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September 2012 incidents

 

September 2012 Incidents

 

2012.09.26     Confusing contamination data

TEPCO has published data on the contamintion along the deconatamination facility line, which are hard to understand.

The data are Bq/cm³. Basically one would expect that the level of decontamination is decreasing aloung the decontamination line, but the table shows different. The admin has no good idea, how to interpret the data, e.g. why do the data for Ru-106 increase. Common sense dictates that the values in the High Temperature Incinerator Buildung should show the peak values.

2012.09.26     Unit 3 Spent Fuel Pool Clean-up Incident

TEPCO has started the clean-up of the unit SPF and in a first step is removing the debris from the SPF. The picture shows the debris before the works started.

The high radiation level requires this work has to be performed by heavy machinery operated by remote control, as shown in the picture.

This is a demanding task and so the inevitable happened. On September 22nd a steel beam of 470 kg slipped from the crane when lifted and dropped into the SPF. As there is a water layer of about 5 m on top of the fuel elements this incident did not cause damage. TEPCO has investigated the impact and did not find any signs of additional destruction. This is reasonable as the water layer is absorbing the energy of the beam diving into the water. Anybody who has practiced high diving knows that a high diver is immediately stopped when entering the water and goes down only 2 or 3 meters. So TEPCOs findings are resonable.

In addition TEPCO has published for the first time details on the position of the FE racks in the SPF. The picture showns the details.

In case of damge done by the steel beam to the fuel elements this would probably result in a radioactive release and this radiation peaks should be reflected in the time plots of the monitoring system. The time plots show no indication of a radiation peak. TEPCO was lucky.

It is quite interesting to see the effect of rainfall on the monitored radiation level. Since radioactivity does not disappear other than by radioactive decay this means addditonal radioactivity on the ground at the site.

2012.09.24     Actual Water Storage Situation

For more than one year TEPCO has been operating the make-shift decontamaintion system, which is shown in the picture. The basements of the reactor and turbine building are filled with highly contaminated water. This water is sepated from oil, decontaminated by removal of cesium and desalinated  and stored in the decontaminated water storage.

 

Since the start of the accident ground water has been leaking into the reactor and turbine buildings at a rate of 370 m³/d.

In this way TEPCO is able to cool the molten core but it is also left with the huge problem of storing the treated wate which is still contaminated at level that does not allow to dump it into the sea. The picture shows the actual sitation. TEPCO hass stored about 200.000 m3 by end of September 2012.

The good news is the level of radioactivity in the contaminated water in ther reactor building basement is decreasing: It has gone down from ~ 2*10⁵ Bq/cm³ on  September 25^th, 2011 to ~ 1*10⁵ on June 19^th, 2012 and now it is down to ~0.5 *10⁵, Practically speaking, this open cooling system based on feed an bleed is washing the radioactivity out of the reactor, it ends up in the filter and is stored in the highly radioactive sludge tanks.

Basically, most of the unused area of the Fukushima plant is covered by tanks by now and it is clear that this cannot go on for the next years. So TEPCO has to do develop a plant how to avoid the groundwater ingress into the basements.

Now TEPCO has come up with a plan for an intermediate solution. In order to avoid radioactive leakage from the basements into the ground water the ground water level has to be kept above the water level in the basement, or vice versa. TEPCO is going to use the existing ground water pit (red circles in the picture) to lower the gound water level below the turbine building basement level. The ground water level cannot be brought down below the level of the reactor building:

1) The actual cooling technique is based on the leakage of highly radiactive water from the pressure vessels via the contianent in to the reactor building basement.

2) The reactor building basement is in a non-permeable layer with the ground water flowing above this layer towards the sea,

The picture shows the ground water level to be achieved by using the pits. The option of a slit fence blocking the ground water has no langer been discussed.

The TEPCO report answers the question how the ground water is leaking into the building. It shows that the leak tightness of the basements was lost in the earthquake which caused a lot of large cracks (technically speaking large cracks) of 0-3 mm which are now being sealed with sealing coating. The second source are the penetrations in the walls which also have suffered in the earthquake and are sealed or plugged to reduce the ground water ingress. The aim is to make the turbine building leak tight again and then reduce the water level in the basements below the turbine building basement bottom.

It now turns out as a severe design flaw that the reactors have been build into the ground water and not above the maximum ground water level with an addtional margin of safety.

A substantial reduction of the ground water level and an isolating slit fence around the reactors would be a real milestone on the road to bring the plant into a reliable safe state.

TEPCO has published the radiation data of the unit 4 SPF:
[September 21 (Before the incident)]
Cesium 134: 2.4X10³Bq/cm³, Cesium 137: 4.0X10³Bq/cm³, Iodine 131: Below the detection limit (3.4X10¹Bq/cm³)
[September 22 (After the incident)]
Cesium 134: 2.2X10³Bq/cm³, Cesium 137: 3.6X10³Bq/cm³, Iodine 131: Below the detection limit (1.4X10¹Bq/cm3)
[September 23]
Cesium 134: 2.5X10³Bq/cm³, Cesium 137: 4.2X10³Bq/cm3, Iodine 131: Below the detection limit (1.6X10¹Bq/cm³)
[September 24]
Cesium 134: 2.4X10³Bq/cm³, Cesium 137: 3.9X10³Bq/cm³, Iodine 131: Below the detection limit (1.6X10¹Bq/cm3)

The data indicate TEPCO was lucky, but at the same time give  raise to the next question: where does the contamination of the unit 4 SPF water come from as there was no meltdown in unit 4  After one year of cooling with a closed cooling loop and contiuous coolant filtering the admin would expect the level of cesium below detection limit.

2012.09.18     Unit 4 on the verge of collapse?

Public disussions and TEPCO report including a picture tampered with Photoshop have left the independent safety analyst with the feeling that TEPCO is covering up substantial structural deficits in unit 4. On a closer look five different issues have been addressed:

1) earthquake resistance
Some months after the explosion TEPCO came up with a plan to enhance the earthquake resistance of the unit 4 building by putting some concrete below the SPF. This plan was welcomed by the US NRC. It was never explained why this reinforcement was implemented, at least there are no report on major strucural deficits of unit 4 at that time.

This picture shows the planned steel structure and the concrete pouring.

This picture shows the pouring of concrete in the middle of 2011.
Since there is no evidence of structural damage of unit 4 and reason dictates the earthquake damage of the three similar reactor structures of unit 2, unit 3 and unit 4 has to be similar the only explanation for this reinforcement is appeasing the public in the same way as has been done by the mission of the helicopters and the Tokio fire fighters.

Technically the reinforcement of unit 4 is the first choice amoung the tree units due to the low radiation levels in unit 4 - the radiation levels in unit 2 or 3 would not allow such an reinfocement to be carried out - and the high number of used fuel elements in the unit 4 SPF - about 1300 as compared to 300 or so.

Basically this reinforcement has to be considered as acting for the sake of action and opportunism.

2) Tilt
The second issue is the hypothesis unit 4 may be tilting and loosing its stability (as is the case with the leaning tower of Pisa). If unit 4 is tilting this would be a serious safety issue. If TEPCO had the feeling unit 4 is continuously tilting TEPCO would install adequate instrumentation (e.g. GPS or laser markers). BUT: TEPCO has made some simple measurements of the SPF level and found it is not inclined. Very inaccurate method and unreliable.
Basically, there are no indications of unit 4 tilting.

 

3) Bulge

TEPCO has found a substantial bulge of the unit 4 west side concrete wall. The location is shown in the picture. It is just above the truck entrance door which TEPCO has tried to make disappear by Photoshop (red arrow).

A bulge of a concrete wall is a serious damage, but the measurements indicate that it is only on the outside of the wall with a maximum extension of about 50 mm.

There are two options: either the bulge has existed since the construction was carried out or it was caused by the hydrogen explosion. If it was caused by the explosion there should also be a substantial displacement on the inside and a lot of cracks, which have not been reported.

Anyway: one has to take this bulge serious as it is in a critical location just below the unit 4 SPF and on top of the truck door in the concrete wall. It should be closely monitored by laser equipment.

4) Cracks
If there was a real problem with the structural strength of unit 4 there should be a lot of cracks visible, but TEPCO does not report any serious cracks, only some minor cracks which are not a problem.

5) Concrete Strength

TEPCO has also investigated the concrete strength - without explaining why this might constitute a problem in a concrete structure without substantial cracks. TEPCO found that the concrete strenth is about 40 N/mm2 - much better thant the required 22.1 N/mm2. This is not astonishing: The poured concrete is always stronger than required by the design. The design values are guaranteed values and if the contractor does not comply he will have to cut out the concrete again - as they did in Olkiluoto unit 3 in Finland lately.

In addition, concrete has the nice property of "carbonizing". In the course of time it will take up carbon dioxide from the ambient air and it will turn harder and harder and become finally indestructable as the German bunkers on the Fench coast from WWII.

Summary:

The concerns unit 4 structure is not sound are not well funded - if the published informations are correct. Nuclear experts are always concerned on structural stability having in mind the findings from the Sandia containment experiments, in which a 1/4 model containment was blown up by overpressurization.

This looks spectacular - but one has to keep in mind this destruction being much worse than that of unit 4.