[en] The paper presents the independent analyses performed by the Institute for Nuclear Research Pitesti, Romania, to evaluate some suggested improvements of the basic CANDU - Cernavoda type reactor, called here CANDU 6. The activities covered three main areas: (i) Advanced fuel cycles for PHWRs. (ii) Power uprating for the CANDU 6 reactor. (iii) Improvements of the CANDU 6 Reactor Safety. Advanced fuel cycles included higher burnup fuel research, reactor physics analyses and technical and economical evaluation. SEU, U + Pu and Thorium (OTT and SSET) fuel cycles were considered. One of the objectives of R and D programmes of INR Pitesti is the conversion of the CANDU 6 reactor to the SEU fuel cycle without major alterations to the existent system. Up to now, activities carried out are related to evaluation of the equilibrium power distribution for 0.9% SEU fueled CANDU core; fuel management schemes studies, including equilibrium conditions and refueling simulations; evaluation of the effect on heat transport conditions; fuel bundle operating conditions evaluations; reactivity worth evaluation for the reactivity devices; fuel behaviour modelling and fuel bundle design; fuel bundle experiments, including out-of-pile tests and irradiation tests in the INR TRIGA reactors. Power uprating for the CANDU 6 reactors are included: increasing number of fuel channels and changes of the refuelling scheme, for a better power flattening, with acceptable burnup penalty. Improvement of the CANDU 6 reactor safety are related to: void effect analyses, including studies for SEU fuel cycle; use of burnable absorbers in fuel etc.; core monitoring and flux mapping systems; PSA and living PSA; non-destructive examination of fuel channels; severe accidents analyses. (author). 10 refs, 1 tab

[en] A Goal Programming formulation to CANDU fuel management optimization is proposed. Four objectives are considered, respectively : the feed rate, CPPF (Channel Power Peaking Factor), BPPF (Bundle Power Peaking Factor) and CPR (Critical Power Ratio). This problem is investigated using a numerical approach to optimization established on STepMethod and the use of loss matrix. The optimization technique developed is more adequate for fuel management analysis for fissile enriched fuel cycles, in which cases the relative importance of the objectives could be modified. Numerical results are presented for 0.93% SEU fuelled CANDU 6Mk1 core and weapons-grade plutonium burning in CANDU 6Mk1 core, using standard 37 rod fuel bundle. (author). 12 refs., 3 tabs., 7 figs

[en] Commission (CNSC) mission is independent evaluation of licensees submittals in support of their safety cases. Reactor physics analysis, criticality safety, fuel and material science are among the core competencies essential to the pursuit of the CNSC mission. Although the licensees have the responsibility for safety and supporting technical analyses, the licensees and, indeed, the public expect the regulator to have capabilities in these areas that are at or very near the state-of-the-art and to use objective evidence, including advanced computational methods and accurate experimental evidence. Therefore, CNSC has enhanced over the last two decades its technical capabilities in these areas to make technically sound regulatory recommendations.

[en] The main objective of reactor physics analysis is the evaluation of flux and power distribution over the reactor core. For CANDU reactors sophisticated computer programs, such as FMDP and RFSP, were developed 20 years ago for mainframe computers. These programs were adapted to work on workstations with UNIX or DOS, but they lack a feature that could improve their use and that is 'user friendly'. For using these programs the users need to deal with a great amount of information contained in sophisticated files. To modify a model is a great challenge. First of all, it is necessary to bear in mind all the geometrical dimensions and accordingly, to modify the core model to match the new requirements. All this must be done in a line input file. For a DOS platform, using an average performance PC system, could it be possible: to represent and modify all the geometrical and physical parameters in a meaningful way, on screen, using an intuitive graphic user interface; to reduce the real time elapsed in order to perform complex fuel-management analysis 'at home'; to avoid the rewrite of the mainframe version of the program? The author's answer is a fuel-management computer package operating on PC, 3 time faster than on a CDC-Cyber 830 mainframe one (486DX/33MHz/8MbRAM) or 20 time faster (Pentium-PC), respectively. (author). 5 refs., 1 tab., 5 figs

[en] Utilization of burnable poison for the fissile enriched fueled CANDU 6 Mk1 core is investigated. The main incentives for this analysis are the reduction of void reactivity effects, the maximization of the fissile content of fresh fuel bundles, and the achievement of better power shape control, in order to preserve the power envelope of the standard 37 rod fuel bundle. The latter allows also the preservation of construction parameters of the standard core (for example: number and location of reactivity devices). It also permits the use of regular shift fueling schemes. The paper makes analyses of MOX weapons-grade plutonium and 1.2% SEU fueled CANDU 6 Mk 1 cores. (author). 6 refs., 4 tabs., 10 figs

[en] The thesis presents the working methodology and applicable solutions aiming to the optimization of burning of nuclear fuel in CANDU type nuclear power plants for both fuel cycle in currently operating plants and for future advanced nuclear fuel cycles. The first part of this thesis gives the computational methods for determination of the fuel burnup at discharge. Then the author introduces original methods for determination of power limits and discusses the use of generalized perturbation theory in comparison with the procedure of goal programming which allows the optimization of fuel management by taking into consideration a set of appropriate objectives. Subsequently, the computational methods are illustrated for the case of CANDU reactor fuelled with natural uranium and with slightly enriched uranium (SEU: 0.9% and 1.2%) and for the case of fuels containing consumable poison (35 ppm¹⁰ B). The results are shown to be very promising with respect to both burnup level (up to 22,000 MW d/t) and the decrease of void effect. In conclusion the work opens new ways in the future development of heavy water reactors regarding both nuclear safety and fuel consumption efficiency. (N. Andreescu) 75 Figs., 8 Tabs., 62 Refs

[en] The good neutron economy and the use of on-power refueling allow the CANDU reactor a wide variety of fuels and fuel cycles to be adapted to with no need of major alterations in the standard core design. A wide variety of thorium fuel cycles can be implemented in the CANDU reactor. These may be classified as (a) cycles without reprocessing, using separate fissile and fertile channels or separate fissile and fertile bundles, and (b) cycles with reprocessing or thorium conventional cycles, based on U-233 recycle and concurrently requiring an external fissile source such as enriched uranium or plutonium. Just on the line stands the self sustaining thorium cycle which, once the equilibrium is reached, no longer requires fissile addition from outside. With regard to the SSET (Self Sustaining Equilibrium Thorium) cycle the main problems reside in obtaining appropriate discharge burnup from the economic point of view and conversion rates to account for the through-cycle losses at the foreseeable technological performance of the reprocessing facilities. Three possible solutions of using thorium fuel in the CANDU reactor are examined in the paper: the use of different enrichment fueling zones, the use of a thorium blanket instead of the D20 reflector, and the use of metallic thorium fuel. The above solutions emerged from the need of obtaining high conversion rates and from that of keeping the main core parameters within the operational limits of the standard CANDU reactor. (author). 10 refs, tabs

[en] This Ph.D. thesis presents the general principles guiding the optimal management of the fuel in CANDU type reactors with slightly enriched uranium. A method is devised which is based on the specific physical characteristics of this type of reactors and makes use of the multipurpose mathematical programming satisfying economical and nuclear safety requirements. The main goal of this work was the establishing of a refueling optimal methodology at equilibrium maintaining the reactor critical during operation. It also minimizes the fuel cycle cost through minimization of the utilized fissile material and at the same time by maximizing the reactor duty time through an optimal chain of refilling operations. This work can be considered as a contribution to a future project of CANDU type reactor core based on slightly enriched uranium. 74 Figs., 9 Tabs., 62 Refs

[en] A Guaranteed Shutdown State (GSS) for a CANDU reactor is one in which the reactor will remain in a stable sub-critical state independent of reactivity perturbations caused by any possible changes in core configuration, core properties or process system failures. The usual procedure incorporated in the CANDU reactor design and specified in the operating procedures for entry into GSS is to over-poison the moderator (typically with Gadolinium nitrate solution). An additional guaranteed shutdown state for the currently in operation CANDU reactors, Rod-Based Guaranteed Shutdown State (RBGSS), using all solid rods, i.e., SDS1, MCA and adjusters, has been proposed and implemented at Pickering and Darlington reactors. The CANDU reactor designer has also considered this option and included it as a new feature in the Enhanced CANDU-6 design. The paper discusses the regulatory requirements for GSS and CNSC staff expectations related to information in support of an application for regulatory approval of RBGSS as an additional GSS in existing CANDU reactors. (authors)

[en] The primary objective of the ''in-core fuel management'' studies for the CANDU core is to determine fuel loading and fuel replacement strategies which will result in minimum total unit energy cost while operating the reactor in a safe and reliable fashion. Two types of calculations are mainly required in fuel management analyses: a) those used to determine the nominal power and burnup distributions, and b) those used to determine instantaneous distributions which include the time varying fine structure of the power distribution. A method for equilibrium power and burnup distributions determination is presented for the first type of calculations, based on computing the macroscopic cross-sections from the bundle power and burnup history. The computation model presented was programmed into the SERA 3-D code, which was developed at INPR. A series of results for the second type of calculations are presented, which were obtained by applying the random age approximation and the autorefuel methods in determining the instantaneous power distributions. Some improvements are proposed for these models on the basis of the above mentioned results. For the sake of numerical illustration a CANDU slightly enriched uranium core configuration is presented, the physics parameters of which were evaluated on the basis of fuel management analyses. (author). 10 refs, 7 figs, 3 tabs