Reactor scram

The frequency of anticipated transients is addressed by EPRI NP2330, 1982 io give information on the type and frequency of initiating events that lead to reactor scram. 

In a protected loss of electric load event the reactor scrams and power drops to decay heat levels. To avoid rapid temperature changes in the IHX the control system should function to scale back... 

The safety demonstration tests in the HTTR are conducted to demonstrate an inherent safety feature, that is an excellent feature in Shutdown of the HTGRs, as well as to obtain the core and plant transient data for validation of safety analsis codes and for establishment of safety design and evaluation technologies of the HTGRs. The safety demonstration tests consist of Reactivity insertion test - control rod withdrawal test and Coolant flow reduction test as shown in Figure 6. In the control rod withdrawal test, a central pair of control rods is withdrawn and a reactivity insertion event is simulated. In the gas-circulators trip test, primary coolant flow rate is reduced to 67% and 33% of rated flow rate by running down one and two out of three gas-circulators at the Primary Pressurized Water Cooler without a reactor scram, respectively. 

One is the secondary- coolant reduction test by partial secondary loss of coolant flow in order to simulate the load change of the nuclear heat utilization system. This test will demonstrate that the both of negative reactivity feedback effect and the reactor power control system brings the reactor power safely to a stable level without a reactor scram, and that the temperature transient of the reactor core is slow in a decrease of the secondary coolant flow rate. The test will be perfonned at a rated operation and parallel-loaded operation mode. The maximum reactor power during the test will limit within 30 MW (100%). In this test, the rotation rate of the secondary helium circulator will be changed to simulate a temperature transient of the heat utilisation system in addition to cutting off the reactor-inlet temperature control system. This test will be performed under anticipated transients without reactor scram (ATWS). 

In the design of the HTTR hydrogen production system (HTTR-H2), SG is installed as the thermal absorber downstream the chemical reactor in the secondary helimn gas loop to mitigate the temperature fluctuation within 10°C at the SG outlet, because the temperature rise above 15 °C compared with the normal temperature at the reactor inlet causes the HTTR reactor scram [3],... 

The safety concept considers two nuclear shutdown systems, a set of six reflector rods for reactor scram and power control and a KLAK system of small absorber balls for cold and long-term shutdown. Decay heat removal is made via the heat exchanger, an auxiliary cooling system, and the panel cooling system inside the concrete cavern, or, in case of a failure of these systems, passively by heat transfer via the surface of the reactor vessel. 

Totally 104 failures have occurred over a total operation period. The abnormal operation events due to equipment and system failures resulted in 28 plant shutdowns of which 18 shutdowns involved reactor scrams (5 events involved manual reactor emergency shutdowns). In the remainder the plant power reductions took place. 

Some features of impurities behaviour in the BN-600 reactor primary circuit are observed during reactor transients, such as increase of temperature and power, shut-down/start-up modes of the loops, reactor scram etc. 

If at least one loop or the reactor is shut down hydrogen and nitrogen content increase is sometimes observed in the cover gas. For instance, in February 1989, when one loop was shut down followed by the reactor scram, contents of hydrogen and nitrogen increased respectively from 2 10 vol.% to 3.2 10 vol.% and from 0.03 to 0.05 vol. %. The origin of these phenomena has not yet been discovered, and studies on impurities behaviour in the reactor are going on. 

Rupture of the EDRS pipe of 50mm diameter is presumed to initiate the LOCA. The transient is shown in Fig. 7. The reactor pressure decreases rapidly immediately after rupture, and it does slowly from about 40 seconds to 520 seconds due to change of break flow phase from the water to the steam. Reactor scram is initiated by the signal of reactor pressure low level at 100 seconds. The EDRS of an intact loop starts to operate at 300 seconds. 

Incident. While increasing power, erratic neutron monitoring readings eventually led to reactor Scram . There was a partial core meltdown due to intermittent blocking of reactor coolant flow by a loose baffle in the base of the vessel. Some radiation released. 

The reactor cooling system is composed of the MCS, ACS and VCS as schematically shown in Fig. 1. The MCS is operated in normal operation condition to remove heat from the core and send it into the environment. The ACS and VCS have incorporated safety features. The ACS is initiated to operate in case of a reactor scram. Besides one out of two components of VCS has sufficient capacity to remove residual heat, the ACS is provided to cool down the core and core support structure. A helically coiled intermediate heat exchanger (IHX) whose heat-resistant material is Hastelloy-XR developed by the JAERI has been installed in S tember 1994. Nuclear heat application tests using the HTTR, are planned to be carried out, and accordingly a heat utilizaticxi system will be connected to the IHX. The fuel fabricaticm started in June 1995 and will complete in 1997. 

The temperature of the primary water, as well as the primary pressure, initially tend to increase and subsequently to decrease after the reactor scram has operated a few seconds from the start of the accident. 

In general, it is possible to ensure that the additional reactivity due to a control rod expulsion is of the order of 0.15 per cent (but, in any case, well below 0.6 per cent, which would originate a prompt criticality ). The accident reactivity excursion is mitigated by the Doppler coefficient and is terminated by the reactor scram. Roughly 10 per cent of the fuel can be damaged (DNBR < 1) and the effective whole-body doses outside the plant may reach 10-20 mSv in two hours at the edge of the exclusion area. 

At the moment of reactor scram total RCCA worth was equal to 8,7% and exceeded the calculated value (7,5%). 

The EPR harmonization process led to another concept considering the reactor scram as a pressure reducing measure, which allows to reduce the overall discharge capacity, provided the reliability and diversity of the provisions related to the reactor scram are similar to those of the protection of the core. 

PM EQA The Reactor Protection System (RPS) initiated a reactor scram as aresult of the turbine trip. All MSIVs closed due to the loss of RPS power). 

Various transients analysed are one primary or secondary or boiler feed water pump trip, one primary or secondary pump seizure, rupture of one primary pump discharge pipe, offsite power failure, uncontrolled withdrawal of a control and safety rod, total loss of feed water to SG, one primary or secondary pump acceleration from 20 % power and feed water flow increase to 125 % in one loop. Based on these studies reactor scram and LOR parameters are identified. Reactor is scrammed, i.e., by gravity drop of all control safety rods (CSR) and diverse safety rods (DSR), only for events involving fast transients and flow blockage in the core. For all the other events LOR (lowering of all the control and safety rods) is used for the reactor shutdown. The safety criteria is to ensure the availability of two diverse reactor trip parameters for every DBE (fig 9). 

The status of safety-related systems and fiinctions is presented in a similar way, in accordance with the organization of the Emergency Operation Procedures (EOP). The parameters that are of immediate interest in a disturbance situation, are presented in a direct form. This means that the reactor pressure vessel with in- and outflow connections, together with neutron flux, water level, and reactor pressure, as well as control rods fiilly in (or not), are displayed directly. Other safety functions are indicated as normal, disturbed or failed in a similar way as for the plant overview, with detailed information at the reactor operator s desk. In this context, it can be noted that the computer-based reactor scram function via the reactor protection system (RPS) has been supplemented by a scram backup system that is implemented in hard-wired equipment. 

Reactivity control system X Reactor scram function RPS, scram valves, hydraulic insertion of control rods (non-safety scram backup via el mech drives), boron mjection system... 

The PIUS plant is also provided with instrumentation systems, protection, logic, and actuation systems for reactor shutdown, residual heat removal, containment isolation, etc. in a similar way as present-day LWR plants. Their importance for ensuring safety is significantly reduced in a PIUS plant. The equipment of these instrumentation, monitoring, protection, and actuation systems is separated from that of other systems and located in separate, physically well protected compartments at the bottom of the reactor building. The reactor protection system (RPS), with a two-out-of-four coincidence logic, has the task of initiating power level reduction, reactor shutdown or reactor scram when reactor process parameters exceed set limits, in order to prevent further departure from permissible conditions. 

The remaining safety-grade functions are performed by the reactor protection system (it initiates opening of the scram valves to achieve a reactor scram), the containment isolation system (it initiates isolation of the containment by closing isolation valves), the reactor vessel safety valves (based on pressure-activated components), and the passive reactor pool cooling function. These functions are not needed for the protection of the core, however. 

A separate command centre at the site will communicate with the computers of the I C system to permit personnel in this centre to follow operations in the plant, but not affect it, with one exception initiation of a reactor scram. 

Three feed water pumps are installed which are driven by steam turbines. Two pumps are used in normal operation. Each has a flow capacity of 50% of full reactor power operation and one feed water pump trip does not cause any reactor scram. 

For heat-removal accidents, with complete loss of all means for heat removal the primary circuit remains leak-tight for approximately 5 hours (reactor scrammed). For loss-of-integrity accidents, with complete failure of water injection, there is a time reserve of approx.. 3 hours. To prevent these accidents the systems described above in item 2 may be used, as well as a special water supply system to the reactor cavity thus providing for cooling the reactor vessel from outside. 

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. 

Reactor Scram, Containment Isolation, Pressure Relief and Depressurization... 

Reactor scram is ensured by gravity driven insertion of absorber rods into the core. 

The reflector drive mechanism consists of two hydraulic systems that are used for startup and shutdown, and six motor systems that are used during normal operation. Each motor system corresponds to a reflector segment. The reflector is moved upward by the hydraulic pump for startup. For normal shutdown of the reactor, the reflector is moved down by means of the hydraulic system. For reactor scram, the scram valves of the hydraulic systems open, and cause the reflector to move downward rapidly. As the reflector goes down Im, the reactor enters the subcritical cold shutdown state. 

---