Negative reactivity shutdown

On three occasions in Summer 1989, the reactor was stopped by automatic emergency shutdown, the negative reactivity threshold (-10 pcm) being exceeded. This reactivity variation was very fast first a minimum after 50 ms followed by an increasing oscillation, and then a decrease, caused by the control rod drop, 200 ms after the start of the transient. The first two events were thought to be spurious (a neutronic chamber fault) and the reactor was restarted. The normal plant instrumentation did not allow proper recording of the transient so following the second trip special instrumentation was instiled. After the third trip, the reactor was shut down in order to identify the cause of the events. 

It was found that the phenomenon could be explained by gas entrunment through the core, after accumulation under the diagrid. The void coefficient explained the transient loss of reactivity. After some reactor improvements related to this explanation, the reactor was allowed to restart at the end of 1989, but the event occurred again in September 1990, after 182 EFPD of operation. 

An expert conunittee was then set up, and an extensive study of all the possible phenomena was started. Also it was decided to fit the plant with special monitoring equipment including fast recording systems, and to perform tests. Around 200 data were concerned. Tests on vessel and component mock-up were also planned. 

The tests were performed with the reactor shut down, critical at zero power (since October 1991) and at 350 MWth power (for around 12 days - F ruary 1993). In the same time, checks were performed on the plant, especially on the reactor, its components and auxiliaries. Reactor tests were very satisfactory they proved the good beha our of the instrumentation, and data are now stored as reference of steady state power and emergency shutdown conditions. 

By the end of 1993, studies had not led to a clear explanation of the phenomenon false reactivity variations (a neutronic mask between core and naitronic chambers, or a spurious signal) are thought to be impossible. Among real reactivity variations, a sodium void effect or variation of the relative displacement of fuel and control rods are also thought to be impossible. There remains only a radial core volume variation, the origin of which (a pressure wave ) has not been found. 

---

Control of the core is affected by movable control rods which contain neutron absorbers soluble neutron absorbers ia the coolant, called chemical shim fixed burnable neutron absorbers and the intrinsic feature of negative reactivity coefficients. Gross changes ia fission reaction rates, as well as start-up and shutdown of the fission reactions, are effected by the control rods. In a typical PWR, ca 90 control rods are used. These, iaserted from the top of the core, contain strong neutron absorbers such as boron, cadmium, or hafnium, and are made up of a cadmium—iadium—silver alloy, clad ia stainless steel. The movement of the control rods is governed remotely by an operator ia the control room. Safety circuitry automatically iaserts the rods ia the event of an abnormal power or reactivity transient. 

Under stationary power operation conditions, the reactivity absorbed by xenon and samarium varies between two and three per cent. However, after shutdown, the reactivity of xenon may increase many times showing the well-known peak at about 11 hours. The negative reactivity due to samarium increases asymptotically up to a few per cent. 

For calculations of this type, the evaluation of the shutdown effect of the depressurization is interesting. The depressurization, in fact, causes a loss of primary liquid and a pressure decrease which increase the steam volume in the core (the void content of the core is increased) with consequent introduction of negative reactivity and shutdown of the chain reaction. These evaluations can be done taking into account that results consistent with refined calculations are obtained by assuming that the core shutdown occurs for an average void ratio a in the primary system of 30 per cent. The value of a can be calculated by the following formulae ... 

Fast shutdown Fast insertion in the nuclear reactor core of negative reactivity, thus causing the immediate stop of the fission chain reaction. 

System capable of shutdown the reactor immediately. The rest belongs to the Adjust and Control System capable to introduce enough negative reactivity to keep the reactor in shutdown mode, with appropriate safety margin, during all cooling conditions. 

A decrease in the Na density and a drop of its level in the core cause reactivity and power growth, and Na boiling may lead to a quick criticahty excursion with possible fuel failure and release of radioactivity. In the case of lead, such an accident is impossible due to the high temperature of Pb boiling, which exceeds the temperature of core failure. Besides, when the core is surrounded by a lead reflector, whose level depends on that in the core, its leaks and the core uncovering result in increase of neutron leakage, input of negative reactivity, and shutdown of the reactor (Adamov et aL 1997). 

The reactor protection system comprises of two independent, fast acting, diverse, and physically separate shutdown systems SDS-1 and SDS-2, each of which is capable of performing the above functions independently. SDS-1 comprises of 28 cadmium rods. The second system SDS-2 is capable of high speed injection of gadolinium nitrate solution (in heavy water) directly into the moderator through six horizontal nozzles. There are four cadmium control rods which can be dropped or driven into the core to supplement the negative reactivity capability of the protection system in case of prolonged shutdown. 

Possibility and simplicity of SG leaky section isolation L AT WS Increased values of negative reactivity coefficients, passive ERHR systems L Pnmary transients Greater pnmary inertia L Secondaiy transient Multichannel shutdown and RHR systems L Loss of electnc sources - Implementation of passive systems (battery power sources) L Total loss of heat sink Passive heat removal to ambient air, non-cntical L Total loss of S G feedwater non-cntical - Passive residual heat removal L Station Blackout non-cntical - Passive shutdown and RHR systems L... 

The PRISM shutdown systems are backed up by the inherent negative power reactivity feedback of the reactor core. This inherently negative reactivity feedback brings the core to a safe, stable, power state following accidents. 

It is necessary to load 31 fuel assemblies with guide tubes for control rods (GT assemblies) to gain a satisfactory shutdown margin under cold zero power shutdown conditions. The Reactor Protection System comprises two independent fast acting shutdown systems. Shutdown System-1 (SDS-1) is based on mechanical shutdown rods with boron carbide based absorbers in 31 fuel assemblies it provides sufficient negative reactivity with all rods inserted, with one maximum worth rod not available, in the cold shut down condition. Shutdown system-2 (SDS-2) is based on liquid poison injection into the moderator. 

Incorporation of several passive and inherent safety features, such as low power density in the core, good thermal characteristics of the metal fuel bonded by sodium, negative reactivity coefficients by temperature, passive shutdown heat removal by both natural circulation of the coolant and natural air draft, and a large coolant inventory are some important provisions for simplicity and robustness of the 4S design. 

The minimum negative reactivity in the reactivity control devices available for insertion should be such that the degree of subcriticality assumed in the safety analysis report can be reached immediately after shutdown from any operational state and in any relevant accident conditions. 

Element drop techniques may be used to measure large negative reactivities and are often used to directly measure shutdown margin. The basic idea is to bring the reactor to an exactly critical condition at a suitable power level then drop all control elements at once and follow the decay of the power level after the drop. 

NRC95) UWNR OTM, Reactor Physics II UWNR OTM, Miscellaneous I Shutdown margin is negative reactivity ... 

The amount of negative reactivity added on a scram is greater then the Shutdown Margin. 

Any condition which adds negative reactivity increases shutdown margin. 

The shutdown margin is the negative reactivity provided in addition to the negative reactivity necessary to maintain the reactor in a subcritical condition without time limit, with the most reactive control device removed from the core and with all experiments that can be moved or changed during operation in their most reactive condition. 

Describe the limits of positive and negative reactivity with regard to prompt criticality, delayed criticality, and minimum shutdown period. 

Shutdown margin, SDM, is the amount of negative reactivity that would be inserted into a reactor core if all rods were dropped from critical height. Technical specifications for a reactor specify the minimum value of SDM if all rods are inserted and if one rod remains stuck in the fully withdrawn position. Technical specifications also specify the maximum excess 3eactivity permitted in a core. Thus, as illustrated in Figure 6.3, a reactivity balance exists between the minimum required SDM and the maximum allowed In practice, the minimum SDM... 

If sufficient excess core reactivity is overcome the negative reactivity inserted by xenon fol shutdown, the reactor can be started up. At the time the reactor is in a somewhat depleted iodine condition formation by iodine decay is below equilibrium rates, power level is returned to the preshutdown level, full flux exists and xenon is burned out by neutron capture maximum rate. The result is a rapid drop in xenon con and reactivity as shown in Figure 8.4. The increased... 

The causes of loss of production are as follows The largest is now the plant shutdown for investigation of the cause of the negative reactivity trips. 

To enhance the negative reactivity feedback at elevated temperatures, gas expansion modules (OEMs) were added at the core periphery to compensate for the large positive Doppler feedback associated with the decreasing fuel temperature of oxide cores during fission shutdown. The gas expansion modules have a vapour space which will expand with loss of core inlet pressure to increase the core neutron leakage. 

A period of up to three days must elapse before the reactivity returns to the value it had before the shutdown. If it is a requirement that it be possible to start the reactor up again at any time during this period, a high percentage of excess reactivity has to be built into it in order to overcome the xenon transient. This is known as xenon override capacity. In normal operation this built-in reactivity excess has to be held down by a large equivalent negative reactivity in the form of control rods or some other mechanism. 

The first shutdown system (FSS) this system consists of gravity driven neutron-absorbing elements. In CAREM-25, this system provides a total negative reactivity at cold shutdown of 6880 pcm, with all rods inserted. 

Loss of heat sink in case of a total loss of feedwater to the steam generators, the RHRS is demanded, cooling the primary system, reducing reactor pressure to values lower than those of hot shutdown. In case of the hypothetical failure of FSS, reactor power is reduced due to the negative reactivity coefficients, without compromising the fuel elements. The SSS will guarantee medium and long-term reactor shutdown. 

---