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Enhancing Nuclear Safety

The severe accident research programme called Phébus FP aims at reducing uncertainties concerning the evaluation of radioactive fi ssion product releases in the event of a Pressurised Water Reactor (PWR) core meltdown, as well as improving IRSN’s expertise and crisis management capacities in this fi eld. To reach such objectives, tests reproducing the main physical phenomena governing apressurised water reactor core meltdown, including the transfer of fi ssion products from the fuel to the reactor containment and the evolution of these fi ssion products have been carried out in an experimental device that is representative of a PWR and installed in the PHEBUS reactor. Led in collaboration with numerous french and foreign partners (EDF, European Commission, United States, Japan, Canada, South Korea and Switzerland), this programme includes a total of 5 tests.

Severe accident research programmePHEBUS FP

ObjectivesA fi rst series of 3 tests (FPT-0,FPT-1 and FPT-2) made it possible to study the infl uence of the irradiation rate and the environment (oxidising or reducing) upon fuel degradation, the release and transport of fi ssion products (FP) and the behaviour of these fi ssion products in the reactor containment. These three tests were carried out using an Ag-In-Cd alloy control rod representative of Westinghouse-type PWR control rods, whose infl uence upon the behaviour of fi ssion products was demonstrated.Another test (FPT-4) focused on the ultimate phase of the accident and studying the release of low-volatile

fi ssion products and transuranics from a debris bed and a pool of molten fuel.The fi fth and last test (FPT-3) concerned the infl uence of a boron carbide (B

4C) control rod – material used both in newer

PWRs and in boiling water reactors operating in Europe and certain Russian reactors operating in Eastern Europe – upon fuel degradation and fi ssion product behaviour.The results of the Phebus FP tests provide a considerable source of information needed to improve the simulation of core meltdown accidents. Computer programmes deve-loped within the IRSN in this fi eld, such as ICARE/ CATHARE and ASTEC, benefi t greatly from such test results.

Sequence of a testA test can be divided up into two successive phases : The “ degradation ” phase lasts several hours during which the temperature of the test fuel is increased – by increasing the core power of PHEBUS – until the fuel liquefi es and material starts to relocate (between 2,300 and 2,500°C), leading to the release and transport of fi ssion products into the reactor containment. At the end of this phase, the PHEBUS reactor is shutdown.

The “ containment ” phase lasts several days during which variables of interest are measured to better unders-tand FP and structural material deposition phenomena, as well as iodine chemistry in the reactor containment.

A series of measurements illustratingthe behaviour of fi ssion productsduring a Phebus FP test.

Experimentaldevice of20 fuel rodsinstalled in thecentre of thereactor core

Phenomena studied in the Phebus FP programme

Reactor vessel

Transport of FP deposits

in the systemPWR vessel

Coredegradation

- FP emissions- generation

of H2

Absorptionof surface

iodine

Reactorcontainment

Emanation of radioactive

iodin

FP transportand deposition in the circuit

Break

Primary system

Sump

Time (s)

Flow

(µg

/s)

Tem

pera

ture

in °

C

Time (s)

Steam andradioactive

aerosolsin suspension

Num

eric

al ra

diog

raph

s

Tom

ogra

phs

l- + Gamma radiation 1/2l2

Zr + 2H2O ZrO

2 + 2H

2 + heat

Hygrogen

NotesApproximately 5 years are required to analyse and interpret the results of one test.