Method of void fraction measurement by gamma ray attenuation in large size liquid metal MHD power generators of gravity type

Two-phase flows consisting of high density liquid metals like lead or lead alloys are used in Liquid Metal Magnetohydrodynamic power generators of gravity type. Non-intrusive measurement of void fraction constitutes an important part of monitoring the industrial power unit. Gamma ray attenuation method of void fraction measurement is most suitable for this purpose. However, due to very large path lengths of two-phase region (0.2-0.25 m) gamma ray beam of 1.33 MeV energy, very large source strength of the order of kilo curie is required. This makes diagnostic system very bulky and also gives rise to radiation handling problems. A modified equation for void fraction is developed where the counts corresponding to the liquid metal alone which decides the overall strength, is replaced by the counts corresponding to some reference void fraction. Using this equation, depending upon the void fraction value, the source strength can be reduced to as low as a fraction of a curie. Experiments have been conducted with {sup 60}Co with 75 mCi and 20 mCi source strength in a mercury-nitrogen liquid metal simulation facility to verify the accuracy of measuring the void fraction based on the modified equation. The values matched well with the values based onmore » the standard equation.« less