Fukushima Daiichi Under Volcanic Pumice Risk, What Does That Mean?

Volcanic floating pumice stones have been causing problems for coastal communities in Japan since August. The floating pumice began showing up in Okinawa near Japan’s southernmost territory first. Japan’s Agency for Marine-Earth Science and Technology predicts the pumice raft may show up around Chiba prefecture by the end of November. Chiba is directly east of Tokyo. The agency provided a video of their pumice raft plume path prediction on their website.

The plume prediction model shows it to be offshore of Fukushima prefecture by November 30th. This does have some potential to create problems at the Fukushima Daiichi disaster site. TEPCO acknowledges this here. The primary concern is units 5 and 6. Units 1 to 3 suffered meltdowns and no longer depend on any ocean cooling. The cooling system for the reactor vessels of those units is a closed-loop system that reuses treated contaminated water. This system has no dependency on the ocean to complete the cooling process. The spent fuel pools for units 1 to 3 have closed-loop cooling systems that are also not dependent on the ocean for heat exchanging. Unit 4 had no fuel in the reactor vessel during the 2011 disaster and has had all of the spent fuel removed from the spent fuel pool.

Units 5 and 6 had all of the fuel removed from the reactor vessels and was placed in the respective spent fuel pools. Unit 5 holds 1542 units of spent fuel while unit 6 holds 1884, which includes some offloaded fuel from unit 4. Additionally, 168 pieces of unused fuel in unit 5’s spent fuel pool and 428 pieces of unused fuel at unit 6 are among the total amounts. The newest fuel in these two spent fuel pools is roughly 10 years old. We explain the decay heat process of spent fuel in this report.

Both reactor units use the RHR cooling system to cool the spent fuel pools. This consists of a pump system in the reactor building basement and a seafront pump facing the port. Both units 5 and 6 had replacement pumps installed at the seafront after the 2011 disaster. In 2019 the reactor building RHR pump in unit 6 failed and needed to be replaced. In 2014 the RHR pump for unit 5 failed. TEPCO was able to “share” the RHR system from unit 6 to cool unit 5’s spent fuel pool. This kept the pool within regulated temperature ranges.

The main concern with units 5 and 6 would be the potential heat up of the spent fuel pools if the pumice raft were to hit Fukushima Daiichi. Both units have significant amounts of spent fuel in their pools with both being over 1000 fuel assemblies each in inventory. By contrast, unit 1 has 392 assemblies in the spent fuel pool, and unit 2 has 615. Over ten year old spent fuel has less ability to generate heat. Typically by the 5-year mark spent fuel can be stored in dry casks due to the decay in heat generation potential.

If the pools were to lose cooling access, water could be replaced by portable piping or hose. In a longer-term situation, a portable heat exchanger similar to those used on the spent fuel pools of units 1 to 3 could be brought in. The time frame for TEPCO to implement such a plan is unknown as that level of emergency planning has not been released to the public.

The common pool is the other cause for concern. The building that houses the common pool has an extensive series of cooling units on the roof. It is assumed that this system isn’t dependent on any seafront cooling exchange but we do not have enough detailed information about the systems to state this with 100% certainty.

The port area for units 5 and 6 does have a silt fence in place that would help reduce pumice infiltration into the areas of the cooling pumps. It could greatly reduce the possibility of pumice reaching the pump intakes but it isn’t 100% effective.

If pumice infiltration appears likely TEPCO has a plan to install additional fencing and oil collection type booms to block the port areas. TEPCO’s report also cites that the metal fence on the pump intakes for units 5 and 6 have a 9mm hole spacing that may be sufficient to stop larger pieces of pumice from reaching the pumps. As of 11.23.21 TEPCO has not released any updates to the situation at the plant related to the pumice issue. The plume is expected to pass by the plant over the next week leaving this an open issue to monitor.

Hat tip to Fukushima Diary who initially discovered the issue.

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