December 6, 2013
This past Monday, Japan’s Nuclear Regulation Authority announced a new worst-case nuclear accident scenario. But, this one does not concern nuclear power plants. It purports a disaster at the Tokai nuclear fuel reprocessing facility.
Tokai has been shut down since 2007. The reason for the shuttering was on-going technical problems. Just when these difficulties were about to be resolved and the facility restarted, the Fukushima accident happened, bringing with it a wave of earthquake fears. Tokai cannot resume operation until all mandated seismic retrofits have been approved by the NRA. However, the influx of nuke restart requests by electric utilities have put the Tokai inspection on the back burner.
Now adding insult to injury, the NRA has concocted a worst case accident scenario based on assumptions related to the back-up of radioactive liquids at Tokai waiting to be handled. There are 3.5 tons of liquids containing Plutonium to be converted into a powder intended for use in the making of mixed oxide fuel for reactors. In addition, 430 tons of liquids containing fission products sit in limbo. The NRA studied the worst possible calamity that might occur with each volume, no matter how unlikely, and posted their conclusions. Unfortunately, the NRA report is available only in Japanese, but the Asahi Shimbun ran a lengthy article about it on Tuesday.
Here is the Asahi’s version of the scenario in a nutshell. In both cases, the assumption is that a natural catastrophe strikes the facility, resulting in a prolonged full-station blackout just like Fukushima Daiichi. The Plutonium liquid is speculated to begin boiling in as little as 23 hours, spewing radioactive isotopes into the atmosphere. The fission product liquid is assumed to begin boiling in about 55 hours. The NRA says the Plutonium liquid could spawn a hydrogen explosion at the 11-hour mark that would damage its containment system and provide a pathway for a radioactive release. A fission-product liquid-based hydrogen explosion is postulated to happen in 38 hours. If the meteorological conditions are also in a worst-possible condition, the combined atmospheric releases could contaminate Tokyo, which lies about 130 kilometers southeast of Tokai. The entire scenario is predicated on the Tokai staff doing nothing and bringing in no outside power sources to alleviate the blackout. Assuming that the Asahi article is relatively accurate, the report is sure to touch off a new wave of antinuclear scare-mongering both inside and outside Japan.
The Asahi reports leaves the door open for several important questions.
First, where is the heat coming from to cause the postulated boiling of each mixture? Decay heat would not seem to be a realistic source due to the 240,000 year half-life (T1/2) of Pu-239, 6540 year T1/2 for Pu-240 and 387,000 T1/2 for Pu-242…the three main isotopes in the mix. (Only Pu-239 is fissionable) As half-life increases, radioactive intensity decreases and the heat production drops with it. Decay heat alone doesn’t seem capable of causing Plutonium-containing liquid tanks to boil. On the other hand, the half-lives of the isotopes in the fission product liquid are many times shorter than Plutonium isotopes and would produce more decay heat. Even then, the probability of sufficient decay heat to cause boiling seems questionable because the shortest-lived, greatest heat-producing isotopes have decayed away to nothing. If the wildly speculative boiling of the volumes did take place, it would seem that the 23-hour assumption would apply to the fission product liquid and the 55 hour time-frame apply to the Plutonium liquid. If there is another source of heat which could be considered, I’m not aware of it.
In addition, how would hydrogen would be generated in sufficient quantity to cause a containment-shattering explosion? Both liquids are the result of the removal of the fuel pellets from their Zirconium tubes, followed by chemically converting the material into liquid form, followed by separation of the Plutonium from the fission product matrix. The hydrogen which caused the explosions at F. Daiichi came from the extremely high-temperature oxidation of the Zirconium tubes in the fuel cores on units 1, 2 & 3. But, the Tokai liquids have no Zirconium to deal with. So, how would the hydrogen be produced…and produced in the huge quantities needed for detonation?
Next, if the potential ramifications of the speculated Tokai accident are so grave, why has the NRA put the station’s request for restart and subsequent reduction of the liquid volumes at the bottom of the review pile? Japan’s Atomic Energy Agency, owner/operator of Tokai, wants their inspection request moved toward the top of the list. They have also asked the NRA to have some of the most-stringent quake mitigation requirements delayed in order to get the facility running ASAP. JAEA says it can process the Plutonium liquid in 18 months. The vitrification of all the fission product liquid would take a lot longer, but every day of operation would reduce the severity of the worst-case scenario. The high level nuclear waste topic has been vaulted to the top of the nuclear issue heap in Japan, so it would seem appropriate to get Tokai back in operation and temper the situation. But…alas…the processing of the JAEA request languishes.
Lastly, why did the NRA undertake this speculative, assumption-predicated project in the first place? The agency says they only have the resources to handle only a handful of restart applications at a time. Wouldn’t that time and money have been better spent on getting qualified nukes back in operation and begin turning around the fossil-fuel-based $1 billion per month trade deficit? I think the NRA has wasted their time and needs to reconsider their priorities.
Reference:
http://ajw.asahi.com/article/behind_news/social_affairs/AJ201312030046
