1. Introduction. 2. Requirements to Storage of the Spent Nuclear Fuel. S. Borsuk, O. Ignatchenko, O. Gorbachenko

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1 Checking of the Spent Fuel Assemblies Burnup Based on the Results of the Neutron Flux Measuring Using IAEA Fork Detector at Zaporozhye NPP, Units 1, 2, 4 and 6 S. Borsuk, O. Ignatchenko, O. Gorbachenko Zaporozhye NPP, Zaporozhye, Russian Federation 1. Introduction A main prime costs component of the electric power production at the nuclear power plant is circulation cost of the spent nuclear fuel. For expenses reducing of spent nuclear fuel export, almost all countries, exploited WWER-1000, began works of creation of the intermediate spent fuel storehouses (storage during fifty years then or remaking or the finally bury). At Zaporozhye Nuclear Power Plant for intermediate storage selected a variant of the spent fuel storage in the dry storage container, where the nuclear fuel is in inert gas (Figure 1). 2. Requirements to Storage of the Spent Nuclear Fuel According to Regulations of the safety upon storage and transportation of the nuclear fuel on the Figure 1. Storage container 1 - Temperature sensor; 2 - Air input; 3 - Concrete storage ground; 4 - Air output; 5 - Container lit; 6 - Protection lit of the MSB; 7 - Guiding pipes block for SFA; 8 - Guiding pipe; 9 - MSB case; 10 - Ventilate storage cask. objects of the atomic energy, general principle of ensure of the safety consists in that the effective neutron multiplication factor must be less 0.95, in normal use and in design emergency. Therefore, the nuclear safety upon loading and storage of the nuclear fuel in the spent nuclear fuel dry storage (SNFDS) storage container may be ensured some of followings ways: Incomplete loading of the storage casks. Number of the spent fuel assemblies select in such a way that effective neutron multiplication factor was less 0.95; Using heterogeneous absorbers by way of installation and fixing enough number of the absorber rods in the spent fuel assemblies; Using homogeneous absorbers (with the checking of the necessary concentration of the absorber) by way of intake absorber in the composition of the materials; Checking of the spent fuel assemblies burnup, if it use in the capacity of the nuclear safety parameter, with the help of the burnup checking device, before place the spent nuclear fuel in the storage. The nuclear safety motivation of the storage system without the burnup discount of the spent nuclear fuel means that in the all calculations of the storage system multiplication character, in the normal use and in the design emergency, all the spent nuclear fuel must be regarded as the fresh fuel. Evidently it bring to too high effective neutron multiplication factor value and therefore to the admissible capacity reduction of the storage system. And in the contrary, the nuclear fuel burnup checking upon safety analysis of the storage bring to the storage capacity increase and to the rise of the intermediate storage economic efficiency. Fuel assemblies number with the low enrichment, what possible to loading to the containers of such type without burnup account on Zaporozhye Nuclear Power Plant, is limited. The main part of the fuel assemblies has the high enrichment. And reducing of the fuel assemblies quantity what can be load to the container brings to the reducing of the storages economic efficiency. For full loading of the fuel assemblies with the high enrichment to the storage container, at Zaporozhye Nuclear Power Plant makes the nuclear safety motivation with the burnup account of the spent fuel assemblies. 120

2 3. Burnup Account and Burnup Checking with Reference to Zaporozhye NPP Burnup Account upon Accounting Motivation of the Nuclear Safety Burnup calculation for fuel assemblies makes on personal computers on programs for simulation of a fuel cycle with the checking followings experimental data: Power curve; Absorber rods position during the cycle; Temperature of cooling water on the active area input; Cooling water consumption and boric acid concentration. The fuel assemblies burnup calculates for ten areas of fuel assembly height. For realization of the conservative approach to the nuclear safety analysis, in particular of the effective neutron multiplication factor calculation of the storage container fuel loading, for each of the spent fuel assemblies, burnup takes as invariable on the fuel assembly height and this burnup value is average between upper and lower areas of the fuel assembly, because it is least burnup value in the fuel assembly. In the burnup profiles, received foregoing method makes isotope compound calculation of the spent fuel assemblies for loading to the storage container. In the burnup process of the nuclear fuel during irradiation in the active area and in the storage process of the fuel in the storage pond, as a result of reduction of the isotope total amount, actinides formation and accumulation of fission products, fall multiplication characteristics of the nuclear fuel. Upon nuclear safety analysis of the fuel loading of the storage container with the nuclear fuel burnup checking, take into account that isotopes, what can be calculated with enough accuracy, and with long lifetime Method of the Burnup Checking and Equipment Figure 2. The Fork detector For the burnup checking use methods of the neutron flux registration and gamma rays registration of the spent fuel assemblies. Single absolutely Figure 3. Scheme of Fork detector location 121

3 non-destructive method is method of the gamma rays registration, based on isotope ratios define by gamma radiation measurement. Regrettably those ratios heavily depend on irradiation history. In addition this method allows define burnup value only in peripheral fuel rods. Burnup checking upon NPP Power Plant makes by neutron flux measurements radiated of the spent fuel assemblies. Whereas Zaporozhye NPP do not has own equipment for the neutron flux measurements, upon burnup checking use data, kindly provided by International Atomic Energy Agency and received upon measurements by the Fork Detector (Figure 2). The Fork detector is a transportable system designed to interrogate spent fuel assemblies in storage ponds and verifies attributes such as operatordeclared exposure and cooling time. The Fork detector system consists of the Fork detector head, hermetic extension pipes, what allow place the Fork detector near an assembly in storage pond and a GRAND-3 electronics unit. The Fork detector is placed around the central region of a spent fuel assembly partially raised from its storage rack (Figure 3). The Fork detector measures thermal neutron signals with a pair of bare-fission chambers, epithermal and fast neutron signals with a pair of cadmium covered fission chambers and gamma ray signals (current) with two ion chambers. The pairing of detectors on two sides of an assembly is for minimizing the sensitivity to assembly-detector geometry Execution Order of the Checking in the Storage Container Loading Process A spent fuel assembly after the storage in the storage pond more than five years radiates neutrons, generated in the result of the spontaneous fission of the curium (two hundred and forty four) isotopes. The Fork detector registers neutron flux, radiated from the spent fuel assembly, and neutron intensity put in the correspondence with the spent fuel assembly burnup value on the checking area. Executed at Zaporozhye NPP neutron intensity measurements of the fuel assemblies depending on assembly position in relation to the Fork Detector allowed define length of the fuel assembly area, what the Fork Detector see (Figure 4). In the measurement process the spent fuel assemblies were put between a Fork Detector jags, and then moved out on fifty, one hundred, one hundred and fifty, two hundred and two hundred and fifty millimeters from the Fork Detector aside. Results of the neutron intensity measurements of the spent fuel assemblies in these positions present on the Figure 4. Results of the neutron intensity measurements depending on distance from spent fuel assemblies to the Fork detector head 122

4 graph. On the ground of these measurements may be made conclusion, what a length of visible area of the fuel assembly is five hundred millimetres, so upon function approximation dependency of the neutron intensity from the burnup, in the capacity of burnup value must be taken average burnup of the central area of the fuel assembly on length five hundred millimetres. Order of the burnup checking upon loading of the spent fuel assemblies to the multi-assembly sealed basket (Figure 5). In the loading pit of the storage pond installed reloading container with the multi-assembly sealed basket and the Fork detector. A spent fuel assembly extracts from storage place of the storage pond and move to the measurement position. Herewith makes checking of the cell number in the storage pond, whence extracted the spent fuel assembly, number of this assembly, its enrichment and number of absorber rods what installed to the assembly. With the aid of the Fork detector define the neutron intensity for the checking area of the spent fuel assembly. By received data, directly before assembly loading to the multi-assembly sealed basket, execute burnup checking the spent fuel assembly. If the burnup checking executed successfully, then the fuel assembly load to multiassembly sealed basket by the loading scheme. Otherwise upon presence the spent fuel assembly, intended for change, an assembly what didn t pass the burnup checking unload to the storage pond, and an assembly intended for change subject to burnup checking. If the change is not provided the cell of the multi-assembly sealed basket stay empty. These actions chain makes to the ending of the loading of the multi-assembly sealed basket by the loading scheme. 4. Data Processing of the Neutron Intensity and Burnup Checking Results Since isotopes, what radiate neutrons, have a different half-life, part of the neutrons radiated Curium (two hundred and forty four) change during the cooling time and is more than ninety five percent upon the cooling time more than three years. Therefore, since part of the neutrons, radiated Curium (two hundred and forty four), dependent on the cooling time of the spent fuel assembly in the storage pond, neutron intensities at the measurement moment, for each of enrichments, recalculates to the neutron intensities at the moment of the fuel assemblies unloading from reactor to the storage pond. On account of the neutron intensities data at the moment of fuel assemblies unloading from reactor to the storage pond and on account of the calculated burnup value on the checking area with the aid of the approximation by power-mode function build approximation curve and curves what limiting permissible interval of the neutron intensities values, for each of the fuel assemblies enrichments (Figures 6-9). In the approximation curve and in curves of the permissible interval, for each of the spent fuel assemblies intended to loading to the multi assembly sealed basket, define upper and lower limiting values of the neutron intensity at the moment of assemblies unloading from reactor to the storage pond, corresponding to the assembly burnup on the checking area. Received upper and lower limiting value of the neutron intensity at the moment of the fuel assemblies unloading from reactor to the storage pond recalculate to upper and lower limiting values of the neutron intensity at the moment of the measurement. Received values of the neutron intensity are limiting at the moment of the burnup checking execution. Success criterion upon burnup checking is hit of a point in the limits of the permissible interval. If the point gets out from the interval then a spent fuel assembly doesn t load to the storage container. 5. Problems of the Method Measurements of the neutron intensity on Zaporozhye NPP make by two Fork detectors with not identical sensitivity. Therefore, that statistics, what accumulated upon measurements by one detector can not use with statistics, what accumulated by other detector; Statistical method of the burnup checking based on use enough statistics for each kind of the spent fuel assemblies. At the present time for some kinds of the spent fuel assemblies the statistics is not enough; At the present moment the neutron flux measurements are possible only on the central area of the spent fuel assembly. As burnup calculation makes for ten regions of the fuel assembly, so correctly to make the neutron flux measurements in the every region; The date processing from moment of the neutron flux measurements to moment of the spent fuel assemblies loading to the storage container takes much time. In the present time develops the program of the mathematical data processing in the on-line mode. 123

5 124 Figure 5. Burnup checking scheme

6 Figure 6. Approximation curve and experimental data for assemblies enrichment 3.00% Figure 7. Approximation curve and experimental data for assemblies enrichment 3.30% 125

7 Figure 8. Approximation curve and experimental data for assemblies enrichment 4.23% Figure 9. Approximation curve and experimental data for assemblies enrichment 4.40% 126