NUKEFACT #33

THE ANATOMY of a REACTOR SCRAM

last update October 15, 1998
Reactor scram is used for rapid reactor shutdown ... the ultimate safety action used to prevent reactor core damage in abnormal or emergency situations, as typically associated with an over-power condition or an under-cooling condition. A scram involves the rapid insertion of substantial negative reactivity, usually via spring or hydraulic assisted injection of all control rods to the fully inserted position. Scram quickly diminishes the fission process, and substantially reduces power production. Scram brings the reactor to the minimum possible fission power level and most negative reactivity condition in the shortest time possible. Scram may be initiated automatically by the reactor protection system or manually by the reactor operator. Because reactor scram is such an important operational safety feature, it warrants special attention in the Class Room … particularly from the standpoint of key reactor behavioral responses.

For purposes of discussing the several features of reactor scram, we use a specific example as generated by The Reactor Trainer (see Figure 33.1). The horizontal axis is time ... scaled from 0 to 30 minutes. The vertical axis is scaled for the four parameters tracked on this real time graphic, color coded as follows:

blue ---- reactor rate
green --- Intermediate Range power
yellow -- primary coolant temperature
red ----- control rod position

INITIAL CONDITIONS

The initial conditions at the instant of a scram can cover a broad range of operational possibilities, from steady state at 100% rated power to a rapid excursion in power below the POAH. As the most general case, and that with specific safety implications, we utilize the rapid excursion as an example. The initial conditions are as follows:

rho = +0.0030 (constant)
lambdaeff = 0.1744 seconds-1 ... see NUKEFACT #30
SUR = +3.9 DPM (T = +6.2 seconds) ... see NUKEFACT #6
Power < POAH (Intermediate Range)

The initial positive reactivity exceeds normal operational limits ... representing an abnormal situation requiring scram. The initial reactor rate is stable because reactivity is constant.

On Figure 33.1 the initial stable rate is pegged at +2 DPM because the actual rate is off the meter scale. The scram occurs as power reaches 1x10-6 amps on the Intermediate Range, which for this model is just below the Point-of-Adding-Heat (POAH).

SCRAM PARAMETERS

The negative reactivity inserted by scram is reactor specific. Typically, scram values will range between rho = -0.0250 and rho = -0.1000. On Figure 33.1 insertion of all control rods from the initial supercritical position to the fully inserted position introduces the following reactivity change:

delta rho of SCRAM = -0.0280
delta-t of SCRAM = 1 second
The average negative reactivity rate during scram is:

delta-rho/delta-t (SCRAM) = -0.0280/1 = -280x10-4 delta-rho/second

With power below the POAH throughout the transient, no negative reactivity feedback from the Doppler coefficient, the moderator coefficient, and/or the void coefficient occurs. The only reactivity change occurring is due to control rod insertion.

FINAL CONDITIONS

With all control rods fully inserted, fission power decays to the equilibrium source multiplication level ... which is determined by the non-fission neutron source strength and by the value of the shutdown reactivity.

rho = -0.0250 (constant)
lambdaeff = 0.08 seconds-1
SUR = 0 DPM (T = infinite seconds)
Power = equilibrium source multiplication (Source Range)

EXAMPLE SCRAM

With the situation thus defined, we can now discuss detail of the reactor scram. Keep in mind that during a scram, the reactor operator has knowledge of the reactor rate and the power, but not of reactivity, reactivity rate, or the precursor decay constant. Utilizing typical values of reactivity and reactivity rate in a simple example provides the means for understanding the general physical processes associated with the scram. The following numbered items correspond to events marked by bold numbers along the Intermediate Range trace on the scram transient graphic, Figure 33.1.

1. Instantaneous halt in power excursion - termination of the power excursion occurs at the instant inward rod motion begins (Figure 33.1). This is important during rapid power excursion because fractions of a second in delay can result in a great increase in the maximum power of the transient ... and in more likelyhood of core damage. Termination of the power excursion occurs before any significant reduction in the initial supercriticality. At the power peak of 1x10-6 amps, primary temperature remains constant, indicating that scram occurred just below the Point-of-Adding-Heat.

The reason that scram action causes an immediate halt in the excursion can be explained by examining the condition for power reversal ... as defined in NUKEFACT #32, Power Reversal in the Delayed-Critical Region.

33.1

Condition for Power Reversal in the D-C Region

Using the parameters of our example gives:

33.2

The negative reactivity rate required for reversal of the excursion is rho-dot = -5.2x10-4 delta-rho/second. The average reactivity rate of the scram is -280 x 10-4 delta-rho/second, or, as is typical of scrams, far in excess of the reversal requirement. Thus, the power excursion stops at the instant that inward rod motion begins.

2. Immediate power reversal - with the very large negative reactivity rate of the scram, power reversal is simultaneous with the halt in the excursion. And the large negative rho-dot causes reactor rate to peg downscale at -1 DPM as the Intermediate Range power drops (Figure 33.1). The initial reactor rate for rho-dot = -280 x 10-4 delta-rho/second is in excess of -200 DPM.

3. Prompt drop in fission power - although a scram takes the form of an extremely rapid ramp-in of control rods, occurring over the short interval of one second, the power response can be derived using the prompt drop approximation of NUKEFACT #27, Prompt Jump is NOT Reactor Rate Response. The negative reactivity introduction is taken to be a step change that produces a prompt drop in fission power. The prompt jump (drop) factor is:

33.3

Prompt Jump Factor

For the conditions of the example, the prompt "drop" factor is:

33.4

Prompt "Drop" Factor for Example

Thus, the power at the instant the control rods reach the fully inserted position is 0.11 of the initial power, or Pf = 0.11 x 1 x 10-6 = 1.1 x 10-5 amps. As can be seen on Figure 33.1, the power drops by about one decade. Shortly after the rods reach the fully inserted position, the reactor rate moves upscale from its peg (due to elimination of negative rho-dot) but remains negative.

4. Maximum rate of decay in fission power - after the prompt drop in power, a transient reactor rate exists for about 4 minutes, as power decays through 2 decades, from the Intermediate range into the Source range, Figure 33.1. However, with constant shudown reactivity, the reactor rate then reaches a constant value of -0.32 DPM (-81 seconds) as the power continues to decrease, indicating the reactor is subcritical on a stable rate. Reactor power decays exponentially through about three decades at this rate. During this time, with all control rods fully inserted, the fission power cannot be forced to decay at a faster rate, being controlled by the slowest decaying precursor group.

5. Shortest possible time to attainment of equilibrium power - as fission power decay reaches 1x102 cps on the Source range, non-fission source neutrons begin to become a significant part of the total neutron population and the rate of power decay slows, Figure 33.1. The reactor rate begins to gradually move toward 0 DPM (infinite seconds). Finally, the power level reaches a constant 2 x 10 1 cps, being produced by equilibrium multiplication of non-fission source neutrons. The power at equilibrium subcritical multiplication is given by:

33.5

Power at Equilibrium Subcritical Multiplication

Using the parameters of our example, with a non-fission source strength of 1x108 neutrons per second gives:
33.6

Equilibrium Power for Example

The time lapse from initiation of reactor scram at the power maximum of 1x10-6 amps to attainment of equilibrium subcritical multiplication at 2.2x101 cps is about 20 minutes. This interval is dependent on power at scram initiation and on the non-fission source strength. For example if the scram had been from 100% power, or about 3 decades higher in power than the example, the decay transient would have extended for about another 10 minutes. Each decade requires approximately 3 minutes for decay, (1 decade/0.31 DPM). ... continued after Figure 33.1


FIGURE 33.1 - REACTOR SCRAM in the D-C REGION

OPERATIONAL CONSIDERATIONS

The operator has several means for determining that a reactor scram has occurred, including:

1. scram alarm activation
2. rod position indicators at zero inches / rod bottom lights on
3. negative reactor rate / reactor power decreasing
The power shown on Figure 33.1 via the Intermediate range trace is direct fission power, which as shown, typically decays to very low levels in some relatively short time interval after scram (< 30 minutes). However, there is another form of power that is generated by accumulated fission product decay that has a much slower decrease after reactor scram. This is decay heat. It is substantial (approximately 7% of the average power before scram), slow to decay, and must be taken into account in providing core cooling after reactor scram.

SUMMARY

Reactor scram:

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