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

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

lambda_{eff}= 0.1744 seconds^{-1}... see NUKEFACT #30

SUR = +3.9 DPM (T = +6.2 seconds) ... see NUKEFACT #6

Power < POAH (Intermediate Range)

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 = -0.0280The average negative reactivity rate during scram is:

delta-t of SCRAM = 1 second

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)

lambda_{eff}= 0.08 seconds^{-1}

SUR = 0 DPM (T = infinite seconds)

Power = equilibrium source multiplication (Source Range)

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 |

Using the parameters of our example gives:

33.2 |

The negative reactivity rate required for reversal of the excursion is rho-dot = -5.2x10

**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 |

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

33.4 |

Thus, the power at the instant the control rods reach the fully inserted position is 0.11 of the initial power, or P

**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 1x10^{2} 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 |

Using the parameters of our example, with a non-fission source strength of 1x10

33.6 |

The time lapse from initiation of reactor scram at the power maximum of 1x10

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

1. scram alarm activationThe 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

2. rod position indicators at zero inches / rod bottom lights on

3. negative reactor rate / reactor power decreasing

**SUMMARY**

Reactor scram:

- is an emergency action that provides rapid reactor shutdown

- terminates a power excursion at the instant inward rod motion begins

- immediately turns power because of the large negative rho-dot

- produces a prompt drop in power

- forces power to decay at the maximum possible rate

- attains equilibrium subcritical multiplication in the shortest possible time

- does not immediately reduce decay heat