Measurement of the subcriticality of power-stressed reactors with an analog reactimeter

Conclusions A technique has been developed for measuring the effects of reactivity in a subcritical reactor with an analog reactimeter. It is based on the compensation of the current applied to the reactimeter input from the neutron detector. The compensation of current produced by neutrons of the subcritical multiplying assembly formalizes the algorithm for reactivity calculation, making it an adequate model of a reactor with a source and making it possible to determine the subcriticality without prior entry into the critical state. In this case the measurements are made in the presence of neutron sources characteristic of power-stressed reactors. The regular devices of the control and safety system could be used to produce unsteady variation of the neutron flux.All of this permits the proposed method to be extended to zero-power reactors and to power-stressed reactors. Once the subcriticality has been measured an analog of the neutron source is introduced into the reactimeter. This instrument measures the effects of reactivity in the subcritical state without the reactor being previously put into the critical state, monitors the entry of the reactor into the critical state by checking the reactivity, and makes all the measurements usually made with analog reactimeters. If the intensity of the source does not change during measurements (5–10 min) when chambers sensitive to rays (e.g., KNK-56 chambers) are used as neutron-flux detectors, then the accuracy of -ray compensation does not affect the results of the measurements.Translated from Atomnaya Énergiya, Vol. 45, No. 5, pp. 375–376, November, 1978.     

Experimental determination of subcriticality at subcritical PWRs in Korea

A direct subcriticality measurement system (SMS) based on the Feynman-α method has recently been developed by KEPRI. It was applied to six commercial pressurized water reactors in Korea. However, the obtained Feynman curves failed to give proper multiplication factors. The objective of these tests was to investigate the performance of the Feynman method to predict directly the subcriticality of a given subcritical reactor by using the neutron pulse counts only without any reactor perturbation in the large commercial reactors. Recently, two methods developed by Hokkaido University and Westinghouse Electric Corporation. These methods have a defect due to being based on the modified neutron source multiplication method. To overcome this defect and derive operational benefits is necessary to estimate the subcriticality of a subcritical core directly from the neutron pulse counts only. The performance of the developed SMS was verified in the Kyoto University Critical Assembly and applied to eight 1000 MWe Optimized Pressurized Water Reactors (OPR1000) in Korea. The obtained results show that the SMS based on the Feynman method can be a useful tool to estimate the reactivity of a subcritical power reactor. Although the discrimination level of the signal-processing unit in OPR1000 suffers from noise and gamma ray effects, SMS can provide good Feynman curves and effective multiplication factors. However, the SMS has failed to give the reactivity for the entire measured data set. Improving the SMS and investigating the effects of different discriminator level settings at SPU in OPR1000 will be topics for further study.     

Measurement of large negative reactivity of an accelerator-driven system in the Kyoto University Critical Assembly

Large negative reactivity of a subcritical system driven by a pulsed 14 MeV neutron source has been measured in the Kyoto University Critical Assembly. The subcriticality of the accelerator-driven system (ADS) ranged in effective multiplication factor roughly from 0.98 to 0.92, which corresponded to an operational range of an actual ADS proposed by Japan Atomic Energy Agency. As the measurement technique, pulsed neutron method, power spectral analysis for pulsed neutron source, accelerator-beam trip method were employed. From neutron count decay data obtained by the pulsed neutron experiment, not only the prompt-neutron decay constant of fundamental mode but also a higher spatial mode could be derived. The subcriticality was also determined from the fundamental decay constant. The measured cross-power spectral density consisted of a familiar correlated reactor-noise component and many uncorrelated delta-function-like peaks at the integral multiple of pulse repetition frequency. The fundamental prompt-neutron decay constant, i.e., the subcriticality determined from the latter uncorrelated peaks was consistent with that obtained by the above pulsed neutron experiment. However, the magnitude of the former correlated component was reduced with an increase in the subcriticality and eventually this component became almost white at deeply subcritical state ranging in the multiplication factor under 0.95. Consequently, the determination of the decay constant from the correlated component was impossible under such a subcritical state. As data analysis method for the beam trip experiment, both the conventional integral count method and the least-squares inverse kinetics method (LSIKM) were employed. The LSIKM analysis led to the consistent subcriticality with that obtained by the pulsed neutron experiment, while the integral count method significantly underestimated the subcriticality. This underestimation originated from a residual background count, which was maintained after the beam trip. The LSIKM was mostly not influenced by such a slight count rate.     

On the evaluation of ADS subcriticality

Based on a recent derivation of perturbation methods generally applicable for the analysis of subcritical reactors, at steady state or transient conditions, a simple procedure is described for the evaluation of their subcriticality level during normal operation. The precision of the method is shown to increase with the system approaching criticality conditions.     

Feasibility Study on Subcriticality Monitoring with a Digital Reactivity Meter

A conventional digital reactivity meter is based on a simple principle to solve inverse point reactor kinetics equations and it can monitor reactivity continuously on a real time basis. Then, feasibility was studied for a conventional digital reactivity meter to be used as a subcriticality monitor. It was necessary to overcome some problems; for example, the applicability of the point reactor kinetics equations must be verified for the system where neutron distribution is dependent on the subcriticality. We showed that the problems can be solved or can be taken into account. The subcriticality calculated by the reactivity meter might not be accurate for the measurement of the actual value of the subcriticality itself, however, it is accurate enough for the purpose of subcriticality monitoring. We believe that the monitoring on a real time basis is more important for subcriticality monitoring than the accuracy of the value of the monitored subcriticality. Based on the study, we proposed that a digital reactivity meter can be used as a subcriticality monitor.     

Optimal Parameter Ratios Guaranteeing Safety During Reactor Startup

A method and relations are proposed for determining the optimal ratio of the parameters for putting a reactor into a critical state safely: the initial subcriticality before reactor startup, the reactivity variation step, the initial pulse frequency, the initial counting rate of the number of pulses, the frequency of the spurious signal pulse, and the sensitivity of the pulsed startup channel, the neutron-flux density at the detector site, the neutron source strength, and the frequency of the pulses in the startup channel that gives the required speed of formation of the emergency signal according to the time interval as the reactor is put into a critical state. The methods and formulas for determining the optimal parameters are obtained taking into account the statistical nature of the frequency of the detector pulses and of the spurious output signal.     

Experimental study of neutron counting in a zero-power reactor driven by a neutron source inherent in highly enriched uranium fuels

Even a zero-power reactor core containing highly enriched uranium has a weak neutron source inherent in uranium 235, and consequently, a neutron counter placed closely to the core without external neutron source registers a certain counting rate. The study of the counting is very important for zero-power reactor physics experiments with a high precision. In this experimental study, first, at a shutdown state of the UTR-Kinki reactor without start-up neutron source, a pulse height distribution of output signals from a neutron proportional counter was measured to confirm that these signals resulted from neutron detections. At several subcritical states of the UTR, then, the Feynman-α analysis was carried out to confirm that the neutrons detected by the counter must be fission neutrons multiplied by fission chain reactions. The correlation amplitude measured in the Feynman-α analysis was much higher than that measured in a previous drive by start-up source. Further, it was also confirmed that the subcriticality dependence of neutron counting rate followed the source multiplication formula. This feature indicated that the one-point model was very successful in the subcritical range including the shutdown state.     

Subcriticality measurement using time-domain decomposition-based integral method for simultaneous reactivity and source changes

ABSTRACT

To estimate the subcriticality in dollar units for an arbitrary state-change, the time-domain decomposition-based integral method (TDDI) is proposed using the point kinetics theory based on the fundamental mode approximation. In a general transient subcritical system, reactivity, neutron source intensity, and point kinetics parameters can vary simultaneously. Furthermore, the state-change may not necessarily be a stepwise change. For such a transient, the TDDI method can estimate the subcriticality after the transient using only the time variation of the neutron count rate. Therefore, the proposed method is useful to approximately estimate the subcriticality in a system where a detailed core configuration is unknown. To investigate the applicability of the TDDI method, transient experiments with simultaneous reactivity and source changes or to two successive safety rods dropping were performed at the Kindai University Training and Research Reactor (UTR-KINKI). By comparing with reference values using excess reactivity and control rod worth, it was validated that the subcriticality values obtained by the TDDI method better agree with the reference values than the previous integral method.     

Ways of altering the coefficients of reactivity in RBMK reactors

Conclusions In RBMK reactors there are many possibilities of acting on the coefficients of reactivity, primarily on the steam-void coefficient . Some of these methods can be implemented only by building new reactors and are irreversible (e.g., changing the lattice pitch of the fuel assemblies). Other ways of great interest make it possible to operatively act on the coefficients of reactivity even in existing reactors. These include such strong, but economically acceptable, measures as keeping some auxiliary absorbers in the reactor core or increasing the operational reactivity margin as well as economically effective measures involving an increase in the density of the fuel and in the initial enrichment. An extremely great effect on the steam-void coefficient of reactivity is displayed by such operating modes as maintaining the mean water density in the reactor and the energy distribution over the height at a required level.Translated from Atomnaya Énergiya, Vol. 46, No. 6, pp. 386–389, June, 1979.     

On-line subcriticality measurement using a pulsed spallation neutron source

To investigate the applicability of the pulsed neutron source method using a pulsed spallation neutron source for an on-line subcriticality monitoring system of an accelerator-driven system, a subcritical experiment is conducted using the Kyoto University Critical Assembly in combination with the fixed-field alternating gradient accelerator. Reactivity values obtained from different traditional techniques, the area-ratio method and the α-fitting method, are discussed with respect to the applicability to on-line subcriticality monitoring. The results show that the area-ratio method robustly and accurately monitors subcriticality in shallow subcritical states with negative reactivity of up to a few dollars; however, this method faces problems with temporal fluctuations, spatial dispersion, and sensitivity to the proton-beam current with increasing depth of subcriticality. On the other hand, it is shown that the α-fitting method alleviates such problems in deep subcritical states. Moreover, a proposed fitting technique using the maximum-likelihood estimation method based on the Poisson distribution is robust enough to be applicable to on-line subcriticality monitoring for negative reactivity levels of up to roughly nine dollars.     

Space-Dependent Effect Observed in Subcriticality Measurements for Loosely Coupled-Core System

The results of the subcriticality measurements by various techniques performed for the UTR-KINKI reactor, a light-water-moderated and graphite-reflected coupled-core reactor, are presented. The result of the source-multiplication measurement indicates that the apparent dependence of the subcriticality on the detector position is significantly observed even under a condition 2$ subcritical, and that the location of neutron source remarkably influences the sub- criticality obtained. On the other hand, the spatial dependence in the source-jerk measurement is slight relative to that in the source-multiplication and the rod drop measurements. Furthermore, the result of the Feynman-α measurement suggests that much more samples should be acquired for the reduction of the experimental uncertainty to the same level as that in the above measurements.     

Neutronic design of a sub-critical, source driven reactor at the RA-8 facility

We present a LEU-ADS design based on an existing Argentine experimental facility, the RA-8 pool type zero power reactor. The versatility of this reactor allows measurement of different core configurations using different fuel enrichment, burnable poison rods, water perturbations, different control rods types in critical or subcritical configurations with an external source.To assess the feasibility of the LEU-ADS, multiplication factors, kinetic parameters, spectra, and time flux evolution were computed. Two external sources were considered: an isotopic source, and a D-D pulsed neutron source.Parameters for different core configurations were calculated, and the feasibility of using continuous and pulsed neutron sources was verified.     

Reactivity Behavior During RBMK Startup

During startup of an RBMK reactor, the reactivity varies from –(4–7)_eff to 0–0.1_eff. Positive reactivity is introduced locally – by extracting control rods. Since the physical dimensions of an RBMK reactor are large, a local change in the properties can produce a large change in the spatial distribution of the neutron flux in the core. The possible range of variation of the reactivity of a subcritical and a critical reactor with one control rod extracted is analyzed for the actual states of the power-generating units of a nuclear power plant with RBMK reactors. It is shown that the extraction of some rods in an RBMK reactor in subcritical and critical states can increase the reactivity by 1_eff or more.     

Determination of critical mass and neutron flux distribution by means of physical models

The design of nuclear reactors, especially new reactors, requires experimental measurements in order to obtain accurate values of the pertinent parameters. In the present paper we present a new method for the preliminary determination of the critical mass of a reactor and the neutron flux distribution; this method is based on the use of physical models. In carrying out these experiments use is made of a model of the reactor which does not contain fissionable material. The working channels in the model are filled with a neutron absorber whose cross section simulates the absorption cross section for neutrons in the fissionable material. The production of fast fission neutrons is simulated by means of a neutron source which is moved along the channels. The distribution of thermal neutrons is measured by means of detectors which are sensitive to thermal neutrons. If the source strength and the absolute value of the neutron flux are known, it is possible to find the critical mass of the reactor.This method has been checked in a reactor with uranium hexafluoride. The value of the critical mass found experimentally was found to be in good agreement with the value obtained when the reactor was started up.The proposed method can also be useful in preliminary investigations of reactor designs, the choice of optimum lattice parameters, etc. The technique is extremely simple and does not require fissionable material or high neutron fluxes.     

Conditions for effective utilization of the electronuclear blanket system as a neutron source

The efficiency of an intermediate-energy electronuclear setup with a blanket as an alternative source of neutrons is discussed on the basis of experience in designing the electronuclear neutron generator at the Institute of Theoretical and Experimental Physics. A classification of electronuclear setups is introduced. The factors determining the efficiency of the driver-target-multiplying blanket scheme at low and intermediate driver energies are examined. To obtain high neutron fluxes, the possibility of compensating an inadequate driver current by decreasing the subcriticality store is examined and the conditions for realizing such a possibility while preserving the fundamental requirement of nuclear safety are formulated. The concept of the criterion of dynamical safety, in contrast to the criterion of static safety ordinarily assumed for subcritical systems, is introduced. A program of precision investigations for studying operating regimes of a blanket under conditions of a low store of subcriticality is formulated on the basis of the criterion of dynamical safety. The results of an implementation of this program are important for the assessment of the technological possibility and desirability of replacing research reactors with subcritical setups based on an accelerator. 2 figures, 6 references. State Science Center of the Russian Federation-Institute of Theoretical and Experimental Physics. Translated from Atomnaya énergiya, Vol. 87, No. 4, pp. 255–262, October, 1999.     

On the multiplication factor and reactivity definitions for subcritical reactor systems

An analysis is made on the merit of different functions adopted for weighting neutron processes in subcritical nuclear reactor systems, as it appears in expressions of relevant integral quantities, such as reactivity worths, prompt neutron lifetimes, etc. All weight functions may be shown to depend on some sort of explicit or implicit, real or fictitious, system control. Associated with the importance function relevant to the reactor power control, the multiplication factor k_sub and generalized reactivity values ρ_gen are defined. The difference (1-k_sub)/k_sub is shown to be the more appropriate index for generally representing the ADS subcriticality. However, in certain circumstances, when an accidental event is studied in which the criticality condition may be surpassed during the transient, it appears more appropriate to take into account the standard multiplication factor (k_eff) and the reactivity values to which the transient is associated and for the definition of which the standard adjoint flux is adopted.     

Spatiotemporal Energy Distribution in a Pulsed Coupled Reactor System

The basic characteristics of nonstationary neutron transport in a subcritical component of a coupled pulsed reactor system are studied. The investigations are performed on the reactor-laser setup of stand B. The existence of a phenomenon which is unusual for nuclear reactors – a spherical wave of fission – in the subcritical assembly is confirmed experimentally. The basic characteristics of this wave are determined experimentally and confirmed computationally for a three-core pulsed reactor–laser system.     

A new version of the reflected core inhour equation and its solution

The study of the neutron evolution in nuclear systems is a challenging mathematical and physical problem since it constitutes a fundamental topic in the design of critical and subcritical reactors. The object of this paper is to derive a mathematical framework to characterize a new version of the two-point inhour equation to determine its roots for reflected reactors. To attain such scope, an analytical approach is always used for analyzing the diffusion theory model with one energy group of neutron and G delayed family of precursors. The modified two-point inhour equation is deduced and an analysis of its roots is performed taking into account the ability to modify source reactivity variation. In addition, the work proposed a number of delayed neutron models using more than the traditional six groups, where the effect of the reflector is considered mathematically equivalent to an additional delayed pseudo-neutron group. For the case of source reactivity variation an asymptotic reactor period is deduced as a solution of this modified inhour equation. The exact solution of the two-point reactor kinetic equations is obtained; in which each transient in the neutron population is a linear combination of exponential functions at different time constants, their respective contributions depends on their previous history. Finally, the numerical results obtained with these algorithms are applied and verified for four cases of reflected reactors.     

Cascade subcritical liquid-salt reactor for burning transplutonium actinides

A cascade subcritical liquid-salt reactor designed for burning long-lived components of the radioactive wastes of the nuclear fuel cycle is examined. The cascade scheme of the reactor makes it possible to decrease by a factor of three the power of the driving accelerator as compared with conventional accelerator-blanket systems of equal power. The fuel composition of the reactor consists of 20% Np, Am, Cm, and other transplutonium elements and 80% plutonium, which are dissolved in a salt melt NaF(50%)-ZrF₄(50%). For a 10 MW proton accelerator, 1 GeV proton energy (10 mA current) and subcriticality depth 0.05, the thermal power of the reactor is 800 MW, which permits burning ∼70 kg/yr Np, Am, Cm, and other transplutonium actinides, i.e., service five VVéR type reactors of equal power. __________ Translated from Atomnaya énergiya, Vol. 101, No. 2, pp. 116–125, August, 2006.