Method of and apparatus for segregating radioactive iodine isotopes

The invention relates to a method of and apparatus for the regregation of radioactive iodine isotopes from fluid samples. The method consists in leading the degassed and in certain cases also otherwise prepared sample into a column filled with an amorphous solid medium for binding quantitatively the cations, fluoride anions and contaminations of colloidal state, then the effluent flow continuously through an energy-selective gamma detector and continuously recording the signals generated in the detector by the radioactive iodine isotopes. The proposed apparatus comprises a sampling unit (MV), a degassing vessel (G), fluid transfer pumps (P1, P2), pipings, cocks (CS), a bubble removing cell (BC), a column (K) with amorphous material filling, an energy-selective detector system (GD) for measuring gamma radiation, and a signal processing and recording unit (JR).

BACKGROUND OF THE INVENTION 
The invention relates to a method of and an apparatus for the quick and 
continuous segregation of radioactive iodine isotopes present in different 
fluid samples and especially in those taken from the primary coolant of 
the water-water-type nuclear power plants base on pressurized water 
reactors, primarily for analytical purposes. The radioactive concentration 
of iodine isotopes--being of high importance from technological and/or 
operational safety aspects--can directly be measured by means of the 
method proposed by the present invention and using a simple measuring 
method applied to liquid samples treated in the equipment realizing said 
method. 
The radioactive iodine isotopes in the primary water circuit of the 
water-water-type nuclear power plants are fission products of the fuel. 
Their quantity (radioactive concentration), further the relative amounts 
of the various iodine isotopes with respect to each other, reliably 
indicate inhermeticity of the cladding of fuel elements, and sensibly 
follow the service condition of the reactor in its steady-state and 
transient periods of operation. Consequently, the qualitative and 
quantitative analysis of iodine isotopes constitute an important item of 
power-plane radio-analytics. The analysis--considering the above 
tasks--should be performed within shortest time possible, in continuous 
service and with good selectivity and sensitivity. A few theoretical and 
practical methods for the analysis of this kind are known for the isotopes 
.sup.131 I, .sup.132 I, .sup.133 I, .sup.134 I and .sup.135 I occurring in 
water-water-type nuclear power plants. 
Soviet authors, V. V. Aksionov et al. have proposed in 1982 a method and 
equipment offering a partial solution of the above task. (Radiatsionnaya 
bezopasnost'i zashchita AES, Vol. 7. Energoisdat, Moscow, 1982; referred 
to in the INIS IAEA--in the International Nuclear Information System of 
the International Atomic Energy Agency, Vienna--under the number 
15:014268). Their method is suitable for continuous detection of the 
radioactive concentrations of the .sup.132 I and .sup.134 I isotopes. By 
means of a special device, the intensity of gamma photons are measured, 
applying in the primary-circuit piping section two NaI(T1) scintillation 
detectors operating in the given energy inverval in gated mode, making use 
of the emission of cascade gamma photons by isotopes .sup.132 I and 
.sup.134 I. Correspondingly, the detectors are connected in coincidence 
connection to a suitably selected electronic signal processing unit. The 
measuring point is arranged between the cation and anion exchange resin 
columns of the water-cleaning equipment having its circuit partially 
arranged in the primary circuit, in order to reduce radioactivity 
resulting from the cations. The interference effects of some isotopes of 
short half-period (such as .sup.16 N, .sup.17 N) is eliminated by imposing 
a delay on the investigated samples, inserting a 10-minute by-pass 
section. By means of this method, the activity of .sup.132 I and .sup.134 
I isotopes can be selectively measured, and a few per cent of overall 
activity is claimed as sensitivity limit. 
T. Bereznai et al. (Energia es Atomtechnika, Budapest, 30, 1977, p. 38.) 
published method of and an apparatus equipment for solving the assumed 
radio-analytical tasks occurring in the PAKS Nuclear Power Plant of Paks 
(Hungary). The aim of the method is to provide continuous analyzis of the 
nuclides present in the primary-circuit coolant of the ractor, the 
determination of iodine isotopes being only a part of this overall task. 
The equipment incorporates detector systems permitting continuous 
measurements at two points, one at the inlet of the by-pass section 
serving for the specific purpose of the investigation, and the other at 
the absorbent placed into the flow of gases separated from the sample and 
drained. In the course of processing the sample, the iodine isotopes are 
bound by a "iodine filter" which has not been specified. 
Aksionov and this co-workers claim their method to be suitable only for 
analyzing the two iodine isotopes mentioned above, whereas in the course 
of measurements aimed at checking reactor operation and safety, 
quantitative analysis of as many of the entire set of iodine isotopes as 
possible, preferably all of them, would be required. A further deficiency 
of the method lies in the unsatisfactory degree of obtainable selectivity 
and sensitivity. From among the detectors performing continuous 
measurements, incorporated in the equipment realizing the method developed 
by Bereznai and his co-workers, the detector directly measuring 
gamma-radiation of unprocessed samples is only suitable for the 
determination of the so-called matrix activity of isotopes representing 
the overwhelming part of radioactive concentration in the sample, and also 
its sensitivity to detect iodine isotopes is very poor. Any continuous 
measurement of the iodine content of the iodine filter would only ppermit 
the plotting of cumulated (integrated) radio-activity, and also the 
sensitivity of continuous measurements would be effected by the presence 
of effluent passing across the iodine filter and still containing a 
considerable part of matrix form activity. 
In the course of developing the present invention, the aim has been to 
determine, in shortest time possible and continuously, the activity of 
iodine isotopes or some quantity proportional to their activity, in 
steady-state and transient operating conditions (i.e. during shut-down or 
start-up periods) of the reactor, said iodine isotopes being of 
outstanding importance from among all radioactive isotopes present in the 
primary coolant of the pressurized water nuclear reactors. 
SUMMARY OF THE INVENTION 
The idea of the invention is, in essence, the recognition that from liquid 
samples, containing beside iodine isotopes disturbing components: 
radio-active isotopes, components in gaseous phase, cations, fluoride 
anions and colloidal corrosion products present in the solution the iodine 
isotopes can be segregated quickly and continuously and held in the sample 
by means of gass rinsing and by letting the liquid medium pass through a 
column filled with amorphous zirconium phosphate or other high specific 
surface active solid medium, so that the rate and quantity of each iodine 
isotope can be determined selectively. 
The method proposed by the present invention and serving for the 
segregation of radioactive iodine isotopes (especially from the 
primary-circuit coolants of pressurized water reactors of nuclear power 
plants) in the course of which the given gases (among others radioactive 
noble gases) are expelled from the sample taken continuously under 
suitable conditions and, in the given case pre-treated, by means of 
passing bubbles of some inert gas through the sample, and adjusting the 
pH-value of the sample to neutral or some suitable alkaline level, all 
that in some known way--is based on passing the sample at a suitably 
chosen flow rate--preferably resulting in a contant duration of about 5 to 
15 minutes--through a column filled with amorphous zirconium phosphate or 
other active solid material for binding the cations, fluoride anions and 
colloidal-state contaminations, e.g. unsolved corrosion products; the 
effluent leaving the column and containing in considerable proportion 
iodine isotopes only--after a complete segregation period of about 20 
minutes permitting the decay of radioactivity of the generally anionic 
isotopes having half-periods up to a few minutes only--is led through a 
gamma detector expediently of the flow-cell type, for continuous recording 
of the signals generated in the detector by the radioactive iodine 
isotopes, either in lumped form by means of a calibrated counter adjusted 
corresponding to the different gamma energies of the various iodine 
isotopes and operated in gated mode, or selectively, according to the 
gamma energy of each individual iodine isotope (expediently by means of at 
least one ratemeter coupled to the recorder). 
The novel apparatus intended for accomplishing the method proposed by the 
invention comprises a sampling unit of continuous operation, providing 
samples of suitable pressure and temperature, a degassing vessel connected 
in series to the former through a piping, and provided with a nitrogen 
inlet producing gas bubbles in the just treated part of the sample, 
performing thereby its required mixing and also permitting level control, 
a liquid transfer pump also connected in series with the former, a reagent 
storage container arranged for being connectible to the continuously 
operated system through a piping and a cock and to be drained by means of 
a pump, a bubble removing cell connected in series to the liquid transfer 
pump through a piping, a separator column filled with amorph circonium 
phosphate connected in series to the former through a piping, a 
continuously operating, flow-type energy-selective detector system for 
measuring gamma radiation, connected in series with the former through a 
piping and, finally, connected to the detector, a signal-processing and 
recording unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The apparatus proposed by the invention and capable of carrying out the 
method of segregating radioactive iodine isotopes (FIG. 1) comprises a 
sampling unit MV interconnected with a pipeline transporting a fluid 
medium containing among others radioactive iodine isotopes. Such pipeline 
can be the primary liquid coolant circuit of a pressurized water reactor 
serving as heat energy source of a water-water-type nuclear power plant. 
The sampling unit MV continuously takes samples of suitable pressure and 
temperature and passes them into a degassing vessel G having an inlet for 
introducing gas as nitrogen (N.sub.2). The nitrogen bubbles ensure 
agitation of the treated sample and permit to control the liquid level in 
the vessel. The degassing vessel G is connected through a piping to a 
transfer pump P1 and thereby to a cock CS receiving appropriate reagent 
from a container R by means of a pump P2. The fluid sample mixed with the 
reagent in the cock CS is transported to a bubble removing cell BC 
wherefrom the mixture is transferred to a segregation column K containing 
a filling made of amorphous zirconium phosphate. The segregation column K 
is connected--again through a piping--to a continuously operating 
flow-type energy-selective gamma radiation detecting system GD and a 
signal processing and recording unit JR. 
Instead of nitrogen it is possible to apply other appropriate inert gas. 
The degassing vessel G serves for expelling the gases dissolved in the 
sample (among them the radioactive isotopes of the noble gases). The 
pH-value of the samples taken is adjusted always to a neutral or alkaline 
level if necessary by means of the appropriate reagent placed in the 
container R. The samples treated in the required manner (having e.g. the 
required pH-value adjusted, being free of bubble and noble gases) are 
transported into the segregation column K with a speed ensuring full 
contact time generally from at least about 5 minutes to about 15 minutes. 
The active filling of the column containing high specific surface 
absorbent as zirconium phosphate is capable of binding the cations, 
fluoride anions and different colloid-state corrosion products from the 
sample. Before detecting the sample treated it flows about 20 minutes in 
order to premits the decay of the very short and short half-period 
radioactive isotopes remaining in the effluent in spite of the previous 
segregation steps and capable of disturbing the measurements related to 
iodine. 
The samples are introduced into the radiation detecting system GD for 
detecting gamma radiation of different energy levels. In the fluid carrier 
medium they contain in considerable percentage the radioactive iodine 
isotopes only. The detection system includes calibrated counters operated 
in gated mode and adjusted according to the different gamma-energy levels 
of the different radioactive iodine isotopes in order to determine their 
radiation level. 
With reference to FIG. 2 the following non-limiting example should be 
helpful in better understanding the essence of the invention. 
EXAMPLE 
A laboratory model was constructed for carrying out the method proposed by 
the invention. This model takes into account the specific conditions of 
the water-water-type nuclear power plant built-up in Paks (Hungary) 
operating with pressurized water nuclear reactors. The method should serve 
for measuring the radiation levels of the radioactive iodine isotopes in 
the primary coolant circuit of the Paks plant and it was realised in an 
arrangement shown in FIG. 2. The measurement was carried out as described 
here below. 
The sample is cooled after taking and the cooled sample is led into a 1000 
cm.sup.3 glass bubbling vessel 2 at a flow rate of about 3 cm.sup.3 /min. 
At a flow rate of at least 10 dam.sup.3 /min, air is passed across a glass 
filter arranged at the bottom of the vessel 2. The surplus of the 
air/liquid mixture leaves the vessel at its top and enters a degassing 
unit, from where the part of the water sample is recycled into the primary 
circuit. From the noble-gas-free and bubble-free part of the solution 
collecting under the filter, the sample is pumped into a delay pipe 
section 3 by a peristaltic pump 4 at a flow rate of about 1 cm.sup.3 /min. 
By another peristaltic pump 5 connected to an Y-pipe 7, suitable reagents 
can be fed from a reagent storage tank 6 into the sample, e.g. for 
adjusting its pH-value. Before segregation, a portion of the sample can be 
picked up in a sampling vessel 9 through a two-way cock 8 for the purpose 
of reference measurements. By setting the two-way cock 8 into its other 
position, the sample is led into a column 10 of 6 mm inner diameter, with 
the spaces between quartz wad filter beds being previously filled up with 
10 g of amorphous zirconium phosphate. The effluent is led through a 
hollow NaI(T1) scintillation detector 11 consisting of a teflon tube of 2 
millimeter inner diameter and provided with a spirally arranged flow cell. 
From said detector the effluent gets through a shut-off cock 12 into a 
sampling vessel 13. The scintillation detector 11 obtains its supply 
voltage from an analyser 14. The energy selective signals of the 
scintillation detector 11 are led in a differential gated mode into 
counters or ratemeters 15, 16, 17 adjusted to the characteristic gamma 
lines of the iodine isotopes 131.sub.I, 132.sub.I and 135.sub.I, then the 
signals of the counters or the analogue signals of the ratemeters are 
plotted in the function of time by recorders 18, 19 and 20. 
The segregation of iodine isotopes by means of the method and apparatus 
proposed by the invention is in several aspects more advantageous than 
other methods known so far. After segregation--having determined its 
efficiency through checking measurements using semi-conductor 
detectors--the sample solution containing iodine is--within the 
sensitivity limits of the measurement--free of N, O, Na and K matrix 
components, i.e. of cationoc radioactive components in general, and its 
content of noble gases and fluoride ions belonging to the matrix 
components is low enough to leave the sensitivity of the measurement 
uneffected even in reactor shut-down periods associated with very low 
radioactive iodine concentrations. The method leads itself to continuous 
and parallel measurement of preferably at least three iodine isotopes, and 
by inserting a required number of signal processing units (and respective 
circuit elements), simultaneous measurement of all occurring iodine 
isotopes is feasible. Also, the apparatus for implementing the method can 
be arranged to contain two segregating columns connected parallel, filled 
up with charges of identical composition, permitting regeneration of 
exhausted charge without interrupting continuity of measurement and 
continuous supervision of operation. The metod is economical, it is 
characterized by low demand on time and work, it is based on exclusively 
inexpensive and simple pieces of equipment, resulting in low installation 
and operating costs.