Severe Nuclear Accident Research Articles

Influence of structural, textural and chemical parameters of silver zeolites on the retention of methyl iodide

An experimental screening of silver-exchanged zeolites towards the trapping of methyl iodide was carried out with the general aim of limiting as much as possible the iodine release in case of severe nuclear accident. For the first time, a systematic study was implemented in order to better understand the effects of some important parameters such as the zeolite structural type (FAU-X and Y, MOR, *BEA, MFI and FER) and chemical properties (namely the silver content and its speciation) on the adsorption behaviour. Characterization of 18 zeolitic sorbents, before or after iodine retention tests, was performed using elemental analyses, N2 porosimetry at −196 °C, XRD, DR-UV-Vis spectroscopy and TEM. The distribution of silver species, namely as isolated Ag+ cations or charged clusters in the internal porosity, and/or to a lesser extent as metallic nanoparticles, was found to depend mainly on the zeolite structural type and the Si/Al ratio and to a lesser extent on silver content.CH3I retention properties were evaluated from dynamic adsorption tests carried out at 100 °C. Quantitative exploitation of breakthrough curves showed that the silver faujasite zeolites of the Y type (Si/Al ratio = 2.5) with more than 15 wt% silver were the most efficient in order to achieve high adsorption capacities both at breakthrough and saturation. More generally, it is found that dispersed silver entities efficiently catalyze the dissociation of CH3I and the production of AgI precipitates.

Study of a “hot” particle with a matrix of U-bearing metallic Zr: Clue to supercriticality during the Chernobyl nuclear accident

This paper is dedicated to the 30th anniversary of the severe nuclear accident that occurred at the Chernobyl NPP on 26 April 1986. A detailed study on a Chernobyl “hot” particle collected from contaminated soil was performed. Optical and electron microscopy, as well as quantitative x-ray microbeam analysis methods were used to determine the properties of the sample. The results show that the particle (≈240 x 165 μm) consists of a metallic Zr matrix containing 2–3 wt. % U and bearing veins of an U,Nb admixture. The metallic Zr matrix contains two phases with different amounts of O with the atomic proportions (U,Zr,Nb)0.73O0.27 and (U,Zr,Nb)0.61O0.39. The results confirm the interaction between UO2 fuel and zircaloy cladding in the reactor core. To explain the process of formation of the particle, its properties are compared to laboratory experiments. Because of the metallic nature of the particle it is concluded that it must have formed during a very high temperature (>2400∘C) process that lasted for only a very short time (few microseconds or less); otherwise the particle should have been oxidised. Such a rapid very high temperature process indicates that at least part of the reactor core could have been supercritical prior to an explosion as it was previously suggested in the literature.

Characterisation of aerosols produced in a simulated severe nuclear accident using electron microscopy

The chemical composition of aerosols formed by revaporisation and subsequent condensation of fission products and simulant material was investigated using electron microscopy. A stage dedicated only to the collection of aerosol particles on carbon coated copper grids was designed and tested. As raw material for these experiments core material from the TMI-2 accident, fission product deposits from the Phébus FPT3 tests and Cs2MoO4 powder were used. The examination of aerosols was made by transmission and scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results indicate the range of species encountered at different temperatures showed the necessity for a time dependent sampling method. The investigated aerosols were in the nanoparticle range. A clear pattern was found for the use of fission product deposits from the Phébus FPT3 experiments. In oxidising atmosphere in the temperature range of 450K to 910K a species of 2nm sized spherical caesium-rhenium particles were observed that coagulated to spherical about 20nm sized particles. Other elements such as Mo and Te occurred at various temperature ranges and atmospheres. Also the vaporisation of caesium molybdate, as predicted in thermodynamic equilibrium calculations, was examined.

Effect of nitrogen compounds on transport of ruthenium through the RCS

Ruthenium is a fission product that can be released from the fuel in case of a severe nuclear accident. In this work the impact of the atmosphere composition, including air radiolysis products, on the transport of ruthenium through a primary circuit was examined. Experiments were performed at temperatures 1300, 1500 and 1700 K in a slightly humid air. In the experiments significant effect of nitrogen oxides (N2O, NO2) and nitric acid on the ruthenium chemistry in the model primary circuit was observed. The obtained results indicate a strong effect of air radiolysis products on the quantity partitioning of transported ruthenium to gaseous and aerosol compounds.

Severe accident consequence mitigation by filtered containment venting at Canadian nuclear power plants

Having the capability to use filtered containment venting during a severe nuclear accident can significantly reduce its overall consequences. This study employs the MAAP-CANDU severe accident analysis code and the ADDAM atmospheric dispersion code to study the progression of: an unmitigated station blackout accident at a generic pressurized heavy water reactor, the release of radioactive material into the environment, the subsequent dispersion of the fission products through the atmosphere and the subsequent consequences (evacuation radius). The goal is to evaluate the application of filtered venting as an accident mitigation technology. Several aspects of filtered containment venting system design, like flow capacity, initiation protocols, filter efficiency, mass loading, and thermal loading are considered. An efficient and robust filtered containment venting system can reduce the amount of radiological materials emitted during an accident by 25 times or more, and as a result considerably reduce the off-site consequences of an accident.

Radiological environment within an NPP after a severe nuclear accident

The radiological environment following a severe nuclear accident can be visualised on building layouts. The direct radiation in an area (or room) can be visualized on the layout by a colouring scheme depending on the dose rate level (for example orange for high gamma dose rate level and purple for an intermediate gamma dose rate level). Following the Fukushima accident, a need for update of these layouts has been identified at the Swedish nuclear power plant of Forsmark. Shielding calculations for areas where access is desired for severe accident management have been performed. Many different sources of radiation together with different types of shielding material contribute to the dose that would be received by a person entering the area. External radiation from radioactivity within e.g. pipes and components is considered and also external radiation from radioactivity in the air (originating from diffuse leakage of the containment atmosphere). Results are presented as dose rates for relevant dose points together with a method for estimating the dose rate levels for each of the rooms of the reactor building.

Neutron resonance analysis for nuclear safeguards and security applications

Neutron-induced reactions can be used to study the properties of nuclear materials of interest in the fields of nuclear safeguards and security. The elemental and isotopic composition of these materials can be determined by using the presence of resonance structures. This idea is the basis of two non-destructive analysis techniques which have been developed at the GELINA neutron time-of-flight facility at JRC-Geel: Neutron Resonance Capture Analysis (NRCA) and Neutron Resonance Transmission Analysis (NRTA). A combination of NRTA and NRCA has been proposed for the characterisation of particle-like debris of melted fuel formed in severe nuclear accidents. In this work, we present a quantitative validation of the NRTA technique which was used to determine the areal densities of Pu enriched reference samples used for safeguards applications. Less than 2% bias has been obtained for the fissile isotopes, with well-known total cross sections.

Study on the Code System for the Off-Site Consequences Assessment of Severe Nuclear Accident

인접 국가인 일본의 후쿠시마 원전에서 극한 자연재해로 인한 중대사고가 발생하면서, 국내에서 중대사고 및 확률론적 안전성 평가 (PSA, Probabilistic Safety Assessment)에 대한 중요성이 재인식되었다. 국내에서는 원전의 소외결말을 평가하는 3단계 PSA에 대한 연구개발이 최근까지 거의 이루어지지 않았다. 본 논문에서는 국외 3단계 PSA 전산코드 중, 미국의MACCS2 (MELCORE Accident Consequence Code System 2), 유럽의 COSYMA (COde SYstem from Maria) 그리고 일본의OSCAAR (Off-Site Consequence Analysis code for Atmospheric Releases in reactor accidents)에 대한 간략한 분석과 미국의 MACCS2에 대한 단점 및 한계점 분석을 수행하였다. 국내·외 전문가들에 의해 공통적으로 지적되어 온 MACCS2의 한계점은 다수호기사고와 사용후핵연료 저장조로부터의 방출 모사의 불가능, 그리고 대기확산모델을 단순 가우시안 플륨모델을 기본으로 한다는 것이며, 이중 일부는 MACCS2업데이트 버전을 통해 개선되어 왔다. Food chain 모델의 모사의 제한, 해양 및 수계 확산모델의 부재, 제한된 범위의 경제영향평가 등 또한 개선되어야 할 사항이다. 기술보고의 결과는 국내 3단계 PSA 관련 기술 개발을 위한 기초자료로 활용될 수 있을 것으로 기대된다.

Application of Yeongdong Illite to Remove Radiocesium for Severe Nuclear Accidents

본 연구는 원자력 중대 사고 시, 환경에 유출된 방사성 세슘의 확산을 억제하기 위해 충북 영동지역 일라이트의 활용 가능성을 평가하였다. 영동 일라이트는 운모질 편암의 열수변질 작용에 의해 형성되었으며, 주요 구성 광물은 석영, 장석, 일라이트이다. 저농도 세슘 용액을 사용한 회분식 흡착실험 결과, 영동 일라이트의 흡착 분배 계수(Kd)는 약 4,200 L kg-1으로 다른 점토 광물에 비해 비교적 높은 값을 가지며, 이는 일라이트에 존재하는 풍화된 모서리면(FES)의 영향으로 판단된다. 영동 일라이트와 세슘의 흡착등온선은 비선형 흡착 특성을 나타내며 단일 표면 한계 흡착 능력이 250,000 μg kg-1으로 우수한 흡착능을 보여주어 방사성 세슘 흡착제로서의 사용 가능성을 입증하였다. 이러한 결과는 추후 방사능 누출 사고 등의 긴급 상황 발생 시, 영동 지역 일라이트를 오염 확산 방지 및 정화작업에 사용하기 위한 평가 자료로 활용될 것으로 기대된다.

Experiments on silver-indium-cadmium control rod failure during severe nuclear accidents

A large-scale bundle experiment with silver-indium-cadmium absorber rod in the QUENCH facility as well as a series of small-scale single absorber rod tests with prototypical materials have been conducted. A variety of parameters like temperature history, initial contact between cladding and guide tube and possibility of inner oxidation of the guide tube was investigated in the single-rod tests. Different failure modes from local melt-through to explosive failure were observed depending on the sample type. No interaction between Ag-In-Cd melt and stainless steel was seen up to 1750K, near to the melting temperature of the steel. The cladding did not balloon before failure. Metallographic post-test analysis identified various phases formed during the high-temperature interactions between the involved materials. Furthermore, solidus and liquidus temperatures of the Ag-In-Cd alloy were determined accurately by DSC.

Авария на ас Фукусима-дайити как стресс-тест для национальной системы защиты населения при тяжелой аварии на атомной станции

Авария на ас Фукусима-дайити как стресс-тест для национальной системы защиты населения при тяжелой аварии на атомной станции

Study of the iodine kinetics in thermal conditions of a RCS in nuclear severe accident

During the PHEBUS-FP integral severe accidents simulation tests, gaseous iodine was detected in earlier stages of the simulated accident, coming from the experimental circuit modelling a reactor coolant system. One possible explanation is the existence of some kinetic limitations which promote the persistence of gaseous iodine at low temperature. This paper sums up some analytical and modelling works performed to check this assumption. Results show that the chemical speciation of iodine cannot be calculated by assuming chemical equilibrium, kinetics have to be considered, in particular for oxidising atmosphere with an excess of steam. A kinetic model for gaseous iodine is proposed and qualified by comparison with experimental works.Such modelling should be considered to calculate the transport of iodine through the reactor coolant system for a severe accident because it can significantly impact iodine source term evaluations.

Impact of Ag and NOx compounds on the transport of ruthenium in the primary circuit of nuclear power plant in a severe accident

Ruthenium is a semi-volatile element originating as a fission product in nuclear reactors that can be released in case of a severe nuclear accident. In this work, the impact of atmosphere composition on the transport of ruthenium through the primary circuit was examined. The effects of silver nanoparticles representing aerosols and NO2 gas as a product of air radiolysis were studied. Quantification of ruthenium transported both as gas and aerosol was performed. Chemical composition of ruthenium species was evaluated. The transport of gaseous ruthenium through the facility increased significantly when NO2 gas was fed into the atmosphere. When both silver aerosols and NO2 were fed into the atmosphere, the transport of ruthenium in gaseous and aerosol forms was promoted. It was concluded that the composition of atmosphere in the primary circuit will have a notable effect on the speciation of ruthenium transported into the containment building during a severe accident and thus on the potential radioactive release to the environment.

Changes to soundscapes at outdoor gathering spaces in Fukushima, Japan, caused by the severe nuclear power plant accident

Five years have passed since the accident at the Fukushima Daiichi Nuclear Power Plant, which caused changes to daily life in the city of Fukushima, Japan. These daily life alterations caused changes to the soundscapes of Fukushima, which are still evolving now. In this study, the soundscape change at Fukushima’s outdoor gathering spaces is discussed using field recordings by the author. Shortly after the accident, few human voices could be heard at outdoor gathering spaces, but natural sounds such as birdsongs could be heard as usual per the time of year. Park decontamination started during summer 2011. In some parks where decontamination was successfully completed, people’s voices and sounds of children playing returned during spring 2012. However, in other parks where decontamination was done but ineffective, the lack of human voices and artificial sounds continued until radiation levels decreased sufficiently. In this way, soundscape change at outdoor gathering spaces represents people’s attitude toward the radioactive contamination; soundscape recording documents not only sonic environments but also people’s lives.

The Effects of the Passage of Time from the 2011 Tohoku Earthquake on the Public’s Anxiety about a Variety of Hazards

This research investigated whether the Japanese people’s anxiety about a variety of hazards, including earthquakes and nuclear accidents, has changed over time since the Tohoku Earthquake in 2011. Data from three nationwide surveys conducted in 2008, 2012, and 2015 were compared to see the change in societal levels of anxiety toward 51 types of hazards. The same two-phase stratified random sampling method was used to create the list of participants in each survey. The results showed that anxiety about earthquakes and nuclear accidents had increased for a time after the Tohoku Earthquake, and then decreased after a four-year time frame with no severe earthquakes and nuclear accidents. It was also revealed that the anxiety level for some hazards other than earthquakes and nuclear accidents had decreased at ten months after the Earthquake, and then remained unchanged after the four years. Therefore, ironically, a major disaster might decrease the public anxiety in general at least for several years.

Leaching of actinides and other radionuclides from matrices of Chernobyl “lava” as analogues of vitrified HLW

The paper commemorates the 30th anniversary of the severe nuclear accident at the Fourth Unit of the Chernobyl Nuclear Power Plant. Results of the investigation of radioactive glassy lava-like materials formed as a result of the accident are presented. Leaching of Pu, Am and Cs in distilled water and sodium carbonate solution at 25°C from matrices of black and brown Chernobyl “lava” is comparable, however, leaching of Cs from black “lava” in seawater at 25 and 90°C is almost 2 times higher than that for brown “lava”. Chemical alteration of black and brown “lava” in seawater is much more intensive that in distilled water and sodium carbonate solution. Comparison of leaching behavior of Chernobyl “lava” and aged samples of nuclear glass is not clear so far because of the lack of data. Further study of Chernobyl “lava” as analogues of vitrified high level waste (HLW) and possibly, Fukushima’s corium is needed.

Physico-chemical properties of Chernobyl lava and their destruction products

The paper commemorates 30th anniversary of severe nuclear accident at the 4th Unit of Chernobyl NPP. Results of the investigation of radioactive glassy lava-like materials formed as a result of the accident at their current state are presented. Complementary analytical methods: vibrational spectroscopy, XAFS, SEM, EBSD, X-ray tomography, provide new information about structure of the Chernobyl lava matrix and inclusions. Most of these techniques are applied to the lava samples for the first time and allow to derive consistent model of the lava and to resolve some of existing controversies. The glassy matrix of the lava is an anhydrous depolimerised metaluminous glass with signs of devitrification. Principal inclusions are (U,Zr)O2−x and (Zr,U)O2−x solid solutions with tetragonal and monoclinic structures, UO2 and U-rich zircon. Cracks around large inclusions are observed; some of them could be caused by volume expansion during tetragonal to monoclinic transition in zirconia. The lava accumulations are characterized by different stability against spontaneous destruction, with the pumice-like lava being the least stable, generating radioactive aerosols and particles with sizes up to 250 μm.

Development of an aerosol decontamination factor evaluation method using an aerosol spectrometer

During a severe nuclear power plant accident, the release of fission products into containment and an increase in containment pressure are assumed to be possible. When the containment is damaged by excess pressure or temperature, radioactive materials are released. Pressure suppression pools, containment spray systems and a filtered containment venting system (FCVS) reduce containment pressure and reduce the radioactive release into the environment. These devices remove radioactive materials via various mechanisms. Pressure suppression pools remove radioactive materials by pool scrubbing. Spray systems remove radioactive materials by droplet−aerosol interaction. FCVS, which is installed in the exhaust system, comprises multi-scrubbers (venturi-scrubber, pool scrubbing, static mixer, metal−fiber filter and molecular sieve). For the particulate radioactive materials, its size affects the removal performance and a number of studies have been performed on the removal effect of radioactive materials.This study has developed a new means of evaluating aerosol removal efficiency. The aerosol number density of each effective diameter (light scattering equivalent diameter) is measured using an optical method, while the decontamination factor (DF) of each effective diameter is evaluated by the inlet outlet number density ratio. While the applicable scope is limited to several conditions (geometry of test section: inner diameter 500mm×height 8.0m, nozzle shape and air-water ambient pressure conditions), this study has developed a numerical model which defines aerosol DF as a function of aerosol diameter (d) and submergences (x).

Fission products behaviour in UO2 submitted to nuclear severe accident conditions

The objective of this work was to study the molybdenum chemistry in UO2 based materials, known as SIMFUELS. These materials could be used as an alternative to irradiated nuclear fuels in the study of fission products behaviour during a nuclear severe accident. UO2 samples doped with 12 stable isotopes of fission products were submitted to annealing tests in conditions representative to intermediate steps of severe accidents. Samples were characterized by SEM-EDS and XAS. It was found that Mo chemistry seems to be more complex than what is normally estimated by thermodynamic calculations: XAS spectra indicate the presence of Mo species such as metallic Mo, MoO2, MoO3 and Cs2MoO4.

Interaction of ruthenium tetroxide with iodine-covered surfaces of materials in nuclear reactor containment building

ABSTRACTVolatile iodine and ruthenium species are likely to be released from the fuel during a severe nuclear accident. Both iodine and ruthenium are expected to deposit on the surfaces in the containment building of the nuclear power plant. It is assumed that, due to the different release times from the fuel, ruthenium will reach the containment at the time when surfaces are already deposited with iodine species. The influence of ruthenium tetroxide on elemental iodine-covered surfaces in the containment of nuclear power plants was studied in this work. The ability of ruthenium tetroxide to oxidize iodine deposits on zinc, aluminum, copper and epoxy paint at high humidity conditions was evaluated. Quantification of both iodine and ruthenium deposits was done by the means of gamma spectroscopy. The chemical speciation of deposited elements was observed with SEM, XPS and EDX techniques. Experiments showed that ruthenium tetroxide oxidized iodine deposits into the volatile forms of iodine on zinc and aluminum samples and higher iodine oxides in the case of copper and epoxy paint samples. A major increase of ruthenium uptake on iodine-exposed surfaces in comparison to clean surfaces was observed.