3.1 Description

The RA-8 critical assembly has been designed and constructed as an experimental facility to measure neutronic parameters of the CAREM NPP, under contract and supervision of the National Atomic Energy Commission (CNEA) of Argentina. It may be used, with relatively minor changes, as a facility to perform experiments for other light water reactors. It provides a reactor shielding block and reactor tanks that can be adapted to hold custom designed reactor cores.

The RA-8 critical facility is located in the PILCA IV Sector of the PILCANIYEU TECHNOLOGICAL COMPLEX, in the Province of Rio Negro, Argentina, at approximately 30 km East of San Carlos de Bariloche. It occupies the main hall of a building shared with the Laboratory for Thermalhydraulic Tests (LET), described in Section 2 of this report, and other special facilities for CAREM Project Geometry and location of core shielding inside the main hall are such that radiation dose levels are acceptable in adjacent rooms, for all operational conditions.

General Characteristics of the RA-8, are:

Low operating power, which makes cooling systems unnecessary.

Extinction systems. •

Rapid insertion of control rods Dumping of the moderator

Regulation and safety rods. There are at present 13 mechanisms to drive control rods in and out of the core. The Control System allows the definition and use of some of the control rods as Regulation Rods, and some as Safety Rods. The number of rods assigned to each function depends on the specific core being tested.

Argentine Regulatory Authority (ENREN) imposes the following requirements for the design of critical facilities:

• Negative reactivity introduced by control rods must be higher than 50% of the critical assembly reactivity excess.

• Core reactivity with control rods must be negative and higher than 3000 pcm.

• Core must remain subcritical in at least 500 pcm after extraction of the control rod of maximum negative reactivity

• Reactivity worth of control rods defined as regulation rods must be such that their insertion makes the core remain subcritic in at least 500 pcm.

• Movement of any control rod mechanism must not produce a reactivity insertion higher than 20 pcm/sec.

Operating modes. There are two possible ways of operation:

Operation by critical height. (Reactivity is determined by the moderator level surrounding the core)

Operation with control rods.( Reactivity is regulated by the amount of absorbing material introduced in the core)

Water System: water level is controlled by the RA-8 Control System. During operation, water fills simultaneously two

concentric and connected tanks. The inner tank is designed to hold the core, its structural components, and nuclear instrumentation. The water in the outer tank serves the purposes of shielding and reflector. Filling of the tanks is performed in two stages: a first stage of fast pumping, followed by a second stage of slow pumping to approach operating level. Safety Logic takes into account the position of safety and control rods to allow pumping of the moderator into the reactor tanks. Tanks are emptied by the opening of two butterfly valves, centred in the inner tank, which dumps water into the hold-up cistern, below the reactor block. It lakes no more than 4 seconds to empty the inner tank.

The water system also has provisions to add boron and to clean, drain and recirculate water. Water temperature can be varied in up to approximately 75 °С. The hold-up cistern has its own water recirculation system.

Data acquisition is done simultaneously and independently by two means: the “hard logic” Instrumentation and Control system needed to operate the facility, and the Control and Data Acquisition System, a microprocessor based system, by means of which the reactor operator will be informed of reactor and experiment related parameters.

At present the facility is being completed and the cold initial start-up is programmed for the end of the present year (1995). The core elements (Fuel Rods and absorbers) for the RA-8 are in manufacturing process and expected to be finished in the first half of next year (1996). The experimental program is foreseen to last for about one year and a half.

3.2. Experimental Program

The cores to be used for CAREM related experiments are made from fuel rods with the same radial geometry of the ones for CAREM, but shorters with a length of 80 cm. The pitch of the core was studied by calculation and is in principle the same as for the CAREM. The core calculations are made with a Diffusion Code (C1TVAP), so the size of the core has to be enough to have good results. A central homogeneous zone is needed to study perturbations as: rods loaded with different concentrations of burnable poisons, absorbing rods, guide tubes, structural materials, etc. The maximum core reactivity covering all the experiments is around 7500 pcm. To meet the requirements given by the ENREN and with the experiments, plate type absorbers made from bare Ag-ln-Cd are used. The distribution of the absorber elements is given in the following figure. The central absorber is not shown.

The studies will be conducted over different cores using two enrichments (E= 1.8 % and E= 3.4 %), some of the defined cores arc:

A. One region of E= 1.8 %.

В Two regions: inner of E= 3.4%, outer of E= 1.8%.

C. One region of E= 1.8% , perturbed with non fuel rods (guide tubes, control rods, burnable poisons) homogeneously distributed in the core.

D Two regions, the inner with E= 1.8% and perturbed with non fuel rods, the outer region with E= 3 .4%.

E. Two region, the inner with fuel rods resembling CAREM fuel elements, the outer with the needed fuel rods to reach enough reactivity to perform experiments with different configurations for the CAREM FE.

A detailed experimental program defines the experiments to be conducted with each core type. Some of the already defined measurements are:

Influence of different boron concentrations at different temperatures. Extensive properties as : critical height, critical buckling and reflector saving. Intensive properties as disadvantage factors, fission ratio (U235 & U 238), epithermal to thermal fissions ratio, epithermal to thermal absorption ratio, average spectra in fuel and moderator. Fluxes and spectra in non fuel rods (control rod guides); similar in macrocells (assembly of non fuel rods with neighbouring fuel rods). Power distribution. Mutual influence in CAREM Control Rods positioning Reactivity changes for different Boron concentrations, different temperatures and different void fractions. Determination of control rods reactivity and fuel rods with different concentrations of burnable poisons.

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3. Benchmarking

The calculation line in INVAP for the CAREM Project and the validations already done are presented.

Nuclear Data

ESIN library originated from the WIMS (1976) updated with data for Ag, In, Cd and Gd from ENDF/B-4 and Nb from WIMCAL-88 (COREA). The WIMS has been in used by INVAP with validated good results in Plate Type FE (for RR) of 20% enrichment and fresh cores. The Gd isotopes were tested using the CONDOR in two numerical benchmarks with good results; however the dispersion of results given by participants are quite wide.

( ell Code C ONDOR 1.3 /9/

A preliminary validation with 91 typical PWR cells (fresh) was done /10/ and /11/, with a difference of400±800 pem Two more benchmarks /12/ and /13/ with mini FE of PWR using burnable poisons were done with satisfactory results Recently more comparisons (49 cases) were performed with results obtained for cores similar to that of the CARJEM /14/ and /15/

Core Code CITVAP

The code was validated for plate type fuel elements of 20% enrichment with good results For 90% enriched FE the results are not as good

The calculations to validate the line are underway in order to reduce the experimental works with the RA-8

Up to this stage of CAREM Project, several design aspects in the internals are recognised that need expen mental verifications The aim of these experiments is to verify the behaviour under normal and abnormal conditions and to define the manufacturing and assembling allowances as well as handling procedures and auxiliary tools Following is a general description of the arrays under construction and the foreseen experiments for each of them

A dummy of a sector of the core containing the following items:

Core support, three FE, upper structures with control rod guides The experiments will be done with water at room temperature The aim is to make fine adjustments in the design and manufacturing and the influences of the different variables in the behaviour of the assembly Also to verify the design of couplings and auxiliary tools This stage will be started by the end of the

A 1:1 in length of a Sector of the Control Rod Drive Structure (for one Control Rod) with the Connecting Rod attached to the dummy core mentioned above and a Drive Mechanism.

The experiments will be earned out in air and in water at room temperature The objectives are to obtain the manufactunng and operational allowances

Examples of the expenments to be conducted dunng this stage are definitions related with alignment, clearances in linear bearings Dynamic Analysis to determine natural frequencies and mode shapes and responses of the system under vanous external excitations

“Warm “ Experiment. T= 80 °С, atmospheric pressure. Characterisation of the mechanism and the driving water circuit at different temperatures. Study of abnormal situations: increase in drag forces; pump failure; Primary level influence; SCRAM valve failure; uncontrolled water flow and temperature; two phases water injection; suspended particles influence; air bubbles influences; drainage blockage.

“Hot” Experiment. A simple loop is under design to reach CAREM nominal operational values in normal and abnormal conditions. The objective are the characterisation of the mechanisms, durability tests, and behaviour of systems under abnormal conditions: breakage of feeding pipes; LOCA, behaviour under relief valves actuation.