{"id":18482,"date":"2018-08-29T16:26:37","date_gmt":"2018-08-29T16:26:37","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=18482"},"modified":"2023-02-03T20:24:47","modified_gmt":"2023-02-03T20:24:47","slug":"neutron-fluence-what-is-fluence","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-operation\/fuel-burnup\/units-of-fuel-burnup\/neutron-fluence-what-is-fluence\/","title":{"rendered":"Neutron Fluence – What is Fluence"},"content":{"rendered":"
Neutron fluence<\/strong>, previously referred to as the neutron dose<\/strong>, is defined as the time integral<\/strong> of the neutron flux density<\/strong>, expressed as the number of particles (neutrons) per cm2<\/sup>. Neutron fluence is primarily defined for material engineering<\/a>\u00a0but is widely used by reactor engineers as a unit of fuel burnup.<\/div><\/div>\n

In nuclear engineering, we have to distinguish between neutron flux density<\/strong><\/a>, neutron intensity<\/strong><\/a>, and neutron fluence.<\/strong><\/p>\n

Neutron Fluence and Fuel Burnup – Neutrons per Kilobarn<\/h2>\n

Neutron fluence can be used as a measure of fuel burnup<\/a> as well since reaction rate is given by the product RR = \u0424 . \u03a3<\/strong>, the rate of burnup is proportional to the neutron flux. The accumulated burnup over a specific period (t) is therefore proportional to the product of flux and time (F = \u0424 . t<\/strong> or F = \u222b\u0424dt<\/strong>). This product is known as the neutron fluence or the total neutron exposure of the fuel. The units of neutron fluence are also neutrons per m2<\/sup>, but, in practice, it is often expressed in neutrons per kilobarn<\/strong>:<\/p>\n

1 n\/kb = 1025<\/sup> neutrons per m2<\/sup><\/p>\n

For example, let assume the fuel in PWR, which is irradiated with flux on the order of 3×1017<\/sup> neutrons.m-2<\/sup>s-1<\/sup> for approximately 4 years. The total fluence of discharged fuel is then 4 n\/kb.<\/p>\n

Neutron Fluence and Irradiation Embrittlement<\/h2>\n

During the nuclear power plant operation, the material of the reactor pressure vessel<\/a> and the material of other reactor internals are exposed to neutron radiation<\/a> (especially to fast neutrons), which results in localized embrittlement<\/strong> of the steel welds in the area of the reactor core. Irradiation embrittlement<\/strong> can lead to loss of fracture toughness. Typically, the low alloy reactor pressure vessel steels are ferritic steels that exhibit the classic ductile-to-brittle transition<\/strong> behavior with decreasing temperature. This transitional temperature is of the highest importance during plant heat up.<\/p>\n

Failure modes:<\/p>\n