Creation path of ³⁵ S from Fukushima not so obvious

Abstract

In PNAS, Priyadarshi et al. (1) reported the detection of unusual levels of atmospheric 35 S in California related to releases from the Fukushima nuclear plant. The authors stated that the 35 S was generated by "leaking neutrons," activating the chlorine in the seawater that was used to cool the Fukushima reactors via the nuclear reaction 35 Cl(n,p) 35 S. The authors calculated "that 4e11 neutrons per m 2 were released," of which "2e8 neutrons per m 2 reacted with 35 Cl."Although we acknowledge it as probable that the detected 35 S originated from Fukushima, we have doubts about the proposed creation and release path. First, there are several possible alternative sources of 35 S:The primary coolant of light water reactors contains veritable amounts of 35 S, typically 0.07-2 MBq/m 3 (or, in other terms, about 7e11 to 2e13 atoms/m 3 ) (2). A typical concentration of Cl in the coolant is about 0.1 ppm, (i.e., 5 decades less than in seawater). However, a reactor at power level generates 1e18 neutrons/m 2 each second, 6 decades more than the calculated "neutron leakage." Hence, the 35 S content in the primary coolant alone could explain the calculated releases. Irradiated fuel itself also contains even more 35 S generated from Cl impurities of the fuel, about 1e19 atoms per ton. This is a huge reservoir of 35 S that might be released if coolant (whether primary or seawater) comes into contact with damaged fuel rods. Finally, the seawater contains dissolved atmospheric 35 S, which will be released immediately once the seawater has evaporated.Additionally, a calculation of the activation of the seawater used for cooling strongly depends on a number of unknown parameters that determine the "irradiation geometry" and reaction rate, most prominently the fuel and core geometry (i.e., the degree of fuel degradation), the water flow rate, moderation, and (self-)shielding effects. Also, the spent fuel pools have to be considered as possible origins, further complicating the analysis. Without any estimation of the size of these uncertainties, probably several orders of magnitude, the results are of little value. Furthermore, the term "neutron leakage" is misleading because its use is not in accordance with common understanding. As far as we understand it, the number given is a neutron fluence (i.e., the neutron flux integrated over the duration of the activation process). In any case, such a number does not provide any new insight as long as it is not compared with any typical value.To conclude, the attempt to gain insights into the accident at Fukushima from airborne radioactivity measurements is very interesting. However, given the uncertainties stated above, we doubt that meaningful quantitative statements are possible from such an analysis. The Fukushima accident is anything but a "well-defined 35 S source." The evidence appears to be too weak to gain "previously undescribed insight" into environmental transformational rates, simultaneously estimating the depositional time scale, oxidation time scale, and a neutron fluen...

Radioactive ³⁵S emitted from the Fukushima Nuclear Power Plant and its re-suspension from the contaminated area

“…In order to interpret the results obtained in this study and to estimate the range of validity of the derived conclusions, the entire process starting at the formation of 35 S atom until deposition of 35 SO 2 or 35 SO 4 2needs to be considered. The most likely formation mechanism of the initial 35 S atom is a conversion from 35 Cl (Strub et al, 2011). The newly formed 35 S atom is oxidized to H 2 SO 4 via SO 2 and SO 3 intermediates:…”

“…Danielache et al (2012) reported several emission scenarios based on these studies. Disagreement exist on whether the 35 S released was already present in the nuclear fuel or was produced from 35 Cl present in the primary coolant of the light wa-ter reactors (Strub et al, 2011). In addition, the seawater, which was injected into the reactor for emergency cooling, contains abundant chlorine that was converted into 35 S and released during the incident.…”

Fukushima’s Forgotten Radionuclides: A Review of the Understudied Radioactive Emissions

“…Although this is a remarkable result of a rather exotic radionuclide monitored in a remote station, the suggested creation path of 35 S caused some dispute . Strub et al argue that the technically unavoidable chlorine content (typically ∼0.1 ppm) of the primary coolant is a more relevant source of chlorine.…”

“…46 Although this is a remarkable result of a rather exotic radionuclide monitored in a remote station, the suggested creation path of 35 S caused some dispute. 48 Strub et al argue that the technically unavoidable chlorine content (typically ∼0.1 ppm) of the primary coolant is a more relevant source of chlorine. Although this is 5 orders of magnitude below the chlorine concentration of seawater, the neutron flux density of an operating reactor is 6 orders of magnitude higher than in shut-down state.…”

“…Although this is 5 orders of magnitude below the chlorine concentration of seawater, the neutron flux density of an operating reactor is 6 orders of magnitude higher than in shut-down state. An even more relevant source of 35 S, according to Strub et al, 48 is the chlorine impurities in the nuclear fuel itself. In any case, the link of elevated atmospheric 35 S concentrations with the Fukushima nuclear accident stands beyond debate.…”