April 5th, 2011 – Notes From Science Experts Call on Fukushima Disaster – SFP Assessment – External Drywell Cooling

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Science Experts Call/April 5, 2011 – Prepared by Doug Burs, INL

I. Spent Fuel Pool Assessment

1. The option of filling a spent fuel pool with borated sand in the event of a high temperature event was identified by the Industry Consortium as a discussion topic. It is not a full recommendation.

2. Filling a pool with sand would likely insulate the fuel and lead to additional fuel damage.

3. There’s a possibility that escape of gaseous fission products could be reduced by addition of sand. However, it’s likely that debris in the pools would prevent placing the sand uniformly
enough to avoid movement of fission products.

4. Installation of high efficiency spray equipment that can get water into the pools would make more sense than sand placement.

5. The pools are designed to carry very heavy loads, so it’s unlikely that the weight of sand would cause loading issues.

II. External Drywell Cooling Update

1. Additional analyses have been performed using pessimistic assumptions about heat transfer. The analyses indicate that drywell cooling might not be able to remove all decay heat.

2. Access ports through the drywell head seal are bolted open during normal operation in US plants. It’s uncertain whether the access port requirement holds in Japan.

3. Piping penetrations through the drywell walls are typically sealed with epoxy. These seals could be breached if the piping sleeves were to be used to inject water.

4. There’s a possibility that drilling holes through the drywell walls or cover could limit the ability to flood the drywell. If drilling was necessary, the penetrations should be placed as high as possible. Stents could potentially be used to plug any holes that were drilled.

5. It’s likely that water injected into the drywell gap would flow in a narrow stream down the drywell wall and provide only limited cooling. A method for spreading the flow would be necessary.

6. Filling the gap from the bottom by injecting water into the drywell drain might be possible. This filling method would avoid the need to drill holes and overcome limitations associated with
streaming of injected water.

7. Shield plugs above the drywell head have to be moved before the gates leading to the spent fuel pools can be moved. As a result, flooding the refueling cavity by opening the gates is not an
option.

8. It’s possible that small shape charges could be used to penetrate the drywell cover. Debris that might fall on the drywell cover after use of shape charges would probably not be heavy enough to
cause significant cover damage.

9. Filling of the drywell gap may require circulation of a large volume of water that would then require treatment.

Ill. The Japanese government has decided to release water containing low levels of radioactivity. There are significant political consequences associated with these releases, but there may be a
need for continuing to expedite release decisions in order to make room for storing more highly contaminated water. Bringing in large storage bladders is an option for to low-contaminated water
storage.

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