The goal of this study is to gather and federate the know-how of European engineering in nuclear experimental facilities in the field of safety test instrumentation, to identify most relevant required instrumentation to develop and to make some proposals and tests of innovative instrumentation, in order to anticipate and identify the future needs for safety experiment.

The first step of the study was the determination of the “most important” sensors to develop short or medium term. Criteria of importance to be considered are : safety issues concerned ; modelled physical phenomena ; relevance of the measurement of the physical phenomenon considered ; and feasibility of the technology used for measurement.

A specific methodology has been used to quantify the importance, relevance and feasibility of each item in order to select the “best compromise” from concerned safety issue until its technical feasibility.

Firstly the four fields of interests were described. Information comes both from bibliography (authorities and institutes of safety) and interviews of experts.

1. Safety issues remain numerous. However, it quickly appeared that the limitation to LWR system and to tests requiring a neutron flux for the physical phenomenon representativeness lead to safety issues of LOCA (Loss of Coolant Accident) and RIA (Reactivity Initiated Accident). Only these two cases are considered in the following study.
2. The physical phenomena were described according to their three main locations: the fuel (energy destocking, fission gas release, expansion, relocation and dispersion), the cladding (oxidation, spalling, embrittlement, deformation and failure) and the coolant (coolability and clad to coolant heat transfer).
3. Measurements important for scientists are listed with the same location description as physical phenomena: the fuel (measuring its inner and surface temperature, pressure, gas production, motion, mass dispersion and grain sizing) ;the cladding ( measuring its inner and surface temperature, radial and axial deformation, oxidation thickness, spalling size) and the coolant (measuring its temperature, pressure, flow, time and space dry out, void fraction and gas).
4. Lastly, the technological aspect was graded according to coefficients of feasibility on the previously identified measurements taking into account specificities of safety tests (in particular kinetics, which are very different between LOCA and RIA tests).

All the items were quantified according to a weighed notation, with arguing among experts. This methodology allows identifying two instrumentations of major importance:

• the cladding surface temperature: the outer surface temperature of the cladding gathers a good compromise between scientific interest and technical feasibility. Welding techniques must be qualified and the demonstration showed that swelling or failure of the cladding are neither promoted nor prevented by welding.
• the internal rod pressure: despite a long practice on experimental devices in all MTRs, this measurement revealed some chronic drawbacks, depending on technology, such as the need for umbilical cable, a long response time, a drift under irradiation or a bad lifetime.
  

Surface cladding temperature

Rumanian institute of INR at Pitesti had formerly carried out such instrumentation by welding thermocouples (TC): the project represented an opportunity to implement this competence again. Specific weld tests of the TC wires by microplasma were carried out and this hot spot was then welded by resistive technique. Analyses showed the repeatability and the firmness of this welding.
In addition, a facility allowed testing pressurized cladding samples with welded thermocouples under a temperature transient close to LOCA conditions (initial pressure and temperature of 4 MPa and 300°C). Tests of ballooning and burst showed that on fresh cladding, this type of welding even close to the deformation zone does not skew the physical phenomenon. Tests with hydrided cladding showed feasibility in terms of welding, but other tests would be necessary to show the harmlessness of the welding with respect to the mechanical behaviour of the hydrided cladding.
The continuation of this work could be the study of tools for a welding in hot cell (on irradiated cladding), the tight routing of thermocouple wires and the thermal calibration of welded thermocouples (by pyrometry for instance).

hot spot welding	thermocouple wires welding on cladding	Burst test result

Internal Rod Pressure

Belgian institute SCK•CEN of Mol dealt with the study of internal rod pressure according to two aspects:

• the experience feedback of internal rod pressure measurement, in particular the LVDT technology pointed out in the former theoretical step. This synthesis included unpublished irradiation tests of sensors in BR2 (Mol).
• the design of a prototype aiming at sensor calibration to adjust the drift.

The results of this study show a partial knowledge of LVDT behaviour under irradiation. Specific tests (fast/thermal flux, ferromagnetic core in various materials) would allow a better prediction of LVDT drift. The prototype proposed by the SCK•CEN takes up the well known technology of institute IFE Halden again by duplicating the bellows and supplying an external gas for calibration. All the procedure of calibration is studied to reach the correction of drift and in fine a more accurate measurement.
The conclusions of this work confirm the very strong advantage of LVDT sensor not only for the pressure measurement but also for many other measurements using the LVDT technology. Preliminary work must be continued by the realization and the qualification of the proposed prototype and by an improved signal processing, these hardware and software approaches being complementary, and following studies must be carried out in a better characterization of the drop of permeability of materials under irradiation.

available and tested (in BR2) LVDT pressure sensors	LVDT signals in BR2 tests	Prototype design

As a conclusion, to address the very wide issues of the necessary developments in the field of safety tests instrumentation, an original and methodical approach led to determine two relevant tracks of development so much for safety issues and the models of scientists as the measurement and the technology of experimenters. It appears that fundamental measurement (temperature of surface fuel cladding and internal rod pressure) are more than ever necessary, with requirements of increased precision and non intrusivity. Beyond the information exchange on embedded instrumentation in MTR, the revamping of fine welding and the study of an innovating sensor’s prototype, this work exposed some R&D tracks to be carried out in short-run. Lastly, the other measurements, quoted in the study, would be relevant to develop mid-term, in particular the use of the optical fibre that by various principles of measurements reaches multiples measurements with the major advantage of non intrusivity.