Absolute measurement of β_eff and l on weapon-grade MOX fuel at the VENUS critical facility by means of the RAPJA technique

Measurements were performed at the VENUS critical facility for the validation of neutron codes in the case of a reactor core loaded with weapon-grade plutonium. Since the effective delayed neutron fraction β_eff and the mean neutron lifetime l are two important parameters in the characterization of the dynamic behaviour of a reactor, an absolute measurement of these parameters is necessary to assure the correct validation of neutron transport codes. To meet this requirement, the RAPJA-technique was used. The RAPJA technique combines a Prompt Jump with a Rossi-Alpha measurement at the stationary subcritical level associated with the antireactivity level obtained after the Prompt Jump. The obtained results are well in accordance with calculated values for MOX-cores at VENUS. The measurements also show that uncertainties of 3 and 1.5% on respectively β_eff and l are achievable on a core loaded with weapon-grade plutonium.