Patent Number: 048062782
Section: summary

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.