5.58. For the management of severe accidents, appropriate instrumentation and procedures should be available to guide operator actions to initiate preventive or mitigatory measures. The instrumentation necessary for the management of severe accidents falls into four categories:

(1) Instrumentation for monitoring the general conditions in the containment;

(2) Instrumentation for monitoring the progression in the values of parameters of interest, specifically in relation to severe accidents;

(3) Instrumentation necessary for operators to execute emergency procedures;

(4) Instrumentation for assessing radiological consequences.

5.59. During and following a severe accident, in order to follow the general conditions in the containment and to facilitate the use of guidelines for the management of severe accidents, essential parameters for the containment such as pressures, temperatures, hydrogen concentrations, water levels and radiation dose rates should be monitored.

5.60. To follow the progression in the values of parameters specific to severe accidents, consideration should be given to the installation of instrumentation to measure the following parameters:

— The status of core depressurization devices (such as relief valves) for the early indication of possible high pressure melting of the core;

— The concentration of combustible gases, in order to assess the likelihood of fast deflagration or detonation;

— Pressure and temperature signals over a wide range, in order to detect possible late failure of the containment;

— The sump water level, as an indication of the amount of water available for long term injection into the core and for containment spraying.

5.61. In order to execute emergency procedures, the operator should have available controls and instrumentation for the containment systems provided specifically for the prevention and mitigation of severe accidents. These may include, for example:

— A filtered venting system;

— A monitoring and control system for combustible gases.

5.62. An assessment of the radiological consequences of a possible severe accident should be conducted in a timely manner to assist in decisions on long term actions for the protection of the population (off-site emergency measures). Instruments for assessing radiological consequences may include:

— Dose rate meters in the containment and in peripheral buildings housing systems that have interfaces with the primary systems;

— Instruments for monitoring conditions in the containment sump water (e. g. temperature and pH);

— Activity monitors for noble gases, iodine and aerosols in the stack(s).

5.63. The larger uncertainties with regard to conditions in the containment following a severe accident should be taken into account by means of appropriate margins in the ranges of operation of the instrumentation, in the domain for which its survivability is demonstrated and/or through protective measures for the instruments (such as shielding). Owing to these uncertainties and the different parameters that it may be necessary to monitor during severe accidents, it may or may not be possible under severe accident conditions to use the instrumentation provided for use in design basis accidents. If instrumen­tation provided for use in design basis accidents is intended to be used in severe accidents, the survivability domain of the instrumentation of the containment systems should be extended as far as is practicable to cope with the containment conditions expected in severe accidents.