2. LESSONS LEARNT FROM OCCUPATIONAL RADIATION PROTECTION IN PAST ACCIDENTS
2.1 Nuclear accidents
2.1.2 Fukushima accident
1. Overview of the Fukushima Daiichi Nuclear Accident
[ Overview of the Fukushima Daiichi Nuclear Power Station ]
Fukushima Daiichi Nuclear Power Station (hereinafter referred to as "Fukushima Daiichi NPS") is located along the central Pacific coast of Fukushima Prefecture, straddling the towns of Futaba and Okuma in the Futaba District. The site is a semi-elliptical shape stretched along the coast and covers approximately 3,500,000 m2.
There are six boiling water reactors (hereinafter referred to as "BWR"). Units 1 to 4 are in the southern area of the power station in the order of Unit 4, 3, 2, 1 from the south to north, and Unit 5 and Unit 6 are located in the northern area of the power station in the order of Unit 5 and Unit 6 starting from the south. Unit 1 has a generator output of 460MW, Units 2 to 5 each have output capacity of 784MW, all having the Mark-I type primary containment vessel (hereinafter referred to as "PCV"). Unit 6 has an output capacity of 1,100MW and is a Mark-II PCV. The total generation capacity of the power station is 4,696MW. The six units commenced commercial operation in succession, starting with Unit 1 starting in March 1971 through Unit 6 starting in October 1979.
When the disaster struck on March 11, 2011, Units 1 to 3 were in operation at ratedpower output, whereas Units 4 to 6 were in outage for periodic inspection.
On March 11, 2011, Units 1 to 3 at Fukushima Daiichi and Units 1 to 4 at FukushimaDaini were in operation. However, due to the Tohoku-Chihou-Taiheiyo-Oki Earthquake occurring at 14:46, whose focal area widely ranged from offshore of IwatePrefecture to offshore of IbarakiPrefecture, all reactors in operation were automatically shut down. Note that no power is required to actuate automatic shutdown (scram) of reactors.
At the same time, all off-site electric power supply (electric power supplied via power
transmission lines and other sources) to Fukushima Daiichi was lost due to the earthquake, but the emergency diesel generators (hereinafter referred to as "EDGs") started up, and the electric power needed to maintain reactor safety was supplied.
Later, at the Fukushima Daiichi, due to a huge tsunami, on the scale of historicalproportions, that subsequently arrived, many power panels were inundated, and allEDGs in operation except for Unit 6 were shut down and it caused loss of all AC power(station black out (SBO)). This caused loss of all cooling functions using AC power.Furthermore, due to flooding of the cooling seawater pumps, the function of transferringresidual heat (decay heat) inside the reactor to seawater (heat removal function) was lost.In addition, at Units 1 to 3, the loss of DC power resulted in the sequential shut down ofcore cooling functions, which were designated to be operated without AC power supply.
Therefore, as a flexible applied action, alternative water injection of freshwater and sea water using fire engines through the Fire Protection (FP) line was conducted.However as it turned out, there remained the situation where water could not be injected into the reactor pressure vessels (RPVs) in Units 1 to 3 for a certain period of time. Consequently, the fuels in each unit were exposed without it being covered by water, and thereby the fuel claddings were damaged. And the radioactive materials in the fuel rods were released into the RPV, and the chemical reaction between the fuel claddings(zirconium) and steam caused the generation of a substantial amount of hydrogen.As a result, since radioactive materials and hydrogen were released from the RPV into the PCV together with steam through the main steam safety relief valves (SRVs), the internal pressure of the PCV increased. At that point, PCV venting1 was attempted several times. It was confirmed that venting reduced the pressure inside Units 1 and 3 PCVs, but it was not confirmed that venting reduced the pressure inside Unit 2 PCV.
Later, in Units 1 and 3, explosions, which appeared to be caused by hydrogen leakage from the PCV, destroyed the upper structures of their reactor buildings. In addition, due to hydrogen which is thought to be inflow from venting Unit 3, another explosion occurred at the upper structure of the reactor building in Unit 4 where all the fuels had been removed from the reactor and stored in the spent fuel pool (SFP) and kept underwater in the SFP.
As for Fukushima Daiichi Units 5 and 6, since one of the Unit 6 EDGs was functioning and feeding its electric power to Unit 5, water could be injected into the cores for both Units 5 and 6. Furthermore, since the function of transferring residual heat in the reactor (decay heat) into seawater was restored, cold shutdown of these units was achieved.
Nevertheless, at the Fukushima Daiichi Units 1 to 3, the accident escalated into a chainof events, and developed into a serious nuclear disaster.
At Fukushima Daiichi, cooling water injection and cooling functions for SFP in each unit and the common SFP were successfully restored through accident response actions.
2. Radiation Control Overview
As a result of the tsunami, core damage, and reactor building explosions that occurred after the earthquake, it became pointless to try to differentiatebetween conventional controlled areas and other areas. Furthermore, APDand borrowed equipment (rechargers) located at the entry management pointsfor controlled areas were inundated with water from the tsunami and rendereduseless, and with the subsequent power loss, management systems used formanaging entry and exit into controlled areas as usual, and aggregatingexposure doses, etc., lost function.
Furthermore, with the loss of power stack radiation monitors and monitoringposts (MP) failed to function, so monitoring cars were deployed to start tomeasure (air dose rate, weather data, etc.) the environment, such as near theboundaries of the power station site. Two monitoring cars, including amonitoring car provided by Kashiwazaki-Kariwa, started to takemeasurements on March 12.
Furthermore, it was decided that all radiation control matters related to thepower station would be handled unilaterally by the ERC at the power stationlocated in the seismic isolated building.In the early morning hours of March 12, since radiation levels within the sitehad risen, it was decided that APD and protective clothing/protectiveequipment worn in accordance with the level of contamination that prior to theearthquake had only been worn in control areas, would be worn when leavingthe seismic isolated building to engage in work.
The large-scale release of radioactive materials and the reactor buildingexplosions led to not only contamination by radioactive materials of the entiresite, but also contamination inside the seismic isolated building.Contamination of the entire site led to an increase in background radiationlevels, making it difficult to evaluate internal exposure using the WBC locatedwithin the facility.
In dealing with the accident a base of operations separate from the seismicisolated building became necessary, so the J Village soccer practice facility located approximately 20km to the south of the Fukushima Daiichi NPS wasselected for this purpose. On March 17, J Village became the base ofoperations for training workers engaged in emergency work, donningprotective equipment and lending out dosimeters when engaging in workwithin the Fukushima Daiichi NPS without going through the seismic isolatedbuilding. As recovery work went into full force, and J Village became home tomany workers and functioned effectively as a base of operations for filling outthe required paperwork for receiving new workers to work inside the powerstation facility amidst the limited space within the seismic isolated building.Furthermore, a mobile WBC used to evaluate the internal exposure of workersengaged in emergency work was lent by the Japan Atomic Energy Agency(JAEA), so measurements could be taken at the OnahamaCoalCenter, etc.
3.Radiation monitoring and protection
3.1.1 Dose rate monitoring
Conditions surrounding the release of radioactive materials were ascertained using normal monitoring posts, however, with the loss of power that immediately followed the earthquake environmental radiation monitoring systems, such as monitoring posts, etc., must function and it became difficult to ascertain the state of the release of radioactive materials. Therefore, in addition to the monitoring car at the Fukushima Daiichi NPS, a car provided bythe Kashiwazaki-Kariwa NPS was also used to conduct manual surveys using a total of two monitoring cars, and mobile monitoring posts were installed in an effort to ascertain the state of the release of radioactive materials near the borders of the site. Gathering data was difficult when taking measurements using monitoring cars since communication tools were unreliable at best and workers from the seismic isolated building had to periodically retrieve handwritten notes from the monitoring cars. Since this data could not be transferred and uploaded to the website using the normal system, the data had to be entered by hand into computers and the results were disclosed on TEPCO’s website.
Thereafter from April 9 all monitoring posts had been restored and the state of the release of radioactive materials was monitored and publicly disclosed.
[ Lessons Learned ]
Reinforcement of environmental radiation monitoring organization
Environmental radiation monitoring systems lost functions when power was lost. They were unable to continuously monitor radiation levels, and two monitoring cars were used. The only available measurement results were compiled by hand until the monitoring posts were restored. Assuming such conditions, radiation needs to be appropriately monitored if a radioactive materials release event occurs at the station. Therefore, it is necessary to reinforce radiation measurement equipment for monitoring such as by deciding in advance alternative methods for monitoring and worker organization in case of power loss.
3.1.2 On site monitoring of environmental sample
Environmental monitoring on site measurement were started to grasp the radiation environment on site with sampling and measurement of samples, such as radioactive materials concentration in air, seawater of the power station coast, sub-drains water, the water accumulated in turbine building basement.
Ge solid-state detectors to analyze gamma ray nuclear species could not be used because background radiation levels were high in addition to the loss of power at the Fukushima Daiichi NPS. As a result, it was decided that samples that were taken were to be transported to the Fukushima Daini NPS and the Fukushima Daini NPS’s Ge solid-state detector used to analyze gamma ray nuclearspecies.
At first,gamma ray analysis of the filter sample were started on March 19to evaluate the radioactive materials concentration in air.The measurement of many samples such as Seawater (Coastal / Offshore) ,Seawater (harbor on-site) ,Sub-drain(on-site), Well water (on-site) ,Soil (on-site) etc. was started sequentially afterwards. And alpha / beta analysis of several samples were started also.
Furthermore, as a result of implementing countermeasures to reduce the level of background radiation in the measurement room, the measurement of publicly disclosed samples, such as seawater, with the Fukushima Daini NPS’s Ge solid-state detector became possible from July 1.Each results of the measurement of environmental samples are publicly disclosed and the raw data from the Ge solid-state factor measurements was disclosed to the headquarters information disclosure corner upon blackening out personal information.
3.2Personal protective equipment
In Fukushima Daiichi Nuclear Power Station, all area was became high density of radioactive substances and radiation protection equipment were required.
At the time of the accident, as the entire areas within Fukushima Daiichi Nuclear Power Station were with high level of radioactive Iodine, so the following was the basic personal protective equipments.
+ Full-face mask with charcoal filter, Coverall, Socks, Shoes, Cotton gloves,
Rubbergloves, Caps
Prior to the disaster of March 11, approximately 1000 Full face masks, approximately 700 charcoal filters, and approximately 6000 Coveralls had been stored at the several buildings on site, however approximately 80% of these personal protective equipments were therefore rendered unusable by the tsunami. Only approximately 20% of these materials that had been stored in the building where affected less by the tsunami were available.Other than the left protective equipments were used, protective equipments support supplies transported from other power stations later were used. And after around March 17 protective clothing and equipment started to continuously arrive.
Various kinds of personal protective equipment as follows were used in the accident of Fukushima Daiichi NPS.
For more information aboutPersonal protective equipment, go to attached CD.
Respiratory Protective Equipment>
In the accident of Fukushima Daiichi NPS, Full-face mask with charcoal filter, Self-contained breathing apparatus (SCBA) and Powered air purifying respirators etc. depending on the concentration of aerial radioactive iodine.
Powered air purifying respirators were introduced after the accident because of this sends cleaned air after removing radioactive substances by filter to the mask of the wearer by an electric fan and lessens the risk of radioactive substances intake and lessens a wearer’s load to breath.
Afterwards, improvement of the work environment including the decontamination of the power station yard was pushed forward sequentially and, from the viewpoint of decrease of the physical load of the worker, installed a no mask area (mask wearing abbreviation area), and it was pushed forward a no mask.
+ Full-face mask respirator with charcoal filter / with dust filter
+ Half mask respirator with charcoal filter / with dust filter
+ Powered air purifying respirators
+ Powered hood mask
+ Self-contained breathing apparatus (SCBA)
+ Circulation-type oxygenrespiratory
Protective Clothes>
In the accident of Fukushima Daiichi NPS, the cover oar was used as a cloth for the radioactive contamination protection of the worker commonly. As for the cover oar, some types including the different types of air permeable degree were adopted.
At the work of the environment that in the wet condition and in contamination level is relatively high condition, the suit of a non-permeable anorak type was wore on thecover oar.
+ Cover oar
+ Anorak
Shielding suit >
In the accident of Fukushima Daiichi NPS, the shielding best and the shielding suit were used as personal shielding equipment depending on work. They were chose and adopted consideration of kind of work, work environment and the physical load of worker.
+ Shielding best made by tungsten
+ Shielding best made by rubber
+ Shielding suit made by special material
While work environments in MCRs and inside and outside of buildings deteriorated extremely rapidly due to release of radioactive material, there were insufficient APDs, charcoal filter full-face masks and other safety gear due to the massive tsunami. The system to properly manage personal dose could not be maintained, resulting in problems such as exceeding dose limits.
[ Lessons Learned ]
Protective equipment (protective clothing, masks, APDs, portable air purifiers,emergency MCR ventilation equipment)
Workers who are obliged to respond in the field, shift operators, and workers at the seismic isolated building are most vulnerable to the impacts of plant abnormalities. Therefore, it is necessary to regularly provide an ample number of various equipment in appropriate locations. Such protective equipment includes protective clothing (normal gear and radiation shielding suit), masks, APDs, and portable air purifiers to improve MCR environment. In terms of the emergency ventilation equipment for the MCR, it is important for equipment to protect the environment of the MCR, which is the frontline center for response. It is positioned as a facility that is prioritized for restoration via power supply cars or other methods.
Furthermore, the seismic isolated building functions as a center for accident responses. The necessary equipment for shielding reinforcement and local fans will be prepared in advance to maintain the environment in the seismic isolated building even if there is release of radioactive materials.
4. Worker’s dose control
4.1 External dose contorl
As the tsunami invaded the buildings, APDs and lending devices (rechargers) that had been prepared for controlling access to controlled areas were rendered unusable. Therefore, APDs were examined for in all power station buildings. There were approximately 320 APDs, including those from the seismic isolated building, that could be found initially but approximately 5,000 APD had been rendered unusable by the tsunami.
Although the approximate 320 APD that could be found were sufficient until around March 15, there were only about 10 APD left for lending and it was determined that the quantity of APDs was insufficient for everyone to have one. Therefore, while APDs were being procured, it was decided that work would be carried out by operation of radiation control through a representative of each group when specific conditions were satisfied until more APDs could be procured. On and after April 1, enough APDs were procured so that each worker could