Wetted Failed Fuel Radiolysis Reference number: NNLC178

NNL are currently contracted to carry out a series of trials to measure radiolytic gas generation.

Because these measurements are to be made using irradiated material there are two significant constraints: • The quantity of material (predominantly a powder) used in each test must be minimised. • The samples and vessels must remain within a hot cell facility (see Appendix for details), therefore: o Sample lines between the text vessels and the measurement system located outside the hot cell will be long (~6-10 m). o Items in the hot cell will receive significant radiation doses and must be suitably radiation hardened. o All operations on equipment located in the hot cell (setup, operation, maintenance and dismantle) must be carried out remotely using master slave manipulators (see Appendix for examples). o Ambient temperature may vary in the range 5°C - 40°C during the year.

NNL has developed a preliminary concept for undertaking the required measurements, show in Figure 1 : Figure 1 Initial Sampling Concept Some relevant characteristics of the proposed system are: • Individual tests will be conducted on a few grams of irradiated material in high integrity vessels with a total volume of ~ 3 cm3 and a free volume of ~ 2 cm3. • Connecting pipework to be narrow-bore, high pressure piping (nominally 1.6 mm, 1/16" bore) • The gas generated, in the test and whose composition needs to be measured, will depend on the test material and may be composed of either: o Hydrogen and sub-stochiometric quantity of oxygen (the degree of sub-stoichiometry is currently unknown); o Roughly equal amounts of hydrogen, carbon dioxide, carbon monoxide and methane, with trace amounts of higher hydrocarbons. • Small quantities of Kr and He may be released from the test materials as a result of radioactive decay, and their presence would need to be accounted for. • The reference sampling process would involve: o Evacuating the manifold and isolating it. o Opening the valve for designated test vessel valve to equalize pressure with manifold following which the valve would be closed to leave a gas sample of around 50µL in the manifold. o Sweep the sample using a carrier gas to it to the measurement instrument. • An equilibrium is expected to form which will limit the overall pressure generated.

It can be assumed that the experiment will be terminated if the pressure exceeds a value which is expected to be less than 50 BarG. • Tests will be conducted for a number of source materials and test conditions, leading to between 20 and 30 simultaneous tests.

Scoping calculations suggests that • gas will be generated at a rate of ~ 10-9 moles per day. • the pressure rise will be ~1.5 x 10-5 Bar.d-1 and • the concentration of each species will increase by o ~90 ppm per week of hydrogen, or o ~20 ppm per week each of hydrogen, methane etc. • The tests may last between 3 and 12 months