Positive scram

The reactor trip system was easily defeated by the operators and was, in the event, too slow to prevent the rapid power surge when it was finally activated. In fact the peculiarities of the control rods may have actually triggered the accident through the positive scram effect. 

The Soviets now accept that positive scram could have played a part in the accident. Cavitation in the pumps may have restricted cooling water flow, causing saturated water to flash to steam. This is a possible alternative initiating mechanism. 

The details of control rod design with an absorber and graphite follower can, under some conditions, lead to an increase in reactivity as the rods enter the core. This is known as positive scram . 

Once the scram button was pressed, the reactor power started to increase strongly because of the aforementioned characteristic of positive scram and because of the progressive increase of the amount of steam in the reactor (caused by the increase of thermal power) and because of the corresponding decrease of water (the water in this reactor is a neutron poison). 

More recent evaluations have shown that an essential contribution to the nuclear power excursion was due to the particular design of the RBMK control rods. Under certain circumstances, insertion of these rods into the reactor core (in order to initiate a reactor scram) will result in an increase in power rather than in a reduction, i. e. the nuclear chain reaction will not be terminated, but accelerated (so-called positive scram effect). 

The two mechanisms may result in substantial and characteristic differences in deuterium distribution. The metal hydride addition-elimination mechanism usually leads to a complex mixture of labeled isomers.195 198 208-210 Hydride exchange between the catalyst and the solvent may further complicate deuterium distribution. Simple repeated intramolecular 1,3 shifts, in contrast, result in deuterium scram-bling in allylic positions when the ir-allyl mechanism is operative. ... 

These control rods thus control the overall reactor power level and provide the principal means of quickly and safely shutting down the reactor. The rods are vertically moved by hydraulically actuated, locking pislou Lype drive mechanisms. The drive mechanisms perform both a positioning and latching function, and a scram function with the latter overriding any other signal (scram signifies prompt shutdown). 

In Apr 95, during a startup, when the reactor was at 7.1 MWt, power increased sharply by 450 kWt in 7 s. No control rod movement was made at this time. Reactor underwent a scram on high positive reactivity and the recorder indicated a i ike of + 10 pcm. Criticality measurements before and immediately after the incident revealed a reactivity gain of about 24 pcm. However, a reduction of this value to 14 pcm was observed upon subsequent measurements. The measurement error itself is of the order of 10 pcm. 

Both the high and low level trips cure connected into the IXX Safety Circuit. Any tvo Beckmans in the tripped position will scram the reactor. Interlocks are provided so that bypassing tvo or more Beckmans at the same time will automatically scram the reactor. The BX and BXA relays are time-delayed... 

PSA studies were carried out based on available data from various fast reactors to establish the probability of plugging in SA. Based on these studies, lowering of scram thresholds for core AT and core mean temperature (0m) from the fuel SA thermocouple was done and reactor operation continued with CCPM at 80 mm position. 

In April 1995, when reactor power was stabilized at 7.1 MWt in the process of power raising to 10 MWt, there was a sharp increase in power of around 450 kWt in 7 s and reactor underwent scram on high positive reactivity (treshold +10 pcm). The reactivity recorder also indicated a spike of 10 pcm. A permanent gain of about 13.8 pcm was observed before and after the incident. 

The positive reactivity scram is manually inhibited during power raising and made effective during steady power operation. 

Following down from the "traise and scram"< position of S-5 is seen to lead to the individual withdrawal circle, provided S-.21 is on "raise" and provided the rod seat interlock (second precaution to ensure that rod is not being raised) is actuating relay R-51. The notation S-21 is used to represent any one of the eight individual shim rod switches, S-21 to S-28. Similar notations apply to the insert interlock MI-1 and the limit switch relay R-21. These conditions on withdrawal simply imply that one is not at the same time trying to run the motors in the opposite direction, and that the rod is not already withdrawn as far as it can go. 

Avoid leaving any shim rods fully inserted. Any such rod is incapable of contributing to the effectiveness of a scram. To have full benefit of prompt scram protection, all rods should be in position to contribute some effect. >... 

Figure 1-1. Drawing of the CPI pile. Scram - this term means fast shutdown of a reactoK various explanations have been proposed for its origin. The most credited one assumes that it derives from the abbreviated name of the CPI safety rod which could be actuated by an axe. In the original design sketches of the pile, the position of the operator of the axe was indicated by SCRAM, the abbreviation of Safety Control Rod Ax Man. The designated operator was the physicist Norman Hilberry, subsequently Director of the Argonne Laboratory. His colleagues used the name Mister Scram. The drawing is courtesy of Prof. Raymond Murray.

The VSR system strength is such that a wet, xenon-free reactor can be held sub-critical by the VSR s alone,with at least 90 per cent of the system control strength effective on demand in two and a half seconds or less. The VSR s may be scrammed into the reactor at any time, even from a partially withdrawn position, The failure or malfunction cf any single rod, or group of rods, does not interfere with the operation of the remainder of the system. 

The fuel subassemblies of the core and of the blanket are placed in the discharge header mounted on the pressure chamber of the reactor vessel (fig.5). The core loading of 220 subassemblies contains fuel elements with UO2 fuel and a blanket material. More recently some MOX experimental fuel elements were also loaded into the core. There are 12 positions in the core for the rods of the control and safety system. Of the 12 control rods 2 are automatic control rods, 6 are reactivity compensators, 3 are scram rods and 1 is a temperature effect compensator. The core is surrounded by the blanket, containing depleted uranium dioxide. The axial blanket is mounted in the subassemblies of the core (Fig. 6) and the radial blanket is formed by fuel subassemblies containing blanket material. Above the core is a central column containing the control rod drives and thermocouples. The column is only as wide as necessary for the absorber rod systems it does not cover the full width of the active core zone. 

In order to initiate a reactor scram, the power supply to the drive is interrupted, thus causing the rod to drop freely into its lowest position in the bore holes of the side reflector due to gravity. Eighteen small sphere shutdown units serve to compensate the reactivity increase due to a cold, unpoisoned core. Graphite spheres with a 10% B4C content and a diameter of approx. 10 mm are used as shutdown elements. The spheres, which are stored in storage containers located above the top thermal shield and over the side reflector, drop freely into the reflector bore holes on demand. 

The Sham Ram-Scram A Positive Reactivity Change on Disassembly, 0. A. 

Closure of all main steam line isolation valves, failure of direct scram based on valve position switches, and reactor scrams from an indirect scram... 

Each steam line has two containment isolation valves, one inside and one outside the containment barrier. The isolation valves are spring-loaded pneumatic piston-operated globe valves designed to fail closed on loss of pneumatic pressure or loss of power to the pilot valves. Each valve has an air accumulator to assist in the closiue of the valve upon loss of the air supply, electrical power to the pilot valves, and failure of the loaded spring. Each valve has an independent position switch initiating a signal into the reactor protection system scram trip circuit when the valve closes. 

Position indicator probe (PIP) Scram position sensing magnet... 

Rod position is sensed by a series of sealed glass reed swifches contained within a tube inside the drive piston. Two switches are spaced every 3 in. (76 mm) with each of the dual switches feeding a separate channel. These signals are multiplexed inside the containment and transmitted to the control room. The rod position information function decodes these data and makes them available to other parts of the RC I function, to the process computer and to the operator. The detection of an invalid inpuf caused by a failed reed switch is indicated. The status of the scram valves and accumulators on the hydraulic control unit is monitored, and these data are available to the operator and the computer. 

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