the boiling water reactor shown in fig1 has a reactor vessel 1 with a core containing fuel elements 2 and control rods 3 and with a plurality of main coolant circulation circuit , one such circuit being shown at 4 in the figure . each main circulation circuit has a pump 5 . the task of the pumps 5 is to see that the reactor core at all times receives sufficient cooling to maintain the core temperature within safe operating limits . in addition to the reactor vessel 1 , the reactor includes a primary circuit 6 which includes , among other things , a steam turbine 7 , a condenser 8 , pump means 9 and a preheater 10 . steam generated in the reactor core delivers its energy to the rotor of the turbine 7 and is condensed , after passage through the turbine , in the condenser 8 by heat - exchange with cooling water flowing in a circuit 11 containing a pump 12 . the condensate from the condenser 8 is fed , after preheating in the preheater 10 , back into the reactor vessel by the pump means 9 . in accordance with the invention , an autoclave 13 ( shown considerably enlarged in fig1 ) for characterization of the reactor vessel is connected in one of the coolant circulation circuits 4 , and in that way is indirectly connected to the reactor vessel 1 . alternatively , the autoclave 13 may be directly connected to the reactor vessel 1 . the outlet of the autoclave 13 may be connected directly to the reactor vessel , or indirectly thereto via the remainder of the coolant circulation circuit ( as shown in fig1 ) or via another point in the primary circuit or to a drainage system or to an exhaust . as will be clear from fig2 the autoclave , which is preferably of austenitic steel , comprises a number of tubes 14 ( test surfaces ) having different electrochemical potentials in relation to the reactor water . between the test surfaces and between these and the inlet 15 and outlet 16 of the autoclave , electrically insulating , tubular spacers 17 are arranged . the spacers 17 can be made , for example , of zirconium dioxide . in one particular case ( given by way of example ) seven tubes 14 are used and in successive order from the inlet to the outlet the test surfaces consist of nickel , chromium , platinum , graphite , quartz , austenitic stainless steel ( swedish standard 2333 ) and titanium . all the tubes 14 , the inlet 15 , the outlet 16 and the central region of each spacer 17 , have the same internal diameter as conduits 18 and 19 which provide the inlet from and the return to the main coolant circuit of the reactor . the parts 14 - 19 thus form together a coherent channel for the water with the same cross - section throughout . with this embodiment , the material transportation conditions from the water to the test surfaces are very well - defined , and therefore the deposit of products on the test surfaces is , in all essential respects , determined by the electrochemical conditions . the autoclave 13 is provided with end walls 20 and 21 , the end wall 21 being removable to allow the parts 14 and 17 to be withdrawn from the autoclave after removal of the end wall 21 . after the test surfaces 14 have been exposed for some time , for example 2 weeks , to the reactor water which is led through the tubes 14 while the reactor is in operation , that is , with shut - off valves 18a and 19a in the conduits 18 and 19 , respectively , open , the valves 18a and 19a are closed . the tubes 14 are removed from the autoclave 13 and the amount of radioactive products which have become deposited on the different test surfaces and which substantially are derived from colloidal particles with different electric charge , is determined by measuring the radioactivity of the different test surfaces . the measurement results thus obtained provide information about the properties of the reactor water under the current operating conditions of the reactor . the results can be used for a number of different purposes . by making repeated determinations of the kind described , changes in the properties of the reactor water can be continuously monitored . it can then be established , when a change in the reactor water is noted , what has caused that change and to take early steps to eliminate the cause of that change . the method and device according to the invention can , of course , also be used for investigating , from the elements detected in the reactor water , how corrosion within the reactor is affected by varying the operating conditions of the reactor . in addition to the determination of radioactive contents , the examination of the products deposited on the test surfaces also permit other chemical and physical tests to be made in order to further establish the nature of the depositions , thus making it possible to monitor the corrosion of particular parts in the reactor in more detail , such as , for example , the oxide growth on those parts made of zircaloy . a chemical analysis of the constituents of a deposition can be made to shed light on processes ( physical or chemical ) occurring in the reactor . it will be apparent that the specific embodiment described above and illustrated in fig2 can be modified in many ways within the spirit and scope of the invention as defined in the following claims .