May 24, 2011 Update – New Information, Unit 4 Hydrogen Explosion, Venting
05/24/2011
Latest in on Japanese Nuclear Accidents
Go to http://josephsmiller.com/Fukushima.html for more information on the event.
As of May 15, 2011, Worker's exposure dose: 30 workers have been exposed to radiation more than 100 move as of 5/11. *Emergency exposure dose limit has been set to 250mSv.
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The current situation by Joe Miller (Remember much of this is spec
ulation based on the information that I have and my experience level)
Unit 4 Spent Fuel Pool
TEPCO indicates that there is no fuel damage in the Unit 4 Fuel Pool. The theory behind this assessment is presented in http://www.world-nuclear-news.org/RS_Theory_for_Fukushima_Daiichi_4_explosion_1705111.html . Basically, TEPCO believes there is no fuel damage in the Unit 4 fuel pool because of photos taken of the top of the Unit 4 fuel pool, which shows no damage at the top of the fuel pool. I think this is wishful thinking by TEPCO. I believe the Zr-water reactor took place close to the center of the fuel bundle 6-8 feet below top of active fuel where fuel rod surface temperatures exceeded 2200 F without significant core melt and significant hydrogen was produced to cause the explosion in Unit 4 reactor building. The top of the rack remained intact while the center of axial fuel rod was damaged. From the photos, the center part of the fuel could not be seen. TEPCO contends that there was no significant Zr-Water reaction in the Unit 4 fuel pool and the hydrogen gas explosion was caused by hydrogen gas migration from the Unit 3 via a ventilation systems shared with Unit 3. Warning its theory was 'presumptive', TEPCO said hydrogen from venting Unit 3 flowed into certain levels of Unit 4 through its Standby Gas Treatment System (SGTS). The Unit 3 data and timeline is shown below. The data shows that an explosion occurred in Unit 3 reactor building on March 14 at 10:46, that the hydrogen explosion in Unit 4 occurred at on March 15 at11:00, which is almost a day later. Once the explosion occurred in the Unit 3 reactor building, all hydrogen in that building would be released to the atmosphere therefore the impetus to force hydrogen into the Unit 4 reactor building would be gone.
Figure 1a Unit 3 Data and Time line
Venting
Just a few comments on venting. Since the Japanese government said that they implemented the same hardened vent configuration as was implemented by the US, I am going to assume that the Fukushima vents for units 1, 2 &3 were designed according to Generic letter 89-016. This letter can be found at http://josephsmiller.com/Fukushima.html . The wording from this GL is as follows " Thus, incorporation of a designated capability consistent with the objectives of the emergency procedure guidelines is seen as a logical and prudent plant improvement. Continued reliance on pre-existing capability (non-pressure-bearing vent path) which may jeopardize access to vital plant areas or other equipment is an unnecessary complication that threatens accident management strategies. Second, implementation of reliable venting capability and procedures can reduce the likelihood of core melt from accident sequences involving loss of long-term decay heat removal by about a factor of 10. Reliable venting capability is also beneficial, depending on plant design and capabilities, in reducing the likelihood of core melt from other accident initiators, for example, station blackout and anticipated transients without scram. As a mitigation measure, a reliable wetwell vent provides assurance of pressure relief through a path with significant scrubbing of fission products and can result in lower releases even for containment failure modes not associated with pressurization (i.e., liner meltthrough). Finally, a reliable hardened wetwell vent allows for consideration of coordinated accident management strategies by providing design capability consistent with safety objectives. For the aforementioned reasons, the staff concludes that a plant modification is highly desirable and-a prudent engineering solution of issues surrounding complex and uncertain phenomena. Therefore, the staff strongly encourages licensees to implement requisite design changes, utilizing portions of existing systems to the greatest extent practical, under the provisions of 10 CFR 50.59." In 1989 it was recognized by the US nuclear industry that venting was necessary to maintain credible accident management capabilities and a harden vent was necessary to ensure that the reactor building was not rendered inaccessible during the venting process.
The proposed venting design is shown in Figure 1b.
Figure 1b Direct Torus Vent System (DTVS)
In GL 89-016, the NRC basically adopted the Boston Edison Company (BECo) installation of A Direct Torus Vent System DTVS as the reference system for venting Mark I containments. GL 89-016 provided details of that design for use by the Nuclear Industry.
As stated in the GL 89-016, this design change provides the ability for direct venting of the torus' to the main 'stack'. Containment venting is one core damage prevention strategy utilized in the BWR Owners Group Emergency Procedure Guidelines (EPGs) as previously approved by the NRC and is required in plant-specific Emergency Operating Procedures (EOPs). The torus vent line connecting the torus to the main stack will provide an alternate vent path for implementing EOP requirements and represents a significant improvement relative to existing plant vent capability. For 56 psi saturated steam conditions in the torus, approximately 1% decay heat can be vented.
This design change (Figure 1b) provides a direct vent path from the torus to the main stack bypassing the Standby Gas Treatment System (SBGTS). The bypass is an 8" line whose upstream end is connected to the pipe between primary containment isolation valves AO-5042 A & B. The downstream end of the bypass is connected to the 20" main stack line downstream of SBGTS valves AON-108 and AON-112. An 8" butterfly valve (A0-5025), which can be remotely operated from the main control room, is added downstream of 8" valve AO-5042B. This valve acts as the primary containment outboard isolation valve for the direct torus vent line and will conform to NRC requirements for sealed closed isolation valves as defined in NUREG 0800 SRP 6.2.4. The new pipe is ASME III Class 2 up to and inclusive of valve AO-5025. Test connections are provided upstream and downstream of AO-5025.
The design change replaces the existing AC solenoid valve for AO-5042B with a DC solenoid valve (powered from essential 125 volt DC), to ensure operability without dependence on AC power. The new isolation valve, AO-5025, is also provided with a DC solenoid powered from the redundant 125 volt DC source. Both of these valves are normally closed and fail closed on loss of electrical and pneumatic power. One inch nitrogen lines are added to provide nitrogen to valves AO-5042B and AO-5025. New valve AO-5025 will be controlled by a remote manual key-locked control switch. During normal operation power to the AO-5025 DC solenoid will also be disabled by removal of fuses in the wiring to the solenoid valve. This satisfies NUREG 0800 SRP 6.2.4, Containment Isolation System acceptance criteria for a sealed closed barrier. An additional fuse will be installed and remain in place to power valve status indication for AO-5025 in the main control room.
A 20" pipe will replace the existing 20" diameter duct between SBGTS valves AON-108, AON-112 and the existing 20" pipe to the main stack. The existing 20" diameter duct downstream of AO-5042A is shortened to allow fitup of the new vent line branch connection.
A rupture disk will be included in the 8" piping downstream of valve AO-5025. The rupture disk will provide a second leakage barrier. The rupture disk is designed to open below containment design pressure, but will be intact up to pressures equal to or greater than those which cause an automatic containment isolation during any accident conditions.
New 8" vent pipe (8"-HBS-44), including valve AO-5025 is safety related. Vent piping downstream of AO-5025, including SBGTS discharge piping to main stack, is also safety related. All safety related piping will be supported as Class I. Nitrogen piping is non-safety related and will be supported as Class II/I.
If Fukushima had this design installed with the ability to open the AOVs to the harden vent with DC power, then I suggest that they choose not to do this and tried to vent through the SBGT system which has low pressure duct work that failed because of the high pressure from containment venting. This would explain why the hydrogen was released into the reactor building.
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Other summaries from JAIF (See summaries attached)
Download JAIF Earthquake Report 91 May 24 18_00
Download JAIF Status May 24 at 12_00a
●TEPCO still looking into emergency cooling system
The operator of the Fukushima Daiichi nuclear power plant is still unable to determine how long an emergency cooling system at the Number 1 reactor remained off after the March 11 earthquake.
Officials of Tokyo Electric Power Company spoke to reporters on Tuesday about the system, which can function without external sources of power.
Operating records at the plant show that the system turned on automatically 6 minutes after the earthquake, at 2:52 PM, and halted 11 minutes later, at 3:03 PM.
The system was back on more than 3 hours later, at 6:18 PM.
TEPCO says that based on hearing from workers, it has confirmed that the system was manually shut down at 3:03 PM.
It said this step was made based on a manual, in order to prevent damage to the reactor, because the temperature of the water to cool the No.1 reactor had dropped sharply.
TEPCO says the system may have been turned on in the 3 hours until 6:18, but that it cannot clearly determine the course of events based on studies of circuits and interviews with workers.
The utility firm says at this point it cannot determine to what extent the emergency system was functioning, and that it will continue investigating.
The firm also said that data taken in the 30 minutes after the earthquake show no irregularities in all safety features of the Number 1 to 3 reactors such as emergency power sources and in major facilities of the plan .
On May 16th, TEPCO disclosed the plant's operating records from immediately after the earthquake. The Nuclear and Industrial Safety Agency has instructed the firm to submit a report after analyzing them further and assessing their effects on nuclear safety.
Tuesday, May 24, 2011 14:00 +0900 (JST)
●Heat exchangers to be installed at No. 2 reactor
The operator of the Fukushima Daiichi nuclear power plant will install 2 heat exchangers at the Number 2 reactor building on Tuesday to lower the temperature of the spent fuel pool.
Last Wednesday, Tokyo Electric Power Company workers entered the reactor building to check radiation levels. But high humidity prevented them from staying longer than 14 minutes.
The humidity is thought to stem from the high temperature of the spent fuel pool and steam from the suppression pool which may have been damaged by explosions after the March 11th earthquake and tsunami.
TEPCO plans to reduce the humidity by installing exchangers in the building next to the reactor.
The utility says it hopes to start using the exchangers this month to reduce the pool's temperature from around 80 degrees Celsius to about 40 degrees Celsius within a month.
TEPCO hopes to install exchangers at the No. 1 and 3 reactors next month and at the No. 4 reactor in July.
Tuesday, May 24, 2011 07:05 +0900 (JST)
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