Defining parameters of polonium release from HLMC into gas derived from IPPE experimental data

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УДК: 621.039.534.6 : 669.774

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Heavy liquid metal coolant (HLMC) containing lead and bismuth form alpha-active isotopes: 210Po, 209Po and 210mBi. 210Po is the most radiotoxic one. There is no significant radiation danger in normal operation until the primary coolant circuit is pressure-tight. However, when designing the reactor gas system and refueling system polonium radiotoxity and alpha-activity presence in the coolant should be considered.
It should be also taken into account that accidents, accompanied by the first circuit coolant leakage or significant depressurization of the gas system may result in formation of alpha-active aerosols in the air and highly radioactive surface contamination upon their precipitation.
Polonium-210 has insignificant gamma radiation and is not dangerous as an external radiation source. The most dangerous case of organism exposure is caused by polonium intake into organism through food, water and inhalation. Contact of polonium with skin and mucous tunics is also dangerous. For this reason while simulating polonium activity migration and developing staff-protection means there is a need to pay special attention to aerosols and gaseous polonium compound – PoH2, obtained as result of PbPo and water vapour interaction.
To substantiate radiation safety and forecast the aftermath of such accidents is essential to develop a model of polonium activity migration through the first circuit in normal operation and under conditions resulting in accidents. The model must be tested by experimental date.
Polonium activity release from HLMC into gas is the main element of this model. Most of the researches of polonium evaporation from smelted HLMC into gas or vacuum were made with lead-bismuth and lead-lithium eutectics. Polonium release from lead is studied to a lesser degree.
An experimental study on polonium evaporation from HLMC into vacuum and into argon atmosphere at 300, 500 and 800 C was performed at IPPE in 1995. The polonium evaporation rate from lead, lead-bismuth and lead-lithium eutectics was measured. The values of effective vapour pressure of polonium in HLMC solution and thermodynamic activity coefficient for polonium in HLMC are presented in the paper. These values were calculated on the basis of experimental data. The calculation results are compared with data on polonium release from liquid metals obtained by different authors.

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