooded by a 14 meter high tsunami on March 11, 4 of its reactors and their spent fuel pools were left without cooling. As a result, the cores of units 1, 2 and 3 overheated and their fuel rods melted. During the overheating, steam was vented from the reactor pressure vessel into the suppression chamber of the containment vessel, until the relatively low design limit of the containment was exceeded. In order to not risk an explosion, the operators at that point decided to release radioactive gasses from the containment through the emergency venting system, which is connected to the exhaust stacks.
If during normal operation of the power plant any gases have to be released, they are treated via the Standby Gas Treatment System (SGTS), which comprises HEPA filters followed by active charcoal filters. The unit 1 SGTS is claimed to capture better than 97% of iodine, while units 2 and 3 have a claimed retention rate of better than 99.9%. Treated gas is sent by electrically operated blowers to the exhaust stack, where winds would usually disperse it over the sea. If you look at aerial pictures of the station (see last picture), you see large pipes running from the units to the stacks. Unit 1 and 2 share one stack, units 3 and 4 another (unit 4 was shut down for repairs at the time). These are low pressure pipes, designed for relatively slow releases. Each unit has two systems capable of handling 0.5 m3/s (unit 1) or 0.75 m3/s (units 2/3) each. Because the SGTS relies on electric blowers it is not usable during a total station blackout.
Next to the fat pipes are much thinner high pressure pipes, which are part of the “hardened venting system” installed by Tepco between 1999 and 2001. They were added specifically for major disasters, when the pressure inside the containment reaches dangerous levels, requiring an urgent release of pressure. The hardened venting system skips the charcoal filters. Its only filtering mechanism is the water pool in the suppression chamber, through which steam vented from the reactor core first bubbles. Its scrubbing effect is much more crude than the normal gas filters. The efficient filtering system can not be used during high pressure containment venting because it would offer too much resistance and could not stand the heat or pressure of an emergency release. Yet I still remember when a government spokesman announced that Tepco was going to vent the unit 1 containment into the atmosphere on March 12, we were told that the venting would be filtered, which wasn’t really true.
A June 2011 report by the Japanese government to the IAEA explains on page IV-13:
TEPCO built new vent pipes extending from the S/C and D/W [suppression chamber and dry well, the two parts of the containment vessel, JW] to the stacks from 1999 to 2001 as PCV [primary containment vessel, JW] vent facilities during severe accidents as shown in Figs. IV-2-13 and IV-2-14. These facilities were installed to bypass the standby gas treatment system (hereinafter referred to as SGTS) so that they can vent the PCV when the pressure is high.
The facilities are also provided with a rupture disk in order to prevent malfunction.
As the pressure dropped in the hot, compressed gases from the containment and they came into contact with the cooler pipes leading to the stack, the gas cooled off and there was condensation inside the pipes. Some radioactive fission products from the melted fuel rods must have been deposited on the pipe walls. Radiation from these deposits is penetrating the pipe walls, causing high levels at the base of the exhaust stack and inside the filter room through which the venting pipe runs.