The primary circuit

The primary circuit of a four-loop PWR essentially consists of the reactor pressure vessel, four steam generators and four main coolant pumps, the pressurizer and, finally, the main coolant lines which interconnect the individual components and through which the hot primary coolant is transported from the reactor pressure vessel to the steam generators (hot leg) and then back to the reactor pressure vessel (cold leg). An isometric drawing of such a primary circuit is shown in Fig. 1.1. The arrangement of the different components inside the reactor building can be seen in Fig. 1.2. Summary descriptions of the primary circuit components have been given by different authors, e. g. by Meyer (1991). 

The closure head of the reactor pressure vessel accomodates the control rod drive mechanism nozzles, the in-core instrumentation nozzles, the RPV water level control nozzles and the venting nozzle. The RPV body and closure head are joined together by means of studs and nuts. During refuelling, the RPV closure head is removed, together with the platform, closure head insulation, control drive mechanisms and the seal rings. 

The core structure supporting the reactor core is located inside the reactor pressure vessel. The lower core support structure mainly serves to support the weight of the fuel assemblies, align and position the assemblies, absorb the impact of the control rods in the event of a reactor trip, and channel the flow of coolant in the reactor pressure vessel. It comprises the lower core support with flow distribution plate, the core barrel and the core shroud with core formers, and it defines the geometry of the reactor core. It remains in the reactor pressure vessel during refu- 

I) Concrete containment 2) Containment steel shell 3) Polar crane 4) Reactor pressure vessel S) Control rod drive mechanism 6) Spent fuel pool 7) Refuelling machine 8) Steam generator 9) Pressurizer 10) Pressurizer relief tank 11) Main coolant pump 12) Main steam line 13) Feedwater line 14) Concrete shield IS) Accumulator 16) Personnel lock 17) Mate rials lock 18) Lifting gantry 19) Fresh fuel assembly storage 20) Borated water storage tank 21) Residual heat cooler 22) Component cooler 23) Safety injection pump (By courtesy of Siemens/KWU) 

The heat generated by nuclear fission in the fuel is removed by the primary coolant, which has a pressure of about 16 MPa and is completely in the liquid 

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Fig. 2. Images of the flaws detected by Augur 4.2 in the primary circuit welds...

The term channel induction furnace is appHed to those in which the energy for the process is produced in a channel of molten metal that forms the secondary circuit of an iron core transformer. The primary circuit consists of a copper cod which also encircles the core. This arrangement is quite similar to that used in a utdity transformer. Metal is heated within the loop by the passage of electric current and circulates to the hearth above to overcome the thermal losses of the furnace and provide power to melt additional metal as it is added. Figure 9 illustrates the simplest configuration of a single-channel induction melting furnace. Multiple inductors are also used for appHcations where additional power is required or increased rehabdity is necessary for continuous operation (11). 

When an HT-VT develops an inter-turn fault on the HV side, there is no appreciable rise in the primary circuit current and w hich may not be delected. But, it... 

Variable inductance (Cj) is adjusted duly to maintain a current of 1 to 5 A in the primary circuit,... 

The primary circuit is closed and filled with a special cooling fluid which, driven by a high-capacity pump, cools the magnet and the Power Mosfet banks. The cooling fluid is a commercially available product (Galden ) whose physical characteristics had been optimized for cooling electric devices wherever it is not possible to use water. It is chemically almost inert, absolutely non-toxic, and electrically non-conductive. 

Radioactive decay contributed to further heat buildup. This caused water in the primary circuit to boil. The pilot-operated relief valve on the... 

After a few years of operation there was a significant increase in radiation fields from the primary circuit piping in the Douglas Point generating station. Other water-cooled reactors around the world experienced similar effects. The principal source of the radioactivity was traced to cobalt-60, formed by neutron absorption in the natural cobalt-59 which arose from hard-facing alloys and was also present as an impurity in boiler materials such as HMDnel, and in carbon steel and other structural materials. The mechanism of this radioactivity transport was found to be corrosion of the cobalt bearing materials, transport... 

By subjecting potassium arsenate in a slowly moving stream of hydrogen to a silent electric discharge, reduction to arsenic occurs.9 Suitable conditions for this result are to apply 80 volts to the primary circuit and 15,000 volts to the reaction vessel, using a transformer instead of an induction coil the hydrogen should pass at the rate of 2 litres per hour. 

They are subject to an ASME I approval if used on the primary circuit of power boilers and superheaters. If the valve is designed according to ASME I, then the valve has a dual-ring control. 

The primary circuit can operate at temperatures of up to 400°C, whilst the motor circuit temperature will not pig. 7 High-pressure canned-motor centrifugal pump integrated into a exceed 60 to 80°C. nuclear fuel rod (Te = 360°C Q = 600 m7h H = 35 m P2 = 125 kW ... 

A reciprocating diaphragm pump (not illustrated here) feeds a sealing fluid into this circuit a sealing fluid which is compatible with the flow product, and which is fed through a threaded shaft seal into the primary circuit. This prevents solid particles from reaching the rotor compartment. 

At the same time the potential of NS, FSV and RC units was determined by potential of each of similar objects because even in case of neighboring NSs a simultaneous release of radioactivity (and, what is more, of fission products) from the primary circuit imder external impacts would be unlikely owing to several SNF safety barriers at each NS. Moreover, NS and MV waterborne storage locations are spread over different piers and bases. Magnitudes of radiation potentials of NS, CMB and RC... 

During operational life these submarines are subject to periodic maintenance operations which can include refuelling. To undertake these operations, the submarine is taken into dry dock and supported on cradles which provide seismic stability. In addition to the dock gates further protection against flooding is provided by concrete caissons. Before any refuelling operations take place, the primary circuit is decontaminated and... 

Prior to placing in dock, Lenin NIB reactors were defueled, and shielding assembly was imloaded from Reactor 1. Electromotors of the primary coolant pump were made dead, automatic power supply was cut off and sealed, drives of control rod groups and actuators of emergency protection system were dismantled and removed from both reactors. The primary circuit was filled with bi-distilled water under a pressure of 15 kgf/cm (reactors 1) and 9 kgf/cm (reactor 2). The third-circuit system was filled with atmospheric-pressme water. 

The paper addresses the aetual status and the main problems of unloading, storage and subsequent management of Spent Nuelear Fuel (SNF) of Russian Nuclear Submarines (NSs) with lead-bismuth Liquid-Metal Coolant (LMC) in the primary circuit. On this basis some topical tasks related to analysis of the sources of potential radiation releases and risk assessment are formulated and discussed. 

During 1962 - 1997 a munber of NSs with lead-bismuth LMC in the primary circuit were in operation in the Russian Navy. Reactors of such NSs developed imder scientific management of the Russian Research Center Institute for Physics and Power Engineering (RRC IPPE, below IPPE), Obninsk, fall into the category of intermediate-neutron reactors [1]. Fuel composition of LMC reactors comprises intermetallic compoimd UBen with enrichment up to 90 % dispersed over beryllium matrix. Some characteristics of such-type NSs are demonstrated in Table 1. 

The NS 900 put into operation in 1970 was the very first vessel of designs 705 and 705K. In 1972 because of failure of NS primary circuit s auxiliary pipelines and impossibility of their repair, the NS was taken out of service after expiration of only 10% of fuel lifetime in the reactor. The RC was cut out of NS, and free cavities of the primary circuit were filled with preservative agents on furfurol basis. A bitumen-layer of about 1000 mm was laid over the whole surface of the upper deck of RC including the reactor upper head. Under such condition the RC is to be stored with nuclear fuel for long within the waterborne storage center. Some characteristics of NS, design 705, are demonstrated in Tables 3, 4 and 5. 

SPI Unit 120. SPI Unit 120 of NS 105 was used at the first phase of the nuclear submarine running. In 1982 after the accident caused by ingress of the primary circuit coolant into RC, the RC comprising SPI unit was cut out of the NS vessel. Assembled with additional nose and stem buoyancy tanks, the RC was laimched and since then has been kept afloat. 

Unit 125. SPl Unit 125 was used at NS 3105 at the second phase of its running. In 1997 the NS reactor was shutdown, coolant drained, cut out of the primary circuit and "frozen". All absorber rods were completely inserted and made immobile via electric cable dismantlement, scram rods and automatic control rods being in the "frozen" alloy. The operational time of the reactor was about 15% of its lifetime. 

TABLE 10. Radioactivity of coolant of the primary circuit due to long-lived nuclides, Bq/kg... 

There is virtually no contact between the primary circuit medium and coolant in case of the first-type cracks. During operation only gaseous and volatile fission products could be released via such cracks to the primary circuit medium. The second-type cracks could cause fuel corrosion and washing out of soluble fission products (cesiiun, strontium) and fuel particles to the coolant circuit. 

Radionuclide release into the atmosphere under the normal NS dismantling process may occur from the primary circuit or gas system of pressiue compensation during the following operations (Table 3). 

Sea disposal could be carried out in the following way, similar to the approach used during the Soviet period as discussed in section 2. The reactor vessels and the primary circuits are drained for water and filled with a material like furfurol or a low melting point metallic alloy to fix the damaged fuel and the control rod and to decrease the release of activity to the environment. If need be, additional material, e.g. concrete may be added to the reactor compartment to reduce the radiation level around the submarine. At the same time the floatability of the submarine must be ensured. After these preparations the submarine is transported out to the sea to a proper place, where the submarine is disposed of by sinking. Depending on the state of the submarine, it may by towed to the disposal area or it may have to be transported in a floating dock or by some other means. 

Similar emergency situation with partial depressurization of SFA canisters and the primary circuit occurred at NS 541. Based on the results of radiation and engineering... 

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