D E T E C T I O N ~,)F G A S LEAKS W I T H TItE AID OF R A D I O A C T I V E I S O T O P E S

The management of the gas supply system of the city of Moscow together with co-workers from the Institfft~.of Physical Chemistry of the Academy of Sciences tlSSR, V. I. Kuznetrov and M. B. Neiman. during the years I9,55 and 1956.developed and proved out in a test section a method for detection of leaking gas with the aid of radioactive isotopes.

Ordinarily in locating leaks in under,,round gas mains the whole suspected length, 1.5 to 2 m, Is bored with samplit~g holes and from each hole asample ofgas is taken which is analyzed for methane and hydrogen content;

Instead of this laborious method it was decided to add to the gas a volatile combination bearing a ~adioactive isotope with sufficiently hard radiation, and to measure the radioactivity directly along the surface of the ground along the path of the gas main. It was assumed that maximum activity would be detected ~t the locations of the leaks.

Cll~llr ~ which had been obtained by trradiation of stable methyl bromide in a nuclear reactor was used at the t racer .

l.lOf mr ' I~"7"

35t

300

250

200

150

fO0

50

0

l.lOS, mr. t h r

I06

2VO"fcounts

i;.o

22 2r Z$ 2a 3O

1.10 "e c~un~

'(X 1"'

O.52

ca ~a 52 #'r .~

Variation of activity along the path. Maximum activities at the 26 m poiut correspol)d to file activity of the sandy covering, and at the 52 m ~)lnt to the activity of the clay. Over the sandy covering the curve 1 was read after O..'1 hot,rs; the curve 2 after 1~ hours. Over the clay cover fl~e curve 1 was read after 0.3 hours; curve 2 after 2.3 hours; curve 3 after 22.8 hours.

Tests to determine the locations of lhe gas Ieahs were conducted oli a specially prepared gas main, 5~ m long. At the 2~ zn a .d ,52 11z points two holes were drilled ~ mm ill diameter. The first hole w,a~ covered with

485

sand and the second with clay, The thickness of catth layer over the main was 1.7 m. The gas mixed with CEI~ Br st in a gas storage was introduced into the main~ In all tests the specific activity did not exceed 0.3 mcurte/n~.

The search for leaks was conducted with valves at the end of the main both closed and open~ the latter being the more realistic situation. For measurement of the activity along the path a potable field type: gamma dosimeter was used, its face graduated in mr/hr.

The figure shows the results of one series of tests. For 18 hours the gas flowed through file main under a pressure of 1.2 atmospheres abs. and with the valve closed at the exit. The curves show that the leak is localized accurately to i o.5 m regardless of the nature of the ground cover.

It was established that the activities of the sand and clay at the leaks, and their changes with time,were different. In all eases without exception, the activity of the sandy covering layer was 4 to5 times higher than the activity of the clay covering, and was retained by the sand longer. Analogous results were obtained in experiments under a pressure of 1.02 atmospheres abs.

The change in activity with time in the area of the leak showed that under a pressure of 3 atmospheres and higher it was possible to make measurements only five minutes after release of tim contaminated gas. and when under lower pressure - in 4 to 5 hours. General activity exceeding background becomes detectable at the

�9 location of the leak within 24 hours of release of the radioactive methyl bromide into the main.

This newly developed medtod can be used in ehecking for leaks during construction or a gas line. Its use in city gas mains is not possible so far because of the high specific activity.

In the future it is proposed to adapt a more sensitive detector than the gamma do~imcter to the measure - ment of the activity. The use of a beta counter will allow lowering of the specific activity to 4 " 10 "t mcurtc]m t.

N. Scrdyak

486