Материал: Апсе ENVIRONMENTAL PROTECTION 2014

is easy to calculate that YFIZZLE = 0,027 for the implosive NED and

YNOM

YFIZZLE = 8,57 ×10−4 for the gun-type NED.

YNOM

Let assume that nominal energy yield corresponds to the energy yield released by plutonium bomb exploded over Nagasaki (Japan) on August 8, 1945, i.e. about 20 kt TNT. Then,

YFIZZLE = 0.027·20 kt = 540 t TNT for the implosive NED; YFIZZLE = 8.57·10-4·20 kt = 17 t TNT for the gun-type NED.

Despite energy yields in the pre-detonation regime are substantially lower than nominal energy yield, nevertheless, the fizzle yields are sufficiently large and dangerous. Energy yields of chemical high explosives in many terrorist attacks were well below 100 kg TNT.

Probability of maximal energy yield

As is known, probability for the CFR to be initiated prematurely, at the time moment t < T , in the system containing internal neutron sources (for example, radionuclides-emitters of spontaneous fission neutrons) can be determined by using the formula:

P(t < T ) = 1 - exp[- N SFN ×T × (КEFF -1)];

within the time interval [0, T ]. Under assumption on linear time de-

pendency КEFF (t) mean value of КEFF can be determined as

КЭФ = 1 + 0.5T / t0 . Then

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Nominal energy yield can be obtained if the CFR is not initiated before a certain minimal time moment, i.e.

90 ×l

ti ³ ti,min = t0 × (1 - prompt )1/ 2 .

t 0

Consequently, P(t < ti,min ) is a probability for the CFR to be initiated before the time moment that can guarantee nominal energy yield, and 1 - P(t < ti,min ) is a probability for the NED to produce nominal energy yield:

Experimental information about generation rate of spontaneous fission neutrons by plutonium isotopes (Table 4.1), about isotope compositions of WG-Pu and RG-Pu U (Table 4.2) can be used to determine total generation rate of spontaneous fission neutrons in critical masses of WG-Pu (~5 kg) and RG-Pu (~8 kg):

N SFN (WG - Pu) = 3.9 ×105 n / s.

N SFN (RG - Pu, PWR) = 2.9 ×106 n / s.

By substituting these values into equation (4.2), the following probabilities can be calculated. Probability of nominal energy yield produced by the implosive NED ( t0 = 10−5 s ) charged with WG-Pu is

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equal to 23,5%. Similar probability for the implosive NED charged with RG-Pu from heavy-water CANDU reactor (minimal value of N SFN ) is

considerably lower, about 1.2·10-3 %.

Probability of nominal energy yield produced by the gun-type NED ( t0 = 10−4 s ) even charged with WG-Pu is evaluated as a very small

value, about 1.3·10-5 %.

Consequently, RG-Pu of any isotope composition is able to produce nuclear explosion only with minimal energy yield (“ fizzle yield”) even in the implosive NED.

So, the following conclusions can be made:

1.Reactor-grade plutonium extracted from SNF of nuclear power reactors is a potentially dangerous material from the standpoint of its military applications.

2.Critical mass of reactor-grade plutonium is insignificantly larger than critical mass of weapon-grade plutonium (9-10 kg via 5-6 kg).

3.Main factor that can complicate any military application of reactor-

grade plutonium is an intense generation of spontaneous neutrons by plutonium isotope 240Pu. These neutrons are able to initiate the CFR prematurely and, thus, substantially reduce the expected energy yield.

4.Nevertheless, even minimal energy yield from any NED charged with reactor-grade plutonium can reach tens or hundreds of TNT tons. Therefore, reactor-grade plutonium must be put under strict control to prevent its diversion from civilian to military purposes.

Control questions to Chapter 4

1.Characterize main physical properties of plutonium isotopes.

2.By what does reactor-grade plutonium differ from weapon-grade plutonium?

3.What are main peculiarities of the implosive and gun-type nuclear explosive devices?

4.What conditions must be satisfied for a nuclear explosion to produce minimal and maximal energy yields?

5.Why is reactor-grade plutonium regarded as a dangerous material for nuclear non-proliferation regime?

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CHAPTER 5. UTILIZATION OF REACTOR-GRADE PLUTONIUM AND WEAPON-GRADE PLUTONIUM IN NUCLEAR POWER REACTORS

As it was already mentioned above, theoretical evaluations have demonstrated that reactor-grade plutonium (RG-Pu) can be used as a charge only in primitive and low-efficient nuclear explosive devices (NED) with relatively small energy yield (at the level of several hundreds of TNT tons). Nevertheless, even so small energy yield represents a serious military threat. Therefore, these theoretical conclusions about RG-Pu applicability for military and terrorist purposes have been supported by the IAEA. In 1972 the IAEA issued the document that confirmed that plutonium of any isotope composition, including RG-Pu extracted from spent fuel of nuclear power reactors, must be regarded as a weapon-usable material, quite like weapon-grade plutonium (WG-Pu). This statement means that strict national requirements to physical protection, control and accountability of WG-Pu must be spread on RG-Pu. Only plutonium containing above 80% 238Pu can be excluded from the IAEA safeguards because 238Pu is an intense heat source (570 W/kg) and intense emitter of spontaneous fission neutrons (2.6·106 n/(s·kg), i.e. 238Pu is more intense neutron source than 240Pu by a factor of 2,5) Therefore, radiochemical plants for SNF reprocessing have been built and put in operation only in those countries which already possessed nuclear weaponry. Also, the IAEA has worked out very strict limitations on accuracy of physical inventory taking for plutonium-bearing materials at the SNF reprocessing plants.

5.1. Utilization of weapon-grade plutonium in nuclear power reactors

In 2007 global stockpiles of WG-Pu were evaluated as ~230 tons (Russia – 120 t, USA – 90 t, Great Britain – 8 t, F rance – 5 t and China

– 4 t). In 1993-2010 the USA and Russia have undert aken some steps towards nuclear disarmament.

In 1993 Russia has decided to withdraw 500 tons of weapon-grade uranium (WG-U) from its nuclear arsenals, dilute WG-U with lowenriched uranium and sell this reactor-grade uranium to the USA for

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energy utilization at commercial NPP. In 1994 the USA administration declared that 174 tons of WG-U are no longer required for national defense purposes. So, this amount of WG-U was diluted with lowenriched uranium and used at commercial NPP. In 2005 the USA decided to withdraw additional 200 tons of WG-U from nuclear arsenals and use them at NPP.

In 1995 the USA and Russia declared that 50 tons (in each country) of WG-Pu are no longer required for national defense purposes. The USA and Russia agreed to use some excessive amounts (34 t in each country) of their WG-Pu in nuclear power industry.

By 2010 about 2300 RG-Pu tons were accumulated in SNF discharged from commercial NPP all over the world. Only ~30% of these RG-Pu FК 2010 годуamounts were extracted from SNF and used in European LWR. So, addition of 34 WG-Pu tons converted into spent RG-Pu after energy utilization in commercial LWR can not significantly increase the scope of works on plutonium utilization.

The US National Academy of Sciences has analyzed some potential ways for safe management and energy utilization of those WG-Pu amounts which have been declared as excessive for national defense purposes. The following four strategies of WG-Pu management were regarded as the most promising options:

1. Incorporation of WG-Pu into SNF compositions.

The SNF-incorporation option presumes introducing WG-Pu into fresh fuel compositions of power LWR and energy utilization of such fuels in the once-through fuel cycle. In this case spent WG-Pu containing fuel is characterized by practically insurmountable physical, chemical and radiation barriers radiochemical extraction and military application of “dirty” plutonium.

2. Short-term irradiation of WG-Pu.

This strategy presumes incorporation of WG-Pu into MOX-fuel composition and short-term irradiation of so modified MOX-fuel in power LWR. The strategy can form the remarkably weaker physical, chemical and radiation barriers against military applications of irradiated WG-Pu than the SNF-incorporation option but time expenses and the number of nuclear power reactors involved into the WG-Pu utilization program could be substantially shortened.

3. Full extermination of WG-Pu.

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