The large amount of cold borated water in the PCRV may be used for condensation of the vapor during accidents with primary coolant boiling due to overheating or depressurization by leakage. For this purpose, the following design modifications were performed:

-the riser was locked by a cover;

-the outlets of by-pass pipes were located several meters below the level of cold borated water;

-the cross-section of by-pass pipes was increased up to 1 m2.

If depressurization or overheating the coolant occurs, these innovations cause the steam-water mixture to pass through the layer of cold borated water.

There are three types of probable leakage:

— destruction of the boron control pipe (050mm) outside the reactor vessel;

— gas pressurizer leakage due to destruction of PCRV cover design or double

(inside and outside the vessel) destruction of steam generator section leg pipe (0200mm);

— double (inside and outside the vessel) destruction of the cooling system

pipe (050mm) under the cold borated water level.

Therefore, large primary leakage is a leakage from the gas pressurizer only. If this leakage occurs, the reactor design will keep the coolant inside PCRV. The steam-water mixture will be squeezed through by-pass pipes into the volume of cold borated water and condensed there.

In the case of very large leakage, when the depressurization speed is very high, there is a probability of squeezing the steam-water mixture through the density lock too (according to a hydraulic resistance of the each channel). During this process full reverse flow in the core is possible. It may cause overheat of the cladding. However an extended cross-section of by­pass pipes allow to pass all the bulk of boiling coolant during several seconds.

Large primary leakage. The scenario of this accident is the following: — depressurizing of the cold borated water and degasing;

-propagation of the depressurization wave into the primary coolant via the by-pass pipes, penetrations in the riser cover (from the top) and the density lock (from the bottom);

-boiling of the primary coolant;

-raising of the primary coolant level up to the by-pass pipes’ position due to coolant density reduction;

-probable stopping of the primary circulation;

-squeezing the steam-water mixture through the by-pass pipes and density lock (according to a hydraulic resistance of the each channel) into the volume of cold borated water and it’s condensation;

-reactor shut-down due to negative fuel and coolant temperature reactivity feedback;

-restoration of the primary circulation;

-compensation of coolant losses by cold borated water;

-pressure reduction down to atmospheric;

-reactor’s heat removal by steam generators.

Small primary leakage. The most probable reason for small primary leakage is a external destruction of the boron control pipe (050mm), which opens into the primary coolant. There is slow depressurization of coolant during this process. The primary coolant level in the riser comes up to the by-pass pipes location, going into the cold borated water volume and condensing there. This process accelerates the depressurization and decreases the coolant outflow. Coolant losses are compensating by cold borated water flow through the density lock. It results in shutdown of the reactor. When the coolant pressure decreases to atmoshperic, the coolant outflow will be stopped. The residual amount of water is enough for the heat removal process.

The probability of leakage via pipes of the borated water cooling system is very negligible: it can be caused only by simultaneous breakage of the pipe inside- and outside the vessel. Breakage in a single place will be detected: there is a checking of pressure in the cooling system. Large losses of cold borated water are excluded by the design: the borated water coolers are located in the upper part of the volume. Therefore, loss of borated water will be limited by the volume above the damage position. When the level of borated water reduces below the rupture location, outflow of borated water will be stopped. The accident will be continued with the same scenario, described for the large leakage. The rate of depressurization will be slower. There is no stoppage of the primary circulation during this accident.

It is apparent that the containment and safety systems may be considerably simplified due to innovation in question.

Decomissioning. Another IRIS feature is a thick (about 3m) borated water layer between the core and PCRV — walls. It allows decrease in PCRY activation significantly. It makes possible decrease of the residual PCRV radio-activity up to an environmentally acceptable level and to simplify reactor decomissioning.

Radioactive waste storage. The large scale PCRV may be used as a long-time storage facility for bumt-up fuel assemblies. Hence, there is no radioactive waste with high activity outside the reactor vessel.

Siting near population sentres. The high level of safety (core melt probability about 108 per reactor*year for PIUS) makes possible siting of this reactor near population centres.