OPEN REPORT SCK•CEN-BLG-1054 OPTIMIZED CONTROL RODS OF THE BR2 REACTOR Analytical, numerical and experimental methods used for the reactivity control of the reactor BR2 S.Kalcheva and E.Koonen September, 2007 SCK•CEN Boeretang 200 BE-2400 Mol Belgium OPEN REPORT OF THE BELGIAN NUCLEAR RESEARCH CENTRE SCK•CEN-BLG-1054 OPTIMIZED CONTROL RODS OF THE BR2 REACTOR Analytical, numerical and experimental methods used for the reactivity control of the reactor BR2 S.Kalcheva and E.Koonen September, 2007 Status: Unclassified ISSN 1379-2407 SCK•CEN Boeretang 200 BE-2400 Mol Belgium © SCK•CEN Studiecentrum voor Kernenergie Centre d’étude de l’énergie Nucléaire Boeretang 200 BE-2400 Mol Belgium Phone +32 14 33 21 11 Fax +32 14 31 50 21 http://www.sckcen.be Contact: Knowledge Centre [email protected] RESTRICTED All property rights and copyright are reserved. Any communication or reproduction of this document, and any communication or use of its content without explicit authorization is prohibited. Any infringement to this rule is illegal and entitles to claim damages from the infringer, without prejudice to any other right in case of granting a patent or registration in the field of intellectual property. SCK•CEN, Studiecentrum voor Kernenergie/Centre d'Etude de l'Energie Nucléaire Stichting van Openbaar Nut – Fondation d'Utilité Publique - Foundation of Public Utility Registered Office: Avenue Herrmann Debroux 40 – BE-1160 BRUSSEL Operational Office: Boeretang 200 – BE-2400 MOL A b s t r a c t At the present time the BR2 reactor uses Control Rods with cadmium as neutron absorbing part. The lower section of the Control Rod is a beryllium assembly cooled by light water. A capsule containing about 190 grams of cobalt granules is inserted between the lower part of the cadmium section and the upper part of the beryllium follower. Due to the burn up of the lower end of the cadmium section during the reactor operation, the presently used rods for reactivity control of the BR2 reactor have to be replaced by new ones. Considered are various types Control Rods with full active part of the following materials: cadmium (Cd), hafnium (Hf), europium oxide (Eu2O3) and gadolinium (Gd2O3). Options to decrease the burn up of the control rod material in the hot spot, such as use of stainless steel in the lower active part of the Control Rod are discussed. Comparison with the characteristics of the presently used Control Rods types is performed. The changing of the characteristics of different types Control Rods and the perturbation effects on the reactor neutronics during the BR2 fuel cycle are investigated. The burn up of the Control Rod absorbing material, total and differential control rods worth, macroscopic and effective microscopic absorption cross sections, fuel and reactivity evolution are evaluated during ~ 30 operating cycles, which is equivalent to ~ 1000 EFPD of reactor operation. The calculations are performed for the full scale 3-D heterogeneous geometry model of BR2 using the Monte Carlo burn up code MCNPX.2.6.E and MCNP&ORIGEN-S combined method. A criterion for choice of the new control rod types is presented. The main procedures for control of the BR2 reactor are revisited and modified to satisfy the new irradiation conditions. Table of Content 1. Introduction………………………………………………………………………………..p.06. 2. Function of control elements……………………………………………………………...p.10. 3. Overview of absorbing materials, used for nuclear control……………………………….p.12. 4. Nuclear Control in the BR2 reactor………………….……………………………………p.15. 5. Analytical and experimental methods for determination of CR worth ...............................p.17. 6. Calculation methodology………………….........................................................................p.52. 7. Neutronics modelling of BR2……………………………………………………………..p.55. 8. Impact of various factors on the control rod parameters in the reactor BR2……………...p.55. 9. Control rod candidate materials and design modifications………………………………..p.56. 10. Evaluation of control rod characteristics of different control rod types during 1000 EFPD of BR2 fuel cycle……………………………………………………………………………..p.57. 11. Changing of the reactor neutronics characteristics for different control rod types during BR2 fuel cycle………………………………………………………………………………………p.63. 12. Control rod effects on neutron fluxes and core power distributions……………………..p.63. 13. Comparison of activity during 1000 EFPD……………………………………………...p.65. 14. Criterion for choice of new control rods of the BR2 reactor…………………………….p.66. 15. Summary………………………………………………………………………………...p.67. 16. Proposed new control rod type – HF+AISI304 (R0=15.6 $)……………………………p.69. 17. References……………………………………………………………………………….p.70. 18. Tables……………………………………………………………………………………p.72. 19. Figures…………………………………………………………………………………...p.85. 5 1. Introduction A nuclear reactor which has to be operated at steady state conditions is initially charged with a significantly larger amount of fuel than required to achieve criticality in order to maintain critical during long time of operation. The fuel loading and fuel enrichment must be estimated to insert into the core sufficient excess reactivity to allow full power operation for a predetermined period. This excess reactivity will compensate the decrease of the multiplication factor of the system due to the negative reactivity feedback of fuel depletion, fission product poisoning and (eventually) temperature and pressure effects. However, to compensate for the excess reactivity at the beginning of an operation cycle, a certain amount of negative reactivity must be introduced into the core, which one can adjust or control by desire. This control reactivity can be used both to compensate for the excess reactivity necessary for long term core operation and also to adjust the power level of the reactor, and finally to shut down the reactor. The determination of control reactivity requirements and the choice of control rod absorbing materials for the various types of control elements is a very important aspect of nuclear reactor core design. Generally, the material selected for control rods should have a good absorption cross section for neutrons and have a long lifetime as an absorber (not burn out rapidly). Materials with very high absorption cross section may not be preferred because they may strongly disturb the neutron flux and the power in the vicinity of the rod and therefore generate big reactivity perturbations in the core. For this reason a special design of the control rod may be required for materials with very high absorption cross section. The same amount of reactivity worth can be achieved by manufacturing the control rod from material with a slightly lower cross section and by loading more of the material. This also results in a rod that does not burn out as rapidly. Another factor in control rod material selection is that materials that resonantly absorb neutrons are often preferred to those that merely have high thermal neutron absorption cross sections. Resonance neutron absorbers absorb neutrons in the epithermal energy range. The path length travelled by the epithermal neutrons in a reactor is greater than the path length travelled by thermal neutrons. Therefore, a resonance absorber absorbs neutrons that have their last collision farther from the control rod than a thermal absorber. This has the effect of making the area of influence around a resonance absorber larger than around a thermal absorber and is useful in maintaining a flatter flux profile. The ability of a control rod to absorb neutrons can be adjusted during manufacture. A control rod that is referred to as a "black" absorber absorbs essentially all incident neutrons. A "grey" absorber absorbs only a part of them. Grey rods sometimes may be preferred because they cause smaller depressions in the neutron flux and power in the vicinity of the rod. This leads to a flatter neutron flux profile and more even power distribution in the core. Materials with a very high absorption cross section may not be desired for use in a control rod, because it will burn out rapidly due to its high absorption cross section, unless the burning isotopes are transmuted into another ones having also high absorption cross section. In Table I are summarized the used control rod materials in different research reactors in the world. It is seen that the most common materials used for control rods are Hafnium and Cadmium. 1.1 Importance of the task The necessity to perform this job has been compelled by the intention to improve the existing reactivity control of the BR2 reactor core. The most commonly used elements for reactivity control in research reactors are presented by rods or plates of strong neutron absorbers (such as 6 boron, cadmium, hafnium, gadolinium, europium or combination of these materials with “grey” absorbers), which can be inserted into or withdrawn from the core. Historically, the earliest reactivity control of the BR2 reactor core has been maintained by control rods with full length made of cadmium as absorbing material. The experience has shown that the lower edge of the control rod, which is exposed to highest thermal neutron flux, is burning out under irradiation mainly due to depletion of the dominant cadmium isotope 113Cd. The neutronography analysis has shown that after about 650 EFPD the cadmium length is reduced by about ~ 295 mm for cadmium thickness ~ 2 mm. Therefore the next control rods have been made with larger cadmium thickness ~ 4 to 5 mm. This allowed prolonging the control life: for the same irradiation period