Safety analyses in support of neutron detector calibration operations at JET

• Neutron calculations to evaluate the dose rate leakage from the shields which contain the neutron source.
• The differences on calculated dose rates using different flux-to-dose conversion factors have been investigated.
• The experimental values were compared to the MCNPX calculations.

Neutron detectors in fusion devices need to be calibrated to provide the absolute neutron yield and the fusion power produced in fusion reactions. A new in situ calibration of the JET neutron detectors was recently performed using a 252Cf neutron source with intensity of about 2.7 × 108 n/s. The source was delivered to the JET facility within a transport flask and the surface radiation levels must fall within transport regulations. Some contingency scenarios required transfer of the source into special shields: the operational shield and the auxiliary shield. In this paper we describe the neutron calculations that have been carried out to evaluate the dose rate leakage from the shields which may contain the neutron source. The calculations have been performed using accurate modelling of the neutron and gamma ray emission from the 252Cf source, and from the three shields. The differences on calculated dose rates deriving from the use of different flux-to-dose conversion factors have also been investigated. A comparison of dose rates calculated and measured is presented from the bare source (in cell) and with the source within its transport flask.