DE2518877C3 - Method for determining the burn-up of nuclear fuel in fuel rod tubes and / or fuel assemblies - Google Patents

Description

or for the river time

10 Φ Τ =

Cr ⁵⁰ Cr ⁵⁰

In - ^ ₅₂ - (O) - In - ^ ₃₃ - (Γ)

The invention relates to a method for determining the burn-up of fuel rod cladding tubes and / or fuel assemblies located nuclear fuel by measuring the neutron radiation caused shift of the isotope concentration of an element with a mass spectrometer.

In this example from »Nuclear Energy«, 16th year (1973), Pages 362 to 369 known method is the uranium content or the content of fission products Determined by mass spectrometry by measuring the isotope shift in the fuel. Such However, measurement is only possible by destroying the fuel rod cladding tubes or fuel assembly boxes.

The object of the invention is to simplify the method mentioned at the beginning.

This is achieved according to the invention in that the shift in the isotope concentration of an im Cladding tube and / or box material containing element is measured. It has surprisingly been shown that the inventive, much simplified method most accurate and the previously known Procedure delivers equivalent results.

As is well known, the following relationship applies to the burnup bu:

bu = (I - e "'■ *'') ■ 100% fifo

(Fissures per originally used fissile atoms).

Here ör is the gap cross section.

The quantity ΦΤ, called the flow time, is the integral of the neutron flow over time, \ _{y £ w}

o ⁵⁰ and o ⁵² represent suitable total reaction cross-sections averaged over the reactor spectrum for the corresponding Cr isotopes.

If two stable isotopes lie next to one another, such as Ni "and Ni * ', Cr * ² and Cr" or Fe ⁵ * and Fe ", the second isotope is formed from the first by the absorption of a neutron, i.e.

20 Fe ⁵⁶ (η, γ) Fc ⁵⁷ .

The following equations then apply:
Fc ⁵ "(r) = Fe ⁵⁶ IO) C"" ⁵ ** ¹

and

- ~~ = Fe ^5h (/) « ^5h Φ - Fe ⁵⁷ α ⁵⁷ Φ. ^M d /

With the solution Fc " (,) = Fe "(Oje -" "· '1 + "Fc ⁵⁷ TO) (τ ^5ΪΓ - -T ⁵ T ' 40 you get Fc ⁵⁷ = ^F C (O) e «" * - ""> - 45 _r r ^5h , ",,, (T '^ - <f

φ Τ =

ΦU) dt.

where Φ ^ είηεη represents mean value.

In the case of high burn-ups, such as those planned especially for fast breeder reactors, shifts occur in the isotope ratios of the metal atoms of the cladding tube and / or box material, which are caused by the different strengths of absorption of neutrons of individual isotopes such as Cr ⁵⁰ and Cr ⁵² or Ni ⁶² and Ni ^{64 are} caused.

The following applies approximately to the change in isotope concentrations

Cr ⁵⁰ Ii) = Cr ⁵¹¹ IO) C """"'
Cr ⁵² If) = Cr ⁵² IO) C "" * '.

It is now very easy to measure the flow time to which the irradiated fuel elements were exposed possible. When the fuel assemblies are discharged, it increases

bo a superficial sample of the cladding tube or box material, which can be obtained by smearing with a rough sampler, and this sample is placed in a mass spectrograph with which isotope ratios can be determined with an accuracy of about 0.01%. In the already achieved today Flußzeiten of 10 ²² n / cm ^2, a difference in the cross sections of less than 10 ²⁴ cm ² is sufficient to enable measurement of the flow time.

Claims (1)

Claim: Method for determining the burn-up of fuel rod cladding tubes and / or fuel assembly boxes Nuclear fuels by measuring those produced by neutron irradiation Shifting the isotope concentration of an element with a mass spectrometer, thereby characterized in that the shift in the isotope concentration of one in the cladding tube and / or Box material contained element is measured. One therefore obtains for the isotope ratio after the reactor time T