Periodic measurement of plant data
From plant data on the temperature, hydrogen concentration, pressure, and other items inside the PCVs that have been continuously obtained since the accident occurrence, it is estimated that Units 1-3 in the plant are kept in a stable cold shutdown condition.
(1) Measurement of Basic Plant Data inside the PCVs
Here is a summary of the temporal changes in the plant data about the conditions inside the PCVs, which reveal that the fuel debris is in a stable state.
a. Temperature
At the time of accident, a power outage occurred, disabling decay heat from fuel assemblies to be immediately removed and thus causing the temperature to rise to higher than 1,000°C. Then, a rapid oxidation reaction occurred between the fuel cladding tube (zircaloy) and steam (Zr + 2H2O -> ZrO2 + 2H2 + 586 kJ/mol), which generated additional heat. This situation caused the fuel melting.
Figure 1 shows the temporal changes in the temperature data around the reactors summarized based on the information published by TEPCO. After the accident, the temperature inside the PCVs started dropping and decreased to 100°C in six months. After that, the temperature has been gradually dropping every year while following the seasonal variations in air and water temperatures. The temperature is staying at a level lower than 50°C at each section inside the PCVs without showing a sharp peak. Since zircaloy does not react with water at low temperatures, it is estimated that no oxidization reaction occurs and thus no additional heat is generated.
Figure 2 shows the (decay) heat from the elements that make up the fuel assemblies loaded at the time of accident. Immediately after the halt of the nuclear reactors, short-lived nuclides generated much decay heat; in five years, the amount of the heat generated decreased to lower than one thousandth of the heat at the time of the accident. Now, only long-lived nuclides are surviving. Since they have long half-lives and therefore decay slowly, decay heat is also expected to gradually decrease. It is estimated that the temperature will further drop in the future over time.
b. Hydrogen Concentration and PCV Pressure
Irradiating water with gamma rays causes hydrogen to be generated by radiolysis. The PCVs are filled with water to cool the fuel debris inside them. In addition, the doses inside them are high as this page shows. With these factors, there is a fear that hydrogen may be generated inside them. Based on the fact that hydrogen has a lower combustible limit of as low as 4%, the PCVs have been filled with nitrogen since the accident occurrence to dilute hydrogen to prevent a hydrogen explosion. Figure 3 and Figure 4 show changes in hydrogen concentration and pressure inside the PCVs, respectively. The hydrogen concentrations are low enough, indicating that hydrogen has been effectively diluted by the inclusion of nitrogen. From the viewpoint of confining FPs, it may be effective to remove hydrogen out of the PCVs. However, doing so may cause the pressure inside the PCVs to be lower (negative) than the (normal) atmospheric pressure. This may allow air that includes oxygen to enter the PCVs through, for example, sealed sections of them, resulting in mixture of hydrogen and oxygen. For this reason, the pressure insidethe PCVs is kept slightly higher (slightly positive) than the atmospheric pressure as Figure 4 shows.
With these facts, it is estimated that Units 1-3 are kept in a stable cold shutdown condition.
[Based on data published by TEPCO]
Figure 1　　Changes in the Ambient Temperature of the Nuclear Reactors at the Fukushima Daiichi NPS
[Based on data published through JAEA-Data/code 2012-018]
Figure 2　　Heat from the Fuel, FPs, and Radiated materials inside the Reactors
[Based on data published by TEPCO]
Figure 3　　Changes in the Hydrogen Concentration inside the PCVs
[Based on data published by the Meteorological Agency and TEPCO]
Figure 4　　Changes in the Pressure inside the PCVs