The premise behind this research is the characterisation and integration of a unique detector system on board a submersible, remotely-operated vehicle (ROV) for the end purpose of fuel debris characterisation at Fukushima Daiichi. Currently, at Fukushima Daiichi. Whilst precise knowledge of the location of the core debris at Fukushima is not known it is commonly assumed that fuel has leaked through into the base of the pedestal below and it is suggested that it may have moved outside of the pedestal into the lower plenum. The flooding of the reactor floors immediately following the Fukushima accident adds an extra element of complexity for the detection system requiring it to be submersible and to hold any detector system in water-tight confinement. The research presented here focusses on the use of a CeBr₃ inorganic scintillator detector with a unique configuration of an in-built HV supply for ease of integration within an ROV in a submerged environment. The detector has been tested in several environments: small wave tank for source identification and a TRIGA reactor and a ⁶⁰Co irradiator. It is hoped that the CeBr₃ detector will constitute one component of an on-board detector payload to determine the suitability for the localisation and identification of fuel debris inside the cores at Fukushima.