## Reactivity

In preceding chapters, the classification of states of a reactor according to the effective multiplication factor – k_{eff} was introduced. The effective multiplication factor – k_{eff} is a measure of the change in the fission neutron population from one neutron generation to the subsequent generation. But sometimes it is convenient to define the **change in the k _{eff}** alone, the change in the state, from the criticality point of view.

For these purposes reactor physics use a term called **reactivity** rather than k_{eff} to describe the change in the state of the reactor core. **The reactivity** (**ρ** or** ΔK/K**) is defined in terms of k_{eff} by the following equation:

From this equation it may be seen that **ρ** may be positive, zero, or negative. The reactivity describes the **deviation of an effective multiplication factor from unity**. For critical conditions the reactivity is equal to zero. The larger the absolute value of **reactivity** in the reactor core, the further the reactor is from **criticality**. In fact the reactivity may be used as a measure of a **reactor’s relative departure from criticality**.

It must be noted the reactivity can be calculated also according to the another formula.

This formula is widely used in neutron diffusion or neutron transport codes. The advantage of this reactivity is obvious, it is a measure of a **reactor’s relative departure** not only from criticality (k_{eff} = 1), but it can be related to any sub or supercritical state (**ln(k _{2} / k_{1})**). Another important feature arises from the

**mathematical properties of logarithm**. The logarithm of the division of k

_{2}and k

_{1}is the difference of logarithm of k

_{2}and logarithm of k

_{1}.

**ln(k**. This feature is important in case of addition and subtraction of various reactivity changes.

_{2}/ k_{1}) = ln(k_{2}) – ln(k_{1})See more: D.E.Cullen, Ch.J.Clouse, R.Procassini, R.C.Little. Static and Dynamic Criticality: Are They Different?. Lawrence Livermore National Laboratory. UCRL-TR-201506. 11/2003.

## Units of Reactivity

Mathematically, reactivity is a **dimensionless number**, but it can be expressed by various units. The most common units for **research reactors** are units normalized to the **delayed neutron fraction (e.g. cents and dollars)**, because they exactly express a departure from prompt criticality conditions.

The most common units for **power reactors** are units of **pcm** or **%ΔK/K**. The reason is simple. Units of **dollars are difficult to use**, because the normalization factor, **the effective delayed neutron fraction**, significantly **changes with the fuel burnup**. In LWRs the delayed neutron fraction decreases with fuel burnup (e.g. from **β _{eff} = 0.007** at the beginning of the cycle up to

**β**at the end of the cycle). This is due to isotopic changes in the fuel. It is simple,

_{eff}= 0.005**fresh uranium fuel**contains only

^{235}U as the fissile material, meanwhile during fuel burnup the importance of fission of

^{239}Pu increases (in some cases up to 50%). Since

^{239}Pu produces significantly less delayed neutrons (

**0.0021**for thermal fission), the resultant core delayed neutron fraction of a multiplying system decreases (it is the weighted average of the constituent delayed neutron fractions).

β_{core}= ∑ P_{i}.β_{i}

## dollar ($)

The unit of reactivity which has been **normalized** to the delayed neutron fraction. Reactivity in **dollars = ρ / βeff**. The cent is 1/100 of a dollar. This is very useful unit, because the reactivity in dollars (rather in cents) determines exactly the response of the reactor on the reactivity insertion. Conversion of dollars to pcm depends on βeff. For reactor core with β_{eff} = 0.006 (0.6%) **one dollar** is equal to about **600 pcm**. It is very important amount of reactivity, because if the reactivity of the core is one dollar, the reactor is prompt critical.

**BOC and β _{eff} = 0.006**

**k _{eff} = 0.99** ρ = (k

_{eff}– 1) / k

_{eff}= -0.01 ρ = -0.01 / 0.006 =

**-1.67 $**=

**-167 cents**

**EOC and β _{eff} = 0.005**

**k _{eff} = 0.99** ρ = (k

_{eff}– 1) / k

_{eff}= -0.01 ρ = -0.01 / 0.005 =

**-2.00 $ = -200 cents**

## %ΔK/K

The unit of reactivity in **percents** of the effective multiplication factor. For example, the subcriticality of **k _{eff} = 0,98** is equal to

**-2%**in units of

**%ΔK/K**. Since this is

**very large amount of reactivity**, these units are usually used to express significant quantities of reactivity like power defects,

**xenon worth**,

**integral worth of control rods**or

**shutdown margin**. For operational changes that affect the effective multiplication factor this unit is inappropriate, because these changes are of the lower order.

**k _{eff} = 0.99** ρ = (keff – 1) / keff = -0.01 ρ = -0.01 * 100% =

**-1 %**

## percent mille (pcm)

The unit of reactivity which is **one-thousandth** of a percent **%ΔK/K** (equal to 10^{-2}x10^{-3} = **10 ^{-5}** of k

_{eff}). The unit of

**pcm**is used at many

**LWRs**because reactivity insertion values are generally quite small and units of pcm allows reactivity to be written in

**whole numbers**. The operational changes such as control rods movement causes usually reactivity insertion of the order of units of pcm per one step. The fact that the effective delayed neutron fraction changes with the

**fuel burnup**have an important consequence. Due to the difference in

**β**a response of a reactor on the same reactivity insertion (in units of pcm) is different at the beginning (

_{eff}**BOC**) and at the end (

**EOC**) of the cycle.

For example,** one step** of control rods causes **greater response** at EOC than at BOC. Despite the fact that we assume in both cases, that one step causes the same reactivity insertion (e.g. +10pcm). Moreover, this assumption is not always correct, because the control rods worth increases with fuel burnup.

(10 pcm = 1.43 cents for **β _{eff} = 0.007**; 10 pcm = 2.00 cents for

**β**)

_{eff}= 0.005**keff = 0.99** ρ = (keff – 1) / keff = -0.01 ρ = -0.01 * 105 = **-1000 pcm**

## Reactivity in Reactor Kinetics

In preceding chapters, the classification of states of a reactor according to **the effective multiplication factor – k _{eff}** was introduced. The effective multiplication factor –

**k**is a measure of the change in the fission

_{eff}**neutron population**from one neutron generation to the subsequent generation. Also the reactivity as a measure of a reactor’s relative departure from criticality was defined.

In this section, amongst other things it will be briefly described how **the neutron flux** (i.e. the reactor power) changes if **reactivity** of a multiplying system is not equal to zero. An understanding of the **time-dependent behavior** of the neutron population in a nuclear reactor in response to either a **planned** change in the reactivity of the reactor or to **unplanned** and abnormal conditions is of the most importance in the nuclear reactor safety. This subject is usually called **reactor kinetics** (without reactivity feedbacks) or **reactor dynamics** (with reactivity feedbacks and with spatial effects).

**Nuclear reactor kinetics** is dealing with transient **neutron flux changes** resulting from a departure from the critical state, from some reactivity insertion. Such situations arise during operational changes such as control rods motion, environmental changes such as a change in boron concentration, or due to accidental disturbances in the reactor steady-state operation.

## Point Kinetics Equation – One Delayed Neutron Group Approximation

**The simplest equation governing the neutron kinetics of the system with delayed neutrons is** **the point kinetics equation**. This equation states that the time change of the neutron population is equal to the **excess of neutron production** (by fission) **minus neutron loss** by absorption **in one mean generation time with delayed neutrons (ld)**. The role of **ld** is evident. Longer lifetimes give simply slower responses of multiplying systems. The role of reactivity (k_{eff} – 1) is also evident. Higher reactivity gives simply larger response of multiplying system.

If there are neutrons in the system at t=0, that is, if n(0) > 0, the solution of this equation gives **the simplest point kinetics equation with delayed neutrons (similarly to the case without delayed neutrons):**Let us consider that **the mean generation time with delayed neutrons is ~0.085** and k (k_{∞} – neutron multiplication factor) will be step increased **by only 0.01%** (**i.e. 10pcm or ~1.5 cents**), that is k_{∞}=1.0000 will increase to k_{∞}=1.0001.

It must be noted such reactivity insertion (10pcm)** is very small** in case of LWRs (e.g. one step by control rods). The reactivity insertions **of the order of one pcm** are for LWRs **practically unrealizable**. In this case the reactor period will be:

**T = l _{d} / (k_{∞}-1) = 0.085 / (1.0001-1) = 850s**

This is a very long period. In ~14 minutes the neutron flux (and power) in the reactor would increase by a factor of e = 2.718. This is completely different dimension of the response on reactivity insertion in comparison with the case without presence of delayed neutrons, where the reactor period was 1 second.