[en] The present invention evaluates radioactivity of activated ¹⁶N or ¹⁵C in a secondary main steam system when primary coolants are leaked in a steam generator, to rapidly detect the leakage of primary coolants. That is, the concentration of radioactivity of activated primary coolants is at first determined based on the data of measuring instruments which are controlled by a nuclear reactor primary coolant system data measuring and controlling device. A concentration at a certain position is determined based on the concentration at a position of a heat transfer tube and dose rate measuring value for radioactivity of leaked activated primary system measured by a γ^- ray detector which is controlled by a reactor secondary system data measuring and controlling device. Then, the concentration at a position of the steam generator is determined. Then, the amount of leakage at the heat transfer tube is evaluated by reversely calculating the concentration at a position of the steam generator. When the result of the evaluation is abnormal, this is informed by an abnormality alarm device. (I.S.)

[en] The present invention provides a measuring method for an effective multiplication factor in a spent fuel loading subcritical system for ensuring critical safety and a neutron monitor disposal method used in the method described above. That is, when subcriticalinity of the spent fuel loading subcritical system is evaluated, a neutron generation rate, the shape and dimension of the spent fuel loading subcritical system and a group constant are applied to a fixed source mode calculation for a neutron transport/diffusion calculation. In this case, a calculation having a fission cross sectional area in a fuel region defined as zero is added. Then, an effective multiplication factor corresponding to a place where a neutron flux counting rate is actually measured is evaluated at a measuring position of the neutron monitor. Further, the neutron monitor is moved in an axial direction during a step of initial loading stage of spent fuel, to measure neutron fluxes. An axial position having a greatest reverse multiplication gradient is determined based on the value for the result of the measurement at the initial step. In the succeeding steps, a neutron monitor is arranged and fixed to the position for the measurement. (I.S.)

[en] An area for a spent fuel storage pool is sectioned into an ordinary rack area for disposing spent fuel assemblies taken out from a reactor core and a preliminary storage rack area having the same constitution as a cask for containing spent fuel assemblies. Preceding to cask-containment, the spent fuel assemblies are temporarily transferred once in the preliminary storing rack area from the ordinary rack area to ensure subcriticality and then contained in casks. In addition, those fuels having a higher burn-up degree are disposed coaxially to the central portion and those having not higher burn-up degree are disposed at the outer circumferential portion. The spent fuel assemblies can surely be contained in the casks, or the process of containing the spent fuel assemblies to the casks or the subcriticality after the containment can be evaluated thereby capable of further ensuring the subcriticality. The spent fuel assemblies can be transferred or stored safely and reliably at a good efficiency. (N.H.)

[en] The monitoring device of the present invention comprises a reactor core/reactor system data measuring and controlling device, a radioactivity concentration calculation device for activated coolants for calculating a radioactivity concentration of activated coolants in a main steam and reactor water by using an appropriate physical model, a radioactivity concentration correlation and comparison device for activated coolants for comparing correlationship with a radiation dose and an abnormality alarm device. Since radioactivity of activated primary coolants is monitored at each of positions in the reactor system and occurrence of leakage and the amount thereof from a primary circuit to a secondary circuit is monitored if the reactor has secondary circuit, integrity of the reactor system can be ensured and an abnormality can be detected rapidly. Further, radioactivity concentration of activated primary circuit coolants, represented by ¹⁶N or ¹⁵C, is always monitored at each of positions of PWR primary circuits. When a heat transfer pipe is ruptured in a steam generator, leakage of primary circuit coolants is detected rapidly, as well as the amount of the leakage can be informed. (N.H.)

[en] N-14 generated in reactor core moderators is divided and transferred to a steam system and a reactor water system when it leaves a reactor pressure vessel. The steam system is an important subject relative to the influence on circumstances at the periphery of a power plant and as a cause of radiation exposure of maintenance personnels due to sky shining of N-14 from turbine buildings. Then, the rated value of the reactor pressure is set greater than 73.1 kg/cm².g to reduce the ratio of volatile nuclides generated in a reactor water in the vicinity of the reactor core transferring to a turbine system. That is, if the reactor is controlled under pressure, since the solubility of NH₃ is increased, the concentration of N-14 in main steams is decreased, to reduce the inflown amount of N-14 and its ratio of transferring to the steam system is decreased. This can reduce the effects thereof on the circumstances at the periphery of the power plant and prevent the radiation exposure of the maintenance personnels. In addition, the thickness of shieldings at the periphery of the main steam and construction cost are reduced. (N.H.)

[en] The device of the present invention rapidly detects leakage of primary coolants due to rupture of heat transfer pipes in a steam generator of a PWR type reactor. That is, a neutron detector is disposed, as a dose rate measuring system, to a secondary main steam system, a secondary main steam pipeline, or a turbine. A calculation processing system compares the data obtained therefrom and a normal state, to judge the presence of an abnormal symptom due to leakage and calculate radioactivity concentration in the main steams at a measuring point based on the dose rate. With such procedures, if a heat transfer pipe in the steam generator should be ruptured, radioactive materials in the primary coolants reach the position of the neutron detector in several seconds. Based on the result, progress of the leakage is forecast, to estimate the scale of the ruptured portion with lapse of time. Since neutrons are an object of the measurement in the device of the present invention, the device does not undergo influence of gamma rays released from a radiation source nuclide present in natural radiation rays and the primary system. (I.S.)

[en] A shroud is disposed coaxially at the inner side of a pressure vessel, and a reactor core having substantially circular cross section is disposed in the shroud. The reactor core is constituted with a large number of fuel assemblies each having a square cross section. A cross-like control rod is inserted to the central gap of the fuel assembly at the center or a portion of the outer circumference of the reactor core. A natural uranium with less emission of fast neutrons, reprocessed and recovered uranium having not more than 1.5wt% of uranium 235, thorium or depleted uranium generated upon enrichment of uranium is used for fuels of fuel assemblies in adjacent with the pressure vessel and the shroud among fuel assemblies at the outer circumference of the reactor core. With such a constitution, the maximum value of localized fast neutron flux irradiation to the shroud and the pressure vessel is reduced. Accordingly, safety and extension of the service life of the reactor can be attained. (I.N.)

[en] In the present invention, a flow of a nuclear fuel solution flowing in a mixer-settler is accurately measured by a neutron monitor to monitor the stability of the flow. A first neutron detector d₁ is disposed near a mixer portion. A second neutron monitor d₂ is disposed at a settler portion at a position sufficiently apart from the mixer portion. In the monitoring method, assuming a counter rate for each of the monitors as C₁, C₂, and the ratio between the counter rates as C₁/C₂=R, R=C₁/C₂ is determined at a standard condition, and normality of the flow is monitored by R(counter rate) or F=(C₁ - C₂/R)/(C₁ + C₂/R). If both of C₁ and C₂/R exceed a predetermined value, an alarm is outputted. In this case, F is an off set factor. Further, if one of them is abnormal, an alarm circuit is bypassed, to inspect the circuit by a neutron source. (I.S.)

[en] Purpose: To determine and monitor the local power peaking coefficients by a method not depending on the combination of fuel types. Constitution: Representative values for the local power distribution can be obtained by determining corresponding burn-up degrees based on the burn-up degree of each of fuel assembly segments obtained in a power distribution monitor and by the interpolation and extrapolation of void coefficients. The typical values are multiplied with compensation coefficients for the control rod effect and coefficients for compensating the effect of adjacent fuel assemblies in a calculation device to obtain typical values for the present local power distribution compensated with all of the effects. Further, the calculation device compares them with typical values of the present local power distribution to obtain an aimed local power peaking coefficient as the maximum value thereof. According to the present invention, since the local power peaking coefficients can be determined not depending on the combination of the kind of fuels, if the combination of fuel assemblies is increased upon fuel change, the amount of operation therefor is not increased. (Kamimura, M.)

[en] The present invention concerns a criticality preventing device used in an extraction step for reprocessing of nuclear fuels. An acidity of nitric acid solution supplied to an extractor (a mixer settler or a pulse column), concentrations of nuclear fuel materials, such as uranium and plutonium therein and a concentration of respective ingredients in the nitric acid solution and an organic solvent are monitored by a process control measuring device. Nuclear fuel material concentration distribution calculator calculates the distribution of the nuclear fuel material concentration of the nitric acid solution and the organic solvent in the extractor based on the result of the monitoring. The nuclear fuel material concentration distribution in the extractor is inputted to an effective multiplication factor calculation device and an effective multiplication factor corresponding to each concentration is calculated. An alarm generation system compares the effective multiplication factor with a previously-incorporated effective multiplication factor upper limit value, and alarms if it exceeds the value. This can previously prevent the critical state. (I.N.)