Determination of radioactivity concentrations of fission products in the primary coolant of nuclear power plants

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Journal o f Radioanaly tical and Nuclear Chemistry, Articles, Vol. 147, No. 2 (1991) 347-353 DETERMINATION OF RADIOACTIVITY CONCENTRATIONS OF FISSION PRODUCTS IN THE PRIMARY COOLANT OF NUCLEAR POWER PLANTS D. BODIZS,* S. DI~SI,* G. KEOMLEY,* T. PINTI~R** *Institute of Nuclear Technics, Technical University Budapest, Budapest (Hungary) **Paks Nuclear Power Plant, Paks (Hungary) (Received April 10, 1990) A semi-automated instrumental gamma-spectrometric measuring system with a sample changer has been developed for determination of the radioactive fission products coming from different parts of the primary coolant circuit. The measuring geometry assures higher sensitivities under normal operation conditions and lower ones for significant fuel cladding failures. The minimal detection limits are in the r~nge of some ten Bq/1. I n t roduct ion For the safe operation of Nuclear Power Plants (NPPs) the permanent knowledge of the fuel cladding behaviour is basically important. 1 Automated or semi-automated measuring systems for the determination of the radioactive components of the primary coolant have been cited in the literature. 2 -6 The measuring devices are able to indicate the fuel cladding failures immediately by means of fast data supplement. NEEB 2 reported the automated measurement of the concentration of the fission products S8Rb, STKr, SaKr, 925r, 1311,132I, 13sI, 13SCs, 133Xe and laSXe in the primary coolant of a NPP in the FRG using a gamma-spectrometric device. A gamma-spectrometer containing a semiconductor detector and a computer has been applied 3,4 to determine the radioactive concentrations of 10-12 fission products instrumentally in the primary coolant of the Obrigheim NPP. KUHLMANN s described a computerized multichannel gamma-spectrometer for measurement of fission products in the primary coolant, too. A measuring system of similar purposes has been applied at Paks NPP. 6 Recent developments concerning this equipment - based on previous measuring experiences - will be discussed in this article. The novelty of the measuring system described, as compared to the others presented in the literature, means especially two features. On the one hand, it greatly Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest D. BODI ZS et al. :DETERMINATION OF RADIOACTIVITY CONCENTRATIONS extends the measurable range of activity concentration with one detector, but with the two (kinds) measuring geometries: On the other hand, the measurement control ensures both periodical and continuous measuring. It is especially of great importance when one has to apply longer cooling times to determine isotopes having longer half- lives (e.g. ~ 31 I) with better sensitivity. Experimental Measuring device Fission products in the primary coolant can be analyzed selectively and rapidly (without chemical separation) by gamma-spectrometers of high energy resolution. The schematic drawing of the measuring system is shown in Fig. 1. Coolant samples coming from different parts of the primary circuit can be analyzed by means of a sample changer. According to a given program, coolant samples of normal pressure and temperature are transferred to the sample holder situated in front of the detector. The sample changer controls falling up, emptying and rinsing of the sample-holders. Low bockgrouno chamber From sample ~ Detector I~ = changer : = . . i WaShinWClter : f ~300m T changer Noise . control ~/CAN MCA BERRA 80 ~-~ PDP lt/23 J t Peripheries Fig. 1. Scematic drawing of the measuring apparatus 348 D. BODIZS et al.: DETERMINATION OF RADIOACTIVITY CONCENTRATIONS The sensitivity of the measuring system is basically determined by the detector. Under normal operation conditions a sensitivity as high as possible is desirable. It depends on the energy resolution, the efficiency and the background level. The gamma- spectrum of the primary coolant is generally rich in gamma-lines, so that only semi- conductor detector of good energy resolution can be considered to be suitable. For the determination of the sensitivity of the detector the method of CURRIE 7 has been applied. The lowest measurable activity is: LD Ami n - (1) e ' tm "f 'r where LD = 2.71 + 3.29 x/-H, H - the backgrounff under the full energy peak (pulse); tm - time interval of the measurement (s), e - absolute full energy peak efficiency of the detector at the given gamma-line, f~ - gamma-abundance. According to Eq. (1) it can be stated that Amln depends on the background under the photopeak. In Fig. 2, the full-energy peaks determined by two detectors of equal efficiencies and different energy-resolutions can be seen in the presence of a back- ground radiation of a H(E) energy-distribution. Since H2 is higher than H~ and the peak areas are equal, the lowest detectable activity is increased according to the H2/H1 ratio for detector 2. c t- O e- u Q. u~ C 8 [ Eo V 2 D Channel number (E l ) Fig. 2. Full energy peaks: R 1 < R 2 ; el = e2 349 D. BODIZS et al.: DETERMINATION OF RADIOACTIVITY CONCENTRATIONS Ami n depends even on the detector efficiency. According to Eq. (1), the higher the efficiency, the better sensitivity can be achieved. It is obvious that to improve the sensitivity, the background should be reduced. If the operational conditions differ significantly from normal ones, e.g., there are fuel element failures of a larger scale - under accidental conditions, the radioactivity of the primary coolant is multiplied compared to the "normal" value. To reduce the error caused by the high dead time, the sensitivity should be reduced, too. According to these points of view a HPGe detector of 2.0 keV energy resolution (at an energy of 1333 keV) and 10% relative efficiency, made by the firm Canberra was chosen. The detector and the sample holders are situated in a low background shielding chamber made of 8 cm thick iron wall with an inner cover of 2 ram-thick lead and 1 mm-thick red copper. Under normal operational conditions coolant sam- ple flows from the sample changer through a teflon-tubing of an inner diameter of 4 mm consisting of ten coils. Under "accidental" situation the sample is contained in one coil of the above mentioned tubing. The detector signals are transmitted through a preamplifier, a spectroscopy amplifier (Canberra Model 2010) and a noise-suppression unit developed in our institute for the ADC of a MCA type Canberra 80. There is a distance of some hundred meter between the detector and the analyzer. For data transmission a novel noise-pickup free analogue signal transmitting system has been elaborated. Analogue signals of the spectroscopy amplifier are transmitted through a cable matching stage using a so-called twin axial cable into the pulse shaping operational amplifier next to the analyzer. In one core of the cable the detector signals are passed together with noise, while in the other one only the electronic noise signals are transmitted. Under suitable compensation, noise pulses of the two cables cancel each other in the pulse shaping amplifier. Further noise suppression is achieved by the pulse shape discriminator between the shaping amplifier and the ADC. Gamma-spectra collected in the analyzer are evaluated on-line by the PDP-11/23 computer based on Spectran-F code of the firm Canberra. Efficiency determination Efficiency functions for the measuring geometries mentioned (teflon tubing of 1 and 10 coils) were determined by standard solutions containing known activities of certain isotopes. The gamma-lines of the highest abundances of radionuclides 2 2 Na, 24Na, 42K, 54Mn, 57C0, 6~ laaBa, 137Cs, lS2Eu and 241Am were used for the calibration. 350 D. BODIZS et al.. DETERMINATION OF RADIOACTIVITY CONCENTRATIONS To the measured points the following functions were fitted by means of SPECTRAN-F code: log e = ao + al log E + a2 (log E) 2 + a3 (log E) a (2) in the 60-350 keV energy range; and log e = bo + ba log E + b2 (log E) 2 + b3 (log E) a + b4 (log E) 4 O) in the 350-2000 keV range. The gamma-energies of most of the fission products to be measured are found in the low and medium energy ranges, e.g., those of ~33Xe (81 keV), 88Kr (196 keV), 13SXe (250 keV), laXI (365 keV), 132I (668 keV), xaaI (875 keV), la4I (1073 keV), X38Cs (1050 and 1436 keV). The efficiency ratio for the two measuring geometries is about 6-7 in these ranges. This means that about 10 times higher activity can be measured passing the fluid through the tubing of a single coil than through the tubing containing 10 coils while the dead-time remains equal. Results The activity concentration of the primary coolant sample depends very much on its origin. As a result of technological processes the primary coolant contains sodium and potassium, thus radionuclides 24 Na and 42 K respectively are formed by (n, 3') nuclear reactions. The energies of the gamma-radiations of both isotopes are high, just like that of 4 x Ar. The ganuna-lines of most fission products are found in the Compton region of the three radionuclides. That is why the value of Am in changes according to the origin of the sample. If the sample is taken after the ion-exchanger columns of the primary coolant circuit, 24Na and 42K isotopes do not disturb. According to laboratory experiments Ami n is about 120 Bq/1 for 13s Xe, about 60 Bq/1 for 1311 and about 40 Bq/1 for 13 s I if the samples are taken after the ion- exchangers. In Table 1 the final results of the test measurements are given for demonstration, which were measured on the primary coolant of a nuclear power plant. The spectrum was taken after about two days of cooling. It can be seen that the activity concentra- tions are generally low. Among the fission products there are only a few radioisotopes present above the detection limit. Owing to technological processes; 24Na and 42K activities are the highest. The activity concentration of corrosion products (e.g. S4Mn, 351 D. BODIZS et al.: DETERMINATION OF RADIOACTIVITY CONCENTRATIONS Table 1 Final results of the measurements pAK~I AToNERoItU~ RAOIOKENIA 22-FE~-90 14-'47'.42 S I IP I I | 40TV20 9002201400 R i te co | l i cked on 22;'~rE0-90 at 14:29109 ~ocovod to 2, d lu l~ 0,4858 hour i ~EFORE the s tar t of COLLECT, RADIONOCL I OE ANALYS IS REPORT Nucl ide ket |v t t~ Concentrat ion in ~0 /L Erter.~v ComParison Oeca~ (k iP) Nel lured Errol ' cor rec ted Er ror E~:~'OCt O i f f 1. F ISS ION OASES AR-41 LLDD. BODIZS et al.: DETERMINATION OF RADIOACTIVITY CONCENTRATIONS s 9 Fe) is also low. These measured results show the undefected cladding of the fuel elements and the purity of the primary circuit of the nuclear power plant. The laboratory testing of the equipment has been finished, the continuous opera- tion of the system is to be started soon. References 1. Detection and Location of Failed Fuel Elements, IAEA Report of a Panel, Vienna, 1968. 2. K. H. NEEB, BMFT-Forschungsbericht K-73-11, 1973. 3. G. BUDNICK, K. H. NEEB, Atom und Strom, (1974) No. 3-4. 4. K. H. NEEB, S. HILLER: VGB Kraftwerstechnik 54 Heft 12, Dez. 1974. 5. H. S. KUHLMANN, R. I. CORNWELL, A. J. GOLDBERT: IEEE Trans. Nuel. Sei., 25 (1978) No. 1. 6. BODIZS D., Dr S., KEOMLEY G., BOGANCS J.: Energia 6s Atomteehnika 35 (1982) 248 (in Hungarian). 7. L. A. CURRIE: Anal. Chem., 40 (1968) 583. 353


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