NEWS FLASH: Radiation levels measured inside the containment and shielding of a damaged nuclear reactor are HIGH. That is about as unexpected as finding out that the temperatures inside a coal-fired furnace are high enough to cause instant death to any unprotected living creature, including human beings.
There is absolutely no reason for the public to be concerned about high radiation levels measured inside a nuclear reactor.
Unfortunately, even well-resourced and established publications like the Washington Post cannot resist the lure of using a scary headline – Japanese nuclear plant just recorded an astronomical radiation level. Should we be worried? – to introduce a typically “balanced” news report that turned what could have been an opportunity to spread knowledge into a “he said, she said” opinion piece that leaves most readers in the dark about who to believe.
After quoting several experts who provided credible responses and interpretations about the recently announced radiation dose rate measurements, the reporter chose to conclude with the following statement from one of the usual suspects.
Indeed, Ai Kashiwagi, an energy campaigner at Greenpeace Japan, said the findings showed how little the government and Tepco knew about what was happening inside the reaction.
“The prime minister said everything was under control and has been pushing to restart nuclear plants, but no one knew the actual state of the plant and more serious facts could come out in the future,” she said. “It’s important to keep an eye on radiation-monitoring data and how Tepco’s investigations go.”
Relatively Routine Progress Report Of A Continuing Effort Turned Into NEWS!
Tepco, the Japanese utility company that owns the severely damaged electricity generation station known as Fukushima Dai-ichi continues to issue reports about its progress in cleaning up the industrial site. It has been nearly six years since a 45 foot high tsunami washed over the installed barrier to destroy the plant’s emergency generators and external power connections.
The resulting damage to the four reactor units that lost power to all cooling and indication systems for several days was unprecedented and extensive. As has been known for years, the site clean up effort is expected to take several decades. There will be new discoveries and learning opportunities throughout that period. Some of the progress reports will provide information that surprise some or most of the people who read them.
One part of Tepco’s planned work has always been to eventually send cameras and sensors into inaccessible areas of the damaged reactor units at the station.
On January 30, 2017, Tepco released a several page presentation with illustrations that described results of sending a small remote controlled camera into the primary containment vessel (PCV) in the pedastal area directly under the reactor pressure vessel (RPV).
The camera not only senses photons (light) to produce images, but it also senses gamma radiation with a +- 30% accuracy. At the levels expected for a device that would be placed closer to the damaged reactor fuel core than any other previous device, that accuracy is good enough. For such a high range measuring device, the key characteristic is to be able to produce a measurement without becoming saturated with an off-scale radiation dose rate.
The remote inspection showed that the control rod drive housings, the position indicator cables and the control rod drive exchange mechanism had not moved from their normal locations in an undamaged plant. There were some deposits on some surfaces and part of the grating that is located under the CDM housings was deformed.
Investigators noticed some water dripping, presumably from the bottom of the reactor pressure vessel. The RPV and the PCV were reported as being stably cooled.
The measured radiation level was 530 Gy/hr. (Note: Tepco and the media reported it as 530 Sv/hr, but Gy (Gray) and Sv (Sievert) are equivalent units for gamma dose rate.)
There are no external impacts from the radiation levels measured inside the PCV under the damaged core because they are adequately shielded by the containment vessel and the buildings.
A portion of the hot corium apparently exited the pressure vessel and distorted the grating below the CDM housing area. This photo (recommend looking at a full sized version) shows the current state of the grating.
If there had been any large scale melt through, the CDM housings and position indication cables would not be in undisturbed locations.
The deformation in the grating indicates that there was a period of time when the pedastal area got hot enough to warp the steel tendons used in typical grating and there was something heavy enough to bend the softened metal.
The dripping water noted indicates that there are some tiny penetrations, most likely in the areas where the CDM housings are welded to the pressure vessel. If the penetrations were any size at all, water would be flowing, not dripping.
The surprisingly high radiation readings indicate that there is a large amount of radioactive cesium in the pedastal area. That low melting point, soluable element would have escaped from fuel rods when they were damaged. It would have dissolved into water used to keep the reactor stably cooled over the past 6 years and would have been able to drip down to accumulate in the pedastal area.
Radiation levels measured under an undamaged core are far lower, even though undamaged cores contain just as much cesium. The difference is that the radiation flux from cesium that is retained inside fuel pellets, inside fuel cladding, and in the designed core configuration has a lot of distributed shielding and distance between it and a radiation detector located in the CDM housing area.
Cesium that has left the fuel rods and accumulated under the reactor pressure vessel is virtually unshielded. The dose rate is lower now than it was six years ago because short lived Cs-134 is mostly decayed away. However, the longer lived Cs-137 has only gone through one fifth of its first 30 year half life. Radiation doses measured in the pedastal area of Fukushima Dai-ichi Unit 2 will not decrease anytime soon, but they will also not hurt anyone because there is sufficient distance and shielding to reduce the radiation to safe levels.
Note: Post has been updated with a better photo of the deformed area of the grating and a changed interpretation of how much of the melted core exited the RPV.
ANS Nuclear Cafe Feb 7, 2017 – Radiation Levels Not Soaring At Fukushima Daiichi