Daily variation of radiation dose rate after the Fukushima Nuclear Accident M. Yamauchi Swedish Institute of Space Physics (IRF), Kiruna, Sweden poster R370@GI1.4 (EGU2015-10171), Friday (2015-4-17)

Daily variation ≈ instrumental Daily variation of the dose rate measurements is normally attributed to instrumental effect because dosimeter (its count rate) is sensitive to temperature, and is even affected by humidity/background solar radiation. In fact, the well-calibrated dosimeter at FNPP2 (10 km south of FNPP1) showed no daily variation. However, we have reasons that part of this daily variation can be real because: (1) Not perfect correlation with temperature or solar radiation (2) Atmospheric electric field (PG) indicates daily lifting of radioactive dust (3) Inter-regional transport by local wind is detected  Need to examine with weather data

Atmospheric electric field indicates daily re-suspension Daily PG variation is due to change in ion density in the air. before Different peak LT between before and after the radioactive contamination

Noon-peak after deposition = lift of radionuclide Conductivity near the ground is extremely low (~ 10-14/Ωm). Therefore, small increase in ion density at ground/air by 137Cs or 131I significantly decreases/increase impeding ground-level (1 m high) vertical E-field (PG) radionuclide when wind blows ion production during wind role of dust surface/ air during day only natural (radon) firmly attached only near surface attach ions to reduce  increase in the morning newly deposited can be lifted if soil is dry in air if dust are lifted increase ions and  decrease when wind blows

Evidence of diffusive inter-regional transport At 150 km away from the FNPP1, ratio of radiation dose rate between different region approach to unity: transport from highly contaminated region to surroundings Alternating wind direction  from high-dose sites to low-dose sites in average (diffusive process)  Ratio of two dose values should approach to unity

Summary Close look at daily variation together with weather * Even Iitate's daily variation of dose rate is not well correlated with temperature or precipitation. * On the other hand, correlation with wind is not very good at Fukushima-shi.  We have no good answer for the enhancement of daily variation Summary The major part of the daily variation in the radiation dose rate is attributed to instrumental effect. However, air transport (indicated from weekly trend of radiation dose rate) and daily lift of radionuclide (indicated in the atmospheric electric field data) could also contribute a part of daily variation. Although the amount of such daily lift was not serious for the Fukushima case, this should be watched if large nuclear accident occur in the future.

Vertical Electric field (= potential gradient: PG) Global current: 1kA Ionospheric potential:200kV = about 100 V/m under clear sky Rain cloud: Ordinary cloud = dipole electric field (local generator) Thunderstorm = strong dipole electric field (global generator) Clear sky: Dry air = electrostatic problems

Ion density n: dn/dt = q - αn2 - βnN q: production (by cosmic ray, radon, and -ray) α：neutralization β：attaching to aerosol (density N) negative ion positive ion   + + + +     + + + + E + + atmospheric electric (E) field   aerosol aerosol     + +   + + + +     + + molecule

minor release from the FNPP-1 3 month after the accident

Faster decay at Takahagi is due to higher I/Cs ratio Faster decay at Takahagi is due to higher I/Cs ratio. Combining with the soil sampling data which shows the same I/Cs ratio between Hirono and Iwaki (Iwaki has the same decay rate as Kitaibaraki), the plume on 20 March had different I/Cs ratio between it east part (red route) and west part (orange route). Thus the data can even show the internal structure of the plume.

More about plumes soil sampling exponential decay = 8-day as expected initial I/Cs ratio More about plumes exponential decay = 8-day as expected Soil sample data are classified into different regions.

Iitate anomaly * Not approaching to unity * Episode of departing from unity  keep supplied from surroundings  from FNPP-1? / from trees? Other anomalies PG dropped to zero on 14 and 20 March (see Poster) PG recovery despite enough ionizing radiation Daily variation with peak at local noon Reset of recovery and daily variation: new deposition!

Close like with wind anti-correlation from Fukushima to Iitate Correlate with wind in Iitate but not in Fukushima-shi  could be instrumental effect? (by temperature)