Development Of Analytical Methods Of Radiocesium And Plutonium And Their Application In The Environment Around Fukushima Prefecture

The massive earthquake and consequent giant tsunams occurred on 11 March 2011 in the northeast coast of Japan,resulting in serious damage to the Fukushima Dai-ichi Nuclear Power Plant(FDNPP).Consequently,the FDNPP accident caused serious contamination in the environment through atmospheric fallout and direct discharge of highly contaminated liquid wastes,containing radiocesium and plutonium.Source identification and activity determination of these two radionuclides play an extremely important role in radiological assessment and environmental remediation.Additionally,from the point of radiation protection,the determination of radiocesium and plutonium activities in Fukushima environment samples is of great significance to evaluate their potential radiativity and biological toxicity.Besides,radioceiusm and plutonium have been extensively applied in studying soil erosion,interaction of sea and land,and sediment geochronology.Analytical method for plutonium in environmental is relatively stable and reliable,however,determination of ¹³⁵Cs concentration and ¹³⁵Cs/¹³⁷Cs atom ratio still exist difficulties up to now.Due to the long half-life of ¹³⁵Cs(2.3×10⁶ years)and low energy of beta decay,it is extremely difficult to obtain its activity by radiometric method.With the rapid development of mass spectrometry in recent decades,accurate determination of ¹³⁵Cs and ¹³⁷Cs by this technique becomes practical,but there are still challenges and difficulities such as isobaric and polyatomic interferences,that greatly limited its application in risk assessment as well as environment study as a tracer.In this thesis,for the first time,an analytical method for simultaneous determination of radiocesium and Pu isotopes in river suspended particles collected from Fukushima Prefecture was developed based on ICP-MS/MS and SF-ICP-MS.However,this established method can not determine the ¹³⁵Cs and ¹³⁷Cs at global fallout level(20 Bq/kg),hence,we made further improvement on this analytical method.Specifically,the sensitivity of ICP-MS/MS was greatly improved as well as experimental condition,and ion exchange and extraction chromatography were used for chemical separation and purification.With this method,the trace ¹³⁵Cs and ¹³⁷Cs originated from global fallout can be accurately determined using APEX-ICP-MS/MS.Finally,radiocesium and plutonium in environmental samples(river suspended particles,lake sediments and seawater)obtained from Fukushima Prefecture and its surrronding regions were determined.The source term,spatial distribution and environmental behavior of these radionuclides were discussed.The main achievements can be briefly summarized as follows:(1)Radiocesium(¹³⁵Cs,¹³⁷Cs)and plutonium(²³⁹Pu,²⁴⁰Pu)isotopes were sequentially pre-concentrated by ammonium molybdophosphate(AMP)and Fe(OH)₃ coprecipitation,respectively.Combined ion exchange columns(2 mL AG MP-1M and 2 mL AG 50WX8)were used for radiocesium separation,and two-stage anion exchange chromatography(2 mL AG 1X8 and 2 mL AG MP-1M)for plutioium separation.Decontamination factors of Ba,Sb and Sn that can cause various interferences for radiocesium reached to 10⁴.The interfering element of uranium for plutonium can be effectively eliminated with the decontamination factor of 10⁶.The analytical results of standard reference materials(IAEA-soil-6 and IAEA-375)demonstrated feasibility and accuracy of simultaneous determination of radiocesium and plutonium using the established method.(2)For determination of low level(20 Bq/kg)radiocesium(¹³⁵Cs,¹³⁷Cs),a highly sensitive analytical method was improved by combining the APEX-Q desolvation sample introduction system with ICP-MS/MS.Compared to the standard instrument setup of ICP-MS/MS,the APEX-ICP-MS/MS enables a 10-fold sensitivity increase.Two-stage ion-exchange(2 mL AG MP-1M and 10.5 mL AG 50WX8)and one-stage extraction chromatographic(2 mL Sr-resin)separations were developed to remove major matrix and interfering elements from environmental samples(10-40 g).This separation method presented high decontamination factors(10⁴-10⁷)for major matrix elements(Al,Ca,K,Mg,Na and Si)and interfering elements(Ba,Mo,Sb and Sn).The high sensitivity of APEX-ICP-MS/MS and effective removal sample matrix allowed reliable analysis of ¹³⁵Cs and ¹³⁷Cs isotopes with extremely low detection limits(0.002 pg).The accuracy and applicability of the APEX-ICP-MS/MS method was validated by analysis of seven standard reference materials(soils,sediment and plants).For the first time,ultra-trace determination of ¹³⁵Cs concentration and ¹³⁵Cs/¹³⁷Cs atom ratio(average:2.58±0.37)(Decay corrected to March 11,2011)at global fallout source environmental samples was achieved with the ICP-MS technique.(3)The obtained ¹³⁵Cs/¹³⁷Cs atom ratios of river suspended particles collected from Fukushima within the range of 0.329-0.391 were comparable with that of the damaged Unit 1-3 reactors(0.333-0.375)in the FDNPP.Meanwhile,the activities of ¹³⁷Cs(23.4-152 Bq/g)in river suspended particles were obviously higher than that(0.1 Bq/g)in Japanese soil before the accident.Therefore,the analytical results of ¹³⁵Cs/¹³⁷Cs atom ratio and 13 7Cs activities indicated that these detected radiocesium were mainly derived from the FDNPP accident,and also demonstrated the horizontal migration of these Fukushima-originated radiocesium by inland flow.Spatial distribution of ¹³⁵Cs/¹³⁷Cs atom ratios showed that radiocesium in the northwestern of the FDNPP mainly from the Unit 2 reactor,while in the east coastal region,the relatively higher ¹³⁵Cs/¹³⁷Cs atom ratios indicated radiocesium was mainly derived from the Unit 1 reactor.For plutonium results in river suspended particles,the measured ^239+240Pu activities(0.085-0.330 mBq/g)showed agreement with that(0.004-4.31 mBq/g)due to global fallout in Japanese soil pre-accident period.The atom ratios of ²⁴⁰Pu/²³⁹Pu were in the range of 0.182-0.199,suggesting that global fallout(ca.0.18)was still the main source.Thus the plutonium contamination in river suspended particles caused by the Fukushima accident can be neglected.(4)The radioactivity contamination about Lake Inba(200 km away from the FDNPP)was investigated.The detected ¹³⁴Cs/¹³⁷Cs activities ranged from 0.67 to 1.52,suggesting that these radiocesium mainly originated from the FDNPP accident(¹³⁴Cs/¹³⁷Cs activity around 1).For vertical distribution of radiocesium,a significant mixing layer from the surface down to 6 cm was observed A remarkable decreasing trend of ¹³⁷Cs activities with depth occurred after 6 cm layer.The higher inventories of ¹³⁷Cs(2.1-3.8×10⁴ Bq/m²)demonstrated the input of radiocesium was greater than that of output in Lake Inba,and most of the Fukushima-derived radiocesium was accumulated through particle scavenging instead of being removed by hydraulic flushing.The ^239+240Pu activities(0.001-0.307 Bq/kg)in the sediments were all within background level(less than 7 Bq/kg)before the accident.The activities of ^239+240Pu in sediments in upper layers(approximately 0-16 cm)presented no obvious changes,after 16 cm in the sediment cores,the ^239+240Pu activities presented a sharp decrease.The higher ^239+240Pu activities in upper layers of the sediment cores were likely to be caused by greater Pu input in the early 1960s due to large-scale nuclear explosions over the past ca.50 years.The atom ratios of ²⁴⁰Pu/²³⁹Pu ranged from 0.175 to 0.210,suggesting that Pu originated from global stratospheric fallout rather than the FDNPP accident.Compared the profile of 239+240pu activity with that of ¹³⁷Cs activity in the sediment columns,there was apparent difference between the vertical distributions of these two radionuclides,and the main reason accounted for the difference was source term and deposition time.Therefore,it is worth noting that early deposited Pu isotopes can be applied as an available indicator to predict the geochemical behavior of radiocesium in Lake Inba in the future.(5)The measured ^239+240Pu concentration(0.78-2.86 mBq/m3)in seawater samples collected around(30-200 km)the FDNPP site were all within the background level(less than 12 mBq/m3),and ²⁴⁰Pu/²³⁹Pu atom ratios(0.199-0.235)indicated that global fallout and Pacific Proving Grounds were the main sources of Pu contamination.Therefore,the Fukushima accident did not cause significant Pu contamination in the northwestern Pacific,while the accident caused obvious radiocesium contamination.However,some highly contaminated radioactive water stored in the plant site and leached rainwater with radioactivity might be input into the Pacific in future.Therefore,it is of vital importance to continuously monitor Pu isotopes composition and concetration level in Japan coastal areas.