Teemu Kärkelä

Production and characterization of plutonium dioxide particles as a quality control material for safeguards purposes.

Plutonium (Pu) dioxide particles were produced from certified reference material (CRM) 136 solution (CRM 136-plutonium isotopic standard, New Brunswick Laboratory, Argonne, IL, U.S.A., 1987) using an atomizer system on December 3, 2009 after chemical separation of americium (Am) on October 27, 2009. The highest density of the size distribution of the particles obtained from 312 particles on a selected impactor stage was in the range of 0.7-0.8 μm. The flattening degree of 312 particles was also estimated. The isotopic composition of Pu and uranium (U) and the amount of Am were estimated by thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICPMS), and α-spectrometry. Within uncertainties the isotopic composition of the produced particles is in agreement with the expected values, which were derived from the decay correction of the Pu isotopes in the CRM 136. The elemental ratio of Am to Pu in the produced particles was determined on the 317th and 674th day after Am separation, and the residual amount of Am in the solution was estimated. The analytical results of single particles by micro-Raman-scanning electron microscopy (SEM)-energy-dispersive X-ray spectrometry (EDX) indicate that the produced particles are Pu dioxide. Our initial attempts to measure the density of two single particles gave results with a spread value accompanied by a large uncertainty.

Radiolytical oxidation of gaseous iodine by beta radiation

Abstract Iodine is one of the most radiotoxic fission product released from fuel during a severe nuclear power plant accident. Within the containment building, iodine compounds can react e.g. on the painted surfaces and form gaseous organic iodides. In this study, it was found out that gaseous methyl iodide (CH3I) is oxidised when exposed to beta radiation in an oxygen containing atmosphere. As a result, nucleation of aerosol particles takes place and the formation of iodine oxide particles is suggested. These particles are highly hygroscopic. They take up water from the air humidity and iodine oxides dissolve within the droplets. In order to mitigate the possible source term, it is of interest to understand the effect of beta radiation on the speciation of iodine.

A Scoping Study of the Chemical Behavior of Cesium Iodide in the Presence of Boron in the Condensed Phase (650°C and 400°C) Under Primary Circuit Conditions

Abstract This work is a contribution toward understanding the chemical reactions on the primary circuit surfaces involving gaseous iodine release during a severe nuclear reactor accident. Cesium iodide was used as a nonradioactive precursor material in order to highlight the effects of carrier gas composition (Ar/H2O, Ar/H2O/H2, and Ar/Air), temperature, the initial cesium/iodine (Cs/I) molar ratio by adding cesium hydroxide, and the presence of boron on the molar composition of the deposited iodine compound and on the release of gaseous iodine from the deposit. The results from the experiments involving only cesium iodide as a precursor revealed a slight decomposition of cesium iodide and a release of gaseous iodine. Furthermore, the measured gaseous iodine mass decreased with the addition of hydrogen to the carrier gas at 650°C. At 400°C, the amount of released material (aerosol and gas) was decreased. However, whereas at 650°C the sampled iodine existed mainly as aerosols, the mass concentration recorded from the experiment at 400°C indicated a predominance of gaseous iodine. When the initial Cs/I molar ratio was significantly greater than unity (1.5 < Cs/I < 4.5), the mass of produced gaseous iodine was barely detectable, suggesting a reaction between cesium hydroxide and the gaseous iodine released from cesium iodide decomposition. In the presence of boron, the transport of gaseous iodine was increased as a result of the formation of glassy cesium borate in the evaporation crucible. The presence of steam and its quantity were shown to have an enhancing influence on the cesium borate formation and on the release of gaseous iodine.

A Complementary Study to the Chemical Behaviour of Caesium Iodide in Presence of Boron in Condensed Phase (650 °C and 400 °C) under Primary Circuit Conditions: Differential Thermal Analysis and Thermogravimetric Studies

Abstract This work is a contribution to the work performed in a paper on the understanding of the chemical reactions between cesium iodide and boron oxide in condensed phase, under conditions close to the ones prevailing in the primary circuit of a nuclear power plant in case of a severe accident. The thermal degradation of samples made from cesium iodide or cesium iodide and boron oxide mixtures has been investigated using the techniques of thermogravimetric analysis and differential thermal analysis at temperatures from 20°C to 1000°C. The boron-to-cesium molar ratio in the investigated mixture was fixed at about the value of 5 (B/Cs = 5). Apart from the dehydration of boric acid, evidence is presented for the formation of a vitreous compound at 360°C to 420°C, depending on the atmosphere composition. Carrier gas composition also seemed to influence the behavior of the precursor mixture. While under air and argon, the recorded thermograms are similar. In the presence of argon/water vapor, a specific behavior and difference on reactivity is noticed, due to the adsorption of water from the carrier gas at the beginning of the process. It was also pointed out that the addition of water or oxygen delayed the glass formation process.

Experimental Study of the Cadmium Effects on Iodine Transport in the Primary Circuit During Severe Nuclear Accident

In case of a severe accident in a light water reactor, iodine is one of the most important fission products in the context of reactor safety because of its significant total fuel inventory, high specific activity and radiotoxicity. Consequently, understanding its behavior under severe accident transient conditions is a major point in the optimization of the accident management and mitigation.An experimental study has been launched at VTT investigating the behavior of iodine on primary circuit surfaces during a severe nuclear accident. The paper presents results obtained from the heating of metallic cadmium and caesium iodide in a crucible at temperature below 400°C under three different atmospheres. Aerosols and gaseous species released from the reaction crucible were sampled at 150°C on filters and liquid scrubbers and analyzed with HR-ICP-MS.At first, experiments were conducted with one precursor (cadmium or caesium iodide). It was observed that cadmium is predominantly present as aerosol in all atmospheres but represents less than 1% of mass what has been recovered from the facility leaching. To the contrary, caesium iodide experiment showed that sampled iodine exists mainly as gaseous species.By comparison to one-precursor studies, a change in the behavior of element was noticed when cadmium was added to caesium iodide. Different observations let to suggest a reaction between caesium iodide and cadmium in the crucible. The reaction would lead to the formation of compound made of cadmium and iodine in condensed phase below 150°C. More gaseous iodine was found from the sampling line with Ar/H2O/H2 atmosphere than in Ar/H2O atmosphere, suggesting that the cadmium is more reactive toward iodine in hydrogen-free atmosphere.Copyright