LWR-PROTEUS Programme (1997 – 2005)

Integral experiments aimed at validating core analysis tool for modern BWR assemblies and PWR with higher burnup and longer operating cycles.

The increasingly important incentives of improved fuel utilisation and greater operational flexibility have led to a steady increase in the enrichments and complexity of fuel assemblies in modern Light Water Reactors (LWR). In particular, demands such as higher burn-up, longer operating cycles and power uprate programmes are leading to designs well outside the available experimental database. With this background, the primary goal of the LWR-PROTEUS experimental programme was to provide an up-to-date validation base for LWR fuel design and core analysis tools.

The programme has been divided into three phases, which are detailed below:

• Phase I dedicated to measure fission rates and reactivity worth in modern SVEA-96+ BWR fuel assemblies,
• Phase II dedicated to PWR fuel and reactivity and neutron source of burnt fuel segments (Credit Burnup), and
• Phase III dedicated to fission rates and moderator density effects in modern SVEA-96 Optima2 BWR fuel assemblies.

The LWR-PROTEUS programme was a joint effort by PSI and the Swiss Nuclear utilities with particular phases receiving additional support from specific partners. The first phase, for example, was supported by the Elektrizitäts-Gesellschaft Laufenburg (EGL) and Kernkraftwerk Leibstadt (KKL).

In Phase I, modern light water reactor fresh fuel elements were investigated via measurements of detailed power and reaction-rate distributions and reactivity effects at fuel rod level. The accuracy targets were very ambitious: ~0.5% in pin power and ~1% in reaction rate distributions, and ~1% in reactivity effects, even for reactivities in the order of a cent.

The uniqueness of the approach was that authentic power reactor fuel was first investigated in PROTEUS before its use as normal fuel in a power reactor. This approach brings not only scientific benefits, in terms of realism and transferability to power reactor features, but also a considerable economic advantage in terms of test-fuel procurement and disposal costs.

For the LWR-PROTEUS Phase I experiments, the reactor-facility was configured as a driven system containing actual BWR (Boiling Water Reactor) elements in its central test region. It essentially consisted of four different radial zones, each of which presented a different axial development:

• Test zone - Nine, full-size, Westinghouse-Atom designed BWR elements of the type SVEA-96+ were located in a 3×3 arrangement in an aluminium test-tank containing water or polyethylene as moderator. Each SVEA-96+ element comprises 96 fuel pins arranged on a square pitch, a central "diamond" water canal and four water wings separating the four sub-bundles (of 24 rods each). The U-235 enrichment varies both axially (3 enrichment zones) and radially in the approximate range 2-5 %, and some pins additionally contain Gd as a burnable poison.
• Natural uranium buffer - This served to minimise the effect of the thermal driver regions on the harder test-zone spectrum. It consists of tightly packed natural uranium metal rods in air.
• D₂O zone - This serves both as a driver region and as a neutron bridge acting to maintain sufficient thermal flux in the graphite driver, which in turn guarantees sufficient worth for the safety/shutdown rods (which are located in the graphite driver). It is fuelled with 5 % enriched UO₂ fuel rods.
• Graphite driver/reflector - This contains driver fuel, safety, shutdown and control rods and the nuclear instrumentation. The same fuel as in the D₂O driver is used.

It is important to note that, since the actual BWR elements in the test zone were some 4.5 m in length and the fuel in the driver regions somewhat less than 1 m in height, the test tank could be driven axially. This enabled investigation of the axially heterogeneous test elements along their whole length.

Thirteen test zone configurations have been studied during Phase I. On top of the reference configuration with an arrangement of 3×3 uncontrolled SVEA-96+ assemblies, the total and partial insertion of B₄C and Hf control blades, the displacement of an assembly (to simulate channel bowing) and changes in the moderation density have been studied. The configuration details are shown below.

The second phase of the LWR-PROTEUS experimental programme was dedicated to the reactor physics investigation of well characterised highly burnt fuel samples from Swiss nuclear power plants by means of reactivity measurements and chemical assays, with the purpose of complementing and extending Post-Irradiation Examinations.

The chemical assays defined the nuclide compositions of the samples and the measurements in the PROTEUS reactor provided information on the reactivity reduction with the fuel burnup. The results are being used to validate calculation methods for predicting composition and reactivity of fuel with burnup, with special emphasis on the validation of high-burnup physics modelling.

Additional reactivity measurements were performed with specially prepared samples to investigate the contributions of particular fission products and actinides.

The LWR-PROTEUS Phase II project exhibits the following salient features:

• At the same site (PSI), the performance of very accurate reactivity measurements (in PROTEUS) and chemical assays (in [[http://psi.ch/ahl][Hot-Laboratory]), with the most advanced techniques.
• A very attractive batch of PWR (Pressurized Water Reactor) samples of very high burnup (up to 120 GWd/t, initial U-235 enrichment ≤ 4.1 wt%).
• A very complete palette of actinides (17) and fission products (>26) to be analysed, completely covering present day expectations.

Facility Layout

The basic layout of the reactor facility for the Phase II experiments was the same as for LWR-PROTEUS Phase I, but with the central SVEA-96+ fuel element replaced by a special PWR test zone. The neutron spectrum at the centre of this zone was characteristic of a pressurised water reactor.

Different moderators were used in the PWR zone (light water and a mixture of light and heavy water) to simulate different reactor conditions. And a remote controlled sample changer handled the fuel samples and the various other samples and drove them through a vertical guiding tube into the centre of the PWR zone.

Experiments

The reactivity worth of a particular sample was determined by introducing/removing the sample into/out of the centre of the PWR test zone and measuring the reactivity signal of the PROTEUS reactor.

The same two experimental techniques as developed and tested during LWR-PROTEUS Phase I were employed. In the first one, a calibrated, automatic control rod was used to compensate the reactivity change caused by introduction/removal of the sample. The compensation movement was then a measure of the reactivity effect. In the second technique, compensation by the automatic control rod was disabled, and the evolution of the neutron flux density after the sample's position had been changed, was measured instead. Analysis of the measured evolution yielded the reactivity via kinetics formulas.

In order to get experimental results with low experimental errors, the sequence and exact manner in which the different samples were measured was carefully chosen.

In addition to reactivity measurements, the intrinsic neutron source of the spent fuel samples has been measured by simple integral counting and by source multiplication technique (i.e. using the intrinsic neutron source of the spent fuel samples to drive the subcritical reactor).

Measurement Programme

The experimental programme of LWR-PROTEUS Phase II comprised measurements of:

• burnt UO₂ and MOX fuel samples with burnup values reaching >100 GWd/t
• samples doped in the main fission product nuclides
• actinides-only samples
• reference and standard samples (fresh fuel, boron).

It should be noted that the measurement of the reference samples enabled appropriate transferability of the obtained results to the power reactor situation.

The reactivity measurements were performed in three test zone configurations simulating different PWR conditions:

• the cold reactor
• the reduced moderator density in a hot reactor (D₂O/H₂O mixture)
• light water poisoned with boron

Selected References

P. Grimm et al., “Burnup Calculations and Chemical Analysis of Irradiated Fuel Samples Studied in LWR-PROTEUS Phase II”, PHYSOR 2006.

H. Kröhnert, “Analysis of the nuclide composition of highly-burnt PWR fuel examined in connection with the LWR-PROTEUS Phase II project with the spectral code HELIOS,” Diplomarbeit, Institut für Wärme- und Brennstofftechnik, Technische Universität Braunschweig (August 2006).

M. F. Murphy et al., “Reactivity and neutron emission measurements of highly burnt PWR fuel rod samples” Ann. of Nucl. Energy 33 760–765 (2006).

S. Caruso et al., “Non-invasive characterization of burnup for PWR spent fuel rods with burnups > 80 GWd/t”, Proceeding of ICAPP ’06, Reno, NV USA, June 4-8, 2006.

S. Caruso et al. “Validation of ¹³⁴Cs, ¹³⁷Cs and ¹⁵⁴Eu single ratios as burnup monitors for ultra-high burnup UO₂ fuel”, accepted by Annals of Nuclear Energy.

S.Caruso, “Characterisation of high-burnup LWR fuel rods through gamma tomography,” EPFL PhD thesis N° 3762, 2007.

Phase III was similar to Phase I but investigate mainly SVEA-96 Optima2 BWR assemblies instead of SVEA-96+. SVEA-96 Optima2 assemblies have the following salient features:

• burnable poison pins,
• additional by-pass channels to flatten within-assembly pin-power distributions,
• burnable poison pins with different gadolinium enrichment,
• radial variation of fuel enrichment,
• part length fuel rods to equalise the axial power distribution

Experiments

In Phase III three types of measurements were mainly carried out: Pin power and reaction rate 3D distributions, activation foils measurements and moderator variation measurements.

The experimental procedure for the reaction rate distribution measurements was the following. After an irradiation of one hour, ten fuel pins were withdrawn from the SVEA-96 test section of the reactor and were loaded into the automatic pin scanning machine. Here they were passed, one by one, to the measuring position and the gamma-ray spectrum from the fission products and from the Np-239 in the fuel pin was recorded at several elevations. The fission product activity is proportional to the fission power generated in the fuel pin and the Np-239 activity is proportional to the rate of neutron capture in U-238 (U238 + n → U-239 → Np-239 → Pu-239). A total of about 60 pins were measured for each experimental configuration and 3D maps were produced of the power and capture distributions. Of particular interest were the reaction rate distributions near the ends of the part length pins.

Measurements of radial and azimuthal within-pin reaction rates were made using novel types of activation foils made from copper, gold, and U-238. The foils were irradiated between the UO₂ pellets in special demountable fuel pins. After irradiation, the activities of the foils were measured on an automatic sample changer. In the same way as for the scanner, the activities were analysed to produce reaction rates and U-238 capture rates. This helped to validate the fission and reaction rate maps previously obtained and also to measure the radial and azimuthal variations in the different neutronics conditions below and above the ends of the part length pins.

Special hardware allowed the on-line changing of moderator in the test region. Tanks were positioned around the pins of each quarter bundle at the PROTEUS core centre. These tanks were filled with different mixtures of light and heavy water to simulate different moderator voidage conditions. Measurements of the reactivity and the fission rate variations were made.

Selected References

F. Jatuff et al., On the accuracy of reactor physics calculations for square HPLWR fuel assemblies, Ann. Nuc. Energy 33, pp. 198-207 January 2006.

F. Jatuff et al, Effects of Void Uncertainties on Pin Power Distributions and the Void Reactivity Coefficient for a 10x10 BWR Assembly, Ann. Nuc. Energy 33, pp. 119-125 January 2006.

M. F. Murphy et al., “Fission and capture rate measurements in a SVEA-96 Optima2 BWR assembly compared with MCNPX predictions”, Physor 2006.

G. Perret, et al. “Modified conversion ratio measurements in a SVEA-96 Optima2 BWR assembly compared with MCNPX predictions”, PHYTRA Conference, Marrakesh, Morroco, 2007.

In January 2006 the last core of Phase III was installed, and this was a complete departure from the previous phases, in as much as it is a mock-up of a Super Critical Water Reactor (SCWR) assembly constructed entirely using PROTEUS fuel. Although the SCWR-type test zone provided useful insights into the physics of this Generation-IV concept, its main purpose has been to serve as a valuable test bed for measurement techniques that were used in the following programme. It is, in fact, the first core of the LIFE@PROTEUS project.