Scram Transient

Safety evaluation studies have been conducted for confuming the physical phenomena and integrity of the fuel subassemblies, the core internal structures and the heat transport systems during the normal operation, scram transients and the early stage of postulated accidents. On this account, thermohydraulic experiments related to the decay heat removal by natural circulation have been carried out, and the development and validation of the thermohydraulic safety analysis codes is also in progress. 

LOCA (Primary) Loss of Primary Coolant Accident LOCA (Secondary) Secondary Pipe Rupture (water or steam) LOCA (Interfacing) Steam Generator Tube Rupture ATWS Anticipated Transients Without Scram Transients... 

If the reactor is shut down f om equilibrium by the scramming of the horizontal safety rods the short-time behavior of the reactivity transient depends on the strength of the rods inserted Several scram transient curv es are given in Figure 6.9.3 1 (See Figure C-13 of. V0L2). The rise following the... 

Ansver Yes - At equilibrium opo-ration, the iodine and xenon saturation values are well established with kr.c. .T. rates of creation, decay, a id burnout. Since the loss from graphite ccoling is a Sanction of exposure lUstory and pile atmosphere, evaluations can bo made to cal Ulate the scram recovery time. The c stect uncertainty is probably in the calibration of the rods vithdraoTi during approach to critical these are calibrated against the scram transient from time to time. 

Answer The most important fission product to reactor operation is xenon-135 because of its large absorption cross-section - about 3.2 x 10 barns - for thermal neutrons. The effects of the xenon poison transients influence reactor operation in many ways. For example, although the saturated poison effect at equilibrium operation may cause a reactivity loss of 2 to 2-l/2 k (2000-2500 c-mk) in the usual power ranges, the decay of this amount of poison in the shutdown pile will represent a total swing of 4000 to 5000 c-mk from the initial xenon-free pile at startup. This delayed action effect in xenon formation and decay is the major cause for the scram transient, minimum downtime, and turnaround problems encountered in the operation of a hi power reactor. A detailed discussion on the effects of xenon poisoning on pile reactivity may be found in Chapter IV of this series. 

Failure to achieve flow coastdown in all five loops will cause draining of the pressuriser even with four Injection pumps available for makeup as shown in Figure 0 55 Of the cases studied, this is the only one that would cauae flashing of primary coolant during the scram transient. For this reason, the scram flow coastdown signal has been made fall-safe. 

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. 

A long evolving use of PSA was for Anticipated Transients without Scram (ATWS) which extended over 15 years to culminate in NUREG-0460 which was upset by the Salem failure-to-scram incident and the subsequent SECY Letter 83-28. Other special studies have been (a) value-impact analysis (VIA.) studies of alternative containment concepts (e.g., vented containment, NUREG/CR-0165), (b) auxiliary feedwater studies, (c) analysis of DC power requirements, (d) station blackout (NUREG/CR-3220), and (e) precursors to potential core-damage accident.s (NUREG/CR-2497), to name a few of the NRC sponsored studies. 

Control of low-pressure injection during an anticipated transient without scram (ATV i Dry well steel shell to prevent melt-through in a Mark 1 containment... 

The Canadian licensing philosophy requires that each accident, together with failure of each safety system in turn, be assessed (and specified dose limits met) as part of the design and licensing process. In response, designers have provided CANDUs with two independent dedicated shutdown systems, and the likelihood of anticipated transients without scram is negligible. 

Anticipated Transients without Scram for Light Water Reactors, Vol. 1-3, December 1978. Haasl, D. F, et al., Fault Tree Handbook, January 1981. 

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). 

Plant temperature transients that have the greatest potential for causing thermal shock include excessive plant heatup and cooldown, plant scrams, plant pressure excursions outside of normal pressure bands, and loss of coolant... 

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. 

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. 

Comment As the modifications of tha electrical portion of the Pressure Monitor near completion. It will he relatively easy to detemlne whether the so-called coincidence tripping feature will reduce spurious scrams as justified In the MJA a or whether an additional problem area, that of pressure transients, m xBt he analyzed Snglneering effort In the study of origin and control of pressure transients will he the only area to produce a significant reduction of spurious scrams. To outline a program for such a study Is beyond the scope of... 

The reactor normally contains a steam-water mixture so that any fast increase in pressure produces steam condensation, an increase of the water mass present and, because of the negative void coefficient for safety reasons, an increase in the core reactivity. It is easily seen that in a BWR the ATWS accident (transients with failure to scram) is particularly serious and represents one of the dominant severe accidents in overall risk evaluations. An accident caused by the spurious and complete closure of isolation valves on steam... 

The presence of a signiflcant neutron flux together with a fast shutdown actuation signal (Anticipated Transient Without Scram (ATWS) case, that is a transient with the failure of the scram to operate). 

The version of the program described here does not foresee the study of Anticipated Transients Without Scram (ATWS) or the calculation of the pressure in a water tank where the primary liquid from the depressurization system is discharged. For additional calculations of this type, the following notes and formulae may be useful. 

Although very good reliability records exist for scram excitation, some failures of the gravity-driven control rod insertion have been recognized. The failures occurred for the different reasons in particular, the cases of insertion speed reduction and incomplete insertion due to fuel assembly deformation have been reported during last ten years (see for example [3]). Besides, some failure modes may be considered which could prevent all the control rods to insert, and it was the basis for designers to analyze Anticipated Transient Without Scram events. 

Anticipated transients without scram are the most significant group of BDBAs. These include failures of the reactor protection system in addition to the following anticipated operational occurrences ... 

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